Zirconium oxide complex-functionalized MCM-41 nanostructure: An efficient and reusable mesoporous catalyst for oxidation of sulfides and oxidative coupling of thiols using hydrogen peroxide

Article abstract of DOI:10.1002/aoc.3348

Zirconium oxide complex-functionalized mesoporous MCM-41 (Zr-oxide@MCM-41) as an efficient and reusable catalyst is reported for the oxidation of sulfides into sulfoxides using hydrogen peroxide (H₂O₂) as the oxidant, with short reaction times in good to excellent yields at room temperature under solvent-free conditions. Also, a simple and efficient method is reported for the oxidative coupling of thiols into corresponding disulfides in good to high yields using H₂O₂ as oxidant in the presence of Zr-oxide@MCM-41 as recoverable catalyst in ethanol at room temperature. A series of sulfides and thiols possessing functional groups was successfully converted into corresponding products. After completion of reactions the catalyst was easily separated with simple filtration from the reaction mixture and reused for several consecutive runs without significant loss of catalytic efficiency. The mesoporous catalyst was characterized using Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller surface area measurements, X-ray diffraction, transmission and scanning electron microscopies, energy-dispersive X-ray spectroscopy and thermogravimetric analysis.

Full text of DOI:10.1002/aoc.3348

Full paper
Received: 22 March 2015
Revised: 10 May 2015
Accepted: 4 June 2015
Published online in Wiley Online Library: 27 July 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3348
Zirconium oxide complex-functionalized MCM-
41 nanostructure: an efficient and reusable
mesoporous catalyst for oxidation of sulfides
and oxidative coupling of thiols using
hydrogen peroxide
Maryam Hajjami*, Lotfi Shiri and Azar Jahanbakhshi
Zirconium oxide complex-functionalized mesoporous MCM-41 (Zr-oxide@MCM-41) as an efficient and reusable catalyst is
reported for the oxidation of sulfides into sulfoxides using hydrogen peroxide (H2O2) as the oxidant, with short reaction times
in good to excellent yields at room temperature under solvent-free conditions. Also, a simple and efficient method is reported
for the oxidative coupling of thiols into corresponding disulfides in good to high yields using H2O2 as oxidant in the presence
of Zr-oxide@MCM-41 as recoverable catalyst in ethanol at room temperature. A series of sulfides and thiols possessing functional
groups was successfully converted into corresponding products. After completion of reactions the catalyst was easily separated
with simple filtration from the reaction mixture and reused for several consecutive runs without significant loss of catalytic
efficiency. The mesoporous catalyst was characterized using Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller
surface area measurements, X-ray diffraction, transmission and scanning electron microscopies, energy-dispersive X-ray
spectroscopy and thermogravimetric analysis. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: Zr-oxide@MCM-41; hydrogen peroxide; oxidation; sulfoxides; disulfides
their frameworks. In fact, numerous studies have been performed
Introduction
aiming to modify pure siliceous MCM-41 in order to increase its
applicability.[11,12] Also, the potential advantage of these materials
Catalytic processes play a fundamental role in chemical and phar-
maceutical industries.[1,2] While very selective and active homoge-
(simplified recovery and reusability) as compared with homoge-
neous catalysts have been developed, it is often difficult to isolate
the products and to recover the catalyst for reuse.[3] This difficulty
can be overcome using heterogeneous catalysts, which in general
allow a simple separation of products and catalyst.[4] Therefore,
the preparation and engineering of nanostructured porous silicate
materials have been greatly explored as heterogeneous catalysts
owing to their characteristics of high surface area and uniform
mesoporous nano-order.[5,6] Particularly, the MCM-41 molecular
sieve has allowed development of catalysts applicable in many in-
dustrial processes.[5–12] Features such as a hexagonal packed array
of unidirectional mesopores (between 2 and 10 nm) with narrow
pore size distribution, highly ordered structure, high thermal stabil-
ity, high surface area (up to 900 m2 gꢀ1) and high specific pore
volume (up to 1.3 ml gꢀ1) are mainly responsible for its high
adsorption capacity even of bulky molecules.[8] Its large pore size al-
lows passage of large molecules such as organic reactants and
metal complexes through the pores to reach the surface of the
channels.[9] Therefore, some transition metal complexes on func-
tionalized MCM-41 support have been prepared and successfully
used in organic reactions.[10] In this light, some zirconium oxide
complexes on functionalized MCM-41 support can be prepared
and successfully used in organic reactions. Meanwhile, the catalytic
properties of these materials rely on the presence of active sites in
neous systems can lead to environmentally benign chemical
processes. In spite of tremendous effort dedicated to the immobili-
zation of homogeneous zirconium oxide complexes over the last
two decades,[13] very few examples of oxidative coupling or oxida-
tion reactions catalysed by heterogeneous zirconium oxide cata-
lysts have been reported.[14] So, the development of a stable
heterogeneous zirconium oxide catalyst that allows for highly effi-
cient oxidation of a wide range of sulfides and oxidative coupling
of many thiols is worthwhile.
