Studies in structural characterization and correlation with the catalytic activity of an efficient and stable WOx/SBA-15 nanocomposite catalyst

Article abstract of DOI:10.1016/j.jcat.2008.05.015

A new WO_x/SBA-15 nanocomposite catalyst system was developed by hydrothermal synthesis. The material was characterized by X-ray diffraction, high-resolution transmission electron microscopy, sorption analysis, FT-IR, CP-MAS NMR, XPS, and energy

Full text of DOI:10.1016/j.jcat.2008.05.015

Journal of Catalysis 257 (2008) 283–290
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Journal of Catalysis
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Studies in structural characterization and correlation with the catalytic activity
of an efficient and stable WOx/SBA-15 nanocomposite catalyst
∗
Ankur Bordoloi, S.B. Halligudi
Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Pune-411 008, India
a r t i c l e i n f o
a b s t r a c t
new WOx/SBA-15 nanocomposite catalyst system was developed by hydrothermal synthesis. The
Article history:
A
Received 6 March 2008
Revised 7 May 2008
Accepted 10 May 2008
Available online 13 June 2008
material was characterized by X-ray diffraction, high-resolution transmission electron microscopy,
sorption analysis, FT-IR, CP-MAS NMR, XPS, and energy-dispersive X-ray analysis. Various types of
aromatic and aliphatic sulfides were selectively oxidized to sulfoxides in good to excellent yields without
becoming overoxidized to the sulfones using 70% aq. tert-butyl hydrogen peroxide in the presence of
nanocomposite catalyst at room temperature. The catalyst could be easily recovered and reused for
at least eight reaction cycles under the reaction conditions described here with no significant loss of
reactivity.
Keywords:
Tungsten oxide nanocluster
Mesoporous materials
Catalytic oxidation
Sulfides
© 2008 Elsevier Inc. All rights reserved.
Sulfoxides
1. Introduction
[10,11]. Among these, oxidation of sulfur compounds to sulfox-
ides is an interesting and attractive method [12,13]. Sulfoxide is
one of the useful building blocks as a chiral auxiliary in organic
synthesis [14–18] and plays a key role in enzyme activation [19–
22]; however, conventional transformations are only somewhat
successful, due to overoxidation and generation of toxic wastes
[23–25]. The efficacy of heterogeneous tungstate systems has been
dynamically examined, because various tungsten-based homoge-
neous catalysts are known to exhibit high activity in practical
oxidation procedures including epoxidation of alkenes [26,27], ox-
idation of alcohol [27,28], oxidative cleavage or halogenation of
olefins [28], and oxidative desulfurization [29–32]. As a result,
some promising heterogeneous systems, such as insoluble polyox-
otungstates [33,34], immobilized peroxotungstates, triphasic phos-
photungstate [35], and pseudoheterogeneous systems [36], have
been reported recently. Inorganic polymers are effective mediators
for organizing tungsten oxide species into higher-order composite
structures [19–22]. Here we report the synthesis of tungsten oxide
and SBA-15 nanocomposite (hereinafter WOx/SBA-15) with large
pore diameters and describe its catalytic activity in the selective
oxidation of sulfur compounds to corresponding sulfoxides under
ambient reaction conditions.
Creation of nanocomposite materials by introducing nanocrys-
tals into a macroscopic matrix is an area of growing interest in
catalysis. The functionality of nanocomposites has been attributed
to their surface properties, including surface-to-bulk atomic ratio,
polyhedral surface morphology, concentrations of surface defects
(coordinatively unsaturated ions due to planes, edges, corners, an-
ion/cation vacancies, and electron excess centers), and shape selec-
tivity [1,2]. Immobilization of nanoscopic materials in high-surface
area supports with improved catalytic activity and product selec-
tivity is a task of great economic and environmental importance
in the chemical and pharmaceutical industries [3]. There are few
reports describing the use of mesoporous silica as a support for
immobilization tungsten oxide species [4,5]. Zhang et al. described
the synthesis of tungsten-containing MCM-41 with good disper-
sion, but with segregated crystalline WOx detected after mild ther-
mal treatment [6]. The crystalline phases of WOx that form at low
pH in the presence of H2O2 was prevented by the use of oxoper-
oxometalate precursors. Its stability was poor, as evidenced by the
extensive leaching of tungsten species [7]. Recently, atomic layer
deposition (ALD) was used to graft tungsten oxide species onto
mesoporous silica (SBA-15) [8,9].
