Selective oxidation of sulfides to sulfoxides by a molybdate-based catalyst using 30% hydrogen peroxide

Article abstract of DOI:10.1016/j.catcom.2014.03.026

An efficient method is reported for selective oxidation of various types of sulfides to sulfoxides and sulfones in good to high yields using 30% H ₂O₂ in the presence of catalytic amounts of molybdate-based catalyst in acetonitrile as solvent at room temperature. The catalyst can be easily recovered and reused for seven reaction cycles without considerable loss of activity.

Full text of DOI:10.1016/j.catcom.2014.03.026

Selective oxidation of sulfides to sulfoxides by a molybdate-based catalyst
using 30% hydrogen peroxide
Ahmad Bayat, Mehdi Shakourian-Fard, Mohammad Mahmoodi Hashemi
PII:
DOI:
Reference:
S1566-7367(14)00120-4
doi: 10.1016/j.catcom.2014.03.026
CATCOM 3858
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 January 2014
18 March 2014
19 March 2014
Please cite this article as: Ahmad Bayat, Mehdi Shakourian-Fard, Mohammad
Mahmoodi Hashemi, Selective oxidation of sulfides to sulfoxides by a molybdate-
based catalyst using 30% hydrogen peroxide, Catalysis Communications (2014), doi:
1
0.1016/j.catcom.2014.03.026
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ACCEPTED MANUSCRIPT
Selective oxidation of sulfides to sulfoxides by a molybdate-based catalyst
using 30% hydrogen peroxide
*
Ahmad Bayat, Mehdi Shakourian-Fard, Mohammad Mahmoodi Hashemi
Department of Chemistry, Sharif University of Technology, PO Box 11465-9516, Tehran, Iran,
*
Corresponding author: mhashemi@sharif.edu
Abstract
An efficient method is reported for selective oxidation of various types of sulfides to sulfoxides
and sulfones in good to high yields using 30% H O in the presence of catalytic amounts of
2
2
molybdate-based catalyst in acetonitrile as solvent at room temperature. The catalyst can be
easily recovered and reused for seven reaction cycles without considerable loss of activity.
Keyword: Selective oxidation, Magnetic nanoparticle, Molybdate anion, Highly reusable
catalyst, Heterogeneous, Hydrogen peroxide
1
. Introduction
The selective oxidation of sulfides to sulfoxides is an important transformation in organic
chemistry [1,2], because sulfoxides are useful synthetic intermediates for the construction of
various chemically and biologically active molecules [3-5] and also they play key role in
activation of enzymes. Therefore, a large number of methods, procedures and oxidizing agents
have been proposed for this type of transformation. Traditional synthesis is performed by using
stoichiometric amounts of oxidants which are dangerous and can create a great quantity of waste
[
6-8]. Moreover, over-oxidation of the sulfoxides to sulfones is a common problem during the
oxidation of sulfides [9]. From an economical and environmental viewpoint, an acceptable
1

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process involving the use of molecular oxygen or hydrogen peroxide as a primary oxidant in the
presence of a catalyst is particularly attractive. Hydrogen peroxide produces water as by-product
and controlled partial oxidation is easier to achieve than with molecular oxygen and air [10].
This feature has stimulated the development of useful procedures for H O oxidation with
2
2
various types of catalyst systems including transition metal complexes [11-14].
In recent years, various types of molybdenum based catalyst systems have been used in
selective oxidation of sulfides to sulfoxides or sulfones. For example, Na MoO .2H O is widely
2
4
2
used as catalyst for hydrodesulfurization from petroleum [15-18]. This catalyst needs
homogeneous reaction condition for hydrodesulfurization process and so it is difficult to recover
and reuse the catalyst. Use of recycled catalysts is required for reducing the catalytic cost and the
amount of used catalyst in the reactions. One way to attain this goal is to immobilize catalytic
system onto a large surface area solid carrier.
In recent years, magnetic nanoparticles (MNPs) have attracted great interest as support
materials in liquid phase reactions because they have unique physical properties and can be
easily separated from reaction mixture by an external magnet [19,20]. Therefore, the recovery of
catalysts is much easier than by filtration or centrifuge techniques. The Fe O core trends to be
3
4
oxidized or dissolved in acidic conditions during the treatment procedure. One of the materials
can be used for coating of Fe O is SiO . There are several advantages of using a silica shell as
3
4
2
the stabilizer. Silica is chemically inert, and therefore does not affect the redox reaction at the
core surface. The nonmagnetic shell can suppress the particle–particle magnetic bipolar
interaction and prevent them from aggregating and agglomerating. Also, the silica shell acts as a
stabilizer, limiting the effect of the outside environment on the core particles so that the Fe O
3
4
cores are not prone to oxidize as a response to the lowered magnetic performance γ–Fe O . In the
2
3
2