Coupling reactions such as oxidative coupling of thiols into corre-
sponding disulfides and also oxidation of organic functional groups
such as oxidation of sulfides into sulfoxides are useful transforma-
tions for the synthesis of new molecules, as well as for various med-
ical, biological, materials and nanotechnological applications.[15–17]
In particular, sulfoxides are valuable synthetic intermediates for the
production of a range of chemically and biologically active
*
Correspondence to: Maryam Hajjami, Department of Chemistry, Faculty of
Science, Ilam University, PO Box 69315516, Ilam, Iran. E-mail: mhajjami@yahoo.
com; m.hajjami@ilam.ac.ir
Department of Chemistry, Faculty of Science, Ilam University, PO Box 69315516,
Ilam, Iran
Appl. Organometal. Chem. 2015, 29, 668–673
Copyright © 2015 John Wiley & Sons, Ltd.

Zr oxide-functionalized MCM-41 as catalyst for oxidation reactions
molecules including therapeutic agents such as anti-ulcer, anti-
fungal, antibacterial, anti-atherosclerotic, antihypertensive and
anti-atherosclerotic drugs as well as psychotropic and vasodilator
drugs.[18,19] Omeprazole and the pesticide fipronil are two typical
examples of the extensive application of these intermediates in
the pharmaceutical and fine chemical industries.[20,21] Likewise,
disulfide bond formation is important in peptides, bioactive
molecules and oil sweetening processes. Disulfides are used in
sulfonylation of enolates and other anions while some disulfides
have been found to be useful as vulcanizing agents for rubber
and elastomers imparting to them suitable tensile strength.[22] They
are also essential moieties of biologically active compounds for
peptide and protein stabilization anions. Furthermore, disulfides
are more stable to organic reactions such as oxidation, alkylation
and acylation compared to the corresponding free thiols, so the
thiol group can conveniently be protected as a disulfide.[23]
For these reasons, many methods have been developed over the
years for these oxidation reactions to be carried out efficiently.
For example, oxidation of sulfides and thiols has been reported
in the presence of various catalysts and using a variety of oxidants
(e.g. metal oxidants, organic oxidants, peroxides, halogens and
air).[15–23] Concerning green oxidants, hydrogen peroxide (H2O2)
is usually applied for the oxidation of sulfides and thiols as a cheap
and environmentally friendly oxidant, because it is inexpensive and
produces only water as a side product.[24] However, many of these
methods have drawbacks, such as harsh acidic conditions, long re-
action times, need of expensive catalyst, unsatisfactory yields and
use of non-reusable catalysts. Therefore, to overcome these limita-
tions, we decided to design a new system for the oxidation of sul-
fides into sulfoxides as well as for oxidative coupling of thiols into
their corresponding disulfides using H2O2 at room temperature in
the presence of zirconium oxide complex-functionalized mesopo-
rous MCM-41 (Zr-oxide@MCM-41) as catalyst.
filtered and the resulting solid washed several times with n-hexane.