2. Experimental
The oxygen-transfer reaction is one of the most elemen-
tary transformations in synthesis organic chemistry, and several
metal-catalyzed oxidation reactions have been investigated to date
2.1. Materials
Sodium tungstate was purchased from Sisco Research Labora-
tory Pvt. Ltd., India. TEOS, P123 block copolymer [poly(ethylenegly-
col)–poly(propylene glycol)–poly(ethylene glycol), average molecu-
Corresponding author. Fax: +91 20 25902633.
E-mail address: sb.halligudi@ncl.res.in (S.B. Halligudi).
*
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.05.015

284
A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
lar mass 5800], and all of the sulfides were purchased from Aldrich
Chemicals and used as received. The 70% aq. TBHP was procured
from Acros Organics. All of the solvents procured from Merck (AR
grade), India were distilled and dried before use. Titanium silicates
(TS-1 and TS-2) were collected from the Catalysis Pilot Plant (CPP),
National Chemical Laboratory, Pune (India).
to a Raman module FRA 106. The AFM images were obtained on a
Digital Instruments Nanoscope IIIa SPM by tapping mode AFM.
2.4. Typical procedure for oxidation sulfides to sulfoxides
The liquid-phase catalytic oxidation of sulfides was conducted
under atmospheric pressure at room temperature in a 50-mL
reaction vessel equipped with a magnetic stirrer. In a typical
experiment, substrate 2 mmol of substrate was oxidized using
2.2. Catalyst preparation
Hexagonally ordered mesoporous WOx/SBA-15 nanocomposite
materials were synthesized using tetraethylorthosilicate (TEOS) as
the silica source and poly(ethylene oxide)-block-poly(propylene
oxide)-block-poly(ethylene oxide) triblock copolymer (Aldrich, MW
avg. 5800, EO20PO70EO20, P123) as a structure-directing agent. In
a typical synthesis, 4.0 g of P123 block copolymer was dissolved
under stirring in a solution of 30.0 g of water and the required
amounts (20, 10, 5, or 2.5 mL) of aqueous sodium tungstate so-
lution (NaWO4·2H2O, 0.5 M) were simultaneously added into the
mixture under vigorous stirring. After 1 h, 120.0 g of HCl (2 M)
3 eq. of 70% TBHP and 0.05 g of catalyst in CH3OH:CH2Cl2
(1:1) (10 mL) at room temperature. The progress of the reac-
tion was monitored by TLC and gas chromatography (GC) with
a Shimadzu 14B gas chromatograph equipped with a capillary
column (cross-linked 5% diphenyl–95% dimethylpolysiloxane cap-
illary column, 30 m × 0.53 × 1.5 μm film thickness) and a flame
ionization detector. After the reaction was complete, the reac-
tion mixture was filtered off, and the catalyst was washed with
dichloromethane (25–30 mL). The excess solvent was removed un-
der reduced pressure to give the corresponding pure sulfoxide
(Table 1). In most cases, the products were analyzed without no
chromatographic purification by GC (Shimadzu 14B) and GC-mass
spectroscopy (GC-MS) (Shimadzu (GC-17A & QP-5000) with iden-
tical columns. All known reaction products gave satisfactory GC
mass and GCIR (Perkin–Elmer system 2000) spectra compared with
those obtained from authentic samples. For di-p-tolylsulfoxides
and p-nitrophenylmethylsulfoxides, after the usual workup, the
crude sulfoxides were chromatographed on silica gel using a sol-
vent mixture of hexane/ethyl acetate (10:1) as the eluent to afford
the corresponding pure product confirmed by 1H- and 13C-NMR
(CDCl3, 200 MHz).
