Tungstate supported on periodic mesoporous organosilica with imidazolium framework as an efficient and recyclable catalyst for the selective oxidation of sulfides

Article abstract of DOI:10.1002/cplu.201500010

Abstract A catalyst based on immobilization of tungstate ions (WO₄^2-) inside the mesochannels of periodic mesoporous organosilica comprising bridged ionic liquid (1,3-bis(3-trimethoxysilylpropyl)imidazolium chloride) has been synthesized and characterized. This catalyst was then employed for the selective oxidation of organic sulfides to the corresponding sulfoxides or sulfones. The final synthesized catalyst was characterized by various techniques such as nitrogen sorption analysis, transmission electron microscopy, and thermogravimetric analysis. The catalyst was also applied to the selective oxidation of sulfides containing readily oxidizable functional groups such as hydroxyl, allylic, and even challenging aliphatic sulfides. Interestingly, it was found that on changing the reaction medium from aqueous methanol to aqueous acetonitrile, the product selectivity was changed successfully from sulfoxide to sulfone with good to excellent yields. Moreover, the catalyst can also be recovered and reused efficiently in nine subsequent reaction cycles without any remarkable decrease in the catalyst activity and selectivity.

Full text of DOI:10.1002/cplu.201500010

DOI: 10.1002/cplu.201500010
Full Papers
Tungstate Supported on Periodic Mesoporous Organosilica
with Imidazolium Framework as an Efficient and
Recyclable Catalyst for the Selective Oxidation of Sulfides
[
a]
[a]
[a]
Babak Karimi,* Mojtaba Khorasani, Fatemeh Bakhshandeh Rostami,
[b]
[c]
Dawood Elhamifar, and Hojatollah Vali
2
ꢀ
A catalyst based on immobilization of tungstate ions (WO4
)
taining readily oxidizable functional groups such as hydroxyl,
allylic, and even challenging aliphatic sulfides. Interestingly, it
was found that on changing the reaction medium from aque-
ous methanol to aqueous acetonitrile, the product selectivity
was changed successfully from sulfoxide to sulfone with good
to excellent yields. Moreover, the catalyst can also be recov-
ered and reused efficiently in nine subsequent reaction cycles
without any remarkable decrease in the catalyst activity and
selectivity.
inside the mesochannels of periodic mesoporous organosilica
comprising bridged ionic liquid (1,3-bis(3-trimethoxysilylprop-
yl)imidazolium chloride) has been synthesized and character-
ized. This catalyst was then employed for the selective oxida-
tion of organic sulfides to the corresponding sulfoxides or sul-
fones. The final synthesized catalyst was characterized by vari-
ous techniques such as nitrogen sorption analysis, transmission
electron microscopy, and thermogravimetric analysis. The cata-
lyst was also applied to the selective oxidation of sulfides con-
Introduction
[
1]
Since the discovery of ordered mesoporous silica (OMS), a va-
riety of organic–inorganic hybrid mesoporous materials with
high specific surface area, available pore volume, and tunable
pore size have been developed extensively in view of their po-
tions of various sophisticated organosilanes into the nano-
space of OMS have been reported by applying post-grafting
and/or co-condensation, but it has been proven that this ap-
proach could potentially lead to a dramatic pore-blocking
effect and inhomogeneous distribution of organic functional
[
2]
tential applications in various fields from catalysis and ad-
[
3]
[4]
[5]
[8]
sorption to nanoelectronics and controlled drug delivery.
groups throughout the surface of these materials. In addition,
These surfactant-derived organo-functionalized ordered meso-
porous materials are typically synthesized either by terminally
in many circumstances, the long-range order of the resulting
material could be reduced dramatically if an organic content
(organosilane precursors) greater than 25% was employed
bonded siliceous organic groups [(R’O) SiꢀR] through co-con-
3
[9]
densation or post-grafting of surface silanol groups in the
during the co-condensation methods.
[
2d,6]
parent OMS.
Moreover, these materials have attracted great
To overcome these limitations, periodic mesoporous organo-
silicas (PMOs), which were built by the self-assembly of
bridged organosilane precursors [(R’O) SiRSi(OR’) ] in the pres-
interest from the viewpoints of academia and industry, be-
cause many of their characteristics, such as hydrophilicity/hy-
drophobicity, mechanical/hydrothermal stability, and other
chemical or physical properties, can be adjusted intelligently
and simply by varying the nature of the immobilized organic
3
3
ence of surfactants as structure-directing agents (SDAs), were
[10]
developed. Because PMOs often exhibit large internal sur-
face areas and open pore structures, the organic bridging
groups inside the pore wall of mesochannels are readily acces-
[
7]
group (R). In this way, a large number of successful incorpora-
[11]
sible to molecules diffusing through the porous matrix, a fea-
ture that makes these materials a unique platform in preparing
advanced functionalized catalysts for important liquid-phase
[a] Prof. Dr. B. Karimi, Dr. M. Khorasani, F. Bakhshandeh Rostami
Department of Chemistry
[12]
Institute for Advanced Studies in Basic Science (IASBS)
P.O. Box 45195-159, Gava Zang, Zanjan 45195 (Iran)
Fax: (+98)241-4153232
organic transformation. Compared with functionalized OMSs,
the PMO surface reactivity, hydrophilicity/hydrophobicity bal-
ance, structural rigidity, and mass-transfer rate as well as its
surface, optical, electronic, magnetic, and charge-transport
properties, can be readily tuned for specific applications in
E-mail: karimi@iasbs.ac.ir
[b] Dr. D. Elhamifar
Department of Chemistry
Yasouj University, Yasouj 75918-74831 (Iran)
[13]
a more sophisticated way.
