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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.

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