Polymeric ionic liquid nanogel-anchored tungstate anions: A robust catalytic system for oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1039/c4nj01931h

A new heterogeneous catalytic system was prepared by immobilization of tungstate ions on a cross-linked poly(ionic liquid) nanogel. The solid nanogel was easily synthesized by a surfactant-less method. The resulting catalyst was highly active in selective

Full text of DOI:10.1039/c4nj01931h

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Polymeric Ionic Liquid Nanogel Anchored Tungstate Anions: A Robust
Catalyst for Oxidation of Sulfides to Sulfoxides
Ali Pourjavadi*, Mojtaba Nazari-Chamazkoti, Seyed Hassan Hosseini
Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Tehran, Iran
E-mail address: purjavad@sharif.edu; Phone/fax: (982)166165311
Abstract
A new heterogeneous catalytic system was prepared by immobilization of tungstate ions on
cross-linked poly(ionic liquid) nanogel. The solid nanogel was easily synthesized by
surfactant-less method. The resulting catalyst was highly active in selective oxidation of
2 2
sulfides to sulfoxides by H O . The oxidation reaction was successfully performed in solvent
free condition with high yield. The catalyst was recycled and reused for several cycles of
reactions without any significant loss of catalytic activity.
Keyword: Nanogel, Poly(ionic liquid), Oxidation

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Introduction
1
-3
Catalytic oxidation reactions by H O have attracted much attention in recent years. H O
2
2
2
2
is an inexpensive oxidant with high oxygen content and thus can be used as an appropriate
reagent for oxidation reactions. Many different transition metal complexes such as
4,5
6,7
8,9
10-11
manganese,
molybdenum, rhenium and tungsten
have been used as catalyst in
these reactions. However, because of its high efficiency and selectivity in oxidation reactions,
tungsten-based catalysts are more attractive than others. Noyori et al. have used tungsten-
based catalyst for oxidation of cyclohexene to adipic acid in the presence of a phase transfer
12
catalyst
.
This organic solvent- and halide-free method was also used in other oxidation
13
reactions, as well. Other tungsten-based catalysts were also developed for oxidation
1
4,15
reactions.
However most of these catalysts are homogenous and thus it is difficult to
separate them from reaction medium.
An alternative method for improving catalyst separation and reuse is immobilization of
catalyst on solid supports. Tungstate catalyst can be immobilized on a solid support and a
heterogeneous system will result in. Different supports have been used for this purpose such
16,17
18,19
20,21
22
as polymers,
silica particles,
mesoporous silica
and magnetic nanoparticles.
Aside from the great advantages of heterogeneous catalysts, there are also some problems in
using them. They usually have lower catalytic activity than homogenous catalysts. Low
loading and high leaching of the catalyst and slow penetration of organic substrate into the
catalyst are other disadvantages of heterogeneous catalytic systems. So it is important to
design a catalyst with high efficiency and recyclability as well as good stability.
23,24
Recently ionic liquids(ILs) have attracted great attention due to their unique properties.
They have high thermal stability, low flammability and variable polarities. ILs have
negligible vapor pressure and can be used as green solvent. Poly(ionic liquids) (PILs)are
polymers composed of ILs as repeating units. Imparting ILs in polymeric structures can lead

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to support materials for heterogeneous catalytic systems. They have been used as support for
25,26
catalysts in many reactions.
As a new class of materials, PILs have found many
27-29
applications.
They benefit from properties of both ILs and polymers. PILs contain many
ionic groups in their structure and thus can carry many different counterions. As a
consequence, using PILs as solid supports can lead to heterogeneous catalytic system with
high catalyst loading and catalytic activity.
Herein, we report synthesis of a new heterogeneous catalytic system by immobilization
tungstate ions on cross-linked PIL nanogel. The supports are synthesized by polymerization
of ILs monomers in the presence of a cross-linker. Then, tungstate ions are immobilized on
PIL nanogel via a counterion exchange reaction to form an active catalyst. This catalyst is
used in oxidation reaction of sulfides by H
catalytic activity in this reaction.
2 2
O . The synthesized catalyst exhibits high
Experimental
Reagents and Analysis
1-Vinylimidazole was purchased from Aldrich and distilled under low pressure before use.1-
bromododecane
(Merck),
1,4-dibromobutane
(Aldrich)
and
2'-Azobis(2-
methylpropionamidine)dihydrochloride (V50, Kanto, 97%)were used without further
purification.
Chemical structure of particles was characterized by Fourier transform infrared (FTIR)
spectroscopy using an ABB Bomem MB-100 spectrophotometer. Thermal gravimetric
analysis (TGA) was performed by a TGA Q 50 thermogravimetric analyzer with heating rate
of 10 °C/min under nitrogen atmosphere. Shape and morphology of particles were observed
using a Philips CM30transmission electron microscope (TEM).

