Efficient oxidation of sulfides catalyzed by a temperature-responsive phase transfer catalyst [(C18H37)2(CH 3)2N]7 PW11O39 with hydrogen peroxide

Article abstract of DOI:10.1016/j.catcom.2012.09.021

A temperature-responsive phase transfer catalyst [(C₁₈H ₃₇)₂(CH₃)₂N]₇PW ₁₁O₃₉, could act as an efficient catalyst for selective oxidation of sulfides with 30% aqueous H₂O₂. Various kinds of sulfides were successfully oxidized to their corresponding sulfones with over 96% yields in a relatively short time and mild conditions. During reaction at 333 K, the catalyst dissolved completely and the oxidation was conducted homogeneously. Before and after reaction, the catalyst was insoluble with cooling, so it is easily recovered and reused. The catalyst was characterized by elemental analysis, FT-IR and ³¹P NMR.

Full text of DOI:10.1016/j.catcom.2012.09.021

Catalysis Communications 29 (2012) 73–76
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient oxidation of sulfides catalyzed by a temperature-responsive phase transfer
catalyst [(C18H37)2(CH3)2N]7 PW11O39 with hydrogen peroxide
Xiaoling Xue a, Wei Zhao b, Baochun Ma a, Yong Ding a,
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province,
⁎
a
State Key Laboratory of Applied Organic Chemistry and College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
b
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2012
Received in revised form 16 September 2012
Accepted 17 September 2012
Available online 24 September 2012
A temperature-responsive phase transfer catalyst [(C18H37)2(CH3)2N]7PW11O39, could act as an efficient
catalyst for selective oxidation of sulfides with 30% aqueous H2O2. Various kinds of sulfides were successfully
oxidized to their corresponding sulfones with over 96% yields in a relatively short time and mild conditions.
During reaction at 333 K, the catalyst dissolved completely and the oxidation was conducted homogeneously.
Before and after reaction, the catalyst was insoluble with cooling, so it is easily recovered and reused. The
catalyst was characterized by elemental analysis, FT-IR and 31P NMR.
Keywords:
Sulfides
© 2012 Elsevier B.V. All rights reserved.
Hydrogen peroxide
Polyoxometalate
Phase-transfer
Sulfone
1. Introduction
and Anderson-type [IMo6O24]
5− [24] combined with different quater-
nary ammonium cations assembled in emulsions to oxidize some
dibenzothiophene (DBT) derivatives. Mizuno reported that [TBA]4
[γ-SiW10O34(H2O)2] [5] and [γ-PW10O38V2(μ-OH)2]3− [22] homoge-
neously catalyze oxidation of various sulfides. Polyoxometalate-based
ionic liquids [25,26] and H3PW12O40 supported nanoparticles catalysts
were also reported on DBT oxidation [27].
The exploring of the highly efficient catalytic systems for the oxi-
dation of sulfides to sulfoxides and sulfones is of great importance
[1,2], because they are important intermediates in organic synthesis
and biologically active molecules [3–5]. Additionally, oxidative desul-
furization to yield sulfoxides or sulfones, as the better alternative way
compared with traditional hydrodesulfurization process, has aroused
attention because of high desulfurization efficiency [6]. H2O2, com-
monly considered as the “green oxidant”, was widely employed as
terminal oxidant because of its natural advantage such as high atomic
efficiency and H2O as sole by product, so it is undoubtedly to be an
ideal option in sulfides oxidation catalysis [7–9].
Many catalytic systems with H2O2 have been implied on sulfides ox-
idation in the past decades. For example, using ionic liquids [10,11],
mesoporous silica–titania [12,13], LDHs [14,15], polymer-immobilized
peroxotungsten catalyst [16], metal catalysts including Se, Mn, Re
[17–19], etc. Polyoxometalates, having sophisticated structure with
the characteristic of tunability, are widely used in catalysis with
the development of POMs chemistry owing to their inherent redox
and acidic properties [20–22]. In the past decades, using POMs on sul-
fides oxidation reactions have been reported widely in homogeneous
catalysis, emulsion catalysis and heterogeneous catalysis [5,8,22–28].
For example, Li et al. synthesized polytungstophosphate anions [23]
Herein, we endeavored to synthesize a novel temperature-responsive
phase transfer catalyst [(C18H37)2(CH3)2N]7PW11O39, and catalyze oxida-
tion of a series of sulfides including thioether and thiophene derivatives.
This catalyst is composed of bulky organic cation and keggin-type
lacunary [PW11O39]
7− anion, which possess the characteristic of temper-
ature sensitivity in the mixture of aqueous H2O2/1,4-dioxane. Both the
advantage of high efficiency in homogeneous catalysis and catalyst
recycling in heterogeneous catalysis are combined in this system
[29,30]. The catalyst was insoluble in the system at room temperature,
but dissolved with the elevation of reaction temperature. After the reac-
tion finished with cooling, the catalyst precipitated gradually from the
system itself. The catalyst effectively catalyzed the oxidation and could
be easily recovered and reused.
2. Experimental
2.1. Preparation of catalyst
All reagents used in this work are commercially available and
⁎
Corresponding author. Tel.: +86 931 8612029; fax: +86 931 8912582.
E-mail address: dingyong1@lzu.edu.cn (Y. Ding).
were used as received without further purification unless otherwise
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.09.021