[
c] Prof. H. Vali
In recent years, many catalytic systems based on PMOs com-
prising simple groups, such as sulfonic acids, Brønsted or Lewis
bases, and phenylene, as well as bulky organic functional
groups, such as phosphine complex, 1,1’-bi-2-naphthol
(BINOL), 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP),
Department of Anatomy and Cell Biology
and Facility for Electron Microscopy Research
McGill University, 3450 University St., Montreal, QC H3A 2A7 (Canada)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500010.
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[
24]
bis[(salicylidene)ethylenediaminato] (salen), phorphyrin, gua-
nine, and also some metal complexes such as copper, cobalt,
and palladium, have been employed extensively in a variety of
subsequently been developed. On the other hand, the use
[25]
of supported IL phases has also been considered as a plat-
form for tungstate ion immobilization for the activation of
[
14]
[26]
important organic transformations. Along this line, we have
recently demonstrated the synthesis and characterization of
a series of PMOs with ionic liquid (IL) framework and studied
their applications as an efficient support for immobilization of
several types of transition-metal catalysts through an anion-ex-
change technique. In this way, we have recently introduced
the novel functionalized material called PMO-IL, which was
successfully employed as a competent support for the immobi-
lization and stabilization of Pd nanoparticles in CꢀC bond for-
30% H O2 in several oxidative transformations.
Although
2
these approaches have provided substantial improvements to
some extent, the catalyst efficiency and selectivity, especially in
the case of sulfide oxidations, remain a challenging research
area.
We have presented for the first time a new catalytic protocol
composed of tungstate ions inside the hydrophobic channels
of the OMS SBA-15, as a novel catalyst system for the selective
preparation of sulfoxides or sulfones by adjusting the hydro-
[
15]
[16]
[27]
mation reactions,
aerobic oxidation of alcohols,
in aerobic alcohol oxidation, as well as for Au nano-
particles in the synthesis of propargylamines through the
for
phobicity of starting sulfides and/or the solvent polarity. En-
ꢀ
[17]
RuO4
couraged by this interesting observation and finding, we were
2
ꢀ
keen to investigate whether the catalyst comprising WO4 ion
immobilized in the interior of nanospaces of PMO-IL
3
[18]
A coupling reaction.
Quite recently, we also found that
2
ꢀ
tungstate ions supported on a PMO with IL framework
(WO4 @PMO-IL, see Figure 1) could be employed in the same
way for the selective oxidation of sulfide into sulfoxide or sul-
fone with 30% H O as a safe oxidant by adjusting the reaction
2
ꢀ
(
WO4 @PMO-IL) is a recoverable catalyst system for the highly
selective oxidation of various primary or secondary alcohols to
2
2
[18g]
the corresponding aldehydes or ketones by 30% H O .
conditions.
2
2
Selective oxidation of sulfides into the corresponding sulfox-
ides is a promising process from both a laboratory and indus-
trial point of view, since they are important synthetic inter-
mediates and/or products in the synthesis of pharmaceuticals
and agrochemicals, and are particularly useful building blocks
[19]
as chiral auxiliaries in asymmetric organic syntheses.
Al-
though a significant number of traditional oxidants, such as
high-valence metal salts, concentrated HNO , m-chloroperoxy-
3
benzoic acid, sodium metaperiodate, halogens, and nitrogen
pentoxide, have been widely used for this transformation,
many of them suffer from the use of hazardous reagents, gen-
erate unwanted toxic waste, and also show overoxidation of
sulfoxides to the expected sulfones during relatively harsh or
42ꢀ
Figure 1. Schematic representation of the WO @PMO-IL.
[
20]
harmful reaction conditions. To address these limitations and
consider the eco-sustainability, green chemistry, and especially
atom economy, a substantial amount of research has been di-
rected toward developing new and efficient catalytic systems
based on the use of aqueous 30% H O as a final green oxi-
Results and Discussion
First, the preparation of PMO-IL was performed by hydrolysis
and condensing of silica precursors, such as the IL 1,3-bis(3-tri-
methoxysilylpropyl)imidazolium chloride (BTMSPI) and tetra-
methyl orthosilicate (TMOS), in the presence of an acidic solu-
2
2
dant because its remarkable benefits include low cost, environ-
mentally benign properties, high atom economy, and water as
tion of Pluronic P123, which acted as a SDA according to our
[
21]
[15–18,28]
a byproduct. For this reason, various heterogeneous/homo-
geneous catalytic systems based on organic material and
metal as well as self-supported catalysts were used successfully
for the activation of hydrogen peroxide in this transforma-
previous reports.
The resulting PMO-IL was then allowed
to react with an aqueous solution of Na WO to replace some
2
4
of the chloride ions with tungstate ions through a direct ion-
[29]
exchange approach following our recent protocol. Then, the
presence of bridged IL and tungstate ions and structural uni-
formity were concomitantly investigated by employing well-
known techniques such as nitrogen physisorption, transmission
electron microscopy (TEM), and thermogravimetric analysis
(TGA).