The solid was dried under vacuum to afford MCM-41-NH2. Then, for
the preparation of Schiff base@MCM-41, this solid (1 g) was refluxed
with 0.147 g (1 mmol) of isatin and 0.5 ml of acetic acid in ethanol
for 5 h under nitrogen atmosphere. The resulting solid (Schiff
base@MCM-41) was filtered, washed with ethanol and dried in
vacuum. Finally, for the preparation of Zr-oxide@MCM-41, Schiff
base@MCM-41 (1.0 g) was mixed with 0.1 g (1.0 mmol) of
ZrOCl2ꢁ8H2O in 10 ml of dry CH3CN. The mixture was stirred at room
temperature for 5 h. The solid product was obtained by filtration,
washed with ethanol and dried at 60°C.
General procedure for oxidation of sulfides to sulfoxides
A mixture of sulfide (1 mmol), H2O2 (1.2 mmol) and Zr-oxide@MCM-
41 (0.01 g, 1.64 × 10ꢀ4 mol%) was stirred at room temperature under
solvent-free conditions, the progress of the reaction being monitored
using TLC. After completion of the reaction, the catalyst was sepa-
rated by simple filtration and washed with ethyl acetate. The product
was then extracted with ethyl acetate (4 × 5 ml). The organic layer
was dried over anhydrous Na2SO4 (1.5 g). Finally, the organic solvents
were evaporated, affording products in good to high yields.
General procedure for oxidative coupling of thiols into
disulfides
Zr-oxide@MCM-41 (0.02 g, 3.29 × 10ꢀ4 mol%) was added to a mix-
ture of thiol (1 mmol) and H2O2 (1.2 mmol) in ethanol (3 ml). The
mixture was then stirred for the appropriate time at room temper-
ature. The progress was monitored using TLC. After completion of
the reaction, the catalyst was separated by simple filtration and
the mixture was washed with ethyl acetate (5 × 2 ml), and then
the product was extracted with ethyl acetate (4 × 5 ml). The organic
layer was dried over anhydrous Na2SO4 (1.5 g). Finally, the organic
solvent was evaporated, affording products in good to high yields.
Experimental
Results and discussion
Materials
In continuation of our studies concerning the introduction of new
catalysts,[6] in the present work we report the preparation of
Zr-oxide@MCM-41 with high catalytic activity and its application
in organic reactions. The details of the supported catalyst prepara-
tion procedure are presented in Scheme 1. Mesoporous MCM-41
was synthesized using a post-grafting procedure. Furthermore,
the MCM-41 surface was functionalized with commercially available
APTES through siloxane linkages. Ultimately, the reaction of amino
Chemicals and solvents used in this work were obtained from
Sigma-Aldrich, Fluka or Merck and used without further
purification.
Preparation of Si-MCM-41
Mesoporous Si-MCM-41 was prepared according to a literature
procedure.[6] Deionized water was added to 2 M NaOH and
cetyltrimethylammonium bromide (CTAB) as a surfactant tem-
plate and stirred vigorously at 80°C. After clarification,
tetraethylorthosilicate (TEOS) was added slowly and continuously.
After 2 h of stirring, a synthetic solution was obtained with molar
composition TEOS/CTAB/NaOH/H2O = 60/3.0/1.0/1. After cooling
to room temperature, the resulting solid was gathered by filtration,
washed with deionized water and dried at 343 K. This was followed
by calcination at 823 K for 5 h at a rate of 2°C minꢀ1 to remove the
residual surfactant.
Preparation of catalyst
An amount of 1.0 g of 3-aminopropyltriethoxysilane (APTES) was
added to a suspension of 1 g of MCM-41 in 30 ml of n-hexane,
which was allowed to reflux for 24 h under nitrogen atmosphere.
