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LD
Hs
Ca
taly
sis
Ion
-Ex
ch
an
ge
Hydrotalcite
Brief history
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LD
Hs
Ca
taly
sis
Ion
-Ex
ch
an
ge
Hydrotalcite
Brief history
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um
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tati
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LD
Hs
Ca
taly
sis
Ion
-Ex
ch
an
ge
Hydrotalcite
Brief history
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Introduction
1.1 Hydrotalcite-Brief history
1.1.1 Structural features
1.1.2 Catalysis
1.1.2.1 Homogeneous catalysis
1.1.2.2 Heterogeneous catalysis
1.1.2.3 Solid base catalysts
1.1.2.4 Hydrotalcite in base catalysis
1.1.3 Ion-exchange
1.1.3.1 Hydrotalcite as anion exchangers
1.1.3.2 Anionic pollutants
1.1.3.3 Hydrotalcite in noxious anion removal
1.2 Synthesis of hydrotalcites
1.2.1 Co-precipitation under low supersaturation
1.2.2 Hydrolysis methods
1.2.3 Miscellaneous methods
1.3 Physicochemical characterization
1.3.1 Powder X-ray diffraction (PXRD)
1.3.2 Fourier transformed infrared (FT-IR) spectroscopy
1.3.3 Thermo gravimetric analysis (TGA)
1.3.4 ICP-OES Elemental analysis
1.3.5 BET adsorption measurements
1.3.6 Scanning electron microscopy (SEM)
1.3.7 Transmission electron microscopy (TEM)
1.3.8 UV-visible spectrophotometry
1.3.9 CHNS analysis
1.3.10 Temperature programmed desorption (TPD)
1.3.11 Temperature programmed reduction (TPR)
1.3.12 Nuclear magnetic resonance (NMR)
1.4 Scope and objectives of the work
1.5 References
Chapter 1 Introduction
Ph.D Thesis 1
1.1 Hydrotalcite-Brief history
In last three decades, hydrotalcite-like (HT-like) materials otherwise known as
layered double hydroxides (LDHs) have received diverse attention such as in catalysis
[1], adsorption[2], sensing [3], polymer blends [4], medicine [5], human health [6],
biological applications [7, 8], thin films for advanced materials [9, 10], nanoscale
research [11] and so forth. Hydrotalcites (HTs) belong to the large class of anionic
clays, and will be taken as a reference name for many other isomorphous and
polytype compounds. Hydrotalcite-like anionic clays as materials form a subset of
clay minerals. Clay minerals and clays constitute the world’s largest and mostly used
material with versatile features [12]. Clays find their potential application in ceramics,
building materials, adsorbents, ion-exchangers, sensors, decolorizing agents and
catalysis [13]. The two main features that evoke the interest on clays are their
common availability and their extraordinary properties [14]. Clay is defined as
materials with particle size less than 2 μm, although the Wentworth scale defines clay
as materials finer than 4 µm [15, 16]. The clays and clay minerals, either as such or
after modification, are recognized as the materials of the 21st century, because they
are abundant, inexpensive, and environment friendly [14]. Nowadays, clay minerals
are source for the preparation of nanostructured advanced materials including
catalysts, adsorbents, organoclays, pillared clays, intercalation compounds, polymer-
clay nanocomposites, agriculture, civil engineering, etc. [17]. The first and most
widely known application of clay in catalysis is French Houdry cracking process
developed in 1930 [18]. Clay minerals with its different and interesting set of
properties are very effective for wide range of organic reactions. The clay materials as
catalysts, exhibit excellent Bronsted and Lewis acid-base properties upon
pretreatment. The high surface area of clays also means that they can act as effective
supports for (usually inorganic) reagents bringing the benefits of heterogeneous
catalysis to several important reactions [19]. According to Lazlo the catalytic property
of clays is not only due to their surface areas and chemical nature. The main features
that contribute to good heterogeneous catalytic ability for clay minerals are local
concentration effects and low dimensionality (<4 µm). The former decides the
strength of adsorption of reactants on the surface and later gives more interaction for
reactant molecules within the surface, which increases the activity [20]. The
adsorptive power and high water retention capacity of clays are responsible for their
Chapter 1 Introduction
Ph.D Thesis 2
extensive applications. Clays and clay minerals can also be classified as layered
materials
Clays are classified into two categories:
1. Cationic or smectite type of clays having layered lattice structure in which
two dimensional oxy-anions are separated by layers of hydrated cations.
2. Anionic or brucite type of clays in which the charge on the layer and the
gallery anion is reversed, complementary to smectic-type clays [21].
Anionic clays, synthetic and natural layered mixed metal hydroxides
containing exchangeable anions, less well known and diffuse in nature than cationic
clays. Hydrotalcites is magnesium/aluminium hydroxylcarbonate layered material
which can be easily crushed into white powder was first discovered in Sweden in
1842 [22]. The stoichiometry of HT, [Mg6Al2(OH)2]CO3.4H2O, was first correctly
determined by Manasse in 1915, who was also the first to recognized that carbonate
ions were essential for its structure [23]. In 1930 Aminoff and Broome [24]
recognized the existence of two polytypes of HTs with rhombohedral and hexagonal
symmetry (proposed by Manasse) through X-ray investigation. In 1942, through the
synthesis of large number of compounds of HT-like structures, Feitknecht proposed
that the structure was assumed to be exist of consecutive layers of brucite [Mg(OH)2],
and aluminum hydroxide [Al(OH)3] and gave the name double sheet structures [25,
26]. In 1969, Allmann and Taylor showed that all the cations were located in the same
layer, and anions together with water molecules lies in the interlayer region through
single crystal X-ray diffraction (XRD) [27-29]. The first patent on HT as catalyst
came in 1970 as an optimal precursor for the preparation of hydrogenation catalysts
[30]. Nearly 40 years later, still many of the fine details of the structure such as the
range of possible compositions and stoichiometry, the extent of ordering of metal
cations within the layers, the stacking arrangements of the layers, the arrangement of
the anions and water molecules in the interlayer galleries are not fully understood. A
literature search of layered materials, over the period of 2000-2006 (limiting to
English as language), revealed nearly 20,000 papers which includes layered
perovskites (LP), pillared clays (PILC) and LDH materials, of which about 85% were
dealing on catalysis . It is also evidenced from the literature that the similar layered
hydroxides like layered perovskites (LP) and pillared clays (PILC) materials are still
mainly at the lab-scale development stage, while HTs find a broad range of
application [31]. This shows the importance of working on clays and clay based
Chapter 1 Introduction
Ph.D Thesis 3
compounds. Hydrotalcites constitute a class of extremely useful materials for
scientists especially in field like catalysis, and ion-exchange. ISI Web of knowledge
[v. 5.5] database search on these materials with the topic search keyword of
“Hydrotalcite* OR Layered double hydroxide*” showed number of articles published
on HTs and LDHs up to February 29th
-2012 has 10,608 records, (Figure 1.1).
Number of articles published up to 2000 was around 1150 and in the new century
literature says that LDHs emerged as one among the promising materials that have
diverse applications. In 2001, 246 articles got published and it increased to 783 in
2010 and increased further to 841 by Dec-2011; this shows the importance of working
on these materials [32].
1998 2000 2002 2004 2006 2008 2010 2012
0
100
200
300
400
500
600
700
800
900
1000
1100
Up
to
19
99
Art
icle
s a
nd
Pate
nts
Year
Figure 1.1 Number of articles and patents published in HTs and LDHs
1.1.1 Structural Properties
Hydrotalcite-like anionic clays are synthetic or natural crystalline materials
consisting of positively charged two-dimensional sheets with water and exchangeable
charge compensating anions in the interlayer region. The identities and ratio of the di-
and trivalent cations and the interlayer anion may be varied over a wide range, giving
rise to a large class of isostructural materials with varied physicochemical properties.
