Immobilization of a vanadium complex onto functionalized nanoporous MCM-41 and its application as a catalyst for the solvent-free chemoselective oxidation of sulfide to sulfoxide

Article abstract of DOI:10.1002/aoc.3422

A complex moiety containing VO(IV) was anchored covalently into organic-modified Si-MCM-41 to prepare a new catalyst. The prepared materials were characterized using various techniques. Several types of aromatic and aliphatic sulfides were successfully oxidized to the corresponding sulfoxides in good to excellent yields using H₂O₂ in the presence of a catalytic amount of the catalyst under solvent-free conditions. The results showed that the OH groups of the various compounds such as 2,2-(phenylthio)ethanol and 2-(methylthio)ethanol remained intact under similar conditions. Meanwhile the catalyst was stable in the reaction system, and could be reused at least four times without significant loss of its activity and chemoselectivity.

Full text of DOI:10.1002/aoc.3422

Full paper
Received: 25 October 2015
Revised: 23 November 2015
Accepted: 26 November 2015
Published online in Wiley Online Library: 13 January 2016
(wileyonlinelibrary.com) DOI 10.1002/aoc.3422
Immobilization of a vanadium complex onto
functionalized nanoporous MCM-41 and its
application as a catalyst for the solvent-free
chemoselective oxidation of sulfide to sulfoxide
M. Nikoorazm*, A. Ghorbani-Choghamarani and M. Khanmoradi
A complex moiety containing VO(IV) was anchored covalently into organic-modified Si-MCM-41 to prepare a new catalyst.
The prepared materials were characterized using various techniques. Several types of aromatic and aliphatic sulfides were
successfully oxidized to the corresponding sulfoxides in good to excellent yields using H2O2 in the presence of a catalytic
amount of the catalyst under solvent-free conditions. The results showed that the OH groups of the various compounds such
as 2,2-(phenylthio)ethanol and 2-(methylthio)ethanol remained intact under similar conditions. Meanwhile the catalyst was
stable in the reaction system, and could be reused at least four times without significant loss of its activity and
chemoselectivity. Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: MCM-41; mesostructured; vanadyl ion; hydrogen peroxide; sulfide
Although many of these methods are quite useful, there still
remains much room for improvements especially in the case of
Introduction
Oxo-vanadium complexes have been extensively studied because
of their potential applicability as catalysts or mediators for oxida-
tions by molecular oxygen, peroxidative oxygenations (both includ-
ing epoxidations), halogenations and carboxylations of aliphatic
and aromatic hydrocarbons, and/or olefins, toward the syntheses
of a variety of organic compounds, such as alcohols, ketones, epox-
ides, aldehydes, organohalides and carboxylic acids,[1] and oxida-
tion of organic sulfides to sulfoxides.[2–5] Some of these systems
operate in liquid media, whereas others involve VO catalysts
immobilized on solid supports. The easily reversible inter-
conversion of the vanadium oxidation states +IV and +V and the
easy formation of epoxide to act as Lewis acidic and basic sites or
undergo radical-mediated transformations during catalytic reac-
tions make vanadium one of the best-suited elements for catalytic
oxidations and oxygen-transfer reactions.
the selectivity and eco-friendliness of the reactions. The goal of
the work reported in this paper was to develop an efficient hetero-
geneous catalytic system for the oxidation of sulfides to sulfoxides
with H2O2 using vanadyl acetylacetonate complex grafted on 2-
amino-3-hydroxypyridine-modified mesoporous silica MCM-41
(VO-2A3HP-MCM-41) as a stable catalyst under solvent-free condi-
tions at room temperature.
Experimental
Materials
The cationic surfactant cetyltrimethylammonium bromide (CTAB;
98%), tetraethyl orthosilicate (TEOS; 98%), hydrogen peroxide
(30% aqueous), 3-chloropropyl-1-triethoxysilane, vanadyl acetylac-
etonate (VO(acac)2), organic sulfides, solvents and other materials
were purchased from Merck, Aldrich or Fluka and were used with-
out further purification.
