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PHYSICOCHEMICAL CHARACTERIZATION AND CATALYTIC APPLICATION OF
NANOCOMPOSITE OXIDES AND CLAY BASED NANOPOROUS MATERIALS FOR
SYNTHESIS OF SOME BIOLOGICALLY IMPORTANT MOLECULES
A THESIS
Submitted by
SATISH SAMANTARAY
for the award of the degree
of
DOCTOR OF PHILOSOPHY
DEPARTMENT OF CHEMISTRY
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA ROURKELA – 769 008, ODISHA
February 2012
Dedicated To
My family
Dr. Braja Gopal Mishra Associate Professor
DEPARTMENT OF CHEMISTRY
NIT, Rourkela-ODISHA
THESIS CERTIFICATE
This is to certify that the thesis entitled “PHYSICOCHEMICAL CHARACTERIZATION
AND CATALYTIC APPLICATION OF NANOCOMPOSITE OXIDES AND CLAY BASED
NANOPOROUS MATERIALS FOR SYNTHESIS OF SOME BIOLOGICALLY
IMPORTANT MOLECULES” submitted by Satish Samantaray to the National Institute of
Technology, Rourkela for the award of degree of Doctor of Philosophy is a bonafide record of
research of research work carried out by him under my supervision. I am satisfied that the thesis
has reached the standard fulfilling the requirements of the regulations relating to the nature of the
degree. The contents of this thesis in full or in parts have not been submitted to any other
Institute or University for the award of any degree or diploma.
Rourkela-769 008 Research Guide
Date:
(Dr. B. G. Mishra)
i
aCKNOWLEDGEMENT
As I am finishing up the writing of my thesis, I realize that the completion of this research
that has been carried out at National Institute of Technology, Rourkela is the result of
group work, rather than my individual effort. During this period, I came across with a great
number of lovely people whose contributions directly or indirectly helped my research and
they deserve special thanks. I am indeed grateful to all who have assisted me over this
period.
The first and foremost acknowledgement goes to my esteemed supervisor Prof. Braja
Gopal Mishra for providing his guidance, constant encouragement and technical
suggestions. It has been a rewarding experience working with him and I feel fortunate
enough to have such a deep involvement from my supervisor. It has been a great honour
for me being a scholar of him. I also express my indebtedness to him for his affectionate
and friendly attitude. His patience, empathy and relentless encouragement contributed to
my stay in the laboratory being most happy and memorable one. His involvement with
originality has triggered, nourished and perfumed my intellectual maturity that will help
me for a long time to come. I am also thankful to his family members for their hospitality
whenever I visited them.
It is also my great pleasure to acknowledge to Prof. G. Hota, my co-supervisor for
providing me with the necessary help for making this research work a success.
I wish to express my sincere thanks to Prof. R. K. Patel, Chairman of doctoral committee
and the Ex-Head of Chemistry Department, NIT Rourkela, for his selfless support and
advice and for extending the necessary infrastructure and facilities.
ii
I wish to place on record my thankfulness to doctoral committee members Prof. K. P.
Maiti, Prof. S. Patel and Prof. A. Mondal, for their suggestions and keen interest in my
work.
I would like to express my gratitude to the all the faculty and members of staff of the
Department of Chemistry, National Institute of Technology, Rourkela for their constructive
critics and suggestions.
I wish to express my sincere thanks to Prof. D. K. Pradhan, Department of Physics, NIT,
Rourkela for his interest and advice.
Words are inadequate to express my deep sense of gratitude to Mr. S.K. Swain for his
constant inspiration and help at any adversable condition.
My heartfelt thanks to Mr. R. Pattanaik, Mr. U.K. Sahu, and Mr. R.K. Patel, for their kind
co-operation in acquiring various experimental data presented in this thesis.
I am grateful to SAIF, IIT Mumbai for availing TEM facility.
I am indebted to my labmates Sudhir, Prabhat, Sujan, Anindya, Purabi, Nibedita, Lipika,
Alka and Aparajita for their help and support during my stay in laboratory and making it a
memorable experience in my life.
My heartfelt gratitude to Rabi bhai, Biswajit bhai, Upendra bhai, Sarat bhai, Subrata bhai,
Naresh bhai, Ramesh Bhai, Anil bhai, Sanjaya bhai, Sushanta bhai and Sudhansu dada for
showering their love, help and affection abundantly and making my stay at NIT, Rourkela a
memorable and pleasant one.
iii
My profound and deep feelings are towards my friends Amitabh, Barun, Auro, Sudhir,
Prabhat, Senthil, Prakash, Satya, Abhipsa, Seema & Tapaswini for their friendship and
constant help throughout my doctoral programme.
I want to acknowledge as well as share my happiness with my companions from DBA
Hostel.
Last but not the least; I am ever indebted to my parents, brother, sisters and to my relatives
for their moral support, wish and encouragement.
Satish Samantaray
iv
ABSTRACT
KEYWORDS: Combustion synthesis, Pillared clay, Amorphous citrate process, Sol-gel
combustion, Amidoalkyl naphthols, Acetamidoketones,
Octahydroquinazolinones, Octahydroxanthenes, -aminocarbonyl
compounds, β-nitro alcohols and arylidenemalononitriles
A series of MoO3-ZrO2 nanocomposite oxides with MoO3 content in the range of 2-50
mol% were prepared by coprecipitation, impregnation and combustion synthesis methods. The
synthesized materials were characterized using different analytical techniques such as XRD, UV-
Vis, SEM, TEM, Sorptometric and Raman techniques. The presence of MoO3 in the zirconia
matrix found to selectively stabilize the tetragonal phase of zirconia. The crystallite size, surface
area and the amount and strength of the acidic sites on the catalyst surface strongly depend on
the molybdenum oxide content and the method of preparation. UV-Vis and Raman study of the
composite oxide indicated the presence of isolated molybdate and polymolybdate clusters at low
molybdna content whereas at higher loading presence of bulk type polymolybdates are observed
on zirconia matrix. The effect of fuel nature and content of on the physicochemical
characteristics of the MoO3-ZrO2 nanocomposite oxides was studied by taking three different
fuel (urea, glycine and hexamethylenetetraamine) and F/O ratio in the range 0.5-1.5 in the
combustion method. Nanocomposite oxides with better physicochemical properties are obtained
using glycine as fuel. The catalytic activity of the coprecipitated samples was evaluated for the
one pot synthesis of amidoalkylnaphthols and acetamidoketones. Similarly, the combustion
synthesized samples were used as an efficient and environmentally benign catalyst for the
synthesis of octahydroquinazolinones by multicomponent condensation reaction of dimedone,
urea and arylaldehydes in aqueous media as well as under solvent free condition using
microwave irradiation.
Iron stabilized zirconia materials were prepared by coprecipitation method. Sulfate ions
are grafted to the Fe2O3-ZrO2 surface by post sulfation method. The sulfate grafted Fe-Zr
materials were characterized by XRD, UV-Vis, FTIR, SEM and TEM techniques. XRD study
indicated the formation of a substitutional solid solution and selective stabilization of the
tetragonal phase of zirconia. The presence of iron oxide in zirconia lattice improves the sulfate
retention capacity of host zirconia. The catalytic application of SO42-
/FeZr catalysts was studied
v
for one pot synthesis of octahydroxanthene by condensation of dimedone and aryl aldehydes
under solvent freecondition and microwave irradiation. The catalyst was found to be highly
efficient for synthesis of structurally diverse octahydroxanthene giving good yield and purity of
the products.
Zr-pillared clay (Zr-P) was synthesized by insertion of zirconium oxyhydroxy clusters
into clay interlayer and subsequent thermal activation. The Zr-P materials were treated with
sulfuric acid to graft sulfate ions onto the zirconia nanopillars. The sulfate grafted Zr-pillared
clay (SZr-P) was used as support for phosphomolybdic acid (PMA/SZr-P). The synthesized
materials were characterized by XRD, IR, UV–Vis, SEM and sorptometric techniques. The
expansion of the clay lattice as a result of pillaring was confirmed from X-ray diffraction study.
The interlayer space was found to be retained in the PMA/SZr-P material. All pillared clay
materials show Type-I sorption isotherm typical of microporous materials. The structural
integrity of the phosphomolybdic acid in the clay matrix was ascertained from the IR and UV-
Vis spectroscopy. The PMA/SZr-P material was used as an efficient catalyst for the one pot
synthesis of -aminocarbonyl compounds by Mannich reaction in aqueous media. The
multicomponent condensation of aryl aldehyde, amines and ketones using PMA/SZr-P material
afforded a variety of -aminocarbonyl compounds in excellent yield and purity.
Ceria based nanocomposite oxides, CeO2-CaO and CeO2-MgO, of different compositions
were prepared by amorphous citrate process and citrate gel combustion process. These materials
were characterized by using different analytical techniques such as XRD, TGA, UV-Vis-NIR,
SEMand TEM. The XRD analysis of the CeO2-CaO oxide indicated the presence of a
substitutional type solid solution for ceria rich composite oxides where as for the calcium oxide
rich composite oxides, the presence of crystalline CaO was observed along with the solid
solution phase. In case of CeO2-MgO material, a minor contraction in the lattice parameter was
observed for the ceria component which has been ascribed to the formation of a small amount of
nonequilibrium MgxCe1-x/2O2 solid solution. The catalytic activity of CeO2-CaO materials was
evaluated for the aqueous phase synthesis of 2-amino-2-Chromenes in aqueous media. Similarly,
CeO2-MgO nanocomposite oxides were used as heterogeneous catalyst for synthesis of β-nitro
alcohols andarylidenemalononitriles. For the synthesis of both classes of compounds, the
composite oxides showed excellent catalytic activity giving good yield and purity of products
under ambient condition.
vi
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS………………………………………………………………. i
ABSTRACT ………………………………………………………………………………. iv
TABLE OF CONTENTS…………………………………………………………………. vi
LIST OF TABLES.............................................................................................................. xii
LIST OF FIGURES …………………………………………………………………....... xv
ABBREVIATIONS …………………………………………………………………......... xx
NOTATIONS ……………………………………………………………………………...
xxi
CHAPTER 1 INTRODUCTION
PART A: Catalytic chemistry of zirconia based materials
1.1 General Introduction ……………………………………………………… 1
1.2 Structural properties and phase transformation of zirconia.....……………. 2
1.3 Importance of zirconia based materials in catalysis...……………………... 6
1.4 Surface and structurally modified zirconia in catalysis…………..……….. 8
1.4.1 Sulfate grafted zirconia …………………………………………………… 9
1.4.2 WOx grafted zirconia…...…………………………………………………. 16
1.4.3
MoO3 species grafted on zirconia ...………………………………………. 18
PART B Catalytic chemistry of clay materials
1.5 Introduction to the clay materials…………………………………..……. 21
1.6 Structure and classification of clay materials ...………………………… 22
1.7 Disadvantages of clay materials as catalysts …………………………….. 25
1.8 Modification of Clay Materials …………………………………………… 25
1.9 Pillaring of clay by inorganic polycations ………………………………... 26
1.10
Catalytic applications of pillared clays …………………………………… 31
vii
PART C Ceria in catalysis
1.11 Introduction ………………………………………………………..……… 37
1.12 Structure of ceria…………………………......…………………………… 38
1.13 Thermal stability and reduction characteristics of doped ceria…....……… 40
1.14 Metal-ceria interaction…...………………... ...…………………………… 42
1.15 Ceria in catalysis.…………………………………………………..……… 43
1.15.1 Three way catalysis...……………………... ...…………………………… 44
1.15.2 Oxidation of environmental pollutants…........……………………………. 46
1.15.2 Ceria in organic transformations…….…........……………………………. 47
1.16
Objectives of the present study.…………………………………..………. 51
CHAPTER 2 MATERIALS AND METHODS
2.1 Preparation of catalytic materials……..…………………………………… 55
2.1.1 Preparation of ZrO2, MoO3 and MoO3 (x mol%)-ZrO2 catalysts ………… 55
2.1.1.1 Coprecipitation method…..……………………………………………….. 55
2.1.1.2 Wet impregnation method…………….…………………………………… 55
2.1.1.3 Combustion method…… …………….…………………………………… 56
2.1.2 Preparation of sulfated zirconia catalyst (SO42-
/ZrO2).....…………………. 57
2.1.3 Preparation of sulfate grafted Fe2O3 (x mol%)-ZrO2
catalysts (SO42-
/xFe-Zr)……………………………………………………
57
2.1.4 Preparation of Zr-pillared clays and their modified analogues……………. 58
2.1.4.1 Preparation of Zr-pillared clay……………………...……………………... 59
2.1.4.2 Preparation of sulfate grafted zirconia pillared clays (SZr-P)………...…... 59
2.1.4.3 Preparation of phosphomolybdic acid dispersed on sulfate grafted
Zr-pillared clays …………………………………………………………...
59
2.1.5 Preparation of CaO, CeO2 and CeO2(x mol%)-CaO catalyst…………….. 60
2.1.6 Preparation of MgO, CeO2 and CeO2 (x mol%)-MgO catalysts…………... 60
2.1.6.1 Sol-gel combustion synthesis….………...................................................... 60
2.1.6.2 Amorphous citrate process………………………………………………… 61
2.2 Characterization of the Catalysts …………………………………...…….. 62
viii
2.2.1 Powder X-Ray Diffraction (XRD) ……………………………………….. 62
2.2.2 UV-Vis-NIR Spectroscopic Analysis ……………………………….......... 62
2.2.3 Thermogravimetric Analysis ………………………………..…………….. 63
2.2.4 Infra Red Spectroscopy …………………………………………………… 63
2.2.5 Raman Spectroscopy………. ……………………………………………... 63
2.2.6 Gas Chromatography……………………………………………………… 64
2.2.7 NMR Spectroscopy………………………………………………………... 64
2.2.8 Sorptometric Studies ……………………………………………………… 64
2.2.9 Scanning Electron Micrograph (SEM-EDX) ……………………………... 64
2.2.10 Transmission Electron Micrograph ……………………………………….. 64
2.2.11 Surface Acidity and Basicity ……………………………………………… 65
2.2.11. n-Butylamine Method …………………………………………………….. 65
2.2.11.2 Trichloroacetic Acid Method ……………………………………………... 65
2.2.11.3 Brønsted Acidity of Pillared Clays by TG Analysis of n-butylamine ……. 65
2.3 Catalytic Activity………. ………………………………………………… 66
2.3.1 Synthesis of amidoalkyl naphthols ……………………………………….. 66
2.3.2 Synthesis of -acetamidoketones ………………………………………… 67
2.3.3 Synthesis of octahydroquinazolinones ….…………………………….….. 67
2.3.4 Synthesis of -aminocarbonyl compounds.………………………………. 68
2.3.5 Synthesis of 2-amino-2-chromenes ………………………………………. 68
2.3.6 Synthesis of arylidenemalononitriles……………………………………… 68
2.3.7 Synthesis of β-nitro alcohols………………………………………………. 69
2.3.8
Synthesis of octahydroxanthenes………………………………………….. 70
CHAPTER 3 CATALYTIC STUDIES ON ZrO2 BASED NANOCOMPOSITE
OXIDES
PART A: ZrO2-MoO3 composite oxides
3.1 Introduction ……………………………………………………………… 72
3.2 Results and Discussion..…………………………………………………… 74
3.2.1 XRD study……………………………………… ………………………… 74
ix
3.2.2 UV-VIS Study………………… ………………………………………….. 80
3.2.3 Raman Study………………………... ……………………………………. 83
3.2.4 TEM study……………………………………………………....…………. 86
3.2.5 Catalytic Studies ………………………………………..…………………. 88
3.2.5.1 Multicomponent synthesis of 1-amidoalkyl 2-naphthols………………….. 88
3.2.5.2
Synthesis of -acetamidoketones ..………………………………………... 94
PART B: Combustion synthesized ZrO2-MoO3 composite oxides
3.3 Introduction ………………………………………………………………. 99
3.4 Results and Discussion ………………………………………..………….. 102
3.4.1 XRD study ………………………………………………………………… 102
3.4.2 UV-Vis Study .………………………………………………………..…... 110
3.4.3 SEM Study………………………………………………………………… 113
3.4.4 TEM study……………………………………………………………...….. 116
3.4.5
Catalytic studies for synthesis of octahydroquinazolinones ……………… 119
PART C: Sulfate grafted Fe2O3-ZrO2 nanocomposite oxides
3.5 Introduction ……………………………………………………………….. 126
3.6 Results and Discussion ………………………………………..………….. 127
3.6.1 XRD study ………………………………………………………………… 127
3.6.2 IR Study .………………………………………………………………….. 131
3.6.3 UV-Vis-DRS study…...…………………………………………………… 133
3.6.4 SEM study……………………………………………………………...….. 135
3.6.5 TEM study………………………………………………... ………………. 136
3.6.6 Catalytic studies for the synthesis of xanthenediones…..……………...….. 137
3.7 Conclusion………………………………………………....………………. 142
x
CHAPTER 4 CATALYTIC APPLICATION OF Zr-PILLARED CLAY BASED
NANOPOROUS MATERIAL FOR SYNTHESIS OF -
AMINOCARBONYL COMPOUNDS
4.1 Introduction ……………………………………………………………….. 143
4.2 Results and Discussion .…………………………………………………… 147
4.2.1 XRD study …………….………………………………………………….. 147
4.2.2 FT IR study………………………………………………………………... 150
4.2.3 UV-Vis study……………………………………………………………… 152
4.2.4 Nitrogen sorption study…………….……………………………………… 154
4.2.5 SEM study…………………...…………………………………………..... 156
4.2.6 TEM study…………………………………………………………………. 158
4.3 Catalytic studies for the synthesis of -aminocarbonyl compounds..…….. 159
4.4
Conclusion…………………...…………………………………………..... 162
CHAPTER 5 CATALYTIC APPLICATION OF CeO2 BASED
NANOCOMPOSITE OXIDES FOR SYNTHESIS OF SOME
BIOLOGICALLY IMPORTANT MOLECULES
PART A: CeO2-CaO nanocomposite oxides
5.1 Introduction ………………………………………………………………. 164
5.2 Results and Discussion …………………………………………………… 166
5.2.1 XRD study……………………………………………………………..…. 166
5.2.2 TG analysis ……………………………………………………………….. 175
5.2.3 UV-Vis-DRS study……………………………………………………….. 176
5.2.4 SEM and TEM study……………………………………………………... 179
5.2.5
Catalytic studies for the synthesis of 2-amino-2-chromenes……………... 182
PART B: CeO2-MgO nanocomposite oxides
5.3 Introduction ………………………………………………………………. 187
5.4 Results and Discussion …………………………………………………… 189
5.4.1
Characterization of the composite oxides prepared by amorphous citrate
and gel combustion process ……………………………………………….
189
xi
5.4.1.1 XRD study……..………………………………………………………..... 189
5.4.1.2 UV-Vis-DRS study ……………………………………………………..... 195
5.4.1.3 SEM and TEM study……………………………………………………... 197
5.4.2 Knoevenagel condensation reaction……………………………………… 201
5.4.3 Henry reaction…………………………………………………………….. 204
5.5
Conclusion………………………………………………………………… 207
CHAPTER 6 SUMMARY AND CONCLUSIONS …………………………… 209
REFERENCES ………………………………………………....... 217
LIST OF PUBLICATIONS …………………………………….. 252
xii
LIST OF TABLES
Table
Title
Page No.
1.1
1.2
1.3
1.4
1.5
1.6
1.7
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Applications of zirconia based materials in catalysis ………..
Catalytic applications of sulfated zirconia based materials……
Catalytic applications of WO3-ZrO2 materials…………………
Different types of pillaring species used for the preparation of
pillared clays …………………………………………………..
Properties of the parent clay before and after pillaring………...
Catalytic applications of various pillared clay based catalysts...
Ceria based materials used in organic transformations………..
Percentage tetragonal phase in the xMoZr and xMoZrI
catalysts calculated using the Toroya’s method………………..
Calculated crystallite size and lattice strain by Fourier line
profile analysis from broadened X-ray profile…………………
Physicochemical characteristics and catalytic activity of
xMoZr catalysts………………………………………………..
Solvent free synthesis of amido alkyl naphthols using 20MoZr
catalyst…………………………………………………………
Solvent free microwave assisted synthesis of amido alkyl
naphtholsusing 20MoZr catalyst……………………………….
Effect of the reaction media on the synthesis of amido alkyl
naphthols(Table 3.4, entry 10) using 20MoZr catalyst………..
Solvent free one pot synthesis of β-acetamido ketones using
20MoZr catalyst……………………………………………….
Different Fuels used in combustion synthesis…………………
The lattice parameters, unit cell volume, crystallite size and
rms strain of ZrO2-G and 10MoZr-G materials………………..
8
12
17
29
30
35
50
79
80
89
91
92
93
96
101
108
xiii
3.10
3.11
3.12
3.13
3.14
3.15
3.16
3.17
4.1
4.2
5.1
5.2
5.3
5.4
5.5
The lattice parameters, unit cell volume, crystallite size and
rms strain of xMoZr-U materials………………………………
Physicochemical characteristics and catalytic activity of
various catalysts screened for the synthesis of
octahydroquinazolinone under microwave irradiation and
solventfree condition…………………………………………...
10MoZr-G (F/O=1) catalyzed synthesis of
octahydroqunazolinones under solventfree condition and
microwave irradiation………………………………………….
10MoZr-G(F/O=1) catalyzed synthesis of
octahydroqunazolinones in aqueous media…………………….
Catalytic activity of SO42-
/xFe-Zr nanocomposite catalysts…...
Effect of microwave power on catalytic activity of SO42-
/10Fe-
Zr catalyst………………………………………………………
Effect of catalyst amount on catalytic activity of SO42-
/10Fe-Zr
catalyst…………………………………………………………
SO42-
/10Fe-Zr catalyzed synthesis of xanthenediones under
solvent free condition and microwave irradiation……………..
Physicochemical characteristics and catalytic activity of the
pillared clay materials………………………………………….
PMA/SZr-P catalyzed synthesis of -aminocarbonyl
compounds……………………………………………………..
Physicochemical characteristics of xCe-Ca-O composite
oxides…………………………………………………………..
Catalytic activity of xCe-Ca-O composite oxides…………….
20Ce-Ca-O catalyzed synthesis of 2-amino-2-chromenes in
aqueous media………………………………………………….
Structural parameters of xCe-Mg-O nanocomposite oxides…...
Crystallite size and lattice strain of xCe-Mg-O-A
nanocomposite oxides………………………………………….
109
121
122
123
139
139
140
140
150
161
172
184
185
192
195
xiv
5.6
5.7
10Ce-Mg-O catalyzed Knoevenagel condensation reaction…...
10Ce-Mg-O-A catalyzed Henry reaction in aqueous media…...
203
206
xv
LIST OF FIGURES
Figure
Title
Page No.
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
3.1
3.2
3.3
Hard sphere model for the fluorite structure of ZrO2……………
(a) The fluorite structure of ZrO2……………………………….
(b) The tetrahedral sites………………………………………….
Fluorite structure of ZrO2………………………………………..
Monoclinic and Tetragonal unit cell of ZrO2……………………
Schematic presentations of the Bronsted and Lewis acidic sites
in sulfated zirconia proposed by Arata and Hino……………….
Schematic presentations of the Bronsted and Lewis acidic sites
in sulfated zirconia proposed by Clearfield et al………………..
Silicate sheet of a clay mineral (a) corner sharing of SiO4 unit
and (b) top view of the basal oxygen plane …………………..
Alumina octahedral sheets in clay mineral ……………………
Structure of 2:1 clay mineral montmorillonite ………………..
Schematic representation of the process of pillaring of the clay
sheets …………………………………………………………
Fluorite structure of ceria ……...……………………………....
Effect of air to fuel ratio on the conversion efficiency of three-
way catalyst ………………………………….………………...
X-ray diffraction patterns for (a) ZrO2, (b) 2MoZr, (c) 5MoZr,
(d) 10MoZr and (e) MoO3 materials prepared by coprecipitation
method ………………………………………..............................
X-ray diffraction patterns of (a) 10MoZr, (b) 10MoZrI, (c)
20MoZr, (d) 20MoZrI, (e) 50MoZr and (f) 50MoZrI materials…
UV-Vis spectra of the MoO3-ZrO2 composite oxide materials:
(a) ZrO2, (b) 2MoZr, (c) 5MoZr, (d) 10MoZr, (e) 20MoZr and
(f) 50MoZr in panel I and (a) 10MoZr-I, (b) 20MoZr-I, (c)
50MoZr-I and (d) MoO3 in panel II……………………………
3
4
4
6
11
11
22
23
24
27
39
45
76
78
82
xvi
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
Raman Spectra of (a) 2MoZr, (b) 5MoZr, (c) 10 MoZr and (d)
20MoZr (In the inset (e) 20MoZrI, and (f) MoO3)
materials…………………………………………………………
Transmission electron micrograph of (a) ZrO2 and (b) 20MoZr
materials.…………………………………………………………
X-ray diffraction patterns of (a) ZrO2-C, (b) ZrO2-U, (c) ZrO2-
H, (d) ZrO2-G, (e) 10MoZr-C, (f) 10MoZr-U, (g) 10MoZr-H
and (h) 10MoZr-G (T and M refers to tetragonal and monoclinic
phases of zirconia, respectively)…………………………………
X-ray diffraction patterns of (a) ZrO2-U, (b) 2MoZr-U (c)
5MoZr-U, (d) 8MoZr-U (e) 10MoZr-U and (f) 20MoZr-U (All
compositions prepared using urea as a fuel, T refers to
tetragonal phase)…………………………………………………
X-ray diffraction patterns of 10MoZr catalyst prepared using
hexamethylene tetraamine as fuel (Panel I) and glycine as fuel
(Panel II) at different fuel to oxidizer ratio in the range of
F/O=0.5-1.5 (T refers to the tetragonal phase)…………………..
Fourier line profile analysis plots for the ZrO2-U, ZrO2-H and
ZrO2-G materials………………………………………………...
pV~L plot for 10MoZr-G materials at different F/O ratio ……...
As~L plot for 10MoZr-G materials at different F/O ratio……….
UV-Vis spectra of MoZr catalyst (a) ZrO2-U, (b) ZrO2-H, (c)
ZrO2-G, (d) 2MoZr-U, (e) 5MoZr-U, (f) 10MoZr-U and (g)
20MoZr-U materials…………………………………………….
UV-Vis spectra of (a) ZrO2, (b) 10 MoZr-G (F/O= 0.5), (c) 10
MoZr-G (F/O= 1) (d) 10 MoZr-G (F/O= 1.5) and (e) pure MoO3
materials………………………………………………………….
Scanning electron micrographs of (a) ZrO2-U, (b) ZrO2-H, and
(c) ZrO2-G ………………………………….……………………
Scanning Electron micrograph of (a) 10 MoZr-G (F/O= 0.5), (b)
10 MoZr-G (F/O= 1) and (c) 10 MoZr-G (F/O= 1.5) materials…
Transmission electron micrographs of (a) ZrO2 and (b) 10MoZr-
G (F/O=1), (c) 10MoZr-U (F/O=1), (d) 10MoZr-H (F/O=1) and
(e) 10MoZr-I …………...………………………………………..
85
87
103
104
105
106
107
108
110
112
114
115
118
xvii
3.17
3.18
3.19
3.20
3.21
3.22
3.23
3.24
4.1
4.2
4.3
4.4
4.5
4.6
4.7
5.1
X-ray diffraction patterns for (a) ZrO2, (b) 2Fe-Zr, (c) 5Fe-Zr,
(d) 10Fe-Zr, (e) 20Fe-Zr and (f) 50Fe-Zr ……………………….
XRD patterns indicating a gradual shift to higher 2θ value for
the (111) peak of tetragonal zirconia phase with iron oxide
content (a) ZrO2, (b) 2Fe-Zr, (c) 5Fe-Zr, (d) 10Fe-Zr, (e) 20Fe-
Zr and (f) 50 Fe-Zr……………………………………………….
Variation of cell volume with iron content for the xFe-Zr
nanocompoite oxides…………………………………………….
XRD patterns of (a) SO42-
/2Fe-Zr, (b) SO42-
/5Fe-Zr, (c) SO42-
/10Fe-Zr, (d) SO42-
/20Fe-Zr and (e) SO42-
/50Fe-Zr……………..
FTIR spectra of (a) SO42-
/ZrO2, (b) SO42-
/2Fe-Zr, (c) SO42-
/5Fe-
Zr, (d) SO42-
/10Fe-Zr, (e) SO42-
/20Fe-Zr and (f) SO42-
/50Fe-Zr
UV-Vis spectra (a) SO42-
/ZrO2 (b) SO42-
/2Fe-Zr, (c) SO42-
/5Fe-
Zr, (d) SO42-
/10Fe-Zr, (e) SO42-
/20Fe-Zr and (f) SO42-
/50Fe-Zr
SEM of (a) SO42-
/ZrO2 (b) SO42-
/2Fe-Zr(c) SO42-
/5Fe-Zr, (d)
SO42-
/10Fe-Zr, (e) SO42-
/20Fe-Zr and (f) SO42-
/50Fe-Zr
Transmission electron micrograph of SO42-
/10Fe-Zr material…..
XRD patterns of the (a) Clay, (b) Zr-P and (c) PMA/SZr-P
materials………………………………………………………….
Structure of the [Zr4(OH)8(H2O)16]8+
pillaring polycation …….
FT-IR spectra of (a) clay, (b) Zr-P, (c) SZr-P and (d) PMA/SZr-
P materials……………………………………………………….
UV-Vis spectra of (a) clay (b) Zr-P, (c)SZr-P and (d) PMA/SZr-
P materials……………………………………………………….
N2 adsorption–desorption isotherms of (a) clay (b) PMA/SZr-P
and (c) Zr-P………………………………………………………
Scanning electron micrographs of (a) Zr-P, (b) SZr-P and (c)
PMA/SZr-P………………………………………………………
Transmission electron micrograph of PMA/SZr-P material……..
X-ray diffraction patterns of (a) CaO, (b) 5Ce-Ca-O, (c) 10Ce-
Ca-O, (d) 20Ce-Ca-O (e) 50Ce-Ca-O, (f) 80Ce-Ca-O and (g)
CeO2……………………………………………………………..
127
129
130
131
132
133
135
136
148
149
151
153
155
156
158
167
xviii
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12
5.13
5.14
5.15
Variation of (I) relative peak area and (II) integral width with
ceria content in the composite oxide…………………………….
Fourier line profile analysis plot of mixed oxide phase in the Ce-
Ca-O composite oxides………………………………………….
Fourier line profile analysis plot of CaO phase in the Ce-Ca-O
composite oxides………………………………………………...
Variation of the crystallite size and lattice strain with ceria and
calcium oxide content in the Ce-Ca-O composite oxide………..
TG analysis plot of the amorphous precursor of 20Ce-Ca-O
material…………………………………………………………..
UV-Vis-DRS spectra of (a) 5Ce-Ca-O, (b) 10Ce-Ca-O, (c)
20Ce-Ca-O, (d) 50Ce-Ca-O, (e) 80Ce-Ca-O and (f) CeO2……...
Plots of [F(R)h ]2 as a function of photon energy for (a)5Ce-
Ca-O, (b) 10Ce-Ca-O, (c) 20Ce-Ca-O (d) 50Ce-Ca-O, (e) 80Ce-
Ca-O and (f) CeO2……………………………………………….
Scanning electron micrographs of (a) CaO, (b) 5Ce-Ca-O, (c)
10Ce-Ca-O, (d) 20Ce-Ca-O, (e) 50Ce-Ca-O, (f) 80Ce-Ca-O and
(g) CeO2………………………………………………………….
Transmission electron micrograph of 20Ce-Ca-O……………….
XRD patterns of (a) MgO, (b) 5Ce-Mg-O, (c) 10Ce-Mg-O, (d)
20Ce-Mg-O (e) 50Ce-Mg-O and (f) CeO2………………………
Fourier line profile analysis plot of the Ce-Mg-O composite
oxides (Panel I, AS vs L and panel II, pV vs L)………………….
XRD patterns of (a) MgO-A, (b) 5Ce-Mg-O-A, (c) 10Ce-Mg-O-
A, (d) 20Ce-Mg-O-A, (e) 50Ce-Mg-O-A and (f) CeO2-A………
UV-Vis-DRS spectra of (a) 5Ce-Mg-O, (b) 10Ce-Mg-O, (c)
20Ce-Mg-O, (d) 50Ce-Mg-O and (e) CeO2 (panel I and II is for
the materials prepared by amorphous citrate and sol gel
combustion, respectively)………………………………………..
Scanning electron micrograph of (a) MgO, (b) 10Ce-Mg-O, (c)
50Ce-Mg-O and (d) CeO2 prepared by gel combustion method...
169
171
173
174
175
177
178
179
182
190
191
194
196
198
xix
5.16
5.17
Scanning electron micrograph of (a) MgO, (b) 5-Ce-Mg-O, (c)
10-Ce-Mg-O, (d) 50-Ce-Mg-O and (e) CeO2……………………
Transmission electron micrograph of (a) 10Ce-Mg-O and (b)
10-Ce-Mg-O-A materials………………………………………..
199
200
xx
ABBREVIATIONS
AR : Analytical Reagent
CT : Charge Transfer
DRS : Diffuse Reflectance Spectroscopy
FID : Flame Ionization Detector
FTIR : Fourier Transform Infrared Spectroscopy
GC : Gas Chromatography
OSC : Oxygen Storage Capacity
SEM : Scanning Electron Micrograph
TEM : Transmission Electron Micrograph
TWC : Three Way Catalyst
XRD : X-Ray Diffraction
PILC : Pillared Interlayer Clay
TG : Thermogravimetry
MCC : Multicomponent Condenstion
xxi
NOTATIONS
eV : electronvolt
m2/g : square meter per gram
K : Kelvin
g : gram
A/F : air to fuel ratio
Hz : Hertz
nm : nanometer
Å : Angstrong
d : interplanner spacing
r : radius
cm : centimetre
KJ : kilo Joule
h : hour
: Bragg angle
: frequency
cc : cubic centimetre
1
CHAPTER 1
INTRODUCTION
PART A: CATALYTIC CHEMISTRY OF ZIRCONIA BASED MATERIALS
1.1 GENERAL INTRODUCTION
Metals and their oxides dominate the vast panorama of heterogeneous catalysis. Noble metals
including Pt, Pd, Rh and Ru are employed in various catalytic processes such as methane
reforming, abatement of automobile emissions, hydrogenation of CO, oxidation and water
purification (Trovarelli, 2002; Mallikarjuna and Varma, 2007; Mishra and Ranga Rao, 2007;
Kim and Ihm, 2011; Li et al., 2011). Metals are employed in the form of finely dispersed
particles on high surface area, porous, thermally stable metal oxides, zeolites or carbon supports
in order to expose large fraction of metal atoms to reactant molecules (Berlier et al., 2005; Amali
and Rana, 2008; Rivallan et al., 2008). Metal oxides generally exhibit both electron transfer and
surface polarizing properties which are of direct relevance in redox as well as acid-base catalytic
reactions (Climent et al., 2011). The redox properties of oxides have been exploited in catalytic
purification systems for complete oxidation of toxic materials. The composite oxide systems
with inherent redox properties have also been used for selective oxidation of organic compounds
and to synthesize important fine chemicals (Trovarelli, 2002; Kašpar and Fornasiero, 2003; Di
Monte and Kašpar, 2005; Busca, 2007; 2010). The surface acid-base properties of oxides have
been taken advantage in carrying out selective organic transformations (Hottori, 1995; Busca,
2007; Climent et al., 2011). Many metal oxides are useful as supports for precious metals. The
traditional role of a support is to finely disperse and stabilize the active metal particles by
inhibiting their agglomeration during high temperature catalytic applications. The use of metal
oxides as catalyst and support is a widely investigated field of research in catalysis. Among the
2
metal oxides which are widely employed as catalysts and support, the most prominent are silica,
alumina and zirconium dioxide. Zirconium dioxide is an important oxide which has been used
extensively for heterogeneous catalytic reactions. Application of zirconium dioxide has been
quite promising in catalysis and many other areas due to its versatile structural and surface
chemical properties as well as good thermal stability (Sahu et al., 2000; Li et al., 2001; Shukla et
al., 2005; Ferna´ndez-Garcı´a et al., 2004; Chang and Doong, 2005; Di Monte and Kašpar,
2005a; Wang et al., 2007). The modified versions of zirconium dioxide, viz. the sulfated ZrO2,
zirconia substituted mixed oxides such as CexZr1-xO2 solid solutions (0 x 1), various
transition metal stabilized zirconia and hydrous zirconium oxide have been reported to be
effective for several organic reactions, combustion and gas phase reactions (Ranga Rao et al.,
2003; Sun et al., 2005; Di Monte and Kašpar, 2005; 2005a; Naik et al., 2008; Lamonier et al.,
2008; Adamski et al., 2011). The partial substitution of ZrO2 into CeO2 improves the thermal
stability, oxygen storage capacity and redox properties of ceria considerably. This is considered
as a significant contribution to three-way catalyst (TWC) technology (Kašpar et al., 2003; Di
Monte and Kašpar, 2005). In this thesis, a series of zirconia and ceria based nanocomposites and
modified pillared clays have been prepared and their catalytic properties have been evaluated for
synthesis of some biologically important molecules. In this chapter, an overview of the
understanding and recent advances in the field of catalysis by zirconia, ceria and clay based
materials is described.
1.2 STRUCTURAL PROPERTIES AND PHASE TRANSFORMATION OF
ZIRCONIA
Zirconia (ZrO2) is one of the most promising high temperature ceramic oxide having tremendous
industrial and technological applications. Zirconia as a ceramic possesses several surface and
3
structural characteristics such as reduced wear and erosion, high refractive index, high thermal
stability, good oxygen ion conductivity and good mechanical properties (Sahu et al., 2000; Li et
al., 2001; Shukla et al., 2005; Garcia et al., 2009; Heshmatpour et al., 2011). These properties of
zirconia are responsible for its technological application in cutting tool, refractory material, solid
electrolytes in oxygen sensors and solid oxide fuel cells, thermal barrier coating, ceramic
biomaterial and as heterogeneous catalyst and support (Li et al., 2001; Lopez et al., 2001; Shukla
et al., 2005; Ferna´ndez-Garcı´a et al., 2004; Chang and Doong, 2005; Wang et al., 2007; Garcia
et al., 2009). Zirconia exhibits three polymorphic forms depending on temperature ranges:
monoclinic (m-ZrO2, space group: P21/c stable up to 1400 K), tetragonal (t-ZrO2, space group:
P42/nmc, stable between 1400 K –2640 K), and cubic (c-ZrO2, space group: Fm3m, stable above
2640 K) (Li et al., 2001; Di Monte and Kašpar, 2005a). The high temperature stable form is
cubic and possesses a fluorite structure. The fluorite structure of zirconia is shown in Fig. 1.1.
Figure 1.1 The hard sphere model for the fluorite structure of ZrO2
The structure of ZrO2 can be described as the Zr4+
ions forming a cubic close pack structure
(FCC lattice) with the O2-
ions occupying all the tetrahedral holes. In the FCC unit cell, the Zr4+
4
ions occupy all the corners and the face center position of the cube. If the cube is divided into
eight symmetric smaller cubes through its centre, then the oxygen ions occupy the centers of all
the smaller cubes (Fig. 1.2).
(a) (b)
Figure1.2 (a) The fluorite structure of ZrO2 and (b) The tetrahedral sites (Red Dot: O2-
and cyan dot: Zr4+
)
The fluorite structure of ZrO2 can be described in an alternate way where the Zr4+
ions are
present in the centre of the cube with O2-
ions are present at all the corner of the cube. Half of
such cube centers are vacant whereas half of them are occupied by Zr4+
ions. One unit cell of
ZrO2 consists of eight such cubes (Fig. 1.3).
Figure 1.3 The fluorite structure of ZrO2 (Green Dot: O2-
and blue dot: Zr4+
)
5
With decreasing temperature, ZrO2 undergoes a cubic to tetragonal (c→t) phase transition at
around 23800C and a tetragonal to monoclinic (t→m) phase transition at around 1205
0C (Sahu et
al., 2000; Li et al., 2001; Chang and Doong, 2005; Wang et al., 2007). At the cubic to tetragonal
phase transition, the fluorite cubic structure distorts to the tetragonal structure with the tetragonal
c-axis parallel to one cubic <001> axes. The characteristics of this phase transition are (i)
displacement of oxygen ions from the fluorite site, (ii) occurrence of cell doubling, Z =1 to Z =2,
(iii) ferro elastic property in the tetragonal phase, and (iv) lower shift of the phase transition
points by incorporating oxygen defects and/or metals ions such as Y3+
, Mg2+
, and Ca2+
.
On the other hand, the tetragonal to monoclinic phase transition is characterized by a martensitic
phase transition accompanied by large hysteresis in the phase transition temperature and the
generation of large shear and volume elastic strains in the monoclinic phase (Li et al., 2001;
Chang and Doong, 2005; Garcia et al., 2009). The tetragonal to monoclinic phase transformation
is governed by the free-energy change of the entire system, Gt→m, which depends on the
chemical free-energy change, Gc, the strain energy change, Use and the energy change
associated with the surface of the inclusion Us, The net free energy change is given by .
Gt→m= −| Gc| + Use + Us
It has been observed that the martensitic transformation temperature for the t→m can be brought
down significantly by adding additives of suitable characteristics which facilitates the
stabilization of the tetragonal phase. The monoclinic and tetragonal unit cell of ZrO2 is shown
schematically in Fig. 1.4.
6
Figure 1.4 The monoclinic and tetragonal unit cell of ZrO2
Out of these polymorphs, the metastable tetragonal zirconia exhibit superior physicochemical
properties suitable for application in heterogeneous catalytic processes.
1.3 IMPORTANCE OF ZrO2 BASED MATERIALS IN CATALYSIS
Zirconia as an oxide material possesses all the required characteristics such as high thermal
stability, extreme hardness, stability under reducing conditions, and surface acidic and basic
functions to be used as heterogeneous catalyst as well as support for catalytically active phases.
Zirconia has been used as a support as well as catalyst for several industrially important reactions
including isomerization (Gregorio et al., 2008), oxidation (Brown et al., 1999), ammoxidation
(Chary et al., 2004), gas combustion (Brown et al., 2009), water gas shift reaction (Graf et al.,
2009), esterification (Rajkumar et al., 2008), alcohol dehydration (El-Sharkawy et al., 2007;
Dabbagh et al., 2011), and dehydrogenation (Ranga Rao et al., 2008; Reddy et al., 2008). Table
1.1 gives a list of different zirconia based catalytic materials and their catalytic applications.
Although ZrO2 has been extensively used as heterogeneous catalyst for a variety of organic
reactions, its application however has been limited by the presence of mild acidic and basic sites
on its surface. In this regard in the recent years, efforts have been focused to increase the surface
7
acid strength of zirconia by various structural and surface modifications. The most prominent
among them is anchoring catalytically active metal oxides and anions such as WO3, SO42-
and
MoO3 at sub monolayer level to generate newer acidic sites (Reddy and Patil, 2009; Kumar et
al., 2006; El-Sharkawy et al., 2007; Naik et al., 2008). The results achieved are quite remarkable
considering the fact that strength of the order of 100% H2SO4 can be achieved by these
modifications which is rarely found in any heterogeneous catalyst. The grafting of sulfated
species on ZrO2 produces strong acidic sites often termed as “super acidic”. These acidic sites are
capable of catalyzing carbonium ion reactions under mild conditions. The relatively simple
method of preparation of sulfated zirconia renders it as one of the attractive classes of
heterogeneous catalysts (Reddy and Patil, 2009). Tungsten oxide species dispersed on zirconia
supports (WOx-ZrO2) comprise another interesting class of surface modified zirconia catalysts
(Soultanidis et al., 2010). In addition to the surface treatment, the formation of nanocomposite
oxides by doping of suitable metal ions into the zirconia lattice can generate new active sites.
Zirconia based composite oxides such as CeO2-ZrO2 (Ranga Rao et al., 2003; Reddy et al.,
2008), SiO2-ZrO2 (Ordomsky et al., 2010), TiO2-ZrO2 (Reddy et al., 2008), Al2O3-ZrO2
(Debbagh et al., 2011) have been synthesized and used for various catalytic reactions such as
coumarins synthesis, hydrogen transfer reaction, aldol condensation, decomposition of CFCs,
NOx abatement, dehydrogenation and alcohol dehydration.