Their general formula is as;
[M(II)1- x M(III)x(OH)2]x+
[Ax/n] n-
]x-
.mH2O
where M(II) and M(III) represent divalent and trivalent metal ion respectively, A is
the interlayer anion with charge n, x represents M(II)/M(III) ratio and m, the water of
crystallization. The value of ‘x’ can be in the range 0.2-0.4. For values outside this
Chapter 1 Introduction
Ph.D Thesis 4
range, pure hydroxides or other compounds with different structure are obtained [33,
34]. The main criteria for elements to crystallize in this network are their ionic radius.
M(II) and M(III) having an ionic radius not too different from that of Mg2+
may be
accommodated in the octahedral sites of the close-packed configuration of the OH-
ions in the brucite-like layers to form HT-like compounds. The ionic radii of M(II)
cation between 0.65 to 0.80 Å and M(III) cation of 0.62 to 0.69 Å (with the main
exception of Al 0.53 Å) can form LDHs [33]. Higher ionic radii (Cd and Sc) seems to
be incompatible with the formation of true brucite-like layers. Recently Guo et al.
reported CdCr and ZnCdCr LDHs. The material showed magnetic properties and also
delaminated in formamide [35]. LDHs can also be obtained with a Li-Al monovalent-
trivalent, [36] Co-Ti divalent-tetravalent [37] and Zn-Mo associations [38]. The single
phase formation of hydrotalcite occurs, after proper selection of M(II) and M(III)
ions. The water molecules are localized in the interlayer sites, which are not occupied
by anions. The amount of water accommodated in the interlayer can be calculated on
the basis of number of sites present in the interlayer.
Structurally, hydrotalcites possess a brucite-like (Mg(OH)2) layered network
wherein a partial substitution of bivalent ion by trivalent ion, say Al3+
, occurs and the
resulting excess positive charge in the layers is compensated by anions located in the
interlayer [39, 40]. The affinity for the interlayer anions can be derived for both mono
and divalent anions (the divalent anions are more strongly held in the interlayer than
the monolayer anions, and carbonate is held most strongly) [41]. The schematic
representation of the hydrotalcite structure is shown in Figure 1.2. The affinity of
interlayer anions within the interlayer of hydrotalcite is:
CO32-
>> SO42-
>> OH- > F
- > Cl
- > Br
- > NO3
- > I
-
Hydrotalcites consists of magnesium ions surrounded approximately
octahedrally by hydroxide ions. These octahedral units form infinite layers by edge-
sharing, with the hydroxide ions sitting perpendicular to the plane. The layers then
stack on top of one another to form the three dimensional structure. The metal cations
occupy the octahedral holes between alternative pairs of OH planes and thus occupy a
triangular lattice identical to that occupied by the OH- ions.
Chapter 1 Introduction
Ph.D Thesis 5
Figure 1.2 Schematic representations of hydrotalcites
The brucite-like sheets can stack on one over the other with two different
symmetries, rhombohedral or hexagonal. The brucite-like layers in LDHs may be
stacked in different ways, which gives raise to variety of possible polytype structures.
LDHs usually crystallize in two different polytypes, one with two-layer hexagonal
stacking sequence (polytype 2H) and another with a three-layer rhombohedral
sequence (polytype 3R). All sites in the (110) plane of the close packed hydroxide
layers may be represented as A, B or C related by lattice translations of (1/3, 2/3, 0) or
(2/3, 1/3, 0) and the locations of octahedral holes occupied by metal cations can be
described analogously as a, b or c. Thus single brucite layer can be represented as
AbC (since, if close packed hydroxyl groups occupy A and C sites, the cations must,
of necessity, occupy b sites). AbC layers may be stacked in various ways giving rise
to a large number of possible polytypes. These polytypes may be classified in terms of
the number of sheets stacked along the c axis of the unit cell. If the opposing OH
groups of adjacent layers lie vertically above one another (say both in C sites), a
trigonal prismatic arrangement (denoted by =) results; if the hydroxyls are offset (say
one layer in C sites and those of an adjacent layer in either A or B sites) then six OH
groups form an octahedral arrangement (denoted by ~). Thus brucite itself can be
denoted as …AbC~AbC~… or 1H, where “1” denotes a one layer polytype and the
“H” denotes a stacking sequence with hexagonal symmetry. Bookin and Dirts [42, 43]
have systematically derived all of the possible polytypes for other stacking sequences.
There are three possible two layer polytypes, each of which has hexagonal stacking of
layers, which can be denoted as,
Chapter 1 Introduction
Ph.D Thesis 6
….AbC=CbA=AbC…. 2H1
….AbC~AcB~AbC…. 2H2
….AbC~BcA=AbC…. 2H3
The interlayers in the 2H1 polytype are all prismatic and those in the 2H2 polytype are
all octahedral, whilst in the 2H3 polytype both types of interlayers are present. There
are nine possible three-layer polytypes. Two of these have rhombohedral symmetry
(3R):
….AbC=CaB=BcA=AbC…. 3R1
….AbC~BcA~CaB~AbC…. 3R2
the remaining seven have hexagonal symmetry:
….AbC~AcB~AcB~AbC…. 3H1
….AbC~AcB~CaB~AbC…. 3H2
….AbC~AcB=BcA~AbC…. 3H3
….AbC~AbC=CbA=AbC…. 3H4
….AbC~AcB=BaC~AbC…. 3H5
….AbC~AcB~CbA=AbC…. 3H6
….AbC~AbA~BcA=AbC…. 3H7
For the 3R1 polytype, the interlayers are all prismatic and in the case of 3R2, 3H1 and
3H2 they are all octahedral; other polytypes involve both types of interlayers. Bookin
and Drits have also described the large number of possible six-layer polytypes, some
of which have rhombohedral symmetry (6R) and the remaining hexagonal symmetry
(6H).
Likewise in the case of hydrotalcite, if some Mg2+
ions are replaced by a
higher valent ion having similar radius different anionic clay minerals can be
synthesized (such as Fe3+
in pyroaurite and Cr3+
in stichtite) [44]. Crystallographic
parameters of different known anionic clay minerals with similar stacking order of are
given in Table 1.1. The parameter ‘a’ gives the cation-cation distance, typical for
hydrotalcite-like materials [45, 46]. The value of ‘a’ parameter has no relation with
the interlayer anions and doesn’t change with anions. However for ‘c’ parameter,
which corresponds to the thickness of the layers, the value depends on the change and
size of interlayer anions. Variation of ‘c’ parameter with different interlayer anion is
given in Table 1.2 [41]. The thickness of the corresponding interlayer region for HT is
the difference between ‘c’ and the thickness of the brucite-layer (4.8 Å.) [12]. A wide
variety of anions like inorganic anions (CO32-
, SO42-
, NO3-, OH
-, CrO4
2-, WO4
2-,
Chapter 1 Introduction
Ph.D Thesis 7
S2O32-
etc.), isopoly anions (V10O282-
, Mo7O242-
etc.), heteropoly anions (PMo12O403-
,
PW12O403-
etc.), complex anions (Fe(CN)63-
, Fe(CN)64-
, IrCl62-
etc.) and organic
anions (carboxylates, porphyrins, pharmaceutically active functional anions and alkyl
sulfates etc.) can be intercalated in the LDH layers.