74
X. Xue et al. / Catalysis Communications 29 (2012) 73–76
Table 1
C
g
o
Oxidation of benzothiophene under different solvents with [(C18H37)2(CH3)2N]7
PW11O39
n
i
o
t
a
e
l
i
n
a
.
g
H
Entry
Solvent
Conversion (mol%)
Selectivity (mol%)
1
2
3
4
5
6
7
8
1,2-dichloroethane
acetonitrile
acetone
water
ethanol
toluene
ethyl acetate
1,4-dioxane
97
67
26
56
62
41
74
99
99
99
99
87
99
99
99
99
duringreaction
Homogeneous
catalyst recycle
Before reaction
Heterogeneous
After reaction
Heterogeneous
a
Reaction conditions: 333 K, 0.5 h, substrate 1 mmol, H2O2 2.5 mmol, 2 ml solvent.
Conversion and selectivity were determined by GC analysis and bromobenzene was
used as internal standard.
Scheme 1. Representation of the temperature-responsive phase transfer catalytic
process.
stated. First, 30 g of Na2WO4·2H2O was dissolved in 37 ml of distilled
water with stirring. Then 0.75 ml of 85% H3PO4, followed by 5.5 ml of
acetic acid, was added to the stirring solution. After a while the solu-
tion became cloudy and gradually a heavy white precipitate formed.
The solid was collected and first dried under vacuum for 24 h, then
it was left in air for 24 h, finally, it was dried at 140 °C for about
1.5 h to get the solid of Δ-Na8H[PW9O34]·19H2O [31]. Then, 0.56 g
of Δ-Na8H[PW9O34]·19H2O was dissolved in 55 ml of water. To this
solution, 0.9 g of dioctadecyl dimethyl ammonium chloride dissolved
in 10 ml of tert-butanol was added slowly. The mixture was stirred
vigorously for 4.5 h at 40 °C. A white solid was filtered off and then
washed with an excess amount of water, then dried in vacuum [29].
The yield was 81%. IR spectrum (KBr, cm−1): 1079, 1043, 1033,
944, 896, 854, 803, 760, 721, 590, 547, 508, 433, 410. 31P NMR: −
9.7 ppm. Calcd for [(C18H37)2(CH3)2N]7[PW11O39]: C, 48.90; H, 8.58;
N, 1.50; P, 0.47; W, 30.98. Found: C, 47.93; H, 8.54; N, 1.44; P, 0.44;
W, 30.21.
detector equipped with SE-54 capillary (30 m×0.32 mm×0.25 μm).
Chemical elemental analysis of the catalyst was done on an ICP-
atomic emission spectrometer (IRIS ER/S).
2.3. Catalytic reaction
Catalyst (8 μmol), 1,4-dioxane (2 ml), substrate (1 mmol), and
H2O2( 2.5 mmol, 30% aq.) were charged in the reaction flask. The re-
action was carried out at 333 K for 0.5 h. After reaction, with the
dropping of temperature, the catalyst gradually precipitated from
solution. It was separated by centrifugation and washed with Et2O,
then dried under vacuum and used for the next oxidation cycle. The
filtrate was analyzed by GC using bromobenzene as internal standard.
The organic products were obtained by vacuum rotary evaporator,
and identified by 1H NMR. (See supporting information).
3. Results and discussion
2.2. Characterization techniques
It's known that the reactivity of common sulfur compounds in
oxidation is commonly in the following order: benzothiophene
bdibenzothiophenebdiphenylsulfidebmethylphenylsulfide [15]. Be-
cause this oxidation reaction was generally considered to proceed via
an electrophilic addition reaction of oxygen atoms, so the sulfides
with higher electron density on the sulfur atom should react easier
[23]. Here we chose the relatively inert sulfide of benzothiophene
(BT), as the model substrate to investigate. Excess oxidant H2O2 was
used to afford it's unavoidably decomposing under the heating temper-
ature (substrate: H2O2=1: 2.5).
Infrared spectra were recorded on a Bruker VERTEX 70v FT-IR spec-
trometer. The catalysts were measured using 2–4% (w/w) KBr pellets
prepared by manual grinding. 31P NMR and 1H NMR spectra were
recorded on a Bruker AVANCE III 400 M NMR using CDCl3 as the solvent
and Me4Si as the internal standard. GC analyses were performed
on a Shimadzu GC-9AM gas chromatogram with a flame ionization
Table 2
a
Oxidation of sulfur containing compounds with [(C18H37)2(CH3)2N]7 PW11O39
.
Firstly, the catalyst of [(C18H37)2(CH3)2N]7PW11O39 was tested in
different solvents (Table 1). BT was almost completely oxidized in
1,2-dichloroethane (entry 1), but this solvent was carcinogenic and
could dissolve this catalyst at room temperature which hindered
Entry Substrate
1
Product
Conversion (mol%) Selectivity (mol%)
97
99
99
99
98
99
97
99
99
5
99
99
99
99
99
99
99
99
99
99
–
2
3
Conversion
Selectivity
4
100
90
80
70
60
50
40
30
20
10
0
5
6
7
8
9
10b
11c
1
2
3
4
5
6
no
cycle times
a
Reaction condition: 333 K, 0.5 h, substrate 1 mmol, H2O2 2.5 mmol, 8 μmol catalyst,
b
c
2 ml 1,4-dioxane as solvent.
without catalyst. without H2O2. Conversions and
Fig. 1. Catalyst recycles for oxidation of benzothiophene. Reaction conditions were the
same with those in Table 2.
selectivity were determined by GC analysis. Products were identified by 1H NMR.

