Carbon-supported metal-modified lacunary tungstosilicic polyoxometallates used as catalysts in the selective oxidation of sulfides

Article abstract of DOI:10.1016/j.molcata.2015.02.021

Lacunary tungstosilicic polyoxometallates modified with transition metal ions [SiW₁₁O₃₉M(H₂O)]^6-, where M = Mn²⁺, Fe²⁺, Co²⁺or Cu²⁺, were synthesized and supported on activated carbon to obtain the SiW₁₁MC catalysts. The samples were characterized by FT-IR, XRD, N₂ adsorption-desorption measurements, and the acidic properties were determined using the isopropanol dehydration test reaction. The activity and selectivity of the catalysts were evaluated in the selective oxidation of a series of sulfides to sulfoxides or sulfone. The reaction was carried out in acetonitrile as solvent using H₂O₂ 35% p/V as a clean oxidant. The conversion values decreased in the following order: SiW₁₁MnC > SiW₁₁FeC > SiW₁₁CuC > SiW₁₁CoC. The catalysts were reused without appreciable loss of their catalytic activity. It was found that the activity of the catalysts decreases in parallel with the increment in the reduction temperature values. The most easily reducible catalyst displayed the highest conversion values. We found a convenient and selective procedure for oxidizing sulfides to sulfoxides or sulfones using aqueous hydrogen peroxide and a catalytic amount of lacunary tungstosilicic polyoxometallates supported on carbon at low temperatures (20-50 °C) in a reasonably short reaction time

Full text of DOI:10.1016/j.molcata.2015.02.021

Accepted Manuscript
Title: Carbon-supported metal-modified lacunary
tungstosilicic polyoxometallates used as catalysts in the
selective oxidation of sulfides
´
Author: Romina Frenzel Angel G. Sathicq Mirta N. Blanco
Gustavo P. Romanelli Luis R. Pizzio
PII:
S1381-1169(15)00078-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.02.021
MOLCAA 9437
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Accepted date:
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21-2-2015
23-2-2015
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P.Romanelli, Luis R.Pizzio, Carbon-supported metal-modified lacunary tungstosilicic
polyoxometallates used as catalysts in the selective oxidation of sulfides, Journal of
Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.02.021
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Carbon-supported metal-modified lacunary tungstosilicic
polyoxometallates used as catalysts in the selective oxidation of
sulfides
Romina Frenzel, Ángel G. Sathicq, Mirta N. Blanco, Gustavo P. Romanelli*, Luis R. Pizzio*
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA),
CCT-La Plata-CONICET, Departamento de Química, Facultad de Ciencias Exactas, Universidad
Nacional de La Plata, 47 Nº 257 (B1900AJK) La Plata
Corresponding authors:
Dr. Luis R. Pizzio and Gustavo P. Romanelli,
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA),
CCT-LaPlata-CONICET, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,
47 Nº 257 (B1900AJK) La Plata, Argentina.

e-mail: lppizzio@quimica.unlp.edu.ar and gpr@quimica.unlp.edu.ar
Graphical abstract
Fx1
Highlights
Metal modified lacunary tungstosilicic polyoxometallates supported on carbon
Catalytic selective oxidation of sufides to sulfoxides and sulfones
[SiW11O39]8- was modified with transition metal ions where M= Mn2+, Fe2+, Cu2+or Co2+
The catalysts can be reused without appreciable loss of the catalytic activity
The reaction is useful due to status of compounds in biological molecules synthesis
Abstract
Lacunary tungstosilicic polyoxometallates modified with transition metal ions
[SiW11O39M(H2O)]6-, where M= Mn2+, Fe2+, Co2+or Cu2+, were synthesized and supported on
activated carbon to obtain the SiW11MC catalysts. The samples were characterized by FT-IR,
XRD, N2 adsorption–desorption measurements, and the acidic properties were determined using
the isopropanol dehydration test reaction.
The activity and selectivity of the catalysts were evaluated in the selective oxidation of a
series of sulfides to sulfoxides or sulfone. The reaction was carried out in acetonitrile as solvent
using H2O2 35% p/v as a clean oxidant. The conversion values decreased in the following order:
SiW11MnC > SiW11FeC > SiW11CuC > SiW11CoC. The catalysts were reused without
appreciable loss of their catalytic activity. It was found that the activity of the catalysts
decreases in parallel with the increment in the reduction temperature values. The most easily
reducible catalyst displayed the highest conversion values. We found a convenient and selective
procedure for oxidizing sulfides to sulfoxides or sulfones using aqueous hydrogen peroxide and a
catalytic amount of lacunary tungstosilicic polyoxometallates supported on carbon at low

