Polyoxometalate [γ-SiW10O34(H 2O)2]4- on MCM-41 as catalysts for sulfide oxygenation with hydrogen peroxide

Article abstract of DOI:10.1016/j.molcata.2014.05.015

The polyoxometalate (POM) catalyst, [γ-SiW₁₀O ₃₄(H₂O)₂]^4-, was introduced into the pores of both as-synthesized (I) and amine functionalized MCM-41 (II). The resultant catalysts were characterized with powder X-ray diffraction, nitrogen sorption, and diffuse-reflectance UV-vis spectroscopy. Both catalysts were tested for reusability through repeated catalytic conversions of methyl phenyl sulfide to methyl phenyl sulfone with hydrogen peroxide. While the physisorbed catalyst (I) exhibits steadily decreasing turnover frequency (TOF), the POM catalyst supported on MCM-41 functionalized with a protonated amine (II) exhibits markedly improved reusability. This chemisorbed catalyst effectively showed no change in TOF between the second (21) and the sixth reactions (22). Additionally, sulfoxidations with catalyst II were investigated with a small set of substrates focusing on compounds including dibenzothiophene, which serves as a model refractory sulfide.

Full text of DOI:10.1016/j.molcata.2014.05.015

Journal of Molecular Catalysis A: Chemical 392 (2014) 188–193
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
4
−
Polyoxometalate [␥-SiW O (H O) ] on MCM-41 as catalysts for
10
34
2
2
sulfide oxygenation with hydrogen peroxide
∗
Dylan J. Thompson, Yang Zhang, Tong Ren
Department of Chemistry, Purdue University, West Lafayette, IN 47907, United States
a r t i c l e i n f o
a b s t r a c t
4
−
Article history:
The polyoxometalate (POM) catalyst, [␥-SiW10O34(H O) ] , was introduced into the pores of both as-
2 2
Received 29 March 2014
Received in revised form 17 May 2014
Accepted 20 May 2014
synthesized (I) and amine functionalized MCM-41 (II). The resultant catalysts were characterized with
powder X-ray diffraction, nitrogen sorption, and diffuse-reflectance UV–vis spectroscopy. Both catalysts
were tested for reusability through repeated catalytic conversions of methyl phenyl sulfide to methyl
phenyl sulfone with hydrogen peroxide. While the physisorbed catalyst (I) exhibits steadily decreasing
turnover frequency (TOF), the POM catalyst supported on MCM-41 functionalized with a protonated
amine (II) exhibits markedly improved reusability. This chemisorbed catalyst effectively showed no
change in TOF between the second (21) and the sixth reactions (22). Additionally, sulfoxidations with
catalyst II were investigated with a small set of substrates focusing on compounds including dibenzoth-
iophene, which serves as a model refractory sulfide.
Available online 2 June 2014
Keywords:
Polyoxometalate
Sulfide oxygenation
Hydrogen peroxide
MCM-41
Heterogeneous catalysis
© 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Catalytic oxygenation of sulfide to the corresponding sulfoxide
with heterogenized materials including slower reactions, more
expensive and tedious synthesis, as well as a liability of deactiva-
tion and leaching, there is a great potential in achieving an easily
recyclable and stable catalyst [16]. One of the simplest methods
to immobilize POM catalysts involves passive diffusion of poly-
oxometalate anions into the pores of mesoporous silica, the latter
of which have been widely used as the catalyst support since the
discovery of MCM-41 [17–20]. Such physisorbed catalysts are gen-
erally reported to be reusable up to 3–5 cycles. The most common
problem for polyoxometalates physisorbed onto MCM-41 is the
loss of catalyst via leaching [21–23]. This is especially the case
when the catalyst is soluble in the reaction medium. One way to
alleviate this matter is to functionalize the surface of silica with
an alkyl amine or other functional groups bearing positive charges
on the surface for supporting a chemisorbed catalyst [24–26].