The immobilization of catalyst or catalyst precursor complexes
on supports is a common and suitable procedure for combining
the advantages of homogeneous and heterogeneous catalysis.
Several types of inorganic supports, such as silica gel,[6] USY
zeolites,[7,8] alumina[6] and mesoporous silicas,[9] have been used
for oxo-vanadium catalysis. MCM-41 gives much opportunity for
the preparation of heterogeneous catalysts due to outstanding
properties like uniform pore structure, large surface area and wide
range of pore dimensions that ensure easy accessibility of reactants
to active sites, and also provides a unique inorganic support that
can anchor the active sites on inorganic oxides.[10,11]
Instrumentation
Powder X-ray diffraction (XRD) of the VO-2A3HP-MCM-41 catalyst
was conducted using a Philips diffractometer with Cu Kα radiation
at 40 kV and 30 mA. The particle size and morphology were inves-
tigated using a JEOL JEM-2010 scanning electron microscopy (SEM)
*
Correspondence to: M. Nikoorazm, Department of Chemistry, Ilam University, PO
Box 69315516, Ilam, Iran. E-mail: e_nikoorazm@yahoo.com
Sulfoxides are useful building blocks especially as chiral auxil-
iaries in organic synthesis.[12–15] Sulfoxides are important intermedi-
ates in the synthesis of various natural products,[16–19] and they also
can play a key role in the activation of enzymes.[20]
Department of Chemistry, Faculty of Science, Ilam University, PO Box 69315516,
Ilam, Iran
Appl. Organometal. Chem. 2016, 30, 236–241
Copyright © 2016 John Wiley & Sons, Ltd.

VO-2A3HP-MCM-41 as nanostructured catalyst for the sulfoxidation reaction
instrument at an accelerating voltage of 200 kV. The measurement
of product components was carried out using energy-dispersive
spectroscopy (EDS) with a Tacnai TF20 high-resolution transmission
electron microscope. Thermogravimetric analysis (TGA) curves
were recorded using a PL-STA 1500 device (Thermal Sciences). Fou-
rier transform infrared (FT-IR) spectra of samples were recorded in
KBr discs using a Nicolet Impact 410 spectrometer. Transmission
electron microscopy (TEM) images were obtained using a Philips
CM10 microscope operating at 200 kV. Specific surface area of sam-
ples was obtained using the Brunauer–Emmett–Teller (BET)
method using a NOVA 1000 instrument.
Synthesis of MCM-41 modified with (3-chloropropyl)
triethoxysilane
Pure silica MCM-41 was prepared by adding 1 g of CTAB to an
aqueous solution containing 3.5 ml of NaOH solution (2 M) and
deionized water (480 ml). The resulting mixture was stirred con-
tinuously at 80°C. Then, 5 ml of TEOS was slowly added into the
solution. The resulting mixture was refluxed for 2 h at 80°C. The
collected product was washed with deionized water and cal-
Scheme 1. Procedure for preparation of VO-2A3HP-MCM-41 catalyst.
cined at 823 K for 5 h at a rate of 2°C minꢀ1 to remove the resid-
ual surfactant. This mesoporous material was designated as Si-
MCM-41. Post-synthesis organic modification of MCM-41 was per-
formed by refluxing 4.8 g of MCM-41 and 4.8 g of 3-chloropropyl-
1-triethoxysilane in n-hexane for 24 h to obtain a white solid of
the functionalized MCM-41 (nPrCl-MCM-41). The resulting material
was filtered, washed with n-hexane and dried under vacuum.