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Synthesis of IL monomer and cross-linker
IL monomer was synthesized by reaction of 1-vinylimidazole and1-bromododecane. In a
typical run, 20mL methanol was poured into a round-bottom flask and 2.82g (30 mmol) 1-
vinylimidazole and 7.44g (30 mmol)1-bromododecane were added to it. The resulting
mixture was stirred at 60 °C for 20h. Then, the flask was cooled to room temperature and its
content was poured in 250 mL diethyl ether. In order to precipitate the product, the resulting
solution was placed in refrigerator for 5 h. The solid product was filtered and washed with
diethyl ether three times and dried in vacuum at 50 °C.
The cross-linker, 1,4-butanediyl-3,3`-bis-l-vinylimidazolium dibromide (BVD), was
synthesized by reaction of 1-vinylimidazole and 1,4-dibromobutane with molar ratio of 2:1.
The detail of procedure is similar to the synthesis of IL monomer.
Synthesis of catalyst
A round-bottomed flask equipped with a condenser was charged with 50 mL water. 2g IL
monomer and 0.3 g BVD were added and argon gas was purged into the solution for 20 min.
Then, V50 (0.03g) was added to the solution and the flask was placed in an oil bath at 70 °C.
After 24 h, the solid product was separated by centrifuge and washed three times with
methanol. The resulting cross-linked PIL nanogel (NPILBr) was dried in vacuum at 50 °C.
An anion exchange reaction was used to immobilize tungstate on the synthesized PIL
2 4 2
nanogel. 1.0 g powdered PIL nanogel was dispersed in 50 mL water and 1.5 g Na WO .H O
was added to it. The solid product was vigorously stirred (1200 rpm) for 3 days at room
temperature. The solid product (NPILW) was separated by centrifuge and washed three times
with water (5×100mL) and twice with methanol (2×20mL) and dried in vacuum at 50 °C.

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Oxidation reactions catalyzed by PILW
The synthesized PILW catalyst was used in oxidation reactions of sulfides by H
round-bottomed flask, 3mmol H , 1mmol sulfide and 10 mg PILW were loaded and the
2
2
O . In a
2
O
2
mixture was stirred at room temperature for a defined time. After the completion of the
reaction, methanol was added and catalyst was separated by centrifuge. The solid catalyst
was washed with methanol, dried at 50 °C and used for another run. The product mixture in
the filtrate were extracted by ethyl acetate and analyzed by gas chromatography (GC).
Results and discussion
Synthesis of catalyst
IL monomer and cross-linker were synthesized by quaternization reaction of 1-vinyl
30
imidazole with 1-bromododecane and1,4-dibromobutane, respectively. Cross-linked PIL
nanogel was synthesized by dispersion polymerization of 3-n-dodecyl-1-vinylimidazolium
bromide in water (Fig. 1). Since ionic liquid monomer has long alkyl chain, it acts like
surfactant. Therefore, aggregation of monomers in water produces nanosized micelle which
after copolymerization PIL nanogel is obtained. These nanoparticles were used as supports
for immobilization of tungstate anions. Tungstate anions were immobilized on the PIL
nanoparticles by anion exchange reaction in which bromide was exchanged by tungstate. The
long alkyl chains in catalyst structure provide a hydrophobic surface on PILW which improve
the adsorption of hydrophobic substrates on the surface of catalyst.