It is worth mentioning that the synthesis of macroporous silica
with lacunary ␥-decatungstosilicate covalently attached to the wall
through siloxy tether has been reported, and the silica supported
decatungstosilicate is active in olefin epoxidation by hydrogen per-
oxide [27]. In order to avoid crowding the pores, it is important to
keep the loading of the amine linker to a minimum [28] and not
overload the POM catalyst [26]. These modifications can result in
an increase in pore congestion [29] and loss of structural integrity of
the mesoporous support [30,31]. Both the physisorb and chemisorb
approaches were employed in this study, and the reusability and
versatility of the resulting heterogeneous catalysts were investi-
and sulfone is of interest for a variety of reasons. Firstly, due to regu-
lations mandating a decreased permissible threshold concentration
of sulfur in fuels [1], alternative methods for deep desulfurization
such as oxidative desulfurization [2,3] are desirable [4]. Secondly,
the detoxification of chemical warfare agents like mustard agent
is achieved through its conversion to the corresponding sulfoxide
[
5]. Lastly, the conversion of sulfides to chiral sulfoxides produces
key intermediates in medicinal chemistry [6,7]. Polyoxometalates
POMs) constitute a broad and interesting field with abundant
(
catalytic opportunities, and are specifically attractive in redox reac-
tions due to their high stability under these conditions [8,9]. The
current work focuses on the di-vacant Keggin based lacunary poly-
4−
oxometalate shown in Fig. 1, [␥-SiW10(H O) O34] (1), which has
2
2
been reported to activate hydrogen peroxide for the epoxidation of
olefins [10,11] and step-wise oxygenation of organic sulfides [12].
In homogenous reactions, 1 is selective with nearly quantitative
efficiency in the use of hydrogen peroxide [10,12,13].
As with all homogeneous reactions, recovery and reuse of
catalyst can be quite difficult, making the investigation of hetero-
geneous catalysts worthwhile [14,15]. While there are challenges
∗ Corresponding author. Tel.: +1 765 494 5466.
E-mail address: tren@purdue.edu (T. Ren).
gated with a focus on H O oxygenation of organic sulfides.
2
2
http://dx.doi.org/10.1016/j.molcata.2014.05.015
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

D.J. Thompson et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 188–193
189
2.3. Impregnation of MCM-41
◦
As synthesized MCM-41 (0.6 g) was heated to 100 C for an
hour and then suspended in 10 mL of acetonitrile containing
4−
0
.12 g of [␥-SiW10O34(H O) ]
[22]. This was stirred for 24 h
2
2
and then filtered and rinsed with acetonitrile to yield 0.6 g [␥-
4−
SiW10O34(H O) ] @MCM-41 (I). The amount of 1 affixed inside
2
2
the pores of MCM-41 was estimated by the change in absorbance
at about 270 nm of the solution from before and after impregnation
implying a 6% loading.
2
.4. Impregnation MCM-41-NH +
3
Fig. 1. Structure of [␥-SiW10(H2O)2O34]4 taken from Ref. [10].
−
4−
The [␥-SiW10O34(H O) ]
catalyst was heterogenized onto
2
2
MCM-41-NH3+ based on modified procedures from literature [34].
Briefly the catalyst was affixed through ion-exchange by stirring
2
. Experimental
3
2
.0 g of MCM-41-NH3+ in a 20 mL acetonitrile containing 0.6 g 1 for
4−
4 h. At the end of this time, the [␥-SiW10O34(H O) ] @MCM-41-
2
2
2.1. General
+
NH4 (II) was filtered off, and rinsed with acetonitrile: recovered
3
.1 g. The amount of 1 affixed inside the pores of MCM-41-NH3+
Reagents for the syntheses of the catalyst and the synthe-
was estimated in the same fashion as I, and corresponds to an 8%
loading.
sis and functionalization of the MCM-41 were purchased from
Sigma-Aldrich. Acetonitrile solvent was also purchased from
Sigma-Aldrich. Sulfide substrates for the reaction, and the inter-
nal standard, 1,2 dichlorobenzene, were purchased from ACROS
Organics.
2.5. Oxygenation reactions
The syntheses of MCM-41 [17,32,33] and the tetrabutylamm-
Oxygenation reactions of organic sulfides were carried out simi-
4−
onium (TBA) salt of [␥-SiW10O34(H O) ]
catalyst [10] were
2
2
larly to similar homogeneous reactions described in literature [12].
The heterogeneous catalyst II was used in quantities that were cal-
culated to give approximately 1 mol% loading of 1 with respect to
completed according to literature procedures. The infrared spectra
4−
of the [␥-SiW10O34(H O) ] catalyst and its precursors were taken
2
2
with ATR-IR on a JASCO FT/IR 6300 to authenticate the products
from each synthetic step. The diffuse reflectance UV–vis spectra of
the MCM-41 material as well as the impregnated MCM-41 cata-
lysts I and II were taken with a JASCO V-670 spectrophotometer
fitted with a JASCO ILN-725 integrating sphere. The powder X-
ray diffraction (XRD) measurements of both catalysts and parent
MCM-41 materials were taken in PANalytical MRD X’Pert Pro High
Resolution XRD at 45 kV and 40 mA, with Cu K␣ radiation. Nitro-
gen sorption measurements of the same materials were taken on
0
.5 mmol organic sulfide substrate. All reactions were carried out
in duplicate.