Table 1.1 Comparison of composition, crystallographic parameters, and symmetry for
different anionic clays [23, 45]
Mineral Chemical composition Unit cell
parameters
Symmetry
a (Å) c (Å)
Hydrotalcite Mg6Al2(OH)16CO3·4H2O 3.054 22.81 3R
Manasseite Mg6Al2(OH)16CO3·4H2O 3.10 15.6 2H
Pyroaurite Mg6Fe2(OH)16CO3·4H2O 3.109 23.41 3R
Sjogrenite Mg6Al2(OH)16CO3·4H2O 3.113 15.61 2H
Stichtite Mg6Cr2(OH)16CO3·4H2O 3.10 23.4 3R
Barbertonite Mg6Cr2(OH)16CO3·4H2O 3.10 15.6 2H
Takovite Ni6Al2(OH)16CO3·4H2O 3.025 22.59 3R
Reevesite Ni6Fe2(OH)16 CO3·4H2O 3.081 23.05 3R
Meixnerite Mg6Al2(OH)16(OH)2·4H2O 3.046 22.92 3R
Desautelsite Mg6Mn2(OH)16CO3·4H2O 3.114 23.39 3R
Table 1.2 Values of ‘c’ with different interlayer anions [23, 47]
Anion OH- CO3
2- F
- Cl
- Br
- I
- NO3
- SO4
2- ClO4
-
c(Å) 7.55 7.65 7.66 7.86 7.95 8.16 8.79 8.58 9.20
1.1.2 Catalysis
Catalysis is one of the most important fields in chemistry, which has very
good applications in industrial research [48]. Catalysis, derived from the Greek word
‘kata’ (cata) means down, and ‘lyein’ (lysis) means loosen. The Swedish chemist
Berzelius (1779-1848) first used the word katalysis meaning breaking down or
loosening in 1836 [49]. During that period in 1814, it was observed by other
researchers that in the presence of acid, the conversion of starch to sugar enhances
and in presence of finely powdered platinum, alcohol got oxidized to acetic acid [50].
Berzelius correlated these observations made by other chemists [51, 52] and
introduced the concept, catalysis. According to German chemist Ostwald who first
scientifically defined catalysis in 1894, catalyst is a substance which alters the rate of
Chapter 1 Introduction
Ph.D Thesis 8
approaching of chemical equilibrium without itself being changed or substantially
consumed in the process [53]. In a reaction the catalyst generally enters into chemical
combination with the reactants but ultimately regenerated so that the amount of
catalyst remains unchanged. The first application of catalysis was done in 1820s by
Dobereiner, who introduced “tinderbox” which was commercially used for the
purpose of lightning fires and smoking pipes. A jet of hydrogen produced by zinc and
sulphuric acid was directed on to the supported platinum where it catalytically
combined with oxygen to yield gentle flame (Million of tinder boxes sold in 1820).
Followed by this several important industrial applications on catalysis emerged such
as;
Industrial oxidation of HCl to Cl2 using clay brick impregnated with cupric
salt as catalyst (Deacon process 1871).
Karlsruhe Fritz Haber prepared copious quantities of ammonia from nitrogen
and hydrogen in presence of a reduced magnetite (Fe3O4) catalyst using high
pressure apparatus (1909).
Industrial synthetic production of methanol using zinc oxide–chromium oxide
catalyst at 400 oC and 200 bar pressure (1923).
Synthetic zeolites were first reported for the selective isomerization of
hydrocarbons in 1960.
Over a billion (109) kilograms of fructose were produced in USA for soft drink
from corn syrup using immobilized glucose isomerase as catalyst (1980).
Some of the path breaking highlights in catalysis reactions are Haber process
for the production of ammonia from gas phase nitrogen [54], Ziegler-Natta catalysts
for the polymerization of olefins [55], Ostwald process for the oxidation of ammonia
to nitric oxide for production of nitric acid [56] and the introduction of Fischer-
Tropsch process [57]. For a good catalyst it must have the following properties;
High and stable activity
High and stable selectivity
Controlled surface area and porosity
Good resistance to high temperature and pressure
High mechanical strength
No uncontrollable hazards
Chapter 1 Introduction
Ph.D Thesis 9
Commercially catalyst makes it possible for the reaction to proceed at rates
high enough to permit their commercial exploitation on a large scale, resulting in
economic benefits. It is also interesting to note that over 90% of industrial processes
involve catalysts in one form or another and the number is rising; interestingly most
of the metals available in nature are involved in catalytic systems in one way or other.
Catalysts are broadly classified into two namely homogeneous catalysts
(dissolve in the liquid reaction mixture) and heterogeneous catalysts (in the form of
insoluble solids).
1.1.2.1 Homogeneous catalysis
Homogeneous catalysis is wherein the reactant and catalyst are in the same
phase, which is usually the liquid phase. In general, homogeneous catalysts are more
active and more selective; it is due to the homogeneity of the active site. But it has
several demerits also. The separation of the catalyst is difficult and hence purification
becomes tough and energy intensive and expensive to separate catalysts from
products.
1.1.2.2 Heterogeneous catalysis
In heterogeneous catalysis, the catalyst and the reactant may be in different
phase. The catalyst will be in solid phase and the reactants may be in liquid or gas
phases. Therefore, heterogeneous catalysts are also broadly referred as solid catalysts.
Heterogeneous catalysts are widely used in refining, petrochemical industry and
pharmaceutical industries. Eventhough it is less active and selective than
homogeneous catalyst, it is economical because of easy separation of products and
reusable. The main advantages of solid catalysts over conventionally used
homogeneous catalysts are: environmental friendly, reusable, non-corrosive and easily
separable from product mixture and also possess higher activity, selectivity, and
longer catalyst life. Heterogeneous catalysts allow high regio and chemo selectivity
due to shape selectivity of porous solids, and poly functionality (acid, base, redox
etc.) allowing multi step reactions.
Heterogeneous catalytic processes can broadly be classified into redox
catalysts and acid-base catalysts [58]. In redox catalysis, the catalyst influence the
bond breaking of reactant molecules with the formation of unpaired electrons, and
further formation of new bonds with the participation of electrons [59]. Acid-base
Chapter 1 Introduction
Ph.D Thesis 10
catalysts either posses a tendency to donate a proton/to accept an electron pair, or
accept a proton/to donate an electron pair as per Bronsted (-Lowry) concept for acids
& bases. These definitions are adequate for an understanding of the acid-base
phenomena shown by various solids, and are convenient for a clear description of
solid acid and base catalysis. However, it should be noted that the same site could
serve as a Bronsted base as well as a Lewis base, depending on the nature of the
adsorbate in a reaction [60].
Solid acid and base catalysis are fast growing research area due to increasing
environmental awareness and concerns. Up to now, more than hundreds of solid acid
and base catalysts are reported in the literature; among them, most are concerned with
or pertain to solid acid catalysts. The reason for rapid growth in the area of solid acid
catalysis is due to great progress/developments in refining and petrochemical
industries in the last 50 years, for example in cracking process. The range of such
materials available include the acidic forms of ion-exchanged resins, zeolites and
mesoporous silicates, modified oxides such as zirconia, immobilized forms of Lewis
acids such as metal halides, Bronsted acids including phosphoric, triflurosulphonic
acids and heteropoly acids (HPAs). Fewer efforts have been made on heterogeneous
base-catalyzed reactions when compared to acid catalysis. Tanabe and Hoelderich
made a statistical survey till 1999, over different type of catalysts (Figure 1.3) in
industrial processes [61]. The total number of commercial processes related to solid
base (8%) and bi-functional catalysts (11%) is much less than that of solid acid
catalysts (81%).
Solid base catalysts
Solid acid-base bifuctional catalysts
Solid acid catalysts
81%
11% 8%
Figure 1.3 Industrial processes based on the heterogeneous catalysts [61]
Chapter 1 Introduction
Ph.D Thesis 11
Nevertheless, the application of heterogeneous base catalysts to the synthesis
of fine and specialty chemicals is receiving increased attention as it is a route to the
design of safer, cleaner, and more sustainable environment friendly industrial
processes. The commercial processes those used solid base catalysts are shown in
Table 1.3.