Then, the reaction mixture was cooled to room temperature,
Scheme 1. Synthesis of Zr-oxide@MCM-41.
Appl. Organometal. Chem. 2015, 29, 668–673
Copyright © 2015 John Wiley & Sons, Ltd.
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M. Hajjami et al.
groups with isatin led to Schiff base ligand-functionalized MCM-41
(Schiff base@MCM-41). Finally, Zr-oxide@MCM-41 was prepared
using the reaction of Schiff base@MCM-41 with ZrOCl2ꢁ8H2O. The
catalyst was characterized using transmission electron microscopy
(TEM), scanning electron microscopy (SEM), powder X-ray diffrac-
tion (XRD), energy-dispersive X-ray spectroscopy (EDS), thermogra-
vimetric analysis (TGA), Fourier transform infrared (FT-IR)
spectroscopy and nitrogen adsorption–desorption isotherms.
The EDS analysis of the catalyst indicates that the amount of C, N, O,
Si, Cl and Zr is 45.08, 3.96, 28.83, 18.39, 2.61 and 1.13 mass%, respec-
tively. These peaks suggest successful attachment of zirconium on
the particle surface.
As shown in Figure 3, a series of characteristic peaks including
(1 0 0), (1 1 0) and (2 0 0) reflecting the existence of mesoporous
MCM-41 structure are clearly identified. Meanwhile, a broad peak
at 2θ for 1 1 0 can also be clearly observed, which is assigned to
amorphous Zr-oxide@MCM-41. Evidently, the samples show typical
long-range order of hexagonal structure.[25]
The intensity of the (1 0 0) plane becomes weaker when metal is
loaded on MCM-41, presumably resulting from the collapse of the
structure of MCM-41. However, compared to the parent material,
some broadening and decreased intensity of the 1 0 0 diffraction
peak is observed, as well as the disappearance of the 1 1 0 and 2
0 0 peaks. The d-space values for these peaks are 38.85, 22.26 and
18.73 Å, respectively. A unit cell parameter (a0) of 44.86 Å is calcu-
lated using the following equation:[26]
Characterization of Zr-oxide@MCM-41
TEM and SEM images of MCM-41 and Zr-oxide@MCM-41 are shown
in Figures 1 and 2. These images confirm the formation of
single-phase Zr-oxide@MCM-41 nanoparticles, with spherical mor-
phology. The average nanoparticle diameter of Zr-oxide@MCM-41
is around 40–50 nm.
In addition, the EDS spectrum of the catalyst shows the presence
of zirconium species in the catalyst (Fig. S1, supporting information).
2dꢂ100
pffiffiffi
a0 ¼
(1)
3
Furthermore, Fig. 3 shows the XRD pattern of recovered Zr-
oxide@MCM-41. The curve clearly indicates the structural stability
of catalyst after being reused. The position and relative intensities
of all peaks confirm this well.
One indication of bond formation between the nanoparticles
and the support can be inferred from TGA. The TGA curves of
MCM-41, MCM-41-NH2, Schiff base@MCM-41, Zr-oxide@MCM-41
and reused Zr-oxide@MCM-41 show the mass loss of the organic
functional group as it decomposes upon heating (Fig. S2,
supporting information). TGA of Zr-oxide@MCM-41 shows three
steps of weight losses and confirms presence of supported organic
functional groups (Fig. S2). The first weight loss at temperatures
below 200°C, with an observed weight loss of 7.04%, is due to the
removal of physically adsorbed solvent and surface hydroxyl
groups and the evaporation of residual absorbed water.[27] The
second weight loss occurs between 300 and 580°C, showing a
weight loss of 6.1%, which may be related to the decomposition
of (3-aminopropyl)triethoxysilane groups. The third weight loss
between 580 and 730°C, with an observed weight loss of 5.3%, is
related to the supported organic groups and Zr-oxide complex
Figure 1. TEM images of MCM-41.