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present work, we report our results about Fe O @SiO functionalized with ammonium
3
4
2
molybdate as a recoverable heterogeneous catalyst for selective oxidation of sulfides to
sulfoxides in the presence of 30% H O .
2
2
2
2
. Experimental
.1 Reagents and analysis
All chemicals and solvents were purchased from Sigma-Aldrich and used without further
purification. Molybdenum content of the catalyst samples were determined by X-ray
fluorescence (XRF ARL8410). A TGA-Q5 thermogravimetric analyzer was used to study the
-1
thermal properties of the compounds under an inert N atmosphere (at 20 mL min ) and heating
2
-1
at a rate of 10 °C min . X-ray diffraction (XRD) patterns of prepared catalyst were recorded
with a APD 2000, using Cu Kα radiation (50 kV, 150 mA) in the range 2θ=10-120°. The SEM
analysis was done with a KYKY-EM3200 with maximum acceleration voltage of the primary
electron between 20 and 25 kV. Energy-dispersive X-ray spectroscopy (EDX) on the catalyst I
was performed by Sirus SD. FT-IR spectra were recorded with ABB Bomem MB100 Fourier
Transform Analyzer.
2
.2 Preparation of magnetic nanoparticles (MNPs)
The MNPs were prepared as reported [21]. Briefly, FeCl .6H O (4.8 g, 0.018mol) and
3
2
FeCl .4H O (1.8 g, 0.0089 mol) were added to 100 mL deionized water and vigorously stirred
2
2
(
700 rpm) under N atmosphere until the salts dissolved completely. Then, 10mL of 25%
2
NH OH was added quickly into the reaction mixture in one portion, which MNP black
4
precipitate was formed immediately. After continuously stirring by mechanical stirrer for 1h, the
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precipitate was separated by external magnet and washed with the double distilled water (five
times), then vacuum-dried at 50 °C for overnight.
2
.3 Fe O magnetic nanoparticles (MNPs) coated by silica (Fe O @SiO )
3 4 3 4 2
The prepared MNPs (1g) were homogeneously dispersed in the mixture of ethanol
80mL), deionized water (20mL) and pH of solution was adjusted to 10 by adding aqueous
(
ammonia. Tetraethoxysilane (TEOS, 4.2 mmol) was added to the solution and stirred at 50 °C
for 6 h to obtain Fe O @SiO MNPs. After washing with ethanol and water for several times,
3
4
2
Fe O @SiO MNPs were dried at 60 °C for overnight.
3
4
2
2
.4 Preparation of Fe O @SiO coated by 3-aminpropyltriethoxysilane (Fe O @SiO -
3 4 2 3 4 2
APTES)
The functionalization of Fe3O4@SiO2 (1g) were performed with 3-
aminpropyltriethoxysilane (4.4 mmol, 1mL) in dry toluene for 24 h. The solid material was
washed with toluene, methanol and ethanol and then dried overnight in an oven at 60 °C. This
product will be referred as Fe O @SiO -APTES.
3
4
2
2
.5 Preparation of catalyst I
The resulting Fe O @SiO -APTES (1g) was reacted with excess of HCl (1M, 5mL) solution
3
4
2
for 4 h and then separated by magnet, washed with water and ethanol, and then dried in vacuum
desicator at 60 °C for overnight to afford surface bound ammonium chloride. Finally, the
prepared material (1g) was ultrasonically dispersed in water (15mL) and then Ag MoO (0.15g,
2
4
0
.4mmol) was added to the dispersed material in water at room temperature. The mixture was
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stirred for 1 h and then was magnetically separated and washed several times with water and
ethanol and then dried to afford the catalyst I (Scheme 1).
(Scheme 1)
2
.6 General procedure for the oxidation of sulfides to sulfoxides
To a solution of sulfide (1 mmol) and 30% H O ( 3 equivalent ) in CH CN (10 mL),
2
2
3
catalyst I (0.189 g, 3 mol% Mo) was added and the mixture was stirred at room temperature for
the time specified. Completion of the reaction was indicated by Thin Layer Chromatography
(
TLC) (n-hexane/ethylacetate 3.5:1) and GC. After completion of the reaction, Et O was added
2
and the catalyst was separated by a small magnet placed at the bottom of the flask. Finally, the
excess of solvent was removed under reduced pressure to give the corresponding pure sulfoxide.
Further purification was achieved by chromatography on silica gel with n-hexane/ethylacetate.
All reaction products were identified by GC chromatogram, IR spectra and melting point as
compared with authentic samples.
3
3
. Results and discussion
.1 Catalyst characterization
2
-
The IR spectrum of catalyst I shows peaks which are characteristic of MoO4 anion, which
-1
clearly differs from that of Fe O @SiO -APTES. The absorption peaks in 884 and 939 cm (Fig.
3
4
2
2
-
1
), were attributed to the M-O stretching frequency in MoO4 species [22]. Also, peaks at 577
-1
and 1101cm were attributed to stretching frequency of Fe-O and Si-O, respectively [23].
5