Table 1.3 Commercial processes using solid base catalyst [62]
Process Catalyst year
Alkylation of phenol with methanol MgO 1970
Iso-Butyraldehyde to iso-butylisobutyrate ZrO2 1974
Dehydration of 1-hexylethanol ZrO2 1986
Alkylation of cumene with ethylene Na/KOH/Al2O3 1988
Isomerization of safrole to iso-safrole Na/KOH/Al2O3 1988
Isomerization of 2,3-dimethyl-1-butene Na/KOH/Al2O3 1988
Isomerization of 3,5-vinylbicyclo[2.2.1]heptene Na/KOH/Al2O3 1988
Reduction of carboxylic acid to aldehyde ZrO2-Cr2O3 1988
Thiols from alcohols with hydrogen sulfide Alkali/ Al2O3 1988
Dehydration of propylamine-2-ol ZrO2-KOH 1992
Esterification of ethylene oxide with alcohol Hydrotalcite 1994
Cyclization of imine with sulfur dioxide Cs-zeolite 1995
Alkylation of o-xylene with butadiene Na/ K2CO3 1995
Isomerization of 1,2-propadiene to propyne K2O/ Al2O3 1996
Dehydrotrimerization of iso-butyraldehyde BaO-CaO 1998
1.1.2.3 Solid base catalysts
Base catalysts play a decisive role in a number of reactions essential for fine-
chemical synthesis as compared to acid catalysts which finds applications largely in
petroleum refining. Solid-base catalysts find application in reactions including
isomerization, aldol condensation, Knoevenagel condensation, Michael condensation,
oxidation, and Si-C bond formation [63]. Base catalyzed condensation is one of the
well known methods for C-C bond formation. The first study of solid base catalyst
includes sodium dispersed on alumina in 1950s for double bond isomerization of
alkenes [64], calcium oxide and magnesium oxide for 1-butene isomerization [60].
Later Zeolites that have been ion exchanged with alkali metal salts show weak
activity whose base strength can be increased by increase in alkali weight percentage
Chapter 1 Introduction
Ph.D Thesis 12
[65]. MgO was also used in series of base catalyzed reactions [66]. The study on the
developments of solid base catalysts were extended to single metal mixed oxide up to
1970s. Busca recently reviewed the basic properties of different solids which found
application in industrial catalysis [67] and one among them is LDHs. Different solid
base catalysts available in literature are given in Table 1.4.
Table 1.4 Types of solid base catalysts [61-63]
Typical catalyst Details of the catalyst
Single metal
oxide
MgO, CaO, SrO, BaO, Al2O3, La2O3, YbO2, ZrO2
Mixed oxides MgO-Al2O3, Al2O3-B2O3, ZrO2-MgO, ZrO2-NaOH, ZrO2-KOH,
SiO2-Al2O3, Al2O3-NaOH-Na, Al2O3-KOH-K, MgO-TiO2
Non-oxides KF/Al2O3, KNH2/Al2O3, Lanthanide imide, nitride on zeolite,
Si2N2O, AlPON, ZrPON, AlGaPON
Zeolites K, Rb and Cs–exchanged zeolite X, Y; nitriles impregnated on
zeolites
Supported alkali
metal ions
On silica, alumina, alkaline earth oxide
Mesoporous
materials
Functionalized by amino groups, MgO/SBA–15
Basic supported
catalyst
KF/ Al2O3, Na/NaOH/ Al2O3, Na/MgO, Na/ K2CO3
Clay and modified
clay
Hydrotalcite, Chrysotile, Sepiolite
Other Modified natural phosphate (NP), Calcined NaNO3/NP, Chitosan
1.1.2.4 Hydrotalcites in base catalysis
Compared to other basic materials like sepiolite, modified zeolites and organic
resins, LDH derived materials show improved thermal stability and diffusion
resistance [68]. Specific metals/anions can be incorporated in the octahedral layer and
thus impart catalysis activity [69, 70]. The anion-exchange strategy is especially
successful in the context of heterogenizing homogeneous catalysts [71]. These
materials on calcination (weight loss of up to 40%) leads to formation of highly
dispersed, large surface area and porous nano dimensional mixed metal oxides, which
can be potentially used for many base catalyzed reactions [72]. The hydrotalcite (HT)
Chapter 1 Introduction
Ph.D Thesis 13
as such is a solid base or, depending on the elemental composition of its octahedral
layers, may have redox properties. HTs are excellent materials to design bifunctional
redox-base catalysts [58, 73-75]. Potential applications of HTs range from the
production of large-scale basic chemicals to the synthesis of small-scale specialty
chemicals [12].
A particular advantage for HTs as base catalysis is that the number and
strength of basic sites can be tuned precisely to a specific reaction. The basic property
of these materials was initially envisaged by Nakatsuka et al. [76], Reichle [77] and
Laylock et al. [78], for catalytic polymerization and aldol condensation reactions. The
basicity of LDH is affected by the calcination procedure, typically at 673-773 K and
by structural and compositional parameters [79, 80]. Cations like Co, Zn or Ni give
less basicity than Mg; less basic catalysts are also obtained from Cl- or SO4
2-
precursors than from CO32-
or OH-containing materials [77] and the basicity also
depends on the Mg/Al ratio [81]. The correlation of the LDH basic properties with the
Mg/Al ratio, however, is not always straightforward [77].
The basicity for hydrotalcite in the as-synthesized form is due to hydroxyl
groups (OH-) that provide Bronsted basic sites [82-84]. In case of calcined LDHs, the
basicity is due to the presence of strong O2-
Lewis basic sites, Mn+
Lewis acid sites,
and Mn+
-O2-
pairs [85]. Figure 1.4 represents schematic picture of catalytic sites
available in hydrotalcite-like materials.
Different alternate synthetic procedures like microwave and sonication leads
to materials with different basic characters [86-88]. Homogenous particles of ~80 nm
average particle size and with higher defects was produced using sonication showed
high basicity [88]. Sol-gel derived materials showed higher surface area than the co-
precipitated materials that are beneficial in catalysis [89]. Reconstruction of thermally
treated hydrotalcite (also known as memory effect) increased the basicity with an
increase in hydroxide ions [90]. Ebitani et al. showed that reconstructed LDH found
to be active for the aldol reactions to produce α-hydroxy carbonyl derivatives in the
presence of water and also can promote the aqueous Knoevenagel and Michael
reactions using nitriles [91].
Chapter 1 Introduction
Ph.D Thesis 14
Figure 1.4 Catalytic sites available in hydrotalcite and derived forms
The basicity of hydrotalcite-like materials was assessed using different
techniques. Among these techniques for correlating basicity with activity, most
common ones are CO2-TPD, calorimetry, NMR and FT-IR spectroscopy using NH3
and pyrole as probe molecules [92-94]. Despite many reports on calcined forms,
necessary correlation between the activity and basicity of as-synthesized LDHs is
scarce and challenging [95]. Earlier attempts for finding the basicity of calcined HT-
like materials include decomposition of 2-methyl-3-butyn-2-ol, 2-propanol,
isophorone isomerization, phenol adsorption, allylbenzene isomerization [96-101].
The presence of both Lewis and Bornsted type basic sites in LDHs and ability to host
different metal ions extended its applications in both base catalyzed reactions and
oxidation reactions [102].
