View
218
Download
3
Category
Preview:
Citation preview
www.elsevier.com/locate/micromeso
Microporous and Mesoporous Materials 73 (2004) 181–189
Hydrothermal synthesis, characterization and catalytic propertiesof urano-silicate mesoporous molecular sieves
Dharmesh Kumar a, S. Varma a, G.K. Dey b, N.M. Gupta a,*
a Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Indiab Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
Received 25 February 2004; received in revised form 10 May 2004; accepted 28 May 2004
Available online 17 July 2004
Abstract
The paper reports on the physico-chemical characteristics, H2-reduction behavior and the catalytic properties of mesoporous
MCM-41 having uranyl groups (UO2þ2 ) incorporated into its framework positions by employing a direct hydrothermal route. XRD,
N2 sorption and TEM studies confirmed the formation of a highly ordered mesophase material whereas the presence of uranyl
groups in the host matrix became evident from the results of 29Si MAS NMR, XRD, DR UV–Visible and IR studies. Further, it is
demonstrated that the uranyl ions in framework positions undergo H2 reduction at a lower temperature, compared to those an-
chored at the silica wall surface of MCM-41 through impregnation route. The samples were also found to be active for the liquid-
phase oxidation of toluene using tertiary butyl hydrogen peroxide as an oxidizing agent; the activity and the selectivity depending
upon the solvent used as reaction medium. In a typical case using ethyl acetoacetate as a solvent, a selectivity of greater than 80% for
production of benzaldehyde was observed after 24 h of reaction at 90 �C.� 2004 Elsevier Inc. All rights reserved.
Keywords: Mesoporous MCM-41; Uranyl group Incorporation; TPR studies; Catalytic activity; Toluene oxidation
1. Introduction
The incorporation of divalent (Co, Ni), trivalent (Al,
Ga, Cr, Fe) or tetravalent (Ti, V, Sn, Zr etc.) metal ions
into the silica network of M41S type matrices has been
attempted widely, with an objective to impart specific
catalytic properties to these host materials [1–7]. Com-
paratively fewer studies have however been taken up onsuch modifications utilizing the elements of lanthanide
and actinide series [8,9]. In our recent publications, we
reported on the dispersion of UOx moieties within the
pore system of MCM-41 and MCM-48 materials by
employing the alternative methods of template exchange
and wet impregnation [10–13]. These modified meso-
porous materials were found to exhibit a catalytic
behavior for oxidation/decomposition of methanol,different from that of the host material or the bulk oxide
of uranium [14].
*Corresponding author. Tel.: +91-22-25505146; fax: +91-22-
25505151/25519613.
E-mail address: nmgupta@magnum.barc.ernet.in (N.M. Gupta).
1387-1811/$ - see front matter � 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2004.05.010
In continuation, we report now on incorporation of
uranyl groups in the silica framework positions of
MCM-41 host, achieved through a hydrothermal route.
The samples were characterized for their physico-
chemical properties by employing the techniques of
powder XRD, N2 sorption, TEM, FTIR, 29Si MAS
NMR and DR UV–Visible spectroscopy. The temper-
ature programmed H2-reduction (TPR) studies wereperformed in order to monitor the reduction behavior of
uranyl species. Comparative studies were performed on
the samples prepared through impregnation route, in
which case the uranyl groups existed in an anchored
state at the walls of the silica host matrix in addition to
nanocrystallites of U3O8 dispersed in the pore system.
Our aim in the present investigation was also to
evaluate the effectiveness of the uranyl incorporatedMCM-41 mesoporous materials as oxidation catalysts.
The uranyl ions, either immobilized in zeolitic materials
or alternatively in aqueous phase, are known to be ac-
tive for thermal oxidation and photo-oxidation reac-
tions involving hydrocarbons, alcohols and ethers etc.
[15–20]. We, therefore, felt prudent to examine the
182 D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189
catalytic behavior of these novel urano-silicate meso-
porous materials. The temperature dependent partial
oxidation of toluene in liquid phase was employed as a
test reaction for this purpose, because of the industrial
importance of likely reaction products, such as benzal-dehyde and benzyl alcohol. We also kept in mind that
the transition metal based catalysts, such as Cr-substi-
tuted mesoporous materials, V and Ti based mixed
metal oxides and substituted alumino-phosphates, are
known to be active catalysts for this reaction, where the
variable valance state of transition metal plays an
important role [21–25]. In that respect, we visualized an
active catalytic behavior of our catalysts, because of theexistence of uranium in its multiple valance states.