8
Table 1.1 Applications of zirconia based materials in catalysis
Catalyst Catalytic reactions References
WO3/ZrO2 Isomerization Di Gregorio et al., 2008
MoO3/ZrO2 Methane oxidation Brown et al., 1999
MoO3/ZrO2 Ammoxidation of toluene Chary et al., 2004
Pt/ZrO2
Pd/ZrO2
Water gas shift reaction
Methane oxidation
Graf et al., 2009
Guerrero et al., 2006
SO42-
/ZrO2, MoO3/ZrO2 Methane combustion Brown et al., 2009
H3PW12O40/ZrO2 Esterification Rajkumar et al., 2008
MoO3-ZrO2;
Al2O3–ZrO2
Alcohol dehydration
Alcohol dehydration
El-Sharkawy et al., 2007
Debbagh et al., 2011
H3PW12O40/CeZrO2;
TiO2-ZrO2, MnO2-ZrO2,
Styrene synthesis
Oxidative dehydrogenation
Ranga Rao et al., 2008
Reddy et al., 2008
H3PW12O40/ZrO2-Al2O3 Epoxidation of trans-stilbene Parida and Mallick, 2009
Cu/ZrO2 Methanolsynthesis Hui-dong et al., 2010
Cu-Ni/ZrO2 Hydrodechlorination Mallick et al., 2011
H3PMo12O40/ ZrO2
H4W12SiO40/ ZrO2
Alkylation of phenol
Veratrole benzoylation
Devassy et al., 2006
Devassy and Halligudi, 2005
1.4 SURFACE AND STRUCTURALLY MODIFIED ZIRCONIA IN CATALYSIS
Surface and structural modification of zirconia to generate catalytic materials with enhanced
surface properties is one of the widely investigated fields of research. The chemical reactivity of
the surface hydroxyl group of zirconia has been extensively exploited to graft various
9
catalytically active species onto the zirconia surface. The hydrous form of zirconia is mainly
used as a precursor to prepare surface modified zirconia. It has been observed that grafting
anionic species such as sulfate, phosphate ions onto the zirconia surface generate new acidic sites
of higher strength (Corma, 1995; Yadav and Nair, 1999; Reddy and Patil, 2009). Similarly,
various transition metals such as Ni, Fe, La, Mn, Gd, Co, Cu, Y and Mo can be incorporated into
the zirconia lattice to prepare catalytic materials with superior physicochemical properties and
catalytic activity (Lamonier et al., 2008; Brown et al., 2009; Lucio-Ortiz et al., 2010; Adamski et
al., 2011). It has been reported that the incorporation of transition metals in to zirconia matrix
selectively stabilizes the metastable tetragonal phase of zirconia along with an increase in the
surface area and number of acidic sites (Wyrwalski et al., 2005; Kenney et al., 2005; Naik et al.,
2008; Lamonier et al., 2008). These transition metal stabilized zirconia materials have been
found to show superior catalytic activity compared to pure zirconia. In this section, a detailed
discussion on various surface and structurally modified zirconia along with their catalytic
properties is described.
1.4.1 SULFATE GRAFTED ZIRCONIA
Grafting of sulfate species onto the surface of metal oxides is one of the attractive methods to
generate catalysts with strong acidic properties. Several surface grafted metal oxides such as
SO42-
/ZrO2, SO42-
/SnO2, SO42-
/Fe2O3, SO42-
/TiO2, SO42-
/CuO, SO42-
/Al2O3 has been synthesized
and their catalytic applications have been evaluated (Magnacca et al., 2003; Kumar et al., 2006;
Khder et al., 2008; Valente et al., 2009; Reddy and Patil, 2009; Arata, 2009; Tyagi et al., 2011;
Xia et al., 2011). Among sulfated metal oxides, the sulfated zirconia is one of the most widely
investigated catalytic system. The catalytic activity of sulfated zirconia has been studied for
various industrial reactions such as isomerization, alkylation, acylation, esterification, nitration,
10
condensation, cracking and hydrocracking reactions (Yadav and Nair, 1999; Reddy and Patil,
2009; Wang et al., 2009; Jadhav et al., 2010; Sinhamahapatra et al., 2011). Sulfated zirconia can
be prepared by treatment of the host oxides with sulfuric acids or ammonium sulfate solutions
(Nicholas and Marks, 2004). Acid treatment of the hydrous form of the metal oxides has been
reported to produce catalysts with better physicochemical properties. The hydrous form of the
metal oxides can be prepared by heat treatment of hydroxide precursors of the metal ions at
lower temperature in the range of 300-500oC. There have been numerous investigations on the
acidic properties of sulfated metal oxides using TPD, FTIR of adsorbed probe molecules and
NMR spectroscopy. Generation of acidic sites on sulfated oxides, thought to proceed by a two
stage reaction mechanism involving grafting of the sulfate species during impregnation step
followed its dehydration at higher temperatures (Corma, 1995; Reddy and Patil, 2009) (shown
below).
First step: Impregnation followed by drying
Zrn(OH)4n + H2SO4 Zrn(OH)4n-2x(SO4)x + 2xH2O
Second step: Calcination above 400oC
Zrn(OH)4n-2x(SO4)x ZrnO2n-x (SO4)x + 2(n-x)H2O
The sulfated metal oxides are known to display both Lewis and Bronsted acidic properties.
Several surface reaction scheme have been proposed in literature to account for both form of
acidity observed in these materials. The most prominent among them are by Arata and Hino
(Arata and Hino, 1990) (Fig. 1.5) and Clearfield et al. (Clearfield et al., 1994) (Fig. 1.6) which
takes into account the generation of surface Lewis and Bronsted acidic sites. Sulfated ZrO2 and
SnO2 have been used as catalysts for several industrially important chemical reactions such as
alcohol dehydration, olefins hydration, transesterification, Friedel–Crafts acylation and
11
isomerization reactions (Li et al., 2004; Kumar et al., 2006; Khder et al., 2008; Garcia et al.,
2008; Reddy and Patil, 2009; Wang et al., 2009; Tyagi et al., 2011). Table 1.2 gives a list of
different sulfated zirconia based catalytic materials.
Zr
OO
H+H S
O O
O
Zr+
Bronsted acidic sites
Lewis acid sites
Figure 1
Figure 1.5 Schematic presentations of the Bronsted and Lewis acidic sites in sulfated
zirconia proposed by Arata and Hino
Zr
O
O
Zr
O
S
O OH
OZr
O
O
Zr
OO
Zr
H
OH
O
Zr
O
300oCZr
O
O
Zr
O
S
O O
OZr
O
Zr
OO
Zr
OH
O
Zr
O
Scheme 1
Figure 1.6 Schematic presentations of the Bronsted and Lewis acidic sites in sulfated
zirconia proposed by Clearfield et al.
This acidic strength of sulfated zirconia depends on several parameters such as preparation
conditions, sulfate density, surface exposed and nature of crystalline phases of ZrO2 (Wang et
al., 2009; Ahmed, 2011). The sulfate retention capacity, surface area as well as the number of
active sties on the sulfated zirconia can be enhanced by preparing transition metal doped zirconia
and subsequent sulfate grafting or by supporting a small amount of noble metals onto the
sulfated zirconia.
12
Table 1.2 Catalytic applications of sulfated zirconia based materials
Catalyst Catalytic reactions References
SO42-
/ZrO2 Isomerization Li et al., 2004
Alkylation Yadav et al., 2003
Acylation Zane et al., 2006
Carbonylation Luzgin et al., 2004
Transesterification Garcia et al., 2008
Prins condensation Jadhav et al., 2010
Acetalization reactions Sinhamahapatra et al., 2011
Acetylation Tyagi et al., 2011
Desulfurization Wang et al., 2009
Condensation (Biginelli)
Knoevenagel
Kumar et al., 2006
Reddy et al., 2006
Ru, Rh, Pd, Pt/ SO42-
/ZrO2 Selective reduction of NOx Ohtsuka, 2001
Co, Mn, In, Ni/ SO42-
/ZrO2 NO reduction Li et al., 2003
Pd/Co/ SO42-
/ZrO2 oxidation of dichlorobenzene Aristiza´bal et al., 2008
SO42−
/ZrO2–Al2O3 Beckmann rearrangement Reddy et al., 2009
Pt/SO42-
/ZrO2 n-hexane isomerization Föttinger et al., 2004
Pd/SO42-
/ZrO2 – Al2O3 Isomerization reaction of sulfur-
containing light naphtha
Watanabe et al., 2004
Pd, Pt/SO42-
/ZrO2 Isomerization reaction Watanabe et al., 2004a
Pd – Au, Pt - Au/SO42-
/ZrO2 Synthesis of H2O2 Bernardotto et al., 2009
Mo, Cu, Fe, Co/SO42-
/ZrO2 Synthesis of dimethyl anisole Chen et al., 2011
Fe, Mn/SO42-
/ZrO2 n-butane isomerization; Garc´ıa et al., 2001
Pt/SO42-
/ZrO2 n-pentane isomerization Vijay and Wolf, 2004
13
In this regard, zirconia based composite oxides are recently used in place of pure zirconia for
preparation of sulfated nanocomposite oxides. Gao et al. have reported that the presence of small
amount of Al2O3 in zirconia matrix increases the catalytic activity and stability of the catalyst for
n-butane isomerization (Gao et al., 1998). The catalytic activity has been ascribed to the
distribution of the acid sites with different strengths and to an increase in the number of acid sites
with intermediate acid strength. The enhanced catalytic activity for sulfated-alumina promoted
zirconia system has also been related to the stability of surface sulfate complex and retardation of
the tetragonal phase transformation to the monoclinic phase (Olindo et al., 2000; Yu et al.,
2008). Moreno et al. have reported that in alumina promoted sulfated zirconia the formation of
Zr–O–Al bond increases the positive charge on Zr which lead to the stability of sulfate complex
(Moreno et al., 2000). Reddy et al. have studied the promoting effect of Al2O3 and TiO2 on the
catalytic activity of sulfated zirconia catalyst for vapour-phase Beckmann rearrangement of
cyclohexanoneoxime to -caprolactam (Reddy et al., 2009a). The ammonia-TPD results of the
promoted catalyst revealed the presence of more number of acid sites with medium acid strength
in case of TiO2 promoted SZ catalyst where as the presence of super acid sites was noted for
alumina promoted SZ catalyst. The TiO2 promoted SZ catalyst exhibited more
cyclohexanoneoxime conversion withgood -caprolactam selectivity which correlates well with
the number of medium strength acid sites. Ahmed has reported the preparation of sulfated-hafnia
promoted zirconia material by precipitation method and studied their catalytic activity for n-
butane isomerization at 250°C. The presence of hafnia increases the surface sulfate density
which in turn increases the amount and strength of Brönsted acid sites on the catalyst surface
(Ahmed, 2011). Pasel et al. has prepared Cu and Ga doped sulfated zirconia catalyst and studied
their catalytic activity SCR of NOx. The doped sulfated zirconia catalyst shows higher catalytic
14
activity compared to unprompted sulfated zirconia which has been attributed to the increase in
BET surface area and strong acidic sites due to Cu and Ga ion doping (Pasel et al., 2000). Pereira
et al reported the effect of iron incorporation on the physicochemical properties and catalytic
activity of sulfated zirconia catalyst. The presence of iron oxide modifies the sulfated zirconia
surface, increases the thermal stability of the catalyst and generates new acidic sites of medium
strength. The iron-doped sulfated zirconia catalysts have been found to display higher catalytic
activity for toluene disproportionation compared to sulfated zirconia catalyst. It has also been
observed by Hsu et al. and Adeeva et al. that addition of manganese and iron oxide to sulfated
zirconia increased the catalytic activity during the n-butane isomerization (Hsu et al., 1992;
Adeeva et al., 1994). Recently, Chen et al. have performed environmentally benign selective
oxidation of m-xylene in methanol with metals promoted sulfated zirconia using one pot
electrochemical synthesis route. They have prepared a series of SO42-
/M-ZrO2 (M = Mo, Cu, Fe,
Co, and Cr) by coprecipitation and post synthesized sulfation steps. Out of different promoted
catalyst, SO42-
/Mo-ZrO2 show the highest catalytic activity (Chen et al., 2011). Sulfated zirconia
catalyst has also been modified by supporting various transition and noble metals such as Pt, Pd,
Ir, Rh and Co, Ni, Mn to enhance the stability of the active sites and catalytic activity. The metal
particles in combination with the surface acidic sites of sulfated zirconia can act as bifunctional
catalyst catalyzing a variety of chemical transformations.Vijay and Wolf have studied the
promoting effect of Pt on sulfated zirconia for n-pentane isomerization. They have observed that
Pt/SZ catalyst shows stable long-term activity for n-pentane isomerization which has been
ascribed to the formation of metal-acid sites ensembles on the catalyst surface which arrest
deactivation rate (Vijay and Wolf, 2004). The catalytic activity of the Pt loaded SZ catalyst has
also been studied for butane skeletal isomerization (Garcia, 2003; Li et al., 2005), n-hexane
15
isomerization reaction (Fottinger et al., 2004) and isomerization–cracking of long chain n-
paraffins (Busto et al., 2011). Pure palladium metal or palladium metal based bimetallic system
supported on sulfated zirconia has been investigated for several important catalytic
transformations. Belskaya et al. have studied the promoting effect of Pd metal supported on
sulfated zirconia (Pd/SZ) isomerization catalyst. The presence of Pd improves the activity of the
catalyst and reduces coke formation. The reduction temperature of the palladium precursor to Pd
particles has been found to be crucial for the activity of the catalyst for n-hexane isomerization.
A low reduction temperature of 70-1500C lead to formation of well dispersed metal particles
where as at higher reduction temperature the decomposition of sulfate ions and formation of PdS
surface species have been observed from XPS study. The authors have described that formation
of palladium metallic particles at low reduction temperatures is one of the major advantage in
employing Pd/SZ catalyst for isomerization reaction compared to other noble metal supported SZ
catalyst. Aristiza´bal et al. have studied the oxidation of ortho-dichlorobenzene over Pd-Co
bimetallic catalyst supported on sulfated zirconia catalyst (Aristizabal et al., 2008; Aristizábal et
al., 2008a). The Pd-Co/SZ catalysts showed a strong interaction between cobalt and sulfate,
which lead to enhanced sulfur retention and maintain catalyst activity under dry and wet
conditions (Aristizábal et al., 2008). Bernardotto et al. have studied the effect of Pt and Au
addition on the catalytic activity of Pd/SZ catalyst for H2O2 production from hydrogen and
oxygen under mild condition. The addition of gold in 1:1 amount to palladium improved both the
productivity and selectivity for the H2O2 production process. The selective catalytic reduction of
NOx by methane on noble metal (Ru, Rh, Pd, Ag, Ir, Pt, and Au) loaded sulfated zirconia (SZ)
catalysts has been studied (Ohtsuka, 2001). The experimental result clearly suggested that a
combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NOx by
16
methane. SCR of nitric oxide with methane in the presence of excess oxygen has also been
studied on a series of non-noble metal (Co, Mn, In, Ni) catalysts supported on sulfated zirconia
(SZ). The Co/SZ catalyst was descried to show superior activity and show more tolerance to the
presence of water and SO2 compared to zeolite supported cobalt catalyst (Li et al., 2003).
1.4.2 WOx GRAFTED ZIRCONIA
WOx/ZrO2 (WZ) is another modified form of zirconia family which shows promising catalytic
activity for industrial catalysis. Although WZ is much less active than sulfated zirconia, it offers
an important advantage as the WOx units in WZ are much more stable than the sulfate groups in
sulfated zirconia at high temperatures and in reductive atmospheres (Hino and Arata, 1987;
Soultanidis et al., 2010). WOx species on ZrO2 support create superacid sites by the interaction
of WO3 with ZrO2 when the latter oxide was crystallized from an amorphous form into a
tetragonal form (Hino and Arata, 1987). The WOx species at saturation coverage on zirconia
inhibit ZrO2 sintering and the tetragonal to monoclinic phase transformation. It has been also
observed that, at lowsurface tungsten oxide coverage (<2 W/nm2), surface mono tungstate
species predominates and only Lewis acid character is found on the surface. At intermediate
tungsten oxide surface coverage (3–4 W/nm2), surface poly tungstate species become
predominant and both Brønsted acid sites and Lewis acid sites are present. At higher tungsten
oxide coverage (>4 W/nm2), crystalline WO3 nanoparticles (NPs) are present on top of the
surface tungstate layer (Wachs et al., 2006). The presence of strong acid sites has been reported
for WOx-ZrO2 materials prepared by impregnation of zirconium oxyhydroxide (ZrOx(OH)4-2x)
with solutions containing tungstate anions (Ross-Medgaarden et al., 2008). Scheithauer et al.
reported the preparation and catalytic application of tungstated zirconia catalysts containing 3.6
to 23.9 wt% WO3 loadings for isomerization of alkanes. They have observed that both Lewis and
17
Brønsted acid sites with enhanced acid strength are created upon WO3 loading, the relative
proportion of which can be controlled by proper choice of the preparation parameters
(Scheithauer et al., 1998). The addition of a small amount of platinum (<1 wt % Pt) to
preoxidized WOx-ZrO2 samples leads to the formation of active, stable and selective catalysts
for n-alkane isomerization reactions. The Pt/WOx-ZrO2 has been shown to catalyze efficient
hydrogen transfer steps preventing extensive cracking of adsorbed carbocations (Barton et al.,
1999; Herna´ndez et al., 2006). The catalytic activity of WOx-ZrO2 and Pt/WOx- ZrO2 have
been studied for several industrially important chemical transformations include isomerization,
hydrogenation, alkylation and selective reduction of NOx (Chang et al., 1996; Baertsch et al.,
2001; Bordoloi et al., 2006; Naik et al., 2008; Ross-Medgaarden et al., 2008; Soultanidis et al.,
2010). Table 1.3 gives a list of catalytic reactions studied using WO3-ZrO2 as catalyst.
Table 1.3 Catalytic applications of WO3-ZrO2 materials
Catalyst Catalytic reactions References
WO3/ZrO2
WOx/ZrO2
WOx/Al2O3, TiO2, Nb2O5
WO3/ZrO2
WO3/ZrOx(OH)4−2x
WO3/ZrO2
WOx/ZrO2
n-pentane isomerization
o-xylene isomerization
oxidative dehydration
veratrole acylation
methanol dehydration
isomerization
n-pentane isomerization
Scheithauer et al., 1998
Baertsch et al., 2001
Wachs et al., 2006
Bordoloi et al., 2006
Ross-Medgaarden et al., 2008
Gregorio et al., 2008
Soultanidis et al., 2010
WOx/ZrO2
Pt/WOx-ZrO2
Esterification of acetic acid
Isomerization-cracking of C8-C12
paraffins
Praserthdam et al., 2010
Grau et al., 2008
18
1.4.3 MoO3 SPECIES GRAFTED ON ZIRCONIA
Molybdnum oxide species dispersed in the zirconia matrix (MoO3/ZrO2) is another class of
interesting solid acids which display well defined strong surface acid sites and catalytic activity
for a variety of catalytic transformation. Molybdnum oxide in a supported state shows a variety
of surface molecular structure and active sites. MoO3 based heterogeneous catalyst has been
extensively investigated for its application in industrial catalytic process such as hydro-
desulfurization (Grange et al., 1980), metathesis (Grunert et al., 1992), oxidation and oxidative
dehydrogenation reactions (Banares et al., 1993; Brown et al., 2009). Molybdenum oxide
supported on a variety of support such as ZrO2, SiO2, TiO2, CeO2 and CeO2-ZrO2 has been
reported in literature. Out of different supported MoO3 system, the MoO3/ZrO2 catalyst has been
extensively investigated for several industrially important reactions. Brown et al. reported the
synthesis of MoO3/ZrO2 catalyst by impregnation of ammonium heptamolybdate to hydrous
zirconia surface and evaluated the catalytic activity for methane combustion (Brown et al., 1999;
2009). Among a series of catalyst including Fe/Mn promoted sulfated zirconia, the MoO3/ZrO2
catalyst is highly active for the reaction which has been ascribed to the formation of a MoO3
monolayer on zirconia surface and the subsequent generation of superacidic properties. Calfat et
al. has reported the synthesis of MoO3/ZrO2 catalysts by co-precipitation at different Mo/Zr
ratios. They have mentioned that the MoO3/ZrO2 catalysts show higher surface area than pure
ZrO2. Surface area depends on the MoO3 content and it is found to increase with increase in Mo
content. Presence of MoO3 selectively stabilizes the metastable tetragonal phase of zirconia
(Calfat et al., 2000). Chen et al. has studied the oxidative dehydrogenation of propane on
zirconia-supported molybdenum oxide catalysts. The structure of MoOx species dispersed on
zirconia depends strongly on the Mo surface density and on the temperature of thermal
19
treatment. Raman study of the catalyst indicated the presence of isolated molybdates and
polymolybdate domains at Mo surface densities below 5 Mo/nm2 and at higher surface densities
presence of bulk type MoO3 and ZrMo2O8 spinels are observed (Chen et al., 2000). At higher
loading, the relative concentration of the MoO3 and ZrMo2O8 species can be altered by proper
choice of calcinations temperature. At similar Mo surface densities, samples containing
predominantly ZrMo2O8/ZrO2 show higher turnover rates and lower initial propene selectivities
than those containing MoO3 species (Chen et al., 2000). Bhaskar et al. has reported the
characterization and catalytic application of molybdenum oxide catalysts supported on zirconia.
It has been observed that at low Mo loading (<8 wt %), the molybdenum oxide is in a well
dispersed state on the surface. TPD study suggests the presence of moderate and strong acidic
sites on the catalyst surface and an increase in the number of strong acidic sites with molybdena
loading. The catalytic activity for ammoxidation of 3-picoline and the selectivity towards
nicotinonitrile was found to correlate well with the number of strong acidic sites (Bhaskar et al.,
2001). Sohn et al. synthesized zirconia modified with MoO3 by impregnation method. The
spectroscopic studies indicated that the addition of a small amount of molybdenum oxide (3 wt
%) to zirconia, increases both the number and strength of acidic sites of the catalyst. The
supported catalyst exhibit both Brönsted and Lewis acid sites on the surface of MoO3/ZrO2. The
catalytic activities for cumene dealkylation correlated well with the acidity of catalysts measured
by ammonia chemisorption method (Sohn et al., 2003). Kenney et al. have synthesized ZrO2–
MoO3 catalysts with different molybdenum loadings (0–18.7 atom % Mo) by co-precipitation
method and studied their catalytic activity for methylcyclopentane conversion. They have
observed that the presence of molybdenum stabilizes the tetragonal polymorph of ZrO2, and
specific surface area of the supported catalyst increases up to 15.8% loading. The increase in
20
surface area has been ascribed to the ability of molybdenum to stabilize ZrO2 grains below a
critical size and prevent grain growth (Kenney et al., 2005). Samaranch et al. investigated the
effect of MoO3 loading on the structural, acidic and catalytic properties of sol–gel synthesized
MoO3-ZrO2 oxides. They have observed that with MoO3 increasing loading upto 20 wt%, the
surface area of the catalyst increased with simultaneous formation of new Brönsted acid sites
which correlate well with the catalytic behavior of materials in the 2-propanol dehydration
(Samaranch et al., 2006). El-Sharkawy et al. reported the structural characterization and catalytic
activity of molybdenum oxide supported zirconia catalysts prepared by impregnation method.
The incorporation of MoO3 into ZrO2 led to enhancement of the surface acidity of the catalysts.
It has been found that for identical MoO3 content, the surface acidity gradually increased as the
temperature of thermal treatment was increased from 400 to 550°C. The catalytic activity of the
MoO3/ZrO2 material has been evaluated for cumene dealkylation and ethanol dehydration
reaction (El-Sharkawy et al., 2007).
21
PART- B
1.5 INTRODUCTION TO THE CLAY MATERIALS
Clays are the most common minerals on earth’s surface and have been used by man for
centuries. Clays are highly versatile materials and are used in millions of tons in different areas
of application. Clays find wide applications in building ceramics, paper coating and filling,
drilling muds, foundry moulds, and pharmaceuticals. These materials are also the most primitive
materials to be used as adsorbents and catalysts or catalyst support for various industrial
applications (Vaccari, 1999; Gil et al., 2010). The high surface area and polarity of the clay
structures help in retaining the ionic species such as K+, +
4NH and Ca2+
which are vital for the
plant growth (Carvalho et al., 2003). Clay minerals are also known to be very good adsorbents
for toxic organic chemicals such as chlorinated compounds, heavy metal ions (eg. Pb2+
, Hg2+
)
and nuclear waste (Komarneni et al., 2000; 2001; Gil et al., 2010). Salt loaded, acid-treated and
ion exchanged clays have been found to be efficient catalysts for many organic reactions
(Lambert and Poncelet, 1997; Vaccari, 1999; Issaadi et al., 2006). Clays are divided into two
main groups: cationic and anionic clays (Rajamathi et al., 2001). The cationic clays are widely
available in nature and contain negatively charged alumino-silicate layers (Vaccari, 1999). The
negative charge in the layer is balanced by the presence of cations in the interlayer of these
materials. These materials exhibit surface acidic properties due to the presence of structural
hydroxyl groups. The anionic clays on the other hand, are relatively rare in nature but simple and
inexpensive to synthesize (Rajamathi et al., 2001). These materials have positively charged
metal hydroxide layers with balancing anions and water molecules located interstitially. The
work described in the thesis mainly concerned with the structural modification and catalytic
application of the cationic clays.
22
1.6 STRUCTURE AND CLASSIFICATION OF CLAY MATERIALS
Clays are two dimensional hydrous layer silicates belonging to the phyllosillicate family (Mott,
1988). The basic framework of clay consists of silicate layers formed from the condensation of
the extremely stable SiO4 tetrahedral units. The structure of a typical silicate plane and the top
view of the basal oxygen plane of silicate layer of a clay mineral are shown in the Fig.1.7.
(a)
(b)
Figure 1.7 Silicate sheet of a clay mineral (a) corner sharing of SiO4 unit and (b) top
view of the basal oxygen plane
Each tetrahedron in the silicate layer has a spare oxygen atom which is unshared and pointed
normally away from the tetrahedral siloxane sheet. These silicate planes then condense with
different octahedral planar units through the unshared oxygen atoms to from different classes of
clay materials. The octahedral layer is formed form the edge sharing of the MO6 (M= Al, Mg
23
etc.) octahedra. The MO6 octahedra are polymerized in the plane by sharing four of its edges to
form the octahedral layer (Mott, 1988). Each octahedron in the layer contains two unshared
oxygen atoms pointing normally above and below the plane of polymerization. This results in the
generation of two negative charges to be satisfied by the metal cations. This is why trivalent
aluminum occupies only two third of the available octahedral holes and forms dioctahedral clay
minerals whereas magnesium occupy all the octahedral holes to form trioctahedral clay minerals.
The most widely used clay material in catalysis is montmorillonite. The structure of
montmorillonite consists of an alumina layer sandwiched between two-silicate layers. The
structure of an alumina layer is shown in the Fig. 1.8.
Figure 1.8 Alumina octahedral sheets in clay mineral
The tetrahedral sheet of the polymerized SiO4 unit and the octahedral sheet of the polymerized
AlO6 unit are the basic building blocks of the montmorillonite clay. The alumina plane
condensed with two silicate planes to form the clay sheet. These clay sheets are then stacked in
the z-direction to form the structure of montmorillonite. The structure of montmorillonite clay is
shown in Fig. 1.9.
24
Figure 1.9 Structure of 2:1 clay mineral montmorillonite
The space between each sheet of clay is called the interlayer spacing. Isomorphous substitution
of Al3+
ion by lower valent ions such as Fe2+
, Mg2+
in the octahedral layer of clay sheet is a
common phenomenon in clay minerals. This results in the development of negative charge in the
clay sheet, which is usually satisfied by cations of alkali and alkaline earth metals. These charge-
compensating cations are found in the interlayer and are easily exchangeable with other cationic
species. The total amount of negative charge originated as a result of isomorphous substitutions
is called the cation exchange capacity (CEC) of the clay material (Figueras, 1988). The
montmorillonite type clay minerals are called 2:1 clay minerals. The other possibility is that one
silicate layer can condense with one alumina layer. Such clay minerals are called 1:1 clay
minerals (Mott, 1988). One such clay mineral used in catalysis is Kaonilite. Clay minerals have
been divided into several groups, sub-groups and species depending upon the layer type, charge
and occupancy and the type of charge compensating cations (Figueras, 1988).
25
1.7 DISADVANTAGES OF CLAY MATERIALS AS CATALYSTS
Although clay materials have been used as solid acid catalysts for variety of organic reactions,
they possess certain inherent disadvantages, which limit their application in heterogeneous
catalysis (Figueras, 1988; Gil et al., 2008; 2010). Extensive dehydration of the clay sheets takes
place at high temperatures leading to the loss in Brønsted acidity. This limits their application as
catalysts at higher temperatures. The forces between individual clay sheets are purely
electrostatic and Van der Wall type. These forces are very week in nature and cannot retain the
interlayer space at high temperatures. At elevated temperatures complete collapse of clay
structure takes place resulting in the loss of Brønsted acidity and shape selective property. The
interlayer dimension of the clay materials are typically between 3-5 Å, which is smaller than the
kinetic diameter of most of the bulky reactant molecules. This result in diffusional constraints
and most of the interlayer catalytically active sites remain unutilized during catalysis.
1.8 MODIFICATION OF CLAY MATERIALS
In last two decades, different types of surface as well as interlayer modifications have been done
to increase the thermal stability, acidity and catalytic properties of clay materials. The most
important modifications reported in literature are the exchange of interlayer cations by inorganic
and organic cationic species (Mata et al., 2007; Guerra et al., 2008; Rezala et al., 2009; Bineesh
et al., 2010; Galeano et al., 2011), acid treatment (Rajesh et al., 2008; Panda et al., 2010;
Nguetnkam et al., 2011), supporting active species on clay surface for catalysis (Ramaswamy et
al., 2002; Sowmiya et al., 2007; Molina et al., 2009; Aznárez et al., 2011) and pillaring by
inorganic polycations (Mishra and Ranga Rao, 2004; 2007; Gil et al., 2010). While the first three
processes essentially increase the acidity and catalytic activity, the pillaring of clay by inorganic
26
polycations provides a multitude of advantages in terms of increasing surface area,
microporosity, thermal stability, acidity and catalytic activity.
1.9 PILLARING OF CLAY BY INORGANIC POLYCATIONS
Pillared clays are a class of microporous materials studied extensively in the last two decades as
catalysts and supports for various organic transformations. These materials are prepared by
exchanging the interlayer cations of the clay materials by inorganic polyoxocations (Mishra and
Ranga Rao, 2004, 2007; Rezala et al., 2009; Gil et al., 2010). The intercalated polycations
increase the basal spacing of the clays and upon heat treatment they are converted to stable metal
oxide clusters. These oxide clusters called pillars hold the individual clay sheets and prevent
them from collapsing during high temperature applications.
Two important properties of clay materials are responsible for pillaring process to occur.
1. Swelling in polar solvent: As mentioned previously, the forces between clay layers are purely
electrostatic. When clay particles are dispersed in polar solvents such as water, solvent molecules
enter in to the interlayers of clay and reduce the electrostatic forces. Under such conditions the
individual clay layers move apart resulting in the swelling of clay minerals.
2. Exchangeability of the interlayer cations: The negative charges in the clay sheet originate
from isomorphous substitution in the octahedral layer. However, these charges are not point
charges but delocalised over the oxygen plane. In the polar solvent, charge delocalisation
provides considerable mobility to the interlayer cations which can be exchanged by other cations.
The process of pillaring is shown schematically in Fig. 1.10.
27
Figure 1.10 Schematic representation of the process of pillaring of the clay sheets
The parent clay containing Na+, Ca
2+ and K
+ ions in the interlayer are dispersed in polar solvent
such as water which results in the swelling of these materials. Under such conditions, the
inorganic cationic clusters are intercalated into the clay layers by replacing the interlayer cations
and giving rise to the intercalated clay (Gil et al., 2010). Upon calcination, the inorganic
polyoxoclusters are converted into stable oxide clusters. The oxide clusters are chemically
bonded to the adjacent clay sheets acting as nano-pillars.
Clay pillared with a variety of inorganic polycations of Al (Katdare et al., 2000; Olaya et al.,
2009; Sowmiya et al., 2007; Mishra and Ranga Rao, 2007), Zr (Cool and Vansant, 1996; Mishra
and RangaRao, 2004; Singh et al., 2006; Guerra et al., 2008), Ti (Jagtap and Ramaswamy, 2006;
Bineesh et al., 2010; Ouidri et al., 2010), Cr (Mata et al., 2007), Fe (Tabet et al., 2006; Galeano
et al., 2011), Ta (Guiuet al., 1997), V (Choudary and Valli, 1990), Ga (Bradley and Kydd,
1993a), Si (Sterte and Shabtai, 1987) and Nb (Christiano et al., 1985) are reported in literature.
These polycations are generally prepared by controlled hydrolysis of the corresponding metal
[Al13O4(OH)24(H2O)12]7+
Na+ K+ Ca2+ Na+ K+ Ca+2 Water
500o C
Step I
Step II
Step III
28
cations in solutions (Figueras, 1988). The final property of the pillared clay depends largely upon
the nature of the polycations which in turn depend upon the method of preparation (Figueras et
al., 1988; Gil et al., 2008). So far, Al-pillared clays have been extensively investigated in the
literature. The pillaring solution for the Al-pillared clay contains [Al13O4(OH)24(H2O)12]7+
polycationic species (Vaccari, 1999). These polycations are prepared by the partial base
hydrolysis of a dilute solution of aluminum chloride. In case of alumina pillared clay, it has been
clearly demonstrated that the preparation and concentration of [Al13O4(OH)24(H2O)12]7+
polycation in solution depends upon several parameters such as degree of hydrolysis,
temperature, time of aging, concentration of the ion in solution, type of counter ion and method
of preparation of the pillaring solution (Sterte, 1988). The [Al13O4(OH)24(H2O)12]7+
polycation
has a Keggin ion type structure in which one AlO4tetrahedra is symmetrically surrounded by
twelve AlO6 octahedra in a dodecahedral coordination (Botteroet al., 1980). A moderate
temperature of 60-80o C, relatively higher (2.5 M) concentration of aluminum ion and OH/Al = 2
favors the formation of this cation in solution (Sterte, 1988). The Zr-pillared clays are prepared
by the intercalation of [Zr4(OH)8(H2O)16]8+
cationic species into the clay interlayer. These
oligomeric species are present in the dilute zirconium oxychloride solution. Subsequent
treatment of the pillaring solution such as aging, refluxing and hydrothermal treatment is
beneficial in terms of the greater hydrothermal stability, crystalline nature and interlayer
expansion of the pillared clay (Cool et al., 2002). The different types of polycations used for the
preparation of the pillared clays are given in Table 1.4.
29
Table 1.4 Different types of pillaring species used for the preparation of pillared clays
The characteristic changes that occur as a result of pillaring include higher thermal stability,
microporosity, surface area and acidic properties of the parent clay. Table 1.5 shows a general
comparison of the properties of the parent clay and pillared clay materials often observed in the
literature.
Pillar type Pillaring species References
Al
Zr
Fe
Ti
Ga
Cr
Ta
Si
Nb
[Al13O4(OH)24(H2O)12]7+
[Zr4(OH)8(H2O)16]8+
[Fe3(OAC)7OH.2H2O]+
[TiO(OH)x]x +
[Ga13O4(OH)24(H2O)12]7+
[Cr4O(OH)5(H2O)10]5+
[Ta8O10(OR)20]
SiO2 sol of different composition
Nb6Cl12n+
Katdareet al., 2000
Mishra and RangaRao, 2007
Cool and Vansant, 1996
Mishra and RangaRao, 2004
Mishra et al., 1996
Kooliet al., 1997
Bradley and Kydd, 1993a
Pinnaviaet al., 1985
Guiu et al., 1997
Sterte and Shabtai, 1987
Christianoet al., 1985
30
Table 1.5 Properties of the parent clay before and after pillaring
Physical and chemical
properties
Parent clay Pillared clay
Interlayer spacing
Surface area
Pore size
Micropore volume
Brønsted acidity
Lewis acidity
Thermal stability
2-3 Å
30-100 m2/g
4-8 Å
< 0.02 cm3/g
0.1-0.2
0.05-0.1 mmol/g
< 300oC
8-13 Å
250-300 m2/g
16-25 Å
> 0.08 cm3/g
0.4 - 0.6 mmol H+/g
0.8-0.6 mmol/g
< 800oC
The evidence of the intercalation of the inorganic polycations into the clay interlayer can be
obtained from the X-ray diffraction study. Since the size of the polycation (10-12 Å) is higher
than the interlayer ions such as Na+, Ca
2+ and K
+ in parent clay, the intercalation of the former
results in the expansion of clay structure in z-direction (Gil et al., 2008). This expansion results
in a shift of the d001 reflections to lower 2 value. Considerable expansion and desegregation of
the of the clay materials takes place during pillaring process resulting in materials with high
surface area and pore volume. It is evident from Table 1.5 that nearly three times increase in the
surface area is observed for pillared materials. Pillared clays are generally microporous in nature.
The adsorption isotherm observed for the pillared clays is type-I according to Brunauer, Deming,
Deming and Teller (BDDT) classification indicating the presence of micropores (Gregg and
Sing, 1982). The acidic property of clay and pillared clay have been studied by several methods
such as TPR of ammonia (Figueiredo et al., 2009), FTIR of the adsorbed basic probe molecules
(Figueiredo et al., 2009; Tomul and Balci, 2009), and Breen’s method (Breen, 1991; Mokaya
31
and Jones, 1995). The general conclusions arrived at from these studies are: (i) pillared clays are
more acidic compared to the parent clay materials both in terms of the strength and number of
acidic sites; (ii) the oxide clusters (pillars) in the clay interlayer are highly defective and contains
acidic centers; (iii) pillared clay offers both Lewis and Brønsted acidic sites while the parent clay
contains mostly Brønsted acid sites and (iv) most of the Lewis acid sites are associated with the
pillars. IR Spectroscopy has been used to discriminate the acidic and nonacidic –OH groups in
clay minerals.
1.10 CATALYTIC APPLICATION OF PILLARED CLAYS
Pillared clays have been used as solid acid catalysts for a variety of reactions of industrial
importance and of environmental concern (Vaccari, 1999; Issaadi et al., 2006; Gil et al., 2008;
2010). The inherent acidic property of the pillared clays is capable of promoting acid catalyzed
reactions. Pillared clays with uniform micropores can be considered as large pore zeolites to
carry out many shape selective reactions involving large reactant. In addition, Pillared clays due
to their high surface area and uniform pore structure can be used as effective supports for
catalytically active components such as oxides, metals and organometallic complexes
(Ramaswamy et al., 2002; Issaadi et al., 2006; Sowmiya et al., 2007; Molina et al., 2009; 2009a;
Kanda et al., 2009; Molina et al., 2010; Aznárez et al., 2011). The surface acidic property of
pillared clay combined with the supported metallic particles can be used for bifunctional
catalysis. Alumina pillared clay based catalytic systems have been used as catalysts for several
reactions such as catalytic wet peroxide oxidation, CO oxidation, hydroxylation of benzene,
epoxidation of cyclooctene, oxidation of propene, hydrodechlorination of 4-chlorophenol,
hydrogenation, and hydrodesulfurizationof dibenzothiophene. (Pan et al., 2008; Ramaswamy et
al., 2008 ; Mata et al., 2009; Gil et al., 2009; Tomul and Balci, 2009; Figueiredo et al., 2009;
32
Galeano et al., 2010; Molina et al., 2010; Aznárez et al., 2011; Romero-Pérez et al., 2012). In
addition to their catalytic properties the pillared clay materials have also been evaluated as
hydrogen storage materials and as efficient adsorbents for removal of dye from aqueous
solutions (Gil et al., 2010; Gil et al., 2011). Galeano et al. has synthesized Al-Fe, Al-Cu and Al-
Fe-Cu mixed cation pillared clays, and studied their catalytic activity for catalytic wet peroxide
oxidation of methyl orange from aqueous solutions (Galeano et al., 2010). The presence of Fe in
the microenvironment of the pillared clay has been found to significantly enhance the wet
oxidation process. The interaction between the pillars and the Fe3+
ions has been found to be
crucial for site isolation and their catalytic activity. Mojovi´c et al. studied the Al-Cu-pillared
clay for effective degradation of toluene by catalytic wet peroxide and elctrooxidation routes.
The Cu2+
ions intercalated inside the constrained environment of the Al-pillared clay have been
found to show good catalytic activity for both the process (Mojovi´c et al., 2009). Tomul and
Balci have used the Al-, Cr- and Cr/Al-mixed pillared clay as catalyst for CO oxidation. FTIR
analysis indicated an increase in the number and strength of Lewis and Brønsted sites in Cr
containing pillared clay as compared to the parent Al-pillared clay. The highest activity of CO
oxidation was measured for Cr-Al-mixed cation pillared clay (Tomul and Balci, 2009). Mixed
Al-Fe and Al-Fe-Ce pillared clays have been evaluated for the phenol oxidation reaction in a
diluted aqueous medium (Olaya et al., 2009). The incorporation of low quantities of Ce and Fe
has a beneficial effect on the crystallinity, increase of the surface area, porosity and catalytic
activity of the Al-pillared clay (Olaya et al., 2009). The Fe-Al and Cu-Al pillared clays are also
found to be active catalysts for deep oxidation of chlorinated hydrocarbons (Barrault et al.,
2000). The leaching of copper during the reaction was very low for the Cu-Al pillared clays
indicating the strong interaction between the copper and Al-pillars.
33
Pillared clays have also been used as supports for several catalytically active metal oxides which
include MnO2 (Gandia et al., 2002), MoO3 (Saleno et al., 2001), V2O5 (Vicente et al., 2003;
Bineesh et al., 2011), V2O5-WO3 (Long and Yang, 2000) and CoO (Hayashi et al., 1999). The
dispersion of the oxide components and their catalytic activity depend upon the type of clay
material and the pillaring species used. Gandia et al. (2002) have studied complete oxidation of
acetone over MnO2 catalyst supported on a series of Al-and Zr-pillared montmorillonites and
saponites. The MnO2 catalyst supported on montmorillonite clay invariably shows better
catalytic activity compared to saponite. Among the pillaring species the Zr-pillared clays are
more effective as supports for acetone oxidation compared to Al-pillared clays. Alumina pillared
clay has been examined as possible support for MoO3 based HDS catalysts (Saleno et al., 2001).
A strong interaction between Mo species and alumina pillared clay has been observed in the TPR
study of the catalyst material. TiO2 pillared clays have also been extensively investigated as
supports for catalytically active V2O5, V2O5-WO3 and Cr2O3-Fe2O3 oxides for catalytic reduction
of NO with ammonia (Cheng et al., 1996; Long and Yang, 2000; Vicente et al., 2003). The V2O5
catalyst supported on Ti-PILC has been found to show better activity for the selective catalytic
reduction of NO compared to the commercial V2O5 based catalysts employed for this reaction
(Long and Yang, 2000). Addition of WO3 to the V2O5 catalyst is further found to improve the
hydrothermal stability and resistance to sulfur poisoning of the pillared clay based catalysts. The
Ti-PILC clays due to their high surface area (300 m2/g), uniform micropores (~ 2 nm) and better
hydrothermal stability are found to be attractive supports compared to other pillared clays. The
catalytic activity of sulfated tin oxide particles (STO) supported on Al-pillared clay have been
studied for the solvent-free synthesis of 3,4-dihydropyrimidin-2(1H)-ones, thiochromans, and
coumarins (Sowmiya et al., 2007). The STO particles are present in a well dispersed form in the
34
pillared clay matrix and acidic properties of the pillared clay have been found to supplement the
catalytic activity of the supported catalyst.
In the past few years there have been extensive studies on the preparation, characterization and
catalytic activity of supported mono and bimetallic systems on clays (Gil et al., 2001; Louloudi
et al., 2003 ; Mishra and RangaRao, 2007; Molina et al., 2009; 2009a). Ni-metal supported on
Al-pillared montmorillonite and saponite has been studied for hydrogenation of benzene
(Louloudiet al., 2003) and CO2 reforming of methane to form synthesis gas (Wang et al., 1998).