Corma et al. studied the basic property of the materials by carrying out the
condensation of benzaldehyde with ethyl acetoacetate in presence of Mg-Al LDH
catalyst. They found that this material shows basic sites with pKa values up to 16.5,
which are normally uncommon among the other commercial basic zeolite materials
Chapter 1 Introduction
Ph.D Thesis 15
[80]. With zeolite as catalyst, the only reaction observed was Knoevenagel
condensation, while calcined LDHs showed other reactions like Michael-type addition
and Claisen condensation, which requires stronger basic sites. The total amount of
basic sites and their strength distribution in the material were determined by carrying
out the Knoevenagel condensation reaction with methylenic groups of different pKa
values in presence of increasing amount of benzoic acid. By increasing the Mg/Al
ratio in the LDH, the number of basic sites with 9.0 ≤ pKa ≤ 13.3 increases, whereas
the amount of basic sites within 13.3 ≤ pKa ≤ 16.5 decreases.
Kustrowski et al. studied the acidic and basic properties of the MgAl mixed
oxides derived from LDHs with different interlayer anions using both NH3 and CO2
TPD [103]. These results showed that concentration of basic sites are in the order of
CO32-
> Cl- > HPO4
2- > Terephthalate > SO4
2- and the acidic sites are in the order of
CO32-
> Terephthalate > Cl- > HPO4
2- > SO4
2-. Both CO3
2- and Cl
- showed higher
basic sites whereas HPO42-
, SO42-
and terephthalate showed weak basic sites.
Kustrowski et al. also studied the variation of Mg/Al ratio over the acidic and basic
property of the LDH using NH3 and CO2 TPD [104]. Materials calcined at different
temperatures showed different acid-base behaviours. For materials calcined at 550 oC
the concentration of acidic sites are in the order of 3.5:1 > 3:1 > 4:1 > 2:1 Mg:Al
ratios, whereas concentration of basic sites are in the order of 2:1 > 3:1 > 3.5 :1 > 4:1.
Choudhary et al. attempted to increase the basicity of MgAl hydrotalcite by
incorporating tert-butoxide and was reported that so formed MgAl-O-t-Bu was active
for base catalyzed organic transformations like cyanoalkylation, Henry reaction,
transesterification, and aldol condensation. They reported that the basicity of MgAl-
O-t-Bu catalysts was much higher than as-synthesized, calcined and rehydrated MgAl
hydrotalcites. The tert-butoxides which is an organic base provides the basicity [105-
108].
Chimentao et al. studied the effect of using mechanical stirring or ultrasound
during reconstruction of the mixed oxides and showed that this leads to an
enhancement in the catalytic activity. Modifications in the structure and basicity of
the resulting materials, together with an increased surface area and improved
accessibility to the active sites lead to higher activity. Increasing the rehydration time
during stirring or sonication also strongly affects the final catalyst and the
performance of these materials has been disclosed for the epoxidation of styrene
[109].
Chapter 1 Introduction
Ph.D Thesis 16
Figueras et al. conducted detailed investigation on the basic properties of
hydrotalcite like materials and published two reviews on the basic properties and the
catalytic applications of hydrotalcite-like materials for fine chemical synthesis. The
authors examined the efficiency of HT for different organic transformations like
aldolization, condensation, hydrogenation, oxidation, and so forth [110, 111].
Onda et al. used activated hydrotalcites for the lactic acid production from D-
glucose in flow reactor at 323 K in aqueous media. The number of accessible
Bronsted-base sites was determined by the ion-exchange method with sodium salts,
based on the OH/Al ratio. The catalytic activity for the lactic acid production showed
a linear increase with the number of the Bronsted-basic sites [112].
Kantam et al. reported an eco-friendly, simple and efficient catalytic system
for selective aerobic oxidation of alcohol to aldehyde. The racemic-BINOL over
CuAl-HT in presence of K2CO3 as base gave selectively aldehyde under mild
conditions [113]. In another study they supported lithium diisopropylamide (LDA)
over MgAl3-HT and studied for aldol, Knoevenagel, Henry, Michael,
transesterification and epoxidation reactions. They reported that these composite
catalysts were effective for C-C and C-O coupling reactions [114].
Recently Kaneda et al. used silver nanoparticles supported on LDH that
efficiently catalyzed chemoselective reduction of nitroaromatic compounds into the
corresponding amines using CO/H2O as a reductant. The basicity of LDH facilitates
the formation of active AgH- species [115]. Solid base-metal combination of copper
nano particles supported on LDH was used for oxidant free alcohol dehydrogenation
[116]. LDH supported gold nanoparticles acted as a reusable catalyst for the synthesis
of lactones from diols using molecular oxygen as an oxidant under mild conditions
and reported that the basicity of supports and the size of the Au particles are key
factors in promoting the above oxidative lactonization [117].
Zeng et al. showed that calcined LDH with Mg/Al molar ratio of 4.0 exhibited
the highest catalytic activity in the synthesis of propylene glycol methyl ether from
etherification of propylene oxide with methanol. The catalytic activity of the material
was correlated with the amount of the basic sites determined by Hammett indicators
[118].
Our group in CSMCRI, has expertise in exploiting LDHs for various base
catalyzed transformations such as condensation [119], double bond isomerization of
alkenyl aromatics [120-122] and also in redox calaysis. Several reports have been
Chapter 1 Introduction
Ph.D Thesis 17
published recently on hydroxylation of phenol and benzene using various transition
metals containing multi-metallic hydrotalcites [123-127].
1.1.3 Ion-exchange
Ion-exchange may be defined as reversible interchange of ions between a solid
phase and a solution phase, where an atom or molecule from solution is exchanged for
a similarly charged ion attached to an immobile solid particle. Ion-exchange process
was first studied over inorganic solids like soil, rocks, clays, zeolite and so forth
[128]. In most cases the term ion-exchange is used to denote the processes of
purification, separation, and decontamination of aqueous and other ion-containing
solutions with solid. Materials having ion-exchange capability possess wide spread
application especially in waste-free technologies as well as in fields like nuclear
chemistry and polymeric materials.
The important features for material to be good ion-exchangers are,
The structure should be hydrophilic
Controlled and effective ion exchange capacity
Rapid rate of exchange
Chemical stability
Physical stability in terms of mechanical strength and resistance
Consistent particle size and effective surface area
Reproducibilty
Among ion-exchangers, cationic exchangers like aluminosilicates possess
great industrial importance [129, 130]; while that for anionic exchangers are less
familiar. Anion exchangers are a class of ion-exchangers consist of fixed ions and
anions that can undergo exchange. Anion exchange materials are classified as either
weak base or strong base depending on the type of exchange group. In general these
are solid bases (Bronsted or Lewis) over wide range of pH [128]. An anion exchange
reaction can be written as:
The effectiveness of anion-exchangers is expressed in terms of anion exchange
capacity (AEC). AEC is a measure of total content of exchangeable ions, and is
Chapter 1 Introduction
Ph.D Thesis 18
conventionally expressed in terms of the total number of equivalents of ion, usually in
milli-equivalents per gram, meq/g.
1.1.3.1 Hydrotalcite as anion exchangers
Hydrotalcite-like materials in the current research arena form widespread class
of anion exchangers. Ion-exchange process in HT is given schematically in Figure
1.5. The structural features of LDHs makes it a good exchanger for anions through
different mechanism like anion exchange with interlayers, reconstruction of heat
treated materials, surface precipitation, surface adsorption and so forth. Anion
exchange capacity (AEC) of hydrotalcite-like (HT-like) materials usually are around
2-3 meq/g (much less than theoretical maximum of 3.6 meq/g [131]), which is
comparable to that of commercial anion exchangers [132].