Figure 1 shows the XRD patterns of MCM-41 and VO-2A3HP-
MCM-41. They have one strong and two weak peaks. All three
XRD reflections represent the hexagonal lattice of MCM-41. The
spectra display hexagonal symmetry with (100), (110) and (200) dif-
fraction peaks that relate to 2θ values of 2.77°, 4.57° and 5.21°, re-
spectively. The decrease in the peak intensities in the XRD pattern
of VO-2A3HP-MCM-41 is an indication of the MCM-41 pore surface
silanol groups reacting with vanadyl ions.[20]
Preparation of heterogeneous VO-2A3HP-MCM-41 catalyst
The FT-IR spectrum of the MCM-41 sample (Fig. 2(a)) indicates
the characteristic bands of the MCM-41 framework related to Si O
stretching of surface Si OH groups and three peaks at 1059, 962
and 454 cmꢀ1 correspond to asymmetric stretching, symmetric
stretching and bending vibrations of Si O Si, respectively. The pres-
ence of anchored 3-chloropropyl(triethoxy)silane (nPrCl) is con-
firmed by the C H stretching vibration appearing at about
2900 cmꢀ1 (Fig. 2(b)). The free ligand signals at 1265 cmꢀ1 (C O)
and 1650 cmꢀ1 (C N) emphasize that 2A3HP is successfully an-
chored on modified MCM-41 (Fig. 2(c)). The band near 3468 cmꢀ1
of the free ligand is assigned to ν(NH2) bending; this band is shifted
to lower wavenumber in the spectrum of the complex due to the
coordination of the amino group nitrogen atom to the metal ion
(Fig. 2(d)).[21]
For the preparation of 2A3HP-MCM-41, 0.5 g of nPrCl-MCM-41 was
refluxed with triethylamine (2 mmol, 0.202 g) and 2-amino-3-
hydroxypyridine (2A3HP; 1 mmol, 0.110 g) in toluene for 24 h. The
resulting solid was washed with ethanol, collected by filtration
and dried under vacuum. Ultimately, VO-2A3HP-MCM-41 was pre-
pared by refluxing VO(acac)2 (0.63 mmol, 0.168 g) with 0.250 g of
2A3HP-MCM-41 in ethanol (20 ml) for 20 h. The obtained VO-
2A3HP-MCM-41 product was washed with ethanol, collected by fil-
tration and dried under vacuum (Scheme 1). Characterization of the
VO-2A3HP-MCM-41 catalyst was performed using TGA, FT-IR, XRD,
SEM, BET and TEM techniques.
Catalytic oxidation of organic compounds
In order to examine the catalytic activity of the catalyst, VO-2A3HP-
MCM-41 (10 mg) was added to a mixture of sulfides (1 mmol) and
H2O2 (0.4 ml) under solvent-free conditions with stirring at room
temperature. After completion of the reaction, the catalyst was sep-
arated using simple filtration and washed with dichloromethane
(3 × 10 ml).
Results and discussion
Characterization of VO-2A3HP-MCM-41
Scheme 1 shows the preparation route of VO-2A3HP-MCM-41.
Characterization of the VO-2A3HP-MCM-41 catalyst was performed
using TGA, FT-IR, XRD, SEM, BET and TEM techniques. The results in-
dicate that, using this method (sol–gel), VO(acac)2 is successfully
immobilized onto the MCM-41.
Figure 1. XRD patterns of MCM-41 and VO-2A3HP-MCM-41.
Appl. Organometal. Chem. 2016, 30, 236–241
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M. Nikoorazm, A. Ghorbani-Choghamarani and M. Khanmoradi
Figure 2. FT-IR spectra of (a) MCM-41, (b) nPrCl-MCM-41, (c) 2A3HP-MCM-41 and (d) VO-2A3HP-MCM-41.