Specific reaction conditions were: 0.20 g catalyst
I
or
II, 0.50 mmol organic sulfide substrate, and 0.40 mmol 1,2-
dichlorobenzene for internal standard. The reactions were run in
5
mL of acetonitrile at room temperature, and the heterogeneous
catalyst was kept in suspension with vigorous stirring. Each reac-
tion was initiated by the rapid addition of 2.0 equiv. of hydrogen
peroxide. Samples were taken by removing 20 ␮L of the reaction
mixture, followed by diluting to 60 ␮L with acetonitrile, shaking
and centrifuging. After centrifugation, the samples were decanted
into fresh centrifuge tubes to keep the sulfoxide/sulfone species
from adsorbing to the silica.
Six recycling reactions were completed with both catalysts I
and II. These reactions used methyl phenyl sulfide for the model
substrate. The recycling procedure involved filtering off the het-
erogeneous catalyst from the MeCN solutions of both the parallel
reactions, and then after rinsing and drying, separating the material
into halves for the next parallel reaction [35].
◦
a Micromeritics ASAP 2010 at 77 K after degassing at 200 C for
1
5 h.
Reaction samples were analyzed by Agilent 7890A GC sys-
tem equipped with a flame ionization detector. Separation of
sulfide, sulfoxide and sulfone was achieved using an Agilent HP-
5
column with dimensions of 30 m × 0.320 mm with 25 ␮m film
thickness.
2.2. Functionalization of MCM-41
Finally a few other substrates: methyl p-tolyl sulfide, phenyl
sulfide, benzothiophene, and dibenzothiophene were used to test
the versatility of catalyst II. The reactions of the last three substrates
Starting with 3.5 g calcined MCM-41, the first step was to
maximize the surface silanol population by refluxing in 20 mL
of 6 M HCl [21]. The material was then filtered and rinsed with
de-ionized water until the pH of the rinse water became neu-
◦
were completed at 40 C to accelerate the otherwise slow reactions.
tral (4 mL × 15 mL). The material was then dried overnight at
◦
1
10 C to remove as much water as possible. In order to ensure
3. Results and discussion
complete removal of the water, the material was refluxed in
toluene (30 mL) with a Dean-Stark trap for 4 h to azeotropically
remove any remaining water. Afterwards, the Dean-Stark trap
is removed, and APTES (115 ␮L, 0.48 mmol) was added to the
toluene suspension of MCM-41 and refluxed for 24 h. The resulting
material was filtered, air dried, and then protonated by suspend-
ing in dichloromethane (10 mL) with 2 equiv. of concentrated
nitric acid with respect to the amount of APTES originally added
3.1. Synthesis of catalyst [ꢀ-SiW10O34(H2O)2]4 @MCM-41 (I)
−
The catalyst [␥-SiW10O34(H2O)2]4 was immobilized upon
MCM-41 by suspending MCM-41 in a acetonitrile solution of cat-
alyst 1 for 24 h followed by filtration and rinsing with MeCN.
The catalyst loading was estimated by monitoring the change in
absorbance at 270 nm in the acetonitrile solution after uptake, and
the decrease in absorbance (32%) corresponds to an estimated load-
ing of 6% (0.019 mmol/g).
−
(
74 ␮L). [34] The product of this synthesis is designated MCM-41-
+
NH3
.

1
90
D.J. Thompson et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 188–193
Scheme 1. Functionalization and impregnation of MCM-41 silica. Conditions: (a) 6 M HCl reflux; (b) dried in oven at 100 C and refluxed in tolune with attached Dean-Stark
trap; (c) reflux in toluene with APTES; (d) suspend in DCM with 2 equiv. HNO3; (e) suspend in acetonitrile solution of 1.
.2. Synthesis of catalyst [ꢀ-SiW10O34(H O) ] @MCM-41-NH +
4
−
3
2 2 4
(
II)
The catalyst II was prepared by first functionalizing the MCM-
4
1 with aminopropyl triethoxy silane (APTES) in toluene reflux
as shown in Scheme 1. The functionalized product was then
protonated with nitric acid in dichloromethane. Catalyst 1 was
introduced into the pores of the functionalized MCM-41 and the
loading was estimated in the same manner as that for I [30]. The
absorbance of the impregnation solution showed a 40% decrease
in absorbance, which corresponds to an approximate 8% uptake
(
0.025 mmol/g).