2. Experimental
2.1. Sample preparation
The hydrothermal synthesis of uranium contain-ing MCM-41 samples was accomplished using the
following gel composition, 1SiO2:0.27CTAB:0.13Na2O:
0.262TMAOH:xUO2:60 H2O, where x ¼ 0:004, 0.007
and 0.014. In a typical synthesis, fumed silica (Cab-O-Sil,
Fluka) was dissolved in TMAOH (Fluka) and NaOH
(S.D.Fine), and stirred for about 1 h to get solution-A. In
another vessel, solution-B was prepared by dissolving
organic template CTAB (Lancaster) in water, followed byaddition of uranyl acetate solution (0.1 molar) and stirred
for another 2 h. Solution-A was then added slowly with
stirring to solution-B, the pH of the gel obtained was
adjusted to 11, which was then stirred vigorously for
another 2 h and autoclaved at 110 �C for 96 h. The yellow
colored mass thus obtained was filtered, washed with
distilled water, dried and finally calcined at 550 �C for 1 h
in N2 and then for 7 h in O2. The calcined samples wereanalyzed for U-content by spectrophotometry and were
designated as HUM1, HUM2 and HUM3, where 1, 2
and 3 refer to the uranium content of about 2, 3.6 and 5.8
wt%, respectively.
For comparative purpose, samples were also pre-
pared through impregnation route following the proce-
dure described in our earlier publications [12,13]. In
brief, 1.0 g of template free MCM-41 host sample wasstirred with uranyl acetate solution (15 ml, 0.1 molar).
The sample was then filtered, washed thoroughly with
distilled water, dried and then calcined at 550 �C in O2.
The uranium content of the sample thus obtained, des-
ignated as IUM, was �10 wt%.
2.2. Characterization
2.2.1. X-ray diffraction (XRD)
The powder XRD patterns were recorded on a Phi-
lips Analytical Diffractometer with CuKa radiation
(k ¼ 1:54056 �A) in the 2h region of 2–10�, with a scan
speed and step size of 0.2� min�1 and 0.02�, respectively.The XRD patterns were also recorded in the 10–70�region in order to monitor the formation of any bulk
oxide.
2.2.2. Diffuse reflectance ultraviolet visible (DR UV–
Visible) spectroscopy
A Jasco V-570 spectrophotometer was employed for
recording of DR UV–Visible spectra. The powder
samples were loaded between two quartz windows and
the spectra were plotted in 200–700 nm wavelength re-
gion using BaSO4 as reference.
2.2.3. 29Si magic angle spin nuclear mass resonance
(MAS NMR)
The solid-state 29Si MAS NMR spectra were mea-
sured on a Bruker model DPX-300 Spectrometer at a
resonance frequency of 59.6 MHz. The powdered sam-
ples were placed in 7.0 mm diameter zirconia rotors and
spun at a rate of 5.0 kHz. The chemical shifts weredetermined using tetraethyl orthosilicate (d ¼ �82:4ppm from TMS) as a reference compound.
2.2.4. Fourier transform infrared (FTIR) spectroscopy
The FTIR spectra were recorded in 4000–400 cm�1
region on a Jasco-610 spectrophotometer. Compressed
KBr pellets containing �5 wt% of a sample were used
for this purpose. Each spectrum was collected by co-adding of 100 scans at a resolution of 4 cm�1.
2.2.5. Nitrogen sorption
The nitrogen sorption studies were carried out at 77
K on Sorptomatic model-1990 instrument. The samples
were dehydrated under vacuum at 373 K for 1 h and
then degassed (523 K) for 7 h. The BET surface area,
pore volume and the pore size distribution (BJH meth-od) were determined with the help of these data.
2.2.6. Transmission electron microscopy (TEM)
The TEM studies were performed on a Jeol 2000 FX
microscope operating at 120 kV. The samples for anal-
ysis were prepared by ultrasonicating 300 mesh size
material in ethanol and then dispersing on a carbon film
supported on copper grid.
2.3. Temperature programmed reduction (TPR)
A TPDRO-1100 instrument (Thermoquest, Italy)
equipped with a thermal conductivity detector was used
for recording of TPR profiles. A 50 mg eliquote of
sample was placed in a quartz microreactor and pre-
treated at 550 �C in oxygen flow (16 h) and then in he-lium (550 �C, 1.5 h). The reduction profiles were then
plotted in the flow of H2(5%) +Ar(20 mlmin�1) at a
heating rate of 6 �Cmin�1. The effluent was passed
D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189 183
through a sodalime trap in order to remove the water
formed during the reaction.
2.4. Catalytic reactions
The partial oxidation of toluene was carried out in
liquid phase, using 50 mg of calcined HUM2 and
HUM3 samples in the presence of various solvents such
as ethyl acetoacetate, chlorobenzene and acetone. Ter-
tiary butyl hydrogen peroxide (TBHP) was employed as
an oxidant. Parallel experiments were also performed
using molecular oxygen as an oxidant. The mixture
containing the catalyst, solvent (5 ml), toluene (20mmol) and oxidant (TBHP, 40 mmol) was taken in a
round bottom flask and refluxed at 90 �C for 24 h under
constant stirring. After completion of the reaction, the
catalyst was filtered out and the products were analyzed
by gas chromatography (Chemito-8510) employing an
OV-17 column and also on a GC–MS, equipped with a
HP-5 capillary column.