temperatures
(20–50
ºC)
in
a
reasonably
short
reaction
time.
Keywords: Supported metal-substituted polyoxometallates, carbon, sulfide oxidation, hydrogen
peroxide
1.
Introduction
The development of new catalytic systems for selective organic synthesis is one of the
challenging tasks in Organic Chemistry. In recent years, the search of ecofriendly benign
chemical methodologies has received much attention from chemistry communities, because they
are essential for the conservation of the global ecosystem. From this viewpoint, in the last two
decades, very useful procedures involving catalysis and hydrogen peroxide, as oxidant, have
been developed. They promote the efficient organic substrate oxidation, due to their low cost,
effective oxygen content, safety in storage and operation, and mainly the ecofriendly character of
hydrogen peroxide [1–3].

Catalysis using heteropolyacids and related compounds is a field of increasing
importance worldwide. Numerous developments are being carried out in industrial processes and
basic research. Heteropolyacids present a very strong acidity and redox properties, which can be
changed by varying the chemical composition of the heteropolyanion. Different transformations
catalyzed by heteropolyacids in heterogeneous and homogeneous phases have been reviewed [4–
7]. Recently, our research group has used Keggin and Wells-Dawson heteropolyacids/aqueous
hydrogen peroxide systems, in a serie of oxidation processes, including phenol [8], alcohol [9],
amine [10], sulfide [11], and 1,4-dihydropyridine [12] oxidation.
Transition metal-modified lacunary heteropolycompounds with Keggin structure, obtained
by modification of the [PW11O39]7- lacunary phase with ions of the transition metals Ni (II), Fe
(III), Co (II), Zn (II), Mn (II) or Cu (II) have been used as catalysts in the oxidation of
cyclohexane with aqueous H2O2 [13], in methyl isobutyrate and cyclohexene with oxygen [14],
and in the oxidation of 2-(methylthio)-benzothiazole with aqueous H2O2 [15]. However,
transition metal-modified lacunary heteropolycompounds with Keggin structure, obtained by
modification of the [SiW11O39]8- lacunary phase with ions of the transition metals, have been
little studied in catalytic oxidation processes.
Meanwhile, sulfoxides and sulfones are important intermediates in organic chemistry due to
their application in basic and applied research, especially because they are versatile intermediates
for the preparation of biologically important products [16], including therapeutic agents such as
antiulcer, antibacterial, antifungal, antiatherosclerotic, antihypertensive and cardiotonic agents as
well as psychotropics and vasodilators [3, 17-18]. Omeprazole and dapsone are two typical
examples of the extensive application in the pharmaceutical industry. Omeprazole (one of the
most widely prescribed drugs internationally) is a proton pump inhibitor used in the treatment of
dyspepsia, peptic ulcer disease, and gastroesophageal and laryngopharyngeal reflux [19], and