The physicochemical characteristics of the support play an important role in controlling the
catalytic activity of the Ni catalyst. The Ni particles present near the acidic sites of the pillared
clays are highly active for the hydrogenation of benzene. Pan et al. studied the Cu supported Al-
pillared clay as efficient catalyst for the direct hydroxylation of benzene to phenol using H2O2 as
an oxidant. In comparison with a series of conventional support such as silica, alumina and clay,
the pillared clay supported catalyst show highest catalytic activity which has been ascribed to the
effective dispersion of active copper metal in the micropores of Al-pillared clay (Pan et al.,
2008). Pillared clay supported noble metals such as Pd and Pt have been investigated for various
industrial and environmentally important chemical reactions such as selective catalytic reduction
of NO with methane (Bahamonde et al., 2001), selective hydrogenation of alkynes with bulky
moities (Kiralyet al., 1996), combustion of methyl ethyl ketone (Gil et al., 2001),
hydrodesulphurization (Kanda et al., 2009), nitrate reduction in aqueous phase (Mishra and
Ranga Rao, 2007) and hydrodechlorination (Molina et al., 2009; 2009a). The catalytic activity
of the pillared clays has been described in several reviews (Figueras, 1988; Vaccari, 1999; Cool
et al., 2002; Issaadi et al., 2006; Gil et al., 2008; 2010). Table 1.6 summarizes some of the
catalytic studies carried out on different pillared clays.
35
Table 1.6 Catalytic applications of various pillared clay based catalysts
Pillar
Type
Catalyst
Reaction
Reference
Al
Zr
Al-PIL
Pt/Al-PIL
Pt/Al-PIL
Pd/Al-PIL
STO/Al-PIL
Cu/Al-PIL
PdCu/Al-PIL
Co-Al-PIL
Mn/Al-PIL
Fe-Al-PIL
Mn/Al-PIL
Pd/AlCe-PIL
Zr-PIL
Alkylation of toluene
Dehydration of 1-pentanol
Hydrodecholrination of 4-ClPhOH
Hydrodesulfurization
Hydrodecholrination of 4-ClPhOH
Decomposition of isopropanol
Synthesis of dihydropyrimidin-
ones, thiochromans and coumarins
Hydroxylation of benzene
Nitrate reduction
Oxidation of propene
Oxidation of acetone
Wet air oxidation of phenol
Total oxidation of acetone, ethyl
acetate and toluene
deep oxidation of benzene
Synthesis of dihydropyrimidinones
Alkylation of benzene
Oxidation of acetone
Deep oxidation of nitrogen-
containing VOCs
Dye degradation
Horioet al., 1991
Jones and Purnell, 1994
Molina et al., 2009
Kanda et al., 2009
Molina et al., 2009a
Issaadi et al., 2006
Sowmiya et al., 2007
Pan et al., 2008
Mishra and Ranga Rao,
2007
Gil et al., 2008
Gil et al., 2008
Guo et al., 2006
Chen et al., 2009
Zuo et al., 2008
Singh et al., 2006
Guerra et al., 2006
Gil et al., 2008
Huang et al., 2009
Gil et al., 2011
36
Ti
Zr-Fe-PIL
Ti-PIL
V2O5/Ti-PIL
TiO2-Al-PIL
Cu, Ni Fe/ Ti-
PIL
Cu-PIL
MnO2/TiO2-
PIL
V/Fe-PILC
Selective oxidation of H2S
Alkylation of Phenol
Wet peroxide oxidation
Alkylaromatics oxidation
Photocatalytic oxidation
Oxidation of aniline
Oxidation of H2S
Epoxidation of cyclohexene
Photocatalytic degradation of
methylene blue
Selective catalytic reduction of
NOx
Wet hydrogen peroxide catalytic
oxidation
Selective catalytic reduction of
NOx
Catalytic oxidation of H2S
Bineesh et al., 2010
Mishra and Ranga Rao,
2004
Molina et al., 2006
Rezala et al., 2009
Ouidri andKhalaf, 2009
Jagtap and Ramaswamy,
2006
Bineesh et al., 2011
Ouidriet al., 2010
Fatimahet al., 2010
Valverde et al., 2003
Caudo et al., 2008
Boxiong et al., 2011
Bineesh et al., 2011
37
PART C: CERIA IN CATALYSIS
1.11 INTRODUCTION
Cerium dioxide is a rare earth oxide which shows promising applications in environmental and
redox catalysis (Trovarelli, 1996; 2002; Kašpar et al., 2000; De Monte and Kasper, 2005). Ceria-
based oxide materials have been extensively investigated because of their structural and chemical
properties (Trovarelli, 1996; 2002; Kašpar et al., 2000), reduction behaviour and non-
stoichiometry (Perrichon et al., 1994; Ranga Rao, 1999), oxygen storage capacity (Ranga Rao et
al., 1994; Kašpar et al., 2003; Mamontov et al., 2003; Kim et al., 2007) and metal-ceria
interactions (Ranga Rao et al., 1996; Bernal et al., 1999). The prominent role of ceria has been
recognized in three-way catalysis, catalytic wet oxidation, water-gas-shift reaction,
oxidation/combustion catalysis and solid oxide fuel cells (Bekyarova et al., 1998; Trovarelli,
2002; Fornasiero et al., 2003; She et al., 2009; Ayastuy et al., 2010; Chen et al., 2011; Wang et
al., 2011; Kim and Ihm, 2011; Tran et al., 2011; Li et al., 2011; Arena et al., 2012; Rovira et al.,
2012). The structural features of ceria in combination with oxygen storage and release properties
are crucial for various catalytic reactions. Ceria is believed to help in preserving the catalyst
surface area, pore size distribution and catalytic activity under extreme operating conditions such
as in three-way-catalysis (Kašpar et al., 2000; 2003). It has been observed that the incorporation
of transition metals such as zirconium into ceria lattice is found to improve the thermal stability,
redox and acid-base properties of ceria significantly (Ranga Rao et al., 1996; Cutrufello et al.,
1999; De Monte and Kasper, 2004; 2005a). A number of CeO2-based systems such as CeO2-
ZrO2, CeO2-Al2O3, CuO/CeO2-ZrO2, CuO/CeO2/Al2O3, CeO2-La2O3, CeO2-HfO2, Fe2O3/CeO2,
Ru, Pd, Pt, Ir/CeO2, Cu/CeO2, Ni/CeO2 and Au/CeO2 have been examined for their catalytic
properties (De Monte and Kasper, 2005; Renard et al., 2005; Barbier et al., 2005; Reddy et al.,
38
2007; Campo et al., 2008; Ayastuy et al., 2010; Tabakova et al., 2010; Zhou et al., 2010; Rao et
al., 2011; Devadas et al., 2011; Kim and Ihm, 2011). The redox and acid-base properties of ceria
and their modification upon incorporating a second oxide component into its lattice is believed to
be due to the changes occurring in the lattice structure and non-stoichiometric properties of ceria.
Recently, efforts have also been focused to synthesize ceria based nanocomposite oxides having
better physicochemical properties for diverse applications (De Monte and Kasper, 2005; Kim
and Ihm, 2011). This section of the chapter provides an overview of some of the structural and
catalytic aspects of ceria-based mixed oxides.
1.12 STRUCTURE OF CERIA
The structural properties and non-stoichiometry of cerium dioxide has been investigated by
several authors (Ricken et al., 1984; Korner et al., 1989; Perrichon et al., 1994; Mogensen et al.,
2000; Andreeva et al., 2002; Kim et al., 2007). Valuable information regarding the redox
property and the oxygen mobility in the lattice of ceria has been obtained from these studies.
Ceria is pale yellow color solid due to O2-
Ce4+
charge transfer and is known to crystallize in a
fluorite structure (CaF2) with a space group of Fm3m. The unit cell structure of ceria is shown in
Fig. 1.11. In the face centered cubic (FCC) structure of ceria, Ce4+
ions form a cubic close
packing arrangement and all the tetrahedral sites are occupied by the oxide ions whereas the
octahedral sites remain vacant. The unit cell of ceria can be considered as simple cube in which
the face center positions and corners are occupied by Ce4+
ions. The tetrahedral sites can be
visualized by dividing the cube into eight smaller cubes. The body center positions of all the
smaller cubes are occupied by oxide ions and alternate corners are occupied by Ce4+
ions.
39
Figure 1.11 Fluorite structure of ceria
Under reducing conditions, ceria is known to form nonstoichiometric oxides of general
composition CeO2-x where 0<x<0.5. The extent of reduction of ceria and the formation of
nonstoichiometric phases have been studied by using several techniques such as TPR, XRD and
magnetic measurements (Ranga Rao et al., 1994; Fornasiero et al., 1995; Ranga Rao, 1999;
Trovarelli, 2002; Kašpar et al., 2003). The fluorite structure of ceria is retained up to 900 K
under reducing atmosphere. However, the lattice parameter was found to increase with reduction
temperature (1.8 % increase at 873 K) indicating an expansion in the FCC lattice (Perrichon et
al., 1994). The increase in the lattice parameter is due to the reduction of Ce4+
ion to Ce3+
ion.
The larger ionic size of Ce3+
(radius 1.14 Å) compared to that of Ce4+
(radius 0.97 Å) results in
the expansion of lattice. The non-stoichiometric phases formed during the reduction process can
be easily oxidized to pure CeO2 phase upon exposure to air or under mild oxidizing conditions.
Ce4+
O2-
40
However, the reversibility of this process is decreased when ceria is reduced at much higher
temperatures. A cubic Ce2O3 phase has been detected in the XRD measurements for high surface
area ceria sample reduced at 1070-1170 K (Perrichon et al., 1994). Reoxidation experiments
conducted for the reduced sample in air show that the cubic Ce2O3 phase reoxidizes slowly
compared to the expanded ceria phases. Reduction at temperatures 1273 K leads to the
formation of hexagonal Ce2O3 phase. This phase is stable at room temperature and possesses
identical structure as that of La2O3. The ability of the cerium ion to switch between Ce4+
and
Ce3+
depends on the ambient oxygen partial pressure is represented as:
CeO2 CeO2-x + x/2 O2 (1)
The amount of oxygen released in the forward reaction and the oxygen consumed in the reverse
reaction is generally referred as the oxygen storage capacity (OSC) of ceria material (Trovarelli,
1996; Fornasiero et al., 1998; Kašpar et al., 2000). The oxygen lability and the possibility of
large deviations from its stoichiometry are the main reasons for the prevalent use of ceria in
three-way catalysts (TWC) (Matsumoto, 1997; Kašpar et al., 2000; 2003).
1.13 THERMAL STABILITY AND REDUCTION CHARACTERISTICS OF DOPED
CERIA
The thermal stability and oxygen storage capacity of ceria strongly decreases upon thermal aging
due to the growth of ceria crystallite and loss in active surface area (Shyu et al., 1988). The
growth of ceria crystallite and the thermal sintering process can be considered as due to the mass
transport at an atomic scale caused by a concentration gradient set at higher temperature
(Maestro and Huguenin, 1995). Improvement of the thermal properties of ceria and retention of
active surface area at high temperature is thus necessary to exploit the redox property of ceria for
41
various applications especially in TWC. The thermal stability of ceria can be improved by the
incorporation of aliovalent dopant into ceria lattice (Hong and Virkar, 1995; De Monte and
Kasper, 2004; 2005a). It has been observed that incorporation of zirconia in ceria lattice creates a
higher concentration of defects, which stabilizes ceria against sintering, enhances the thermal
stability and redox properties (Kašpar et al., 2000; De Monte and Kasper, 2004; 2005). Ceria is
known to form a substitutional type solid solution with zirconia (Ranga Rao et al., 2003; De
Monte and Kasper, 2005a). The ceria-zirconia solid solutions show better oxygen storage
capacity and reduction behavior compared to pure ceria and is regarded as a potential substitute
for ceria in TWC applications (Fornasiero et al., 1998; Martinez-Arias et al., 2001; 2002). The
modification of the redox properties of ceria is attributed to the increase in the oxygen mobility
in the bulk of the oxide due to the introduction of zirconia into ceria lattice (Martin and Duprez,
1996). Theoretical studies on ceria zirconia solid solutions showed that the activation energy for
oxygen migration in the bulk decreases almost monotonically with the zirconia content
indicating the facile oxygen diffusion through the bulk of the material (Balducci et al., 1997).
The redox properties of ceria-zirconia solid solutions have been improved tremendously in the
presence of loaded metals such as rhodium (Ranga Rao et al., 1994; Ranga Rao, 1999). In the
recent years mixed oxides of Fe, Mn, Ni, W, Co, Zn and Cu with ceria and modified ceria have
also been studied for their redox and catalytic properties. The presence of above transition metal
not only improves the thermal properties of ceria but also helps in improving the catalytic
properties. Several catalytic transformations such as photo-oxidation of basic orange dyes, wet
oxidation of phenol, aniline and ammonia, CO oxidation, methane decomposition, catalytic
reduction of NOx, hydrogen transfer reactions, water gas shift reactions are promoted by the
presence of transition metal ions in ceria lattice (Mishra and Ranga Rao, 2006; Tang et al., 2010;
42
Zhang et al., 2011; Arena et al., 2012). The interaction between the transition metal ions and
ceria lattice is crucial for the catalytic activity of the composite oxides (Arena et al., 2007; Kim
et al., 2009; Ayastuy et al., 2010; Martı´nez et al., 2011; Li et al., 2011; Arena et al., 2012).
1.14 METAL-CERIA INTERACTION
Supported metals are used in large scale in heterogeneous catalysis. The role of the support is to
disperse the metal particles and inhibit them from sintering. The supports widely used for this
purpose are SiO2, Al2O3, TiO2, Nb2O5, CeO2 and ZrO2. Out of supports widely studied, ceria
occupies a prominent position as a support material because of its redox properties, and oxygen
storage capacities that can control the dispersion, morphological, and electronic properties of
supported metals, and thus influence their catalytic activity. For example: ceria as support can
enhance the catalytic performance of Ni catalysts for the steam reforming of ethanol (Zhou et al.,
2010). It has been observed that for steam reforming over Ni catalyst, ceria as a support can
provide lattice oxygen to oxidize the carbon deposit/coke species on Ni surfaces. The nucleation
and growth of Ni nanoparticles can also be controlled by the ceria. The interaction between ceria
and Ni is crucial for sintering behavior, electronic properties, extent of dispersion and catalytic
activity of the active Ni phase (Zhou et al., 2010). Ceria supported with Pt, Pd, and Ru (Barbier
et al., 2005) metals have been extensively investigated for metal-support interactions and
catalytic properties. Ceria plays active role and influences the catalytic properties of supported
noble metals via metal-support interactions. In many ceria supported catalyst, mutual promoting
effect between the ceria and metal component has been found to be responsible for higher
catalytic activity (Ranga Rao, 1999; Fornasiero et al., 1998; Yeung et al., 2005; Avgouropoulos
et al., 2008; 2009; Tang et al., 2010; Mayernick and Janik, 2011; Longo et al., 2011; Salazar-
Villalpando, 2012). For example, in Pd/CeO2-x/Al2O3 system the synergistic effect between the
43
supported Pd metal and cerium dioxide has been observed during methane oxidation reaction
(Haneda et al., 1998). In addition to the metal support interaction, the supported metal can also
influence the physicochemical properties of ceria such as reducibility and the oxygen storage
capacity (Fornasiero et al., 1998; Ranga Rao, 1999). The presence of a small amount of Rh has
been found to modify the low temperature reducibility of ceria (Fornasiero et al., 1995; 1998).
The M/CeO2 systems have been extensively used as catalysts for several industrially and
environmentally important chemical reactions such as wet air oxidation of phenol, aniline, and
ammonia, water-gas shift (WGS) and preferential CO oxidation (PROX) reactions,
hydrogenation, three way catalysis, nitrate reduction, methane oxidation and steam reforming of
ethanol for hydrogen production in fuel cell applications (Tabakova et al., 1997; Trovarelli,
2002; Kašpar et al., 2000; 2003; Campo et al., 2008; Zhang et al., 2011; Zhou et al., 2010;
Nousir et al., 2008; Chen et al., 2011; Devadas et al., 2011 ; Wang et al., 2011 ; Mayernick and
Janik, 2011; Arena et al., 2012).
1.15 CERIA IN CATALYSIS
The catalytic applications of ceria have been reviewed in literature (Trovarelli, 2002; Kašpar et
al., 2000; 2003; Kim and Ihm, 2011). The main applications of ceria in industrial catalysis are
connected to the removal of pollutants such as CO, NOx and unburned hydrocarbon (HC)
originating from automobiles and also the removal of SOx from FCC process. Ceria also forms
an important component in the catalyst formulation for oxidation of basic orange dyes, CO
oxidation and dehydrogenation of ethyl benzene to styrene (Trovarelli et al., 1999). The use of
ceria has also been extended to the removal of organic compounds in waste water by catalytic
wet oxidation (Kim and Ihm, 2011). Besides the oxidation properties of ceria, the surface acid-
base properties of ceria have also been explored for synthetic organic transformations (Wang et
al., 2009; Gangadharan et al., 2010; Reddy et al., 2010; Postole et al., 2010).
44
1.15.1 THREE WAY CATALYSIS
The most important application of ceria based materials is in the field of three-way catalysis. The
catalytic and technological aspect of three-way catalysts has been reviewed from time to time
(Shelef and McCabe, 2000; Kašpar et al., 2000; 2003; Chen et al., 2011; Wang et al., 2011). The
three important pollutants present in automobile exhaust are (i) uncombusted hydrocarbons
(HC), (ii) carbon monoxide (CO) and (iii) nitrogen oxides (NOx). The three-way catalysts
(TWC) are designed to simultaneously convert these pollutants to environmentally acceptable
products such as carbon dioxide, water and nitrogen. The TWC formulation contains Pt/Rh,
Pt/Pd/Rh, Pd/Rh and Pd-only noble metal catalysts dispersed either on the surface of alumina
pellets or on an alumina washcoat anchored to a monolithic substrate made of cordierite
(2MgO.2Al2O3.5SiO2) or metal (Kašpar et al., 2000; 2003). High surface area -alumina is used
as support for this purpose because of the higher thermal stability of the -alumina under
hydrothermal condition. The thermal stability and surface area of -alumina has been further
improved by adding oxides of cerium, barium, lanthanum and zirconium as stabilizing agents
(Kašpar et al., 2000). Ceria and ceria based mixed oxides such as CeO2-ZrO2 is used as oxygen
storage promoters in the TWC system (Kašpar et al., 2000; 2003). Unlike the majority of
industrial catalytic reactions, which operate under static condition, the automotive exhaust
catalyst is exposed to varying conditions of temperature, pressure and reactant concentration. For
optimum performance of the three way catalyst, a stoichiometric air-fuel ratio (A/F) value of
14.6 is required. If the air-fuel ratio is close to the stoichiometric value of 14.6, the catalyst
converts all the pollutants to CO2, H2O and N2 gases with high efficiency. The conversion
efficiency plot for HC, CO and NOx as a function of air-fuel ratio is given in Fig. 1.12. The plot
shows the highest conversion of pollutants at the stoichiometric value of 14.6. Deviation from
45
the stoichiometric value affects the performance of the three way catalyst resulting in the
incomplete conversion of the pollutants. Under fuel lean condition the conversion of NO is
affected whereas under fuel rich condition the oxidation of CO and hydrocarbons remains
incomplete. The role of ceria and ceria based materials in TWC is to widen the A/F window and
help maintaining the conversion efficiency of the catalyst.
13.0 13.5 14.0 14.5 15.0 15.5 16.00
20
40
60
80
100
Operation Window
HC
CO NO
Co
nve
rsio
n E
ffic
ien
cy (
%)
Air to fuel mass ratio
Figure 1.12 Effect of air to fuel ratio on the conversion efficiency of three-way catalyst
(Kaspar et al., 2000)
Ceria has the ability to store excess oxygen under fuel lean condition and made it available under
fuel rich condition for the oxidation of CO and hydrocarbons. This happens because of its ability
to switch between Ce4+
and Ce3+
oxidation states depending on the oxygen partial pressure in the
exhaust gas composition. In the exhaust environment, the following reactions have been
46
proposed to account for the storage and release of oxygen (Trovarelli, 1996, Di Monte and
Kašpar, 2005).
Oxygen storage
Ce2O3 + 0.5 O2 2CeO2
Ce2O3 + NO 2CeO2 + 0.5 N2
Ce2O3 + H2O 2CeO2 + H2
Oxygen release
2CeO2 + H2 Ce2O3 + H2O
2CeO2 + CO Ce2O3 + CO2
1.15.2 OXIDATION OF ENVIRONMENTAL POLLUTANTS
Catalytic wet oxidation of environmental pollutant is one of the promising techniques for
removal of organic pollutant from aqueous sources. For effective CWO process, noble metals
(Pt, Pd, and Rh) are widely used as catalysts which are highly expensive. In the past years, there
have been effort to replace noble metals by metal oxides or a combination of both has been used
to maximize the efficiency of the catalyst. Among different metal oxides studied, CuO, MnOx,
and CeO2 are the most prospective materials to compete with noble metals as catalyst. Ceria
based mixed oxides and supported metals show promising catalytic wet air oxidation (CWAO)
activity for several types of pollutants such as phenol, p-coumaric acid, halogenated liquid
wastes, nitrogenous compounds, azo dyes, and refractory organic pollutants in industrial
wastewaters (Neri et al., 2002; Goi et al., 2006; Kim et al., 2009; Kim and Ihm, 2011; Tran et
al., 2011; Arena et al., 2012). Ceria in combination noble metals such as Ru, Rh, Pd, Ir, and Pt
and oxides of Cr, Mn, Fe, Co, Ni, Cu, Zn and Mo has been reported to be efficient catalysts for
the oxidation of dissolved organic compounds in industrial discharge (Trovarelli, 2002; Kim and
47
Ihm, 2011). It has been reported that ceria based mixed metal oxides such as CeO2-ZrO2, CeO2-
TiO2, CuCeOx, and MnCeOx are most favourable for the CWAO of organic pollutants (Kim and
Ihm, 2011). The high activity of ceria based composite oxides in wet oxidation of organic
compounds has also been related to the reaction of the molecular oxygen, with the vacant sites
that are generated as a result of incorporation of the second component into ceria lattice
(Leitenburg et al., 1996; Trovarelli, 2002; Kim and Ihm, 2011). The interaction of oxygen with
ceria lattice and formation of different adsorbed oxygen species on the surface is vital for the
catalytic activity for oxidation reaction.
1.15.3 CERIA IN ORGANIC TRANSFORMATIONS
The surface acid base character of metal oxide arises due to the presence of hydroxyl groups and
coordinatively unsaturated cations (Mn+
) and oxygen anions (O2-
). The exposed coordinatively
unsaturated cations can act as acceptors for electron pairs of adsorbed reactant molecules (Lewis
acid site) while the exposed coordinatively unsaturated oxygen anions, being more basic than
bulk ions, can behave as Lewis base sites. The surface hydroxyl groups due to the surface
polarization effect may act as Brønsted acid sites (Kung, 1989). The acid strength (i.e. electron
acceptor strength) of a surface site depends upon the effective positive charge on the metal cation
(Mn+
) and/or their coordination on the catalyst surface. Similarly, the base strength (or electron
donor strength) is related to the effective negative charge on the anion (O2-
) and/or their
coordination on the surface (Choudhary and Rane, 1991). The presence of surface imperfections
on the polycrystalline metal oxides such as steps, kinks and corners provide sites for ions of low
coordination n+
LCM and 2-
LCO , and are responsible for the presence of surface acid-base sites of
different strengths (Che and Tench, 1982). The other property of the metal oxides that can
contribute to the surface basicity is the mobility of surface oxygen. This property is linked to
48
Mn+
O2-
bond on the oxide surface and depends on the number and strength of the bond on the
surface of the solid (Martin and Duprez, 1996).
The surface acid-base properties of metal oxides are generally probed by using spectroscopic,
temperature programmed methods and test reactions (Corma, 1995; Busca, 2010). Various probe
molecules such as pyridine and ammonia are used for the determination of acidic sites where as
pyrrole and CO2 are used for the determination of basic sites on catalyst surface. The ring
vibration mode and N-H stretching frequency of pyridine and pyrrole are sensitive to the strength
of the surface sites and are excellently determined by FTIR spectroscopy. The vibrational shift of
these IR bands to lower wavelength has been taken as a measure of the strength of the site
involved. Temperature programmed desorption (TPD) of CO2 and NH3 has been extensively
used for the determination of basic and acidic sites on the catalyst surface. Both qualitative and
quantitative information about the acid-base sites can be obtained from the TPD experiments.
The temperature of desorption during a TPD experiment can be taken as a measure of the
strength of the site involved while the amount of the desorbed probe molecules at that
temperature can be used to calculate the number of surface sites. The amphoteric nature of
alcohols allows their interaction with both acidic and basic sites on a catalyst surface.
Dehydrogenation and dehydration of secondary alcohols, especially propan-2-ol, is frequently
employed as a model reaction to probe surface acid-base property. It is believed that propan-2-ol
dehydration to propene probes the acid character of the oxide surface while dehydrogenation to
acetone characterizes the surface basicity of the oxide material (Ouqour et al., 1993; Haffard et
al., 2001). However, the redox property of oxide catalysts has been reported to contribute to the
dehydrogenation process (Fikis et al., 1978; Ouqour et al., 1993; Moro-oka, 1999; Haffard et al.,
2001). Therefore both redox and acid-base properties of the metal oxides have to be taken into
49
account to establish the relationship between surface properties and catalytic activity observed.
Ceria is a strong redox promoter with mild acid-base properties and has been considered as a
catalyst component for several organic transformations. Some of the organic transformations
carried out on ceria based materials are summarized in Table 1.7. The redox property (Ce3+
Ce4+
) and weak basicity of CeO2 are found to be responsible for promoting a number of
reactions. Ceria is reported to activate methanol and responsible for higher catalytic activity
during the alkylation of phenol over CeO2-MgO mixed oxide catalysts (Sato et al., 1998; 1999).
Abimanyu et al, has investigated the catalytic performance of CeO2-MgO mixed oxide for the
synthesis of dimethyl carbonate via transesterification. The mixed oxide catalyst with 25% ceria
content shows highest dimethyl carbonate selectivity. They have mentioned that the catalytic
activity depends on the surface basicity and the base strength distribution rather than the surface
area of the catalysts (Abimanyu et al., 2007). The dehydration of 4-methylpentan-2-ol over ceria-
zirconia solid solutions has also been explained based on surface acid-base and redox properties
of ceria (Cutrufello et al., 1999). The incorporation of zirconia into ceria lattice is found to
generate new basic sites of higher strength and homogeneity compared to pure ceria. The
creation of defect centers in the solid solution and the change in the electronic property has been
proposed to be responsible for the creation of new active sites in the ceria-zirconia system.
Ranga Rao et al. reported that the presence of ceria in ZnO matrix is found to accelerate the rate
of cyclohexanol conversion and hydrogen transfer reactions (Mishra and Ranga Rao, 2006). The
redox and acid–base properties of individual oxides can be tuned to produce surface acid-base
and redox sites in mixed oxide systems to carry out organic reactions. Ceria and ceria-based
materials are ideal choice for such study and larger utilization in organic transformations remains
a field for further study.
50
Table 1.7 Ceria based materials used in organic transformations
Catalyst Reaction studied References
CeO2
CeO2-ZnO
CeO2/Al2O3 and
V2O5/CeO2/Al2O3
CeO2/Al2O3
CeO2-MgO
Ru/CeO2-MgO
CeO2-SnO2
CeO2-ZrO2
Reactivity of alcohol
Reactivity of acetone
Steam reforming of ethanol
oxidative coupling of methane
Hydrogen transfer reaction
Degradation of acridine orange
Oxidative dehydrogenation of
ethylbenzene
NO reduction
Alkylation of phenol
Transesterification of ethylene
carbonate
Ammonia synthesis
Hydroxymethylation of anisole
Dehydration of alcohol
Hydrogen transfer reaction
Esterification of acetic acid
CO hydrogenation
Knoevenagel condensation
synthesis of diethyl carbonate
Aldol condensation and
ketonization
4-methylpentan-2-ol dehydration
Auroux et al., 1996
Zaki et al., 2001
Llorca et al., 2001
He et al., 2004
Mishra and Ranga Rao, 2006
Faisal et al., 2011
Reddy et al., 2007
Shen et al., 2009
Sato et al., 1998, 1999
Abimanyu et al., 2007
Saito et al., 2006
Jyothi et al., 2000a
Cutrufello et al., 1999; 2001
Ranga Rao et al., 2003b
Sugunan and Varghese, 1998
Khaodee et al., 2009
Postole et al., 2010
Wang et al., 2009
Gangadharan et al., 2010
Reddy et al., 2010
51
CeO2-NiO
CeO2-CaO
Cu/CeO2
MnCeOx
CeO2-SiO2
Oxidative dehydrogenation of
ethane
Conversion of isopropanol
Transesterification reaction
Hydrogenationof benzaldehyde
Transesterification reaction
4-methylpentan-2-ol dehydration
Solsona et al., 2012
Deraz, 2012
Yu et al., 2011
Saadi et al., 2000
Cannilla et al., 2010
Reddy et al., 2010
1.16 OBJECTIVES OF THE PRESENT STUDY
The main objectives of this study is to prepare surface and structurally modified zirconia
materials and evaluate their catalytic application for synthesis of biologically important
molecules under environmentally benign conditions. Zirconia is one of the promising materials
for application as catalyst and support. The catalytic activity of zirconia can be enhanced
significantly by surface modification by grafting active species such as molybdate, tungstate and
sulfate species. Similarly, doping transition metal into the zirconia lattice can lead to metastable
tetragonal phase stabilization and increase in catalytic activity. In this thesis, the effect of Mo
and Fe addition on the physicochemical properties and catalytic activity of the zirconia has been
explored. The catalytic activity of the modified zirconia has been evaluated for synthesis of
amidoalkyl naphthols, β-acetamidoketones, octhydroquinazolinones and octahydroxanthenes by
multicomponent condensation approach under solvent free conditions or using water as a green
solvent.
Pillared clays are a class of solid acid catalysts which possess thermally stable lattice structures,
uniform micropores and acidic properties. These materials are simple to prepare, cost effective
and can be used as catalyst for a variety of organic reactions. The thermal stability and acidic
52
property of the pillared clays can be suitably tuned for particular application. In this work, post
synthetic modification of zirconia pillared clay by grafting sulfate species to the nanopillars and
dispersion of phosphomolybdic acid in the micropores of sulfated Zr-pillared clay has been done
to enhance its catalytic properties. The use of the modified Zr-pillared clay as heterogeneous
catalyst for the synthesis of β-aminocarbonyl compounds have been explored in aqueous media.
In this investigation, the promoting effect of ceria has been explored for biologically important
chemical transformations requiring surface acid-base property. Ceria base materials exhibit
interesting structural, redox and acid-base properties which are vital for a variety of catalytic
reactions. The oxygen storage and release capacity of these materials is excellent and is exploited
in exhaust catalysis. In addition, the role of ceria as a catalyst and promoter has been well
recognized in environmental catalysis. In past few years, there has been an increasing effort
towards understanding the acid-base property of ceria based mixed oxides. The current
investigations have been undertaken considering the vast scope of ceria-based materials in
catalytic science and applications.
The results obtained in this investigation are described in the following chapters.
Chapter 1: This chapter describes briefly the structural and catalytic chemistry of zirconia and
ceria-based materials and pillared clays. The current understanding of the various properties and
applications of these materials in catalysis are highlighted with specific examples.
Chapter 2: The experimental procedures employed in the preparation of zirconia and ceria-
based materials and pillared clays are described in this chapter. A brief description of the
instrumental techniques used in the characterization of these materials and the methods of
53
analysis of the results are described. The experimental procedure used for performing the
synthesis of amidoalkyl naphthols, β-acetamidoketones, octhydroquinazolinones
octahydroxanthenes, β-aminocarbonyl compounds, 2-amino-2-chromenes, β-nitro alcohols and
arylidenemalononitriles are presented. The product analysis protocols are also outlined.
Chapter 3: This chapter describes the physicochemical characterization and catalytic
applications of MoO3-ZrO2 nanocomposite oxides and sulfate grafted Fe doped zirconia (SO42-
/Fe-Zr) materials. The chapter has been divided into three parts. Part A deals with the study of
the MoO3-ZrO2 nanocomposite oxides synthesized by coprecipitation and impregnation methods.
These materials have been characterized using XRD, Raman, UV-Vis, SEM, TEM and
sorptometric techniques. The catalytic activity data for the synthesis of amidolakyl naphthols and
β-acetamidoketones has been presented. Part B of this chapter describes the physicochemical
characterization and catalytic application of combustion synthesized MoO3-ZrO2 nanocomposite
oxides for aqueous phase synthesis of octahydroquinazolinones. The last part of the chapter deals
with characterization and catalytic application of the sulfate grafted Fe doped zirconia catalyst
(SO42-
/Fe-Zr) for the microwave assisted solvent free synthesis of octahydroxanthenes.
Chapter 4: The effect of post synthesis modification by sulfate grafting and supporting
phosphomolybdic acid on the physicochemical properties and catalytic activity of Zr-pillared
clay has been described in this chapter. The modified Zr-pillared clay (PMA/SZr-P) material
have been characterized by XRD, IR, SEM, TEM , UV-Vis, TGA and sorptometric techniques.
The catalytic activity data for the synthesis of β-aminocarbonyl compounds has been presented.
54
Chapter 5: This chapter describes the promoting effect of ceria in CeO2-MgO and CeO2-CaO
mixed oxides for the environmentally benign synthesis of 2-amino-2-chromenes, β-nitro alcohols
and arylidenemalononitriles. The chapter has been divided into two parts. Part A deals with the
study of the CeO2 (x mol%)-CaO composite oxides as catalysts. The CeO2 (x mol%)-CaO
materials have been prepared by amorphous citrate process and characterized using different
analytical techniques. The catalytic test results for the synthesis of 2-amino-2-chromenes in
aqueous media are presented. Part B of this chapter describes the promoting effect of ceria in
CeO2–MgO mixed oxide catalysts. The CeO2–MgO materials have been prepared by two
different methods namely gel combustion and amorphous citrate method. The catalytic
application of the CeO2-MgO nanocomposite oxides have been demonstrated for the synthesis of
β-nitro alcohols and arylidenemalononitriles.
Chapter 6: The results obtained from various investigations presented in this thesis are
summarized in this chapter.
55
CHAPTER 2
MATERIALS AND METHODS
2.1 PREPARATION OF CATALYTIC MATERIALS
2.1.1 PREPARATION OF ZrO2, MoO3 AND MoO3 (x mol%)- ZrO2 CATALYSTS
2.1.1.1 COPRECIPITATION METHOD
The MoO3-ZrO2 catalysts were prepared by coprecipitation method using zirconium oxychloride
(ZrOCl2.8H2O) and ammonium heptamolybdate ((NH4)6Mo7O24.4H2O) (S.D. Fine Chemicals,
India, 99.9%) as precursor salts and liquid ammonia as precipitating agent. Initially, 500 ml of
double distilled water was adjusted to pH 9.0 by addition of liquid ammonia. To this solution
required amount of precursor salt solution was added dropwise (20 ml/h) under constant stirring.
The pH of the solution was continuously monitored and maintained at the same pH by drop wise
addition of ammonia solution. After the completion of the precipitation process, the aqueous
mixture was allowed to stir for 12 h followed by multiple washing in double distilled water (till
Cl- free). The co-precipitated materials were recovered by filtration and then dried at 120
oC for
12 h in hot air oven and calcined at 500oC for 2 h to obtain the MoO3-ZrO2 catalysts. Using this
procedure MoO3-ZrO2 catalyst with MoO3 content of 2, 5, 10, 20 and 50 mol% were prepared.
The MoO3-ZrO2 catalysts are referred to as xMoZr in the text, where x represent the mol% of
MoO3 present in the composite oxide.
2.1.1.2 WET IMPREGNATION METHOD
In this method, the required amount of ammonium heptamolybdate was added to ZrO2 aqueous
suspension and then stirred for 6 h. The aqueous suspension was then heated with constant
stirring to remove water. The resulting material was dried at 120oC for 12 h followed by
calcination at 500oC for 2 h to obtain the final catalyst. The MoO3-ZrO2 catalysts prepared by
56
this method are referred to as xMoZrI in the text, where x represent the mol% of MoO3 present
in the composite oxide and I represent the impregnation method
2.1.1.3 COMBUSTION METHOD
Zirconyl nitrate (ZrO(NO3)2·xH2O), glycine and hexamethylene tetramine (HMTA) were
procured from Merck India. Ammonium heptamolybdate and urea were obtained from S.D. fine
chemicals ltd., India. All the chemicals were used as received without any further purification.
Pure ZrO2 and MoO3-ZrO2 composite oxide materials were prepared by solution combustion
synthesis using ZrO(NO3)2.xH2O, ammonium heptamolybdate as salt precursor and urea, glycine
and hexamethylene tetraamine as fuels. In a typical preparation procedure, required amounts of
zirconyl nitrate, ammonium heptamolybdate and urea were dissolved in minimum amount of
water and the mixture was kept in a furnace, preheated to 400°C. The mixture instantaneously
gets ignited producing a foamy material of MO3-ZrO2 simultaneously releasing a lot of gaseous
products. The fuel to oxidizer equivalence ratio (F/O) was calculated by summing the total
oxidizing and reducing valencies in the fuel and dividing it by the sum of the total oxidizing and
reducing valencies in the oxidizer salt (Jain et al., 1981, Patil et al., 2008).
The valency of the elements have been assigned by assuming the oxidation state of the elements
C, H, O, N, Zr and Mo to be +4, +1, -2, 0, +4 and +6 respectively, in the final product (Jain et
al., 1981). The F/O ratio was varied from 0.5 to 1.5 to study the effect of fuel content on the
physicochemical characteristics of the MoO3-ZrO2 composite oxide materials. Using this
procedure, MoO3-ZrO2 composite oxides with MoO3 content of 2, 5, 8, 10, 20 mol% were
57
prepared using urea as fuel. The MoO3-ZrO2 materials are referred to as xMoZr-Y in the text,
where x represents the mol% of MoO3 present in the composite oxide and Y stands for the fuel
employed for the preparation of these composite oxides (Glycine (G), Urea (U) and
Hexamethylene tetraamine (H)).
2.1.2 PREPARATION OF SULFATED ZIRCONIA CATALYST (SO42-
/ZrO2)
The sulfated zirconia catalyst was prepared by equilibrium adsorption of sulfate species on the
surface of hydrous zirconium oxide samples. The hydrous zirconia catalysts were prepared by
precipitation method using ZrOCl2.8H2O and liquid ammonia solutions. Required amount of
zirconyl chloride solution were added drop wise to 200 mL of deionised water whose pH was
previously adjusted to 10.00 by addition of liquid ammonia solution. During the addition of
zirconyl chloride solution, the pH of the mixture was found to decrease due to the hydrolysis of
the zirconium salt. The pH was maintained at 10.00 by controlled addition of ammonia solution
to the reaction mixture. The precipitated solution was stirred for 12 h at room temperature
followed by filtration and washing with double distilled water until free from chlorine (AgNO3
test). The hydroxide precipitate were subsequently dried overnight at 120oC and calcined at
400oC for 2 h to generate hydrous zirconium oxide. The sulfation experiment was carried out by
suspending the ZrO2 powder in 0.5 M H2SO4 solution. After 24 h of stirring, the solid particles
were filtered, washed with 0.05 M H2SO4, dried at 120oC, and calcined for 2 h at 500
oC to yield
the sulfated zirconia catalyst.
2.1.3 PREPARATION OF SULFATE GRAFTED Fe2O3 (x mol%)-ZrO2 CATALYSTS
(SO42-
/xFe-Zr)
The Fe2O3-ZrO2 catalysts were prepared by coprecipitation method using zirconium oxychloride
(ZrOCl2.8H2O) and iron nitrate (Fe(NO3)3·9H2O) (S.D. Fine Chemicals, India, 99.9%) as
58
precursor salts and liquid ammonia as precipitating agent. Initially, 200 ml of double distilled
water was adjusted to pH 9.0 by addition of liquid ammonia. To this solution required amount of
precursor salt solution was added dropwise (20 ml/h) under constant stirring. The pH of the
solution was continuously monitored and maintained at the same pH by drop wise addition of
ammonia solution. After the completion of the precipitation process, the aqueous mixture was
allowed to stir for 12 h followed by multiple washing in double distilled water (till Cl- free). The
co-precipitated materials were dried at 120oC for 12 h in hot air oven and calcined at 500
oC for 2
h to obtain the Fe2O3-ZrO2 catalysts. Using this procedure Fe2O3-ZrO2 catalyst with Fe2O3
content of 2, 5, 10, 20 and 50 mol% were prepared. The Fe2O3-ZrO2 catalysts are referred to as
xFeZr in the text, where x represent the mol% of Fe2O3 present in the composite oxide. The
sulfate grafted Fe2O3 (x mol%)- ZrO2 (SO42-
/xFe-Zr) material was prepared by suspending 5 g
of the xFeZr material in 150 ml of 0.5 M sulphuric acid and stirring the aqueous suspension for
24 h. The solid particles were subsequently filtered and washed with 0.05M sulphuric acid (20
ml portions for 3 times) and calcined at 500oC for 2h to obtain the (SO4
2- /xFe-Zr) materials.
Using this procedure, SO42-
/xFe-Zr materials with Fe2O3 content of 2, 5, 10, 15, 20, and 50
mol% were prepared.
2.1.4 PREPARATION OF Zr-PILLARED CLAYS AND THEIR MODIFIED
ANALOGUES
The natural Na-montmorillonite, (Na0.35K0.01Ca0.02)(Si3.89Al0.11)tet
(Al1.60Fe0.08Mg0.32)oct
O10(OH)2.nH2O (Kunipia-F, Kunimine Industrial Company) was used for the preparation of Al-
and Zr-pillared clays described in the subsequent sections. The cation exchange capacity of the
clay is 120 mequiv (100g clay)-1
. The clay was used as received without any further purification.
59
2.1.4.1 PREPARATION OF Zr-PILLARED CLAY
Pillaring solution: Zirconium oxychloride (S. D. Fine Chemicals, India) was used as source for
zirconium polycations. 0.1 M ZrOCl2.8H2O solution was prepared by dissolving the required
amount of salt in double distilled water. The solution was refluxed for 24 h to prepare the
pillaring solutions.
Pillaring process: In a typical pillaring procedure, 4 gm of the Na-montmorillonite was
dispersed in 200 ml of deionised water to form 2.0 wt% clay slurry. The slurry was stirred at
room temperature for 2 h and sonicated for 15 minutes for better dispersion of the clay particles.
The pillaring solution was then added drop wise to the clay slurry to finally obtain Zr
/montmorillonite ratio 10 mmol/g. During the process of addition of the pillaring solution, the
clay suspension was stirred vigorously. The mixture was kept at constant stirring for 24 h at
room temperature and then filtered. The filtered materials were washed repeatedly with
deionised water (at least 6 times) until free from chloride ions (AgNO3 test), centrifuged and
dried in air and calcined at 450oC for 2 h in air to get Zr- pillared clay.
2.1.4.2 PREPARATION OF SULFATE GRAFTED Zr-PILLARED CLAYS (SZr-P)
The sulfate grafted Zr-pillared clay (SZr-P) was prepared by suspending 5 g of the Zr-P material
in 150 ml of 0.5 M sulphuric acid and stirring the aqueous suspension for 24 h. The solid
particles were subsequently filtered and washed with 0.05M sulphuric acid (20 ml portions for 3
times) and calcined at 400oC for 1h to obtain the SZr-P materials.
2.1.4.3 PREPARATION OF PHOSPHOMOLYBDIC ACID SUPPORTED ON SULFATE
GRAFTED Zr-PILLARED CLAY (PMA/SZr-P)
The 20 wt% Phosphomolybdic acid supported on SZr-P material was prepared by wet
impregnation method. Required amount of PMA was dissolved in 100 ml of double distilled
60
water and to this solution added 2 g of the SZr-P material. The aqueous suspension was stirred
for 12 h at room temperature and then heated with continuous stirring till complete removal of
water. The resulting material was dried overnight in a hot air oven and calcined at 200oC for 1h
to prepare PMA/SZr-P material.
