Silica-supported molybdenum complex: A novel, selective and reusable organic-inorganic hybrid catalyst for eco-friendly oxidation of sulfides and olefins

Article abstract of DOI:10.1016/j.poly.2012.07.065

A novel, highly efficient and reusable molybdenum based catalyst has been synthesized by covalent grafting of 2,6-diacetylpyridine-monosalicyloylhydrazone onto functionalized silica followed by complexation with MoO ₂(acac)₂, and the resulting organic-inorganic hybrid material was found to be highly effective catalyst for oxidation of various sulfides and olefins to their corresponding sulfoxides/sulfones and epoxides, respectively. The catalyst was characterized by elemental analysis (CHN), diffuse reflectance UV-Vis, ¹³C CPMAS NMR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and BET surface area analysis, energy dispersive X-ray fluorescence (ED-XRF), Fourier-transform infrared (FT-IR) and atomic absorption spectroscopy (AAS). Mild reaction conditions, high turnover frequency, high selectivity, easy recovery and reusability of the catalyst render the present protocol very useful to address the industrial needs and environmental concerns.

Full text of DOI:10.1016/j.poly.2012.07.065

Polyhedron 45 (2012) 86–93
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Silica-supported molybdenum complex: A novel, selective and reusable
organic–inorganic hybrid catalyst for eco-friendly oxidation of sulfides and olefins
⇑
R.K. Sharma , Amit Pandey, Shikha Gulati
Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi-110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel, highly efficient and reusable molybdenum based catalyst has been synthesized by covalent graft-
ing of 2,6-diacetylpyridine-monosalicyloylhydrazone onto functionalized silica followed by complexa-
tion with MoO2(acac)2, and the resulting organic–inorganic hybrid material was found to be highly
effective catalyst for oxidation of various sulfides and olefins to their corresponding sulfoxides/sulfones
and epoxides, respectively. The catalyst was characterized by elemental analysis (CHN), diffuse reflec-
tance UV–Vis, 13C CPMAS NMR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy
(SEM) and BET surface area analysis, energy dispersive X-ray fluorescence (ED-XRF), Fourier-transform
infrared (FT-IR) and atomic absorption spectroscopy (AAS). Mild reaction conditions, high turnover fre-
quency, high selectivity, easy recovery and reusability of the catalyst render the present protocol very
useful to address the industrial needs and environmental concerns.
Received 5 June 2012
Accepted 19 July 2012
Available online 27 July 2012
Keywords:
Silica
Molybdenum
Recoverable catalyst
Sulfoxidation
Epoxidation
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
minimize waste. In this context, the development of novel, econom-
ical and energy efficient routes for the selective and green oxidative
Molybdenum complexes have been known as versatile and use-
ful catalysts for oxidative transformations [1]. A wide range of sol-
uble molybdenum complexes with diverse ligands have been
synthesized and employed as homogeneous catalysts for epoxida-
tion and sulfoxidation of various olefins due to their high selectivity
[2–8]. However, these homogeneous catalytic systems suffer from
problems associated with the separation and recovery of the active
catalyst as well as instability at high temperatures. These draw-
backs have so far precluded their industrial utilization. Moreover,
metal contamination of the products is inevitable when using
homogeneous catalysts. Therefore, there is a need to develop
improved strategies for recycling of catalysts for economical and
environmental concerns. In this scaffold, covalent immobilization
of homogeneous catalyst onto inorganic solid support constitutes
a potential strategy to combine the advantages of both homoge-
neous (high activity and selectivity) and heterogeneous (easy sepa-
ration from the reaction mixture) catalysts [9]. In recent years, silica
has been receiving much attention as inorganic solid support mate-
rial for anchoring of metal complexes due to its numerous advanta-
ges such as excellent thermal and mechanical stability, economic
viability, high surface area and large pore volume [10,11].
transformation of sulfides and olefins represents a challenge to di-
verse areas of contemporary chemistry since sulfoxides/sulfones
are biologically important compounds, and epoxides are versatile
precursors in the synthesis of a variety compounds such as resins,
cosmetics, surface coatings, sweeteners, perfumes, drugs, etc.