3. Results and discussion
3.1. DR UV–Visible spectroscopy
Curves a, b and c in Fig. 1 show the DR UV–Visible
pattern of HUM2, HUM3 and IUM samples, respec-
tively. Curve d presents a comparative spectrum ofuranyl acetate sample, which shows the characteristic
structure due to the electron-vibration interaction, typ-
ical of a UO2þ2 moiety [26–28]. A comparison of spectra
a–c with spectra d in Fig. 1 reveals that uranium exists
as uranyl species (UO2þ2 ) in both HUM as well as IUM
200 300 400 500 600
200 300 400 500 600
Abs
orba
nce
(a.u
.)
Wavelength [nm]
a
b
d
c
Fig. 1. DR UV–Visible spectra of (a) HUM2 (b) HUM3, (c) IUM and
(d) uranyl acetate.
samples. At the same time, we observe in Figs. 1a–c an
increase in overlap, a blue shift in various transition
bands and a significant change in the relative intensity of
these bands. These features are indicative of binding of
the uranyl groups at silicate matrix. We may mentionthat such changes in the spectrum of uranyl groups are
known to arise because of the changes in the environ-
ment or the changes in the ligand to which they are
bonded [26–28]. It is thus evident from DR UV–Visible
data that the uranium exists in a hexavalent form in
both HUM as well as IUM samples. Also it can be
observed from Fig. 1a–c that the extent of blue shift is
different in case of HUM and IUM samples, especiallyin the wavelength region 200–350 nm, with IUM sample
showing a greater blue shift. This again may be attrib-
uted to the presence of uranyl groups in two different
chemical environments in HUM and IUM samples.
3.2. Powder X-ray diffraction
Fig. 2 presents 2–7� 2h region powder XRD patternsof calcined Si-MCM-41 (curve a), HUM samples with
different U-content (curves b–d) and an IUM sample
(curve e). All the samples show well-defined XRD pat-
terns with Bragg reflections characteristic of MCM-41
materials [29]. For HUM samples (Fig. 2b–d), peaks are
found to shift to lower 2h values with simultaneous in-
crease in the d-values and in the unit cell parameter (a0)(Table 1), as compared to its silica polymorph. Such anincrease in the unit cell parameter has been reported
earlier by other researchers for incorporation of various
metal ions or radicals into the silicate framework [1–9].
The XRD results thus indicate the incorporation of
2 3 4 5 6 7
100
a
2θ/degree
210200
110
c
b
d
e
Arbi
tary
Uni
t
Fig. 2. XRD patterns in 2–7� 2h region of (a) MCM-41, (b) HUM1, (c)
HUM2, (d) HUM3 and (e) IUM samples.
Table 1
XRD Parameters and physical characteristics of MCM-41, HUM and IUM samples
Samples Unit cell parameter
(a0/�A)aBETsurface area
(m2 g�1)
BJHpore volume
(cm3 g�1)
BJHpore diameter
(�A)
FWTb
(�A)
Si-MCM-41 44.1 1080 1.1 28 16.1
HUM1 45.9 1057 1.0 31 14.9
HUM2 46.2 984 1.0 31 15.2
HUM3 45.8 966 0.96 30 15.8
IUM 43.3 856 0.62 25 18.3
a a0 ¼ 2d1 0 0=ffiffiffi
3p
.b FWT, framework wall thickness ¼ a0 � pore diameter.
184 D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189
uranium into the silica framework positions of MCM-41
host. The above inference finds further support when we
compare the changes in the unit cell parameter of the
HUM samples (Fig. 2b–d) with that of the impregnatedsample, IUM (Fig. 2e), which show a decrease in the
unit cell parameter as a result of uranium loading. In
the case of IUM samples the uranyl groups are bound to
the Si-sites at the wall surface [12,13], the process that
leads to increase in the cross linking of the uncondensed
silanol groups and hence the contraction of overall host
structure [12,13,30]. The comparative XRD studies of
the HUM and IUM samples thus reveal that dependingupon synthesis condition the uranyl groups may exist in
two distinct environments.