[
22]
tion. Although these reports provided significant advances
in the field, some of the processes described suffer from low
selectivity for sulfoxide or sulfone production. Meanwhile,
a number of versatile and efficient catalysts based on oxotung-
states have received considerable attention for the oxidation
of sulfides, which feature high functional compatibilities and
the possibility to control the selectivity of this reaction toward
Figure 2 shows the N adsorption–desorption isotherms and
2
pore size distributions (Barrett–Joyner–Halenda, BJH) for the
[
23]
formation of either sulfoxide or sulfone.
To deliver more
synthesized materials. In particular, N sorption analysis pro-
2
viable “green” protocols for sulfoxidation reactions, several
strategies for the immobilization of tungsten-based catalyst
into/onto inorganic supports, organic–inorganic hybrid materi-
als, and organic polymers with the hope of improving the recy-
clability and durability of the employed catalyst systems have
vides useful information on the textural properties and meso-
scopic quality of the presented materials. The PMO-IL itself dis-
plays a type IV isotherm, with an H1 hysteresis loop, which in-
dicates the formation of mesoporous materials with cylindrical
[29]
pores of relatively uniform pore size and shape.
Further-
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ions. Hence, TEM images of
2
ꢀ
WO4 @PMO-IL along the [001]
and/or [110] directions also
demonstrate a highly ordered
structure with 2D hexagonal
symmetry in the mesoporous
channels, which is in good
agreement with the results of
N physisorption (Figure 3).
2
The TGA of WO @PMO-IL cat-
4
alyst was performed at temper-
atures that ranged from 20 to
6008C under an air flow (Fig-
2
ꢀ
ure S3 in the Supporting Infor-
mation). This TG pattern shows
two main weight losses; the
first one of about 5% below
Figure 2. N adsorption–desorption isotherms (A) and BJH pore size distributions (B) for PMO-IL and WO @PMO-
IL.
2
4
more, a sharp capillary condensation step around P/P ꢁ0.7–
1008C may be a result of the desorption of solvent molecules
such as water, ethanol, and methanol that remained from the
solvent extraction processes, whereas a major weight loss of
about 16 wt% observed from 350 to 6008C could be attribut-
ed to the loss of the organic fragments such as IL precursors
from the final catalyst. From these data, estimated IL amounts
0
0
.8 is evidence for the formation of well-ordered mesoporous
solid materials with narrow pore size distribution, despite the
flexible precursor (BTMSPI) that has been inserted into the
PMO wall networks. This is made possible by the structural ri-
gidity imported to these materials by the siloxane linkages.
The data of BET surface area (SBET) derived from the linear part
ꢀ
1
were determined as approximately 1 mmolg . Finally, the
amount of exchanged tungstate ions in the catalyst was found
of adsorbed nitrogen around P/P ꢁ0.05–0.3 and the amount
0
ꢀ
1
of total pore volume (V ) of P/P ꢁ0.99 show successive forma-
to be 0.07 mmolg by using inductively coupled plasma
t
0
2
ꢀ1
tion of PMO-IL with high surface area of 563 m g , which con-
tains open mesoporous channels with total volume
3
ꢀ1
[30]
1
.12 cm g (Table 1). Interestingly, a narrow pore size distri-
Table 1. Textural properties of the synthesized materials determined
from nitrogen physisorption.
[
a]
2
ꢀ1
[b]
3
ꢀ1
[c]
Material
PMO-IL
S
BET [m g
]
V
t
[cm g
]
D
BJH [nm]
563
391
1.12
0.88
10.6
10.6
2
ꢀ
WO
4
@PMO-IL
[a] SBET =specific surface area. [b] V
t
=total pore volume. [c] DBJH =average
pore diameter.
bution derived from the absorbance branch of the isotherm
for PMO-IL centered around 10.6 nm indicates a highly ordered
structure in the synthesized PMO-IL, which is also in agreement
[
31]
with N physisorption data (Figure 2 and Table 1). In addi-
2
tion, the specific surface area and pore volume of
2
ꢀ
WO4 @PMO-IL in comparison with the parent PMO-IL were
2
ꢀ1
decreased significantly from the initial amounts to 391 m g
3
ꢀ1
and 0.88 cm g , respectively; this result clearly demonstrates
that the tungstate ions have been successfully immobilized
inside the pores of the PMO-IL. Interestingly, a sharp capillary
condensation step in the isotherms and narrowed pore size
distribution (BJH) indicate that structurally ordered and avail-
able mesochannels also remained intact after modification of
the parent PMO-IL with tungstate ions (Figure 2).
TEM images of the catalyst also provided further evidence
that the ordered organizations of the mesopores were retained
after surfactant extraction and/or immobilization of tungstate
42ꢀ
Figure 3. TEM images of WO @PMO-IL along the A) [110] and B) [001] di-
rections (scale bars: 100 nm).
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atomic emission spectroscopy (ICP-AES) of its acid-washed so-
lution.
and that the same reaction in H O/CH CN (1:1) furnished the
2
3
respective sulfone (Table 2, entry 8) highlights the notion that
in order to adjust the selectivity of the reaction toward sulfox-
ide or sulfone the solvent composition needs to be respected.