The resulting mixture was filtered to remove the catalyst, and
the solvent was removed under reduced pressure. It was then
dried in vacuum and purified by column chromatography on silica
gel (60–120 mesh) as the stationary phase (petroleum ether/ethyl
acetate, 50/50). Yield: 99%. Methylphenylsulfoxide: colorless liquid
1H NMR (CDCl3, 200 MHz): δ = 7.47–7.53 (m, 3H), 7.60–7.65
(m, 2H), 2.69 (s, 3H). 13C NMR (CDCl3, 200 MHz): δ = 131.1 (C),
129.3 (CH), 128 (CH), 127.3 (CH), 125.3 (CH), 123.5 (CH), 44 (CH3).
GCMS: m/z 140(M+.), 125, 112, 97, 91, 77, 65, 51.
◦
and 9.1 g of TEOS were added under stirring at 40 C. After 24 h
◦
of constant stirring, the gel composition was kept at 100 C under
static conditions for 48 h. After being cooled to room temperature,
the solid product was recovered by filtering, washing, drying, and
◦
calcining at 550 C. The nanocomposite samples were designated
WOx/SBA-15(v), where v denotes the volume of sodium tungstate
solution [37].
2.3. Catalyst characterization
Tungsten content of the catalyst was determined by energy-
dispersive X-ray analysis (EDAX) using Microanalysis Phoenix sys-
tem. Nitrogen adsorption and desorption isotherms were measured
◦
at −196 C with a Quantachrome Autosorb 1 sorption analyzer.
◦
The samples were outgassed for 3 h at 250 C under vacuum in
the degas port of the adsorption analyzer. The specific surface
area was calculated using the BET model. The pore size distri-
butions were obtained from the adsorption branch of the nitro-
gen isotherms by the Barrett–Joyner–Halenda method. The mor-
phology and elemental mapping of the materials were analyzed
using a Hitachi S-4800 field emission scanning electron micro-
scope (HR-FESEM) with EDAX using an accelerating voltage of
10 kV. The X-ray diffraction (XRD) patterns of the samples were
collected on a Philips X’Pert Pro 3040/60 diffractometer using
CuKα radiation (λ = 1.5418 Å), iron as the filter, and X’celera-
tor as the detector. The high-resolution transmission electron mi-
croscopy (HRTEM) images and the elemental mapping were an-
alyzed using a JEOL model 1200 microscope. The preparation of
samples for HRTEM analysis involved sonication in isopropanol for
30 min, followed by deposition on a copper grid. The accelerat-
ing voltage of the electron beam was 120 kV. X-ray photoelectron
spectroscopy (XPS) measurements were performed on a VG Mi-
crotech Multilab ESCA 3000 spectrometer with a nonmonochrom-
atized MgKα X-ray source. The resolution of the spectrometer was
set at 0.8 eV with MgKα radiation energy of 50 eV. The bind-
ing energy correction was performed using the C1s peak of carbon
at 284.9 eV as a reference. Shimadzu FTIR-8201PC in DRS mode
was used to obtain FTIR spectra of solid samples with a measure-
ment range of 1400–700 cm−1. The NMR spectrum of the catalyst
was recorded using a Bruker DSX-300 spectrometer. Diffuse re-
flectance UV–visible (DRUV–vis) spectra of catalyst samples were
obtained using a Shimadzu UV-2101 PC spectrometer equipped
with a diffuse–reflectance attachment, with BaSO4 as the reference.
The reducibility of the calcined copper catalysts was measured by
the TPR method using a Micromeritics AutoChem 2910 instrument.