Fig. 3 shows the XRD patterns of the respective
samples in 2h region of 10–70�. In addition to the
characteristic broad band in 2h region between 15� and35� due to amorphous silica walls of the pristine mate-
rial, we observe some weak reflections in case of HUM3
(Fig. 3d) and IUM (Fig. 3e) samples. The XRD lines,observed at d-values of 4.15, 3.39, 2.62, 2.07, 1.96, 1.75�A, correspond closely to {0 0 1}, {0 3 1}, {1 3 1}, {2 0 0},
10 20 30 40 50 60 70
b
a
2θ/degree
c
d
(331
)
(022
)
e
(330
)(131)
(130)(001)
Arbi
tary
Uni
t
Fig. 3. XRD patterns in 10–70� 2h region of (a) MCM-41, (b) HUM1,
(c) HUM2, (d) HUM3 and (e) IUM samples.
{0 3 3}, {1 3 3} and {1 6 2} reflections of a-U3O8 [JCPDS
card no. 24-1172]. We have reported earlier, that a part
of the uranyl groups bonded to silica wall surface get
polymerized on calcination and transform eventually tonanocrystallites of U3O8 dispersed in mesopores of host
matrix [12,13]. The high angle XRD results thus indicate
that for a loading of above 5% of U in hydrothermally
synthesized samples, a part of uranyl groups may simi-
larly polymerize, leading thereby to formation of a
U3O8 secondary phase.
3.3. 29Si MAS NMR
The 29Si MAS NMR spectra of calcined Si-MCM-41
and HUM2 samples, shown in curves a and b in Fig. 4,
exhibit three lines with mean values of )92.5, )103.2and )108.5 ppm with a shift of ±0.5 ppm, assigned
respectively to (SiO)2–Si(OH)2 (Q2), (SiO)3–SiOH (Q3)
and Si(SiO)4 (Q4) sites or groups [5,31]. As seen in Fig.
4b, a considerable increase is observed in the relativeintensity of Q4 sites in addition to broadening of this line
in case of HUM sample. These results provide further
evidence in support of the substitution of uranyl groups
at silica framework sites, resulting thereby in the pro-
Fig. 4. 29Si MAS NMR spectra of (a) MCM-41 and (b) HUM2 sam-
ples.
D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189 185
motion of the cross linking of silicate defect sites (Q2
and Q3) at mesoporous wall and increase in the number
of Si sites with four bridging oxygen neighbors (Q4
sites). This phenomenon is akin to that reported earlier
for incorporation and substitution of transition metalsinto the silicate framework of mesoporous materials
[4,5,32].
3.4. FTIR study
Fig. 5 presents the IR spectra of Si-MCM-41 (curve
a), HUM2, HUM3 (curves b and c) and IUM sample
(curve d) in the framework region. In addition tovibrational bands at 1237, 1084, 801, 460 cm�1 charac-
teristic of silica framework in MCM-41 sample and the
band at about 965 cm�1 due to terminal Si–O�Hþ
groups [7,9,32], we notice new IR bands at 1062, 906
and 816 cm�1 in case of HUM samples. The vibrational
band at 1062 cm�1 may be ascribed to the stretching
vibrations of (Si–O–U) groups indicating the binding of
the uranium at silica sites. Also of interest is an increaseobserved in the intensity and blue shift of this band at
965 cm�1 in HUM samples, which is generally taken as
an evidence of the incorporation of metal into the
framework of the microporous and mesoporous metal-
losilicates [33]. These characteristics of IR spectra are
therefore attributed to the linkages of uranyl groups at
the terminal silica sites of MCM-41 walls.
The bands observed at 906 and 819 cm�1 observed inHUM samples (Fig. 5b and c) are assigned to the
asymmetric and symmetric stretching vibrations of
uranyl species [34,35]. These vibrations appear at about
930 and 856 cm�1 in case of uranyl acetate. However it
has been demonstrated earlier that the frequency of the
U@O band in uranyl compounds changes with the
change in the ligand [34,35], and thus a red shift of
about 25–35 cm�1 in the asymmetric and symmetricstretching bands of U@O in case of HUM samples re-
flects on the perturbation of the uranyl ions arising be-
1400 1200 1000 800 600 400
a
Wavenumber (cm-1)
c
b
816906
Arbi
tary
Uni
t
d
1062
965
965
916
Fig. 5. FTIR spectra of (a) MCM-41, (b) HUM2, (c) HUM3, (d) IUM
samples.
cause of their bonding with the framework silicate sites
resulting thereby in the weakening of the U@O bond.