Figure 3. XRD patterns of (a) MCM-41, (b) MCM-41-NH2, (c) Schiff
Figure 2. SEM image of Zr-oxide@MCM-41.
base@MCM-41, (d) Zr-oxide@MCM-41 and (e) reused Zr-oxide@MCM-41.
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 668–673

Zr oxide-functionalized MCM-41 as catalyst for oxidation reactions
grafting to the MCM-41 surface.[6] Finally, the weight loss above
400°C can be assigned mainly to the dehydroxylation of SiOH
groups. The loading of the complex in Zr-oxide@MCM-41 can be
calculated from inductively coupled plasma analysis, which con-
Table 1. Textural properties of synthesized materials obtained using
nitrogen adsorption–desorption analysis
Sample
S
BET (m2 gꢀ1
)
DBJH (nm)
VTotal (cm3 gꢀ1
)
firms a loading of approximately 0.110 mmol gꢀ1
.
MCM-41
917
512
454
402
2.44
2.26
1.41
1.22
0.979
0.758
0.177
0.081
Amine-MCM-41
Imine-MCM-41
Zr-imine-MCM-41
FT-IR spectra of MCM-41, MCM-41-NH2, Schiff base@MCM-41 and
Zr-oxide@MCM-41 are shown in Fig. S3 in the supporting informa-
tion. The vibrational band observed at 3433 cmꢀ1 for MCM-41 is at-
tributed to the silanol ν(OH) stretch. The bands at 1078 and 798
cmꢀ1 correspond to the symmetric and asymmetric modes of the
Si–O–Si bond stretching vibrations.[6] However, the appearance of
new peak at 2980 cmꢀ1 in the FT-IR spectrum of MCM-41-NH2,
which may be related to NH2 bonds, confirms the anchoring of
APTES on MCM-41. In the FT-IR spectra of Schiff base@MCM-41
and Zr-oxide@MCM-41, no bands are observed in the 2980 cmꢀ1
region which would be associated with free amine group, indicat-
ing that isatin has been successfully condensed with supported
APTES and grafted onto MCM-41-NH2. Therefore, the disappear-
ance of the 2980 cmꢀ1 peak related to the NH2 bonds confirms
the formation of Schiff base@MCM-41. Also after imine formation
the C¼N stretching band appears at 1626 cmꢀ1. The FT-IR spec-
trum of the catalyst shows important bands from Zr-oxide@MCM-
41. These are strong evidence that Zr-oxide complex is bonded to
the surface of mesoporous MCM-41. The C¼N stretching band
shows an increase in frequency from 1626 to 1632 cmꢀ1 due to
complexation to metal ion.
Wall thickness ¼ a0 ꢀ DBJH
(2)
These results are strong evidence that Zr-oxide complex is bonded
on the MCM-41.
Catalytic Studies
As a first part of our organic study, the synthesized separable Zr-
oxide complex immobilized on MCM-41 was tested for the oxida-
tion of sulfides into sulfoxides using H2O2 as the oxidant at room
temperature under solvent-free conditions (Scheme 2).
In order to optimize the reaction conditions, initially various
solvents such as acetone, acetonitrile, ethanol, ethyl acetate,
dichloromethane and n-hexane as well as solvent-free conditions
were tested for the oxidation of dipropylsulfide using H2O2 as
oxidant with Zr-oxide@MCM-41 as catalyst. Also, we evaluated the
influence of varying the amount of catalyst on the oxidation of
The nitrogen adsorption–desorption isotherms of MCM-41,
MCM-41-NH2, Schiff base@MCM-41 and Zr-oxide@MCM-41 sam-
ples are illustrated in Figure 4. Based on the IUPAC classification,
these materials display a typical type IV isotherm, which is charac-
teristics of mesoporous material.[28] The Brunauer–Emmett–Teller
dipropylsulfide as
a model compound under solvent-free
conditions at room temperature (Table 2). As evident from Table 2,
in the absence of catalyst the reaction is incomplete even after 24 h
(Table 2, entry 7). The optimum amount of H2O2 (0.2 ml) in the
presence of Zr-oxide@MCM-41 (10 mg, 1.64 × 10ꢀ4 mol%) at
room temperature under solvent-free conditions is ideal for com-
plete conversion of dipropylsulfide to dipropylsulfoxide (Table 2,
entry 10).