[12–15]. The most recent studies have therefore focused on the
catalytic oxidations with environmentally benign oxidants, such
as oxygen, hydrogen peroxide (H2O2) and tert-butyl hydroperoxide
(TBHP) which are safer and economical for industrial applications
[16–18]. Recently, we have reported the synthesis and application
of Silica-supported palladium complex for the synthesis of E-stilb-
enes [19]. In continuation of our research work on the designing
and synthesis of organic–inorganic hybrid materials, and their
applications as metal scavengers, sensors, and catalysts for various
organic transformations [20–27], herein, we report the synthesis,
characterization and application of silica supported molybdenum
catalyst in the oxidation of various sulfides and olefins to their cor-
responding sulfoxides/sulfones and epoxides, respectively.
2. Experimental
Due to the generation of huge amount of toxic wastes and
byproducts arising from chemical processes, chemists have been
enforced to develop environmentally friendly catalytic routes that
2.1. Materials and physical measurements
APTES (98%), silica gel, SiO2 (60–100 mesh), 2,6-diacetylpyri-
dine and sulfides were purchased from Sigma–Aldrich. Olefins
and Ammonium heptamolybdate were obtained from Alfa Aesar.
Salicyloyl hydrazide, H2O2 (30% in water), urea-H2O2 (UHP) & TBHP
⇑
Corresponding author. Tel./fax: +91 011 27666250.
E-mail address: rksharmagreenchem@hotmail.com (R.K. Sharma).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.065

R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
87
O
O
O
Si
SiO2
NH2
N
O
OH
O
N
N
H
APTES@SiO2
DAPSH
DMF
Reflux
2h
N
O
OH
O
O
O
Si
N
N
SiO2
N
H
DAPSH@APTES@SiO2
MoO2(acac)2
Methanol
Reflux, 4h
N
O
OH
O
O
O
Si
N
N
SiO2
N
H
Mo
O
O
CH3OH
Mo-DAPSH@APTES@SiO2
Scheme 1. Preparation of Silica-supported molybdenum catalyst (Mo-DAPSH@APTES@SiO2).
(a)13C CPMAS NMR of APTES@SiO2
20.99 ppm
O
O
O
SiO2
Si
NH2
9.44 ppm
42.68 ppm
APTES@SiO2
Fig. 1. 13C CPMAS NMR spectrum of APTES@SiO2.
(70% in water) were obtained commercially from Spectrochem.
Pvt. Ltd., India and used without purification. The dioxo molybde-
num acetylacetonate complex, MoO2(acac)2 and 2,6-diacetyl pyri-
dine salicyloyl monohydrazone (DAPSH) were synthesized
according to the reported procedures [28,29].
Solid-state 13C cross-polarization magic-angle spinning
(CPMAS) NMR spectra were recorded on Bruker DSX-300 NMR
spectrometer under the following pulse conditions: spectrometer
frequency (SF) 300.1312005 MHz, acquisition time (AQ)
0.0249665 s, number of acquisitions (NS) 1351, pulse duration

88
R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
Fig. 4. SEM image of catalyst (Mo-DAPSH@APTES@SiO2).
Fig. 2. Diffuse reflectance UV–Vis spectra of (a) SiO2 and (b) Catalyst, Mo-
DAPSH@APTES@SiO2.
(P3) 4.25
ls, receiver gain (RG) 16384, time of contact(P15)
2000 s, interval between pulse (D1) 5 s, rotor size 4 mm.
l
Elemental analysis (CHN) was performed using Elementar Anal-
ysensysteme GmbH VarioEL V3.00 instrument. Powder XRD pat-
terns were recorded on Bruker D8 ADVANCE X-ray diffractometer
using graphite monochromatized Cu K
a radiation (k = 1.54183 Å)
over 2h range 5–40o at a scan rate of 2h minÀ1. Surface area was
calculated using the BET method on Gemini-V2.00 instrument
(Micromeritics Instrument Corp.). Samples were outgassed at
100 °C for 3 h to evacuate the physically adsorbed moisture before
measurement. Scanning electron microscopy (SEM) images were
obtained using a ZEISS EVO 40 instrument. The samples were
placed on a carbon tape and then coated with a thin layer of gold
using a sputter coater. Qualitative analysis of catalyst for metal
was performed using energy dispersive XRF spectrometer (Fischer-
scope X-ray XAN-FAD BC). The amount of molybdenum content
was detected using LABINDIA AA 7000 Atomic Absorption Spec-
trometer. The Fourier transform infrared (FT-IR) spectra were re-
corded on a Perkin-Elmer spectrometer using KBr in the range of
4000–400 cmÀ1. The products obtained were analyzed and con-
firmed by GC–MS using Agilent gas chromatography (6850 GC)
with a HP-5MS 5% phenyl methyl siloxane capillary column
Fig. 5. XRD patterns of (a) SiO2 (b) APTES@SiO2 and (c) Mo-DAPSH@APTES@SiO2.