Fig. 5d shows the comparative IR spectrum of IUM
sample, where the band due to uranyl groups is observed
at a 916 cm�1 and also observed is a considerable de-crease in the intensity of 965 cm�1 band. Decrease in the
intensity of 965 cm�1 band clearly indicates binding of
the uranyl groups to the silica wall surface through hy-
droxyl groups. Also the symmetric stretching band of
uranyl group at 816 cm�1 is IR active only when the
axial oxygen atoms in uranyl group lacks a center of
symmetry and exhibits a bent structure. On the other
hand, it is IR inactive in case of a linear uranyl group[26]. The presence of symmetric band and a difference in
the positions of the asymmetric band of uranyl group in
case of HUM and IUM samples thus indicates that the
uranyl groups may be present in a constrained and dif-
ferent environment in case of HUM samples, in contrast
to the IUM samples where the anchoring of the uranyl
groups at the silica walls of mesoporous host has been
established.Based on the above mentioned results, we may thus
infer that the samples prepared by hydrothermal and
impregnation routes consist of uranyl groups in two
different environments. In case of the samples prepared
through impregnation, most of the uranyl groups are
bound linearly to the terminal sites. In case of hydro-
thermally synthesized samples, the uranyl groups are
present in two different environments, one in theframework sites (Type-1) and other at the terminal sites
of the silica wall (Type-2). The mode of binding of
uranyl groups in these two distinct modes in MCM-41
are shown schematically in Fig. 6.
3.5. Nitrogen sorption
The N2 sorption isotherms obtained for Si-MCM-41,HUM2 and IUM samples are shown in Fig. 7. All the
SiO
SiO
O U O
U
O O
OO O
O Si
Si
Si
Si
Type - 1
Type - 2
Fig. 6. Schematic picture of two alternative modes of uranium incor-
poration in mesoporous MCM-41. Type-1––uranyl groups in the
framework sites of silica matrix. Type-2––uranyl groups bonded at the
terminal sites of mesoporous wall of MCM-41.
0.00 0.15 0.30 0.45 0.60
200
300
400
500
Relative Pressure (p/p0)
Volu
me
(cc/
g)
cba
Fig. 7. Nitrogen sorption isotherms of (a) MCM-41, (b) HUM2 and (c)
IUM samples.
10 20 30 40 500.0
0.1
0.2
0.3
c
b
a
dV
(d)[c
c/Å/
g]
Pore Diameter [Å]
Fig. 8. Pore size distribution plots of (a) MCM-41, (b) HUM2 and (c)
IUM samples.
186 D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189
samples exhibit type IV isotherm with a well developed
step in the relative pressure (p=p0) range 0.30–0.50,
characteristic of MCM-41 type mesoporous material
[36]. The position of inflection point and the sharpness
of the curve in this region provide information about thepore size and uniformity of the pore size distribution
[37]. A shift in the inflection point to higher p=p0 values,observed for HUM sample, indicate an increase in the
pore size of the sample as compared to its siliceous
counterpart. On the other hand a shift to lower p=p0values was observed for IUM sample along with some
decrease in the sharpness of the isotherms, indicating a
decrease in the pore size and also inhomogeneity in thepore filling. Also observed was an inflection point at
p=p0 > 0:9 corresponding to capillary condensation in
the interparticle pores for all the samples.
The BET surface area, average pore size distribution
(BJH method), and the specific pore volumes of Si-
MCM-41, HUM and IUM samples are presented in
Table 1. No significant changes are observed in the
surface area and pore volume of HUM samples ascompared to Si-MCM-41 sample, thus indicating the
formation of highly ordered mesoporous material.
However, mean pore size is found to increase from 28 �Ain Si-MCM-41 to about 31 �A in case of HUM samples,
as shown in Fig. 8. This increase in the pore size of
HUM samples can be attributed to lesser contraction of
the pore system upon calcination and removal of sur-
factant molecules due to incorporation of uranyl groupsin the silica matrix. Such a phenomenon has been re-
ported earlier for the incorporation of other metal sys-
tems, such as Ce, V, Ti and Sn etc. into the silicate
framework [7,9,38].
However a considerable decrease is noticed in the
surface area, pore volume and pore size for IUM sample
(Table 1, Fig. 8) and this could be attributed to increase
in the interconnectivity of the silica defect sites (Q2 andQ3) through uranyl groups, which leads to overall con-
traction of the host structure (also seen from XRD).
Secondly upon calcination a part of these uranyl groups
get converted to U3O8 nanosize crystallites which fur-ther fill the pores and hence a decrease in the overall
mean pore size and some disordering in the host struc-
ture, however the filling of the pore is not very
homogenous as reflected from the FWHM of the pore
size distribution plot of this sample (Fig. 8c) and also
from the decrease in the sharpness of the inflection point
for this sample as seen in Fig. 7c. The decrease in the
pore size along with the increase in the framework wallthickness (FWT) of IUM sample as compared to sili-
ceous MCM-41 (Table 1) may be ascribed to the depo-
sition of uranium oxide species in the pore surface of the
channels of MCM-41. However, the corresponding data
for HUM samples in Table 1 show no significant
changes in the framework wall thickness, thus indicating
that the increase in the unit cell parameter (a0) in case of
HUM samples is because of incorporation of uranylions, a phenomenon also observed for other hetero ions,
which are larger than Si4þ ions [7,9]. The results of this
study thus clearly reveal a difference in the characteris-
tics of HUM and IUM samples, synthesized through
two different methods.