Figure 3. SEM images of (a) MCM-41 and (b) VO-2A3HP-MCM-41.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 236–241

VO-2A3HP-MCM-41 as nanostructured catalyst for the sulfoxidation reaction
Figure 3 shows SEM micrographs of MCM-41 and VO-2A3HP-
MCM-41. This analysis shows diameters of approximately
77.01–226.99 nm for the VO-2A3HP-MCM-41 nanoparticles with
a spherical shape. The EDS spectrum (Fig. 4) evidently shows
the presence of vanadium in the catalyst. The sample consists
of carbon with oxygen most likely due to the presence of a few
unreacted oxygen functional groups. It also shows the presence
of nitrogen and silicon indicating the presence of these ele-
ments in VO-2A3HP-MCM-41.
Furthermore the TEM image (Fig. 5) clearly shows that nanopar-
ticles are present, and VO-2A3HP-MCM-41 particles have spherical
shapes with hexagonal channels. It can be seen that the catalyst
is homogeneously dispersed on the surface of the silica matrix,
and no obvious aggregation is observed.
Figure 6. TGA thermograms of (a) MCM-41 and (b) VO-2A3HP-MCM-41.
TGA curves of the MCM-41 support and VO-2A3HP-MCM-41 are
shown in Fig. 6. The pure MCM-41 shows a weight loss (4.0 wt%)
in one step at 25ꢀ100°C. This behaviour indicates a poor capacity
for water adsorption. The TGA curve of VO-2A3HP-MCM-41 shows
a weight loss (around 10 wt%) appearing at a temperature
<150°C corresponding to the loss of water and other solvents.
The other weight loss at about 250–500°C (15 wt%) is related to
decomposition of organic moieties.
In order to determine the textural properties of the catalyst, nitro-
gen adsorption and desorption isotherms of MCM-41 and VO-
2A3HP-MCM-41 were measured and the results are shown in Fig. 7.
BET analysis of MCM-41 shows a surface area of 1372 m2 gꢀ1 and a
pore volume of 1.521 cm3 gꢀ1. In comparison with MCM-41, the rel-
ative pressure is transformed into a lower value for VO-2A3HP-
Figure 7. Nitrogen adsorption–desorption isotherms of (a) MCM-41 and (b)
Figure 4. EDS spectrum of VO-2A3HP-MCM-41.
VO-2A3HP-MCM-41.
Figure 5. TEM image of VO-2A3HP-MCM-41.
Appl. Organometal. Chem. 2016, 30, 236–241
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wileyonlinelibrary.com/journal/aoc

M. Nikoorazm, A. Ghorbani-Choghamarani and M. Khanmoradi
MCM-41, which leads to a reduction of BET specific area, pore vol-
ume and pore diameter. These results are summarized in Table 1.
The wall thickness of MCM-41 and VO-2A3HP-MCM-41 was calcu-
lated using the following equation:
show, increasing the amount of oxidant leads to higher conver-
sions. Methylphenylsulfide is completely converted to sulfoxide
with 5 equiv. of H2O2. An attempt to carry out the reaction with-
out the catalyst proved the reaction could occur rather slowly
(5 h). However, when a similar oxidation reaction is conducted
in the presence of the catalyst (10 mg), the reaction is completed
(with a yield of 98%) in 2 h.
2d100
pffiffiffi
Wall thickness ¼
ꢀ DBJH
3
After optimization of the reaction parameters, the scope of appli-
cability of the catalyst was investigated for oxidation of a variety of
substituted aromatic and aliphatic sulfides (Scheme 2). The results
are summarized in Table 3. Both alkyl and aryl sulfides selectively
convert to the corresponding sulfoxides in excellent yield. The ad-
vantage of this method over conventional homogeneous reactions
is that it provides greater selectivity, proceeds with enhanced reac-
tion rates, gives cleaner products and involves simple manipulation.
Upon completion of the reaction, the catalyst was filtered off, rinsed
with CH2Cl2, dried and reused for at least four times with similar
activity. It is evident that the catalyst possesses high stability in
the oxidation of sulfides with H2O2 which demonstrates that
VO-2A3HP-MCM-41 can be totally recovered after the oxidation
without any decomposition. One of the most important factors
that affect catalytic performance in sulfoxdations is Lewis acidity
behaviour of the oxo-vanadium(IV) complex. This complex can
readily withdraw electrons from the peroxide oxygens making
them more susceptible to be attacked by nucleophilic sulfide
compound. Functional groups such as hydroxyl groups remain un-
affected (Table 3, entries 8 and 9).