was set to an initial oven temperature of 60 °C and was increased
at a rate of 10 °C/min to 250 °C, and the oven was held at 250 °C for
10 min.
2.2. Preparation of Silica-supported 2,6-diacetylpyridine
salicyloylhydrazone molybdenum complex (Mo-DAPSH@APTES@SiO2)
(30.0 m  250
l
m  0.25
l
m) and
a
quadrupole mass filter
Aminopropylated silica gel (APTES@SiO2) was prepared
according to the reported method [30]. The grafting of DAPSH
on APTES@SiO2 was performed by refluxing DAPSH (5 mmol)
with APTES@SiO2 (2 g) in DMF (20 mL) for 2 h. The obtained
equipped 5975 mass selective detector (MSD). The carrier gas
was helium (rate 0.9 ml minÀ1) and the temperature of the injec-
tion port was 250 °C. The temperature program of the column
Fig. 3. ED-XRF spectrum of catalyst (Mo-DAPSH@APTES@SiO2).

R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
89
Table 1
Oxidation of sulfides using Mo-DAPSH@APTES@SiO2 catalyst.
Entry
1
Substrate
Sulfoxidea
Sulfoneb
Conv.c (%)
100
Conv.c (%)
Selectivity (%)
95
TOF (hÀ1
111
)
Selectivity (%)
98
TOFd (hÀ1
113
)
98
S
S
S
2
3
4
100
92
98
98
99
113
104
110
100
99
100
96
113
112
112
CH3
97
99
98
S
5
96
95
109
98
99
111
NO2
S
6
7
82
97
90
95
93
86
98
94
98
98
S
CH3
Cl
110
111
S
CH3
H3C
C
O
8
9
98
94
97
94
111
106
100
97
100
98
113
110
S
CH3(H2C)10H2C
CH2(CH2)10CH3
S
CH3
H2C
OH
10
11
97
94
98
98
110
106
100
95
98
113
107
S
H3C(H2C)2H2C
CH2(CH2)2CH3
100
S
CH3
H3C
12
96
97
109
100
98
113
S
a
b
c
Reaction conditions: sulfide (1 mmol), 30% H2O2 (1.2 equiv), catalyst (30 mg), 0.5 h., rt.
Reaction conditions: sulfide (1 mmol), 30% H2O2 (3.6 equiv), catalyst (30 mg), 0.5 h., rt.
Conversion and selectivity were determined by GC, Conversion = [(initial moles of substrate À final moles of substrate)/initial moles of substrate] Â 100, Selectivity to
sulfoxide = [sulfoxide%/(sulfoxide% + sulfone%)] Â 100.
d
TOF = number of moles of substrate converted per mole of catalyst per hour.
solid (DAPSH@APTES@SiO2) was filtered, washed with ethanol
and dried under vacuum at 110 °C for 4 h. Then, the complexa-
tion of DAPSH@APTES@SiO2 (1 g) was done by refluxing it with
alcoholic solution of MoO2(acac)2 (0.4 mmol) for 4 h. The result-
ing organic–inorganic hybrid catalyst (Mo-DAPSH@APTES@SiO2)
was washed with ethanol and dried in vacuum oven (Scheme 1).

90
R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
R1
R3
R2
R1
R3
O
R2
R4
Mo-DAPSH@APTES@SiO2
TBHP/Solvent-free/60 oC
R4
Scheme 3. Catalytic epoxidation of olefins.
of the functionalized aminopropyl group (which does not appear in
the parent SiO2 spectrum). Also, the band due to silanol groups
present in SiO2 has disappeared in FT-IR spectrum of APTES@SiO2.