Figure 1.5 Typical ion-exchange process over HT
The AEC of LDHs has reliance on the interlayer anions which undergoes
substitution during the anion exchange; usually hydrotalcites with carbonate as the
interlayer anion shows less adsorption capacity due to the high affinity of carbonate
with the layers. Generally carbonate anion is almost impossible to remove by ion-
exchange which has large preference over other anions in layers. However,
hydrotalcites with nitrates or chlorides in the interlayer generally bestow relatively
good anion exchange capacity (than carbonate containing LDHs) and are well
explored for the removal of contaminants [133, 134]. The literature also says that the
calcined hydrotalcites shows good removal capacity for toxic anions through
reconstruction mechanism than as-synthesized forms [135]. It is also known that
anions with higher charge density exchange more strongly than monovalent anions.
The anion exchange capacity for HT has been affected by anion species in the
interlayer and the layer charge or ratio of divalent and trivalent metal ions. Miyata and
Chapter 1 Introduction
Ph.D Thesis 19
co-workers have done extensive studies on anion exchange properties of hydrotalcite-
like materials. They determined the ion-exchange equilibrium constants of HTs with
anions in the sequence as; CO32-
>> SO42-
>> OH- > F
- > Cl
- > Br
- > NO3
- > I
- [41].
The anion-exchange capacity in the case of hydrotalcite-like materials is calculated by
the formula [136],
where x stands for M(II)/M(III) atomic ratio and FW is formula weight
1.1.3.2 Anionic pollutants
Increase in the concentration of harmful ions (both anions and cations) in
water bodies causes environmental concern (or pollution) that not only threatens the
human life but also put at risk the life of aquatic organisms. The main protocol of the
governing bodies around the globe in this 21st century is the preservation of nature, or
other way controlling air, land and water pollution. The present scenario mainly aims
on water pollution which is a major problem to our society as we know how important
water to mankind. The increased industrialization and inadequate disposal of waste is
a serious problem that we are facing in the environmental perspective. Hence the
removal of hazardous anions is a big challenge towards environmental view point.
Number of processes is available for decontamination of noxious anions from
polluted water bodies, mainly coagulation [137], surface precipitation [138], ion-
exchange through resins [139], magnetic separation [140], bio sorption [141], and
sorption process [2]. Sorption process has its own advantages towards water
remediation due to less waste disposal. The water remediation properties of LDHs are
due to the inherent nature of material that have large surface area for the adsorption of
noxious anion, facile exchange of interlayer anion, and tailorable
hydrophobic/hydrophilic nature.
1.1.3.3 Hydrotalcite in noxious anion removal
For the uptake of toxic anions, various materials like carbons, zeolites, carbon
nanotubes, chitin, metal oxides, metal hydroxides, polymer composites, clays as
adsorbent are available [142-150]; among them layered double hydroxides or
hydrotalcite-like materials received more attention towards removal of toxic anion
(oxy-anions) in last few decades. Hydrotalcites as anion exchangers have also gained
Chapter 1 Introduction
Ph.D Thesis 20
significant progress in the research and development for the removal of organic,
inorganic and nuclear wastes from contaminated waters [2]. Hydrotalcites are widely
used for the removal of inorganic monovalent anions like nitrate, fluoride, chloride,
bromide, iodide, divalent oxy-anions like phosphate, chromate, selenate, arsenate,
borate, organic pesticides, dyes, and so forth (Table 1.5).
Table 1.5 Uptake of pollutants (anions) over various hydrotalcite-like materials
Toxic
anions
Hydrotalcite (HT) References
Arsenite As-sythesized and calcined MgAl, hydrocalumite [151, 152]
Arsenate As-sythesized and calcined MgAl, hydrocalumite, MgAl-
NO3
[153-155]
Chromate As-synthesized Ni–Fe, as-synthesized and calcined
MgAl, ZnAl
[134, 135,
156-158]
Phosphate As-synthesized and calcined ZrZnAl, ZnAl [138,
159-162]
Selenite ZnAl, MgAl, ZnFe LDHs, MgAl and ZnAl [163-165]
Selenate As-sythesized and calcined MgAl [165, 166]
Borate As-synthesized and calcined ZnAl, calcined MgAl,
MgAl-NO3, MgAl and MgFe
[167-170]
Nitrate MgAl, CoFe, NiFe, and MgFe, as-synthesized MgAl,
CoFe, NiFe, and MgFe-Cl
[171, 172]
Iodate As-synthesized MgAl LDHs and Mg-Al-NO3 [173]
Fluoride MgAl, NiAl, and CoAl, ZnAl, calcined MgAl [174-177]
Iodide Calcined MgAl [178]
Bromide As-synthesized and calcined MgAl, MgAl [179, 180]
Molybate As-synthesized and calcined MgAl, hydrocalumite [181-183]
Vanadate Calcined MgAl [184, 185]
Dyes MgAl-NO3 [186]
Pesticides MgAl-Cl, MgFe-Cl, MgAlFe-Cl, calcined MgAl [187]
This doctoral work is designed after understanding the importance of
hydrotalcite-like materials in the field of catalysis and ion-exchange. They were used
for industrially important organic transformation exploiting its basicity and as
exchangers for the removal of toxic chromate anions from aqueous conditions.
Chapter 1 Introduction
Ph.D Thesis 21
1.2 Synthesis of hydrotalcites
Hydrotalcite like materials can be prepared by different methodologies,
namely, precipitation at constant pH, precipitation at variable pH, deposition
precipitation reactions, hydrothermal synthesis, anion exchange, sol-gel, structure
reconstruction, electro chemical methods and hydrolysis reactions [188].
1.2.1 Co-precipitation under low super saturation
Conventionally MgAl hydrotalcites are synthesized by co-precipitation of
aqueous alkaline solutions of Mg and Al salts (nitrates or chlorides) at fixed pH under
stirring and in some cases in inert atmosphere (Figure 1.6). Classically, an aqueous
NaOH or KOH solution is used to adjust the pH and an aqueous Na2CO3 or K2CO3
solution is added as the carbonate source [189]. After aging of the slurry, the resulting
material is filtered, washed with deionised water and dried (~373 K). In the case of
nitrate containing HTs, NaNO3 along with NaOH was used as precipitating agent and
all the process like metal addition and aging was done under N2 atmosphere. The
aging step may itself be used as a tool to change the properties of the final material.
For example, microwave irradiation may be used during aging in order to provoke a
reduction in the size of the particles, an increase in the specific surface areas and an
increase in the basicity [190].
M2+ + M3+
Solution (A)
NaOH + Na2CO3
Solution (B)
Slurry
Crystallized Gel
Dry Sample
110°C, 12h Drying
65°C, 18h
Aging
Figure 1.6 Schematic sketch of synthesis of hydrotalcites by coprecipitation at fixed
pH
Hydrotalcites based on other metals (Co, Ni, Cu, etc.) can be synthesized in
similar way by introducing appropriate nitrate salt [191,192]. However it is necessary
Chapter 1 Introduction
Ph.D Thesis 22
to precipitate at a pH higher than that of pH of precipitation of metal hydroxides.
Materials synthesized by this method has several advantages like high surface area,
multiple metal ions containing LDHs can be synthesized simply by taking different
metal nitrate precursors, highly dispersed metal ions containing LDHs and
crystallinity of the material can be tuned by varying the aging time and temperature.
Several tetravalent metal cations containing LDHs were also synthesized successfully
using this method [159, 193].
1.2.2 Hydrolysis methods
Urea on hydrolysis at high temperature releases ammonia which precipitates
the metal cations in the synthesis of LDHs. The urea method was initially developed
by Costantino et al. and has become very popular and undergone several
modifications [194]. A typical synthetic procedure is as follows: An aqueous stock
solution of urea (1.0 M), magnesium chloride (0.1 M) and aluminium chloride (0.1
M) are mixed together at the molar Mg/Al/urea ratio of 4:1:10 with magnetic stirring
at room temperature. After cooling to room temperature, the solid precipitate is
collected by centrifugation and washed with deionized water subsequently. In
aqueous solutions, urea decomposes on heating to give ammonia and HNCO. In
acidic or neutral media, HNCO is converted into CO2, and ammonia takes up a proton
to give NH4+. Both these steps lead to consumption of H
+ and hence increase the pH
of the medium [195, 196]. The so formed NH4+
precipitates the metal nitrates. Urea
hydrolysis is found to be one of the best methods to produce the highly crystalline
LDHs [197].