Application of VO-2A3HP-MCM-41 complex in oxidation
reactions
In continuation of our previous work on oxidation of sulfides into
sulfoxides,[22–24] here we report an easy way to accomplish cata-
lytic sulfoxidation with hydrogen peroxide in the presence of
VO-2A3HP-MCM-41 to afford sulfoxides in high yields and good
chemoselectivity. Initially, the reaction without using any cata-
lyst was conducted, giving the product in very low yield. In order
to find the optimum reaction conditions, we tried to oxidize
methylphenylsulfide as a model substrate with H2O2 in the pres-
ence of the catalyst under solvent-free conditions at room tem-
perature. As evident from Table 2, the model compound
selectively converts to the corresponding sulfoxide in excellent
yield. Performing the oxidation of methylphenylsulfide with var-
ious molar ratio of H2O2 to sulfide shows that the amount of ox-
idant considerably affects the yield of sulfoxide. As the results
Table 1. Textural parameters deduced from nitrogen sorption
isotherms
Sample
SBET
Pore
Pore
Wall
(m2 gꢀ1
)
diameter volume diameter
(nm)a
(cm3 gꢀ1
)
(nm)
MCM-41
1372.275
2.45
1.21
1.521
0.846
0.808
1.64
VO-2A3HP-MCM-41
846.01
aObtained using the Barrett–Joyner–Halenda (BJH) method.
Scheme 2. Oxidation of sulfides to sulfoxides in the presence of
VO-2A3HP-MCM-41.
Table 2. Comparison between various solvents and amount of
VO-2A3HP-MCM-41 for the oxidation of methylphenylsulfide in 2 h
at room temperaturea
Table 3. Oxidation of various sulfides using H2O2, catalysed by
VO-2A3HP-MCM-41
Condition
Solvent
Solvent
Yield (%)b
Entry
Substrate
Product Time Yield
Melting
point (°C)
(min)
(%)a
Dichloromethane
45
1
2
3
4
5
6
7
8
9
10
Methyl(phenyl)sulfide
Tetrahydrothiophene
Methyl(p-tolyl)sulfide
Diphenylsulfide
2a
2b
2c
2d
2e
2f
120
10
96 31–33[18]
93
Oil[18]
95 120–123[18]
86 66–69[25]
Ethanol
90
Acetonitrile
98
45
Ethyl acetate
80
1800
100
30
Solvent free
98
Dipropylsulfide
98
92 61–65[18]
Oil[18]
Amount of VO-2A3HP-MCM-41 (mg)
Amount of oxidant (equiv.)
10
98
Dodecyl(methyl)sulfide
n-Dodecylsulfide
8
82
2g
2h
2i
60
91
90
98
95
Oil[18]
Oil[18]
Oil[18]
Oil[19]
6
78
Absc
No reaction
2-(phenylthio)ethanol
2-(methylthio)ethanol
4-Chlorophenyl(methyl)
sulfide
15
60
3
4
5
73
89
98
2j
50
11
12
Methyl-3-(methylthio)
propionate
2k
2L
240
60
93
94
Oil[18]
Oil[26]
aReaction conditions: methylphenylsulfide (1 mmol), H2O2 (0.4 ml,
1 mmol) at 40°C.
bIsolated yields.
cReaction proceeds in the absence of catalyst.
Allyl(methyl)sulfide
aIsolated yields.
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VO-2A3HP-MCM-41 as nanostructured catalyst for the sulfoxidation reaction
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Appl. Organometal. Chem. 2016, 30, 236–241
Copyright © 2016 John Wiley & Sons, Ltd.
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