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Fig. 1 Preparation of PILW catalyst
Characterization of catalyst
FTIR spectrum of cross-linked PILs is shown in Fig. 2a. Peaks at 2850 cm and 2950 cm
-1
-1
-1
-1
are related to C-H stretching vibration and peaks at 1565 cm and 1640 cm are attributed to
stretching vibration of C=C and C=N bond of imidazolium ring, respectively. The results
show that PIL nanoparticles were successfully synthesized. In the spectrum of tungstate-
-1
immobilized PILs (Fig. 2b), a new peak was appeared at 822 cm . This peak which is also
present in the spectrum of Na WO (Fig. 2c), is attributed to W=O stretching vibration,
2
4
indicating successful exchange of tungstate with bromide in PIL structure.

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2 4 2
Fig. 2 FTIR spectra of PILBr (a), PILW (b), and Na WO .H O (c).
The presence of tungstate in the catalyst was also confirmed by energy dispersive x-ray
EDX) spectroscopy, as depicted in Fig. 3. Also, no Na atom was observed in EDX spectrum
of PILW demonstrating that ratio of imidazolium cation to WO anion is 2:1. Moreover, the
(
4
4
EDX spectrum shows that all bromide anions were almost exchanged with WO .
Fig. 3 EDX spectrum of PILW
TGA spectra of PILBr and PILW (Fig. 4) show a weight loss in about 100 °C for both
samples, which is related to the removal of adsorbed water molecules. The main weight loss,

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however, occurred in the rage of 200-300 °C, is attributed to complete thermal degradation of
PIL network. The thermal degradation of catalyst structure occurs in lower temperature for
4
PILW than PILBr due to the presence of active WO anions in PILW. However, the
degradation of structure of PILW starts at 160 °C, but it is still enough for catalytic purpose.
Fig. 4 TGA curves of PILBr(a), and PILW (b).
Fig. 5 shows TEM image and size distribution diagram of the synthesized PILW
nanoparticles. It is observed that the synthesized particles are nearly spherical and their mean
size is around 100 nm.
Fig. 5 TEM image and size distribution diagram of PILW particles

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-
In order to measure the loading amount of Br in PILBr, titration by AgNO
3
was used as a
standard method (according to Moher method) and it was found that the loading is 2.43
-1
mmol.g . Elemental composition of PILBr and PILW is listed in Table 1. The loading
-
amount of WO catalyst on PIL supports was measured by inductively coupled plasma optical
4
emission spectrometry (ICP-OES). The data obtained by ICP-OES showed that the loading
-
-1
amount of WO
4
ions on PILs is 1.2 mmol.g . XRF analysis also showed that there is no
+
significant amount of Na , confirming almost all characterized tungstate ions are exchanged
2 4
with bromide in the PILW structure and no Na WO is physically adsorbed into the nanogel
structure. The results show that ion exchange reaction was performed with a 2:1 ratio of
-
imidazolium cations to WO anions and yield of the ion exchange reaction is about 96%.
4
Table 1 Results of CHN elemental analysis of catalyst
a
Entry
1
2
Catalyst
PIL/Br
Fresh PIL/W
Recycled PIL/W
%C
%H
10.12
9.89
%N
7.78
6.91
6.80
% W
-
52.80
47.56
48.33
22.1
21.5
3
11.20
a
Content of tungsten atom was measured by ICP-OES.
Catalytic performance of PILW
The synthesized PILW nanogel was used as catalyst in oxidation reaction of sulfides to
sulfoxides using H as oxidant. The catalytic activity of PILW in oxidation of
2 2
O
diphenylsulfide to diphenylsulfoxide in different conditions was investigated as control
reaction and the results are listed in Table 2. As is shown in the Table, conversion and yield
of the reaction in the absence of any catalyst are only 6 and 8 %, respectively. The reaction
was performed with PILBr as catalyst in solvent free condition for 24 h at room temperature
2 2
using 5 mmol H O as oxidant. The result showed that the presence of PILBr did not have
any significant effect on conversion and yield of the reaction, indicating PILBr is inactive and
does not have any catalytic activity in this reaction. Using 30 mg PILW significantly
increased conversion and yield of the reaction to 95 and 99 %, respectively in just 3 h. By