Raman spectra of the catalyst were recorded on powder samples
at room temperature with a Bruker IFS 66 spectrometer connected
3. Results and discussion
3.1. Catalyst characterization
The wide-angle and low-angle XRD patterns of the WOx/SBA-
15 nanocomposite with different loading of WOx are shown in
Figs. 1A and 1B, respectively. All diffraction peaks could be in-
dexed to monoclinic WO3, which are in good agreement with
those of bulk monoclinic crystal (JCPDS card no. 83-0951). The
low-angle XRD measurements show that WOx/SBA-15 materials
◦
exhibited three peaks at a 2θ in the range of 0.5–5 , which can
be indexed to (100), (110), and (200) reflections of the hexago-
nal p6mm space group. The observation is in good agreement with
the XRD pattern of pure hexagonally ordered SBA-15 material re-
ported in the literature [38–40], indicating that the WOx/SBA-15
materials have a well-ordered two-dimensional mesoporous struc-
ture with a hexagonal porous network. Fig. 1B clearly displays the
peaks at higher angles, showing that the intensities of the peaks
increased with increasing WOx loading, confirming the presence
of WOx nanoclusters in the mesochannels of SBA-15. The size of
the WOx crystallites in the SBA-15 porous matrix was calculated
to be 2–5 nm using Scherrer’s equation. The nitrogen adsorption–
desorption isotherms of SBA-15 and WOx/SBA-15 nanocomposite
were found to be type IV isotherms and a exhibit sharp increases
in the amounts of nitrogen adsorbed at a relative pressure (P/P0)
of 0.6–0.9, typical features of curves of mesoporous structures,

A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
285
(A)
(B)
Fig. 1. (A) Wide and (B) low angle XRD patterns of WOx/SBA-15 with different concentration of WOx: (a) WOx/SBA-15(2.5), (b) WOx/SBA-15(5), (c) WOx/SBA-15(10), (d) WOx/
SBA-15(20).
Fig. 3. XPS spectra of W4f of WOx/SBA-15(20).
which were maintained in the WOx/SBA-15 (Fig. 2) [41,42]. The
amount of nitrogen adsorbed decreased with increasing loading of
WOx nanoclusters. Surface areas and pore volumes were lower for
the WOx/SBA-15 samples compared with the SBA-15 sample (Ta-
ble 1). Interestingly, the WOx/SBA-15 materials had larger pore di-
ameters than pure SBA-15. The catalysts were characterized by XPS
(Fig. 3), elemental mapping (Fig. 4), and EDAX to evaluate their el-
emental composition and purity. No peak of any metallic tungstate
was observed in the XPS spectra of the as-prepared WOx nanoclus-
ters, indicating high purity of the material (Fig. 3). The broadening
of the XPS peak could be due to the presence of tungsten species
in various oxidation states. EDAX analysis of the material (Table 1)
demonstrated the presence of W, Si, and O peaks.
Fig. 2. (A) Nitrogen adsorption–desorption isotherms and (B) BJH adsorption pore
size distributions of SBA-15 and WOx/SBA-15: (P) SBA-15, (!) WOx/SBA-15(2.5),
(1) WOx/SBA-15(5), (P) WOx/SBA-15(10), and (E) WOx/SBA-15(20).
HRTEM images of WOx/SBA-15(20) nanocomposite (Fig. 5)
show that the materials retained the hexagonally ordered porous
structure even after the formation of WOx nanoclusters in the
Table 1
Physicochemical properties of catalyst systems
No
Materials
Surface
area
Pore
EDAX analysis (%)
W
Volume
(cm3 g
Diameter
(nm)
Si
O
−1
)
(m2
g
−1
)
Wt
At. wt
Wt
At. wt
Wt
At. wt
1
2
3
4
5
SBA-15
910
632
562
464
321
1.25
1.02
0.98
0.88
0.69
9.2
9.3
10.0
10.0
10.0
0
2.5
8.1
20.5
36.9
0
38.1
40.0
34.7
30.3
26.6
26.0
28.3
25.5
25.3
27.6
61.9
57.5
57.2
49.2
36.5
74.0
71.4
73.6
72.1
66.6
WOx/SBA-15(2.5)
WOx/SBA-15(5)
WOx/SBA-15(10)
WOx/SBA-15(20)
0.3
0.9
2.6
5.8

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A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
Fig. 4. Elemental mapping of WOx/SBA-15(20).