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The FT-IR spectrum of catalyst I also shows C-H stretching vibrations of APTES in around
-1
-1
2
928 cm and 2850 cm . Therefore, the presence of the organic groups on the surface of catalyst
I was confirmed by FT-IR spectroscopy (Fig. 1).
(Fig. 1)
Thermal analysis of the samples gives information about the thermal stability of catalyst I.
In the TGA curve of catalyst (Fig. S1 of supporting information), the weight loss below 100°C
(<1.57%) is attributed to residual physisorbed water and /or organic solvents, which was applied
during the catalyst I prepared. Analysis of TGA diagram strongly suggests that catalyst I is stable
and no further weight loss occurs below 200°C. The largest weight loss was occurred between
2
00 °C and 500 °C which is corresponding to 4.63% of the initial sample weight. This is due to
the decomposition of the grafted APTES organic molecule on surface which is consistent with
covalently bonded the aminopropyl group to the surface of MNPs.
Typical loading of surface-bound amino group was determined by TGA analysis which
-1
shows a loading 0.33 mmol g . However, from the simultaneous XRF analysis of the catalyst I
-1
for molybdenum it was calculated that the loading molybdenum in catalyst I was 0.16 mmol g .
2
-
From this result, it was concluded that ratio between MoO4 anion and the surface-bound
ammonium group is 1:2 (Scheme 1).
The XRD pattern of catalyst match completely with the standard Fe O sample (Fig. S2 in
3
4
supporting information). The XRD spectrum of Fe O clearly matches with the literature data
3
4
from JCPDS PDF-2 Data Base (Card number 89-43191, 19-0629, 79-0419). Presence of
diffraction peaks in XRD spectra of the catalyst I at 2θ=30.2, 35.8, 43.4, 53.2, 57.2, 62.6 were in
good agreement with the corresponding (220), (311), (400), (422), (551) and (440) diffraction
planes of Fe O MNPs.
3
4
6