dapsone, an antibacterial most commonly used for the treatment of leprae infections and in the
treatment of malaria [20] (Figure 1).
(Figure 1)
The most appropriate method for the preparation of sulfoxide and sulfone compounds is the
selective oxidation of the corresponding sulfide. In general, the system formed by the
oxidant/catalyst is the same for the selective oxidation of sulfides to sulfoxides as that of
sulfones. The only change is often the more drastic reaction conditions required for obtaining
sulfones. Hydrogen peroxide may be used as an electrophilic reagent for the direct selective
oxidation of sulfides to sulfoxides or sulfones. However, the rate of reaction can be greatly
improved by the use of a catalyst [3]. Some of the most recent catalysts are: silica-vanadia [21],
oxalic acid dihydrate [22], metalloporphyrin [23], p-toluenesulfonic acid [24], sulfamic acid
[25], titanosilicates [26], heteropolyacids [27], and many others.
The objective of this paper is to study both the use of carbon-supported transition metal-
modified lacunary tungstosilicic polyoxometallates as catalysts in the selective oxidation of
sulfides to sulfoxides or sulfones (Scheme 1) and the effect of the metal introduced in the
heteropolyacid structure, on the activity and selectivity on the reaction test. A relation between
the substrate structure and conversion rate was also found.
(Scheme 1)
2. Experimental
2.1. Catalyst synthesis

2.1.1. Transition metal-modified lacunary tungstosilicic polyoxometallate (TMPOM) synthesis
The TMPOM ions [SiW11O39M]6−, (M = Mn2+, Fe2+, Cu2+, and Co2+) were synthetized
according to the literature data [28–30]. An aqueous solution of H4SiW12O40·23H2O was
alkalinized with aqueous NaHCO3 solution up to a pH value of 5.0–5.5, which resulted in the
formation of the lacunary heteropolyanion [SiW11O39]8−. In order to introduce the transition
metal ion into the octahedral lacuna, the obtained solution of [SiW11O39]8− and an aqueous
solution of the transition metal salt were mixed and stirred at 60–90°C. The sodium salts of the
lacunary heteropolycompounds were obtained by solvent evaporation and recrystallization from
water. They were dried at 70°C before their characterization by thermal and physicochemical
techniques.
2.1.2. Supported catalyst synthesis
The support used was a commercial activated carbon ground to a mean particle size of 1
mm, previously washed with 0.1 N solutions of NaOH and HCl, and then treated with a HNO3
30% solution under heating to reflux for 2 h. It has a surface area of 806 m2/g, and a mean pore
diameter below 2 nm. The carbon was impregnated using the incipient wetness method, with
[SiW11O39M]6− water solutions (0.14 M), in order to add 1.0 10−4 mol of the lacunary
heteropolycompounds per gram of carbon. The catalysts were kept at room temperature till
dryness and then thermally treated at 70 °C for 24 h. They will be named SiW11MC (M = Mn2+,
Fe2+, Cu2+, and Co2+). A fraction of the solids obtained were washed with ethanol, acetonitrile or
toluene, for 24 h. For this leaching experiment, 1 g of solid was placed in contact with 4 mL of
solvent at 20 °C under constant stirring for a selected time.
The tungsten concentration in the solutions, after leaching, was determined by atomic
absorption spectrometry. The calibration curve method was used, with standards prepared in the

laboratory. An IL Model 457 spectrophotometer, with single channel and double beam, and
monochromator with a 330 mm focal distance, was used. The light source was a hollow
monocathode lamp. The analysis was carried out at wavelength 254.9 nm, bandwidth 0.3 nm,
lamp current 15 mA, phototube amplification 800 V, burner height 4 mm, and acetylene–nitrous
oxide flame (11:14).
2.2. Catalyst characterization
2.2.1 FT-IR spectroscopy
Fourier transform infrared spectroscopy (FT-IR) of [SiW11O39M]6− sodium salts and
catalysts dried at 70 °C for 24 h was obtained in the 400–4000 cm-1 wavenumber range using
Bruker IFS 66 FT-IR spectrometer.
2.2.2 X-ray diffraction
X-ray diffraction (XRD) patterns were recorded on the same samples that had been
analyzed by FT-IR. The equipment used to this end was Philips PW-1732 with built-in recorder,
using Cu Kα radiation, nickel filter, 30 mA and 40 kV in the high-voltage source, and scanning
angle between 5 and 60° of 2θ at a scanning rate of 1° per min.
2.2.3. SEM analysis
The distribution of [SiW11O39M]6− anions along the width of the carbon granules was
measured with a Philips Model 505 scanning electron microscope with an energy dispersive X-
ray analysis (EDAX) system, following a technique previously reported [31, 32].
2.2.4. Isopropanol decomposition
The isopropanol dehydration test reaction was carried out in a conventional flow fixed-
bed reactor at atmospheric pressure and 180 °C. The gas carrier was helium and the isopropanol
volumetric flow was 0.126 cm3/min in a total flow of 50 cm3/min. The reaction products were