Fig. 5. HRTEM pictures of WOx/SBA-15(20).
Fig. 6. HRSEM images of (a) SBA-15 and (b) WOx/SBA-15(20).
mesochannels. But the WOx nanoclusters are not visible in the
HRTEM images, confirming that the WOx nanoclusters were not
deposited on the external surface but instead were present in-
side the mesochannels of SBA-15 [43]. The SEM images show that
the morphology of WOx/SBA-15(20) nanocomposite remained the
same as that of the parent SBA-15 (Fig. 6). The AFM image of
WOx/SBA-15(20) clearly shows that the well-organized mesostruc-
ture was maintained on calcination, along with elimination of the
organic template from the pore voids. On calcination, the template
was eliminated, accompanied by the crystallization of WOx, which
decreased the long-range order and resulted in marginal collapse
of mesopores (Fig. 7).
MAS NMR is a very attractive technique for characterizing sur-
face protons in high-surface area materials. Fig. 8 shows 1H NMR–
MAS spectra of the WOx/SBA samples. Fig. 8a shows the spectra
for “as-synthesized” WOx/SBA before and after calcination in air
at 773 K. The “as- synthesized” NMR spectrum of WOx/SBA ex-
hibits two 1H NMR lines centered at 2.5 and −0.56 ppm. The line
at 2.5 ppm (Fig. 8a) is assigned to protons in H2O and Si–OH in-
teracting with one another through hydrogen bonding, with the
latter (−0.56 ppm) reflecting the presence of isolated silanols (Si–
OH). The line at 2.8 ppm seen in calcined sample WOx/SBA-15 is
Fig. 7. AFM image of WOx/SBA-15(20).
due to hydrogen-bonded hydroxyl groups and/or physisorbed wa-
ter. Fig. 8b shows the 1H NMR–MAS spectra of the WOx/SBA-15
nanocomposite samples calcined at 773 K. The sharp line seen at
2.8 ppm is due to hydrogen-bonded hydroxyl groups and/or physi-

A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
287
Fig. 10. UV–vis DRS spectra for three WOx/SBA-15 samples with different tungsten
loadings. [F(R∞)hν] represents the Kubelka–Munk function multiplied by the pho-
ton energy.
Fig. 8. (a) CP-MAS NMR profile of calcined and uncalcined WOx/SBA-15(20),
(b) CP-MAS NMR profile of WOx/SBA-15 with different concentration of WOx.
Fig. 11. Raman spectra of calcined and uncalcined samples of WOx/SBA-15(20). Inset:
Raman spectra of Na2WO4·2H2O.
The optical absorption edge energies of WOx/SBA-15 nanocom-
posite samples were obtained from the DRUV–vis absorption spec-
tra. The optical absorption edge energy is defined as the minimum
photon energy required to excite an electron from the highest oc-
cupied molecular orbital (HOMO; at the top of the valence band in
semiconductor domains) to the lowest unoccupied molecular or-
bital (LUMO; at the bottom of the conduction band). There are
two basic types of electronic transitions: direct and indirect. Di-
rect transitions require only that photons excite electrons, whereas
indirect transitions require concerted vibrations and energy from
the crystal lattice (phonons). For dispersed WOx domains, Barton
et al. used the square root of the Kubelka–Munk function mul-
tiplied by the photon energy, which allows the edge energy to
be obtained by extrapolation to zero absorbance for amorphous
semiconductors. The values thus obtained carry information about
the average domain size of the oxide nanoparticles, although the
values depend on local symmetry and support electronegativity.