3.6. TEM
Fig. 9a and b exhibit the bright field transmission
electron micrograph of HUM2 samples taken in the
direction parallel and perpendicular to the pore axis.
Fig. 9a shows a highly ordered hexagonal pattern
characteristic of MCM-41, while Fig. 9b shows the
channel like patterns in MCM sample joined to each
other, suggesting the formation of a long-range highly
ordered mesophase material [29]. On the other hand,some disordering in the host structure was clearly visible
Fig. 9. TEM monograph of HUM2 sample (a) parallel to pore axis (b)
perpendicular to pore axis.
100 200 300 400 500 600 700 800
c
b
a
480
590
540
Temperature (°C)
Fig. 10. TPR plots of (a) MCM-41, (b) HUM2 and (c) IUM samples.
D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189 187
in the TEM micrographs of HUM3 sample, containing
higher content of U (�6%). Some highly dispersed
nanocrystallites (2–4 nm) of U3O8 were also noticed in
the TEM pictures of HUM3. The presence of U3O8 in
this sample is in conformity with the powder XRD data
of Fig. 3d.
3.7. TPR studies
Fig. 10 presents the TPR profiles of host MCM-41
(curve a) and HUM2 sample (curve b). In curve (c) of
this figure is given a comparative TPR profile of an IUM
sample. As expected, no hydrogen consumption is ob-
served in case of TPR profile of uranium-free Si-MCM-
41 sample. In case of IUM sample we observe tworeduction stages (Fig. 10c), one at a temperature of
about �540 �C corresponding to reduction of the uranyl
groups and another giving rise to a broad peak at about
590 �C due to reduction of nanosize U3O8 crystallites
[39]. Coming to Fig. 10b for HUM2, it is important to
notice a single TPR band at �480 �C, thus revealing
almost exclusive presence of uranyl groups in this sam-
ple. It is also important to notice a lowering in thetemperature of this TPR peak, as compared to the
corresponding peak in case of impregnated sample. This
indicates once again the different chemical environment
of uranyl groups in case of HUM and IUM samples.
The difference in the reduction behavior of uranium
containing samples prepared by two different routes can
thus be attributed to subtle structural difference in the
coordination of the two kinds of uranyl groups, repre-
sented in Fig. 6. It is evident that the stearic over-
crowding due to four �SiO� linkages in structure A of
Fig. 6 may lead to a distortion in the plane of linear
symmetry of axial oxygen atoms of uranyl groups and
hence in a weakening of U–O bonding (as seen by IRstudies) and also in a decrease in stability of these
groups leading thereby to lowering of TPR peak tem-
perature (Fig. 10b). On the other hand, the lack of such
crowding in structure (B) for IUM sample where the
plane of symmetry of axial oxygen atom is maintained
will lead to greater stability of U@O linkages thus jus-
tifying a higher temperature for reduction (Fig. 10c). We
may mention that the TPR behavior of HUM3 samplewas found to be almost similar to that of HUM2 sam-
ple, excepting an increase in the area under TPR profile
as per the higher uranyl concentration in this case.
3.8. Catalytic activities studies
The uranium-free MCM-41 sample showed no
activity for toluene oxidation reaction under the condi-tions of this study. HUM samples, on the other hand,
showed a considerable activity for this reaction using
tertiary butyl hydrogen peroxide as an oxidant, the
conversion depending upon the U-content of a sample.
Benzaldehyde and benzyl alcohol were the main reaction
products, the yield of former being many times more
and accounting for a selectivity of >80%. The conver-
sion of toluene depended upon the solvent employed inthe reaction, and the product yield was negligibly small
when no solvent was employed. Further, the higher
conversions were obtained in ethyl acetoacetate med-
ium, as compared to two other solvents tried out, i.e.
0 8 16 24
0
2
4
6
8
10
Time (hrs)
(a)
c
b
a
% C
onve
rsio
in a
nd P
rodu
ct Y
ield
Time (hrs)0 8 16 24
20
40
60
80(b)
e
d
% S
elec
tivity
of p
rodu
cts
Fig. 11. Time dependent progress of oxidation reaction of toluene in presence ethyl acetoacetate as solvent and TBHP as oxidizing agent. (a) %
Conversion and % yield of (b) benzaldehyde, (c) benzyl alcohol, % selectivity of (d) benzaldehyde and (e) benzyl alcohol.