3.3. Characterizations of catalysts I and II
Both catalysts I and II, as well as pristine MCM-41 were
characterized with nitrogen sorption (Fig. 2a) and powder X-ray
diffraction (XRD) (Fig. 2b). From the nitrogen sorption, the BET
2
2
surface area of MCM-41, I, and II were 962 m /g, 540 m /g, and
2
3
3
4
0
18 m /g, and the pore volumes were 0.85 cm /g, 0.46 cm /g and
3
.50 cm /g, respectively, indicating the decreases in surface area
and pore volume upon the loading of catalysts (Table 1). The pore
diameters of MCM-41 and I are ca. 27 A˚ , but is reduced to 22 A˚ for
II. The nitrogen sorption isotherms of I and MCM-41 are quite sim-
ilar in shape, indicating that physical loading of catalyst induced
minimal change of the mesoporous structure. On the other hand,
the nitrogen sorption isotherm of II is significantly different from
that of MCM-41 or I, especially in the deviation of the shape of
the hysteresis loop from the typical H -type in MCM-41 and I.
1
These changes imply a partial loss of structural integrity and an
overall pore shrinking, likely occurred during the functionalization
process. This supposition is confirmed by the powder XRD study.
Compared with the diffraction pattern of MCM-41, the character-
◦
istic (1 0 0) peak around 2Â of 2.1 is diminished in intensity and
severely broadened for catalyst II, and the (1 1 0) and (2 0 0) peaks at
◦ ◦
3
.6 and 4.3 disappeared, evident of the deterioration of the meso-
porous ordering. In contrast, the diffraction pattern of catalyst I is
almost identical to that of MCM-41.
3.4. Sulfide oxygenation reactions with catalyst I
In order to test the physisorbed catalyst, I was subjected to
repeated catalytic cycles of the conversion of methyl phenyl sulfide
Fig. 2. (a) Nitrogen sorption isotherms and (b) powder XRD patterns of catalysts I,
II and pristine MCM-41.

D.J. Thompson et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 188–193
191
Table 1
Summary of the physicochemical properties and the loading ratio of MCM-41, catalyst I and II.
2
3
Material
Surface area (m /g)
Pore volume (cm /g)
Diameter (Å)
Loading (wt%)
MCM-41
Catalyst I
Catalyst II
962
540
418
0.85
0.46
0.50
27
27
22
N/A
6
8
Table 2
Catalytic oxygenation of methyl phenyl sulfide with I.a
Reaction number
3 h
5 h
%
MPS
% MPSO
% MPSO2
% MPS
% MPSO
% MPSO2
TOFb
1
2
3
4
5
6
47
21
21
27
32
47
45
64
64
60
55
43
7
14
12
11
9
10
7
6
7
11
19
77
72
77
77
76
69
11
18
15
15
13
10
20
22
21
21
20
18
8
a
4−
Each of the duplicate catalytic reactions were completed with 0.2 g of catalyst I (1 mol% of the catalyst [␥-SiW10O34(H2O)2] with respect to 0.50 mmol MPS) in 5 mL of
acetonitrile. The reaction slurry was stirred for 15 min before initiation with 2.0 equiv. of hydrogen peroxide. After the reaction was completed, the powder catalyst I from
both reactions was collected via filtration, rinsed well with acetonitrile, and air dried. For the subsequent reactions, the material was split in half and the next recycle was
repeated the same way as the first.
b
Turn over frequency (h−1) = {[RR’SO] + 2[RR’SO2]}/{[cat] × time (h)}.
3
h, and 72% sulfoxide 18% sulfone at 5 h. Subsequent recycles (reac-
tions 3–6) showed a slight decrease in formation of both MPSO and
MPSO , which corresponds to a drop in the TOF over 5 h from 22 to
2
1
−
1
8 h
While the catalyst I certainly is reusable, its effectiveness began
.
to decrease noticeably after the fourth reaction. The only exception
is that the second reaction was faster than the first, which is likely
attributed to the opening of the pores [21]. All subsequent reactions
show apparent loss of reactivity at the sample time of 3 h. This may
be due to the loss of 1 through leaching from the silica pores. The
diffuse reflectance UV–vis spectra of catalyst before and after the
recycles were recorded (Fig. 3), which showed a clear decrease in
the absorbance at 270 nm. This lends credence to the postulation
that the catalyst has been washed out of the MCM-41 support.