2.1.5 PREPARATION OF CaO, CeO2 AND CeO2 (x mol%)-CaO CATALYSTS
The CeO2 (x mol%)-CaO materials along with the pure oxides were prepared by amorphous
citrate method. Ceric ammonium nitrate ((NH4)2Ce(NO3)6), calcium nitrate (Ca(NO3)2. 6H2O)
and citric acid monohydrate (C6H8O7.H2O) were obtained from Merck (India) limited. All the
chemicals were used as received without any further purification. A solid mixture of ceric
ammonium nitrate and calcium nitrate of desired molar ratio was mixed with an equimolar
amount of citric acid and heated at 70oC to form a uniform melt. The molten mixture was then
evacuated at the same temperature until it formed an expanded spongy solid material. This
material was immediately transferred to a hot air oven preheated at 160oC. The temperature of
the oven was maintained at 160oC for 2 h which yielded the amorphous citrate precursor. The
amorphous precursor was then calcined at 700oC for 3 h to obtain CeO2-CaO mixed oxide
materials. Using this procedure we have prepared pure CeO2, CaO and CeO2-CaO mixed oxide
phases containing 5, 10, 20, 50 and 80 mol % of CeO2. The CeO2-CaO nanocomposite materials
were referred to as xCe-CaO in the subsequent text where x represents the mol % of CeO2
present in the composite oxide..
2.1.6 PREPARATION OF MgO, CeO2 AND CeO2 (x mol%)-MgO CATALYSTS
2.1.6.1 SOL-GEL COMBUSTION SYNTHESIS
The starting materials, ceric ammonium nitrate ((NH4)2Ce(NO3)6), magnesium nitrate
(Mg(NO3)2.6H2O) and citric acid monohydrate (C6H8O7.H2O) were obtained from Merck (India)
61
limited. Stoichiometric quantity of ceric ammonium nitrate, and magnesium nitrate was mixed
with equimolar amount of citric acid and the mixture was dissolved in 100 mL of deionized
water. The pH of the solution was maintained at ~ 9 by drop wise addition of liquid ammonia
under constant stirring condition. The solution was evaporated at 100oC in a thermostatic oil bath
followed by drying at 120oC in a hot air oven for 2h. The resulting solid was kept in a muffle
furnace at 4000C
for 30 minutes for complete combustion. The obtained material was then
grinded thoroughly and calcined at 5500C for 2h in air to get the desired oxide. We have
prepared pure CeO2, MgO and CeO2-MgO mixed oxide phases containing 5, 10, 20, and 50 mol
% of CeO2 by this method. The CeO2-MgO nanocomposite materials were referred to as xCe-
Mg-O in the subsequent text where x represents the mol % of CeO2 present in the composite
oxide.
2.1.6.2 AMORPHOUS CITRATE PROCESS
The CeO2-MgO nanocomposite oxides were also prepared by amorphous citrate method. A solid
mixture of ceric ammonium nitrate and magnesium nitrate of desired molar ratio was mixed with
an equimolar amount of citric acid and heated at 70oC to form a uniform melt. The molten
mixture was then evacuated at the same temperature until it formed an expanded spongy solid
material. This material was immediately transferred to a hot air oven preheated at 160oC. The
temperature of the oven was maintained at 160oC for 2 h which yielded the amorphous citrate
precursor. The amorphous precursor was then calcined at 550oC for 3 h to obtain CeO2-MgO
composite oxide materials. Using this procedure we have prepared pure CeO2, MgO and CeO2-
MgO composite oxide phases containing 5, 10, 20, 50 and 80 mol % of CeO2. The CeO2-MgO
nanocomposite materials were referred to as xCe-MgO-A in the subsequent text where x stands
62
for the mol % of CeO2 present in the composite oxide and A represents the amorphous citrate
process.
2.2 CHARACTERIZATION OF CATALYST MATERIALS
2.2.1 POWDER X-RAY DIFFRACTION (XRD)
The X-ray diffraction patterns of the various composite oxide materials were recorded using
Philips PAN analytical diffractometer employing Ni filtered CuK radiation ( = 1.5405 Å) in
the 2θ range of 20–70o
at a scan rate of 2 degrees per minute. Since clays and pillared clays show
broad low angle XRD patterns, oriented specimens were prepared for basal spacing
determination. 2wt% slurry of different clay samples in water were spread on a glass slide and
dried in air at room temperature. The slides were then equilibrated with ethylene glycol for 12 h
to obtain the oriented specimen. For this purpose small strips of filter paper were soaked with
ethylene glycol and warped around the glass slide and kept for 12 h. The excess of ethylene
glycol was removed from the slide by gentle contact with dry filter papers and XRD
measurements were performed within 1 hour of the sample preparation. For the pillared clay
samples, the XRD patterns are recorded between 2 =3–20o with a scan rate of 2 degrees per
minute.
2.2.2 UV-VIS-DRS SPECTROSCOPIC ANALYSIS
The UV-Vis spectroscopic studies for all the solid samples were carried out in diffuse reflectance
mode. UV-Vis DR spectra were recorded on a Shimadzu spectrophotometer (UV-2450)
equipped with BaSO4 coated integrating sphere in the spectral range of 200-900 nm. The powder
samples were made to self-support pellets of 12 mm diameter and 2 mm thick. This was mounted
on the integrating sphere by a sample holder. The pellet can be regarded as infinitely thick as
required by the Kubelka-Munk theory (Weckhuysen and Schoonheydt, 1999). The selected
63
recording parameters comprised of spectral bandwidth of 4nm and data point distance of 1nm.
For all the spectra PTFE was used as the reference. The spectra were recorded at ambient
condition in air. The Kubelka-Munk unit was calculated from the absolute reflectance R of an
infinite thick layer according to the equation 2.1
F(R ) = (1- R )2 / 2 R = K/S (2.1)
Where K and S are the effective absorption and scattering coefficients respectively.
2.2.3 THERMOGRAVIMETRIC ANALYSIS
Thermogravimetry analysis of the samples were preformed on Perkin-Elmer TGA-7 apparatus in
air (30 ml per min) with linear heating rate (10oC per min) from room temperature to 800
oC. For
the determination of the acidity of the clay catalysts by TGA of adsorbed n-butyl amine
(discussed in section 2.2.10) nitrogen atmosphere was used and TG analysis was performed from
room temperature to 550oC at a linear heating rate of 10
oC per minute.
2.2.4 INFRA RED SPECTROSCOPY
The FT-IR spectra of different samples (as KBr pellets) were recorded using a Perkin-Elmer
infrared spectrometer with a resolution of 4 cm-1
, in the range of 400 cm-1
to 4000 cm-1
. Nearly
3-4 mg of the sample was mixed thoroughly with 30 mg of oven dried KBr and made into
pallets. The pallets were stored in vacuum desiccators and exposed to IR lamp for 1 minute prior
to the IR measurement.
2.2.5 RAMAN SPECTROSCOPY
The Confocal micro-Raman spectra were recorded on a Horiba Jobin-Yvon LabRam HR
spectrometer using a 17 mW internal He–Ne laser source of excitation wavelength of 632.8 nm.
64
2.2.6 GAS CHROMATOGRAPHY
The GC analysis of different samples were performed by Nucon 5765 equipment fitted with a
FID detector and different packed as well as capillary column. Depending on the sample type,
different column have been used for analysis.
2.2.7 NMR SPECTROSCOPY
1H,
13C NMR spectra were recorded using Bruker spectrometer at 400 MHz using TMS as
internal standard.
2.2.8 SORPTOMETRIC STUDIES
The specific surface areas of the samples were determined by BET method using N2
adsorption/desorption at 77 K on a Quantachrome instrument. The materials were degassed at
120oC for 12 h prior to the sorptometric studies.
2.2.9 SCANNING ELECTRON MICROGRAPH (SEM)
Scanning electron micrographs were taken using JEOL JSM-6480 LV microscope (acceleration
voltage 20 kV). The sample powders were deposited on a carbon tape before mounting on a
sample holder.
2.2.10 TRANSMISSION ELECTRON MICROGRAPH (TEM)
The transmission electron micrographs were obtained with a PHILIPS CM 200 microscope,
working at a 100 kV accelerating voltage. Samples for TEM were prepared by dispersing the
powdered sample in ethanol by sonication and then drop drying on a copper grid (400 mesh)
coated with carbon film.
65
2.2.11 SURFACE ACIDITY AND BASICITY
2.2.11.1 n-BUTYLAMINE METHOD
0.5 g of the catalyst material was kept in contact with a standard n-butylamine solution (0.025 M
in benzene) for 24 h in a sealed container. A 5 ml portion of the supernatant solution was
pipetted out and titrated against trichloroacetic acid solution, using neutral red as indicator
(Ranga Rao et al., 2003). The total acidity was computed from the volume of n-butylamine
consumed and the value was expressed in millimoles per gram of the catalyst.
2.2.11.2 TRICHLOROACETIC ACID METHOD
About 0.5 g of the catalyst was kept in contact with a standard trichloroacetic acid solution
(0.025 M in dry benzene) for 24 h in a sealed container. A 5 ml portion of the supernatant
solution was pipetted out and titrated against n-butylamine solution, using neutral red as
indicator. From the n-butylamine equivalent, the basicity of each catalyst sample was computed
and expressed as millimoles/g. The trichloroacetic acid adsorption method was used to measure
total basicity (Ranga Rao et al., 2003).
2.2.11.3 BRØNSTED ACIDITY OF THE OXIDE AND CLAY MATERIALS BY TG
ANALYSIS OF n-BUTYLAMINE
The Brønsted acidity of the catalysts was measured following the method described by Breen
(Breen, 1991). The catalyst samples were exposed to the reagent grade n-butyl amine for 48 h in
a desiccator. Thermogravimetric analysis of the samples was then performed in a nitrogen
atmosphere from room temperature to 550 C at a rate of 10 C per minute. The percentage
weight loss corresponding to the temperature range 250-450 C were calculated and the proton
acidity is obtained assuming each base molecule interact with a single proton site in the catalyst.
66
Prior to the thermogravimetric analysis the samples were flushed with nitrogen gas for 30
minutes in room temperature to remove the physisorbed n-butyl amine molecules.
2.3 CATALYTIC ACTIVITY
2.3.1 SYNTHESIS OF AMIDOALKYL NAPHTHOL
Method A: Microwave Irradiation
The synthesis of amidoalkyl naphthols was carried out under solvent free condition using
microwave as an energy source. Typically, a mixture of β-naphthol (1 mmol), benzaldehyde (1
mmol), benzamide (1.1 mmol) and 20MoZr (100 mg) catalyst was irradiated at 900 W for the
specified time with an intermittent cooling interval of 2 minutes after every 1 minute of
microwave irradiation. The progress of the reaction was checked periodically using TLC. After
completion of the reaction, 10 ml of methanol was added to the reaction mixture and the
heterogeneous catalyst was filtered. The filtrate was concentrated under vacuum and the crude
residue was purified by crystallization from ethanol:water mixture (1:1) to afford pure N-[(2-
Hydroxynaphthalen-1-yl) phenylmethyl] benzamide.
Method B: Conventional Heating
The synthesis of amidoalkyl naphthols was also carried out under solvent free condition by
conventional heating in a thermostatic oil bath at 800C. Typically, a mixture of β-naphthol (1
mmol), benzaldehyde (1 mmol), benzamide (1.1 mmol) and 20MoZr (100 mg) was heated at 800
C under stirring for the required amount of time. The progress of the reaction was checked
periodically using TLC. After completion of the reaction, 10 ml of methanol was added to the
reaction mixture and the heterogeneous catalyst was filtered. The filtrate was concentrated under
vacuum and the crude residue was purified by crystallization from ethanol:water (1:1) to afford
pure N-[(2-Hydroxynaphthalen-1-yl)phenylmethyl] benzamide.
67
2.3.2 SYNTHESIS OF -ACETAMIDOKETONES
To a solution of the arylaldehyde (2 mmol), aryl ketone (2 mmol), acetyl chloride (4 mmol) in
acetonitrile (4 ml) was added 100 mg of 20MoZr catalyst and heated at 50oC, with stirring for the
required amount of time. The progress of the reaction was followed by TLC. After completion of
the reaction, the MoZr catalyst was filtered and the filtrate was poured into 50 ml of ice-water.
The ensuing solid product was filtered, washed with ice-water and recrystallized from
ethylacetate/n-heptane to give the pure product.
2.3.3 SYNTHESIS OF OCTAHYDROQUINAZOLINONES
Method A: Microwave Irradiation
The synthesis of octahydroquinazolinones was carried out under solvent free condition using
microwave as an energy source. For reaction under microwave condition, a neat mixture of
benzaldehyde (1 mmol), dimedone (1 mmol), urea (1.5 mmol) and 100 mg of catalyst (10MoZr-
G) was exposed to the microwave radiation for the required amount of time. The reaction
mixture was irradiated at 900 W for the specified time with an intermittent cooling interval of
60s after every 60s of microwave irradiation. The progress of the reaction was monitored by
TLC. After completion of the reaction, the crude product from the reaction mixture was poured
in to 25mL of ice cold water. The ensuing solid product was filtered, dried and then dissolved in
hot ethanol. The ethanolic solution was filtered to separate the heterogeneous catalyst and the
filtrate was concentrated under reduced pressure to remove the ethanol and recrystallized.
Method B: In aqueous media
For reaction in aqueous media, the same reaction stoichiometry and similar reaction work up
condition was used. The reactions were carried out under reflux condition under stirring for the
68
specified time using 5 mL of double distilled water as solvent. The catalyst was regenerated by
washing three times with 10 mL portion of ethanol followed by heat treatment at 400oC for 1 h.
2.3.4 SYNTHESIS OF -AMINOCARBONYL COMPOUND
A mixture of acetophenone (1 mmol), benzaldehyde (1 mmol), aniline (1 mmol) and 50 mg of
PMA/SZr-P clay was taken in 4 mL of water and stirred at room temperature for the required
amount of time. The progress of the reaction was monitored by TLC. After completion of the
reaction 10 ml of chloroform was added to the reaction mixture, stirred for 15 minutes and
filtered to separate the catalyst. The product was separated from the organic layer and
recrystallized from ethanol or purified by column chromatography to obtain the pure product.
The catalyst was regenerated by washing with 10 ml of chloroform followed by heat treatment at
200˚C for 1h.
2.3.5 SYNTHESIS OF 2-AMINO- 2-CHROMENES
A mixture of benzaldehyde (1 mmol), malononitrile (1 mmol), -naphthol (1 mmol), and xCe-
CaO (25 mg) in water (5 ml) was heated at 80°C for the required amount of time. After
completion of the reaction, as indicated by TLC, the reaction mixture was extracted with ethyl
acetate (3x5 mL). The organic phase was dried using Na2SO4, filtered, and excess ethyl acetate
was distilled off in vacuo. Filtration of both organic and aqueous phases led to the recovery of
the catalyst. The crude product was recrystallized from methanol to afford the pure product.
2.3.6 SYNTHESIS OF ARYLIDENEMALONONITRILES BY KNOEVNAGEL
CONDENSATION
Method A: Mechanochemistry (Grinding) Method
Stoichiometric amount of benzaldehyde (2mmol), malononitrile (2mmol) and 100 mg 10 Ce-Mg-
O catalyst was grinded at ambient temperature till the reaction mixture changed its physical state
69
to solid form. The progress of the reaction was monitored by TLC and after the completion of the
reaction, 15 ml of ethyl acetate was added to the reaction mixture and centrifuged at 4000 rpm
for 15 minutes to separate the catalyst. The final product was recovered by evaporating the
excess solvent and subsequent crystallization from ethanol to obtain pure benzylidene
malononitrile.
Method B: Microwave Irradiation
Typically, a neat mixture of benzaldehyde (2 mmol), malononitrile (2mmol) and 10 Ce-Mg-O
(100 mg) was exposed to the microwave radiation (900 W) for the specified time with an
intermittent cooling interval of 60s after every 60s of microwave irradiation. The progress of the
reaction was monitored by TLC, on completion of reaction as indicated by TLC, similar workup
condition as described in method A was adopted to get the final product
Method C: Using water as green solvent
In a 25 ml round bottom flask, stoichiometric amount of benzaldehyde(2 mmol), malononitrile(2
mmol) and the catalyst 10Ce-Mg-O (100 mg) in 10ml of water was stirred at ambient
temperature for the specified time. After completion of the reaction as indicated by TLC, the
reaction mixture was treated with 15 ml ethyl acetate and the catalyst was filtered. The organic
phase was separated, dried over Na2SO4, filtered, and excess ethyl acetate was distilled off. The
crude product was recrystallized from ethanol to obtain the pure product.
2.3.7 SYNTHESIS OF β-NITRO ALCOHOLS BY HENRY REACTION
In a 25 ml round bottom flask, stoichiometric quantity of p-nitrobenzaldehyde (1 mmol),
nitromethane (2 mmol) and 50 mg catalyst (10Ce-Mg-O-A) in 5ml of water was stirred at
ambient temperature for the specified time. After completion of the reaction as indicated by
TLC, the reaction mixture was extracted with ethyl acetate (3x5 mL). The organic phase was
70
dried using Na2SO4, filtered, and excess ethyl acetate was distilled off in vacuo. Filtration of both
organic and aqueous phases led to the recovery of the catalyst.
2.3.8 SYNTHESIS OF OCTAHYDROXANTHENES
Typically, a neat mixture of 4-nitrobenzaldehyde (1 mmol), dimedone (2mmol) and SO4-2
/xFe-
Zr (100 mg) was exposed to the microwave irradiation. The reaction was carried out at 900W for
the specified time with an intermittent cooling interval of 60s after every 60s of microwave
irradiation. The progress of the reaction was monitored by TLC. On completion of reaction, the
crude product was dissolved in ethyl acetate (15 ml). The catalyst was separated by
centrifugation at 4000 rpm for 15 minutes. The final product was obtained by evaporating the
excess solvent in vacuum and upon recrystallization from ethanol.
All the compounds synthesized in the section 2.3 are known products and are characterized by
comparing their IR, 1H NMR and melting points with those reported in literature.
71
CHAPTER 3
CATALYTIC APPLICATION OF ZIRCONIA BASED NANOCOMPOSITE OXIDES
FOR SYNTHESIS OF SOME BIOLOGICALLY IMPORTANT MOLECULES
PART A: ZrO2-MoO3 COMPOSITE OXIDES
3.1 INTRODUCTION
Oxides are used in large scale in industrial catalytic processes involving redox and acid-base
reactions. The fundamental aspects of the reactivity of oxides have been extensively studied and
attributed largely to the polarizability and electron transfer ability of the oxide surface (Corma,
1995; Hattori, 1995; Climent et al., 2011). The electron donating or accepting property of an
oxide is determined by the standard redox potential of the cations involved whereas the acid-base
property arises due to the surface unsaturation and presence of surface hydroxyl groups (Tanabe
et al., 1989; Busca, 1999; Busca, 2007). In general, the strength and number of acid-base sites on
a metal oxide surface depends upon intrinsic and extrinsic factors. The intrinsic factor depicts the
strength of the acid-base sites which is related to the degree of covalency of the metal-oxygen
bond, elcetronegativity difference between the cation and oxygen, ionization potential and radius
of the cation. The extrinsic factors which include the method of preparation, pre- and post-
treatment steps, doping with other cations and surface modification decide the number of active
sites on the oxide surface. The coordinately unsaturated cations present on the surface act as
electron pair acceptors (Lewis acid site) whereas the surface hydroxyl groups may act as
Brønsted acid sites. The exposed coordinately unsaturated oxygen ion on the surface exhibit
Lewis basic nature.
Acid-base catalysis by composite oxides is an area widely investigated in heterogeneous
catalysis. The composite oxides exhibit a multitude of advantages over the individual oxide
components. The surface area, porosity and thermal stability of an oxide catalyst can be
significantly improved in composite oxide systems. The most important example of this type is
the doping of aliovalent cations such as Ba2+
to alumina in three-way catalyst to preserve the
72
surface area of -alumina (Kaspar et al., 2000). In addition to the textural property, improved
chemical property and surface reactivity are noted in composite oxide systems (Busca, 1999).
Specific surface properties such as acid-base and redox property, resistance to catalyst poison
and product selectivity can be improved in composite oxides. Although clear definition exists for
predicting the relative acidic or basic properties of oxides, there is no general rule for predicting
the acid-base character of multicomponent oxides. Different qualitative models are proposed
regarding the generation of new acidic/basic character upon mixing different oxides. The most
frequently referred models are the Tanabe and Kung models (Tanabe et al., 1974; Kung, 1984).
Both the models presume dilute solid solutions of one cation in the matrix oxide rather than over
whole range of composition. The Tanabe model is based upon a local approach. A mismatch in
cation valency and coordination number along a hetro-linkage M-O-M’ contributes an excess
charge at the site of the minor component cation, resulting in acidity generation. However, this
model failed to explain the acidity trend in ceria-zirconia solid solutions obtained from
microcalorimetric measurements (Cutrufello et al., 1999). The Kung’s model, on the other hand,
relates acidity generation to the electrostatic potential at the substituted cation sites and changes
in the lattice necessary to balance the stoichiometry (Kung, 1984). A general conclusion of the
model is that for oxides having the same-metal stoichiometry, new acidity (Lewis type) can be
generated only if the host oxide is more ionic than the guest.
Zirconia is one of the most promising ceramic to be used as catalyst and catalyst support (Sahu et
al., 2000; Ranga Rao et al., 2003; Kumar et al., 2006; Naik et al., 2008; Naik et al., 2011). The
catalytic properties of zirconia as a catalyst and support has been established for several
industrially important chemical transformations such as isomerization (Gregorio et al., 2008),
oxidation (Brown et al., 1999), ammoxidation (Chary et al., 2004), gas combustion (Brown et
al., 2009), water gas shift reaction (Graf et al., 2009), esterification (Rajkumar et al., 2008),
alcohol dehydration (El-Sharkawy et al., 2007; Dabbagh et al., 2011), and dehydrogenation
(Ranga Rao et al, 2008; Reddy et al., 2008). Pure ZrO2 exhibits three polymorphic forms
73
depending on temperature ranges: monoclinic (m-ZrO2, space group: P21/c stable up to 1400 K),
tetragonal (t-ZrO2, space group: P42/nmc, stable between 1400 K –2640 K), and cubic (c-ZrO2,
space group: Fm3m, stable above 2640 K) (Bocanegra-Bernal et al., 2002; Li et al., 2001; Di
Monte and Kašpar, 2005a; Wang et al., 2007; Garcia et al., 2009; Heshmatpour et al., 2011). Out
of these three polymorphs, the high temperature polymorphs have adequate properties for
technological applications. Hence, the stabilization of the metastable tetragonal phase of zirconia
has attracted immense interest because of its outstanding electronic, optical, mechanical and
catalytic properties (Sahu et al., 2000; Ranga Rao et al., 2003; Shukla et al., 2005; Chang and
Doong, 2005). Several attempts have been focused towards the stabilization of the metastable
tetragonal zirconia phase at ambient temperature by optimizing the different factors during its
preparation such as the nature and content of stabilizer, pH, mode of preparation, critical
crystallite size, lattice strain, and intrinsic defects such as vacancies in the anionic sub lattice
(Ferna´ndez-Garcı´a et al., 2004). Different chemical methods such as sol–gel (Tyagi et al.,
2006; Heshmatpour et al., 2011), precipitation (Tyagi et al., 2006), hydrothermal (Wang et al.,
2007; Kumar et al., 2006), mechanochemical (Isfahani et al., 2011), metathesis (Chen et al.,
2006) and borohydride (Nayak et al., 2010) synthesis have been extensively adopted to
synthesize nanosize t-ZrO2 material. Although ZrO2 has been extensively used as heterogeneous
catalyst for a variety of organic reactions, however, its application has been limited by the
presence of mild acidic and basic sites on its surface. In this regard in the recent years, efforts
have been focused to increase the surface acid strength of zirconia by various structural and
surface modifications. Various forms of modified zirconia such as sulfated zirconia, WO3 and
MoO3 grafted zirconia has been described in the literature which display enhanced acidic and
catalytic properties (El-Sharkawy et al., 2007; Naik et al., 2008; Soultanidis et al., 2010).
Molybdenum oxide is one of the most promising materials used in many advanced applications
such as gas sensing, anode material in organic solar cells, photo-chromic, electro-chromic
devices and battery system. MoO3 has also been explored as an efficient material in various
74
heterogeneous catalytic processes because of its strong surface acidic and redox properties and
varied surface molecular structure. MoO3 based heterogeneous catalyst has been extensively
investigated for its application in industrial catalytic process such as hydro-desulfurization
(Grange et al., 1980), metathesis (Grunert et al., 1992), oxidation and oxidative dehydrogenation
reactions (Banares et al., 1993; Brown et al., 2009). Molybdenum oxide supported on a variety
of support such as ZrO2 (Liu et al., 1998; Brown et al., 1999; Chen et al., 2000; Calafat et al.,
2000; Chary et al., 2004; El-Sharkawy et al., 2007; Brown et al., 2009), SiO2 (Parghi and
Jayaram, 2010), TiO2 (Kanai et al., 2003; Al-Kandari et al., 2005), CeO2 (Mohamed et al., 2005;
Al-Yassir et al., 2007), CeO2-ZrO2 (Wan et al., 2008; Escribano et al., 2009), mesoporous silica
(Li et al., 2008) has been reported in literature. Out of different supported MoO3 system,the
MoO3/ZrO2 catalyst has been investigated for several industrially important reactions such as
cumene dealkylation (El-Sharkawy et al., 2007), ethanol conversion, ammoxidation of picoline
and toluene (Chary et al., 2004), methylcyclopentane conversion (Kenney et al., 2005), methanol
oxidation reactions and methane combustion reaction (Brown et al., 1999; Brown et al., 2009).
With an aim to explore novel applications of the MoO3-ZrO2 materials, in this section, we have
described the synthesis of this important class of material by coprecipitation and characterized by
several analytical techniques such as XRD, UV, Raman, SEM and TEM. The catalytic activity of
these materials has been studied for the environmental benign synthesis of amidoalkylnaphthols
and β-acetamido ketones.
3.2 RESULTS AND DISCUSSION
3.2.1 XRD STUDY
Fig. 3.1 and 3.2 show the X-ray diffraction patterns of the xMoZr catalyst prepared by
coprecipitation and impregnation methods. The pure ZrO2 prepared by the precipitation method
shows intense and well defined diffraction peaks at 2 values of 24.30, 28.3
0, 31.5
0, 34.9
0 and
50.40 corresponding to the presence of monoclinic phase and at 30.2
0 corresponding to the
75
tetragonal phase. The pure zirconia prepared in this investigation contains a mixture of
monoclinic and tetragonal phases. The percentage tetragonal phase in the ZrO2 sample is
calculated to be 58% using the Toroya method given below (El-Sharkawy et al., 2007).
Where IT and IM stands for the integral intensity of the XRD peaks at the specified 2θ values for
the tetragonal and monoclinic phases, respectively. Addition of MoO3 onto the ZrO2 is found to
have a beneficial effect on the selective stabilization of the tetragonal phase. The 2MoZr catalyst
prepared by the coprecipitation method contains 69% tetragonal phase (Table 3.1). Further
increase in MoO3 content to 5 mol% and above resulted in the complete formation of the
tetragonal phase. No separate crystalline phase corresponding to the crystalline MoO3 and
Zr(MoO4)2 is detected in the XRD patterns of the coprecipitated sample. In case of the MoO3-
ZrO2 catalyst prepared by the impregnation method, the transformation of the monoclinic phase
to tetragonal phase is also observed. However, the phase transformation is less pronounced. The
10MoZrI catalyst contains 69% tetragonal phase whereas the same material prepared by
coprecipitation method contains 100% tetragonal phase. Moreover at higher Mo content the
presence of crystalline MoO3 is clearly observed in the X-ray diffraction patterns of the 20MoZrI
and 50MoZrI sample. There are earlier studies available in literature regarding the structure,
phase composition and transformation of the MoO3-ZrO2 composite oxide (Chen et al., 2000;
Chary et al., 2004; El-Sharkawy et al., 2007; Brown et al., 2009). All these studies clearly point
to the fact that the composition and phase transformation of the MoO3-ZrO2 composite depends
on the method of synthesis, amount of MoO3 loading and calcination temperature. A lower
calcination temperature around 500oC and MoO3 loading greater than 5 % favors the formation
of tetragonal phase.
76
20 30 40 50 60 70
e
d
c
b
ZrO2
2 MoZr
5 MoZr
10 MoZr
Inte
nsi
ty (
a.u
.)
2 (degrees)
MoO3
a
Figure 3.1 X-ray diffraction patterns of (a) ZrO2, (b) 2MoZr, (c) 5MoZr (d) 10 MoZr
and (e) MoO3
El-Sharkawy et al. have studied the effect of calcination temperature and Mo content on the
phase transformation of MoO3-ZrO2 catalyst prepared by impregnation method (El-Sharkawy et
al., 2007). These authors observed the formation of separate crystalline MoO3 and Zr(MoO4)2
phases for 20% MoO3 loading and a calcinations temperature of 700oC. At this calcinations
temperature the tetragonal to monoclinic transformation was also found to be facilitated. Similar
observation has been reported by Yori et al. (Yori et al., 2000). Calafat et al. (Calafat et al.,
2000) have prepared the MoO3-ZrO2 catalyst by coprecipitation method. These authors have
77
reported the selective stabilization of the tetragonal phase for 500oC calcined sample. Calcination
at 700oC leads to the formation of the monoclinic phase of zirconia for sample with higher Mo
loading, however, these authors have not observed the formation of any Mo-Zr spinel phase or
crystalline MoO3 at higher calcinations temperature. Chary et al. have prepared the MoO3-ZrO2
catalyst by impregnation method (Chary et al., 2004). These authors have not observed any
significant change in phase content and transformation at various MoO3 loading under their
adopted condition of preparation. In the presence study the monoclinic to tetragonal phase
transition of ZrO2 was found to be facilitated in presence of MoO3, without the formation of any
new phases. This observation is in accordance with the results reported by El-sharkawy et al.
(El-Sharkawy et al., 2007) and Calafat et al. (Calafat et al., 2000). The selective formation of the
tetragonal phase has been ascribed to the reduction of the grain boundary area which prevents the
mobility of the ions during phase transformation and the subsequent elimination of defects from
ZrO2 particles in presence of MoO3. Srinivasan et al. has reported that in case of sulfated
zirconia catalyst the tetragonal phase is stabilized due to the preferential segregation of the
sulfate ions along the grain boundary (Srinivasan et al., 1995). The presence of MoO3 in the
composite material also inhibits the growth of the zirconia particles. It has been reported that
there is a critical crystallite size of zirconia below which the tetragonal phase is stabilized (Yori
et al., 2000; El-Sharkawy et al., 2007).
78
20 30 40 50 60 70
f
e
d
c
b
10MoZr
10MoZrI
20MoZr
20MoZrI
50MoZr
Inte
ns
ity
(a
.u.)
2 (degrees)
50MoZrI
a
Figure 3.2 X-ray diffraction patterns of (a) 10MoZr, (b) 10MoZrI, (c) 20MoZr, (d)
20MoZrI, (e) 50MoZr and (f) 50MoZrI
In the present study, it is believed that the formation of a well dispersed layer of MoO3 on the
surface and grain boundary region helps in the stabilization of the tetragonal phase. Moreover, in
the coprecipitated sample the nucleation and growth step for zirconia can be delayed
significantly in presence of Mo ions. This can result in small nanosize particles for the composite
oxide.
79
Table 3.1 Percentage tetragonal phase in the xMoZr and xMoZrI catalysts calculated
using the Toroya’s method
Catalyst % tetragonal
phase
ZrO2 58
2MOZr 69
5MoZr 100
10MoZr 100
20MoZr 100
50MoZr 100
10MoZrI 69
20MoZrI 81
50MoZrI 82
The crystallite size and lattice strain of the composite oxides are calculated from the Fourier line
profile analysis of the XRD patterns. The result obtained from these calculations is presented in
Table 3.2. It is clearly observed that the composite oxides display smaller crystallite size as
compared to the pure zirconia. Moreover, the lattice strain is found to be inversely proportional
to the crystallite size of the composite oxide. The Fourier line shape analysis data corroborates
the proposition that the crystal growth of zirconia is inhibited by the presence of MoO3 species
due to dissimilar coordination preference and atomic mismatch along the grain boundary region
which can contribute to the stabilization of the tetragonal zirconia phase.
80
Table 3.2 Calculated crystallite size and lattice strain by Fourier line profile analysis
from broadened X-ray profile
Catalyst Crystallite size
(nm)
r.m.s strain
(<e2>
1/2)
ZrO2 37 5.71 × 10-3
2MoZr 32 5.80 × 10-3
5MoZr 17 6.64× 10-3
10MoZr 20 6.08 × 10-3
20MoZr 19 6.26 × 10-3
50MoZr 17 6.49 × 10-3
3.2.2 UV-VIS STUDY
The UV-Vis spectra of the MoO3-ZrO2 catalyst prepared by coprecipitation method are presented
in Fig. 3.3. Pure ZrO2 prepared by this method shows a sharp and intense band at 215 nm.
Zirconium dioxide is a direct band gap insulator, which shows interband transitions at ~ 220 nm
with an absorption edge at 250 nm (Scheithauer et al., 1998). The absorption band observed at
215 nm is ascribed to the O2-
Zr4+
charge transfer transition. Pure MoO3 shows two well-
defined absorption bands at 240 and 320 nm. Since Mo6+
has a d0 electronic configuration, the
only absorption feature expected is due to the ligand–metal charge transfer transition (LMCT),
O2-
Mo6+
which occur in the range of 200–400 nm. However, depending on the coordination
preference and local symmetry of the Mo (VI) ions different absorption bands are observed in
the electronic spectrum. The peak at 240 nm for pure MoO3 can be assigned to the isolated MoO4
species with tetrahedral symmetry whereas the peak at 320 nm can be assigned to bulk
polymolybdate species present in the sample (Henker et al., 1991; Liu et al., 1998; Li et al.,
2002; Mohamed et al., 2005). The addition of MoO3 to zirconia matrix significantly modifies the
absorption spectra of the composite oxide. For samples with low MoO3 loading (2 and 5 wt%) a
81
prominent band is observed at 226 nm along with a broad shoulder at 280 nm. The sharp and
narrow absorption band at 226 nm can be assigned to the tetrahedrally coordinated isolated
molybdate species with structural distortion. Henker et al. in their study of supported molybdate
species has compared the absorption spectra of Na2MoO4 and Na2MoO4.2H2O (Henker et al.,
1991). It has been observed that the Na2MoO4 with tetrahedral Mo ions shows absorption peak at
245 nm whereas in Na2MoO4.2H2O where the tetrahedral Mo is distorted due to the water of
crystallization shows band maxima at 225 nm. In the present study, the assignment of the peak at
226 nm to structurally distorted tetrahedral isolated molybdate species is in accordance with the
observation by Henker et al. (Henker et al., 1991). The broad peak at 280 nm can be assigned to
small nanosize molybdate clusters dispersed on the surface of the zirconia. Liu et al. have
studied MoO3/ZrO2 system using UV-Vis spectroscopy and assigned the peak at ~ 300 nm to the
presence of heptamolybdate clusters (Liu et al., 1998). In the present case it is likely that such
clusters can exist on the surface of zirconia in a well dispersed state. When the percentage MoO3
loading is increased to 10 percent and higher a broad band with maxima at 280 nm was observed
for all the samples. Earlier studies on the UV-Vis spectra of MoO3 system have revealed three
absorption regions corresponding to the presence of isolated molybdate species (220-250 nm),
polymolybdate clusters (260-290 nm) and bulk type molybdate (>320 nm) (Henker et al., 1991;
Liu et al., 1998). The peak observed at 280 nm for sample with higher loading thus can be
assigned due to the polymolybdate clusters. Although the bulk type MoO3 peak at 320 nm peak
is absent for the samples with higher loading, it is possible that this absorption feature can be
masked by the broad band at 280 nm. This is more likely to occur for the 50MoZr sample.
However, since the XRD study does not show crystalline MoO3 phase, the bulk type MoO3 may
be present in small quantities in these samples.
82
200 400 600 800
I
f
e
d
c
b
ZrO2
MoO3
50MoZr
20MoZr
10MoZr
5MoZr
2MoZr
Inte
nsit
y (
a.u
.)
Wavelength (nm)
a
200 400 600 800
II
d
c
b
a
50MoZrI
20MoZrI
10MoZrI
Figure 3.3 UV-Vis spectra of the MoO3-ZrO2 composite oxide materials: (a) ZrO2, (b)
2MoZr, (c) 5MoZr, (d) 10MoZr, (e) 20MoZr and (f) 50MoZr in panel I and (a) 10MoZr-I,
(b) 20MoZr-I, (c) 50MoZr-I and (d) MoO3 in panel II
The UV–Vis spectra of the MoO3–ZrO2 materials prepared by impregnation method are
presented in Fig. 3.3, panel II. All the impregnated catalysts show the 226 nm band. In addition,
the 20MoZrI and 50MoZrI materials show a prominent peakat 315 nm corresponding to the
presence of bulk type molybdate species (Fig. 3.3 II). In the present study, no absorption feature
corresponding to the bulk type MoO3 species was observed for the xMoZr sample which is
complimentary to the XRD study which does not show any crystalline MoO3 phase. This study
also implies the presence of well dispersed molybdate species on the surface of zirconia in the
form of isolated and cluster molybdates.
83
3.2.3 RAMAN STUDY
The Raman spectra in the range of 700-1200 cm-1
for the xMoZr catalyst prepared by both
coprecipitation and impregnation method is shown in Fig. 3.4. This frequency range covers the
antisymmetric Mo-O-Mo and symmetric and antisymmetric of Mo=O terminal stretching of the
molybdate species. Spectral data below 700 cm-1
are not collected due to the strong scattering
from the zirconia support. Pure MoO3 (Fig.3.4 inset) shows two sharp and intense band at 816
and 990 cm-1
due to the crystalline MoO3 phase (Kim et al., 1989). These bands are observed for
the 20MoZrI catalyst prepared by impregnation method. This observation clearly indicates the
presence of crystalline MoO3 in bulk state in the impregnated 20MoZrI sample. Contrary to this
observation, the coprecipitated sample shows broad Raman features with altered peak position.
The Raman spectroscopic investigations for supported MoO3 system provides good insight into
the dispersion, interaction, symmetry and the type of molecular Mo species present on the
support (Kim et al., 1989; Kim et al., 1994; Brown et al., 1999; Chary et al., 2004). It has been
observed that the surface molecular species present on supported MoO3 systems depends on the
preparation method, the net pH at the PZC of the support and the amount of loading. Kim et al in
their extensive study on supported MoO3 system by Raman spectroscopy have reported that the
pH at the PZC of the support plays a major role in controlling the surface molecular species
(Kim et al., 1994). For MoO3/support materials prepared by equilibrium adsorption method it has
been observed that for support with low pH at PZC like SiO2 and amphoteric oxides such as
TiO2 and ZrO2, the predominantly favored surface species are octahedrally coordinated
polymolybdates (Mo8O264-
and Mo7O246-
) whereas oxides with basic pH at PZC favors isolated
tetrahedral molybdate species. These authors have also reported that under ambient condition the
supported surface molybdate species are present in a hydrated state and closely resemble the
aqueous chemistry of molybdate ions in solution. Chary et al. (Chary et al., 2004) in their study
of MoO3/ZrO2 catalyst prepared by impregnation method have observed a gradual shift in the
Mo=O stretching frequency from 920 cm-1
to 950 cm-1
with increase in the MoO3 content. These
84
authors have inferred that the shift in the peak position is due to polymerization of the molybdate
species at higher loading. At low loading the peak at 920 cm-1
has been assigned to isolated
molybdate species whereas at higher MoO3 content due to polymerization process
polymolybdate clusters are formed. In the present study, 2MoZr catalyst shows two bands of low
intensity at 837 and 915 cm-1
. These peaks have been assigned to presence of isolated molybdate
species present on the surface of zirconia. Kim et al. have compared several tetrahedral and
octahedral coordinated reference molybdate compounds with the supported MoO3 systems (Kim
et al., 1989). Na2MoO4 which contain tetrahedrally coordinated MoO42-
groups shows prominent
bands at 832 and 893 cm-1
. The MoO42-
ions in aqueous solution is also known to possesses
Raman bands at 897 ( s (Mo-O)) and 837 ( as(Mo-O)). Isolated dimeric MoO3-O-MoO3 species
with tetrahedral molybdate groups present in (NH4)2Mo2O7 shows Raman band at 910 cm-1
due
to the Mo=O terminal stretching. In line with the observations by Kim et al (Kim et al., 1989;
Kim et al., 1994) and Chary et al. (Chary et al., 2004), the bands at 837 and 915 cm-1
for 2MoZr
can be assigned to isolated tetrahedral molybdate species present on the surface of zirconia. With
increase in the MoO3 content, the Raman band at 915 cm-1
was found to shift to higher
wavenumber along with increase in the intensity of the band. For 20MoZr catalyst this band is
observed at 960 cm-1
. As pointed out by Chary et al. upon increase in MoO3 content, more
number of surface sites is occupied by the molybdate ions resulting in the formation of
polymolybdates by linkage between adjacent monomolybdate species. Thus the Raman peak
observed in the range of 940-960 cm-1
can be assigned to the oligomeric molybdate clusters
present on the surface of zirconia.
85
800 1000 1200
d
c
b
20MoZr
10MoZr
5MoZr
2MoZr
Inte
nsit
y (
a.u
.)
Raman shift (cm-1
)
a
800 1000
MoO3
20MoZrI
e
f
Figure 3. 4 Raman Spectra of (a) 2MoZr, (b) 5MoZr, (c) 10 MoZr and (d) 20MoZr (In
the inset (e) 20MoZrI, and (f) MoO3)
It has been reported that, supported heptamolybdate (Mo7O246-
) and octamolybdate
(Mo8O264-
) clusters in hydrated state as well as in aqueous solution shows Raman band in the
range of 935-960 cm-1
due to the terminal Mo=O stretch of the distorted MoO6 octahedra,
whereas the terminal Mo=O stretch of the bulk MoO3 is observed at 996 cm-1
. In the present
study, for higher MoO3 loading it is likely that such clusters are present on the ZrO2 surface.
Also it was observed that the peak at 832 cm-1
for 2MoZr reappear at 822 cm-1
for the 20MoZr
86
catalyst. The broad and asymmetric nature of the band indicate that it represent a range of Mo-O
bond. Also it is possible that at this loading the bulk type MoO3 have formed on zirconia surface
which show strong band at 816 cm-1
. However it is likely that the MoO3 crystallite formed on
ZrO2 are of very small size which has escaped detection by XRD. From the Raman analysis it
can be concluded that at low MoO3 content the predominant molecular species is isolated
molybdates whereas at higher loading oligomeric clusters and polymolybdate type MoO3 is
present in zirconia surface.
3.2.4 TEM STUDY
The transmission electron micrograph of the ZrO2 and 20MoZr material is shown in Fig. 3.5.
Pure ZrO2 show particle size in the range of 40-50 nm. Addition of MoO3 to the zirconia matrix
resulted in the formation of 20-30 nm size particles for the 20MoZr materials. The composite
oxide materials show smaller particle size compared to the pure zirconia. The intergrowth and
decoration of MoO3 particles along the grain boundary area of zirconia particles is probably
responsible for smaller particle size observed for the composite oxide. The TEM result is found
to be supplemented by the Fourier line profile analysis of the XRD patterns.
87
(a)
(b)
Figure 3.5 Transmission electron micrograph of (a) ZrO2 and (b) 20MoZr materials
88
3.2.5 CATALYTIC STUDIES
3.2.5.1 MULTICOMPONENT SYNTHESIS OF 1-AMIDOALKYL 2-NAPHTHOLS
Compounds containing 1,3-amino-oxygenated functional groups are frequently found in
biologically active natural products and potent drugs such as nucleoside antibiotics and HIV
protease inhibitors such as ritonavir and lipinavir (Knapp, 1995). It has been reported that
amidoalkyl naphthols can convert to biologically active aminoalkyl naphthol derivatives by
amide hydrolysis reaction, which shows activities such as hypotensive and bradycardiac effect
(Shaterian, 2008). Thus the synthesis of amidoalkyl naphthols is of paramount importance in
organic synthesis. 1-amidoalkyl 2-Naphthols are synthesized by the multi-component
condensation of aryl aldehydes, 2-naphthol, and acetonitrile or amide in the presence of acid
catalysts (Nagarapu et al., 2007). Several Lewis and Brønsted acidic catalysts have been used for
this reaction which include among others Ce(SO4)2, Iodine, K5CoW12O40.3H2O, p-TSA, FeHSO4
and sulfamic acid (Selvam et al., 2006; Nagarapu et al., 2007; Patil et al., 2007; Lei et al., 2009).