188 D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189
chlorobenzene and acetone. Fig. 11a presents the time
dependent progress of toluene oxidation reaction at 90
�C when ethyl acetoacetate was used as a solvent along
with TBHP as an oxidizing agent. It is of interest to
notice a fast progress of reaction during first 15 h of
time, followed by a slower reaction. The product dis-
tribution was also found to change with time and thesedata are included in Fig. 11b. We notice a continuous
increase in selectivity for the formation of benzaldehyde
(curve d) at the cost of the yield of benzyl alcohol (curve
e). These observations may be attributed to the sec-
ondary oxidation of benzyl alcohol on catalyst surface.
As mentioned above, the conversion of toluene de-
pended upon the uranium content in MCM-41. Table 2
presents the results on the oxidation of toluene onHUM2 and HUM3 samples, carried out in three dif-
ferent solvent media and collected after 24 h of reaction
time. As seen in these results, ethyl acetoacetate helped
not only in higher conversion of toluene but also gave
rise to selectivity of greater than 80% for benzaldehyde
formation, as mentioned above. Also, an almost quan-
titative correlation is seen in these data between the
product yield and the uranyl content of the samples,indicating that the UO2þ
2 species indeed serve as the sites
Table 2
Catalytic activity of HUM3 and HUM2 samples for oxidation of toluene in
Sample Solvent % Toluene conversion
HUM3 Ethyl acetoacetate 9.1
Chlorobenzene 5.7
Acetone 2.1
HUM2 Ethyl acetoacetate 5.1
Chlorobenzene 3.2
Acetone 1.6
of oxidation reaction. Analysis of unreacted TBHP
showed that about 65–70% of it remained unused during
the reaction process after 24 h of reaction time, in
accordance with the overall conversion of toluene. It
also needs a mention that no measurable reaction was
observed when oxygen gas was employed as an oxidant
instead of TBHP.In order to check for the stability of the catalyst, a
HUM3 sample was subjected to repeated use in toluene
oxidation reaction and these results are presented in
Table 3. As revealed in these data, while the activity
decreased by �20% after first use of the catalyst, the
conversion and product selectivity changed only mar-
ginally during the subsequent few more cycles of test
reaction. These results confirmed that the catalystsmaintain their activity after initial leaching of the sur-
face uranyl ions during first cycle.
4. Conclusions
Uranium (2–6 wt%)-containing MCM-41 mesopor-
ous materials were synthesized hydrothermally. Changesin the physico-chemical properties of MCM-41 host due
different solvents
% Selectivity
Benzaldehyde Benzyl alcohol
86.8 13.2
78.9 21.1
80.9 19.1
88.2 11.8
75.0 25.0
81.2 18.8
Table 3
Catalytic activity of HUM3 sample for oxidation of toluene at 90 �C,when a sample was subjected to repeated use in ethyl acetoacetate
medium
Sample % Toluene
conversion
% Selectivity
Benzaldehyde Benzyl alcohol
Calcined
HUM2
9.1 86.8 13.2
1st cycle 6.9 84.1 15.9
2nd cycle 6.7 83.6 16.4
3rd cylcle 6.7 82.5 17.5
D. Kumar et al. / Microporous and Mesoporous Materials 73 (2004) 181–189 189
to incorporation of uranium as uranyl ions within the
silica framework matrix were monitored using multiple
techniques such as powder XRD, N2 sorption, DR UV–
Visible, 29Si MAS NMR and FTIR spectroscopy. TEM
studies confirmed the formation of highly ordered
mesophase material. A marginal disordering in pore
structure was, however, noticed in the case of samples
with larger U-content of 5.8 wt%. Also, a small fractionof uranium existed in this sample in form of U3O8
crystallites of 2–3 nm size. Further, the uranyl groups
located in framework position were found to reduce in
H2 at a lower temperature, as compared to linearly
bonded uranyl ions at the silica walls of IUM sample
prepared through impregnation route. The uranyl-con-
taining samples were found to be active for partial oxi-
dation of toluene, the conversion and the selectivitydepending upon the U-content of a sample and the
solvent employed as a reaction medium. A selectivity of
greater than 80% was obtained for formation of benz-
aldehyde, an important intermediate used in the syn-
thesis of a number of pharmaceutical compounds, resins
and dyes etc.
Acknowledgements
Authors thank Dr. P.R. Rajamohan, National
Chemical Laboratory, Pune, for providing NMR data
on our samples.
References
[1] K.M. Reddy, I. Moudrakovski, A. Sayari, J. Chem. Soc., Chem.
Commun. 1059 (1994).
[2] R. Schmidt, H. Junggreen, M. St€ocker, J. Chem. Soc., Chem.
Commun. 875 (1996).
[3] K.A. Koyano, T. Tatsumi, Chem. Commun. 145 (1996).
[4] C.F. Cheng, H. He, W. Zhou, J. Klinowski, J.A.S. Goncalves,
L.F. Gladden, J. Phys. Chem. 100 (1996) 390.