Therefore, to gain better insight into the impact of solvent
composition on the selectivity of this catalyst process, we per-
formed additional experiments. Along this line, it was found
that when water was replaced by methanol under otherwise
the same reaction conditions, the conversion was noticeably
improved whereas the selectivity toward sulfoxide did not vary
significantly and still remained around the excellent value of
2ꢀ
After the initial characterization of the WO4 @PMO-IL, its
catalytic activity was studied in the oxidation of organic sul-
fides to the corresponding sulfoxides and/or sulfones with
aqueous 30% H O as terminal green oxidant. Thus, the oxida-
2
2
tion of methyl phenyl sulfide (1 mmol) as a test model was
first investigated in various solvents and at different tempera-
tures, to find the optimum reaction conditions (Table 2). Initial-
Table 2. Optimization of the solvent, temperature, and amount of hydro-
gen peroxide during oxidation of methyl phenyl sulfide.
96% at 83% conversion (Table 2, entry 9). Surprisingly, the
same reaction when methanol/H O (10:1) was used as the sol-
2
[
a]
[b]
[b]
Entry
Solvent
T [8C] t [h] Sulfoxide [%]
Sulfone [%]
vent system instead of pure water or methanol gave an excel-
lent yield of 92% of methyl phenyl sulfoxide as major product
within 30 minutes (Table 2, entry 10). These data indicate that
on changing the solvent system from aqueous methanol to
aqueous acetonitrile, the reaction selectivity varies from sulfox-
ide to sulfone with retention of the high yield and selectivity
(Table 2, entries 8 and 10). These preliminary results indicate
that there is a distinctive relationship between the solvent
composition and the catalyst activity–selectivity for the selec-
tive formation of either sulfoxide or sulfone using the present
catalyst system. More information about this issue is provided
later.
1
2
3
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O
RT
RT
35
50
50
50
50
50
RT
0.5
1
1
1
0.5
1
1.5
2
0.5
50
81
84
75
30
23
10
4
5
2
5
25
70
77
90
96
3
O/MeCN (1:1)
O/MeCN (1:1)
O/MeCN (1:1)
O/MeCN (1:1)
O/MeCN (1:1)
O/MeCN (1:1)
[
[
[
[
c]
4
5
6
7
8
9
1
d]
d]
d]
[
d]
H
MeOH
H
H
2
O/MeCN (1:1)
80
0
2
O/MeOH (1:10) RT
O/MeOH (1:10) 50
0.5 92
55
8
45
[
d]
11
2
2
2ꢀ
[a] Reaction conditions: methyl phenyl sulfide (1 mmol), WO
4
@PMO-IL
(
1 mol%), 30% H O (1.2 equiv), and solvent (2 mL). [b] Yield determined
2
2
by GC analysis using the standard addition method. [c] 1.5 equiv 30%
was used. [d] 2 equiv 30% H was used.
Kinetic experiments were also performed for oxidation of
methyl phenyl sulfide under the optimized reaction conditions
H
2
O
2
2 2
O
(
Table 2, entries 8 and 10). Although in H O/CH OH solvent
2 3
mixture the oxidation of methyl phenyl sulfide furnishes the
corresponding sulfoxide in high yield and selectivity (up to
92%, 30 min), the yield of methyl phenyl sulfones was less
than 10% (Figure 4A). On the other hand, in H O/CH CN mix-
ly, the oxidation of methyl phenyl sulfide with 30% H O2
2
(1.2 equiv) in the presence of 1 mol% of catalyst in pure water
at room temperature afforded the corresponding methyl
phenyl sulfoxide in 50% yield after 30 minutes (Table 2,
entry 1). Our preliminary investigation showed that the solvent
2
3
ture, the reaction kinetic profile indicates that both sulfone
and sulfoxide were concomitantly produced from almost the
beginning of the reaction with very close reaction rates to
afford the same approximately 50:50 ratio within 25 minutes
(Figure 4B). Interestingly, a gradual selectivity changeover
toward the formation of the corresponding sulfone was ob-
served when the reaction was allowed to proceed for an addi-
tional reaction time up to 120 minutes. Considering these in-
teresting results, it is believed that the concomitant formation
of both sulfoxide and sulfones in the latter case might be the
[
27]
has a crucial role in this transformation. With this promising
result, we then switched our attention to the use of a mixture
of water with organic solvent. In this context it was found that,
in an equal mixture of water and acetonitrile (1:1), which was
recently selected as the optimum solvent system according to
[
27]
our previous tungstate-based catalyst system, the oxidation
of thioanisole proceeded much better and furnished the corre-
sponding sulfoxide in 81% yield within 1 hour under the same
reaction conditions (Table 2, entry 2). Encouraged by these re-
sults, the effect of temperature was then examined (Table 2,
entry 3). Although no significant improvement in both sulfox-
ide yield and reaction conversion was achieved upon increas-
ing the reaction temperature to 358C, when employing either
result of in situ generation of proxamidic acid (CH CNHOOH)
3
through the reaction of CH CN and 30% H O . It is therefore
3
2
2
reasonable to speculate that the in situ generated
CH CNHOOH may contribute to and to some extent be respon-
3
sible for the enhanced reactivity in H O/CH CN relative to
2
3
a higher temperature (508C) or larger amount of 30% H O2
those obtained in H O/CH OH using our catalyst system.