Fig. 10 shows DRUV–vis spectra for the WOx/SBA-15 samples. All
of the WOx/SBA-15 catalysts exhibited main absorption features at
energies ranging from 3.5 to 2.6 eV due to ligand-to-metal charge
transfers in tungsten species on the SiO2 surface, as reported by
Herrera et al. [8] and Barton et al. [44].
Fig. 9. FTIR spectrum of WOx/SBA-15(20).
sorbed water, whereas the small hump seen at 0.28 ppm for the
WOx/SBA-15 samples could be due to water molecules hydrogen-
bonded to W–OH protons [44,45].
−1
FTIR spectroscopy in the skeletal region at 400–700 cm
was
used to follow the variation of the structure of WOx/SBA-15(20).
Typical bands due to siliceous material Si–O–Si were observed for
both the parent and the WOx/SBA-15(20) (Fig. 9): a main band
−1
at 1080 cm−1, with a shoulder at 1200 cm
due to asymmet-
−1
ric Si–O–Si stretching modes and symmetric stretch at 805 cm
,
and an additional band at ca. 960 cm−1, which is widely used to
characterize the presence of transition metal atoms near the silica
framework as the stretching Si–O vibration mode perturbed by the
neighboring metal ions. Thus, the presence of such an IR band due
to perturbed silica vibrations can be attributed to the formation of
WOx nanoclusters in SBA-15.
We performed Raman experiments to determine in which syn-
thesis step (WOx/SBA-15) were the WOx nanoclusters formed. Ra-
man bands corresponding to crystalline WO3 (Fig. 11) appeared
at 806 and 713 cm−1; these bands corresponded to W–O stretching

288
A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
Fig. 12. TGA plot of WOx/SBA-15(20).
Fig. 14. Temperature programmed reduction profile of catalyst: WOx/SBA-15(20).
Scheme 1. Oxidation of sulfide to sulfoxide.
Table 2
WOx/SBA-15(20) catalyzed oxidation of sulfides to sulfoxides with 70% aq. TBHPa
Entry
1
R
R1
Yield (%)b
Me
1st
2nd
3rd
4th
Et
i-Pr
–CH2–C6H5
–CH2–p-Tol
Me
–CH2–CH2CH2
Ph
Me
Me
Me
Me
Ph
99
97c
95c
91c
90c
99
99
95
99
99
92d
95
68
65
68
49
99
92
2
3
4
5
6
7
8
9
10
11
12
13
14
Ph
Ph
Ph
4-Me–C6H4
4-MeO–C6H4
Ph
Fig. 13. X-ray diffraction patterns of WOx/SBA-15(20). The diffractograms were ob-
tained in situ during thermal treatment at the indicated temperatures.
Ph
and bending modes, respectively, in both calcined and uncalcined
materials [43]. The Raman bands of uncalcined materials are of
comparatively lower intensity than those of the calcined materi-
als. However, the Raman bands of the cubic Na2WO4 phase were
observed at 932 and 840 cm−1, corresponding to internal vibra-
tions of WO4 (Fig. 11, inset). This indicates that the formation of
WOx nanoclusters started in the aging step of their synthesis. The
weight loss detected in the thermogravimetric analysis (TGA) of
the calcined WOx/SBA-15(20) sample below 373 K was related to
desorbed water (Fig. 12).
2-Cl–C6H4
4-Cl–C6H4
4-Br–C6H4
4-NO2–C6H4
Et
Et
–CH2–C6H5
–CH2–C6H5
a
Reaction conditions: substrate (2 mmol) using 70% TBHP (3 equiv.) and catalyst
(0.05 g) in CH3OH:CH2Cl2 (1:1) (10 mL) at room temperature.
b
Isolated yields. GC conversion unless otherwise stated.
Recycling experiment.