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SEM image showed that the encapsulated nanoparticles were present as uniform particles (Fig.
2
).
(Fig. 2)
From EDX analysis, the molybdenium peak clearly confirms the presence of a molybdate anion
on catalyst I (Fig. 2). As seen from XRF and EDX analyses, there is a trace of Ag element on the
catalyst because the ratio of Mo and ammonium group on the surface is not 1:2 exactly.
Magnetic measurement of Fe O , Fe O @SiO and catalyst I was investigated with a Quantum
3
4
3
4
2
Design vibrating sample magnetometer (VSM) at room temperature in an applied
magnetic field sweeping from −10 to 10 kOe. All of the magnetic particles exhibited
superparamagnetic behavior and had little hysteresis, remanence and coercivity. The coat of
silica resulted in the decrease in the magnetic strength of the composite because of the weight
contribution from the nonmagnetic portion. The saturation magnetization of catalyst I was
−
1
2
1.5emu g (Fig. S3), which was sufficient for magnetic separation with a conventional magnet.
3
.2 Oxidation of sulfides to sulfoxides using catalyst I
In order to optimize the reaction conditions and test the activity of catalyst I, we examined the
oxidation reaction of methyl phenyl sulfide as a model compound using 30% H O under various
2
2
reaction conditions in terms of time and product yield (Table 1). As seen in Table 1, no
significant amount of sulfoxide was produced in absence of catalyst I.
(Table 1)
7

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The results show (Table 1) that the optimize conditions for the complete conversion of
methyl phenyl sulfide to methyl phenyl sulfoxides is in the presence of 30% H O (3 equiv.) and
2
2
catalyst I (3 mol % Mo, 189 mg) with CH CN as solvent at room temperature. Also, at a higher
3
temperature and higher concentration of H O , the reaction took place at a shorter time with over
2
2
oxidation and sulfone was formed. To assess generality and efficiency of the method, different
aromatic, linear and benzylic sulfides were reacted using 30% H O in the presence of catalyst I
2
2
and CH CN as solvent under optimized reaction conditions (Table 2). As seen in Table 2,
3
various types of sulfoxides were obtained in good to excellent yields. Therefore, these results
show that the protocol was general and efficient.
(Table 2)
The chemoselectivity of the presented protocol was investigated in the selective oxidation of
sulfides containing groups such as OH, COOCH and double bond. These functional groups
3
remained intact during the conversion of sulfide to sulfoxides (Table 2. entries 7-10)
The reusability of the catalyst I was evaluated in methyl phenyl sulfide oxidation at room
temperature employing aqueous 30% H O for 1 mmol of substrate. The catalyst was separated
2
2
by external magnet after each cycle, washed with CH Cl and then placed into a fresh reaction
2
2
mixture. As shown in Fig. 3, the catalyst was reused seven times and no significant loss of
activity was observed.
(Fig. 3)
To explore the catalyst leaching, we completely terminated the reaction by a removal of
catalyst from the reaction mixture at 50% yield of methyl phenyl sulfoxide and the solution was
kept back for oxidation under the same conditions did not increase the product yield. Inductively
coupled plasma-atom emission spectrometer (ICP-AES) analysis of the liquid phase confirmed
8

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that the content of molebdenuim in the solution was 1.5% that this amount is acceptable. These
results confirm that the reaction is attributed to the heterogeneous catalyst and leaching is low.
The efficiency of this method is demonstrated by compassion our results on the oxidation of
methyl phenyl sulfide with data from the literature (Table 3). These results clearly point out the
efficiency of the proposed methodology (Table 3, last entry) in both activity and recyclability of
catalyst I as compared to literature reports involving several heterogeneous systems under
various conditions (Table 3, entries 1-6) [9, 24-28].
(Table 3)
A plausible catalytic mechanism for the oxidation of sulfides to sulfoxides, which is the first
stage of the oxidation process, is depicted in Scheme 2. In the first step, peroxo-molybdate
species is formed and then the subsequent nucleophilic attack of the sulfur atom in the sulfide is
performed on the peroxo species. As known, the role of the metal atom is to form peroxo-metal
species which are able to activate the H O molecule [18,6,29].
2
2
(Scheme 2)
4
. Conclusion
We have developed efficient magnetic catalyst for the oxidation of sulfides to sulfoxides
using H O as oxidant at room temperature. This catalyst can be easily prepared from
2
2
commercially available starting materials. Also, the catalyst I can be considered as
heterogeneous version of Na MoO . Therefore, it is possible to recover and reuse the catalyst for
2
4
9