[5] W. Zhang, T.J. Pinnavaia, Catal. Lett. 38 (1996) 261.
[6] H. Kosslick, G. Lischke, H. Landmesser, B. Parlitz, W. Storek, R.
Fricke, J. Catal. 176 (1998) 102.
[7] K. Chaudhari, T.K. Das, P.R. Rajmohanan, K. Lazar, S.
Sivasankar, A.J. Chandwadkar, J. Catal. 183 (1999) 281.
[8] J.R. Matos, L.P. Mercuri, M. Jaroniec, M. Kruk, Y. Sakamoto,
O. Terasaki, J. Mater. Chem. 11 (2001) 2580.
[9] S.C. Laha, P. Mukherjee, S.R. Sainkar, R. Kumar, J. Catal. 207
(2002) 213.
[10] K. Vidya, S.E. Dapurkar, P. Selvam, S.K. Badamali, N.M.
Gupta, Micropor. Mesopor. Mater. 50 (2001) 173.
[11] K. Vidya, S.E. Dapurkar, P. Selvam, S.K. Badamali, D. Kumar,
N.M. Gupta, J. Mol. Catal.: Chem. 181 (2002) 91.
[12] D. Kumar, S. Bera, A.K. Tripathi, G.K. Dey, N.M. Gupta,
Micropor. Mesopor. Mater. 66 (2003) 157.
[13] D. Kumar, G.K. Dey, N.M. Gupta, Phys. Chem. Chem. Phys. 5
(2003) 5477.
[14] D. Kumar, V.S. Kamble, N.M. Gupta, Catal. Lett. 88 (2003) 175.
[15] W.-D. Wang, A. Bakac, J.H. Espenson, Inorg. Chem. 34 (1995)
6034.
[16] Y. Mao, A. Bakac, J. Phys. Chem. 100 (1996) 4219.
[17] T.M. McCleskey, C.J. Burns, W. Tumas, Inorg. Chem. 38 (1999)
5924.
[18] M. Sarakha, M. Bolte, H.D. Burrows, J. Phys. Chem. A 104
(2000) 3142.
[19] S.L. Suib, A. Kostapapus, D.J. Psaras, J. Am. Chem. Soc. 106
(1984) 1614.
[20] S.L. Suib, J.F. Tanguay, M.L. Occelli, J. Am. Chem. Soc. 108
(1986) 6972.
[21] S. Larrondo, A. Barbaro, B. Irigoyen, N. Amadeo, Catal. Today
64 (2001) 179.
[22] L. Kiwi-Minsker, D.A. Bulushev, F. Rainone, A. Renken, J. Mol.
Catal. A: Chem. 184 (2002) 223.
[23] A. Sakthivel, S.K. Badamali, P. Selvam, Catal. Lett. 80 (2002)
73.
[24] M.H. Zahedi-Niaki, S.M. Javaid Zaidi, S. Kaliaguine, Appl.
Catal. A: Gen. 196 (2000) 9.
[25] Ch. Subrahmanyam, B. Louis, F. Rainone, B. Viswanathan, A.
Renken, T.K. Varadarajan, Appl. Catal. A: Gen. 241 (2003)
205.
[26] S.P. McGlynn, J.K. Smith, J. Mol. Spectrosc. 6 (1961) 164.
[27] B. Je _zowska-Trzebiatowska, A. Bartecki, Spectrochim. Acta 18
(1962) 799.
[28] J.T. Bell, R.E. Biggers, J. Mol. Spectrosc. 18 (1965) 247.
[29] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck,
Nature 359 (1992) 710.
[30] M. Morey, A. Davidson, H. Eckert, G. Stucky, Chem. Mater. 8
(1996) 486.
[31] Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996)
1147.
[32] Z. Luan, J. Xu, H. He, J. Klinowski, L. Kevan, J. Phys. Chem. B
100 (1996) 19595.
[33] M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti, G. Petrini,
Stud. Surf. Sci. Catal. 48 (1989) 133.
[34] E. Rabinowitch, R.L. Belford, Spectroscopy and Photochemis-
try of Uranium Compounds, Pergamon Press, Oxford, 1964, p.
18.
[35] K. Nakamoto, Infrared and Raman spectra of Inorganic and Co-
ordination Compounds, Wiley, New York, 1978, p. 85.
[36] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A.
Pierotti, J. Rouquerol, T. Siemieniewska, J. Pure Appl. Chem. 57
(1985) 603.
[37] Z. Luan, H. He, W. Zhou, C.F. Cheng, J. Klinowski, J. Chem.
Soc., Faraday Trans. 91 (1995) 2955.
[38] C. Pak, G.L. Haller, Micropor. Mesopor. Mater. 44–45 (2001)
321.
[39] D. Kumar, S. Varma, N.M. Gupta, Catal. Today, in press.
Recommended