Recently, montmorillonite K10 has been used as an efficient heterogeneous catalyst for synthesis
of amidoalkylnaphthols (Kantevari et al., 2007). The other heterogeneous catalysts used for this
reaction include FeCl3/SiO2 and HClO4/SiO2 (Shaterian et al., 2008; Nandi et al., 2009). In this
work, we have described the application of the MoO3-ZrO2 materials as heterogeneous catalyst
for synthesis of structurally diverse amidoalkylnaphthols under environmentally benign
conditions (Scheme 3.1).
CHO
R
+
OH
+ R1CONH2
xMoZr
OH
NHCOR1
R
Conventional ormicrowave heating
Scheme 3.1 One pot synthesis of amidoalkylnaphthol using xMoZr catalyst
89
Table 3.3 shows the catalytic activity of the composite oxides for the synthesis of N-[(2-
Hydroxynaphthalen-1-yl)2-chlorophenylmethyl]urea at 80oC under solvent free condition for 1 h
of reaction time. The physicochemical properties of the composite oxide materials are also given
in Table 3.3.
Table 3.3 Physicochemical characteristics and catalytic activity of xMoZr catalysts
aReaction conditions: 2-chlorobenzaldehyde: β-naphthol: urea:: 1:1:1.3, Temperature 80
oC, solvent free
condition, 1ha isolated yield forN-[(2-Hydroxynaphthalen-1-yl)2-chlorophenylmethyl]urea (Table 3.3,
Entry 2) for a reaction time of 1h.
bCalculated with respect to 2-Chlorobenzaldehyde conversion using gas chromatography
c no reaction
The composite oxides show higher surface area and acidity compared to pure zirconia. Addition
of MoO3 to zirconia surface has a profound influence on the acidity of the material. The number
of acidic sites is found to increase with the MoO3 content upto 20 % MoO3. In the composite
oxide, the surface low coordinated Mo atoms can act as Lewis acidic sites whereas the surface
hydroxyl group on the catalyst surface can provide Bronsted acidic sites. For well dispersed
molybdate species, the presence of more number of surface active Mo sites is responsible for
higher acidity of the composite material. When the molybdenum content was increased to 50%
Catalyst Surface
area
(m2/g)
Acidic
sites
(mmol/g)
Yielda
(%)
Reaction rateb
(mmolh-1
m-2
x10-2
)
TOFb
(h-1
)
ZrO2 45 0.17 NRc ---- ---
2MoZr 62 0.26 30 5.3 12
5MoZr 82 0.35 46 5.9 14
10MoZr 95 0.41 65 7.1 16
20MoZr 90 0.47 89 10.0 19
50MoZr 78 0.42 68 8.9 16
90
the acidity was found to decrease marginally probably due to formation of larger polymolybdate
clusters with less active surface area. Similar observation has been noted for supported MoO3
catalyst in the earlier studies of MoO3/ZrO2 and MoO3/MCM-41 system (Chary et al., 2004; Li
et al., 2008). Among all the binary MoZr oxide, the 20MoZr catalyst display highest catalytic
activity which is ascertained from the reaction rate and turnover frequency data. The 20MoZr
catalyst is chosen for further catalytic experiments in the present study. Various reaction
parameters such as the catalyst amount, reaction stoichiometry, temperature and mode of heating
are varied to obtain the optimized protocol. It is observed that for a reaction involving 1 mmol
of the reactants, 100 mg of the catalyst is ideal for efficient condensation of the reactants at 80oC
under solvent free condition. The optimum reactant ratio is found to be 1:1:1.3 for benzaldehyde,
naphthol and urea, respectively. In case of reaction involving benzamide the reactant ratio is
found to be 1:1:1.1. The effect of mode of energy supply is studied by conducting the reaction
under solventfree conditions both using a conventional oil bath and microwave oven. The results
obtained in these studies are presented in Table 3.4 and Table 3.5. It is observed that use of
microwave as energy source considerably shortens the reaction time. For both protocols
involving microwave and conventional heating, a wide variety of substituted benzaldehyde
reacted efficiently to give the amidoalkylnaphthol in high yield and purity. Compared to the
earlier reported heterogeneous catalytic protocols (yields~ 70-90%, time: 2-20 h) (Kantevari et
al., 2007; Shaterian et al., 2008; Nandi et al., 2009), the present method was found to be
advantageous in terms of less reaction time and good yield of product. Moreover, the 20MoZr
catalyst was highly efficient for the reaction of a wide variety of aryl aldehydes with benzamide
to give the corresponding amidoalkyl naphthols in high yield and purity.
91
Table 3.4 Solvent free synthesis of amido alkyl naphthols using 20MoZr catalyst
Entry No. R R1 Time (min) Yield (%)
1 H NH2 90 80
2 2-Cl NH2 60 82
3 4-Cl NH2 60 86
4 4-OCH3 NH2 90 78
5 3-NO2 NH2 60 84
6 4-OH NH2 90 75
7 2-NO2 NH2 60 84
8 4-NO2 NH2 60 88
9 2-OH NH2 90 80
10 H C6H5 120 70
11 2-Cl C6H5 120 76
12 4-OCH3 C6H5 120 72
13 3-NO2 C6H5 90 76
14 4-NO2 C6H5 90 81
In order to study the effect of reaction media on the activity of the 20MoZr catalyst, the reaction
of benzaldehyde, naphthol and benzamide has been taken as a model reaction and carried out in
different solvents with varied polarity and the obtained results are presented in Table 3.6. It is
observed that the reaction is facilitated in presence of a polar reaction media. It has been also
observed that the reaction gives good yield in water and when a mixture of water and PEG
istaken the reaction yield is found to be enhanced.
92
Table 3.5 Solvent free microwave assisted synthesis of amido alkyl naphtholsusing
20MoZr catalyst
Entry No. R R1 Time (min) Yield (%)
1 H NH2 5 76
2 2-Cl NH2 4 80
3 4-Cl NH2 4 82
4 4-OCH3 NH2 6 72
5 3-NO2 NH2 3 80
6 4-OH NH2 6 78
7 4-NO2 NH2 4 86
8 H C6H5 6 72
9 2-Cl C6H5 4 78
10 4-OCH3 C6H5 6 75
11 4-NO2 C6H5 4 82
The favorable interaction between the reactants and the catalyst surface seems to be the reason
for the higher yield in the PEG-water mixture. The recyclability of the 20MoZr catalyst has been
tested for three consecutive cycles (Table 3.4, Entry 8, yields, 88%, 1st; 83%, 2
nd; 80%, 3
rd) and
no significant decrease in yield has observed in the study.
The mechanistic details of the reaction have been described in literature (Kantevari et al., 2007;
Nandi et al., 2009). Briefly, the addition of the 2-naphthol to the aromatic aldehyde results in the
formation of orthoquinonemethides (o-QMs) intermediates (Scheme 3.2). The o-QMs
intermediates subsequently undergo nucleophillic conjugate addition with the amide or urea to
form 1-amidoalkyl-2-naphthol derivatives. The Lewis acidic Mo sites of the catalyst can help in
the activation of the aldehyde facilitating the formation of o-QMs intermediates. The adsorbed o-
93
QMs can undergo electrophillic activation on the catalyst surface which upon conjugate addition
yields the 1-amidoalkyl-2-naphthol derivatives.
Table 3.6 Effect of the reaction media on the synthesis of amido alkyl naphthols(Table
3.4, entry 10) using 20MoZr catalyst
Sl.No Solvent Time (h) Yield (%)
1 Solvent free 2 70
2 Toluene 14 30
3 Dioxan 12 43
4 methanol 12 78
5 ethanol 14 71
6 water 12 68
7 PEG 12 72
8 PEG + water (50:50 v/v) 8 80
O H O H
HO
O H
HO
HHO H
O
H
H
O
-H2O
NH2
O
H2NNH2
O
NH2
O
NH2
O
NH
HO
Lew
is a
cid
i c s
i tes
Lew
is a
cid
ic s
ites
Lew
is a
cid
ic s
ite s
Lew
is a
cidic
sit
es
Lew
i s a
cidic
sit
es
Lew
is a
c idic
sit
es
Lew
is a
cid
i c s
i tes
Scheme 3.2 Probable mechanism path for synthesis of amidoalkyl naphthols on xMoZr
catalyst
94
Physical and spectra data ofsome representative compounds from Table 3.4
N-[(2-Hydroxynaphthalen-1-yl)phenylmethyl] benzamide (Entry 10)
Mp: 241-242oC; IR (KBr, cm
-1): 3420, 3061, 3022, 1629, 1572, 1511, 1435, 1348, 1271, 1049,
822, 696; 1H NMR (CDCl3, 400 MHz): δ =10.3 (s, 1H), 9.0 (d, 1H), 8.1 (d, 1H), 7.9 (d, 2H),
7.84 (d, 1H), 7.8 (d, 1H), 7.6 (t, 1H), 7.4–7.5 (m, 3H), 7.2–7.4 (m, 8H).
1-((2-hydroxynaphthalen-1-yl)(3-nitrophenyl)methyl)urea (Entry 5)
Mp: 167-168oC; IR (KBr, cm
-1): 3426, 3392, 3379, 3196, 1725, 1652, 1603, 1523, 1440, 1349,
1312, 1268, 1092, 1043, 815, 730 cm–1
. 1H NMR (CDCl3, 400 MHz): δ = 10⋅16 (s, 1H), 8⋅01–
8⋅05 (m, 2H), 7⋅80–7⋅88 (m, 3H), 7⋅45–7⋅52 (m, 3H), 7⋅10–7⋅24 (m, 3H), 5⋅99 (s, 2H).
3.2.5.2 SYNTHESIS OF -ACETAMIDOKETONES
Acetamidoketones are a class of organic compounds with interesting biological and
pharmacological properties. These compounds are the potential building blocks for synthesis of
1,3- amino alcohols, β- amino acids and synthesis of various bioactive molecules such as
nucleoside antibiotic drugs such as nikkomycine or neopolyoxines (Dahn et al., 1976). The
synthesis of -acetamidoketones has been accomplished by the Dakin–West reaction by
condensation of -amino acid with acetic anhydride in the presence of a base (Dakin et al.,
1928). Recently, improved synthetic routes have been developed for the synthesis of -
acetamido ketones involving the acid catalyzed multicomponent condensation reaction of aryl
aldehydes, enolizable ketones, acetyl chlorides and acetonitrile. Several Lewis and Bronsted
acids such as CoCl2, heteropoly acids, FeCl3, BiCl3 generated from BiOCl, Sc(OTf)3 and
ZrOCl2.8H2O have been used as catalysts under homogeneous condition for the synthesis of -
acetamidoketones (Mukhopadhyay et al., 1997; Pandey et al., 2005; Ghosh et al., 2005; Ghosh et
al., 2006; Khan et al., 2007). However, due to the inherent disadvantages of separation and
recyclability associated with the homogeneous catalysts there have been afford to develop
heterogeneous catalytic route for the synthesis of these biologically important class of
95
compounds. The heterogeneous catalysts reported for the synthesis of -acetamidoketones
include sulfuric acid absorbed on silica, montmorillonite K-10 clay, ZnO and PWA/ZrO2
(Bahulayan et al., 2003; Khodaei et al., 2005; Yakaiah et al., 2005; Maghsoodlou et al., 2007;
Naik et al., 2011). In this investigation, we have used the MoO3-ZrO2 composite oxides prepared
by the coprecipitation method as efficient and recyclable catalyst for the synthesis of the -
acetamidoketones. Initially, we focused on optimizing the reaction condition by taking the
reaction of benzaldehyde, acetophenone and acetyl chloride in acetonitrile as a model reaction
(Scheme 3.3).
CHO
+
COCH3
R
xMoZr
50oC
+ CH3COCl
CH3CN
ONH
C
O
R
R1
R1
Scheme 3.3 One pot synthesis of -acetamidoketones using xMoZr catalyst
The reaction parameters which are varied to generate the optimized protocol are the catalyst
amount, catalyst composition, stoichiometry of the reactants and reaction temperature. It was
observed that among all the catalyst, the 20MoZr material displayed highest activity under
identical reaction condition. This catalyst also found to possess more acidic sites as compared to
the other xMoZr catalyst (Table 3.3). The preferred ratio of aryl aldehyde, aryl ketone, and
acetyl chloride was found to be 1:1:2 using acetonitrile as a solvent as well as reactant. It was
observed that, for a reaction involving 2 mmol of the aldehyde, 100 mg of the catalyst ideally
suited for the efficient four component condensation at 50oC and further increase in the catalyst
amount does not significantly improve the yield. The generality of the protocol was verified by
introducing different structural variation in the aryl aldehyde and aryl ketone (Table 3.7). The
catalyst is highly active for different substituted carbonyl compounds.
96
Table 3.7 Solvent free one pot synthesis of β-acetamido ketones using 20MoZr catalyst
Sl.
No
R R1 Product Time
(min)
Yield
(%)
1 H H ONHCH3CO
120 76
2
2-Cl H ONHCH3CO
Cl
90 82
3 4-Cl H ONHCH3CO
Cl
90 84
4 3-NO2 H ONHCH3CO
O2N
90 82
5 4-NO2 H ONHCH3CO
O2N
90 88
6 4-OCH3 H ONHCH3CO
H3CO
120 78
7 4-Br H ONHCH3CO
Br
90 83
8 H 4-Cl ONHCH3CO
Cl
120 86
97
9 C6H5 4-NO2 ONHCH3CO
NO2
90 85
10 4-NO2 4-Cl ONHCH3CO
ClO2N
90 88
The mechanistic detail of the reaction is presented in scheme 3.4. The aromatic aldehyde is first
acylated with acetyl chloride to an intermediate I which then reacts with the enolic form of
acetophenone to produce II. Next, acetonitrile attacks II with the elimination of acetate group to
give III. Finally, the hydrolysis of III followed by tautomerization gave the desired β-acetamido
ketone. The 20MoZr catalyst can help in more than one step in the mechanism in expediting the
synthesis of β- acetamidoketones. The aldehydic carbonyl group can be activated by adsorption
onto the Lewis acidic Mo6+
sites of 20MoZr catalyst. Similarly, the process of enolization,
hydrolysis and tautomerism steps can be catalyzed in the presence of the Bronsted acidic sites of
the 20MoZr catalyst. The used catalysts is reactivated by heat treatment at 400oC for 1 h and then
reused for three consecutive cycles without significant loss in catalytic activity (Table 3.7, Entry
5, yields, 88%, 1st; 85%, 2
nd; 82%, 3
rd).
98
Ar
O
H
CH3COCl
ArCl
H
O
O
CH3
Ar H
O+
O
CH3
Ar'
CH2
OH
Ar Ar'
O O+H
H3C
O
Ar Ar'
O O
H3C
O+
H
H3C CN
C
H3C
N+
Ar
Ar'
O
H2O
H3C
C N
O+
HH Ar
Ar'
O-H+
H3C
C N
OHAr
Ar'
O
Tautomerism
Ar Ar'
ONHCOCH3
I
+CH3COOH
I
IIIII
20MoZr
Scheme 3.4 Probable mechanistic path for synthesis of β-acetamidoketones over
xMoZr catalyst
Physical and spectral data of some of the representative compounds
β-Acetamido-β-(phenyl)propiophenone (Table 3.7, Entry 1)
Mp: 103-104oC; IR (KBr, cm
-1): 3430, 1765, 1652, 1503, 1379, 1300, 1189, 1025, 707, 588;
1H
NMR (CDCl3, 400 MHz): δ 2.00 (s, 3H), 3.40 (dd, 1H), 3.73 (dd, 1H), 5.62 (dd, 1H), 6.95
(d1H), 7.30–7.60 (m, 8H), 7.95 (d, 2H) .
β-Acetamido-β-(4-nitrophenyl)propiophenone (Table 3.7, Entry 5)
Mp: 149oC; IR (KBr, cm
-1): 3301, 3065, 2850, 1696, 1650, 1597, 1513, 1351, 1295, 1197, 990,
755, 638; 1H NMR (CDCl3, 400 MHz): δ 2.05 (s, 3H), δ 3.48 (dd, 1H), 3.82 (dd, 1H), 5.60 (m,
1H), 6.9 (d, 1H), 7.40–7.60 (m, 5H), 7.8 (d, 2H), 8.2 (d, 2H) .
99
PART B: COMBUSTION SYNTHESIZED ZrO2-MoO3 COMPOSITE OXIDES
3.3. INTRODUCTION
Combustion synthesis is an attractive method for preparation of homogeneous, finely dispersed
crystalline uni- and multicomponent oxides (Rao, 1994; Patil et al., 2002; Ranga Rao et al. 2003;
Naik et al., 2008; Yermekova et al., 2010; Peng et al., 2007) as well as metals and alloys of
uniform composition and size (Li et al., 2002; Ranga Rao et al., 2004). The specific advantages
of combustion synthesis method over other methods of material preparation are (1) single step
process, (2) less time consuming, (3) formation of metastable phases and (4) formation of
homogeneous nanoparticles (Rao et al., 1994, Patil et al., 2002; Yermekova et al., 2010). Several
classes of materials with diverse properties such as oxides, nitrides, carbides, complex oxides,
metals and alloys have been prepared by combustion synthesis for a variety of technological
applications (Rao, 1994; Patil et al., 2002; Li et al., 2002; Ranga Rao et al., 2003, 2004; Peng et
al., 2007; Naik et al., 2008; Yermekova et al., 2010). In past few years, many novel procedures
have been adopted during combustion synthesis such as use of organic compounds as fuels,
microwave and sonic wave as energy source to obtain materials with desired properties. The
solution combustion method, one of the convenient modifications of self-propagating high
temperature synthesis is widely used for material preparation (Patil et al., 2002). In solution
combustion method, organic compounds containing reducible functional groups are employed as
fuels and metal nitrates are used as oxidizers. The exothermic combustion reaction between the
fuel and the oxidizer results in the formation of unagglomerated solid particles with simultaneous
evolution of large amounts gaseous products (Ranga Rao et al., 2003; Peng et al., 2007; Naik et
al., 2008; Yermekova et al., 2010). The nature and amount of fuel in a combustion mixture can
effectively control the particle size and morphology of the final products. In certain cases, the
phase transformation and formation of metastable phases are facilitated by the amount and nature
of the fuel. For example, the metastable phase formation of zirconia (Venkatachari et al., 1995;
100
Sahu et al., 2000) and titania (Ma et al., 2010) are affected by the amount of fuel. The fuel
content in the combustion mixture can also affect the oxidation state of the metal ions in the final
oxide. Ranga Rao et al. have reported the formation of CuO, Cu2O and Cu phases at different
fuel content using carbohydrazide as fuel (Ranga Rao et al., 2004).
The role of a fuel is to supply energy during their decomposition for the transformation of metal
salt precursors to respective oxides. Sometimes the fuel binds with the metal cation salt precursor
to form coordination compounds with high solubility in solvents like water. Nitrogen containing
fuels are widely employed for this purpose because of their natural tendency to act as bidentated
chelating ligands forming complexes with the metal ions and also their decompositions are
highly exothermic. Patil et al. (Patil et al., 2002) have outlined some important criteria for
selection of fuel which include (1) water solubility (2) low ignition temperature (<500oC) (3)
compatibility with metal nitrates, i.e., the combustion reaction should be controlled and smooth
and should not lead to explosion and (4) yield no other residual mass except the oxide in
question. The specificity of a fuel for synthesis of metal oxides, depends on the the metal–ligand
complex formation and the thermodynamics of the reaction.
In combustion synthesis process, the heat generated by the exothermic decomposition of the fuel
(F) converts the metal nitrates (oxidizer, O) into the target oxide material. A stoichiometric
composition denoted as fuel-to-oxidizer ratio (F/O) is used in the combustion process to achieve
complete combustion of fuel and conversion of oxidizer into final products leaving no residuals
of the starting materials. The stoichiometric composition is calculated using the total oxidizing
and reducing valencies of the elements involved in the fuel as well as in the oxidizer and the
appropriate numerical coefficients from the reaction stoichiometry so as the equivalent ratio, e
becomes unity (Jain et al., 1981). The equivalent ratio ( e) essentially represents the ratio of the
oxidizing part (metal salt) to reducing part (fuel) of the elemental composition which is
expressed as
101
The various types of fuel generally employed in the synthesis of oxide materials and their
reducing valancies compiled in Table 3.8.
Table 3.8 different Fuels used in combustion synthesis
Fuel Formula Reference
Urea
Carbohydrazide
Oxyldihydrazide
Maleic hydrazide
Diformalhydrazide
Tetraformaltrisazine
Glycine
Hexamethylenetetramine
N- tertiary butoxy carbonyl
piperazine
CH4N2O
CH6N4O
C2H6N4O2
C4H4N2O2
C2H4N2O2
C4H12N6
C2H5NO2
C6H12N4
C9H18N2O2
Naik et al.,2008
Ranga Rao et al., 2004
Bera et al., 2000b
Chandran and Patil, 1992
Patil et al., 1997
Ramesh et al., 1995
Xijuan et al., 2001
Prakash et al., 2002
Ranga Rao et al., 2004
In the present work, we have prepared MoO3-ZrO2 composite oxides by the solution combustion
method using different fuels and compared their physiochemical characteristics. The potential
fuels employed in the present study are urea, glycine and hexamethylenetetramine, which are
fairly common among many organics used in solution combustion synthesis. The catalytic
activity of the combustion synthesized MoO3-ZrO2 nanocomposite oxides have been investigated
for the multicomponent one pot synthesis of octahydroquinazolinones under environmentally
benign conditions.
102
3.4 RESULTS AND DISCUSSION
3.4.1 XRD STUDY
The X-ray diffraction patterns of the pure ZrO2 and 10MoZr materials prepared by combustion
as well as conventional impregnation method are presented in Fig. 3.6. Pure zirconia prepared by
the precipitation method show well defined and intense peaks with d values of 3.16, 2.95. 2.85,
2.56, 1.81 and 1.53 Å. These peaks correspond to the presence of both monoclinic and tetragonal
phase of zirconia (Venkatachari et al., 1995; Sahu et al., 2000). The percentage tetragonal phase
present in the sample is calculated to be 58%. The pure ZrO2 prepared using different fuels by
the combustion method exhibit peaks with d values 2.95, 2.58, 1.80 and 1.54 Å corresponding to
the presence of tetragonal phase only. The characteristic reflections at 3.16 and 2.85 Å of
monoclinic zirconia are not observed for the combustion synthesized zirconia samples. This
observation clearly suggests the selective stabilization of the metastable tetragonal phase of
zirconia in combustion synthesis process. Among all ZrO2 synthesized, the ZrO2-G sample
shows considerable broadening of the peaks and low intensity as compared to the ZrO2-H and
ZrO2-U sample. Addition of MoO3 onto ZrO2 matrix has been found to significantly influence
the phase composition of the composite oxide. The 10MoZr-C material prepared by
impregnation method found to contain 83 % tetragonal phase. All the 10MoZr materials prepared
by combustion method show XRD peaks characteristics of tetragonal phase.
103
20 30 40 50 60 70
T
T
T
M
10MoZrG
10MoZrH
10MoZrU
ZrO2-G
ZrO2-H
ZrO2-U
Inte
ns
ity
(a
.u.)
2 (degrees)
ZrO2-C
10MoZrC
a
b
c
d
e
f
g
h
M
T
T
T
T
TT T
M M
T
T T T
Figure 3.6 X-ray diffraction patterns of (a) ZrO2-C, (b) ZrO2-U, (c) ZrO2-H, (d) ZrO2-
G, (e) 10MoZr-C, (f) 10MoZr-U, (g) 10MoZr-H and (h) 10MoZr-G (T and M refers to
tetragonal and monoclinic phases of zirconia, respectively)
The XRD patterns of xMoZr materials with MoO3 content in the range of 2-20 wt%, prepared
using urea as fuel is presented in Fig. 3.7. All the materials synthesized show the characteristic
reflections corresponding to the tetragonal phase. No separate crystalline phase corresponding to
either MoO3 or Zr(MoO4)2 is observed in the XRD study.
104
20 30 40 50 60 70
Figure 2
(311)(220)(200)
(111)
f
e
d
c
b
a
20MoZr-U
10MoZr-U
8MoZr-U
5MoZr-U
2MoZr-U
Inte
ns
ity
(a
.u.)
2 (degrees)
ZrO2-U
T
TT T
T TT
T
Figure 3.7 X-ray diffraction patterns of (a) ZrO2-U, (b) 2MoZr-U (c) 5MoZr-U, (d)
8MoZr-U (e) 10MoZr-U and (f) 20MoZr-U (All compositions prepared using urea as a fuel,
T refers to tetragonal phase).
This result suggests the fact that the MoO3 species are well dispersed in zirconia matrix. In order
to study the effect of fuel content in the combustion mixture on the crystallinity and phase
transformation behavior of the composite oxides, the 10MoZr material has been prepared at
105
different F/O ratio using glycine and HMTA as fuels. The XRD patterns of the synthesized
10MoZr materials are presented in Fig. 3.8.
20 30 40 50 60
T
T
T
T
T
b
Inte
ns
ity
(a
.u.)
2 (degrees)
a
10MoZr Using HMTA as fuel I
T
20 30 40 50 60
TT
T
T
T
T
Figure 3
a
c
d
e
b
c
d
e
F/O=0.5
F/O=0.75
F/O=1.0
F/O=1.25
F/O=1.5
F/O=0.5
F/O=0.75
F/O=1.0
F/O=1.25
F/O=1.5
10MoZr Using Glycine as fuel II
Figure 3.8 X-ray diffraction patterns of 10MoZr catalyst prepared using
hexamethylenetetraamine as fuel (Panel I) and glycine as fuel (Panel II) at different fuel to
oxidizer ratio in the range of F/O=0.5-1.5 (T refers to the tetragonal phase)
The major phase observed in case of both the fuels is tetragonal zirconia. The XRD peak
intensity has been found to increase with fuel content. The increase in exothermicity of the
combustion reaction with fuel content can lead to well crystalline materials which show well
defined intense peaks. In case of HMTA fuel two low intensity peaks are observed at 2 values
of 27.5 and 32.5 degrees. These peaks correspond to the presence of a small amount of
Zr(MoO4)2 spinel phase (JCPDS file No. 39-1438). No separate crystalline phase corresponding
106
to the presence of MoO3 is observed in the XRD patterns of the composite oxide. The crystallite
size and rms stain are calculated from the Fourier line shape analysis for all the combustion
synthesized samples following the Warren and Averbach method (Warren et al., 1950) using
software BRAEDTH (Balzar et al., 1993). The peak position (2θ), full width at half maximum
(FWHM) and intensity are calculated using commercially available software (PEAK FIT) for
each peak of the XRD data. The indexing of all peaks of XRD patterns are carried out using 2
and intensity value of each peak by a standard computer software POWD. The best agreement
between the observed and the calculated interplaner spacing and Bragg angles is found for
tetragonal crystal structure. The calculated volume-weighted distributions, (pV), and Fourier size
coefficient (As) as function of the Fourier length (L) for the different zirconia samples is given in
Fig. 3.9(A) and 3.9(B) respectively.
0 250 500 750 10000.00
0.25
0.50
0.75
1.00
As
L(A0)
ZrO2
G
ZrO2
U
ZrO2
H
0 500 1000 1500 2000 25000.000
0.002
0.004
0.006
0.008
pV
BA ZrO
2G
ZrO2
U
ZrO2
H
L (Ao)
Figure 3.9 Fourier line profile analysis plots for the ZrO2-U, ZrO2-H and ZrO2-G
materials
107
The wide distribution function observed for the ZrO2-H sample indicates that the particles are
polycrystalline with larger particle size. The ZrO2-G material shows a narrower size distribution
function as compared to the ZrO2-U and ZrO2-H materials (Fig. 3.9A). The crystallite sizes of
the ZrO2-G, ZrO2-U and ZrO2-H materials are found to be 9, 30 and 49 nm respectively (Fig.
3.9B).The pV~L and As~L plot for the 10MoZr materials synthesized using glycine as a fuel at
different F/O ratio is presented in Fig. 3.10 and Fig. 3.11. The corresponding Fourier line shape
analysis data for the 10MoZr-G materials along with the pure ZrO2-G material is shown in Table
3.9. For different F/O ratio, first there is decrease in the crystallite size up to F/O ratio 0.75, after
that the crystallite size is found to be increased. The increase in the crystallite size at higher F/O
ratio is due to the increase in exothermicity of the combustion reaction (Sahu et al., 2000). It has
been reported that the adiabatic reaction temperature (Td) increases with fuel content in the
combustion mixture (Sahu et al., 2000; Kishan et al., 2010).
0 200 400 600 800 10000.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
pV
ZrO2
10MoZr (F/O=0.5)
10MoZr (F/O=1)
10MoZr (F/O=1.5)
L(Ao
)
Figure 3.10 pV~L plot for 10MoZr-G materials at different F/O ratio
108
0 100 200 300 400 500 600 700 800 900 10000.0
0.2
0.4
0.6
0.8
1.0
ZrO2
10MoZr (F/0=0.5)
10MoZr (F/O=1)
10MoZr (F/O= 1.5)
Fou
rier
siz
e co
ffic
ien
t (A
s)
L(Ao)
Fig. 2b, Samantaray and Mishra
Figure 3.11 As~L plot for 10MoZr-G materials at different F/O ratio
Table 3.9 The lattice parameters, unit cell volume, crystallite size and rms strain of
ZrO2-G and 10MoZr-G materials
Fuel Materials a in Å c in Å Volume(Å3) Size
(nm)
Strain
<e2>
1/2
Glycine ZrO2 5.1543 5.1809 137.64 9 6.51 × 10-3
10MoZr- G F/O= 0.5 5.1568 5.1785 137.71 7 5.81 × 10-3
F/O= 0.75 5.1644 5.1714 137.93 4 1.26 × 10-2
F/O= 1.0 5.1531 5.1821 137.61 11 7.21 × 10-3
F/O= 1.25 5.1361 5.1820 136.70 15 5.66 × 10-3
F/O= 1.5 5.1518 5.1833 137.57 39 3.88 × 10-3
109
In the present study, it is believed that the higher adiabatic temperature at F/O=1.5 is responsible
for the possible agglomeration of the nanoparticles giving larger particle size. The reverse trend
is observed for the rms strain. The crystallite size calculations are also carried out for 10MoZr-H
materials prepared at different F/O ratio and found to be in the range of 10-50 nm. The effect of
Mo concentration on the microstructural parameters of the MoO3-ZrO2 composite oxides was
studied by using urea as a fuel and varying the MoO3 concentration between 2-20 mol% at a
fixed F/O ratio of 1. The least-squares refined lattice parameters, the unit cell volume, crystallite
size and rms strain of xMoZr-U materials with different percentage of MoO3 is presented in
Table 3.10.
Table 3.10 The lattice parameters, unit cell volume, crystallite size and rms strain of
xMoZr-U materials
With increase in Mo concentration there is a decrease in the crystallite size and increase in the
strain observed for the nanocomposite oxide. The increase in the rms strain may be due to the
higher disorder corresponding to the random and local lattice distortion.
Fuel Materials a in Å c in Å volume(Å3) Size
(nm)
Strain
<e2>
1/2
Urea ZrO2 5.1505 5.1844 137.53 30 3.84 × 10-3
2MoZr 5.1555 5.1796 137.67 37 3.34 × 10-3
5MoZr 5.1568 5.1785 137.71 28 3.98 ×10-3
8MoZr 5.1542 5.1808 137.63 8 7.43 ×10-3
10MoZr 5.1361 5.1820 136.70 6 7.97 × 10-3
20MoZr 5.1338 5.1796 136.51 5 1.26 × 10-2
110
3.4.2 UV-VIS STUDY
The UV-Vis spectra of the combustion synthesized xMoZr materials are presented in Fig. 3.12.
Pure Zirconia is a direct band gap insulator which shows interband transition in the UV region of
the electronic spectrum corresponding to the O2-
(2p) Zr4+
(4d) charge transfer transition.
200 300 400 500 600 700
Inte
nsit
y (
a.u
)
Wavelength (nm)
g
f
e
d
c
b
a
Figure 3.12 UV-Vis spectra of MoZr catalyst (a) ZrO2-U, (b) ZrO2-H, (c) ZrO2-G, (d)
2MoZr-U, (e) 5MoZr-U, (f) 10MoZr-U and (g) 20MoZr-U
111
Among the polymorphic forms of zirconia, the octacoordinated tetragonal (space group Fm3m)
and cubic (space group P42/nmc) phases show UV absorption maxima in the range of 200-210
nm whereas the heptacoordinated monoclinic phase (space group P21/c) shows maxima around
240 nm (Lo´pez et al., 2001). The prominent absorption band observed at peak maxima of 210
nm has been assigned to the O2-
→Zr4+
Charge transfer transition arising out of the tetragonal
polymorph of zirconia. This observation is complementary to the XRD result where selective
stabilization of tetragonal phase has been observed for the combustion synthesized samples. The
broad band observed at 280 nm for ZrO2-U sample can be ascribed to the transition arising out of
structural imperfection and defect centre present in the nanosize crystallite of zirconia. The
presence of structural disorder and defect centres shift the adsorption edge to higher wavelength
due to the presence of localized states between the valence and conduction band. It is likely that
the zirconia particles synthesized by combustion method using urea as fuel can contain defect
sites which contribute to the broad absorption feature in the higher wavelength side of the
spectrum. Pure MoO3 shows two well-defined absorption bands at 240 and 320 nm. The addition
of MoO3 to zirconia matrix significantly modifies the absorption spectra of the combustion
synthesized MoO3-ZrO2 composite oxide. For samples with low MoO3 loading (2 and 5 wt %) a
prominent band is observed at 226 nm along with a broad shoulder at 280 nm. The sharp and
narrow absorption band at 226 nm can be assigned to the tetrahedrally coordinated isolated
molybdate species with structural distortion whereas 280 nm can be assigned to small nanosize
molybdate clusters dispersed on the surface of the zirconia. When the percentage MoO3 loading
is increased to 10 percent and higher, two broad absorption features with maxima at 250 nm and
>300 nm are observed. The peak observed at 260 nm for sample with higher loading thus can be
assigned due to the polymolybdate clusters. Whereas the broad peak with maxima >300 nm can
be assigned to the presence of bulk type molybdate species present on the surface of zirconia.
However it is likely that the bulk type MoO3 can exist in amorphous form which has escaped
XRD detection. The UV-Vis spectra of 10MoZr-G materials prepared at different F/O ratio is
112
presented in Fig. 3.13. There is no apparent change in the absorption feature when the F/O ratio
was varied between 0.5-1.5 indicating the presence of identical molybdate species in these
samples.
200 300 400 500 600 700 800
c
b
d
Inte
nsit
y (
a.u
)
Wavelength (nm)
a
Figure 3.13 UV-Vis spectra of (a) ZrO2, (b) 10 MoZr-G (F/O= 0.5), (c) 10 MoZr-G (F/O=
1) (d) 10 MoZr-G (F/O= 1.5) and (e) pure MoO3 materials
113
The UV-Vis study of the combustion synthesized MoZr sample implies the presence of well
dispersed molybdate species in the form of isolated and polymolybdate clusters on the surface of
zirconia.
3.4.3 SEM STUDY
The scanning electron micrographs of ZrO2-U, ZrO2-H and ZrO2-G are presented in Fig.3.14.
The nature of the fuel is found to have a profound influence on the morphology of synthesized
zirconia particles. The zirconia particles synthesized using urea as a fuel is found to be of low
density and spongy in nature (Fig. 3.14a). Similar observation was noted for HTMA fuel with
numerous elongated macropores present on the surface of the particle (Fig. 3.14b). Zirconia
nanoparticles prepared using glycine as fuel are dense and with well-defined shape (Fig. 3.14c).
The spongy nature of zirconia particles in case of urea can be ascribed to the evolution of large
amount of gases during combustion. Theoretically, for the formation of one mole of zirconia,
8.66, 7.5 and 5.8 moles of gases are generated using urea, glycine and HTMA as fuel
respectively. The higher amount of gases generated leaves the material porous and spongy. The
difference in morphology may be related to the inherent nature of the fuel, their binding
characteristics and amount of gases evolved during their decomposition (Sahu et al., 2000; Patil
et al., 2002; Cattin et al., 2010). Urea and glycine can act as bidentate ligand and can coordinate
to the metal ions in the combustion mixture. However, urea is basic whereas glycine is
amphoteric in nature which can have different degree of bonding with the metal ions. HTMA on
the other hand is a tetra dentate ligand which is known to release ammonia in aqueous solution
(Cotton et al., 2000). Zirconium ions being acidic in nature can undergo considerable hydrolysis
in presence of urea and HTMA. The uncontrolled hydrolysis of the Zr4+
ions and subsequent
combustion can lead to particles with random morphology and sizes.
114
(a)
(b)
(c)
Figure 3.14 Scanning electron micrograph of (a) ZrO2-U, (b) ZrO2-H, and (c) ZrO2-G
115
(a)
(b)
(c)
Figure 3.15 Scanning Electron micrograph of (a) 10 MoZr-G (F/O= 0.5), (b) 10 MoZr-G
(F/O= 1) and (c) 10 MoZr-G (F/O= 1.5) materials
116
The zwiterionic character of glycine molecule on the other hand is useful in effectively
complexing with metal ions of varying ionic sizes. This in turn helps in preventing their selective
precipitation and maintains a compositional homogeneity among the constituents (Kishan et al.,
2010). Hence the uniform morphology of the combustion synthesized particles can be ascribed to
the effective complexation and prevention of hydrolysis of the ions in the combustion mixture.
The scanning electron micrograph of the 10MoZr-G materials prepared using different F/O ratio
is presented in Fig. 3.15. The 10 MoZr-G materials prepared using glycine fuel are of low
density and spongy in nature. The particles are found to be agglomerated and of irregular shape
and size. With increase in F/O ratio the spongy nature is found to increase. Numerous
macropores and intraparticle pores are observed for the 10 MoZr-G samples. The presence of the
macropores can be ascribed to the evolution of large amount of gaseous products during the
combustion process. Theoretically, for the formation of one mole of 10MoZr-G, 6.69, 6.94 and
7.21 moles of gases are generated for F/O ratio of 0.5, 1 and 1.5, respectively. The higher
amount of gases generated leaves the material porous and spongy.
3.4.4 TEM STUDY
The transmission electron micrographs of ZrO2 and 10MoZr (F/O =1) materials prepared using
different fuels are shown in Fig. 3.16. The TEM image of pure zirconia synthesized using
glycine shows the presence of uniform nanoparticles with size in the range of 5-10 nm.
Similarly, for the 10MoZr-G material, nanoparticles with size in the range of 10-20 nm were
observed in the TEM study. The Fourier line shape analysis data is found to corroborate the
TEM results. The 10MoZr-U and 10MoZr-H materials synthesized using urea and
hexamethylene tetraamine show particles with size in the range of 20-30 nm whereas the
10MoZr-I materials show larger particles with size in the range of 50-100 nm (Figure 3.16 e).
The TEM observations indicate that combustion synthesis is quite effective in the preparation of
nanosize particles.
117
(a)
(b)
(c)
118
(d)
(e)
Figure 3.16 Transmission electron micrographs of (a) ZrO2 and (b) 10MoZr-G (F/O=1),
(c) 10MoZr-U (F/O=1), (d) 10MoZr-H (F/O=1) and (e) 10MoZr-I
During combustion synthesis process, the reactants are uniformly dispersed at molecular level in
the combustion mixture. When combustion occurs, the nucleation and growth of the particles
take place only through the short-distance diffusion of the nearby atoms. Due to the shorter life
span of the combustion reaction, the long-distance diffusion of the atoms is not facilitated
resulting in the formation of nanosized materials.
119
3.4.5 CATALYTIC STUDIES FOR SYNTHESIS OF
OCTAHYDROQUINAZOLINONES
In recent years, considerable attention has been focused on octahydroquinazolinone derivatives
because of their potential antibacterial activity against Staphylococcus aureus, Escherichia coli,
Pseudomonas aeruginosa and as calcium antagonist (Kidwai et al., 2005; Yarim et al., 2003).The
octahydroquinazolinones are synthesized by the multi-component Biginelli reaction route
involving one pot condensation of cyclic β-diketones, aromatic aldehydes and urea. Various
homogeneous and heterogeneous catalytic systems used for synthesis of octahydroquinazolinone
and its derivatives include concentrated HCl in ethanol (Yarim et al., 2003), ionic liquid
(Khurana et al., 2010), Nafion-H (Lin et al., 2007), concentrated H2SO4 (Hassani et al., 2006),
TMSCl (Kantevari et al., 2006), ammonium metavanadate (Niralwad et al., 2010) and
PWA/ZrO2 (Naik et al., 2011). Main disadvantages of the reported processes are low yield of
products, longer reaction time, use of strongly acidic conditions, formation of side products and
handling and recycling of the catalyst. In recent years, enlarging attention on environmental
damage and economic factor leads to the development of eco-friendly technology and economic
processes. Hence, the synthesis of octahydroquinazolinone derivatives by the Biginelli reaction
using solid acid catalysts which are stable, recyclable and environment friendly is highly
desirable. In this work, we have studied the application of the combustion synthesized MoO3-
ZrO2 materials as heterogeneous catalyst for synthesis of structurally diverse
octahydroquinazolinones by one pot multi-component condensation of dimedone, aromatic
aldehydes and urea (Scheme 3.5).
120
+
O
OH3C
H3C
+
H2N NH2
X
10MoZr-G
water, Reflux
NH
NH
X
O
H3C
CH3
CHO
R
R
or solventfreemicrowave
Scheme 3.5 One pot synthesis of octahydroquinazolinones using 10MoZr-G catalyst
Catalysts having different active sites and acidic strength were initially screened for the synthesis
of octahydroquinazolinone under solvent free condition using microwave as an energy source.
Table 3.11 shows the physicochemical characteristics and catalytic activity of several zirconia
based catalysts screened for the condensation of benzaldehyde, dimedone and urea. All the
10MoZr materials screened for the reactions are synthesized at the stoichiometric F/O ratio of 1.
For the sake of comparison, the 10MoZr materials is also synthesized by impregnation method
(10MoZr-I). Under identical reaction conditions, the 10MoZr-G (F/O =1) material was found to
be most efficient giving 82% yield of the product in a minimum irradiation time of 180s.
10MoZr-I and sulfated zirconia also show comparable yield however they require considerably
longer irradiation time. Among all the catalyst the 10MoZr-G (F/O =1) material display more
number of acidic sites compared to other catalytic materials used (Table 3.11). Since the catalyst
screened show different surface area and acidity, the reaction rate were calculated in terms of
unit surface area and acidic sites by estimating the benzaldehyde conversion in the reaction
mixture by GC analysis. It was observed that the 10MoZr-G (F/O =1) catalyst shows higher
reaction rates as compared to other materials.
121
Table 3.11 Physicochemical characteristics and catalytic activity of various catalysts
screened for the synthesis of octahydroquinazolinone under microwave irradiation and
solventfree condition
Catalysta Surface
area(m2/g)
Acidity
(mmol/g)
Yieldb
(%)
Irradiation
time (s)
Ratec
(mmolh-1
g-1
)
Ratec
(mmolh-1
m-2
)
ZrO2 50.8 0.21 25 360 30 0.59
SO42-
/ZrO2 45.4 0.39 72 360 75 1.65
10 MoZr-U 54.5 0.37 62 300 86 1.57
10 MoZr- G 49.4 0.45 82 180 170 3.43
10 MoZr- H 44.7 0.26 70 210 128 2.86
10 MoZr- I 95.0 0.32 75 360 78 0.82
Mont K-10 185.0 0.35 68 360 72 0.39
aall composite oxides synthesized by combustion method corresponds to F/O =1 bRefers to pure and isolated yield,
c Calculated with respect to the benzaldehyde conversion in the reaction mixture analyzed using gas chromatography
Based on the results described in the Table 3.11, the 10MoZr-G (F/O =1) catalyst was selected
for further studies. The reaction parameters are optimized by varying catalyst amount and
reaction stoichiometry. It is observed that the for a reaction involving 1 mmol of the reactants,
100 mg of the catalyst is ideal for an efficient condensation of the reactants under solvent free
condition using microwave as an energy source. The optimum reactant ratio is found to be
1:1:1.5 for benzaldehyde, dimedone and urea or thiourea, respectively. Ensuing optimized
conditions for this multicomponent reaction, we explored the scope and limitations of this
optimized protocol using different substituted aromatic aldehydes and urea or thiourea (Table
3.12).