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several reaction cycles without considerable loss of activity. The main advantages of the present
synthetic protocol are mild conditions and easy magnetic separation of the catalyst.
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Figure Caption
Scheme 1. Preparation of Catalyst I
Scheme 2. Proposed mechanism for the sulfide oxidation with H O in the
2
2
presence of molybdate anion
Fig. 1 (a) FT-IR spectra of Fe O @SiO -APTES and (b) the catalyst I
3
4
2
Fig. 2 (a) SEM image and (b) EDX analysis of the catalyst I
Fig. 3 Catalyst reusability
1
3

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Scheme 1.
1
4

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Scheme 2.
1
5

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Fig. 1
1
6

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Fig. 2
1
7

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Fig. 3
1
8

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Table 1. Optimization of the reaction conditions for selective oxidation of methyl
phenyl sulfide (1mmol) to methyl phenyl sulfoxide in the presence of H O (3
2
2
equiv) and the catalyst I at room temperature.
a
b
Entry
Catalyst I %(g)
-
Solvent
Time (h)
Con. (%)
9
Yield (%)
9
Sel. (%)
100
93
1
2
3
4
5
6
7
8
9
CH CN
24
5
5
5
3
3
1mol%(0.063)
1mol%(0.063)
1mol%(0.063)
1.5mol%(0.093)
2mol%(0.125)
3mol%(0.189)
3mol%(0.189)
3mol%(0.189)
Water
CH Cl
59
71
84
88
55
70
82
84
91
92
75
78
98
97
95
96
95
75
78
98
2
2
CH CN
3
CH CN
3
CH CN
1.5
1.5
40 min
50 min
5
94
3
CH CN
96
100
100
3
c
CH CN
3
d
CH CN
3
1
1
0
1
1mol%(0.063) CH Cl /MeOH
1mol%(0.063)
79
51
78
2
2
n-Hexane
5
50
98
a
b
c
d
Sulfoxide yield, Sulfoxide selectivity, Temperature is 60 °C, Temperature is 40
°
C and the amount of 30% H O is 5 equiv.
2
2
1
9

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Table 2. Oxidation of sulfides to the corresponding sulfoxides using 30% H O
and catalyst I .
2
2
a
b
b
Entry
1
Substrate
Product
Time (h)
4
Con. (%) Yield (%) Sel. (%)
O
S
S
Ph
Ph
90
83
85
79
94
S
O
S
2
4
95
O
S
S
3
4
5
6
85
92
96
83
88
88
98
96
92
O N
2
O2N
O
S
S
4.5
Cl
Cl
O
S
S
50 min
S
S
6
7
1.5
1
100
100
91
94
91
94
O
O
S
S
S
S
O
S
S
OH
8
9
OH
2
4
97
93
90
89
93
96
O
S
S
2
0

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O
S
OCH3
S
OCH3
1
0
1
94
82
87
O
O
a
Reaction conditions: sulfide (1 mmol), H O (3 equiv.), catalyst I (3 mol%, 0.189
2
2
g) in acetonitrile as solvent at room temperature, stir.
b
Determined by GC
2
1

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Table 3. Comparison of the activity of various catalysts in the oxidation of
methyl phenyl sulfide using H O .
2
2
Ref
9
Catalyst
Nanomagnetic-based N-
propylsulfamic acid
MCM-Mo
Silica-based tungstate
interphase
Solvent
Temperature (°C)
Time (min) Yield % Reusability
Solvent-free
25
40
25
80
60
90
87
> 99
82
10
4
2
2
4
5
CH CN
3
CH Cl /MeOH
8
2
2
Peroxotungstate supported on
silica
2
2
2
6
7
8
CH Cl /MeOH
8
2.5h
30
91.9
88
6
6
2
2
Tungstate-exchanged
Mg-Al-LDH catalyst
VO(acac)
H O
2
25
H O
2
25
25
15
90
> 99
92
5
7
2
-resin
This work
Fe O @SiO /Mo
CH CN
3
3
4
2
2
2

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Graphical abstract
2
3

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Highlights



An efficient magnetic catalyst is synthesized.
The catalyst recovery is carried out by applying an external magnet device.
Various types of aromatic and aliphatic sulfides are selectively oxidized to
sulfoxides
2
4