The FT-IR spectrum of silica supported molybdenum catalyst (Mo-
DAPSH@APTES@SiO2) shows band around 848–960 cmÀ1 due to
asymmetric and symmetric stretching vibrations of O@Mo@O,
confirming the complexation of DAPSH@APTES@SiO2 with MoO2
(acac)2 [31]. The conservation of 3-aminopropyl group after func-
tionalization of SiO2 with APTES was confirmed by solid state 13C
CPMAS NMR spectroscopy. The 13C CPMAS NMR spectrum of AP-
TES@SiO2 presents three well resolved peaks at 9.44, 20.99 and
42.68 ppm assigned to C1, C2, and C3 carbons of the incorporated
aminopropyl group- O3SiCH2(1)CH2(2)CH2(3)NH2 respectively
(Fig. 1) which authenticate the preparation of APTES@SiO2 [32,33].
BET surface area measurements were also carried out to analyze
the grafting reactions. The surface area of the un-functionalized
SiO2 was approximately 235 m2gÀ1. The surface area of aminopro-
pylated silica gel (APTES@SiO2) and immobilized complex (Mo-
DAPSH@APTES@SiO2) was found to be 132 m2gÀ1 and 125 m2gÀ1
respectively. Generally, anchoring of organic and organometallic
moieties on SiO2 surface blocks the access of nitrogen molecules,
thus reducing the surface area [34,35]. Therefore, the reduction
in surface area according to the sequence SiO2 > APTES@SiO2 > -
Mo-DAPSH@APTES@SiO2 confirmed the functionalization of SiO2
with APTES to give APTES@SiO2, and its further modification to
yield the catalyst, Mo-DAPSH@APTES@SiO2.
Fig. 6. Optimization of amount of catalyst.
2.3. General procedure for the oxidation of sulfides to sulfoxides/
sulfones
A mixture of sulfide (1 mmol), catalyst (30 mg) and solution of
30% H2O2 (1.2 & 3.6 equiv. for sulfoxides & sulfones, respectively)
was stirred at room temperature. The progress of reaction was
monitored by GC. After completion of the reaction, the product
was extracted with ethyl acetate and the combined organics was
washed with brine, dried over anhydrous Na2SO4, and analyzed
by GC–MS.
2.4. General procedure for the oxidation of olefins to epoxides
The diffuse reflectance UV–Vis spectrum of SiO2 does not have
any absorption band in the region of 200–650 nm. The spectrum
of Mo-DAPSH@APTES@SiO2 (Fig. 2) shows weak bands around
200–300 nm due to intra-ligand transitions, and a strong broad
absorption band around 300–400 nm due to ligand to metal charge
transfer transition (O2À ? Mo6+) [36]. The elemental analysis data
of APTES@SiO2 shows the appearance of nitrogen (2.05%) as well as
the carbon surface content (6.14%), providing an evidence for the
APTES immobilization onto SiO2. The ED-XRF spectrum of catalyst
(Fig. 3) confirms the presence of molybdenum in the catalyst, and
its molybdenum content (5.6 wt.% of silica) was further confirmed
and estimated by AAS.
The catalytic reactions were performed in a 25 mL two-necked
round-bottomed flask equipped with a septum, a magnetic stirring
bar, and a reflux condenser. The oxidation was carried out as fol-
lows: catalyst (25 mg) and olefin (1 mmol) were taken in the reac-
tion flask. The reaction was carried out at 60 °C with continuous
stirring. TBHP (1.2 equiv) was progressively added to the reaction
mixture using a syringe. After completion of the reaction, the prod-
uct was extracted with ethyl acetate and the combined organics
was washed with brine, dried over anhydrous Na2SO4, and ana-
lyzed by GC–MS.