No epoxidation product was detected.
c
d
Fig. 13 shows temperature-resolved PXRD patterns of the WOx/
SBA-15(20) sample collected at 298–1173 K at 100-K intervals and
again at room temperature. Clearly, the WOx/SBA-15(20) material
exhibited a monoclinic phase in the temperature range studied,
in agreement with literature reports. The data quality of the scan
was comparatively good, due to the use of the X’celerator detector,
which uses real-time multiple-strip technology to enhance both
the resolution and intensity of the reflections. The multiple plots
of the powder patterns measured in static air indicate the stability
of the monoclinic phase of WOx at temperatures as high as 1123 K.
Above 1023 K, the expected anatase-to-rutile phase transforma-
tion was observed. The powder pattern of the sample obtained
after cooling to room temperature was similar to that seen at high
temperature (298 K), indicating a reversible anatase-to-rutile phase
transformation.
a reducible catalyst or catalyst precursor is exposed to a flow of a
reducing gas mixture (typically a few vol% of hydrogen in an inert
gas) as the temperature is increased linearly. According to previous
reports [45], pure WO3 exhibits three reduction peaks: a shoulder
◦
◦
at 560 C (WO3 → W20O58), a sharp peak at 750 C (W20O58
→
◦
WO2) and a peak at higher temperature (WO2 → W). Pure SiO2
exhibited no detectable TPR peak at temperature below 1000 C;
thus, the reduction peaks observed (Fig. 14) can be attributed to
the reduction of WOx only.
3.2. Catalytic oxidation of sulfides
To evaluate catalytic activity, we selected the oxidation of
methylphenylsulfide using 70% TBHP as a model reaction at room
temperature (Scheme 1). The catalyst gave excellent yields of cor-
responding sulfoxide in 10 h (Table 2, entry 1), with no overox-
Temperature-programmed reduction (TPR) with hydrogen is
widely used to characterize reducible solids and catalysts. In TPR,

A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
289
Table 3
Comparison of different catalysts in oxidation of ethylphenylsulfidea
Entry
Catalyst
Conversion
(mol%)
Selectivity of sulfoxide
(mol%)
1
2
3
WOx/SBA-15(20)
TS-1
TS-2
100
10
90
99
100
80
a
Reaction conditions: substrate (2 mmol) using 70% TBHP (3 equiv.) and catalyst
(0.05 g) in CH3OH:CH2Cl2 (1:1) (10 mL) at room temperature.
idation of sulfides to sulfones. The efficiency of TBHP for the
aforementioned reaction was evaluated based on the amount of
oxidative products produced from methylphenylsulfide divided by
the amount of TBHP consumed. We found 99% conversion of TBHP,
demonstrating the 100% efficiency of TBHP. In the absence of the
catalyst, no significant oxidation was observed under similar reac-
tion conditions.
(A)
In a similar way, various types of structurally diverged aryl alkyl
sulfides underwent smooth oxidation to selectively afford the cor-
responding sulfoxides in excellent yields (Table 2, entries 2–6).
Another useful feature of the present protocol can be seen in the
selective oxidation of allyl phenyl sulfide to corresponding sulfox-
ide; neither overoxidation to the sulfone nor epoxidation of the
double bond and corresponding sulfoxide was obtained in good
yields (Table 2, entry 7). Interestingly, under the present reaction
conditions, even hindered diaryl sulfides furnished corresponding
sulfoxides in excellent yields (Table 1, entry 8). The compounds
that have an electron-withdrawing ring substituent, such as bromo
or chloro, are less reactive (Table 2, entries 9–11). Also of note
is the finding that the strong electron-withdrawing NO2 group on
the phenyl ring had little effect on the efficiency of synthesis (Ta-
ble 2, entry 12). Interestingly, however, the corresponding sulfox-
ides formed under the protocol for the oxidation of dialkyl sulfides.