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decreasing catalyst loading to 20 and 10 mg, conversion and yield of the reaction was still
high without significant changes. However, by further decreasing of catalyst loading to 5 %,
conversion and yield of the reaction were decreased significantly and reached to 87 %. The
results show that the optimized catalyst loading of PILW is 10 mg (1.2 mol%). The effect of
the amount of H O on the reaction was also investigated and it was found that with using 3
2
2
mmol of H
of H to 1.5 mmol considerably reduces conversion and yield of the reaction. So the
optimized amount of H is 3 mmol. Since raising the temperature to 50 °C did not change
2 2
O the conversion and yield of the reaction is still high, but reducing the amount
2
O
2
2 2
O
conversion and yield of the reaction, room temperature was chosen as optimized temperature
for further experiments. The reaction was also performed in different solvents such as water,
acetonitrile, n-hexane and dichloromethane but the best catalytic performance was obtained
in solvent free condition. The results indicated that PILW acts as an efficient catalyst in
oxidation reaction of diphenyl sulfide to diphenylsulfoxide.
a
Table 2 Control experiment of oxidation of diphenylsulfide
Cat. Loading
H
(mmol)
2
O
2
T
( C)
Time
(h)
24
24
3
3
3
3
3
3
3
4
4
Conb.
Yield
(%)
6
7
Entry Catalyst
Solvent
o
c
(
mg)
(%)
1
2
3
4
5
6
7
8
9
1
1
1
1
-
-
5
5
5
5
5
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
water
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
50
r.t
r.t
r.t
r.t
6
8
PILBr
PILW
PILW
PILW
PILW
PILW
PILW
PILW
PILW
PILW
PILW
PILW
30
30
20
10
5
10
10
10
10
10
10
10
99
99
99
87
99
75
99
94
95
81
73
94
95
98
87
98
74
91
92
95
79
71
5
3
1.5
3
0
1
2
3
3
3
3
3
CH
3
CN
n-hexane
CH Cl
5
4
2
2
a
b
Reaction condition: diphenyl sulfide (1 mmol), H
2
O
2
(30%), solvent (3 mL). Conversions were calculated
c
based on initial mmol of sulfide. Yields were determined by GC.
The catalytic performance of PILW in oxidation reaction of other sulfide compounds was
also studied. Different sulfide compounds were used as substrate in oxidation reaction based

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on the optimized condition obtained in control reaction discussed previously. As is shown in
Table 3, all reactions were performed with high conversion and yield; however aromatic
sulfides produced higher yields.
Table 3 Oxidation of sulfides to sulfoxides catalyzed by PIL/W
Entry
Sulfide
Time (h) Con. (%) Yield (%)
1
3
2
99
99
98
98
95
95
2
3
2.5
4
5
2.5
3.5
97
95
93
91
6
7
4
4
99
99
95
96
8
9
4
97
93
5
5
91
94
91
93
1
0
1
1
1
2
5
5
95
85
94
83
a
Reaction condition: sulfide (1mmol), H
mg), room temperature, solvent free. GC yield.
2
O
2
30% (3 mmol), catalyst (10
b

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Recycling and leaching experiments
The recyclability of PILW was investigated in several runs of oxidation reaction of diphenyl
sulfide to diphenylsulfoxide. At the end of the reaction, PILW was separated by centrifuge
and its catalytic activity in next runs was investigated. As is shown in Fig. 6, the catalytic
activity of PILW is preserved even after 8 runs and yield of the reaction does not change
significantly.
Fig. 6 Recycling experiment
The leaching of the catalyst in oxidation reaction of diphenyl sulfide was then studied. The
reaction was stopped at half of the reaction time and the catalyst was separated from the
solution. The remaining mixture was stirred for another period of half the reaction time. The
leaching results are shown in Fig. 7. After removing the catalyst no more products were
produced which indicates that catalyst leaching is negligible. Atomic adsorption spectroscopy
of reaction mixture after catalyst separation also confirmed that no free tungstate ions are
present in the solution. The results showed almost all tungstate ions are successfully
immobilized on the PIL supports and do not exist as free ions in the solution.