122
Table3.12 10MoZr-G (F/O=1) catalyzed synthesis of octahydroqunazolinones under
solventfree condition and microwave irradiation
Entry R X Time(s) Yield (%)
1 H O 180 82
2 2-Cl O 180 86
3 4-Cl O 150 89
4 2- NO2 O 200 82
5 3- NO2 O 180 86
6 4- NO2 O 180 88
7 4- OMe O 240 79
8 2- OH O 240 75
9 4- OH O 220 81
10 H S 270 74
11 4- OH S 300 63
12 4-Cl S 250 81
It was observed that the aromatic aldehydes having electron-withdrawing groups react faster in
the optimized protocol as compared to the aldehydes having electron donating groups. Unlike
those of urea, the reaction of thiourea proceeded at lower rate to give corresponding
octahydroquinazolinones (Table 3.12). The 10MoZr-G(F/O=1) catalyst was further explored for
the synthesis of octahydroquinazolinones in aqueous media. For reactions in water, the
multicomponent condensation is carried out under reflux conditions using 5 ml of the solvent
using the optimized reaction stoichiometry and catalyst amount of 100 mg. Table 3.13 shows the
results of the catalytic studies carried out in aqueous media using 10MoZr-G(F/O =1) catalyst.
123
Table 3.13 10MoZr-G(F/O=1) catalyzed synthesis of octahydroqunazolinones in
aqueous media
Entry R X Time(h) Yield(%)
1 H O 6 74
2 2-Cl O 5 78
3 4-Cl O 4 82
4 2- NO2 O 5 72
5 3- NO2 O 6 76
6 4- NO2 O 5 80
7 4- OMe O 10 66
8 2- OH O 10 70
9 4- OH O 10 68
10 H S 8 68
11 4- OH S 10 62
12 4-Cl S 8 75
It was observed that the catalyst was quite efficient for the one pot synthesis giving good yield of
the products in a time span of 5-12 h. A variety of aldehydes containing electron withdrawing
and releasing groups reacted efficiently to give the corresponding octahydroquinazolinones in
good yield and purity. Comparing both protocols, although the yields of the products are
comparable, the reaction rate is quite sluggish in aqueous media as compared to the microwave
assisted synthesis under solvent free condition. The mechanistic path for the formation of the
124
octahydroquinazolinones over the MoO3-ZrO2 catalyst surface is presented in scheme 3.6.
Mechanistically, initial condensation of the aldehyde and urea leads to acyliminium intermediate.
ArCHO H2N NH2
O
N
Ar H
O
NH2
Ar
NH
OH2N N
H
NH
ArO
OH
NH
NH
ArO
OO
-H2O
O OH
-H2O
O
O
10MoZr-G
Scheme 3.6 Probable mechanistic path for synthesis of octahydroquinazolinones on the
10MoZr-G catalyst surface
The MoZr material is known to possess both Bronsted and Lewis acidity. The surface hydroxyl
groups on the composite oxide surface can act as Bronsted acidic sites where as the Mo6+
metal
cations act as Lewis acidic sites. It is likely that the imine formation is accelerated in presence of
Bronsted acidic sites of the catalyst. The Lewis acidic Mo6+
sites can activate the acylimine
intermediate by coordinating to the nitrogen and oxygen heteroatom. The enolic content of the β-
dicarbonyl compounds is further enhanced in presence of the composite oxide by coordination
with surface metallic sites. Finally, the nucleophilic addition of the enolic β-dicarbonyl to the
acylimine intermediate, followed by cyclization and dehydration, affords the
octahydroquinazolinones derivatives (Scheme 3.6). The recyclability of the catalyst was
evaluated in the aqueous media. The catalyst after filtration from the reaction media is
regenerated by washing with 10 mL ethanol followed by heat treatment at 400oC for 1 h. The
regenerated catalyst is used for three consecutive cycles without any significant loss in catalytic
activity (Table 3.13, entry 1, 74%, 1st, 70% 2
nd, 67%, 3
rd)..
125
Physical and spectral data of some selected compounds
7,7-Dimethyl-4-phenyl-4,6,7,8-tetrahydro-1H,3H-quinazoline-2,5-dione (Table 3.12, entry 1):
Mp: 291-292oC, IR (KBr): 3325(br), 3262(br), 2963(br), 1710(s), 1670(s),1605(vs), 1442(w),
1374(s), 1228(s), 764(s), 689(w), 484(w) cm-1
, 1HNMR (400MHz, CDCl3): 0.92 (s, 3H, CH3);
1.06 (s, 3H, CH3); 2.15 (q, 2H,CH2); 2.35 (q, 2H, CH2); 5.25 (d, 1H, CH); 7.35-7.20 (m, 5H, Ar);
7.49 (s, 1H, NH); 9.42 (s,1H, NH)
4-(2-Chloro-phenyl)-7,7-dimethyl-4,6,7,8-tetrahydro-1H,3H-quinazoline-2,5-dione (Table
3.12, entry 2): Mp: 281-282oC, IR (KBr):3350 (br), 3250 (br), 2962 (br), 1710(s), 1670 (s), 1612
(vs), 1452 (s), 1371 (s), 1223 (s), 755 (s), 566 (w), 476 (w), 1HNMR (400MHz, CDCl3+DMSO-
d6): 0.98 (3H, s, CH3); 1.05 (3H, s, CH3); 2.10 (2H, q, CH2); 2.42 (2H, q, CH2); 5.52 (1H, d,
CH); 7.25-7.42 (4H, m, Ar); 7.72 (1H, d, NH); 9.50 (1H, s, NH).
7,7-Dimethyl-4-phenyl-2-thioxo-2,3,4,6,7,8-hexahydro-1H-quinazolin-5-one (Table 3.12, entry
10): Mp: 284-286oC, IR (KBr): 3255(br), 3172(br), 2962(br), 1623(vs), 1562(s), 1452(s),
1370(s), 1142(m), 1095(sh), 1055(w), 695 (w), 516 (w), 427 (w) cm-1
, 1HNMR (400MHz,
CDCl3+DMSO-d6): 0.96(s, 3H, CH3); 1.10 (s, 3H, CH3); 2.24 (q, 2H, CH2); 2.35(s, 2H, CH2 );
5.28 (d, 1H, CH); 7.38-7.26 (m, 5H, Ar ); 9.51(s, 1H, NH); 10.32(s, 1H, NH )
126
PART C: SULFATE GRAFTED Fe2O3-ZrO2 NANOCOMPOSITE OXIDES
3.5 INTRODUCTION
Fine and specialty chemicals synthesis involving non-polluting and atom-efficient catalytic
protocols isone of the most promising research areas in synthetic chemistry. Recently, solid acids
are increasingly used in many fine chemical synthesis processes. Among several solid acids such
as heteropolyacids, clays, metal oxides and zeolites used for fine chemical synthesis, the sulfate
grafted metal oxides are one of the most extensively investigated catalytic systems. These
catalysts contain strong acidic sites on their surfaces often termed as “superacidic” which are
capable of catalyzing carbonium ion reactions under mild conditions. The relatively simple
method of preparation of these catalysts makes them one of the attractive classes of
heterogeneous catalysts. Sulfated metal oxides such as sulfated SnO2, TiO2, Fe2O3 and ZrO2 have
been extensively used in many heterogeneous catalytic processes (Kumar et al., 2006; Khder et
al., 2008; Wang et al., 2009; Reddy and Patil, 2009; Arata, 2009; Tyagi et al., 2010). The
sulfated metal oxides have been found to catalyze a number of fine chemical synthesis processes
under mild conditions compared to other conventional catalyst materials. Sulfated metal oxides
such as sulfated zirconia and tin oxide have been used as catalyst for the synthesis of
dihydroprimidones, bis(indolyl methane), glycosides and oligosides, regioselective ring opening
of aziridines, dehydration of xylose to furfural, alkylation of diphenyl oxides, dihydropyridines
and selective protection of alcohols, phenols and aromatic aldehydes (Nagai et al., 2001; Yadav
and Sengupta, 2002; Lin et al., 2003; Reddy et al., 2005; Kumar et al., 2006; Das et al., 2008;
Negron-Silva et al., 2008). Although there are reports on the preparation, characterization and
catalytic application of nanosize sulfated metal oxide, there is a lot of scope to explore the
application of this class of material as catalyst for synthesis of biologically relevant molecules.
With an aim to explore novel catalytic applications of sulfated metal oxides, in the present
investigation we have synthesized Fe2O3-ZrO2 nanocomposite oxide by coprecipitation method
127
followed by sulfate grafting onto the surface of the composite oxide. The SO42-
/ Fe2O3-ZrO2
materials were characterized using XRD, UV, FT-IR, SEM and TEM analytical techniques. The
catalytic activity of these materials has been studied for the environmental benign synthesis of
xanthenediones under solvent free condition using microwave as energy source.
3.6 RESULTS AND DISCUSSION
3.6.1 XRD STUDY
Fig. 3.17 shows the X-ray diffraction patterns of the Fe-Zr-O composite oxides prepared by
coprecipitation method.
20 30 40 50 60 70
2 (degrees)
Inte
nsit
y (
a.u
.)
e
f
a
d
b
c
Figure 3.17 XRD patterns of (a) ZrO2, (b) 2Fe-Zr, (c) 5Fe-Zr, (d) 10Fe-Zr, (e) 20Fe-Zr
and (f) 50Fe-Zr
128
Pure zirconia prepared by precipitation shows the characteristic reflections corresponding to the
presence of a mixture of monoclinic and tetragonal phase. The iron doped zirconia materials on
the other hand show reflections corresponding to the presence of tetragonal phase of zirconia
only. No diffraction peaks corresponding to the presence of crystalline Fe2O3 species is observed
upto 20 mol% iron oxide content in the composite oxide. However, the 50FeZr material shows
diffraction peaks at 2 values of 24.2, 33.1, 35.7, 40.9, 49.4, and 54.1 degrees corresponding to
the rhombohedral structure of the -Fe2O3 (hematite) phase in the composite oxide (Wu et al.,
2006). The phase transformation behavior of zirconia has been widely studied in literature
(Navı´o et al., 2001; Ferna´ndez-Garcı´a et al., 2004; Chang and Doong, 2005; Figueroa et al.,
2005). The selective stabilization of the tetragonal phase of zirconia depends on several factors
like critical crystallite size, and the presence of phase stabilizer either in the bulk or at the surface
(Ferna´ndez-Garcı´a et al., 2004). The tetragonal phase of zirconia is also stabilized in presence
of aliovalent impurity ions in zirconia lattice (Ferna´ndez-Garcı´a et al., 2004; Figueroa et al.,
2005). The presence of aliovalent dopants such as Fe3+
ions induces oxygen ion vacancy in the
crystal lattice of zirconia which in turn helps in the stabilization of the tetragonal phase. For
oversized dopants, the oxygen vacancy resides near the Zr ion whereas for undersized dopants,
both the ions compete for the oxygen vacancies, resulting in 6-fold oxygen coordination and a
large distortion of the surrounding next nearest neighbor cation network. With increase in the
iron oxide content in the composite oxide, the diffraction peaks of tetragonal zirconia was found
to shift progressively to higher 2 values (Fig. 3.18). The peak position (2θ), full width at half
maximum (FWHM) were calculated for each peak using the software peakfit and the indexing of
the peaks were carried out using POWD software.
129
28 30 32
2 (degrees)
Inte
nsit
y (
a.u
.)
a
b
c
d
e
f
Figure 3.18 XRD patterns indicating a gradual shift to higher 2 value for the (111) peak
of tetragonal zirconia phase with iron oxide content (a) ZrO2, (b) 2Fe-Zr, (c) 5Fe-Zr, (d)
10Fe-Zr, (e) 20Fe-Zr and (f) 50 Fe-Zr
Fig. 3.19 shows the variation in the cell volume with iron content in the composite oxide. The
cell volume was found to decrease linearly with increase in the iron content. The decrease of cell
volume is due to the contraction in lattice occurred due to substitution of Zr4+
ions by Fe3+
ions
in zirconia lattice. Iron oxide is known to form a substitutional type solid solution with zirconia
(Navı´o et al., 2001; Figueroa et al., 2005; Chen et al., 2008a). Fe3+
ions (0.65 Å) being smaller
in size compared to Zr4+
ions (0.79Å) the substitution of Fe3+
ions results in shrinkage of the
lattice.
130
2 4 6 8 10 12 14 16 18 20136.0
136.5
137.0
137.5
138.0
138.5
139.0
Iron mol%
Ce
ll V
olu
me
Figure 3.19 Variation of cell volume with iron content for the xFe-Zr nanocompoite
oxides
Fig. 3.20 shows the XRD patterns of the sulfate grafted FeZr materials (SO42-
/xFe-Zr). All the
materials show the diffraction peaks corresponds to the presence of tetragonal phase of the host.
There is no apparent change in the peak position and intensity as a result of grafting of sulfate
ions.
131
20 30 40 50 60 70
2 (degrees)
Inte
nsit
y (
a.u
.)
e
d
c
b
a
2FeZrS
5FeZrS
10FeZrS
20FeZrS
50FeZrS
Figure 3.20 XRD patterns of (a) SO42-
/2Fe-Zr, (b) SO42-
/5Fe-Zr, (c) SO42-
/10Fe-Zr, (d)
SO42-
/20Fe-Zr and (e) SO42-
/50Fe-Zr
3.6.2 FT-IR STUDY
The FT-IR spectra of sulfate grafted xFe-Zr materials in the spectral region of 900-1800 cm-1
are
presented in Fig. 3.21. All the sulfate grafted Fe-Zr samples show a prominent and broad band
around 1635 cm-1
. This band can be assigned to the bending vibration of the structural O-H
group and the coordinated water molecules present on the surface of the sulfate grafted materials.
In addition, the sulfate grafted composite oxides exhibit a series of IR bands in the frequency
range of 900-1450 cm-1
characteristic to the vibrational features of anchored sulfate groups
(Morterra et al., 1997; Li et al., 2004; Jadhav et al., 2010; Tyagi et al., 2010; Sinhamahapatra et
al., 2011).
132
900 1200 1500 1800
Wave number (cm-1
)
f
e
d
c
b
In
ten
sity
(a
.u.)
a
16361401
1205
11401050
990
Figure 3.21 FTIR spectra of (a) SO42-
/ZrO2, (b) SO42-
/2Fe-Zr, (c) SO42-
/5Fe-Zr, (d) SO42-
/10Fe-Zr, (e) SO42-
/20Fe-Zr and (f) SO42-
/50Fe-Zr
In the present study, a group of spectral bands are observed in the region of 950-1250 along with
a well-defined IR band at 1401 cm-1
.The band at 1401cm-1
can be attributed to the partially
ionized S=O stretching vibrations of the anchored bidentate sulfate group (Morterra et al., 1997;
Li et al., 2004) . In the spectral region of 950-1250 cm-1
, a series of discrete peaks are observed
at 990, 1050, 1140, 1205 cm-1
which are assigned to the various vibrational modes of the S–O
bonds of sulfate species connected to the zirconia surface. The FTIR spectral analysis indicates
133
the presence of anchored bidentate sulfate species on the composite oxides with partially ionized
bonds. This ionic structure of sulfate group in presence of adsorbed water molecules is believed
to be responsible for the Bronsted acidity in sulfated zirconia catalysts (Mishra et al., 2003).
3.6.3 UV-VIS-DRS STUDY
The UV-Vis-DRS spectra of SO42-
/xFe-Zr materials with different Fe2O3 content are presented in
Fig. 3.22. Pure sulfated ZrO2 shows a sharp and intense band at 214 nm with an absorption edge
around 300 nm. ZrO2 is a direct band gap insulator which shows interband transition in the UV
region of the spectrum. The monoclinic form of ZrO2 has two direct interband transitions at 5.93
eV and 5.17 eV, whereas the tetragonal form has a band gap of 5.1 eV (Scheithauer et al., 1998).
200 300 400 500 600 700 800 900
F(R
) (a
.u.)
wave length (nm)
a
b
c
d
e
f
Figure 3.22 UV-Vis spectra (a) SO42-
/ZrO2 (b) SO42-
/2Fe-Zr, (c) SO42-
/5Fe-Zr, (d) SO42-
/10Fe-Zr (e) SO42-
/20Fe-Zr and (f) SO42-
/50Fe-Zr
134
In the present case the peak a 214 nm can be assigned to the O2-
Zr4+
charge transfer transition
arising from the host zirconia matrix. The presence of Fe2O3 in the zirconia matrix significantly
modifies the UV absorption feature of the zirconium dioxide. The SO42-
/xFe-Zr materials with
low Fe2O3 content (2 and 5 mol%) shows a prominent band in the range 260-280 nm. This band
can be assigned to isolated octahedral Fe3+
species anchored to the zirconia surface. Iron in Fe3+
state is expected to show two ligand to metal charge transfer transitions corresponding to the
t1 t2 and t1 e transitions (Bordiga et al., 1996; Gao and Wachs, 2000; Kumar et al., 2004;
Gervasini et al., 2009). However, depending upon the coordination environment of the Fe3+
species these transitions differ widely in energies and appear in different region of the spectrum.
Ferric ion in an isolated state gives peaks below 300 nm. For example, Fe3+
in isomorphous
substituted in the silicate frame work shows two peaks at 215 and 241 nm (Bordiga et al., 1996).
Similarly, Fe3+
in isolated octahedral state shows LMCT transitions around 280 nm in Fe3+
doped
alumina sample (Gao and Wachs, 2000). Thus the CT bands are red shifted as the number of
oxygen ions coordinated to the Fe3+
increases. It has also been reported that the Fe3+
present in
small nonstoichiometric nanooxide clusters of the type FexOy shows charge transfer transition in
the range of 300-400 nm whereas bulk type Fe2O3 particle shows UV absorption above 450 nm
(Bordiga et al., 1996; Gao and Wachs, 2000). In the present study, the peak between 260-280 nm
can be assigned to the isolated octahedral iron species. As the Fe2O3 content increases, this peak
found to be red shifted and reappear in the range of 300-400 nm. With increase in Fe2O3 content
it is likely that small FexOy type nanoclusters can exist in a well dispersed state in the composite
oxide matrix. For SO42-
/50Fe-Zr composite oxides a broad band was observed at 490 nm which
indicate the presence of bulk type Fe2O3 particles. From the UV-Vis study, it can be concluded
that the SO42-
/xFe-Zr materials at low iron oxide loading predominantly contain well dispersed
isolated Fe3+
species. At higher iron oxide content, the small FexOy type nanoclusters along with
bulk type particles are present on the zirconia matrix.
135
3.6.3 SEM STUDY
The scanning electron micrograph of SO42-
/xFe-Zr material is shown in Fig. 3.23. The pure
sulfated zirconia shows agglomerated particles with irregular shape and size (Fig. 3. 23).
Addition of iron species to sulfated zirconia matrix changes the morphology of the sulfated
zirconia significantly. At low iron content, the material appears to be nonhomogeneous with
irregular shape and size. With increasing the iron content to 10 mol% there appears to be certain
homogeneity in particle shape and size. The presence of iron seems to yield some morphological
control in the composite oxides as can be observed that the 50FeZr material show well dispersed
particles of spherical shape.
(a) (b)
(c) (d)
136
(e) (f)
Figure 3.23 SEM of (a) SO42-
/ZrO2 (b) SO42-
/2Fe-Zr(c) SO42-
/5Fe-Zr, (d) SO42-
/10Fe-Zr,
(e) SO42-
/20Fe-Zr and (f) SO42-
/50Fe-Zr
3.6.4 TEM STUDY
The transmission electron micrograph of SO42-
/10Fe-Zr material is shown in Fig. 3.24. The
electron micrograph indicates the presence of 20-40 nm size particles with irregular shape. The
particles are present in a highly agglomerated state.
Figure 3.24 Transmission electron micrograph of SO42-
/10Fe-Zr material
137
3.6.5 CATALYTIC STUDIES FOR SYNTHESIS OF XANTHENEDIONES
Xanthenes and their derivatives such as1,8-dioxo-octahydroxanthenes (xanthenediones) are an
important class of biologically active heterocyclic compounds which shows potential
antibacterial, antimicrobial, anti-inflammatory, antiviral, anti-depressants, antimalarial and
agricultural bactericide activities as well as efficiency in photodynamic therapy and antagonists
for paralyzing action of zoxazolamine (Lamkbert et al., 1997; El-Brashy et al., 2004; El Ashry et
al., 2006). These heterocyclics have also been used as dyes, flouerescent materials for
visualization of biomolecules and in laser technologies because of their valuable spectroscopic
properties (Ahmad et al., 2002; Menchen et al., 2003; Bhowmik and Ganguly, 2005).
Xanthenediones are found as a constituent unit in a large number of natural products and hence
they occupy a prominent position in the field of medicinal chemistry (Wang et al., 1997). Owing
to its attractive biological and pharmaceutical activity, the synthesis of this category of
heterocyclic compounds has received much attention in recent years. The most efficient way of
synthesizing xanthenediones involves the condensation of dimedones with aryl aldehydes in
presence of acidic catalysts. The various catalytic systems employed for this condensation
reaction include among others Amberlyst-15, ionic liquid, polyaniline-p-tolulenesulfonate salt,
Fe3+
-montmorillonite, HClO4–SiO2 and PPA–SiO2, sodium hydrogen sulfate and ZrOCl2.8H2O
(Das et al., 2006; John et al., 2006; Kantevari et al., 2007; Das et al., 2007; Song et al., 2007;
Fang et al., 2009; Mosaddegh et al., 2012). Most of the catalysts studied so far are homogeneous
catalysts or supported catalysts or polymers which suffer from drawbacks such as longer reaction
time, harsh reaction conditions, toxic solvents, deactivation, and handling of the catalyst. In this
investigation, we have evaluated the catalytic activity of SO42-
/xFe-Zrmaterials for the synthesis
of xanthenediones by condensation of dimedone and aryl aldehydes under solvent free conditions
using microwave as energy source (Scheme 3.7). Initially, the condensation reaction of
dimedone (2 mmol) and 4-NO2-benzaldehyde (1mmol) is taken as a model reaction and various
138
reaction parameters such as catalyst composition, catalyst amount, microwave power and
reaction stoichiometry are varied to obtain the optimized protocol.
CHO
+
O
O O
O O
SO42-/xFe-Zr
Microwave
R
R2
Scheme 3.7 One pot synthesis of xanthenediones using SO42-
/xFe-Zrcatalyst
Table 3.14 shows the catalytic activity of sulfate grafted zirconia based nanocomposite oxides
having different iron content. It has been observed that under the identical reaction condition,
SO42-
/10Fe-Zr nanocomposite oxide shows highest activity in a short reaction time for this
reaction. This catalyst is also found to show highest percent of sulfur retaining capacity on the
surface determined from the EDX analysis. The catalytic activities of the sulfated Fe-Zr
materials are found to correlate well with the sulfate content in the composite oxide (Table 3.14).
As described Arata and Hino, the sulfate ions act as bidentate ligands and coordinate to the
surface of the metal oxides. The surface metal ions which are coordinated to the sulfate species
act as Lewis acidic sites where as the coordinated water molecules are responsible for generation
of Bronsted acidity (Arata and Hino, 1990). In the presence study, the observation of an increase
in the sulfate retention capacity in presence of iron oxide suggests a possible enhancement in
acidity of the composite oxide which is reflected in the higher catalytic activity of the sulfate
grafted composite oxide.
139
Table 3.14 Catalytic activity of SO42-
/xFe-Zr nanocomposite catalysts
Entry Catalyst Sulphur
content (wt%)
Time (min) Yield (%)
1 SO42-
/ZrO2 1.8 20 72
2 SO42-
/2Fe-Zr 2.1 20 76
3 SO42-
/5Fe-Zr 2.3 18 79
4 SO42-
/10Fe-Zr 3.0 12 90
5 SO42-
/20Fe-Zr 2.6 12 84
6 SO42-
/50Fe-Zr 2.2 15 80
Hence, the SO42-
/10Fe-Zr catalyst was chosen for further catalytic experiments in the present
study. Table 3.15 shows the effect of microwave power on the model reaction. The microwave
power was varied between 360W to 900W. It has been observed that at higher irradiation power
(900W), the reaction time is considerably shortened and yield of the product is improved. Thus,
we have fixed the microwave power at 900 W for further reaction.
Table 3.15 Effect of microwave power on catalytic activity of SO42-
/10Fe-Zr catalyst
Entry Power(W) Time (min) Yield (%)
1 900 12 90
2 720 13 83
3 540 14 77
4 360 22 76
Table 3.16 shows the screening of catalyst amount on the model reaction. The result revealed
that, at low catalyst amount (50 mg) the time taken for completion reaction is high and yield is
less. For reaction involving 75 and 100 mg catalyst, the reaction time was found to be identical
however marginally better yield was observed in the latter case. Hence, in the present study, we
have fixed the catalyst amount 100 mg for efficient condensation of reactants.
140
Table 3.16 Effect of catalyst amount on catalytic activity of SO42-
/10Fe-Zr catalyst
After optimizing the various parameters, in order to further validate the generality of this
protocol, we have explored different aryl aldehydes bearing electron donating and withdrawing
groups. Under identical reaction conditions, all the aryl aldehydes reacted efficiently to give the
corresponding xanthenediones in high yield and purity (Table 3.17).
Table 3.17 SO42-
/10Fe-Zr catalyzed synthesis of xanthenediones under solvent free
condition and microwave irradiation.
Entry R Time (min) Yield (%)
1 H 14 80
2 2-Cl 16 80
3 4-Cl 12 83
4 2-NO2 15 84
5 3-NO2 11 90
6 4-NO2 12 90
7 4-OCH3 18 75
8 4-OH 14 80
9 2-OH 17 77
10 4-F 15 83
Entry Catalyst
amount
Time (min) Yield (%)
1 100 12 90
2 75 12 82
3 50 18 65
141
The para substituted substrates are found to give better yield of the products compared to the
ortho substituted aldehydes. After completion of the reaction, the catalyst was regenerated by
washing with 10 ml of ethyl acetate followed by heat treatment of the catalyst at 450oC for 2 h.
The regenerated catalyst was used for three consecutive cycles without any significant loss in
catalytic activity (Table 3.17, entry 6, 90%, 1st, 88% 2
nd, 83%, 3
rd).
Physical and spectral data of some selected compounds
3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-phenyl-2H-xanthene-1,8(5H,9H)-dione (Table 3.17,
Entry 1)
Mp: 200-202oC, IR (KBr): 3034, 2970, 1670, 1470, 1360, 1200, 1170, 1142, 740, 700 cm
-1;
1HNMR (400MHz, CDCl3): 1.00 (s, 3H, CH3); 1.12 (s, 3H, CH3); 2.16 (q, 4H, CH2); 2.48 (s,
4H, 2CH2); 4.76 (s, 1H, CH); 7.11-7.32 (m, 5H, Ar)
9-(4-chlorophenyl)-3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-2H-xanthene-1,8(5H,9H)-dione
(Table 3.17, Entry 3)
Mp: 231-232oC, IR (KBr): 3030, 2982, 1691, 1666, 1640, 1620, 1492, 1365, 1198, 1151, 1094,
1021, 747 cm-1
.; 1HNMR (400MHz, CDCl3): 1.04 (s, 3H, CH3); 1.10 (s, 3H, CH3); 2.11 (q, 4H,
CH2); 2.38 (s, 4H, 2CH2); 4.63 (s, 1H, CH); 7.09-7.16 (m, 2H, Ar)
3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-(3-nitrophenyl)-2H-xanthene-1,8(5H,9H)-dione
(Table 3.17, Entry 5)
Mp: 171-172oC, IR (KBr): 3035, 2965, 1680, 1630, 1537, 1365, 1210, 1162, 765, 735, 705 cm
-
1.;
1HNMR (400MHz, CDCl3): 0.94 (s, 3H, CH3); 1.05 (s, 3H, CH3); 2.11 (q, 4H, CH2); 2.43
(s, 4H, 2CH2); 4.75 (s, 1H, CH); 7.31 (t, 1H, Ar); 7.74 (d, 1H, Ar); 7.90-7.93 (m, 2H, Ar)
142
3.7 CONCLUSION
In the present investigation, surface and structurally modified zirconia materials have been
synthesized and their catalytic activities are evaluated for the synthesis of some biologically
important molecules. The MoO3-ZrO2 nanocomposite oxides synthesized by both coprecipitation
and combustion synthesis method exhibit the selective stabilization of the tetragonal phase of
zirconia. The particle size of the composite oxide calculated from Fourier line shape analysis of
the XRD profiles as well as TEM. The particle sizes are found to be in the range of 5-50 nm
depending on the method of preparation. Glycine as a fuel is particularly effective for the
synthesis of uniform nanoparticles with size in the range of 5-10 nm. The molybdina component
is present in a highly dispersed state in the form of isolated tetrahedral and polymolybdate
clusters in the composite oxide material. The combustion synthesized MoO3-ZrO2 composite
oxide has been explored for a rapid, convenient and environmentally benign synthesis of
biologically important octahydroquinazolinones under solvent-free conditions and in aqueous
medium. The combustion synthesized 10MoZr-G catalyst was highly active for the
multicomponent reactions generating a variety of structurally diverse octahydroquinazolinones
with good yield and purity of the products. The MoO3-ZrO2 composite oxide prepared by
coprecipitation method was found to be efficient heterogeneous catalytic system for synthesis of
a variety of amodialkyl naphthols and β-acetamidoketones. The sulfate grafted Fe2O3-ZrO2
nanocomposite oxides were prepared by coprecipitation and post sulfation steps. The Fe3+
ions
substitute for the Zr4+
ions in the zirconia lattice to form a solid solution. The presence of Fe2O3
improves the sulfate retension capacity as well as catalytic activity of the composite oxides. The
sulfate grafted Fe2O3-ZrO2 nanocomposite oxides is highly active for the microwave assisted
solvent free synthesis of a variety of octahydroxanthenes. The synthetic protocols developed in
this investigation using surface and structurally modified zirconia materials as catalyst are found
to be advantageous in terms of simple experimentation, preclusion of toxic solvents, recyclable
catalyst and high yield and purity of the synthesized compounds.
143
CHAPTER 4
CATALYTIC APPLICATION OF Zr-PILLARED CLAY BASED NANOPOROUS
MATERIAL FOR SYNTHESIS OF -AMINOCARBONYL COMPOUNDS
4.1 INTRODUCTION
Pillared clays are a class of microporous materials with high surface area and acidic property.
The intercalation of inorganic cationic clusters as pillars imparts thermal stability generating new
micropores and acidic sites in the clay materials (Gil et al., 2000; Cool et al., 2002; Mishra and
RangaRao, 2004; Sowmiya et al., 2007; Gil et al., 2008). In the past years, these materials have
been increasingly used as catalysts for several important catalytic reactions. The expansion in the
clay structure as a result of pillaring is typically of few molecular dimensions which have been
exploited successfully for shape selective reactions involving larger reactant molecules
(Choudary et al., 1990; Kikuchi and Matsuda, 1988; Gonzalez et al., 1992). In addition to the
shape selectivity, the chemical nature of the pillars can induce new property in the clay structure
in the form of catalytically active acidic and redox sites. The structural flexibility of the clay
lattice is largely responsible for the generation of pillared clay with diverse chemical property.
The Al- and Zr-pillared clays are the most widely investigated pillared clay system. The catalytic
activity of Al- and Zr-pillared clay have been studied for alkylation, isomerization,
disproportionation, dehydrogenation, hydrodesulfurization, esterification, nitrate reduction, one
pot synthesis of dihydropyrimidinones, synthesis of propylene glycol methyl ether,
thiochromans, and coumarins (Suzuki and Mori, 1990; Katoh et al., 1994; Matsuda et al., 1995;
Vijaya et al., 1998; Sun Kou et al., 2003; Singh et al., 2004; Mishra and Ranga Rao, 2004;
Sowmiya et al., 2007; Mishra and Ranga Rao, 2007; Kanda et al., 2009; Timofeeva et al.,2011).
In recent years, there have been efforts to modify the property of the pillared clays for specific
144
applications (Cool et al., 2002; Gil et al., 2008). The physicochemical characteristics of the
pillared clays are dependent upon several factors such as the genesis of the polycations, method
of intercalation, type of precursor material, washing and drying steps (Figueras, 1988; Cool and
Vansant, 1998; Gil et al., 2000; Cool et al., 2002). Addition of a second cation to the pillared
clay has also been done to improve the catalytic activity and thermal stability of the pillared clay.
The interaction of the second cationic components with the pillars in the microenvironment of
the clay is very crucial for the property of the solid, and indeed it has been shown that pillared
clay with different basal spacing and microporosity can be prepared with mixed cation pillaring
(Gil et al., 2008). The microporous nature and acidic properties of the clay materials can also be
tailored by altering the pillar density or in other words the lateral spacing between the pillars
(Suzuki et al., 1988; Horio et al., 1991; Jones and Purnell, 1994; Sychev et al., 2000; Sychev et
al., 2001, Mishra and Ranga Rao, 2004) It has been reported that the ion exchanged property of
the clay materials can be altered by heat treatment of ion exchanged clays containing smaller
cations in the interlayer. These cations upon heat treatment are fixed to the clay layer and are
inaccessible for exchange by other ionic species (Sposito et al., 1983; He et al., 2001). The
amount of the cation fixed in the interlayer can be varied by suitable choice of the pretreatment
temperature of the ion exchanged clay. Under such conditions, pillaring of clay can be done to
alter the pillar density in the interlayer of the clay materials (Jones and Purnell, 1994; Sychev et
al., 2000; Sychev et al., 2001). The main consequence of changing the pillar density in the clay
material is that the acidic and microporous nature of the pillared clay can be varied to the desired
value. In addition to the above described modifications, the catalytic activity of the pillared clay
materials can be modified by grafting various catalytically active species to the nanopillars and
dispersing catalytically active components in the microenvironment of clay materials. Several
145
catalytically active metal oxides such as sulfated SnO2, Ce-Cr-O, V2O5 and metal nano particles
such as Cu, Cu-Pd, Pd, Co, Pt dispersed in the micropores of the pillared clay have been
synthesized and their catalytic applications have been evaluated (Issaadi et al., 2006; Mishra and
RangaRao, 2007; Sowmiya et al., 2007; Pan et al., 2008; Gil et al., 2008; Molina et al., 2009;
2009a; Huang et al., 2010). It has been observed that 1 wt% Pd supported Al- and Zr-pillared
clay show enhanced acidity and catalytic activity for isopropoanol decomposition compared to
the parent pillared clay (Issaadi et al., 2006). Similarly, in case of Al-pillared clay supported
PdCu bimetallic system, the bimetallic particles show superior catalytic activity and selectivity
for nitrate reduction (Mishra and RangaRao, 2007). It has been also reported that, 4.8 % of Pt
supported on Al-pillared clay shows 90% efficiency for the hydrodechlorination of p-
chlorophenol using formic acid as hydrogen source at mild conditions (Molina et al., 2009). The
Cu supported Al-pillared clay system, shows high catalytic activity for the direct hydroxylation
of benzene to phenol (Pan et al., 2008). The presence of sulfated tin oxide in the pillared clay
lattice has been found to significantly increase the acidity and catalytic activity of Al-pillared
clay for synthesis of several biologically important molecules (Sowmiya et al., 2007). These
investigations on the supported pillared clay system clearly points to the fact that pillared clay
materials due to their high surface area and microporous nature are efficient support for
catalytically active components.
Heteropoly acids (HPAs) are a class of polyoxometallates with interesting tunable acidic and
redox properties (Kozhevnikov,1998). Several types of heteropoly acids with diverse structural
and chemical properties have been reported in open and patent literature. From catalysis point of
view, Keggin type heteropoly acids have been extensively investigated because of their higher
structural stability, Bronsted acidity, oxidation potential and resistance to deactivation by
146
hydrolysis (Kozhevnikov, 1998; Mizuno and Misono, 1998). Heteropoly acids such as
H3PW12O40 and H3PMo12O40 are known to possess well-defined Brønsted acidic sites. Like
strong mineral acids, HPAs are capable of generating carbocations from adsorbed olefins and
arenes (Kozhevnikov, 1998). The Brønsted acidity of HPAs in solid and liquid state has been
studied using several analytical techniques such as indicator titration, TPD, FT-IR study of
adsorbed probe molecules, microcalorimetry and NMR spectroscopy (Kozhevnikov, 1998;
Mizuno and Misono, 1998; Blaser and Studer, 1999). These studies revealed that the Bronsted
acidity of HPAs are stronger than most of the conventional solid acids such as SiO2-Al2O3,
H3PO4/SiO2, and HX and HY zeolites. Although heteropoly acids display promising surface and
catalytic properties, their application for catalytic reactions is limited by their low surface area
and solubility in most of the polar solvents. The heterogenization of the heteropoly acids is
highly desirable both from the point view of increasing surface area and recyclability aspects
(Mizuno and Misono, 1998; Naik et al., 2011). In general, heteropoly acids interact strongly with
supports at low loading levels, often resulting in grafting of these materials on solid surface.
Solids having basicity such as Al2O3 and MgO tend to decompose HPAs and are not preferable
to be used as support (Mizuno and Misono, 1998). The strength and number of acid centers of
HPAs can be controlled by parameters such as extent of hydration, type of support, thermal
pretreatment and the structure and composition of heteropolyanions. In recent years, there have
been extensive efforts to utilize the potential of the HPAs in fine chemical synthesis. New
synthetic protocols have been developed recently for synthesis of wide range of organic moieties
using heteropoly acids as catalysts which include among others deprotection of t-
butyldimethylsilane, dihydropyrimidone synthesis, regioselective aerobic oxygenation of
nitrobenzene to 2-nitrophenol and oxidation of aliphatic, benzylic and allylic alcohols using
147
dimethyl sulfoxides as oxygen transfer agents (Khenkin and Neumann, 2002; Kishore Kumar
and Bhaskaran, 2005; Khenkin et al., 2005; Kumar et al., 2006; Rocha et al., 2010; Naik et al.,
2011).
With an aim to enhance the catalytic efficiency of the Zr-pillared clay materials by post
treatment procedures, in the present investigation, we have prepared Zr-pillared clays by
insertion of [Zr4(OH)8(H2O)16]8+
into clay interlayer. The Zr-pillared clay is subsequently
modified by sulfate grafting and dispersion of phosphomolybdic acid (PMA) in the micropores
of the pillared clay materials. The catalytic activity of the modified pillared clay has been
evaluated for the synthesis of β-aminocarbonyl compounds by multicomponent condensation of
aryl aldehydes, ketones and aniline in aqueous media under ambient condition.
4.2 RESULTS AND DISCUSSION
4.2.1 XRD STUDY
The X-ray diffraction patterns of the parent clay along with Zr-P and PMA/SZr-P materials are
shown in Fig. 4.1 and the corresponding d001 spacing values are given in Table 4.1. The parent
clay shows relatively broad and intense reflections at 2 = 6.8o with basal spacing of 12.9Å. This
diffraction peak in figure4.1(b) has shifted to lower 2 values for the Zr-pillared samples
indicating an expansion in the layer structure as a result of pillaring. The Zr-pillared clay
prepared by intercalation of a refluxed ZrOCl2 solution shows a basal spacing of 20 Å. This
value is higher than the 17-18 Å typically observed for calcined Zr-pillared clay. Zirconium ion
is known to form stable tetrameric species in moderately concentrated acidic solution(Muha and
Vaughan, 1960). The structural formula of the complex is described as [Zr4(OH)8(H2O)16]8+
with
size 0.89 x 0.89 x 0.58 nm3.
148
4 8 12 16 20
c
b
a
Inte
nsi
ty (
a.u
.)
2 (degrees)
Figure 4.1 X-ray diffraction patterns of (a) clay (b) Zr-P (c) PMA/SZr-P materials
These tetrameric species are well characterized in solution and are considered to be the major
oligomeric species present in the pillaring solution (Vaccari, 1998; Cool and Vansant, 1996). The
structure of the complex ion in solution is described as square planar with Zr4+
ions located at the
corners of the square which are joined by two bridging OH ions along each edge (Fig. 4.2)
(Muha and Vaughan, 1960). The eight-fold coordination of Zr4+
ion shown in Fig. 4.2 is satisfied
by the oxygen atoms of four surrounded water molecules. Zirconium ions form larger polymeric
149
OH-
Zr4+
H2O
species in solution upon aging at elevated temperature under mild acidic condition (Baes and
Mesmer, 1976).
Figure 4.2 Structure of the [Zr4(OH)8(H2O)16]8+
pillaring polycation
The higher basal spacing observed for the Zr-P sample may be due to the further polymerization
of the tetrameric species under reflux conditions. Earlier studies on Zr-pillared clays prepared
from a refluxed ZrOCl2 solution show similar basal spacing in the range of 20-25 Å which has
been ascribed to the further polymerization and difference in the staking of the Zr-pillars in the
clay interlayer (Bruch and Warburton, 1986; Moreno et al., 1999; Mishra and Ranga Rao, 2003).
The PMA/SZr-P material prepared by supporting PMA on sulfate grafted Zr-pillared clay shows
a relatively broad reflection at 2 values of 4.7o with a basal spacing of 18.8 Å.
150
Table 4.1 Physicochemical characteristics and catalytic activity of the pillared clay
materials
Clay sample Basal spacing (Å) Surface area (m2/g) Pore volume (cm
3/g) Yield (%)
Clay 12.9 30.0 0.048 ----
Zr-P 20.0 162.5 0.195 42
SZr-P 20.2 170.2 0.202 68
PMA/SZr-P 18.8 132.5 0.074 78
The interlayer spacing observed for this material is 9.2 Å. This value suggests that the layer
structure of clay materials is retained in the PMA/SZr-P material due to the structural rigidity
achieved by pillaring process.
4.2.2 FT-IR STUDY
The IR spectra of the Zr-P, SZr-P and the PMA/SZr-P material along with the parent clay are
presented in Fig. 4.3. In the stretching frequency region (Fig.4.3 Panel I) all the materials shows
two intense and well defined bands with maxima at 3625-3640 and 3450 cm-1
. These bands can
be assigned to the O-H stretching vibration of the structural hydroxyl groups and the physisorbed
water molecules present in the interlayer of the clay sheets (Miller et al., 1982; Trillo et al.,
1992). The structural hydroxyl groups in the pillared clay contain contribution from the hydroxyl
group of the clay sheet as well as from the pillars (Bodoardo et al., 1994). Comparing the
intensity of these bands for the different clay samples, it is clear that the pillaring has a positive
impact on the water retention capacity of the clay minerals.
151
4000 3500 3000
I
d
c
b
a
wavenumber (cm-1
)
Inte
nsi
ty (
a.u
.)
1500 1750 2000
IIIII d
c
a
b
600 800 1000
d
c
b
a
Figure 4.3 FT-IR spectra of (a) clay, (b) Zr-P, (c) SZr-P and (d) PMA/SZr-P materials
In addition, the intensity of the band due to structural OH group at 3640 cm-1
is found to increase
with the process of pillaring and subsequent treatment with sulfate ions and PMA supporting
onto the surface. It has been reported that grafting of SO42-
ions induce significant electrophilic
character to the adjacent surface Zr ion which upon adsorption of water and subsequent
decomposition can provide surface hydroxyl group (Arata and Hino, 1990). Hence the IR band
with enhanced intensity observed at 3640 cm-1
for PMA/SZr-P material can be ascribed to the
combined contribution from the sulfate grafted pillars, PMA and clay sheets. The IR band
observed at 1630 cm-1
in Fig. 4.3 (panel II) is due to the bending vibration mode of OH groups.
Montmorillonite clay is known to possess two types of hydroxyl groups. One of them is more
152
labile with IR absorption pattern similar to that of liquid water and has been ascribed to the water
molecules present in the outer coordination spheres of the interlayer cations (Kloprogge et al.,
1999). The other type is more firmly held and associated with the water molecules directly
coordinated to the exchangeable cations. The later type also contributes significantly to the
absorption band at 1630 cm-1
and also responsible for the acidity of the material (Trilloet al.,
1992). The intense band observed at 1630 cm-1
for the PMA/SZr-P materials suggests the
possibility of increase in acidity due to sulfate grafting and anchoring of phosphomolybdic acid.