Hexamethylenetetramine (HMT), also called as hexamine, found to hydrolyze
at higher temperature in aqueous solution release, ammonia similar to urea, which
makes the solution alkaline. In a typical procedure metal nitrates or chlorides with
hexamine in molar ratio of (M2+
+ M3+
): HMT with 0.15 M: 0.45 M were taken in a
Teflon inner vessel with a stainless steel outer vessel and allowed to react at 140 oC
for 24 h. Pioneering synthesis of LDH using hexamine was done by Iyi et al. [198]
and they have achieved hexagonal crystalline MgAl-LDH. Hexamine on hydrolysis
gives ammonia and formaldehyde; formaldehyde on reductive amination with
ammonia (Leuckart reaction) gives formic acid and subsequently leads to carbonate
containing LDH. It was also proposed that slow release of ammonia leads to highly
crystalline hexagonally shaped LDH. Followed by Iyi, several works were reported on
Chapter 1 Introduction
Ph.D Thesis 23
the synthesis of LDH as well as brucite-like mixed hydroxides using HMT hydrolysis
[199, 200].
1.2.3 Miscellaneous methods
Apart from these three important methods, several other methods are also
reported. The sol-gel synthesis of materials based on the hydrolysis and condensation
of molecular precursors is used to prepare a wide range of inorganic materials. This
procedure gives sols and these sol colloidal particles suspended in a liquid, progress
through a gelation process to form two interpenetrating networks between the solid
phase and the solvent phase. This technique limits the amount of alkali required and
thus sometimes preferred for industrial synthesis processes [201, 202]. The synthesis
procedure is as follows: Magnesium ethoxide is dissolved by acid hydrolysis with
HCl (35% in water) or with HNO3 (65%) in 120 ml of ethanol; the solution is refluxed
at 353 K, under constant stirring. A second solution, containing a suitable amount of
aluminium acetylacetonate in 80 ml of a mixture acetone/ethanol 1:1 in order to
obtain Mg2+
/Al3+
ratios in the range 3-6, is then slowly added, and the pH is adjusted
to about 10 with NH4OH. The solution is then refluxed at 353 K for 17 h, until the gel
was formed. Gel is then repeatedly washed with ethanol and dried overnight at 353 K
and gel can also be exchanged with Na2CO3 aqueous solution to get carbonate in the
interlayers of LDH [203]. Sol-gel synthesis of ZnAl-LDH without impurity phase was
prepared [204] which were not possible previously. Recently, Valente et al.
synthesized the novel multi-metallic LDHs with nanocapsular morphology using sol-
gel method [205, 206] and one of the advantages here is that these materials shows
high surface area than both co-precipitation and urea/hexamine hydrolysis materials.
Other methods include synthesis of LDH nanomaterials with uniform
crystallite size through separate nucleation and aging steps [207] and explored for
various applications. NiAl-LDH was synthesized by Mizuhata et al. using liquid
phase deposition (LPD) using aluminium metal [208]; highly stable LDH obtained
through this method can be applied in nickel-metal hydride batteries. Davila et al.
synthesized MgAl-mixed oxides using combustion method using sugar as fuel [209]
and was further converted to LDH using memory effect. Recently O’Hare synthesized
LDHs with unique morphologies using reverse microemulsion method [210]; the
surfactant/water ratio enables them to obtain nanometer sized LDH platelets typically
with 40-50 nm diameter and 10 nm thickness. MgAl and ZnAl-LDH were synthesized
Chapter 1 Introduction
Ph.D Thesis 24
using lazer ablation in water [211] and are also synthesized through electrochemical
methods [212].
1.3 Physicochemical characterization
1.3.1 Powder X-ray diffraction (PXRD)
The powder X-ray diffraction (PXRD) is a diagnostic tool for the phase
identification of these compounds. The PXRD patterns of all clay minerals possesses
layered structure that generally show sharp, symmetric peaks at lower angels (2θ) and
broad, asymmetric peaks at higher diffraction angles. The thickness of the brucite-
like layers (4.8 Å) [213], and the interlayer space vary depending on the size and
orientation of anion (2.8 Å shows presence of carbonate anion with its molecular
plane parallel to the brucite-like layers). Lattice parameter ‘a’ is the distance between
the neighboring cations in the brucite-like layers, which can be estimated from the
ionic radii of the cations in the brucite-like lattice [191] and their molar fractions in
the samples while the parameter ‘c’ is three times the distance between the adjacent
brucite-type layer, controlled mostly by the size (and orientation) of the interlayer
anion and the electrostatic forces operating between the interlayer anion and the
layers. PXRD was carried out in a Philips X’Pert MPD system using Cu K radiation
( = 1.5406 Å). The operating voltage and current were 40 kV and 30 mA,
respectively. A step size of 0.04˚ with a step time of 2 seconds was used for data
collection. The data were processed using the Philips X’Pert (version 2.2e) software.
Identification of the crystalline phases was made by comparison with the JCPDS files
[214]. PXRD of some of the samples was also carried out in Rigaku-Miniflex II using
Cu K radiation ( = 1.5406 Å). The operating voltage and current were 30 kV and 15
mA, respectively. A step size of 0.04˚ with a step time of 2 seconds was used for data
collection (scan speed, 1.2 deg/min).
1.3.2 Fourier transformed infrared (FT-IR) spectroscopy
Although infrared (IR) analysis is not a primary tool for the characterization of
LDHs, yet it has been routinely used especially for the identification of the foreign
anions in the interlayer space and its interaction with the brucite-like sheets. Besides
that, information about the type of bonds formed by the anions and about their
orientation can also be discerned. FT-IR absorption spectra of the samples were
recorded in a Perkin-Elmer FT-IR spectrometer (Model-FT-1730). The powdered
Chapter 1 Introduction
Ph.D Thesis 25
samples were ground with KBr in 1:20 ratio and pressed into pellets for recording the
spectra. 64 spectra (recorded with a nominal resolution of 4 cm-1
) were accumulated
and averaged to improve the signal-to-noise ratio.
1.3.3 Thermogravimetric analysis (TGA)
The thermal behavior of LDHs is generally characterized by two transitions:
The first one being reversible, endothermic, at low temperature corresponds to the
loss of interlayer water, without collapsing the structure and the second one
endothermic, at higher temperature is due to the loss of hydroxyl group from the
brucite-like layer as well as of the anions. The nature of these two transitions depends
on many factors such as: M(II)/M(III) ratio, type of anions, low temperature treatment
(hydration, drying etc.) and atmosphere (in case of oxidizable element such as
Cr(III)). Thermogravimetric analysis (TGA) was carried out in Mettler TGA/SDTA
851e and the data were processed using Star
e software, in flowing nitrogen at a flow
rate of 60 ml/min and at a heating rate of 10 oC/min.
1.3.4 ICP-OES elemental analysis
Elemental chemical analyses of the samples were determined using
inductively coupled plasma optical emission spectrometry (ICP-OES; Perkin Elmer,
OES, Optical 2000 DV). The samples were digested in minimum amount of
concentrated HNO3 and diluted using milli Q water (conductivity ~ 18 m Ohm) and
analyzed.