2 3
2
(
1.5 equiv) a gradual selectivity changeover toward the forma-
On having the optimized reaction conditions, we then ex-
tended the scope of the present catalytic system to the selec-
tive oxidation of a range of sulfides to the corresponding sulf-
oxides or sulfones (see Scheme 1). As shown in Table 3, various
types of sulfides including aryl, alkyl, diaryl, and dialkyl sulfides
furnished the corresponding sulfoxide in moderate to excellent
yields (52–98%) and good to excellent selectivities (83–98%)
under the conditions demonstrated in entry 10 of Table 2. No-
tably, this method was equally applicable to the oxidation of
tion of the corresponding sulfone was observed (Table 2, en-
tries 3–5). Consistent with these results, it was found that the
use of only 2 equivalents of 30% H O was sufficient for pro-
2
2
viding excellent selectivity for the corresponding sulfone, al-
though somewhat longer reaction times were necessary to
ensure complete substrate conversion (Table 2, entries 6 and
7
). The fact that the oxidation of methyl phenyl sulfide in
water afforded the corresponding sulfoxide (Table 2, entry 1)
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the corresponding sulfoxide in
excellent yield and selectivity
(
Table 3, entry 10).
As already mentioned, in the
equal mixture of water and ace-
tonitrile (1:1) in the presence of
two equivalents of hydrogen
peroxide and 1 mol% of our
catalyst, methyl phenyl sulfide
was oxidized to the correspond-
ing sulfone in excellent yield
and selectivity (Table 2, entry 8).
Considering these initial promis-
ing results and with the aim to
study the generality of this pro-
cedure, selective oxidations of
various challenging sulfides
2 2
Figure 4. Reaction progress as a function of time in the oxidation of methyl phenyl sulfide using 30% H O in
A) water/MeOH and B) water/MeCN.
were explored under identical reaction conditions (Table 4, en-
tries 1–5). The butyl phenyl sulfide as relatively sluggish sub-
strate was oxidized selectively to the corresponding sulfone in
quantitative yield and excellent selectivity (Table 4, entry 2). In-
terestingly, under the same reaction conditions, allyl phenyl
Table 3. Oxidation of various sulfides to the corresponding sulfoxides
2ꢀ
catalyzed by WO
4
@PMO-IL in H
2
O/CH
3
OH (1:10).
[
a]
Entry
1
Sulfide
t [h]
Sulfoxide [%]
Selectivity [%]
92
[
[
[
b]
b]
b]
0.5
92
2
3
4
0.5
0.5
1.5
79
57
93
92
89
95
Scheme 1. A possible mechanism for catalytic oxidation of sulfide using
2ꢀ
WO
4
@PMO-IL.
[c]
[
c]
c]
5
6
2
1
94
96
91
sulfides with electron-withdrawing functional groups such as
bromide and nitro, which gives the respective sulfoxide com-
pounds in good to excellent yields with a high level of selectiv-
ity under the same reaction conditions (Table 3, entries 4 and
[
[
91
c]
7
8
9
1
85
85
98
98
98
84
5). Even in the case of more challenging sulfides comprising
[
b]
0.5
1
94
70
98
52
sensitive functional groups, such as olefin and hydroxyl func-
tionalities, the corresponding sulfoxides were obtained in
good to excellent yields and selectivities. In the case of allylic
sulfides neither the epoxidation of the double bond nor the
overoxidation of sulfoxide to sulfone were observed (Table 3,
entries 8 and 9) even after prolonged reaction times. It is also
worth mentioning that under essentially identical reaction con-
ditions, highly selective sulfoxidation of sulfides such as 4-
[b]
[
b]
1
0
1
[
d]
[c]
11
2
[
c]
1
1
2
3
2
5
70
89
83
(methylthio)benzyl alcohol and 2-(phenylthio)ethanol that bear
[e]
[c]
60
an oxidizable primary alcohol unit can also be achieved with-
out formation of any measurable amount of the respective car-
bonyl compound (Table 3, entries 6 and 7). Even in the case of
less reactive substrates such as diaryl sulfides, the correspond-
ing sulfoxides were obtained in moderate yields and high che-
moselectivity (Table 3, entries 11 and 12). In addition, dibutyl
sulfide as a model for aliphatic sulfide can be converted into
[
a] Reaction conditions: sulfide (1 mmol), 30%
H
2
O
2
(1.2 equiv),
2ꢀ
WO
4
@PMO-IL (1 mol%), and H
2
O/CH
3
OH (1:10, 2 mL) at room tempera-
ture (258C). [b] Yield determined by GC analysis using the standard addi-
tion method. [c] Yields referred to isolated pure products after column
chromatography. [d] 3 equiv 30% H
and 2 mol% of catalyst were used.
2 2 2 2
O was used. [e] 3 equiv 30% H O
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and it especially shows hydrothermal stability of the catalyst
during the recycling process (Figure S5). Moreover, the TG pat-
tern of the recovered catalyst indicated that the catalyst com-
positions were stable under the reaction conditions (Figure S6).
Table 4. Oxidation of various sulfides to the corresponding sulfones cata-
2
ꢀ
lyzed by WO
4
@PMO-IL in H
2 3
O/CH CN (1:1).
[
a]
Entry
1
Sulfide
t [h]
Sulfoxide [%]
Selectivity [%]
100
2ꢀ
In addition, TEM images of the recovered WO4 @PMO-IL after
the tenth reaction cycle indicate the presence of a two-dimen-
sional ordered structure with estimated pore size of 10 nm,
which is in good agreement with nitrogen physisorption data,
and predominantly confirms the stability and robustness of the
catalyst under the described reaction conditions and recycling
procedure (Figure 5). The high recyclability of the catalyst can
also be understood from the low leaching of tungstate species,
which was found to be less than 0.5 ppm by ICP-AES.