We applied these optimized reaction conditions to various sul-
fide substrates to explore the catalytic activity of Zr-oxide@MCM-
41. The various functional groups on the sulfides are well tolerated
affording good to excellent yields of the corresponding sulfoxides.
The results are summarized in Table 3. Moderate to good yields of
the desired product are obtained using aliphatic, aromatic and cy-
clic sulphide substrates. Also, furfurylmethylsulfide affords good
yield of furfurylmethylsulfoxide (Table 3, entry 3). To show the
chemoselectivity of the presented protocol, a sulfide containing hy-
droxyl group was subjected to the oxidative reaction. The hydroxyl
group remains intact during the conversion of sulfide to sulfoxide
(Table 3, entry 9).
(BET) surface area (SBET) for MCM-41, MCM-41-NH2, Schiff
base@MCM-41 and Zr-oxide@MCM-41 is 917, 512, 454 and
402 m2 gꢀ1, respectively. That is, a decrease in SBET is observed
with functionalization. Physicochemical and structural parameters
of the samples obtained from nitrogen isotherms are summarized
in Table 1. The decrease in surface area of modified MCM-41 is
attributed to the immobilization of APTES and Zr-oxide complex
onto the mesoporous walls.
The wall thickness of MCM-41 is 2.05 nm, calculated using the
following equation:[26]
1400
adsorption (MCM-41)
1200
desorption
adsorption (Amine-MCM-41)
desorption
adsorption (imine-MCM-41)
desorption
1000
adsorption (Catalyst-MCM-41)
desorption
800
As a second part of our organic investigation towards the devel-
opment of new methods for the coupling of thiols, we tested the
catalytic activity of Zr-oxide@MCM-41 in the oxidative coupling of
thiols into corresponding disulfides. Oxidative coupling of thiols
into disulfides using H2O2 in the presence of this catalyst at room
temperature in ethanol is shown in Scheme 3.
600
400
200
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, (p/p0)
Figure 4. Nitrogen adsorption–desorption isotherms of (a) MCM-41, (b)
MCM-41-NH2, (c) Schiff base@MCM-41 and (d) Zr-oxide@MCM-41.
Scheme 2. Synthesis of Zr-oxide@MCM-41.
Appl. Organometal. Chem. 2015, 29, 668–673
Copyright © 2015 John Wiley & Sons, Ltd.
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M. Hajjami et al.
Table 2. Optimization of reaction conditions for the oxidation of
Table 4. Oxidative coupling of thiols into disulfides using H2O2
dipropylsulfide (1 mmol) as a model compound at room temperature
catalysed by Zr-oxide@MCM-41 in ethanol at room temperature
Entry
Solvent
Catalyst (mg)
Time (min)
Yield (%)a
Entry
Thiol
Product Time Yield
(min) (%)a
M.p. (°C)
1
Acetone
10
10
10
10
10
10
—
5
40
40
40
40
40
40
24
40
40
40
40
17
21
24
35
38
83
15
45
56
97
98
1
2
3
4
5
6
7
8
Phenylmethanethiol
4-Methylbenzenethiol
Naphthalene-2-thiol
2-Mercaptobenzoic acid
Benzenethiol
4a
4b
4c
4d
4e
4f
50
30
40
35
30
20
30
40
95
99
97
96
98
99
97
90
68–71[33]
36–38[16]
139–141[16]
285–290[16]
56–58[16]
170–172[33]
Oil[16]
2
Acetonitrile
Ethanol
3
4
Ethyl acetate
CH2Cl2
5
6
n-Hexane
Benzo[d]thiazole-2-thiol
2-Mercaptoethanol
2-Mercaptoacetic acid
7
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
4g
4h
8
Oil[16]
9
7
10
11
10
20
aIsolated yield.
aIsolated yield.