1.3.5 BET adsorption measurements
Specific surface area and pore size analysis of the samples were measured by
nitrogen adsorption at -196 oC using a sorptometer (ASAP-2010, Micromeritics). The
samples were degassed under vacuum at 120 oC for 4 h prior to measurements in
order to expel the interlayer water molecules. The BET specific surface area (SA) was
calculated by using the standard Brunauer, Emmett and Teller method [215] on the
basis of adsorption data. Pore volume (PV), micropore area and mesopore area were
determined by using the t-plot method of De Boer [216]. Average pore size
distributions (APD) were calculated from the desorption branch of the isotherms
using the Barret, Joyner and Halenda (BJH) method [217].
Chapter 1 Introduction
Ph.D Thesis 26
1.3.6 Scanning electron microscopy (SEM)
The scanning electron microscopic studies were done in a scanning electron
microscope (Leo series VP1430) equipped with EDX facility (Oxford Instruments),
having silicon detector. The samples were coated with gold using sputter coating
before analysis to avoid charging effects during recording. Analyses were carried out
with an accelerating voltage of 20 keV and a working distance of 17 mm, with
magnification values up to 100,000x.
1.3.7 Transmission electron microscopy (TEM)
Transmission electron microscope (TEM) images were obtained with a JEOL
JEM-2100 microscope with acceleration voltage of 200 keV using carbon coated 200
mesh copper/gold grids. The samples were ultrasonically dispersed in ethanol for 5
min and deposited onto carbon film using capillary and dried in air for 30 min.
Elemental mapping analysis were done using STEM mode of HRTEM using an
energy dispersive X-ray (EDX) detector (Oxford EDX detector: Model INCAx-
Sight).
1.3.8 UV-visible spectrophotometry
UV-vis spectra were recorded following the reflectance technique in a
Shimadzu UV 3101PC instrument using 5 nm slits and BaSO4 as reference.
1.3.9 CHNS analysis
The elemental analysis was carried out using Perkin-Elmer CHNS/O analyzer
(Series II, 2400). The sample weighed in milligrams housed in a tin capsule is
dropped into a quartz tube at 1020 °C with constant helium flow (carrier gas). A few
seconds before the sample drops into the combustion tube, the stream is enriched with
a measured amount of high purity oxygen to achieve a strong oxidizing environment
which guarantees almost complete combustion/oxidation even for thermally resistant
substances. The combustion gas mixture is driven through an oxidation catalyst
(WO3) zone, then through a subsequent copper zone which reduces nitrogen oxides
and sulphuric anhydride (SO3) formed during combustion, to elemental nitrogen and
sulphurous anhydride (SO2) and retains the oxygen excess. The resulting four
components of the combustion mixture are detected by a thermal conductivity
detector in the sequence N2, CO2, H2O and SO2. In case of oxygen which is analyzed
Chapter 1 Introduction
Ph.D Thesis 27
separately, the sample undergoes immediate pyrolysis in a Helium stream which
ensures quantitative conversion of organic oxygen into carbon monoxide separated on
a GC column packed with molecular sieves.
1.3.10 Temperature programmed desorption (TPD)
Temperature programmed desorption (TPD) was carried out in an AutoChem
2910 (Micromeritics, USA) instrument. The sample was pretreated by passage of
high-purity (99.995%) helium (50 ml/min) at 100 oC for 2 h. After pretreatment, the
sample was cooled and started adsorption of a mixture of CO2-He (10 vol.% CO2) at
80 oC for 1 h and subsequently flushed with He (50 mL/min) at 105
oC for 2 h to
remove physisorbed CO2. TPD analysis was then carried out from ambient
temperature to 900 oC at a heating rate of 10
oC /min. The amount of CO2 desorbed in
the effluent stream was monitored and analyzed with the TCD and quantified by
deconvolution method.
1.3.11 Temperature programmed reduction (TPR)
Temperature programmed reduction (TPR) analysis was carried out in a
Micromeritics 2900 TPD/TPR instrument. The reducing agent was H2-Ar (5 vol. %)
from L’Air Liquide (Spain) and gas flow (50 ml min-1
), sample weight (15-20 mg)
and heating schedule (10 oC min
-1) were chosen according to literature to optimize
resolution of the curves. Calibration of the instrument was carried out with CuO (from
Merck).
1.3.12 Nuclear magnetic resonance (NMR)
Solid-state 7Li and
27Al NMR spectra were recorded on a Bruker Avance-II
500 MHz spectrometer equipped with a double resonance CPMAS probe. 27
Al spectra
was acquired using single pulse excitation method, with standard zg program, at MAS
frequency 12 kHz and for 7Li, 8 kHz.
27Al chemical shift was measured with reference
of Al(NO3), at 0 ppm and 7Li measured with reference LiCl, at 0 ppm.
Chapter 1 Introduction
Ph.D Thesis 28
1.4 Scope and objectives of the work
The thesis divulge two important applications of hydrotalcite-like (HT-like)
materials (otherwise known as layered double hydroxides; LDHs), namely for base
catalysis and ion-exchange. The inherent properties of these materials like high
surface area, acid-base functionality, ability to accommodate homogeneously different
metal cations in the layers and improved basic property through memory effect were
explored for catalysis. The superior anion exchange ability of these materials is
explored for the removal of toxic anionic pollutants for aqueous systems. In this
doctoral work, Chapter 2 & 3 deals with base catalysed isomerization reaction of
alkenyl aromatics under thermal and microwave irradiation. The iso products of
alkenyl aromatics have great commercial value and Ni and Mg containing HT-like
materials as solid base catalysts are potentially explored for these reactions, with an
endeavour to replace environmentally unfriendly conventionally practiced alkali-
based homogeneous catalysts. Kinetics of isomerization of eugenol is studied and
scale up of the reaction is endeavoured to have practical attractiveness. Chapter 4
deals with the structure-property-activity relationship study over hydrotalcite-like
materials for these reactions that are less traversed earlier. The science/chemistry
behind isomerization activity with the structure/property of hydrotalcite is discussed.
The operando/in situ DRIFT-FTIR spectroscopy is used for the first time as a tool to
deduce the surface-structure-activity relationship for NiCuAl catalysts for
isomerization reaction. Attempts are made to fill some gap in correlating the basicity
and activity for as-synthesized Mg and Ni containing hydrotalcites by studying
isomerization of allylbenzene. Further support on catalytic property with activity is
assessed by exploring LiAl-HTs synthesized through urea hydrolysis for base-
catalysed condensation reactions both in their as-synthesized and calcined forms.
Chapter 5 discusses the efficiency of NiAl HT-like materials for the removal of
chromate, a toxic pollutant, through anion exchange; kinetics and adsorption
equilibrium phenomenon are also studied. Chapter 6 discusses the utilization of
chromate uptake by CoAl HT-like materials and further use of adsorbed materials for
catalysis, a concept that is proposed for the first time. This ideology of combined
approach of using layered double hydroxides for both environmental remediation and
the derived material as heterogeneous catalyst is highly beneficial in green chemistry
perspective.
Chapter 1 Introduction
Ph.D Thesis 29
The objectives of the present thesis are:
Synthesis of hydrotalcites and their physiochemical characterization
Isomerization of alkenyl aromatics of potential interest over NiAl HTs
Kinetic studies for isomerization of eugenol over MgAl4 and scale up studies
Microwave assisted isomerization of alkenyl aromatics over MgAl and NiAl
HTs
Structure-surface-activity correlation studies over HT-like materials for base
catalyzed reactions
Operando DRIFT-FTIR studies for NiAl and NiCuAl HTs
Removal of toxic chromate using NiAlNO3 HTs as efficient adsorbents
Valorization of CoAlCrO4 HTs derived after chromate removal for selective
oxidation of benzyl alcohol
Chapter 1 Introduction
Ph.D Thesis 30
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