[
[
[
b]
b]
b]
2
96
100
100
89
2
3
1.5
1
100
100
100
[
d]
e]
[c]
[c]
4
5
6.5
[
9
87
100
[
a] Reaction conditions: sulfide (1 mmol), 30%
H
2
O
2
(2 equiv),
ꢀ
4 2 3
@PMO-IL (1 mol%), and H O/CH CN (1:1, 2 mL) at 508C. [b] Yield
2
WO
determined by GC analysis using the standard addition method. [c] Yields
referred to isolated pure products after column chromatography.
2 2
[d] 3 equiv 30% H O and 2 mol% of catalyst were used. [e] 4 equiv 30%
2 2
H O
and 2 mol% of catalyst were used.
sulfide was completely converted to the expected sulfone
within 1 hour without any oxidation of double bonds to epox-
ide or diols (Table 4, entry 3). Furthermore, relatively less reac-
tive diphenyl sulfide and 4-nitrodiphenyl sulfide were carefully
oxidized to their sulfones with good yield and excellent selec-
tivity (Table 4, entries 4 and 5).
An important feature of the heterogeneous catalyst is its re-
cyclability and stability after the reaction process through
simple filtration and separation of the catalyst from the reac-
tion medium for use several times. Hence, the recyclability of
42ꢀ
Figure 5. TEM image of the recovered WO @PMO-IL after the tenth reac-
tion cycle in the oxidation of methyl phenyl sulfide to the related sulfoxide
2 2
by using 30% H O (scale bar: 20 nm).
2
ꢀ
WO4 @PMO-IL was examined for the selective oxidation of
methyl phenyl sulfide (1 mmol) to the corresponding sulfoxide
under optimized reaction conditions. At this point, after each
run the catalyst was isolated through simple filtration, then
washed with dichloromethane (3ꢁ10 mL) and subsequently
dried under vacuum overnight (12 h). Interestingly, the de-
scribed catalyst was carefully reused for a subsequent nine
cycles and resulted in 92, 92, 92, 93, 92, 90, 94, 94, and 81%
yields, respectively, without any significant decrease in its effi-
ciency and selectivity. For evaluating the effect of reaction con-
ditions on the catalyst structure, a sample of the separated cat-
We were also very interested to compare the catalytic per-
formance of our system with the ones already reported in the
literature in terms of the observed turnover frequencies (TOFs).
At this point, selective oxidation of methyl phenyl sulfide to
the related sulfoxide in the presence of 30% H O was selected
2
2
(Table 5). As can be clearly seen, the TOF in our protocol is su-
perior to those of the W-supported catalysts on silica gel, lay-
ered double hydroxide (LDH), MCM-41, as well as SBA-15
(Table 5, entries 1–6) under nearly the same reaction condi-
tions, whereas it is inferior to those obtained from either deca-
alyst from the last cycle was characterized by N sorption anal-
2
ysis, TEM, and TGA. Data from N adsorption–desorption analy-
tungstate or peroxotungstate catalysts supported on SiO or
2
2
sis indicate that the recovered catalyst shows a typical type IV
isotherm with hysteresis loop H1, which demonstrates that the
recovered catalyst is still characterized as an ordered mesopo-
rous material (Figure S4). A particularly sharp capillary conden-
sation step indicates that the nanostructural uniformity of the
catalyst did not change after ten reaction cycles. The BET data
polymer, respectively (Table 5, entries 7 and 8). This might be
owing to higher loading and thus a higher number of active
sites in the catalyst systems comprising either decatungstate
or peroxotungstate species. In particular, the superior catalytic
performance of the present catalyst system in comparison with
the W-supported catalysts on nonfunctionalized silica materi-
als, such as silica gel, MCM-41, and SBA-15, clearly highlights
the advantage of imidazolium motifs inside the framework of
PMO-IL in achieving the observed superior catalytic activity.
Considering the short reaction times, good recyclability, excel-
lent yields, and great selectivities as well as high TOFs ob-
tained at room temperature for our catalyst system in the oxi-
2
ꢀ1
show a surface area of 290 m g
and pore volume of
3
ꢀ1
0.7 cm g . Moreover, the pore size distribution was estimated
as 10.6 nm according to the BJH method derived from the ad-
sorbed branch. On considering the pore size distribution
(10.8 nm) for the catalyst and recovered catalyst, it is clearly in-
dicated that the pore regularity and uniformity remains intact
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Table 5. Oxidation of methyl phenyl sulfide to the related sulfoxide by using 30% H
2
O
2
in the presence of various supported tungsten-based catalysts.
Entry
Catalyst
Cat. amount
mol%]
Solvent
T
[8C]
t
[h]
Conv.
[%]
Select.
[%]
TOF
Ref.