We found that under the similar reaction conditions, diethyl sulfide
was converted to diethyl sulfoxide in excellent yield (Table 2, en-
try 13), and dibenzyl sulfide furnished the corresponding sulfoxide
in high yield (Table 2, entry 14). We studied catalyst stability and
recyclability using standard procedures; the results clearly demon-
strate the practical reusability of the reported catalyst system. We
also studied the oxidation of methylphenylsulfide with 70% TBHP
using TS-1 and TS-2 under similar reaction conditions. The re-
sults, presented in Table 3, show that TS-1 had negligible activity,
whereas TS-2 had comparable activity to WOx/SBA-15(20) but with
lower sulfoxide selectivity [46,47]. Undoubtedly, our proposed cat-
alyst system is very active in the oxidation of methylphenylsulfide
to give sulfoxides selectively at room temperature.
(B)
Fig. 15. (A) Wide and (B) low angle XRD patterns of recycled WOx/SBA-15(20):
(a) 1st recycle, (b) 3rd recycle, and (c) 7th recycle.
Table 4
EDAX analysis data of recycled catalyst WOx/SBA-15(20)
Recycles
EDAX analysis (%)
W
Si
O
Wt
At. wt
Wt
At. wt
Wt
At. wt
1st
3rd
7th
36.9
36.8
36.8
5.8
5.7
5.7
26.6
26.6
26.5
27.6
27.6
27.5
36.5
36.7
36.8
66.6
66.7
66.8
SBA-15(20) sample with a freshly prepared reaction mixture, and
then carried out the reaction at room temperature for 24 h. The
reaction mixture was then filtered to separate catalyst, and the
filtrate was analyzed for the presence of tungsten. The catalyst
separated as described earlier after the first recycling was treated
in another fresh reaction mixture, and the filtrate was analyzed
for tungsten. As before, this catalyst was recycled 7 times. ICP-AES
analysis of all of the recycling experiments demonstrated the com-
plete absence of tungsten in the reaction mixture.
The foregoing results confirm the stability of the WOx/SBA-15
(20) system, with no leaching of active species WOx seen even af-
ter the seventh recycling. These results demonstrate the practical
reuse of the reported catalyst system under the reaction conditions
used here.
3.3. Catalyst stability and recyclability
WOx/SBA-15(20) was used to establish the stability of the cat-
alyst in recycling experiments. XRD and EDAX experiments were
performed to establish the stability of the catalyst system after re-
cycling. Fig. 15 presents wide-angle and small-angle XRD patterns
of the first, third, and seventh catalyst recyclings. The wide-angle
XRD patterns are comparable with those obtained for the fresh cat-
alyst (Fig. 1). The small-angle XRD patterns show a decrease in the
intensities of the peaks, possibly due to a marginal loss of order
in the material after recycling. But the presence of three well-
resolved peaks in XRD patterns of the catalysts after the seventh
recycling indicates the retention of a well-ordered WOx/SBA-15(20)
structure. The EDAX data (Table 4) also support the good structural
integrity of the material after recycling.
4. Conclusion
We carried out two additional experiments to investigate the
stability of WOx/SBA-15(20) system toward a strong oxidant such
as TBHP. To check the leaching of WOx species from the cat-
alytic system into the reaction medium, we treated a fresh WOx/
We have successfully demonstrated that highly dispersed WOx
nanoclusters in the mesochannels of SBA-15 can be prepared by in
situ hydrothermal method in a highly acidic medium. The catalyst
exhibits high catalytic activity for the oxidation of sulfur com-

290
A. Bordoloi, S.B. Halligudi / Journal of Catalysis 257 (2008) 283–290
pounds due to the presence of highly dispersed tungsten oxide
species in the SBA-15 mesochannels. The enhanced chemoselec-
tivity at higher substrate conversions achieved over this catalyst
are attributed to the unique large pore diameters and low surface
acidity of WOx/SBA-15, which enable the facile discharge of the
desired products from the channels of the catalyst under ambient
conditions. In addition, the catalyst is recyclable with no loss in
activity. This strategy could be used for similar chemical transfor-
mations in an eco-friendly manner.
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