X. Xue et al. / Catalysis Communications 29 (2012) 73–76
75
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
clear solution gradually became turbid, and finally a large amount of
white solid precipitated (Scheme 1) [29]. The catalyst was washed
after centrifugation and dried for the next cycle.
a
Stability and reuse of the catalyst was evaluated. For substrate of
BT, the conversion decreased to 76% for the sixth cycle, but the selec-
tivity to sulfone kept as high as 99% (Fig. 1). The decrease for activity
could be ascribed to the physical loss of catalyst and change in the
structure of the catalyst (see Figs. 2 and 3). The W content of the
organic phase after catalyst separation was about 45 ppm by ICP
analysis, indicating that leaching of the catalyst exists. While for
methyl phenyl sulfide, which is the most susceptible to be oxidized,
the catalyst can be recycled for over ten times with excellent conver-
sion (over 99%).
The fresh catalyst and used catalysts were characterized with
FT-IR and 31P NMR. As shown in Fig. 2, the IR spectrum of the fresh
catalyst (Fig. 2a) exhibited characteristic peaks at 1079, 1043, 1033,
944 and 900–725 cm−1, which were attributed to ν(P―O), ν(W=O)
and ν(W―O―W), respectively, the peaks at 591 and 547 cm−1
were attributed to ν(W―O―O) [29,32] . There were some differences
between the IR spectra of the fresh catalyst and the first used one. An
apparent strong peak at 840 cm−1 which was attributed to ν(O―O)
was appeared for the latter (Fig. 2b) [29,33]. This strong peak of
peroxy bond vibration absorption also appeared in the IR spectrum
of the catalyst of cycle 2 (Fig. 2c), which was similar to the catalyst
of cycle 1. But in the spectrum of the 6th cycle (Fig. 2d), the vibration
intensity of peroxy bond became very weak, indicating the amount of
peroxo-species decreased. Therefore, the conversion of BT dropped
gradually with the increasing recycling times. 31P NMR of both the
fresh and used catalyst was carried out to thoroughly investigate. A
b
c
d
840
400
600
800
1000
1200
Wavenumber (cm-1)
Fig. 2. IR spectra of (a) the fresh catalyst of [(C18H37)2(CH3)2N]7 PW11O39;(b) the
catalyst of cycle 1;(c) the catalyst of cycle 2;(d) the catalyst of cycle 6.
catalyst recycling. In acetonitrile, the catalyst was actually temperature-
responsive, but the conversion was not satisfying (entry 2). For acetone
and toluene, only part of the catalyst was dissolved, which led to low
conversion (entry 3 and 6). When using water as solvent, the catalytic
system was emulsion, and a low yield of sulfone was obtained with
coproducing a little sulfoxide (entry 4). Only a small amount of the
catalyst precipitated after reaction in ethanol and ethyl acetate (entry
5 and 7), additionally, the conversion was also unsatisfactory. In com-
parison, 1,4-dioxane got the best result among all the solvents tested
(entry 8). What's more, the catalyst was almost completely precipitated
from the solvent after cooling. Therefore, 1,4-dioxane was chosen as the
most suitable solvent in this catalytic system.
After optimization of the reaction conditions, we extended sub-
strates to chain alkane sulfides and aromatic sulfides (Table 2). For
chain sulfides including ethyl sulfide, propyl sulfide and butyl sulfide,
they were all selectively oxidized to their corresponding sulfones with
excellent conversions of over 97% and 99% selectivity to sulfones
(entry 1–3). All the aromatic sulfides such as methyl phenyl sulfide
and substituted phenyl methyl sulfide (4-bromo- and 4-methyl-), as