Montmorillonite clay shows a series of discrete peaks between 700 and 950 cm-1
and is
related to the structural OH-bending mode of the octahedral sheet (Sposito et al., 1983). In case
of parent clay, three IR bands were observed at 912, 840 and 795 cm-1
corresponding to the
bending vibration modes of the Al-Al-OH, Al-Mg-OH and Mg-Mg-OH groups, respectively, in
the octahedral layer of the montmorillonite (Sposito et al., 1983). These bands were found to be
preserved in the Zr-P and SZr-P materials. The PMA/SZr-P material shows additional bands at
963 (Mo=O), 872 (Mo-Oc-Mo) and 802 cm-1
(Mo-Oe-Mo) characteristics of the
phosphomolybdic acid. The occurrences of these bands in the PMA/SZr-P material indicate the
structural integrity of the phosphomolybdic acid in the composite catalyst.
4.2.3 UV-VIS STUDY
The UV-Vis spectra of the Zr-P, SZr-P and the PMA/SZr-P material along with the parent clay
are presented in Fig. 4.4. The parent Na-Montmorillonite clay displays a characteristic broad
band centered at 247 nm (Fig. 4.4a). This band is assigned to (Fe3+ O
2-, OH
- or OH2) charge
transfer transition for the structural iron present in the octahedral layer of the clay mineral
(Sposito et al., 1983). Incorporation of the Zr- nanopillars into the clay interlayer results in a
153
change in the absorption feature of the parent clay. For Zr-P material, an additional band was
observed at 210 nm.
200 300 400 500 600 700
PMA-Zr-P
SZr-P
Zr-P
Clay
d
c
b
a
Inte
nsi
ty (
a.u
.)
Wavelength (nm)
Figure 4.4 UV-Vis spectra of (a) clay (b) Zr-P, (c)SZr-P and (d) PMA/SZr-P materials
This band can be assigned to the Zr4+ O
2- charge transfer transition arising out of the zirconia
nanopillars present in the clay interlayer (Gao et al., 1999). Zirconia is a direct band gap
insulator which shows interband transition in the UV region of the electronic spectrum
corresponding to the O2-
(2p) Zr4+
(4d) transition. Among the polymorphic forms of zirconia,
the octacoordinated tetragonal (space group Fm3m) and cubic (space group P42/nmc) phases
154
show UV absorption maxima in the range of 200-210 nm whereas the hepta coordinated
monoclinic phase (P21/c) shows UV maxima around 240 nm (Lo´pezet al., 2001). It has been
observed that as the coordination number of Zr4+
ion decreases from 8 to 6 the LMCT absorption
maxima progressively shifts to lower energy (higher wavelength) (Gao et al., 1999; Lo´pez et al.,
2001; Li et al., 2001; Zhao et al., 2008). In case of Zr-P material, it is likely that extremely small
zirconium dioxide nanoclusters exist as pillars with octacoordination in the clay interlayer.
Sulfate grafting onto the zirconia pillars results in an increase in the intensity of the LMCT
transition along with a shift in the band maxima to 204 nm (Fig. 4.4c). The apparent increase in
the band intensity along with the blue shift can be ascribed to a change in the coordination
environment of the Zr4+
ions in the pillars. It has been proposed that the sulfate ion act as a
bidentate ligand and attached to two adjacent Zr4+
ions through the oxygen atoms of the SO42-
ions (Arata and Hino, 1990). The sulfate grafting results in a change in the coordination
environment of the zirconium ion which can influence the UV-Vis spectrum. The PMA/SZr-P
material shows two additional bands with maxima at 236 and 311 nm. These bands are
characteristics of various oxygen to metal charge transfer transitions observed for
phosphomolybdic acid (Zhang et al., 2002; Carriazo et al., 2008).
4.2.4 NITROGEN SORPTION
The surface area and pore structures of the clay materials have been studied using nitrogen
sorption. All pillared clay samples show high initial adsorption with nearly horizontal plateau up
to relative pressure of ~ 0.9 (Fig.4.5). This behavior is characteristic of microporous solids
(Gregg et al., 1982).
155
0.0 0.2 0.4 0.6 0.80
10
20
30
40
50
60
70
80
90
100
110
120
130
PMA/SZr-Pc
b
a Clay
Va
ds (
cc/g
)
P/P0
Zr-P
Figure 4.5 N2adsorption–desorption isotherms of (a) clay (b) PMA/SZr-P and (c) Zr-P
The sorption isotherms observed for these pillared clays are Type I according to the BDDT
classification indicating the presence of micropores. It has also been observed that the initial
nitrogen adsorption is enhanced in the order, clay < PMA/SZr-P<Zr-P. The hysteresis found in
these materials is H3 type according to IUPAC classification and attributed to slit shaped pores
or plate like particles with space between the parallel plates (Kloprogge et al., 1999). The surface
areas and pore volumes of various clay samples are presented in Table 4.1. It is seen that surface
area increases in the order of parent clay < PMA/SZr-P<Zr-P SZr-P. Pillaring with the
zirconium polycations has a large impact on the surface area of the clay materials. During the
process of pillaring, the expansion in the clay structure and the desegregation of the clay particles
largely contributes to the enhancement of the surface area and porosity of the clay materials. The
decrease in the surface area (132.5 m2/g) and pore volume (0.074 cm
3/g) observed for the
156
PMA/SZr-P sample in comparison with Zr-P or SZr-P can be ascribed to the partial blocking of
the micropores by the PMA particles.
4.2.5 SEM STUDY
The scanning electron micrograph of the Zr-P, SZr-P and PMA/SZr-P material is shown in Fig.
4.6. The calcined Zr-P material (Fig. 4.6(a)) show large agglomerated particles of irregular shape
and size. After sulfate treatment, these materials show flake like textures on the surface probably
due to delamination of the particles. When crystallize from a dilute suspension of polar solvents,
pillared clay materials tends to arrange themselves in the (001) direction maximizing the face to
face interaction.
(a)
.
157
(b)
(c)
Figure 4.6 Scanning electron micrographs of (a) Zr-P, (b) SZr-P and (c) PMA/SZr-P
158
However, heat treatment causes significant disordering to the face-to-face stacking structure,
most likely because of edge-to-face and/or edge-to-edge interactions (Sychev et al., 1997).
Treatment with dilute mineral acid can cause delamination of the structure resulting in flake like
morphology (Fig. 4(b)). The PMA/SZr-P material (Fig. 4.6(c)) shows irregular particles with
folded morphology probably due to the interaction between the HPA particles and the clay
layers.
4.2.6 TEM STUDY
The transmission electron micrograph of the phosphomolybdic acid particles dispersed in the
micropores of the sulfate grafted Zr-pillared clay is shown in Fig. 4.7.
Figure 4.7 Transmission electron micrograph of PMA/SZr-P material
159
The material is found to contain nanosize phosphomolybdic clusters with irregular shape and
sizes in the range of 5–15 nm. The pillared clay, due to its high surface area and uniform
micropores (170.0 m2/g and 0.202 cm
3/g) is quite effective in the dispersion of the
phosphomolybdic acid clusters in the clay microenvironment. TEM study of the PMA/SZr-P
material clearly indicates the presence of nanosize phosphomolybdic acid clusters in a well
dispersed form in the sulfate grafted zirconia-pillared clay matrix.
4.3 CATALYTIC STUDIESFOR SYNTHESIS OF -AMINOCARBONYL
COMPOUNDS
Carbon–carbon bond forming reactions continue to be the central focus of research in organic
synthesis (Mannich and Krosche, 1912). Among this category of reactions, the Mannich reaction
which involves the synthesis of -aminocarbonyl compounds is of considerable importance
because of its usefulness in synthesis of drugsand biologically attractive molecules containing
nitrogen atom (Arendet al., 1998; Kobayashi et al., 1999; Cordova et al., 2004). Since many
natural products contains nitrogen atom, the Mannich reaction provides a basis for development
of such molecules. The products obtained in the Mannich reaction act as valuable synthetic
intermediate for synthesis of amino alcohols, peptides and lactams and as precursors to
synthesize amino acids. In the Mannich reaction route, the -aminocarbonyl compounds are
synthesized by one pot three component condensation of aldehydes, amines and ketones. Various
catalysts studied for this reaction include Lewis and Bronsted acid, organometallic compounds,
lanthanides, and polymer-support sulfonic acid in different solvents (Kobayashiet al., 1996;
Notzet al., 2001;Donget al., 2005; Wanget al., 2005; Periasamyet al., 2005; Li al., 2009).
However, most of the catalytic system, suffer from drawbacks such as long reaction time,
160
toxicity, harsh reaction condition, low yield, recyclability, expensive metal salt as catalyst and
their usage in stoichiometric amounts.In the recent years, several improved catalytic systems
such as I2-Al2O3, Cu nanoparticles, functionalized ionic liquid and sulfated zirconiahave been
reported in the literature for this reaction (Wanget al., 2007; Wuet al., 2007; Kidwaiet al., 2008;
Kidwaiet al., 2009; Fanget al., 2009; Xiaet al., 2010; Rajbangshiet al., 2011; Sharma et
al.,2011). With an aim to develop environmentally benign protocol involving heterogeneous and
recyclable catalyst, we have used the PMA/SZr-P material as efficient catalyst for the synthesis
of -aminocarbonyl compounds in aqueous media (Scheme 1). Initially, different clay based
materials were applied as catalyst for the condensation of benzaldehyde, aniline and
acetophenone under ambient condition (Scheme 1).
R1 CH3
O
+
CHO
+
NH2
R1
HNO
PMA/SZr-P
room temperature
4 ml H2O
X
X
Scheme 1 Synthesis of -amino carbonyl compounds using PMA/SZr-P as catalyst
It was observed that among all the clay catalysts the PMA/SZr-P material shows highest catalytic
activity (Table 1). The PMA/SZr-P catalyst was chosen for further exploration for the synthesis
of -aminocarbonyl compounds. The catalyst amount in the reaction mixture was varied in the
range of 25-100 mg.
161
Table 2 PMA/SZr-P CATALYZED SYNTHESIS OF -AMINOCARBONYL
COMPOUNDS
Sl. No R1 X Time (h) Yield (%)
1 Ph H 5.0 78
2 Ph 2-NO2 4.0 86
3 Ph 3- NO2 4.0 82
4 Ph 4- NO2 4.0 90
5 Ph 4-OH 5.0 76
6 Ph 4-Cl 3.5 84
7 Ph 2-Cl 3.5 88
8 Ph 4-OCH3 5.0 75
9 CH3 H 5 82
10 CH3 2-NO2 3.5 88
11 CH3 3-NO2 4.5 86
12 CH3 4-NO2 3.5 89
13 CH3 4-OH 3.5 78
14 CH3 2-Cl 3.5 90
15 CH3 4-Cl 3.5 86
For 1 mmol of the reactants, 50 mg of the catalyst is ideally suited for the effectual condensation
of benzaldehyde, aniline and acetophenoneto yield the corresponding -aminocarbonyl
compound in 78% yield (Table 2, entry 1). Further increase in the catalyst quantity in the
reaction mixture does not have any marked impact on the product yield. In order to validate the
162
applicability of the protocol, aryl aldehydes bearing electron donating and withdrawing groups
are used in place of benzaldehyde. Under identical reaction conditions all the aryl aldehydes
reacted efficiently to give the corresponding -aminocarbonyl compounds in high yield and
purity. Further, acetoephenone was replaced with acetone. The reaction was found to proceed
with equal ease both in case of aliphatic and aromatic ketones. After completion of the reaction,
the used catalyst was filtered from the reaction mixture and regenerated by washing with 10 ml
of methanol followed by heat treatment at 200˚C for 1h. The regenerated catalyst was reused for
three consecutive cycles without significant loss in catalytic activity (Table2, entry 9, 1st 82%,
2nd
79%, and 3rd
77%).
In the present work,PMA/SZr-P catalyst was demonstrated as an efficient catalyst for Mannich
reaction in aqueous media at room temperature. The protocol developed using the PMA/SZr-P as
catalyst was found to be advantageous in terms of simple experimentation, preclusion of toxic
solvents, use of aqueous reaction media, high yield and purity of the products.
Spectral data of some of the representative compounds
1) 1, 3-diphenyl-3-(phenylamino)propan-1-one (Table 5.2, Entry 1)
M.P: 168-170oC, IR (KBr): 3384, 3020, 2923, 1670, 1600, 1512, 1290, 860 cm
-1
1HNMR (400MHz, CDCl3): δ 3.45-3.36 (m, 2H, CH2), 4.90 (t, 1H, CH), 6.45 (d, 2H, Ar-H),
6.54-6.61(m, 1H, Ar-H). 7.00-7.06 ( m, 2H, Ar-H), 7.16 (d, 2H, Ar-H), 7.22-7.28 (m, 1H, Ar-H),
7.35-7.43 (m, 2H, Ar-H), 7.47-7.52 (m, 1H, Ar-H), 7.55-7.61 (m, 2H, Ar-H), 7.80 (d, 2H, Ar-H).
2) 3-(4-Nitro-phenyl)-1-phenyl-3-phenylamino-propan-1-one(Table 5.2, Entry 5)
M.P: 104-106oC, IR (KBr): 3374, 1680 cm
-1
163
1HNMR (400MHz, CDCl3): δ 3.35(2H, d), 5.05 (t, 1H), 6.50 (2H, d, Ar-H),6.65-6.70 (m, 1H,
Ar-H), 7.05-7.10 (m, 2H, Ar-H), 7.45-7.49 (m, 2H, Ar-H), 7.54-7.60 (m, 2H, Ar-H), 7.68 (d, 2H,
Ar-H), 7.85 (d, 2H, Ar-H), 8.20 (d, 2H, Ar-H).
4.4 CONCLUSIONS
In this chapter, we have discussed a novel application of the Zr-pillared clay based catalyst for
one pot multicomponent synthesis of -aminocarbonyl compounds by classical Mannich route.
The catalytic efficiency of the Zr-pillared clay was enhanced significantly by sulfate grafting and
subsequent dispersion of phosphomolybdic acid. The expansion of the clay interlayer as a result
of pillaring and the retention of the interlayer structure of the pillared material in the composite
catalyst was confirmed from the XRD investigation. The structural integrity of the
phosphomolybdic acid was ascertained from IR study. The apparent increase in the peak
intensity of the structural OH groups in the PMA/SZr-P material was interpreted as due to the
formation of new structural hydroxyls as a result of pillaring and sulfate grafting. UV-Vis spectra
of the clay materials indicate the presence of extremely small clusters of ZrO2 as pillars in the
clay matrix. Catalytic studies performed on these materials indicate that sulfate grafting and
subsequent dispersion of phosphomolybdic acid substantially enhances the catalytic activity of
Zr-pillared clay. The PMA/SZr-P material was found to be highly efficient for the synthesis of -
aminocarbonyl compounds under ambient condition in aqueous media. A wide variety of
substrates reacted under the optimized protocols to give the corresponding -amino carbonyl
compounds in high yield and purity. The protocol developed in this
investigation using the PMA/SZr-P material was found to be advantageous in terms of simple
experimentation, use of water as green and ecofriendly solvent, recyclable catalyst and high yield
and purity of the product.
164
CHAPTER 5
CATALYTIC APPLICATION OF CeO2 BASED NANOCOMPOSITE OXIDES FOR
SYNTHESIS OF SOME BIOLOGICALLY IMPORTANT MOLECULES
Part A: CeO2-CaO NANOCOMPOSITE OXIDES
5.1 INTRODUCTION
Heterogeneous base catalyzed organic synthesis is a promising field of research with potential
application in pharmaceuticals and related fine chemical industries (Busca, 2010; Corma and
Iborra, 2006; Kumar et al., 2007; Xi and Davis, 2011; Boey et al., 2011). Among several
heterogeneous base catalysts, the alkali and alkaline earth metal oxides and layered double
hydroxides are most promising (Busca, 2010; Corma and Iborra, 2006). Among the basic oxide
materials, MgO has been extensively investigated for several industrial reactions (Busca, 2010;
Kumar et al., 2007; Xi and Davis, 2011). Calcium oxide is another interesting basic oxide which
shows well defined surface basicity and catalytic activity for several important reactions such as
biodiesel production, retroaldol condensation, dimethyl carbonate synthesis and cyclohexanol
conversion reactions (Boey et al., 2011; Wang et al., 2005; Hsin et al., 2010; Albuquerque et al.,
2008). Although CaO display promising catalytic properties, its application is limited by its low
surface area and solubility in H2O and CO2 (Corma and Iborra, 2006). The heterogenization of
the CaO is highly desirable from the point of view of increasing surface area, basic sites, stability
and recyclability aspects. The catalytic activity of CaO supported on a variety of inorganic
matrix such as ZrO2, SiO2, SBA-15, MCM-41, TiO2, V2O5/TiO2, ZnO, Al2O3 and MgO have
been reported in literature (Wang et al., 2005; Hsin et al., 2010; Albuquerque et al., 2008; Reddy
et al., 2006; Alba-Rubio. et al., 2010; Matsuhashi et al., 2011; Taufiq-Yap et al., 2011).
Recently, the promoting effect of ceria in catalytic reactions involving acid-base catalysis has
165
been established (Mishra and Ranga Rao, 2006; Sato et al., 1998; Beier et al., 2009). Several
ceria based composite oxides such as CeO2-ZnO, CeO2-SnO2, CeO2-MgO, CeO2-ZrO2, CeO2-
CuO, CeO2–Fe2O3 and CeO2-NiO have been reported to catalyze organic transformations (Sato et
al., 1998; Jyoti et al, 2000; Mishra and Ranga Rao, 2006; Ayastuy et al., 2011; Li et al., 2011;
Solsona et al., 2012). The redox property (Ce3+
Ce4+
) and weak basicity of CeO2 have been
found to be responsible for promoting a number of reactions. Ceria is reported to activate
methanol and responsible for higher catalytic activity during the alkylation of phenol over CeO2-
MgO mixed oxide catalysts (Sato et al., 1998). The presence of ceria in ZnO matrix has been
found to accelerate the rate of cyclohexanol conversion and hydrogen transfer reactions (Mishra
and Ranga Rao, 2006). The acid-base property of ceria and ceria-based composite oxide
materials have been studied using microcalorimetry (Cutrufello et al., 1999), in-situ IR study of
adsorbed probe molecules such as pyridine, CO2 (Zaki et al., 2001) and using model reactions
(Martin and Duprez, 1997). Although these studies vary to a certain degree on the assessment of
the surface acid-base character, it is certain that the surface acidic property is generated due to
the presence of surface Ce4+
and OH+ species and the basic property due to O
2- and OH
- ions
(Martin and Duprez, 1997). Martin and Duprez (Martin and Duprez, 1996) have studied a series
of catalytically important oxides by using model reactions and CO2 adsorption. These authors
have suggested the presence of strong basic sites on ceria surface. The basic property is also
found to correlate well with the oxygen mobility in ceria lattice (Martin and Duprez, 1996).
Recently, there are reports available on the catalytic applications of CeO2-CaO composite oxides
for biodiesel production, low temperature water gas shift reaction and for application in solid
oxide fuel cells (Zhu et al., 2003; Yu et al., 2011; Linganiso et al., 2011). In the present
investigation, we have synthesized a series of Ce-Ca-O nanocomposite oxides and demonstrated
166
their effectiveness as a heterogeneous basic catalyst for the synthesis of biologically important 2-
amino-2-chromenes in aqueous media. The Ce-Ca-O composite oxides were synthesized by
amorphous citrate method and characterized using XRD, TGA, UV-Vis-DRS, SEM and TEM
experimental techniques.
Amorphous citrate method is one of the versatile, cost-effective, less time consuming, and
solution based technique for synthesis of nanoparticle. In this method, the metal precursor is
mixed with an organic compound containing polycarboxylic acids (citric acid) for the
complexation of the metal ion. This method is most convenient for the synthesis of oxide based
nanomaterial. The amorphous citrate method have been used effectively for the synthesis of
single and composite oxides such as ZrO2, SnO2, CeO2-MgO, CeO2-ZnO having excellent
properties and catalytic activity (Sato et al., 1998; Bhagwat et al., 2003; Bhagwat and
Ramaswamy; 2004; Mishra and Ranga Rao, 2006).
5.2 RESULTS AND DISCUSSION
5.2.1 XRD STUDY
Fig. 1 shows the X-ray diffraction patterns of the CaO, CeO2 and xCe-Ca-O nanocomposite
oxides prepared by amorphous citrate method. The peak position (2θ), full width at half
maximum (FWHM) and intensity of the peaks are calculated using commercially available
software (PEAK FIT) for each peak of the XRD data. The indexing of all peaks of XRD patterns
were carried out using 2θ and intensity value of each peak by a standard computer software
POWD. The pure CaO shows well defined and intense diffraction peaks at 2 values of 32.0,
37.7, 53.7, 64.0 and 67.2 degrees corresponding to the presence of cubic crystal structure
associated with the reflections from (110), (200), (220), (311), and (222) planes, respectively
(JCPDS No: 37-1497 ) (Fig. 1a). In addition to the reflections from the cubic CaO phase, three
167
minor and less intense peaks are observed at 2 values 33.90, 47.0
0, 50.7
0 which can be assigned
to Ca(OH)2 phase (JCPDS file no: 84-1274).
20 30 40 50 60 70
(311)(220)
(200)
g
f
e
d
c
b
Inte
nsi
ty (
a.u
.)
2 (degrees)
a
(110) (200) (220)
(311) (222)** *
* Ca(OH)2
Phase(111)
Figure 5.1 X-ray diffraction patterns of (a) CaO, (b) 5Ce-Ca-O, (c) 10Ce-Ca-O, (d)
20Ce-Ca-O (e) 50Ce-Ca-O, (f) 80Ce-Ca-O and (g) CeO2
168
Pure CeO2 shows the characteristic reflections at 2 values of 28.5, 33.0, 47.3, 56.2 degrees
corresponding to the reflection from (111), (200), (220), and (311) planes respectively, of the
fluorite structure of ceria (JCPDS No: 34-394) (Fig. 1g). Addition of 5% CeO2 to CaO matrix,
results in significant changes in the XRD patterns of CaO. The peak intensity of the CaO phase is
found to decrease drastically whereas a weak reflection associated with fluorite structure of ceria
is observed in the XRD pattern (Fig. 1b). With increase in the Ce:Ca molar ratio, the XRD peaks
associated with the fluorite phase become more intense, and concomitantly those of CaO
decreases significantly. This can be attributed to the higher X-ray scattering factor of Ce4+
compared to the Ca2+
ions (West, 2007). For 50Ce-Ca-O composite oxide, XRD peaks
corresponding to the fluorite structure are only observed (Fig. 1e). Calcium oxide is known to
form substitutional solid solution with ceria. Rodriguez et al have reported the formation of Ce1-
xCaxO2-x type solid solutions for Ce-Ca-O composite oxide synthesized by microemulsion
method (Rodriguez et al., 2003). These authors have reported the formation of solid solution
phase upto 33% doping of Ca into the ceria matrix. The fluorite type ceria structure has been
retained in the mixed oxide and there is no appreciable shift in the XRD peaks as a result of Ca2+
incorporation. The formation of the solid solution has been further verified using XANES and
Raman studies (Rodriguez et al., 2003). In line of the observation by Rodriguez et al, in the
present study it is believed that a substitutional type solid solution is formed for higher ceria
content whereas for calcia rich phases, there is a mixture of CaO and Ce1-xCaxO2-x type solid
solutions in the composite oxide. As the CeO2 and CaO peak position does not change
appreciably in the composite oxide, the relative peak area and the integral width (peak area
divided by height) of the ceria (111) peak and CaO (200) peak in the composite oxides are
calculated and plotted in Fig. 5.2.
169
0 20 40 60 80 100
0.0
0.2
0.4
0.6
0.8
1.0
CaO (200) peak
CeO2 (111) peak
Rel
ati
ve
pea
k a
rea
CeO2 content (mol%)
(I)
0 20 40 60 80 1000.1
0.2
0.3
0.4
0.5
0.6
CeO2 (111) peak
CaO (200) peak
Inte
gal
wid
th
CeO2 content (mol %)
(II)
Figure 5.2 Variation of (I) relative peak area and (II) integral width with ceria content
in the composite oxide
170
At low ceria content, there is rapid increase in relative peak area for the ceria peak, whereas at
higher loading the increase is more gradual. The reverse trend is observed for the integral width.
For CaO although there is rapid decrease in relative peak area, the integral width does not change
appreciably with ceria content upto 50 %. Beyond 50% ceria content, the CaO phase does not
show diffraction peaks in the XRD profiles of the composite oxide. This observation suggests
that at low ceria content, the mixed oxide particles are of extremely small size and the particle
size increases with increase in ceria percentage. The particle size of the CaO phase on the other
hand does not vary much with the ceria content. In order to further validate this observation,
Fourier line profile analysis from the broadened XRD profiles of the composite oxides are
carried out. Using the Fourier line shape analysis method the crystallite size and rms stain of the
mixed oxide and CaO phase are calculated for all the composite oxide samples following the
Warren and Averbach method (Warren and Averbach, 1950) using software BRAEDTH and the
details are available in literature (Balzar and Ledbetter, 1993). The calculated volume-weighted
distributions (PV), and size coefficient (AS) as function of the Fourier length (L) for the mixed
oxide phase for different Ce-Ca-O materials are given in Fig. 5.3.
171
0 200 400 600 800 1000 1200
0.0
0.2
0.4
0.6
0.8
1.0
5Ce-Ca-O
10Ce-Ca-O
20Ce-Ca-O
50Ce-Ca-O
80Ce-Ca-O
CeO2
Fou
rie
r s
ize c
off
icie
nt
(As)
L (A0
)
I
0 200 400 600 800 1000 1200
0.000
0.002
0.004
0.006
0.008
5Ce-Ca-O
10Ce-Ca-O
20Ce-Ca-O
50Ce-Ca-O
80Ce-Ca-O
CeO2
PV
Fourier Length (A0)
II
Figure 5.3 Fourier line profile analysis plot of mixed oxide phase in the Ce-Ca-O
composite oxides
172
The wide distribution function observed for the 80Ce-Ca-O and 50Ce-Ca-O samples (Fig. 5.3II)
indicate that the particles are polycrystalline and nonhomogeneous with larger particle size
distribution. The 5Ce-Ca-O, 10Ce-Ca-O and 20Ce-Ca-O materials show a comparable narrower
size distribution function (Fig. 3II). The crystallite sizes of the mixed oxide phase are calculated
from the intercept of the initial slope of the AS~L curve (Fig. 5.3 I) and are presented in Table
5.1.
Table 5.1 Physicochemical characteristics of xCe-Ca-O composite oxides
Catalyst Crystallite size (nm) Direct Band
gap (eV) CeO2 phase CaO phase
CaO
5Ce-Ca-O
10Ce-Ca-O
20Ce-Ca-O
50Ce-Ca-O
80Ce-Ca-O
CeO2
--
5
9
15
21
22
16
26
33
20
35
--
--
--
--
3.66
3.62
3.57
3.45
3.53
3.57
Upto 20% ceria content, the particle size of the mixed oxide phase are very small (<15 nm)
where as at higher ceria content larger particle size was observed. Similar calculations were
carried out for the CaO phase in the composite oxides. The PV~L and AS~L plots of the CaO
phase in the composite oxides are presented in Fig. 5.4 (panel I and II). Wide distribution
function is observed for the 20Ce-Ca-O sample whereas the 5Ce-Ca-O and 10Ce-Ca-O materials
173
show a comparable narrower size distribution function (Fig. 5.4II). The calculated crystallite
sizes of the CaO phase in the xCe-Ca-O composite oxide are also presented in Table 5.1.
0 200 400 600 800 1000 1200 1400
0.0
0.2
0.4
0.6
0.8
1.0
CaO
5Ce-Ca-O
10Ce-Ca-O
20Ce-Ca-O
Fou
rier
siz
e c
off
icie
nt
(As)
L (A0
)
I
0 200 400 600 800 1000 1200 1400 1600 1800 2000
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030 CaO
5Ce-Ca-O
10Ce-Ca-O
20Ce-Ca-O
pV
Fourier Length (A0
)
II
Figure 5.4 Fourier line profile analysis plot of CaO phase in the Ce-Ca-O composite
oxides
174
The particle size for the CaO phase is found to be in the range of 25-35 nm. The calculated
crystallite size does not vary significantly with ceria content (Table 5.1). The lattice strains of the
CaO and mixed oxide phases in the composite oxides were calculated from the Fourier analysis
and are presented in Fig. 5.5. With increase in particle size the lattice strain is found to be
decreased for both phases present in the composite oxide.
0 10 20 30 40 50 60 70 80 90 1000
5
10
15
20
25
Ceria (mol %)
Cry
sta
llit
e si
ze (
nm
)
1.2x10-3
1.8x10-3
2.4x10-3
3.0x10-3
3.6x10-3
4.2x10-3
4.8x10-3
5.4x10-3
6.0x10-3
size
strain
<e2
>1
/2
0 10 20 30 40 50 60 70 80 90 10010
15
20
25
30
35
Calcium oxide (mol %)
Cry
sta
llit
e s
ize (
nm
)<
e2
>1
/2
size nm
strain
1.6x10-3
1.8x10-3
2.0x10-3
2.2x10-3
2.4x10-3
2.6x10-3
2.8x10-3
3.0x10-3
Figure 5.5 Variation of the crystallite size and lattice strain with ceria and calcium oxide
content in the Ce-Ca-O composite oxide
175
5.2.2 TG ANALYSIS
The TG analysis of the amorphous citrate precursor for the 20Ce-Ca-O material is shown in Fig.
5.6. A major weight loss region is observed between 375-675oC corresponding to the
decomposition of the citrate precursor to yield the composite oxide material. The percentage
weight loss corresponding to this temperature region was calculated to be 27.5 %.
200 400 600 800 100050
60
70
80
90
100
Temperature (oC)
We
igh
t lo
ss (
%)
-250
-200
-150
-100
-50
0
Figure 5.6 TG analysis plot of the amorphous precursor of 20Ce-Ca-O material
This weight loss corresponds to the total decomposition of all the citric acid molecules present in
the amorphous precursor. In the amorphous citrate method, it is believed that the removal of the
water of hydration is key to the dispersion of the metal cations in the citrate matrix. The citric
acid act as a chelating agent and help in the dispersion of the metal component (Sato et al., 1998;
Ma et al., 2003; Mishra and Ranga Rao, 2006). Upon heat treatment at 160oC, the nitrate ions
decompose resulting in liberation of gas molecules. The citrate molecules are retained in the
176
amorphous precursor and upon calcinations they decompose to gaseous products which occur in
the range of 375-675oC.
5.2.3 UV-VIS-DRS STUDY
The UV-Vis-DRS spectra of the xCe-Ca-O nanocomposite oxides are presented in Fig. 5.7. Pure
CaO is an insulator material with band gap of ~ 6.5 eV and does not show any appreciable
absorption in the UV region of the spectrum. Cerium dioxide on the other hand, is a n- type
semiconductor which absorbs strongly in the UV region with absorption threshold near 400 nm
(Fig.7a). The band gap of crystalline bulk phase ceria particle has been reported to be ~ 3.19 eV
(Orel and Orel, 1994). In the present study, pure ceria shows two absorption maxima at 275 nm
and 330 nm. The absorption band at 330 nm can be assigned to the interband transition whereas
the 275 nm peak is due to the Ce4+
O2-
charge transfer transition. Addition of ceria to CaO
matrix resulted in significant change in the absorption feature of the composite oxide. For
samples upto 20% ceria content, a prominent absorption maxima is observed around 290 nm.
This band can be ascribed to the Ce4+
O2-
charge transition transitions occurring on surface low
coordinated Ce4+
sites. Such bands have been observed earlier for ceria nanoparticles supported
on various inorganic matrices such as silica, alumina and zinc oxide (Bensalem et al., 1992; Zaki
et al., 1997; Ho et al., 2005; Mishra and Ranga Rao, 2006; Maensiri et al., 2007; Phoka et al.,
2009).
177
200 300 400 500 600 700 800
c
d
f
e
b
F(R
) (a
.u.)
wave length (nm)
a
Figure 5.7 UV-Vis-DRS spectra of (a) 5Ce-Ca-O, (b) 10Ce-Ca-O, (c) 20Ce-Ca-O, (d)
50Ce-Ca-O, (e) 80Ce-Ca-O and (f) CeO2
It has been reported that small supported ceria nanoparticles show high surface to volume ratio
and a significant fraction of the Ce4+
ions exist on the surface. The coordination number of
surface Ce4+
ions can vary between 4 and 8, the later being the coordination number of bulk Ce4+
in the fluorite structure (Bensalem et al., 1992; Zaki et al., 1997; Mishra and Ranga Rao, 2006).
These surface Ce+4
sites are primarily responsible for the band at 290 nm. Moreover, due to Ca
incorporation, there is a posibility of oxygen vacancy in the lattice. This vacancy can effectively
modify the coordination number of Ce4+
ions and responsible for the observed spectral feature.
When the ceria content is increased further, absorption features corresponding to bulk like ceria
particles are observed for 50Ce-Ca-O and 80Ce-Ca-O samples. In addition, for Ce-Ca-O
178
composite oxide with less than 20% ceria, there is considerable blue shift of the absorption edge.
In order to correlate the band gap energies with grain sizes, the band gap of the composite oxides
are calculated from their UV-Vis spectra. Fig. 5.8 shows the plot of the h vs [F(R)h ]2 for
different composite oxides.
2 3 4 5 60
100
200
300
a
2 3 4 5 60
300
600
900
1200b
2 3 4 5 60
1000
2000
3000
4000
c
2 3 4 5 60
500
1000
1500
2000
2500 d
[F(R
)*h
v]2
2 3 4 5 60
400
800
1200
1600
e
Photon energy (eV)
2 3 4 5 60
500
1000
1500
2000 f
Figure 5.8 Plots of [F(R)h ]2 as a function of photon energy for (a)5Ce-Ca-O, (b) 10Ce-
Ca-O, (c) 20Ce-Ca-O (d) 50Ce-Ca-O, (e) 80Ce-Ca-O and (f) CeO2
The direct band gap energy (Eg) are calculated from the abscise interception in Fig. 5.8 and are
presented in Table 5.1. The calculated values are within the range described in the literature and
imply the existence of small nanoparticles in the composite oxide (Scheithauer et al., 1998; Yin
et al., 2002; Chen et al., 2005; Ho et al., 2005; Maensiri et al., 2007; Phoka et al., 2009).
179
5.2.4 SEM & TEM STUDY
The scanning electron micrographs of the xCe-Ca-O composite oxides are present in Fig.5.9. The
SEM images of the composite oxide indicate the presence of agglomerated particles of irregular
shape and size. The materials are found to be of low density and spongy in nature. With increase
in ceria content the materials become increasingly porous. Numerous macropores are found to be
present on the surface of the particles. In the citrate method, the amorphous precursor contains
well dispersed metal ions in citrate matrix. Upon calcinations, the citrate molecules decompose
to yield a large quantity of gaseous products which leaves the materials porous and spongy.
(a)
(b)
180
(c)
(d)
(e)
181
(f)
(g)
Figure 5. 9 Scanning electron micrographs of (a) CaO, (b) 5Ce-Ca-O, (c) 10Ce-Ca-O, (d)
20Ce-Ca-O, (e) 50Ce-Ca-O, (f) 80Ce-Ca-O and (g) CeO2
The transmission electron micrograph of the 20Ce-Ca-O material is presented in Fig. 5.10. Two
different phases with clear contrast can be seen in the TEM image. There seems to be a
continuous calcia matrix where the mixed oxides nanoparticles are present in a well dispersed
state. The particle sizes of the mixed oxide phase are in the range of 15-25 nm which agrees well
with the particle size data obtained from the Fourier analysis of the sample.
182
Figure 5. 10 Transmission electron micrograph of 20Ce-Ca-O
5.2.5 CATALYTIC STUDIES FOR SYNTHESIS OF 2-AMINO-2-CHROMENES
In recent years, synthesis of biologically important molecules involving environmentally benign
reaction strategy is of considerable focus of research in synthetic chemistry. The synthesis of
aminochromenes has generated lot of interest in synthetic organic chemistry because of its
potential biological properties, such as antimicrobial, antiviral, antidepressant, antitumor,
antiproliferative, antihypertensive, sex pheromonal, anti-tubulin and antioxidative activity. In
addition, chromenes also find applications in cosmetics, pigments, potential biodegradable
agrochemicals (Hafez et al., 1987; Varma and Dahiya, 1998; Kidwai and Saxena, 2005). These
molecules are also found to be the main constituents of many natural products (Ellis, 1977;
Varma and Dahiya, 1998). The synthetic strategy of this heterocyclic nucleus involves the
183
multicomponent condensation reaction of aldehyde, malononitrile, and an activated phenol in the
presence of piperidine using acetonitrile or ethanol as reaction solvent (Ellis, 1977). Synthetic
methods proposed in literature towards the synthesis of this class of biologically important
compounds include the use of CTACl, basic alumina, basic ionic liquid as catalyst/reagent
(Ballini et al., 2001; Magii et al., 2004; Chen et al., 2009). However, most of the earlier
methods require longer reaction time, involve stoichiometric reagents, and toxic solvents, with
moderate yields of the product. Recently, the synthesis of this important class of molecule via
MCC approach has been accomplished using nanosized magnesium oxide and Mg/Al
hydrotalcites (Kumar et al., 2006: Surpur et al., 2009). In the present investigation we have
developed a methodology using Ce-Ca-O as an efficient and recyclable heterogeneous catalyst
for three-component condensation of aldehyde, malononitrile, and α-naphthol in aqueous media.
The catalytic activity of xCe-Ca-O nanocomposite oxide catalysts are evaluated for the synthesis
of 2-amino-2-chromenes by one pot multi-component condensation of aryl aldehydes,
malononitrile and α-naphthol in aqueous media (scheme 5.1).
CHO
+
CN
CN
+
CHO
20Ce-Ca-O
H2O, 800C
O
CN
NH2
R
R
Scheme 5.1 One pot aqueous phase synthesis of 2-amino-2-chromenes
Initially, the Ce-Ca-O catalysts having different compositions were screened for the synthesis of
2-amino-2-chromenes at 80oC for a time period of 1 h. Table 5.2 shows the catalytic activity of
different xCe-Ca-O catalysts screened for the condensation of benzaldehyde, malononitrile and
-naphthol. It is observed that at low ceria content upto 20 mol%, the reaction is found to be
184
accelerated. Among all the catalysts, the 20Ce-Ca-O catalyst shows highest catalytic activity
(Table 5.2).
Table 5.2 Catalytic activity of xCe-Ca-O composite oxides
Catalyst Basic sites
(mmol/g)
Yield (%)
CaO
5Ce-Ca-O
10Ce-Ca-O
20Ce-Ca-O
50Ce-Ca-O
80Ce-Ca-O
CeO2
0.66
0.70
0.76
0.82
0.56
0.36
0.16
60
64
72
78
54
42
---
The catalytic activity correlates well with the surface basic properties of the composite oxides
measured using nonaqueous indicator titration method. It has been reported that the substitution
of Ce4+
into the CaO lattice increases the iconicity of the CaO crystal (Rodriguez et al., 2003).
This may result in better charge separation resulting in the increase in the basicity of the O2-
ions
in the composite oxide. Hence, the higher activity observed for the composite oxide compared to
the pure CaO can be attributed to the change in the physicochemical property of the CaO
material as a result of cerium ion substitution. As observed from the XRD, UV-Vis and TEM
study, the presence of Ce4+
ions effectively modify the particle size and phase composition
resulting in well dispersed mixed oxide particles in CaO matrix. The grain boundary region
between the two phases can provide new catalytically active basic sites. The reaction parameters
185
were optimized by varying catalyst amount and reaction stoichiometry. The catalyst amount in
the reaction mixture was varied in the range of 10-50 mg. For 1mmol of the reactants, 25 mg of
the catalyst is ideally suited for the effectual condensation of benzaldehyde, malononitrile and -
naphthol to yield the corresponding 2-amino-2-chromenes in 78% yield (Table 5.3, entry 1).
Table 5.3 20Ce-Ca-O catalyzed synthesis of 2-amino-2-chromenes in aqueous media
Further increase in the catalyst amount in the reaction mixture does not have any marked impact
on the product yield. The reaction temperature is varied between 60-100oC. At 80
oC, the product
yield is quite significant and hence this temperature is chosen for further investigation. The
optimum reactant ratio is found to be 1:1:1 for benzaldehyde, malononitrile and -naphthol,
respectively. Ensuing optimized conditions for this multicomponent reaction, we explored the
scope and limitations of this reaction using different substituted aromatic aldehydes (Table 5.3).
Entry R Time (min) Yield (%)
1
2
3
4
5
6
7
8
9
10
H
2-Cl
4-Cl
2-NO2
3-NO2
4-NO2
4-OCH3
4-OH
4-Br
4-F
60
60
60
30
30
30
75
75
60
30
78
77
82
80
76
83
81
83
85
83
186
Under identical reaction conditions, aryl aldehydes bearing electron withdrawing and donating
group reacted efficiently to give the corresponding 2-amino-2-chromenes in high yield and
purity. The recyclability of the 20Ce-Ca-O catalyst was tested for three consecutive cycles
(Table 5.3, Entry 1, yields, 78%, 1st; 74%, 2
nd; 72%, 3
rd) and no significant decrease in yield of
the product is observed in the study. Overall, in this investigation we have developed a novel
protocol for synthesis of a wide variety of 2-amino-2-chromenes in aqueous media.
Physical spectral data of some representative compounds are given below
2-Amino-3-cyano-4-phenyl-4H-benzo[h]chromene (Table 5.3, Entry 1): M.P. 208-2100C, IR(KBr):
3438, 3313, 2187, 1655, 1402, 1100, 1H NMR (CDCl3, 400 MHz): 4.77(2H, s), 4.88 (1H, s),
7.04 (1H, d), 7.20-7.31 (5H, m), 7.52-7.60 (3H, m), 7.80 (1 H, d), 8.19 (1H, d).
2-Amino-3-cyano-4-(4-chlorophenyl)-4H-benzo[h]chromene (Table 5.3, Entry 3): M.P. 232-2330C,
IR (KBr): 3449, 3330, 2190, 1659, 1408, 1100, 1H NMR (CDCl3, 400 MHz): 4.79 (2H, s), 4.89
(1H, s), 7.0 (1H, d), 7.15-7.28 (4H, m), 7.54-7.63 (3H, m), 7.81 (1 H, d), 8.20 (1H, d).
187
Part B: CeO2-MgO NANOCOMPOSITE OXIDES
5.3 INTRODUCTION
In the previous section of this chapter, the promoting effect of ceria in the CeO2-CaO
nanocomposite oxides has been described for one pot synthesis of 2-amino-2-chromenes in
aqueous media. Amorphous citrate process which has been used for the synthesis of the
composite oxide was demonstrated to be a good method for the preparation of finely dispersed
composite oxide of uniform composition. This section describes the catalytic activity of CeO2-
MgO nanocomposite oxide catalysts for the synthesis of -nitroalcohols and
arylidenemalononitriles. Since the preparation procedure is important for the final property of the
composite oxide, we have adopted two different methods namely amorphous citrate process and
gel combustion method for the synthesis of the composite oxides. Both the methods involve the
use of citric acid as a complexing agent.