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Fig. 7 Leaching experiment. Blue line: reaction with PILW and red line: reaction without a catalyst.
The elemental analysis of fresh and recycled catalysts revealed that tungsten content of the
catalyst does not significantly change after recycling, as represented in Table 1. Moreover,
there is no significant change in the FTIR spectrum (Fig. 8) and size distribution diagram
(Fig. 9) of the recycled catalyst compared with fresh catalyst, demonstrating the catalyst is
stable even after 8 times use. The resulting particles represent an effective heterogeneous
catalytic system for oxidation of sulfides.
Fig. 8 FTIR spectra of fresh and recycled catalyst

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Fig. 9 Size distribution diagram of fresh and recycled catalyst
Comparison with other catalysts
In order to have a better image of catalytic activity of the prepared catalyst, we compared the
data obtained here by some other catalysts that recently have been used for oxidation of
sulfides to sufoxides. For this purpose we chose catalytic oxidation of methyl phenyl sulfide
2 2
by H O . Catalyst, yield of the reaction and other parameters related to the reaction condition
are listed in Table 4. It is observed that the catalyst used in this study shows equal or even
higher yield than the other catalysts. Considering the other parameters such as solvent, time
and temperature reveals that the catalyst studied here is more advantageous than the others,
since it benefits from a solvent-free condition, low temperature and relatively short reaction
time. The loading of the catalyst is also higher than the other listed catalysts. Oxidation
reaction of different sulfides with high yields is possible by using just a small amount of the
catalyst. The results show that the prepared catalyst is comparable and in most cases is even
advantageous over other catalysts that are already reported in the literature.

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Table 4 Comparison of activity of PILW with other reported catalysts in oxidation of sulfides to sulfoxides
Entry
Catalyst
Catalyst
Amount of
T
Time
Solvent
Yield
(%)
o
Loading used catalyst ( C) (h)
(
mmol/g)
2.05
0.332
0.38
0.75
0.18
1.15
0.4
(mol%)
0.33
1.5
a,31
1
2
3
4
5
6
7
8
9
Fe O /CS/HWO
25
25
25
25
26
40
30
25
25
4
CH CN
83
95.5
72
3
4
3
b,32
BisILs-C H -W2
8
17
2 2 3
1.5 CH Cl :CH OH
c,33
PANW
polymer-anchored Cu(II)
0.1
0.5
3
CH OH
3
34
0.75
0.45
1.15
2
CH CN
3
84
3
5
Mo(VI)O @SBA-15
3
CH CN
89
2
3
d,36
Cr-MPTA-1
3
CH CN
3
95
e,37
[C mim] PM
6
3
0.5
1
CH OH
3
96
f,22
MNP@PILWO4
This study
0.62
1.2
0.93
1.2
Solvent free
Solvent free
99
2
95
a
b
c
d
e
f
Protonated peroxotungestate immobilized on magnetic crosslinked chitosan
Peroxotungestate immobilized on ionic liquid brushes
Peroxotungstate(VI) immobilized on poly(acrylonitrile)
Cr-grafted mesoporous polytriallylamine
Ionic liquid-based polyoxometalate salt
Tungstate immobilized on magnetic poly(ionic liquid)
Conclusion
In summary we prepared a poly(ionic liquid) nanogel with tungstate anions as a
heterogeneous catalytic system for oxidation of sulfides to sulfoxides. The loading of
-1
tungstate on polymeric support was about 1.2mmol.g and the leaching of the catalyst was
negligible. The prepared system represents an efficient highly stable catalyst for oxidation
reaction of sulfides by H
2 2
O . Many different sulfide compounds were used as substrate and in
all cases oxidation was performed with high yield. The recyclability of the catalyst was also

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investigated and it was found that the catalyst can be recovered and reused for several cycles
of reaction and there is no reduction in catalytic activity even after 8 runs.

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