The morphology of Mo-DAPSH@APTES@SiO2 was examined by
SEM. During the preparation of APTES@SiO2 as well as Mo-DAP-
SH@APTES@SiO2, SiO2 beads were subjected to vigorous stirring
but it is evident from SEM image of catalyst (Fig. 4) that no clog be-
tween particles occurred during the grafting reactions, and parti-
cles maintained regular lumpy shape. It signifies that the
particles of silica had good mechanical stability, and they had not
been damaged during surface modification reactions. The struc-
tural features of SiO2 before and after functionalization were
checked by XRD measurements (Fig. 5). The broad peak centered
around 2h = 23o in the XRD patterns of SiO2, APTES@SiO2 and
3. Results and discussion
3.1. Characterization of silica supported molybdenum complex
The FT-IR spectroscopy was employed to examine the covalent
grafting reactions (Supplementary information). The FT-IR spec-
trum of SiO2 exhibits the characteristic band at 973 cmÀ1 assigned
to silanol group. The post-grafting of APTES onto SiO2 is confirmed
by the appearance of new band (in the range of 2900–2800 cmÀ1
)
in the spectrum of APTES@SiO2, which is due to the C–H stretching
O
O
O
Mo-DAPSH@APTES@SiO2
S
Mo-DAPSH@APTES@SiO2
S
S
R
R'
H2O2(1.2 equiv.)/Solvent-free/r.t.
H2O2(3.6 equiv.)/Solvent-free/r.t.
R
R'
R
R'
Scheme 2. Catalytic oxidation of sulfides.

R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
91
Table 2
Recycling of catalyst in the oxidation of sulfides.
Cycle
Sulfoxidea
Conv.c (%)
Sulfoneb
Selectivity (%)
Recovery efficiencyd (wt.%)
Conv.c (%)
Selectivity (%)
Recovery efficiency (wt.%)
1
2
3
4
5
6
98
98
98
97
97
96
95
95
95
95
95
94
99
97
99
98
98
96
100
100
100
99
99
98
98
98
98
98
97
97
98
99
95
97
98
99
a
b
c
Reaction conditions: diphenyl sulfide (1 mmol), 30% H2O2 (1.2 equiv), catalyst (30 mg), 0.5 h., rt.
Reaction conditions: diphenyl sulfide (1 mmol), 30% H2O2 (3.6 equiv), catalyst (30 mg), 0.5 h., rt.
Conversion and selectivity were determined by GC.
d
When the amount of recovered catalyst was not equal to the amount added in the first run, fresh catalyst was added to keep the same amount of catalyst in the reaction
system in each run.
Table 3
Oxidation of olefins using Mo-DAPSH@APTES@SiO2 catalyst.a
Entry
1
Substrate
Epoxide
Conv.b (%)
100
TOFc (hÀ1
)
Selectivity (%)
100
28
2
95
100
27
3
4
5
92
84
80
100
100
100
26
23
22
(CH2)5
a
Reaction conditions: olefin (1 mmol), TBHP (2 equiv), catalyst (25 mg), 2 h, 60 °C.
Conversion and selectivity were determined by GC, Conversion = [(initial moles of substrate À final moles of substrate)/initial moles of substrate] Â 100 and Selectivity to
b
epoxide = [epoxide%/(epoxide% + other product%)] Â 100.
c
TOF = number of moles of olefin converted per mole of catalyst per hour.
Mo-DAPSH@APTES@SiO2 is assigned to the diffraction peak of
amorphous silica, which clearly depicts that topological structure
of SiO2 is retained after grafting reactions. Apparently, by function-
alizing SiO2 and eventually anchoring molybdenum complex, the
intensities of reflections have decreased significantly confirming
the immobilization [37].
Table 4
Recycling of catalyst in the oxidation of olefins.a
Cycle
Epoxidea
Conv.b (%)
Selectivity (%)
Recovery efficiencyc (wt.%)
1
2
3
4
5
6
100
100
98
98
98
100
100
100
100
100
100
98
98
99
97
98
99
3.2. Catalytic studies
98
3.2.1. Catalytic activity of Mo-DAPSH@APTES@SiO2 in oxidation of
sulfides
a
Reaction conditions: cyclooctene (1 mmol), TBHP (2 equiv), catalyst (25 mg),
2 h, 60 °C.
b
In order to obtain best catalytic activity, reaction conditions
were optimized for the oxidation of sulfides. The oxidation of di-
phenyl sulfide was carried out as the model reaction with different
oxidants (H2O2, UHP & TBHP) at room temperature under solvent-
free conditions (Supplementary information). The results showed
that H2O2 is the best oxidizing agent for oxidation of sulfides as
higher conversion was obtained. The effect of different solvents
was also investigated and results are presented in supplementary
information. Since, solvent-free reactions are usually quantitative
and waste free, reaction was also carried out under solvent-free
conditions in the presence of catalyst. Interestingly, under solvent
free conditions, maximum conversion of diphenyl sulfide was ob-
tained. Next, we optimized the quantity of the catalyst (Fig. 6)
and it was observed that on increasing the amount of catalyst, con-
version also increased due to the availability of large number of ac-
tive sites on the porous surface of catalyst. It is noteworthy that in
Conversion and selectivity were determined by GC.