well as phenylsulfide, were all oxidized with almost complete conver-
sion (entries 4–7). Moreover, DBT was thoroughly oxidized into sul-
fones (entry 8). All the good results demonstrated the universality of
this catalyst for selective oxidation of sulfides. It was confirmed that
oxidation of BT hardly proceeded without catalyst under the present
conditions (entry 10), and no product was detected without oxidant
(entry 11). When the reaction started at room temperature, the catalyst
was insoluble. While the temperature increased, the solid catalyst grad-
ually dissolved to form a clear homogeneous solution, which made the
reaction conducted homogeneously. After reaction with cooling, the
single peak at −9.7 ppm appeared in the spectra of the fresh catalyst
7−
(Fig. 3a), which could be attributed to PW
O
. In the 31P NMR spec-
11 39
tra of the first used catalyst (Fig. 3b), the original peak at −9.7 ppm
still exist, and some new peaks at the lower field of −8.3, −7.5, 7.2
and 7.6 appeared. These new peaks could be attributed to some
low W/P ratio polytungstophosphate peroxy species, for example,
[(PO4){WO(O2)2}2{WO(O2)2(H2O)}]3−
,
[(PO4){WO(O2)2}4]3− and
[(PO3(OH)){WO(O2)2}2]2− [34–36]. The original peak at −9.7 ppm
almost disappeared in the 31P NMR for
7−
which represent PW
O
11 39
the 6th cycle (Fig. 3c), revealing the catalyst decomposed to other
phosphorous containing species with the increasing cycle times. 31P
NMR spectra characterization results were in good accordance with
the results in IR spectra.
According to the IR and 31P NMR spectra, we proposed a possible
mechanism for this catalytic system (Scheme 2). The characterization
results indicated that the catalyst structure changed in the first run,
7−
part of the [PW11O39
]
would be decomposed to some low W/P
ratio peroxo species in the presence of H2O2, these peroxy species
are the true catalyst on oxidation of sulfides. First the catalyst was
reacted with H2O2 to form the peroxy phosphotungstic active species,
and then they transfer their active oxygen to the substrates of
sulfides. In this way, the entire catalytic process was completed.
Fig. 3. 31P NMR spectra of (a) the fresh catalyst of [(C18H37)2(CH3)2N]7 PW11O39; (b)the 1st recycled catalyst; (c) the 6th recycled catalyst.

76
X. Xue et al. / Catalysis Communications 29 (2012) 73–76
H2O2
H2O
O
O
H2O2
Mixture of low P/W ratio
polytungstophosphate species
P W
[(C18H37)2(CH3)2N]7PW11O39
O
O
S
R1
R2
S
R1
R2
Scheme 2. Proposed mechanism for oxidation of sulfides with [(C18H37)2(CH3)2N]7[PW11O39].
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Although the catalyst after the first recycle changed compared with
the fresh one, it did not change anymore from the second cycle on.
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4. Conclusions
In summary, a [(C18H37)2(CH3)2N]7PW11O39/H2O2/1,4-dioxane/
sulfides catalytic oxidation system has been developed, which
achieves complete oxidation of a series of sulfides to their corresponding
sulfones in a short time and mild conditions. The polyoxometalate-based
catalyst behaved as solid–liquid–solid phase transfer controlled by tem-
perature under the catalytic conditions. It can be recycled for several
times without obvious loss of activity. This process was considered
to be a potential way in oxidative desulfurization. It is just a study
of models of a real sample of fuel, and that further experiments will
be conducted to demonstrate that the catalyst operates and can be
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.09.021.
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