Scheme 2. Oxygenation of methyl phenyl sulfide (MPS) to methyl phenyl sulfoxide
and sulfone (MPSO and MPSO2).
(
MPS) to methyl phenyl sulfone (MPSO ) by hydrogen peroxide
2
(Scheme 2). In general, methyl phenyl sulfide was treated with
2
equiv. of hydrogen peroxide and 1 mol% of supported catalyst,
and the actual conditions and results are summarized in Table 2. At
the end of each reaction cycle, catalyst was recovered by filtration
and washed with copious amount of acetonitrile.
For reaction 1 (the initial cycle), productions of MPSO and
MPSO2 are 45% and 7% at 3 h, and 77% and 11% at 5 h, respec-
tively. Upon the first recycling (reaction 2), the product distribution
reveals an increased activity with 64% sulfoxide and 14% sulfone at
3.5. Recycling reactions of II
To assess the efficacy of II, catalytic runs with II were performed
in the same fashion as that of catalyst I. For reaction 1 (the initial
cycle), as shown in Table 3, productions of MPSO and MPSO2 are
5
5% and 3% at 3 h, and 78% and 9% at 5 h, respectively. Similar to I,
a slight increase in conversion was observed between reactions 1
and 2. It is noteworthy that there is no loss of catalytic activity from
the second to the last (sixth) reactions.
Overall, the TOFs at 5 h, ranging from 18 to 22 h−1, reveal no sign
of catalyst deactivation. The diffuse reflectance spectral measure-
ments (Fig. 4) showed that the characteristic absorption at about
2
70 nm was less drastically affected than catalyst I over the course
of the recycling reactions, further indicating a much improved
retention of the POM catalyst.
3.6. Stability of I versus II
Shown in Fig. 5a and b are the reaction profiles of four recycles
for each catalyst, where the percentage reaction yield is defined as
SO]% + 2[SO ]%, namely the oxygenation yield. For catalyst I, the
[
2
reaction yield dropped 39% at 3 h and 23% at 5 h between the second
and the sixth reactions. On the other hand, while there is some
variability among the reactions, the percent yield increases 9% at
3
h and 5% at 5 h from the second to the sixth reaction.
In order to investigate the stability of the heterogeneous cata-
Fig. 3. Diffuse reflectance spectra of catalyst I before and after six reactions, along
with that of pristine MCM-41 and solution spectrum of [␥-SiW10O34(H2O)2] in
acetonitrile.
4
−
lyst, it is most productive to investigate reaction progress before

1
92
D.J. Thompson et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 188–193
Table 3
Recycling summary of II with methyl phenyl sulfide for model substrate.
a
Reaction number
3 h
5 h
%
MPS
% MPSO
% MPSO2
% MPS
% MPSO
% MPSO2
TOFb
1
2
3
4
5
6
40
39
36
58
32
27
55
55
56
39
59
62
3
5
7
0
2
6
9
5
8
11
5
4
78
78
77
77
77
81
9
13
12
7
14
14
19
21
20
18
21
22
a
All entries are averages of two reactions. Reactions were completed in an analogous fashion as those described in Table 2.
Turn over frequency (h ) = {[RR’SO] + 2[RR’SO2]}/{[cat] × time (h)}.
b
−1
Fig. 4. Diffuse reflectance of catalyst II before and after six reactions.
completion of the reaction [16]. Since the S → SO reaction is sig-
nificantly faster than the SO → SO reaction, earlier times are
2
dominated by the S → SO reaction, after which the reaction is dom-
inated by the much slower SO → SO . Thus, any deactivation of the
2
heterogeneous catalyst will be more obvious at an earlier time. As
shown in Fig. 5, comparing the recyclability of I to II, the deacti-
vation of Catalyst I is most noticeable at 3 h. At this time the TOF
−
1
for catalyst I decreases from 31 to 20 h while the for catalyst II
−
1
actually the TOF increases slightly from 22 to 25 h
.
The decrease in activity for I is likely caused by the loss of catalyst
back into acetonitrile solution, while the slight increase in activity
for II is possibly due to the reduction of pore blockage during the
course of catalytic runs [21]. This demonstrates that the additional
effort to functionalize the MCM-41 silica is likely to be worthwhile.