Among different solid heterogeneous base catalysts, magnesium oxide (MgO) is a promising
material which has been used for a variety of organic transformations (Corma and Iborra, 2006;
Wang and Yang, 2007, Sutradhar et al., 2011; Chintareddy and Kantam, 2011). The
conventional methods employed for synthesis of MgO usually produce materials with relatively
large particle sizes, small surface areas and inhomogeneous morphologies. These
physicochemical aspects limit its application for efficient base catalyzed processes. In recent
years, significant effort has been made for the synthesis of nanosized MgO with superior
physicochemical properties as compared to bulk MgO. Nanosize MgO has been found to exhibit
enhanced surface basicity and catalytic activity for several important reactions such as hydrogen
transfer, transesterification reaction, dye degradation and green synthesis of anti-inflammatory
agents (Corma and Iborra, 2006; Wang and Yang, 2007, Climent et al., 2007; Moussavi and
188
Mahmoudi, 2009; Díez et al., 2011; Chintareddy and Kantam, 2011). MgO in combination with
other catalytically active metal oxides have also been synthesized in the form of nanocomposite
oxides and their catalytic activity has been evaluated. It has been reported that the dispersion of
the MgO in the matrix of the second oxide component helps improving the number of exposed
basic sites. In some cases synergestic effect between both the oxides components have also been
invoked to account for the higher catalytic activity. The MgO based nanocompsoite oxide
systems such as CaO-MgO, MgO-ZrO2 and La2O3–MgO has been reported as efficient catalyst
for transesterification reaction, hexane conversion and synthesis of polyfunctionalized pyrans
(Ciuparu et al., 2007; Babu et al., 2008; Wei et al., 2011; Taufiq-Yap et al., 2011). In the present
investigation we have studied the promoting effect of ceria in the CeO2-MgO nanocomposite
oxides for the base catalyzed C-C bond formation reactions. The CeO2-MgO oxides have been
studied earlier as heterogeneous catalyst for alkylation of phenol with methanol, synthesis of
dimethyl carbonate and CO oxidation reactions (Sato et al., 1998; Abimanyu et al., 2007; Yu et
al., 2011). Noble and transition metal supported CeO2-MgO catalysts have also been used for
ammonia synthesis, methane oxidation and NO reduction by CO (Saito et al., 2006; Chen et al.,
2009a; Tanaka et al., 2009). It has been reported that the presence of ceria in MgO increase the
number basic sites required for the orthoselective alkylation of phenol with methanol (Sato et al.,
1998). Similarly, an increase in the number and strength of the basic sites have been observed
Ce-Mg-O composite oxide, which has been ascribed to the higher catalytic activity of the
composite oxide for transesterification of ethylene carbonate (Abimanyu et al., 2007). Saito et al.
have observed a synergistic effect between the CeO2 and MgO components in the Ru/Ce-Mg-O
catalyst which has been found to be responsible for the enhanced catalytic activity for ammonia
synthesis (Saito et al., 2006).
189
The sol–gel combustion method is a versatile technique that has been used for the preparations of
multicomponent oxides with desired properties in a simple and economic way (Ranga Rao and
Sahu, 2001; Montanari and Costa, 2005; Ranga Rao and Rajkumar, 2008). This technique
combines the advantages of the chemistry of sol–gel as well as the combustion method and has
been used to produce homogeneous and highly reactive crystalline powders without the
intermediate steps (Montanari and Costa, 2005; Mercadelli et al., 2008; Ranga Rao and
Rajkumar, 2008). In the sol–gel process, a homogeneous cations mixing at molecular level is
achieved by crosslinking a concentrated solution of carboxylate-metal complexes into a three
dimensional gel. The exothermic combustion of the gel leads to the formation of finely dispersed
composite oxides.
5.4 RESULTS AND DISCUSSION
5.4.1 CHARACTERIZATION OF THE COMPOSITE OXIDES PREPARED BY
AMORPHOUS CITRATE AND GEL COMBUSTION PROCESS
5.4.1.1 XRD STUDY
Fig. 5.11 illustrates the X-ray diffraction patterns of the MgO, CeO2 and xCe-Mg-O
nanocomposite oxides prepared by sol gel combustion method. The pure MgO shows well
defined and intense diffraction peaks at 2 values of 36.8, 42.8 and 62.2 degrees due to
reflections from (111), (200), and (220) planes, respectively, from the cubic crystal structure of
MgO (JCPDS No: 77-2179) (Fig. 5.11a). Pure CeO2 shows the characteristic reflections
corresponding to the fluorite structure of ceria (JCPDS No: 34-0394) (Fig. 5.11f). Addition of 5
mol% CeO2 to MgO matrix, results in significant changes in the XRD patterns of MgO. The peak
intensity of the MgO phase is found to decrease significantly whereas a weak and broad
reflection associated with fluorite structure of ceria is observed in the XRD profile (Fig. 5.11b).
190
20 25 30 35 40 45 50 55 60 65 70
Inte
nsi
ty (
a.u
.)
2 (degrees)
(220)(200)(111)
(311)
(220)
(200)
(111)
f
e
d
c
a
b
Figure 5.11 XRD patterns of (a) MgO, (b) 5Ce-Mg-O, (c) 10Ce-Mg-O, (d) 20Ce-Mg-O (e)
50Ce-Mg-O and (f) CeO2
With increasing Ce:Mg molar ratio, the XRD peaks for ceria becomes more prominent, where as
those of MgO decreases significantly (Fig. 5.11c-e). For 50Ce-Mg-O composite oxide, XRD
peaks corresponding to the fluorite structure of ceria are only observed (Fig. 1e). Using the
Fourier line shape analysis method, the crystallite size and r.m.s. stain are calculated for all the
composite oxide samples following the procedure described in section 5.2.1. Figure 5.12 (panel I
191
and II) represents the AS~L and pV~L plots for the ceria phase of different composite oxide
samples.
0 50 100 150 200 250 300 350 400 450 500
0.0
0.2
0.4
0.6
0.8
1.0
L (A0
)
Fo
uri
er
size
co
ffic
ien
t (A
s)
CeO2
5Ce-Mg-O
10Ce-Mg-O
20Ce-Mg-O
50Ce-Mg-O
I
0 100 200 300 400 500 600 700 800
0.000
0.005
0.010
0.015
0.020
0.025 II CeO2
5Ce-Mg-O
10Ce-Mg-O
20Ce-Mg-O
50Ce-Mg-O
pV
Fourier Length L (A0
)
Figure 5.12 Fourier line profile analysis plot of the Ce-Mg-O composite oxides (Panel I,
AS vs L and panel II, pV vs L)
192
Table 5.4 Structural parameters of xCe-Mg-O nanocomposite oxides
Composite oxide Lattice
parameter (Å)
Cell Volume
(Å3)
Size
(nm)
Strain
<e2>
1/2
5Ce-MgO 5.3924 156.80 3 9.20×10-3
10Ce-MgO 5.3937 156.92 5 7.10×10-3
20Ce-MgO 5.4043 157.84 6 3.06×10-3
50Ce-MgO 5.4070 158.08 9 2.74×10-3
CeO2 5.4173 158.98 13 1.73×10-3
The wide distribution function observed for the pure ceria and 50Ce-Mg-O indicate the materials
are polycrystalline and nonhomogeneous with larger particle size. On the other hand, the 5Ce-
Mg-O, 10Ce-Mg-O and 20Ce-Mg-O materials show a comparable narrower size distribution
function (Fig. 3b). The calculated crystallite size, r.m.s strain, lattice parameter and cell volume
of pure CeO2 and xCe-Mg-O composite oxides synthesized by sol-gel combustion method are
presented in Table 5.4. The lattice parameter and cell volume for the ceria component is
calculated using the POWD software. The lattice parameter and cell volume of ceria in the
composite oxide is found to decrease as compared to pure ceria (Table 5.4). There seems to be a
small amount of lattice contraction for the ceria component. This may be due to substitution of
Mg2+
in the ceria lattice in the composite oxide to form a nonequibillibrium solid solution of the
type MgxCe1-x/2O2-x. However, considering the fact that the Ce4+
(ionic radii 0.97 Å) and Mg2+
(ionic radii 0.65 Å) ions differ widely in their size as well as coordination environment, such a
type of solid solution can exist only in a limited composition range. Although, there are earlier
XRD reports available in literature for the Ce-Mg-O composite oxides, there is no general
193
agreement on the structural aspects of this composite oxide (Sato et al., 1998; Saito et al., 2006;
Abimanyu et al., 2007; Chen et al., 2009a; Yu et al., 2011). Some of the reports exclude the
formation of solid solution (Roth et al., 1981; Sato et al., 1998; Saito et al., 2006; Abimanyu et
al., 2007) whereas in other studies the formation of a non-equilibrium solid solutions have been
indicated (Sato et al., 1998; Chen et al., 2009a). Sato et al. have proposed the formation of an
interstitial type non-equillibrium solid solution for the Ce-Mg-O material synthesized by
amorphous citrate method (Sato et al., 1998). These authors have not observed any apparent shift
in the peak position for the ceria phase in the composite oxides. Chen et al. have performed
Rietveld analysis and observed a minor contraction in lattice parameter and a larger microstrain
for the ceria component in the Ce-Mg-O materials (Chen et al., 2009a). The lattice contraction
has been ascribed to the substitution of a small amount of Mg2+
for Ce4+
ions in ceria lattice. In
line of the observation by Chen et al., in the present study the Ce-Mg-O nanocomposite oxides is
believed to contain a nonequilibrium MgxCe1-x/2O2-x solid solution along with the individual
phases of the oxides.
Fig. 5.13 shows the X-ray diffraction patterns of the MgO, CeO2 and CeO2-MgO nanocomposite
oxides (xCe-Mg-O-A) prepared by amorphous citrate method. Similar observations in the XRD
profiles are noted for xCe-Mg-O-A materials as described in the preceding discussion for xCe-
Mg-O maeterials synthesized by gel combustion method. However, both the method shows
different extent of line broadening and hence the Fourier line profile analysis is carried out to
calculate the crystallite size and r.m.s. strain.
194
20 30 40 50 60 70
2 (degrees)
Inte
nsi
ty (
a.u
.)
(220)(200)(111)
(311)
(220)
(200)
(111)
a
b
c
d
e
f
Figure 5.13 XRD patterns of (a) MgO-A, (b) 5Ce-Mg-O-A, (c) 10Ce-Mg-O-A, (d) 20Ce-
Mg-O-A, (e) 50Ce-Mg-O-A and (f) CeO2-A
The calculated crystallite size and strain for the xCe-Mg-O-A materials is presented in Table 5.5.
The composite oxides display particles with size in the range of 4-22 nm. For an identical
composition, the gel combustion method was found to yield particles with small size as
compared to amorphous citrate process.
195
Table 5.5 Crystallite size and lattice strain of xCe-Mg-O-A nanocomposite oxides
Composite oxide Crystallite
Size (nm)
Strain
<e2>
1/2
5Ce-MgO 4 7.66×10-3
10Ce-MgO 7 4.53×10-3
20Ce-MgO 15 2.37×10-3
50Ce-MgO 22 1.11×10-3
CeO2 16 1.55×10-3
5.4.1.2 UV-VIS-DRS STUDY
The UV-Vis-DRS spectra of the xCe-Mg-O nanocomposite oxides prepared by both amorphous
citrate and gel combustion methods are presented in Fig. 5.14 (panel I and II). Pure ceria
prepared by either method shows two broad bands with absorption maxima at 245 and 275 nm
(Fig. 5.14 Ie and IIe). In addition, a broad shoulder is observed at 330 nm. Crystalline cerium
dioxide has a band gap of 3.1 eV and absorbs strongly in the UV region with the absorption
threshold near = 1240/Eg = 400 nm. The distinct absorption bands at 245 nm and 275 nm have
been assigned to O2-
Ce3+
and O2-
Ce4+
charge transfer transitions where as the shoulder at
330 nm can be assigned to the interband transition. These absorption features are characteristics
of the bulk ceria particles (Zaki et al., 1997; Ranga Rao and Sahu, 2001; Mishra and Ranga Rao,
2006).
196
200 300 400 500 600 700
F(R
) (a
.u.)
Wavelength (nm)
a
b
c
d
e
I
200 300 400 500 600 700
II
e
d
c
b
a
Figure 5.14 UV-Vis-DRS spectra of (a) 5Ce-Mg-O, (b) 10Ce-Mg-O, (c) 20Ce-Mg-O, (d)
50Ce-Mg-O and (e) CeO2 (panel I and II is for the materials prepared by amorphous
citrate and sol gel combustion, respectively)
In case of the CeO2-A sample the interband transition is more intense and discriminable as
compared to the ceria samples prepared by gel combustion method. Addition of ceria to MgO
matrix resulted in significant change in the absorption feature of the composite oxide. For
samples upto 20% ceria content (Fig. 5.14 a-c, panel I and II), a prominent absorption maxima is
observed around 290-295 nm. This band can be ascribed to the Ce4+
O2-
charge transition
transitions occurring on surface low coordinated Ce4+
sites. This observation is similar to the
observation made for CeO2-CaO composite oxide in section 5. 2.3 and indicate the presence of
197
ceria particles in a highly dispersed state. For samples with 50 mol% ceria content, the
appearance of bulk like feature can be observed from the UV-vis spectra of the composite oxide
samples.
5. 4.1.3 SEM AND TEM STUDY
The scanning electron micrographs of xCe-Mg-O nanocomposite oxides prepared by gel
combustion route are present in Fig. 5.15. The SEM image of pure MgO indicates the presence
of agglomerated particles with irregular shape and size. With increase in ceria content the
materials become gradually more porous and of low density and spongy nature. The pure ceria
shows flake like morphology along with numerous macropores present on the surface of the
particles. In sol gel combustion method, the precursor contain a homogeneous carboxylate-metal
complexes which on combustion results in the formation of well dispersed composite oxide
particles with simultaneous evolution of large amounts gaseous products which makes the
material porous and spongy in nature. Similar observation is also noted for the samples
synthesized by amorphous citrate method shown in Fig. 5. 16.
198
(b)
(a) (b)
(c)
Figure 5.15 Scanning electron micrograph of (a) MgO, (b) 10Ce-Mg-O, (c) 50Ce-Mg-O
and (d) CeO2 prepared by gel combustion method
(d)
199
(a) (b)
(c)
Figure 5.16 Scanning electron micrograph of (a) MgO, (b) 5-Ce-Mg-O, (c) 10-Ce-Mg-O,
(d) 50-Ce-Mg-O and (e) CeO2
The transmission electron micrographs of the 10Ce-Mg-O and 10Ce-Mg-O-A nanocomposite
oxide are presented in Fig. 5.17. Small nanoparticles with size in the range of 5-10 nm are
observed in the TEM study. The TEM result is found to be complimentary to the Fourier line
profile data. The particles are present in an agglomerated state attached to each other through the
grain boundary. The TEM study clearly indicates the effectiveness of both the methods for
synthesis of nancatalyst materials.
(d)
200
(a)
(b)
Figure 5.17 Transmission electron micrograph of (a) 10Ce-Mg-O and (b) 10-Ce-Mg-O-A
materials
201
5.4.2 CATALYTIC APPLICATION OF xCe-Mg-O GEL COMBUSTION
SYNTHESIZED SAMPLE FOR KNOEVENAGEL CONDENSATION REACTION
Knoevenagel condensation is an important and extensively studied method for the formation of
C-C double bond between carbonyl and active methylene compounds (Freeman, 1981; Tietze
and Beifuos, 1991; Tietze, 1996; Corma and Iborra, 2006; Seki and Onaka, 2007; Parida and
Rath, 2009; Postole et al., 2010). The Knoevenagel products have numerous applications in the
synthesis of fine chemicals in agriculture and medicine and synthesis of carbocyclic as well as
heterocyclic compounds of biological significance (Tietze, 1996; Yu et al., 2000; Song et al.,
2003; Corma and Iborra, 2006; Balalaie et al., 2007; Kumar et al., 2010). This reaction has been
explored for the preparation of substituted alkenes, coumarin derivatives, perfumes, cosmetics
and as key step in natural product synthesis. Knoevenagel condensation is a base-catalyzed
cross-aldol condensation reaction. The conventional organic bases used as catalyst for this
reaction include pyridine, piperidine, amines, and ammonia or sodium ethoxide in different
organic solvents. In recent year, there has been significant effort to design environmentally
acceptable protocols involving heterogeneous catalysts, green solvent or using solvent free
conditions during synthesis. The use of water and highly protic solvent as reaction media for
Knoevenagel condensation has been advantageous in terms of preventing furhter self-
condensation, 1,2-elimination and retro-Knoevenagel condensation reactions (Tietze and
Beifuos, 1991; Gupta et al., 2009). Hence, the use of heterogeneous catalysts in conjugation with
benign reaction media for Knoevenagel condensation is highly desirable. The heterogeneous
catalytic systems reported for this reaction include alumina (Seki and Onaka, 2007), silica
(Martins et al., 2010), ionic liquid (Zhao et al., 2010) and Ni nanoparticle (Kumar et al., 2010).
In this work, we have studied the catalytic application of xCe-Mg-O nanocomposite oxides
202
prepared by gel combustion method for Knoevenagel condensation. The Knoevenagel
condensation of aryl aldehydes with malononitrile was carried out in presence of xCe-Mg-O
catalyst in aqueous media as well as under solid state reaction condition using mechanochemical
method and microwave as energy source (scheme 5.2). Initially, the Ce-Mg-O catalysts having
different compositions were screened for the synthesis of arylidenemalononitriles by
condensation of benzaldehyde with malononitrile using mechanochemical method under ambient
condition. It is observed that among all the catalysts, the 10Ce-Mg-O catalyst shows highest
catalytic activity in a short span of time.
R
CHO
+
CN
CN
CN
CN
RGrinding, Microwave
Solvent (H2O)
10Ce-Mg-O
Scheme 5.2 xCe-Mg-O catalyzed condensation of aryl aldehydes with malononitrile
Hence, the 10 Ce-Mg-O catalyst is chosen for subsequent exploration for synthesis of
arylidenemalononitriles. The reaction parameters are optimized by varying catalyst amount and
reaction stoichiometry. The catalyst amount in the reaction mixture was varied in the range of
25-150 mg. For 2 mmol of the reactants, 100 mg of the catalyst is ideally suited for the effectual
condensation of benzaldehyde, and malononitrile to yield the corresponding
arylidenemalononitriles in 87% yield (Table 5.6, entry 1). Further increase in the catalyst
quantity in the reaction mixture does not have any marked impact on the product yield. The
optimum reactant molar ratio is found to be 1:1 for benzaldehyde and malononitrile,
respectively. Ensuing optimized conditions for this condensation reaction, we explored the scope
203
and limitations of this reaction using different substituted aromatic aldehydes (Table 5.6).Under
identical reaction conditions, aryl aldehydes containing electron withdrawing and donating group
reacted efficiently to give the corresponding arylidenemalononitriles in high yield and purity. In
order to study the effect of reaction media as well as mode of energy supply, the condensation
reactions are carried out under microwave heating as well as in aqueous media under ambient
condition.
Table 5.6 10Ce-Mg-O catalyzed Knoevenagel condensation reaction
Mechanochemical
method
Microwave Solvent (H2O)
Entry R Time (m) Yield (%) Time (m) Yield (%) Time (m) Yield (%)
1 H 6 87 2 82 10 86
2 4-F 3 92 2.5 90 5 90
3 4-OH 2 88 3 85 4 85
4 4-OMe 3 85 1 89 3 92
5 2-NO2 2 88 1.5 87 2 86
6 3-NO2 3 90 2.5 83 3 85
7 4-NO2 3 90 2.5 88 3 88
8 4-Cl 6 88 1 88 3 89
9 3-Br 3 87 2 85 5 84
10 2-Cl 3 87 1.5 87 5 85
For reaction in aqueous media, 5 ml of distilled water is used in the optimized protocol. It is
observed that the use of microwave shorten the time span of the reaction with a marginal
decrease in yield. For reaction under aqueous media, the reaction rates are sluggish as compared
204
to mechanochemical and microwave assisted methods. However, in all the three methods a
variety of aldehydes reacted with malononitrile to give the corresponding products in high yield
and purity. This result effectively demonstrates the application of Ce-Mg-O materials as catalyst
for base catalyzed condensation reaction. The recyclability of the 10Ce-Mg-O catalyst was tested
for three consecutive cycles using the mechanochemical method (Table 5.6, Entry 1, yields,
87%, 1st; 85%, 2
nd; 81%, 3
rd). No significant decrease in yield of the product was observed in the
study indicating the stable activity of the 10Ce-Mg-O catalyst for Knoevenagel condensation.
Physical spectral data of some representative compounds are given below
Benzylidene malononitrile (Table 5.6, Entry 1):
Mp (°C): 82-84; 1H NMR (400 MHz, CDCl3): δ 7. 92 (d, Ar), 7.80 (s, CH), 7.69–7.64 (m, Ar),
7.60– 7.51 (m, Ar).
4-Nitrobenzylidene malononitrile (Table 5.6, Entry 7):
Mp (°C): 160-162; 1H NMR (400 MHz, CDCl3): δ 8.25 (d, Ar), 8. 05 (d, Ar), 7.85 (s, CH).
4-Chlorobenzylidene malononitrile (Table 5.6, Entry 8):
Mp (°C): 162-163; 1H NMR (400 MHz, CDCl3): δ 7. 90 (d, Ar), 7.80 (s, CH), 7.55 (d, Ar).
5.4.3 HENRY REACTION
The Henry (nitroaldol condensation) reaction is one of the most significant C–C bond formation
reactions which involve the synthesis of β-nitro alcohol by condensation of aryl aldehydes and
nitroalkanes (Luzzio, 2001, Ono, 2001). Since many natural products contains nitrogen atom, the
Henry reaction provides a basis for development of such molecules. The β-nitro alcohol
synthesized by the Henry reaction route is used as an intermediate for the synthesis of β-amino
alcohol, amino sugar, ketones, pyrroles, porphyrins and pharmacologically potent compounds
205
including fungicides, insecticides, antibiotics, natural products, and antitumor agents (Ono, 2001;
Luzzio, 2001; Bures and Vilarrasa, 2008). The various homogeneous and heterogeneous catalytic
systems studied for this reaction include amine-MCM-41 hybrids, MCM-41-Cu(salen) complex,
I2/K2CO3, organoamine-functionalized mesoporous catalysts, silica-supported amine catalysts,
and CsF/[bmim][BF4] ionic liquids (Ren and Cai, 2007; Wang and Shantz, 2010; Biradar et al.,
2010; Dhahagani et al., 2011; Shinde et al., 2011). Most of the catalysts reported so far are either
homogeneous catalysts or supported reagents which suffer from drawbacks such as leaching and
recyclability. Herein, we have developed a methodology for the synthesis of nitro aldol products
using amorphous citrate synthesized CeO2 (10 mol%)-MgO nanocomposite oxides as an efficient
and recyclable catalyst in aqueous media (scheme 5.3). Initially, the xCe-Mg-O-A catalysts
having different compositions were screened for nitroaldol condensation, taking the reaction
between p-nitrobenzaldehyde and nitromethane in aqueous media as a model reaction under
ambient condition. Among different catalysts, the 10Ce-Mg-O-A catalyst show best catalytic
activity for the model reaction which is chosen for further study.
H
O
10Ce-Mg-O-A
CH3NO2, H2O
NO2
OH
RR
Scheme 5.4 Henry reaction catalyzed by 10Ce-Mg-O-A materials
The reaction parameters such as the catalyst amount and reaction stoichiometry are varied to
obtain the optimized protocol. It is observed that for a 1 mmol reaction scale, 50 mg of the
catalyst and a 1: 2 molar ratio of p-nitrobenzaldehyde to nitromethane is quite effective for the
206
efficient condensation of both the components. The optimized protocol is used to synthesize a
variety of β-nitro alcohols the yield of which is reported in Table 5.7. Under the optimized
conditions, the aryl aldehydes containing both electron withdrawing and donating groups reacted
efficiently to give the corresponding nitro alcohols in high yield and purity (Table 5.7). In
general, for substrates with electron withdrawing group the yields are marginally better
compared to the substrates with electron donating groups. Many functional variations in the
aldehydic substrates and their good reactivity in the optimized protocol demonstrate the
effectiveness of the 10Ce-Mg-O-A catalyst for the Henry reaction. The protocol developed in
this study using 10Ce-Mg-O-A catalyst is advantageous in terms of simple experimentation,
preclusion of toxic solvents, use of benign reaction solvent, shorter reaction time and high yield
and purity of the products.
Table 5.7 10Ce-Mg-O-A catalyzed Henry reaction in aqueous media
Entry R Time (h) Yield (%)
1 H 5 82
2 4-F 4.5 84
3 4-OH 9 75
4 4-OMe 7 76
5 2-NO2 1.5 80
6 3-NO2 1.5 85
7 4-NO2 1 88
8 4-Cl 6 80
9 2-Cl 6.5 82
10 3-Br 6 81
207
Physical spectral data of some representative compounds are given below
2-Nitro-1-phenylethanol (Table 5.7, Entry 1):
1H NMR (400 MHz, CDCl3): δ 3.0 (br, 1H), 4.52-4.81 (m, 2H), 5.48 (dd, 1H), 7.28-7.42 (m,
5H).
1-(4-fluorophenyl)-2-nitroethanol (Table 5.7, Entry 2):
1H NMR (400 MHz, CDCl3): δ 3.32 (br, 1H), 4.52-4.71 (m, 2H), 5.37 (dd, 1H), 7.00-7.14 (m,
2H), 7.30-7.32 (m, 2H).
2-Nitro-1-(4-nitrophenyl)ethanol (Table 5.7, Entry 7):
1H NMR (400 MHz, CDCl3): δ 3.2 (br, 1H), 4.56-4.65 (m, 2H), 5.62 (dd, 1H), 7.61 (d, 2H), 8.29
(d, 2H).
5.5 Conclusion
In is chapter, two classes of ceria based nanocomposite oxides ie. Ce-Ca-O and Ce-Mg-O have
been explored as heterogeneous catalytic systems for the synthesis of biologically important
molecules. The Ce-Ca-O composite oxides contains a substitutional solid solution of the type
Ce1-xCaxO2-x for higher ceria content where as for calcium oxide rich samples the presence of the
CaO phase along with the mixed oxide phase is observed. The presence of well dispersed ceria
particles in the composite oxide was ascertained from UV-vis spectroscopy. The crystallite size
of the mixed oxide and the CaO phase are in the range of 5-25 and 25-35 nm respectively. The
Ce-Ca-O composite oxides are used as an environmentally benign heterogeneous catalyst for the
synthesis of 2-amino-2-chromenes in aqueous media. The 20Ce-Ca-O material is found to be
highly efficient for the multicomponent reactions yielding a variety of structurally diverse
aminochromenes. The Ce-Mg-O nanocomposite oxides synthesized by the amorphous citrate and
gel combustion methods have been used as catalyst for synthesis of β-nitro alcohol and
arylidenemalononitriles, respectively. The presence of individual phases of the oxides along with
208
a nonequilibrium solid solution was observed form XRD study of the composite oxides prepared
by both methods. The TEM study of the composite oxides indicates the presence of small
naonosize particles with size in the range of 5-15 nm. The gel combustion synthesized materials
shows excellent catalytic activity for the synthesis of arylidenemalononitriles by the
condensation of aryl aldehydes and malononitrile in aqueous media as well as under solvent free
conditions. The composite oxides synthesized by amorphous citrate process are found to be
highly active for synthesis of structurally diverse β-nitro alcohols. In the present investigation,
the ceria based nanocomposite oxides involving alkaline earth metal oxides have been
demonstrated as efficient catalyst for base catalyzed carbon-carbon bond formation reactions.
209
CHAPTER 6
SUMMARY AND CONCLUSIONS
In this thesis, an effort has been made to explore the catalytic activity of surface and structurally
modified zirconia materials for synthesis of biologically important molecules under
environmentally benign conditions. The MoO3-ZrO2 nanocomposite oxides have been
synthesized by three different methods namely coprecipitation, combustion and wet
impregnation method. For the coprecipitated sample, the selective stabilization of the tetragonal
phase of zirconia is observed in the presence of MoO3. The molybdina component exists in a
well dispersed state in the composite oxide without formation of crystalline MoO3 or ZrMo2O8
phase. The molecular nature of the molybdate species in the zirconia matrix has been probed
using Raman and UV-Vis spectroscopic techniques. The MoO3 species are present in the form of
isolated and cluster molybdates for sample containing upto 20 mol% MoO3. For higher loading
bulk type MoO3 are observed in the zirconia matrix. The particle size of the composite oxides are
found to be in the range of 20-30 nm which is smaller than the 40-50 nm particle size observed
for pure zirconia synthesized by precipitation method. The MoO3-ZrO2 composite oxides
prepared by the coprecipitation method show improved physicochemical characteristics
compared to the samples prepared by impregnation method. The surface area and the acidity of
the composite oxides are also higher than the individual components. The catalytic activity of the
MoO3-ZrO2 composite oxides prepared by coprecipitation method has been evaluated for the
multicomponent synthesis of amidolalkyl naphthols and -acetamido ketones. The amidoalkyl
naphthols are synthesized by multi-component condensation of aryl aldehydes, 2-naphthol, and
amide/urea under solvent free conditions using conventional as well as microwave heating.
Similarly, the β-acetamido ketones are synthesized by four component one pot condensation
210
reaction of aryl aldehydes, enolizable ketones, acetyl chlorides and acetonitrile. The results
obtained from the catalytic tests, clearly showed that the MoO3-ZrO2 composite oxide catalyst
was recyclable and highly efficient for the synthesis of both classes of compounds giving good
yield and purity of the products.
The MoO3-ZrO2 composite oxides are prepared further by combustion synthesis method using
glycine, urea and hexamethylene tetramine as fuels at different F/O stiochiometric ratio. The
selective stabilization of the tetragonal phase of zirconia is also observed for the combustion
synthesized composite oxides. Fourier line shape analysis of the XRD profiles and TEM study
indicate that the crystallite size of the composite oxides are in the range of 4–40 nm depending
upon the nature and amount of the fuel employed during combustion. Glycine as fuel is quite
effective in producing nanoparticles with uniform morphology and smaller size. SEM study of
the composite oxide indicate the material to be of low density and spongy in nature. The inherent
nature of the fuel and its complexing capability with the ionic species is crucial for the
production of particle with uniform shape and morphology. The molybdina component is found
to be present in a highly dispersed state in the form of isolated tetrahedral and polymolybdate
clusters in the composite oxide material. The combustion synthesized 10MoZr-G composite
oxide is explored for a rapid, convenient and environmentally benign synthesis of biologically
important octahydroquinazolinones under solvent-free conditions and in aqueous medium. The
octahydroquinazolinones were synthesized by multiomponent condensation of dimedone, urea
and arylaldehydes. The combustion synthesized 10MoZr-G catalyst is highly active for the
multicomponent reactions generating a variety of structurally diverse octahydroquinazolinones
with good yield and purity of the products.
211
The sulfate grafted Fe2O3 (x mol%)- ZrO2 material are prepared by a two step process involving
coprecipitation followed by sulfate grafting onto the surface of the Fe-Zr composite oxide. The
incorporation of iron ions into the zirconia lattice results in the formation of a substitutional solid
solution. The selective stabilization of the tetragonal phase of the zirconia is observed in the
composite oxide. The tetragonal phase is found to be retained in the sulfate grafted composite
oxide samples. The characteristics vibrational features corresponding to the S=O and S-O
stretching indicated the presence of grafted sulfate species on the surface of the Fe-Zr composite
oxides. At low Fe2O3 content, the iron oxide species exist primarily as isolated octahedral Fe3+
ions and small nonstoichiometric (FexOy) nanooxide clusters, whereas at higher iron oxide
content, the presence of bulk type Fe2O3 was observed in the UV-Vis study. The presence of iron
oxide in zirconia lattice improves the sulfate retention capacity of host zirconia. The catalytic
activity of SO42-
/xFe-Zr materials are evaluated for the synthesis of xanthenediones by
condensation of dimedone with aryl aldehydes under solvent free conditions using microwave as
energy source. The SO42-
/xFe-Zr materials materials are found to be highly efficient for the
condensation reaction generating a variety of xanthenediones with high yield and purity. The
specific advantage of the protocols developed in this investigation using surface and structurally
modified zirconia materials are simple experimentation, shorter reaction time, reusable
heterogeneous catalyst, preclusion of toxic solvents and high yield and purity of the products.
Zr-pillared clays have been prepared by the intercalation of [Zr4(OH)8(H2O)16]8+
clusters into the
clay interlayer. In order to increase the catalytic efficiency of the Zr-pillared clay, sulfate ions are
grafted to the zirconia nanopillars and phosphomolybdic acid is dispersed in the micropores of
the pillared clay materials. The intercalation of the zirconium polycation resulted in an expansion
212
of the clay interlayer. The expended interlayer space of the pillared clay material is found to be
retained in the composite catalyst. All the pillared clay materials exhibit Type-I N2 sorption
isotherm characteristics of the microporous materials. Pillaring with Zr-polycation is found to
have beneficial effect on the surface area and pore volume of the parent clay. The structural
integrity of the phosphomolybdic acid in the composite catalyst was ascertained from IR study.
The Zr-pillars exists as extremely small clusters of ZrO2 in the pillared clay matrix. Catalytic
studies performed on these materials indicate that sulfate grafting and subsequent dispersion of
phosphomolybdic acid substantially enhances the catalytic activity of Zr-pillared clay. The
modified pillared clay material is found to be highly efficient for the synthesis of β-
aminocarbonyl compounds under ambient condition in aqueous media. A wide variety of
substrates reacted under the optimized protocols to give the corresponding β-amino carbonyl
compounds in high yield and purity. The protocol developed in this investigation using the
modified Zr-pillared clay material is found to be advantageous in terms of simple
experimentation, use of water as green and ecofriendly solvent, recyclable catalyst and high yield
and purity of the product.
The promoting role of ceria for base catalyzed C-C bond formation reaction is studied for Ce-Ca-
O and Ce-Mg-O nanocomposite oxides for synthesis of three classes of biologically important
molecules. A series of CeO2-CaO nanocomposite oxides are synthesized by amorphous citrate
method and their catalytic activity is evaluated for aqueous phase synthesis of 2-amino-2-
chromenes. The Ce- Ca-O composite oxide contain a substitutional solid solution of the type Ce1-
xCaxO2-x for higher ceria content where as for calcium oxide rich samples the presence of the
CaO phase along with the mixed oxide phase is observed. Fourier line profile analysis of the
213
composite oxide indicates the presence of nanosize particles in the composite oxides. The mixed
oxide phase show crystallite size in the range of 5-25 nm whereas the CaO phase shows
crystallite size in the range of 25-35 nm. The Fourier analysis data is found to be complementary
with the TEM observation. UV-Vis-DRS study of the composite oxide showed well dispersion of
the ceria in the CaO matrix. The characteristics reflections from the low coordinated surface Ce4+
ions are observed in the UV study along with a blue shift in the absorption edge for samples
containing upto 20 mol% ceria. The direct band gaps calculated from the UV spectra are in the
range of 3.45-3.66 eV. The composite oxide materials are porous and spongy in nature. TEM
study of the CeO2(20%)-CaO material indicated the presence of well dispersed mixed oxide
nanoparticles in a continuous CaO matrix. The Ce-Ca-O nanocomposite oxides have been used
as environmentally benign heterogeneous catalyst for the multicomponent one pot synthesis of 2-
amino-2-chromenes by condensation of aryl aldehydes, α-naphthol and malononitrile in aqueous
media. The nanocomposite oxides were found to be highly efficient for synthesis of structurally
diverse aminochromenes with excellent yield and purity.
The Ce-Mg-O nanocomposite oxides are synthesized by gel combustion and amorphous citrate
methods. Structural studies performed on these materials indicate the presence of individual
phases of the composite oxides along with nonequilibrium solid solutions of the type MgxCe1-
x/2O2-x. The crystallite sizes of the composite oxides are in the range of 3-22 nm calculated from
the Fourier line profile analysis of the XRD patterns. For identical composition, the gel
combustion synthesis method provides smaller particles compared to the amorphous citrate
process. The presence of ceria component in a well dispersed form is ascertained from the UV-
Vis study. The ceria particles shows altered UV absorption features corresponding to the low
214
coordinated surface Ce4+
ions. The composite oxides synthesized by either method are of low
density and spongy in nature. Numerous macropores and intraparticle pores are found on the
surface due to the escaping gases formed during calcinations of the precursors. The Ce-Mg-O
materials prepared by gel combustion method are demonstrated as efficient heterogeneous
catalyst for synthesis of arylidenemalononitriles by condensation of aryl aldehydes with
malononitriles. The condensation reactions have been performed under solventfree conditions as
well as using water as reaction media. A variety of aldehydic substrates reacted under the
optimized condition to give the corresponding arylidenemalononitriles in high yield and purity.
The catalytic activity of Ce-Mg-O materials prepared using amorphous citrate method is
evaluated for the synthesis of β-nitro alcohols by condensation of aryl aldehydes and
nitromethane. The composite oxides are found to be highly active for synthesis of structurally
diverse β-nitro alcohols. In this investigation, the applicability of ceria based composite oxides as
catalyst is clearly demonstrated for base catalyzed C-C bond formation reactions.
MAJOR CONCLUSIONS
The applicability of MoO3-ZrO2 composite oxide materials as heterogeneous catalyst is
demonstrated for synthesis of three classes of biologically important molecules. The
coprecipitated MoO3-ZrO2 samples exhibit good catalytic activity for synthesis of amidoalkyl
naphthols and β-acetamidoketones. The combustion synthesized MoO3-ZrO2 composite
oxides are efficient catalyst for the aqueous phase synthesis of octahydroquinazolinones.
Presence of MoO3 in the zirconia matrix selectively stabilizes the tetragonal phase of the
zirconia. The molybdna phase exists in a well dispersed state in the form of isolated and
cluster molybdates in the zirconia matrix.
215
Combustion synthesis is a versatile technique to prepare nano-size MoO3- ZrO2 composite
oxide materials of uniform composition. The nature and amount of fuel controls the particle
morphology and size. Glycine as fuel is quite effective in producing nanoparticles with
uniform morphology and smaller size.
The addition of MoO3 to zirconia matrix enhances the number of surface acidic sites,
modifies the surface area and improves the catalytic activity of the host zirconia.
Iron stabilized zirconia particles synthesized by coprecipitation shows the formation of a
substitutional solid solution and selective stabilization of the tetragonal phase of zirconia.
The presence of iron improves the sulfate retention capacity of the composite oxide.
The sulfate grafted iron stabilized zirconia particles are catalytically active for the synthesis
of xanthenediones under solvent free condition and microwave irradiation.
The Zr-pillared clay materials are modified by grafting sulfate ions onto the zirconia
nanopillars and subsequent dispersion of the phosphomolybdic acid in the micropores of the
pillared clay materials. The modified Zr-pillared clay materials show improved catalytic
properties for synthesis of β-amino carbonyl compounds in aqueous media under mild
conditions.
The applicability of ceria based composite oxide catalysts for base catalyzed C-C bond
formation reaction is studied for the synthesis of 2-amino-2-chromenes,
arylidenemalononitriles and β-nitro alcohols.
The Ce-Ca-O composite oxides synthesized by amorphous citrate method shows the presence
of a substitutional solid solution for ceria rich composite oxide where as biphasic system
comprised of the mixed oxide and calcia phase is observed for the CaO rich composite oxide.
The mixed oxide phase contains nanoparticles with size in the range of 5-25 nm present in a
well dispersed state in the calcia matrix.
216
The Ce-Ca-O composite oxides are efficient catalytic systems for the multicomponent
condensation reaction of aryl aldehydes, α-naphthol and malononitrile in aqueous media for
synthesis of structurally diverse 2-amino-2-chromenes. The catalytic activity of the
composite oxides correlates well with the surface basic properties.
The CeO2-MgO composite oxides synthesized by amorphous citrate and gel combustion
method show the presence individual phases of the oxides along with a non equilibrium solid
solution. The Ce-Mg-O materials synthesized by gel combustion method are catalytically
active for the synthesis of arylidenemalononitriles under solventfree condition as well as in
aqueous media. Similarly, the Ce-Mg-O materials synthesized by amorphous citrate method
are efficient heterogeneous catalysts for synthesis of β-nitro alcohols in aqueous media.
In this thesis, novel protocols have been designed for the synthesis of biologically important
molecules using zirconia, ceria and pillared clay based heterogeneous catalysts. The reaction
protocols developed in this thesis have specific advantages in terms of multicomponent one
pot synthesis, solventfree condition of synthesis, reusable heterogeneous catalysts, preclusion
of toxic solvents, water as green reaction media and high yield and purity of the synthesized
compounds.
217
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LIST OF PUBLICATIONS BASED ON RESEARCH WORK
Refereed Journals
1. Satish Samantaray, D.K. Pradhan, G. Hotaand B.G. Mishra, Catalytic application of
CeO2-CaO nanocomposite oxide synthesized using amorphous citrate process towards the
aqueous phase one pot synthesis of 2-amino-2-chromenes. Chem. Eng. J. 193-194 (2012)
1-9.
2. Satish Samantaray and B.G. Mishra, Combustion synthesis, characterization and catalytic
application of MoO3–ZrO2 nanocomposite oxide towards one pot synthesis of
octahydroquinazolinones. J. Mol. Catal. A: Chem.339 (2011) 92-98.
3. Satish Samantaray, G. Hota and B.G. Mishra, Physicochemical characterization and
catalytic applications of MoO3- ZrO2 composite oxides towards one pot synthesis of
amidoalkyl naphthols. Catal. Commun.12 (2011) 1255-1259.
4. Satish Samantaray, B.G. Mishra, D.K. Pradhan and G. Hota, Solution combustion
synthesis and physicochemical characterization of ZrO2-MoO3 nanocomposite oxides
prepared using different fuels. Ceram. Int.37 (2011) 3101-3108.
5. Satish Samantaray, S.K. Sahoo and B.G. Mishra, Phosphomolybdic acid dispersed in the
micropores of sulfate treated Zr-Pillared clay as efficient heterogeneous catalyst for the
synthesis of β-aminocarbonyl compounds in aqueous media. J. Porous Mater. 18 (2011) 573-
580.
6. Satish Samantaray, P.K. Subudhi and B.G. Mishra, CeO2-MgO nanocomposite oxides
prepared using citrate precursor routes as efficient catalyst towards some base catalyzed C-C
bond formation reactions. Applied Catalysis B (under review).
7. Satish Samantaray and B.G. Mishra, Sulfate grafted iron stabilzed zirconia materials as
efficient heterogenous catalyst for solvent free synthesis of xanthenediones under microwave
irradiation. Catal. Commun. (communicated)
Poster and paper presented/accepted in conference/symposium
1. Satish Samantaray, G. Hota and B.G. Mishra, Combustion synthesis, characterization &
catalytic application of MoO3-ZrO2 composite oxides, National Conference on Recent
Trends in Chemical Science & Technology (RTCST), Dec. 19-20, 2009, Department of
Chemistry, NIT, Rourkela, INDIA.
253
2. Satish Samantaray, S.K. Sahoo, S. Dutta and B.G. Mishra, Phosphomolybdic acid
dispersed in the micropores of Zr-pillared clay an efficient catalyst for the synthesis of
structurally diverse Coumarins, National Conference on Recent Trends in Chemical Science
& Technology (RTCST), Dec. 19-20, 2009, Department of Chemistry, NIT, Rourkela,
INDIA.
3. Satish Samantaray, G. Hota and B.G. Mishra, Catalytic applications of MoO3-ZrO2
nanocomposite oxides towards synthesis of biologically important molecules, National
Conference on Green & Sustainable Chemistry (NCGSC), Feb. 19-21, 2010, Department of
Chemistry, BITS, Pilani, INDIA.
4. Satish Samantaray, G. Hota and B.G. Mishra, Modified Zr-Pillared clay as efficient
heterogeneous catalyst for the synthesis of β-aminocarbonyl compounds, National Seminar
on “Smart Molecules Dec. 25-26, 2010, Department of Chemistry, Apex Institute of
Technology & Management, INDIA.
5. Satish Samantaray, P. Kar, P. K. Subudhi and B.G. Mishra, Selective synthesis and
stability of t- zirconia nanoparticles; structural properties, 13th
CRSI National Symposium in
Chemistry & 5th
CRSI-RSC Symposium in Chemistry,Feb. 4-6, 2011, Department of
Chemistry, NISER & KIIT University, Bhubaneswar, INDIA.
6. Satish Samantaray, P. K. Subudhi and B.G. Mishra, Synthesis and characterization of
tetragonal zirconia nanoparticles: microstructural & optical properties, International
Conference on Chemistry: Frontiers and Challenges,March. 5-6, 2011, Department of
Chemistry, Aligarh Muslim University, Aligarh, INDIA.
Other publications
1. M.A. Naik, Satish Samantaray and B.G. Mishra, Phosphotungstic acid nanoclusters
grafted onto high surface area hydrous zirconia as efficient heterogeneous catalyst for
synthesis of octahydroquinazolinones and β-acetamido ketones, J. Cluster Sci. 22 (2011)
295-307.
2. M.A. Naik, Satish Samantaray, B.G. Mishra and A. Dubey, Phosphomolybdic acid
dispersed in Al-pillared clay (PMA/Al-PILC) as heterogeneous catalyst for the benign
synthesis of benzoxanthenes under solvent free conditions, React. Kin. Catal. Lett. 98 (2009)
125-128.
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