ꢀ
1 [a]
[
[h
]
2
3
ꢀ
[b]
1
2
3
4
5
6
7
8
9
WO
4
@LDH
4.4
1.5
1.5
1
1
1
0.1
0.5
1
H
2
O
RT
RT
8
RT
RT
35
RT
RT
RT
0.5
1.5
2.5
1.5
4
35
98.6
92
82
97
93
92
>99
100
88
98.6
86
100
94
97
92
>99
92
43
[22a]
[23j]
[23h]
[23d]
[24a]
[27]
[23e]
[23j]
this study
ꢀ
[c]
W
W
2
2
O
O
(O
(O
ꢀ
2
)
)
4
4
MIL@SiO
@SiO
2
CH
CH
CH
CH
H
CH
CH
2
2
2
3
Cl
Cl
Cl
2
2
2
/CH
/CH
/CH
3
3
3
OH (1:1)
OH (1:1)
OH (1:1)
43.5
24.5
54.6
25
3
2
2
2
WO
WO
WO
4
3
4
@SiO
2
@MCM-41
2
OH
ꢀ
@SBA-15
2
O
1
101
4
ꢀ
W
10
O
32 @SiO
2
3
3
OH
OH
1.5
0.58
0.5
666.6
1235
200
W(O)(O )(CN)@polymer
2
2
ꢀ
WO
4
@PMO-IL
CH
3
OH/H
2
O (10:1)
[a] Turnover frequency determined using (conversion)ꢂ[(cat. mol%)ꢁ(reaction time)]. [b] LDH=layered double hydroxide. [c] MIL=multilayer ionic liquid.
dation of various organic sulfides, it is reasonable to consider
this system as a high-performance alternative for this purpose.
Syntheses
Synthesis of ionic liquid precursor: The IL BTMSPI precursor was
[15]
[28]
prepared by a few modifications of our last synthesis report. In
a typical experiment, a suspension of sodium imidazolide in dry
tetrahydrofuran (THF) was prepared from the direct reaction of
freshly dried imidazole (2 g) and NaH (95%, 0.77 g) in a flame-dried
two-necked flask containing dry THF (60 mL) under an argon at-
mosphere. CPTMS (5.4 mL) was added to the mentioned stirred
suspension and the resulting mixture was heated at reflux for 30 h.
Then, the reaction mixture was allowed to cool to room tempera-
ture followed by solvent removal under reduced pressure until an
oily mixture containing NaCl was obtained. To this end, CPTMS
Conclusion
In summary, we have reported the synthesis and characteriza-
tion of periodic mesoporous organosilica with ionic liquid
framework (PMO-IL), as an efficient support for the immobiliza-
tion of tungstate ion catalysts through a simple ion exchange
technique. At first, the designed catalyst was characterized by
various techniques such as nitrogen physisorption, TEM, and
2ꢀ
TG analysis. The mentioned WO4 @PMO-IL was carefully
checked in the selective oxidation of organic sulfides into the
corresponding sulfoxide or sulfone derivatives. Our studies
showed that various aliphatic or aromatic sulfides comprising
readily oxidizable groups such as hydroxyl and allylic function-
al groups were selectively converted into the corresponding
sulfoxide with good to excellent yields under the same reac-
tion conditions. Moreover, by changing the reaction medium
from aqueous methanol to aqueous acetonitrile, the substrates
were completely oxidized to sulfones instead of sulfoxides. Fur-
thermore, the presented catalyst could be effectively recovered
and reused at least nine times without remarkable loss in the
catalytic activity. Finally, embedding of the flexible ionic liquid
along with active catalyst in the PMO wall network allows the
catalysis to act as an interphase catalyst during reaction prog-
ress. Further works on the practical applications of PMO-IL as
heterogeneous ionic liquid and also as an innovative support
for the immobilization of other types of transition-metal cata-
lysts through anion-exchange is underway in our laboratories.
(5.4 mL) and dry toluene (60 mL) were added and the resulting
mixture was heated at reflux for 48 h until a two-phase mixture
comprising toluene and IL (BTMSPI) was obtained. Then, the tolu-
ene phase was removed and dry CH Cl2 (60 mL) was added to
2
remove the precipitated NaCl. In the next stage, the CH Cl phase
2
2
was transferred into a well-dried two-necked flask and the volatiles
removed by reduced pressure until the IL (BTMSPI) and unreacted
starting materials were obtained. Finally, the IL was washed by dry
toluene (5ꢁ50 mL) for removal of unreacted starting materials to
1
give almost pure BTMSPI in 78% yield of isolated product. H NMR
(
250 MHz, CDCl , 258C, TMS): d=10.00 (s, 1H; NCHN), 7.46 (d, J=
3
1
.7 Hz, 2H; CHCH), 4.32 (t, J=7.1 Hz, 4H; NCH ), 3.60 (s, 18H;
2
6OCH ), 2.00 (m, 4H; CH CH CH ), 0.62 ppm (t, J=8.1 Hz, 4H;
3
2
2
2
13
SiCH2); C NMR (63 MHz, CDCl , 258C, TMS): d=136.08 (NCHN),
3
122.20 (CHCH), 51.76 (NCH ), 50.77 (OCH ), 24.12 (CH CH CH ),
2 3 2 2 2
5
.81 ppm (SiCH2).
Synthesis of PMO-IL: PMO-IL was also synthesized according to our
[15–18]
previously reported methods.