Table 3. Oxidation of sulfides into sulfoxides using H2O2 catalysed by
Zr-oxide@MCM-41 at room temperature under solvent-free conditions
Entry
Sulfide
Product Time Yield
(min) (%)a
M.p. (°C)
1
2
Methylphenylsulfide
Ethyl 3-(methylthio)
propanoate
2a
2b
300
35
98
94
Oil[29]
Oil[30]
Oil[29]
3
2-((Methylthio)methyl)
furan
2c
30
98
Figure 5. Reuse of catalyst in the oxidation of 4-methylthiophenol.
4
5
6
7
8
9
Benzylmethylsulfide
Dibutylsulfide
2d
2e
2f
35
25
20
25
40
30
96
97
91
95
97
94
115–119[31]
Oil[30]
Oil[32]
Reusability of Zr-oxide@MCM-41
Diethylsulfide
Tetrahydrothiophene
Dipropylsulfide
2g
2h
2i
Oil[29]
To make the developed protocol more economical, the recycla-
bility of the Zr-oxide@MCM-41 catalyst was studied for oxidative
coupling of 4-methylthiophenol in ethanol. After completion of
the reaction, the Zr-oxide@MCM-41 catalyst was separated by
simple filtration. The catalyst was then washed with ethyl
acetate to remove traces of organic compounds, dried and sub-
jected to the next cycle. The catalyst could be reused for up to
six consecutive cycles without significant loss of its catalytic ef-
ficiency (Fig. 5). The recovered catalyst after one use shows a
decrease in XRD peak intensity but no changes in the
mesostructure and it still has ideal mesoporosity (Fig. 3).
Oil[29]
2-(Methylthio)ethanol
Oil[30]
aIsolated yield.
The generality of this approach has been demonstrated by
facile oxidative coupling of various thiols as summarized in
Table 4. The disulfides are obtained in good to high yields. A
variety of thiols including aromatic thiols (Table 4, entries 2–6),
aliphatic thiols (Table 4, entries 7 and 8) and other thiols with
different functional groups such as acidic group (Table 4, entries
5 and 8), amine group (Table 4, entry 4) and hydroxyl group
(Table 4, entry 7) were successfully employed to prepare the
corresponding disulfides in excellent yields. The hydroxyl group
and other functional groups remain intact during the conversion
of thiols into disulfides (Table 4, entry 7). This result indicates that
the present protocol could be applicable to the chemoselective
coupling of thiols in the presence of other functional groups.
Therefore, the results reveal that this methodology is effective
for a wide range of thiols.
Conclusions
A simple, efficient and environmentally benign method has been
developed for the oxidation of sulfides to sulfoxides and for the
oxidative coupling of thiols into their corresponding disulfides
using Zr-oxide@MCM-41 having high catalytic activity. The
notable advantages of these methods are easy-to-handle envi-
ronmentally friendly reagents, cheap and chemically stable
oxidant, operational simplicity, practicability, good to high yields
of products in a short time and inexpensive reagents. More
important, the catalyst can be easily recovered using simple
filtration and reused several times without significant loss of its
catalytic activity.
Acknowledgement
The authors thank the research facilities of Ilam University, Ilam,
Iran, for financial support of this research project.
Scheme 3. Zr-oxide@MCM-41-catalysed oxidation of sulfides into
sulfoxides using H2O2.
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Appl. Organometal. Chem. 2015, 29, 668–673

Zr oxide-functionalized MCM-41 as catalyst for oxidation reactions
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