3.7. Catalytic oxygenation of other substrates
The reactivity of catalyst II was examined with several other
substrates shown in Scheme 3 under conditions similar to those
of MPS. These substrates were chosen for their diverse electronic
and steric characteristics. For methyl p-tolyl sulfide (MpTS), the
para-methyl substituent enhances the electron richness of the sul-
fur center. The other substrates are models of refractory sulfides,
and their oxygenation is relevant to sulfur removal from petroleum
via oxidative desulfurization (ODS) [4].
Fig. 5. Comparison of recyclability of I and II Selected reactions of I (a) and II (b).
the reaction of PPS yielded 58% sulfoxide and 40% sulfone, while
the reaction of DBT produced 90% sulfone with 9% unreacted DBT.
The outcome of these reactions highlights the potential utility of II
in the removal of refractory sulfides via ODS.
The oxygenation of MpTS was performed at room temperature
and the reaction was terminated at 4 h. The oxygenation reactions
of the model refractory sulfides were carried out at a mildly ele-
3.8. Effect on selectivity
◦
vated temperature (40 C) and terminated at 24 h. As shown in
Fig. 6, the smaller sulfides, MpTS and BT, were converted to the
corresponding sulfones exclusively. Among the larger substrates,
In the corresponding homogeneous reactions, catalyst
was reported to convert sulfide to sulfoxide entirely before the
1

D.J. Thompson et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 188–193
193
4. Conclusion
Two approaches of a heterogenized polyoxoanion catalyst 1
onto MCM-41: physisorbed and chemisorbed, have been devel-
oped. While the physisorption of catalyst into the pores of
MCM-41 resulted in minimal alteration to the porous structure,
ligand-grafting process led to partial loss of crystallinity in the
chemisorbed catalyst. The recyclability of both physisorbed and
chemisorbed catalysts were tested, and the latter is clearly advan-
tageous in maintaining full reactivity throughout recycling. Further
investigation of the chemisorbed catalyst revealed its utility for
converting organic sulfides, especially refractory sulfides such as
DBT, into corresponding sulfones, and the impact of mass transport
limitation on the stepwise nature of sulfide oxygenation. Further
investigation will be focused on improving the functionalization
procedure to minimize the loss of structural integrity, increasing
the rates of these reactions, and limiting the loss of selectivity.
Scheme 3. The four additional substrates.
Acknowledgements
We gratefully acknowledge the financial support from Purdue
University and the Army Research Office (W911NF-06-1-0305). We
also thank Prof. Fabio Ribeiro and Mr. Dhairya Mehta of Purdue
School of Chemical Engineering for nitrogen sorption characteriza-
tion.
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with MPS and PPS as substrates and II as the catalyst. As shown in
Table 4, the selectivity for sulfoxide is 90% with MPS, and 75% with
PPS. This is probably explained by the reduced mass-transport
efficiency in a supported catalyst, which enhances the successive
oxygenation of substrate. The loss of selectivity is significantly
greater for PPS since it is bulkier than MPS.
[
[
[
[
[
[
[
[
24] J.A.F. Gamelas, D.V. Evtuguin, A.P. Esculcas, Transit. Met. Chem. 32 (2007) 1061.
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Table 4
Selectivity loss upon impregnation.a
Analyte
Homogeneous [12]
Heterogeneous
(
2001) 1074.
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19 (2010) 119.
MPS
MPSO
MPSO2
Selectivity
0
100
0
13
77
8
3
[
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Sci. 303 (2006) 32.
30] W. Kaleta, K. Nowinska, Chem. Commun. (2001) 535.
b
100%
90%
[
PPS
PPSO
PPSO2
Selectivity
1
99
0
16
61
20
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Chu, D.H. Olson, E.W. Sheppard, J. Am. Chem. Soc. 114 (1992) 10834.
[33] Q.S. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147.
b
100%
75%
[
34] B. Karimi, M. Ghoreishi-Nezhad, J.H. Clark, Org. Lett. 7 (2005) 625.
a
Reactions completed with 1.0 equiv. H2O2 with respect to 0.5 mmol sulfide in
mL acetonitrile. Reactions were sampled at 24 h to ensure completion.
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bitsina, M.S. Melgunov, E.B. Burgina, J. Mrowiec-Bialon, A.B. Jarzebski, C.L. Hill,
J. Catal. 226 (2004) 363.
5
b
Calculated by (% RSOR/% RSOR + % RSO2R) × 100%.