P123 (1.67 g) was dissolved in a mixture of H
6.14 g), and KCl (8.8 g), and the mixture was stirred at 408C until
In a typical procedure, Pluronic
O (10.5 g), HCl (2m,
2
4
a homogeneous solution was obtained. To this end, a premix of
BTMSPI (2 mmol, 0.86 g) and TMOS (18 mmol, 2.74 g) in dried
methanol was added immediately to the above-mentioned solu-
tion and stirred at 408C for 24 h. The resulting mixture was ex-
posed to hydrothermal treatment without stirring at 1008C for
Experimental Section
Materials
7
2 h. The obtained solid material containing surfactant was isolat-
Sodium hydride 95%, Pluronic P123 (MW ffi5800, Aldrich), and tet-
ed by filtration and carefully washed with deionized water. The sur-
factant residue was then extracted from the material through
a Soxhlet apparatus by using ethanol (100 mL) and concd HCl
(37%, 3 mL). In a typical extraction, as-synthesized PMO (1 g) was
washed four times with acidic ethanol over 12 h and then dried
under vacuum for 24 h, to afford PMO-IL as a bright yellow
powder.
av
ramethyl orthosilicate (TMOS) were obtained from Aldrich. Imida-
zole, 3-chloropropyltrimethoxysilane (CPTMS), potassium chloride,
concd HCl (37%), and Na WO ·2H O were purchased from Merck.
2
4
2
Imidazole was first recrystallized in distilled CH Cl and then dried
2
2
in a desiccator under vacuum over dry P O5 for 3 days at room
2
temperature.
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Full Papers
2
ꢀ
2ꢀ
Preparation of WO4 @PMO-IL: WO4 @PMO-IL was synthesized by
a simple ion-exchange technique according to our previous proce-
Angew. Chem. 2006, 118, 3290–3328; e) A. Zamboulis, N. Moitra, J. E.
Moreau, X. Cattoꢂn, M. W. C. Man, J. Mater. Chem. 2010, 20, 9322–9338.
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[23d]
dure with slight modifications.
0.5 g) was added to deionized water (20 mL) and the mixture was
sonicated for at least 10 min. Then Na WO ·2H O (0.090 g,
.26 mmol) in deionized water (3 mL) was added gradually to the
aforementioned suspension and stirred at room temperature for
h. The excess of tungstate solution was filtered, washed with de-
In a typical method, PMO-IL
(
2
4
2
[
4] K. Landskron, B. D. Hatton, D. D. Perovic, G. A. Ozin, Science 2003, 302,
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0
2
[
5
7
ionized water (3ꢁ30 mL), and dried under vacuum at 658C over-
night (12 h). The total tungstate loading of the materials was esti-
mated by inductively coupled plasma atomic emission spectrosco-
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3
47–360.
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[
General procedure for the selective oxidation of sulfides to the cor-
2
ꢀ
responding sulfoxides in water/CH OH: WO4 @PMO-IL (0.143 g,
3
[
9] B. D. Hatton, K. Landskron, W. Whitnall, D. D. Preovic, G. A. Ozin, Adv.
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1
mol%) was added to a solution of sulfide (1 mmol) and 30%
H O (1.2 mmol; exceptionally 3 mmol in the case of phenyl benzyl
2
2
[
[
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Asefa, N. Coombs, M. J. MacLachlan, G. A. Ozin, Chem. Commun. 1999,
sulfide) in H O/CH OH (2 mL, 1:10), and the solution was stirred at
2
3
room temperature for the requisite time. The progress of the reac-
tion was monitored by TLC or GC. After completion of the reaction
the product was first extracted with CH Cl (3ꢁ10 mL) and finally
2
2
2539–2540.
purified by using column chromatography (n-hexane/ethyl acetate,
:1).
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General procedure for the selective oxidation of sulfides to the cor-
2ꢀ
responding sulfones in water/CH CN: WO4 @PMO-IL (0.143–
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4
0
.286 g, 1–2 mol%) was added to a solution of sulfide (1 mmol)
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2
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and H O (2–4 mmol, depending on sulfide) in H O/CH CN (1:1),
2
2
2
3
and the solution was stirred at 508C for the requisite time. The
progress of the reaction was monitored by TLC or GC. After com-
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pletion of the reaction the product was first extracted with CH Cl2
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(
(
3ꢁ10 mL) and finally purified by using column chromatography
n-hexane/ethyl acetate, 8:1).
2ꢀ
General procedure for recycling of WO4 @PMO-IL in the oxidation
of methyl phenyl sulfide to the corresponding sulfoxide: After the
first run, the catalyst was isolated by centrifugation and subse-
quently washed with CH Cl (3ꢁ10 mL) and dried under vacuum
for 12 h. The recovered catalyst was then used in several runs in
same manner reported for the first run.
2
2
1
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Keywords: ionic liquids · oxidation · supported catalysts ·
sustainable chemistry · tungsten
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Full Papers
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Published online on && &&, 0000
&
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ÝÝ These are not the final page numbers!

FULL PAPERS
Interior design: The tungstate ion sup-
ported on periodic mesoporous organo-
silica with an ionic-liquid framework
B. Karimi,* M. Khorasani,
F. Bakhshandeh Rostami, D. Elhamifar,
H. Vali
(
see figure) has been used as an effi-
&
& – &&
cient and recoverable catalyst in the se-
lective oxidation of organic sulfides into
the related sulfoxides or sulfones, ac-
cording to the choice of reaction sol-
vent. The use of 30% H O as a green
Tungstate Supported on Periodic
Mesoporous Organosilica with
Imidazolium Framework as an
Efficient and Recyclable Catalyst for
the Selective Oxidation of Sulfides
2
2
oxidant was also achieved.
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
11
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