When the amount of recovered catalyst was not equal to the amount added in
c
the first run, fresh catalyst was added to keep the same amount of catalyst in the
reaction system in each run.
a blank experiment, no significant sulfoxidation was observed un-
der similar reaction conditions in the absence of catalyst, and only
a low conversion was observed in the presence of 2 equiv of H2O2
after prolonged reaction time. Hence, in all cases, neat reactions
were carried out at room temperature using H2O2 as oxidant in
presence of 30 mg of Mo-DAPSH@APTES@SiO2 catalyst under sol-
vent-free conditions (Scheme 2). After optimization of the reaction
conditions, applicability of catalyst was tested for oxidation of var-
ious sulfides (Table 1). A variety of structurally diverse substrates-
aryl–alkyl, diaryl and dialkyl sulfides undergo effective and

92
R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
Table 5
Comparison of the activity of molybdenum catalysts in the oxidation of sulfides.
Entry Catalytic system
Sulfide
Reaction conditions
Sulfoxide
Sulfone
Ref.
Conv.
(%)
Selectivity
(%)
Conv.
(%)
Selectivity
(%)
1
2
3
[MoO2(L)(CH3OH)]
methyl
phenylsulfide
methyl
phenylsulfide
methyl
phenylsulfide
urea–hydrogen peroxide Ethanol, r.t, 30 min
92
100
95
100
96
100
–
98
–
[38]
[39]
[40]
Oxo-peroxido Mo schiffs base
complex
Mo-oxazoline complex cis-
[MoO2(phox)2
urea–hydrogen peroxide, methanol/
dichloromethane r.t. , 35.25 min
urea–hydrogen peroxide, dichlromethane/
methanol, r.t
100
–
–
4
5
[MoO2(L)(CH3OH)]
Cp’Mo(CO)3Cl
diphenyl sulfide urea–hydrogen peroxide ethanol, r.t, 30 min
diphenyl sulfide hydrogen peroxide, dichloromethane, 35 °C , 5 h
75
100
93
100
95
–
97
–
[38]
[41]
Cp-cyclopentyl
6
Cp’Mo(CO)3Cl
methyl
hydrogen peroxide, dichloromethane, r.t, 1 h
100
–
96
–
[41]
Cp-cyclopentyl
phenylsulfide
Table 6
Comparison of the activity of Silica-supported molybdenum catalysts in the oxidation of olefins.
Entry
Olefin
Catalytic system
Reaction conditions
Conv. (%)
Selectivity (%)
Ref.
1
2
3
4
5
6
7
1-octene
Cyclohexene
1-octene
Cyclohexene
1-hexene
Dodecene
1-octene
silica-supported MoO2Cl2(dmf)2
silica-supported MoO2(acac)2
silica-supported MoO2histidine(acac)
silica-supported MoO2ala(acac)
silica-supported MoO2furfuryl
dichloromethane, TBHP, 55 °C, 6 h
1,2-dichloroethane, TBHP, reflux, 2 h
chloroform, TBHP, reflux, 9 h
chloroform, TBHP, reflux, 9 h
chloroform, TBHP, reflux, 7 h
1,2-dichloroethane, TBHP, reflux, 16 h
1,2-dichloroethane, TBHP, reflux, 14 h
36
90
17
30
42
90
88
100
100
–
–
98
[42]
[43]
[44]
[44]
[45]
[46]
[47]
silica-supported Schiff base-MoO2(acac)
silica supported imidazole-MoO2(acac)2
100
100
selective oxidation to produce the corresponding sulfoxides/sulf-
ones. Further to broaden the domain of present protocol, and mak-
ing it better in comparison to other reported systems a variety of
sulfides containing sensitive functional group were also tested un-
der same reaction conditions. The sulfide containing allyl, keto and
hydoxy groups (Table 1, entries 3, 7, 9) were selectively oxidized to
their corresponding sulfoxides/sulfones without affecting these
reactive functional groups. It is also interesting to mention that
even the presence of strong electron withdrawing NO2 group, pres-
ent in the phenyl ring of diaryl sulfides, does not affect the synthe-
sis of sulfoxide considerably (Table 1, entry 5). The catalytic
oxidation of sulfides under given conditions showed good effi-
ciency in terms of conversion, TOF and selectivity.
cyclooctene, catalyst was separated from the reaction mixture by
filtration (after 15 min). The filtrate was allowed to react further,
and no increment in conversion was observed. Further, the filtrate
was subjected to AAS analysis and absence of molybdenum in the
reaction mixture suggested that no leaching of the catalyst was ob-
served during the reaction. The reusability of silica supported
molybdenum catalyst was also investigated in the oxidation of di-
phenyl sulfide and of cyclooctene. Catalyst was separated by filtra-
tion after the first catalytic run, washed, dried under vacuum and
then subjected to the second run under similar reaction conditions.
The catalytic run was repeated with addition of substrates under
optimized reaction conditions, and the selectivity of the products
was comparable to that of the original one. It was observed that
the activity and selectivity of the catalyst does not change signifi-
cantly after seven consecutive runs (Tables 3 and 4).
3.2.2. Catalytic activity in oxidation of olefins
This study was further extended to the epoxidation of several
linear and cyclic olefins using Mo-DAPSH@APTES@SiO2 as catalyst
(Scheme 3). Initially, for optimization of reaction conditions for
epoxidation, cyclooctene was chosen as the model substrate
(Supplementary information). The reaction was carried out using
various oxidants (H2O2, UHP & TBHP) and in different solvents and
solvent-free conditions. The best results were obtained using TBHP
as oxidant under solvent-free conditions at 60 °C in presence of
Mo-DAPSH@APTES@SiO2 catalyst. Generally, excellent epoxide
selectivity was observed for all aliphatic and aromatic substrates.
The results of catalytic studies using Mo-DAPSH@APTES@SiO2 cata-
lyst reveals that it is highly efficient towards all the substrates with
maximum conversion, TON, and selectivity (Table 2). It is suggested
that the olefin having higher electron donating ability and less
sterically hindered gives higher conversion with high selectivity.
3.4. Comparison with reported molybdenum catalyst
The present catalytic system was also compared with some re-
ported molybdenum catalysts (Tables 5 and 6), and found that our
organic–inorganic hybrid catalytic system (Mo-DAPSH@APTES@
SiO2) is superior to some reported catalytic protocols in terms of
cost, reaction time, selectivity, conversion, turn-over frequency
(TOF) and reusability [38–47].
4. Conclusion
In conclusion, we have prepared a silica supported molybde-
num catalyst which was used in the oxidation of sulfides and ole-
fins using environmentally benign oxidants. Mild reaction
conditions, high activity and selectivity, economic viability, easy
preparation and reusability of the catalyst make the present proto-
col a greener alternative for the synthesis of commercially impor-
tant sulfoxides/sulfones and epoxides.
3.3. Catalyst stability and reusability
To confirm the heterogeneous nature of the catalyst, filtration
test was applied. During the oxidation of diphenyl sulfide and of

R.K. Sharma et al. / Polyhedron 45 (2012) 86–93
[17] A. Massa, E.M. Lorenzo, A. Scettri, J. Mol. Catal. A: Chem. 250 (2006) 27.
93
Acknowledgements
[18] C. Aprile, A. Corma, M.E. Domine, H. Garcia, C. Mitchell, J. Catal. 264 (2009) 44.
[19] R.K. Sharma, A. Pandey, S. Gulati, Appl. Catal. A: Gen. 431–432 (2012) 33.
[20] R.K. Sharma, A. Pandey, S. Gulati, A. Adholeya, J. Hazard. Mater. 209–210
(2012) 285.
The financial assistance from University Grant Commission,
Delhi, India is acknowledged. The authors also thank IISc, Banga-
lore, India for solid state NMR measurements.
[21] R.K. Sharma, A. Goel, Anal. Chim. Acta 534 (2005) 137.
[22] R.K. Sharma, D. Rawat, G. Gaba, Catal. Commun. 19 (2012) 31.
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[24] R.K. Sharma, C. Sharma, J. Mol. Catal. A. Gen. 332 (2010) 53.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.065.
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