quantified by gas chromatography, using a thermal conductivity detector. The specific
isopropanol conversion (CSPROP) was calculated under differential conversion conditions
(conversions lower than 10%) and expressed as mol propene/mol SiW11M. s
2.2.5 Temperature programmed reduction
Temperature programmed reduction (TPR) of the samples dried at 70 °C (100 mg, 60 to
100 mesh) was obtained using a mixture of H2 (5%) in Ar flowing at 90 mL/min. The heating
rate was kept at 10 °C/min until reaching a temperature of 1000 °C. The amount of H2 consumed
during reduction was determined using a thermal conductivity detector.
2.3. Catalytic test
General remarks
All reagents were purchased from Merck and Aldrich and used without further
purification. The product distribution was quantified by GC analysis and reactions were also
monitored by thin layer chromatography analyses. The products were characterized by the
determination of the mass spectra. All yields refer to isolated products after purification.
Sample analysis
Samples were withdrawn from the organic phase at 15-minute intervals. Each volume
sample was approximately 20 μL and it was diluted with 1–2 mL of acetonitrile, the
concentrations were calculated for the determination of the corresponding areas. The capillary
column was a 30 m Chromopack CP Sil 8 CB, 0.32 mm in diameter.
General procedure for the oxidation of sulfide to sulfoxide

A solution of sulfide (1 mmol) and catalyst (100 mg), in acetonitrile (9 mL), was added to
H2O2 35% (w/v) (2 mmol). The mixture was stirred at 20 °C for a time period (see Tables 1 and
2). The solvent was evaporated and then H2O (5 mL) was added. The substrate was extracted
with toluene (2 x mL) and dried with anhydrous Na2SO4; filtration and evaporation afforded the
corresponding sulfoxides. The solid crude sulfoxides were purified by recrystallization to afford
the pure sulfoxides.
General procedure for the oxidation of sulfide to sulfone
A solution of sulfide (1 mmol) and catalyst (100 mg), in acetonitrile (9 mL), was added to
H2O2 35% (w/v) (10 mmol). The mixture was stirred at 50 °C for a time period (see Tables 1 and
2). The solvent was evaporated and then H2O (5 mL) was added. The substrate was extracted
with toluene (2 x mL) and dried with anhydrous Na2SO4; filtration and evaporation afforded the
corresponding sulfoxides. The crude solids were purified by recrystallization to afford the pure
sulfones.
Electronic densities of sulfur atom determination
The densities of the sulfur atoms in the sulfide were determined after optimization of
their conformation. The conformations of the compounds were drawn by means of the “model
build” modulus available in HyperChem 5.0. Each molecular structure was firstly preoptimized
with the Molecular Mechanics Force Field (MM+) procedure, and the resulting geometry was
further refined by For Peer Review Only means of the semiempirical method PM3 (Parametric
Method-3). A gradient norm limit of 0.01 kcal Å−1 was chosen.
Electronic impact mass spectra of the representative compounds

Diphenyl sulfoxide: EIMS, m/z (Relative intensity): 202 (M+, 83%), 185 (18%), 154 (73%), 109
(90%), 97 (48%), 77 (78%), 65 (59%), 51 (100%), 39 (32%).
Diphenyl sulfone: EIMS, m/z (Relative intensity): 218 (M+, 36%), 153 (7%), 125 (100%), 97
(12%), 77 (37%), 51 (23%).
Di-n-butyl sulfoxide: EIMS, m/z (Relative intensity): 162 (M+, 5%), 145 (11%), 106 (32%), 89
(45%), 57 (58%), 41 (98%), 29 (100%).
Benzyl phenyl sulfoxide: EIMS, m/z (Relative intensity): 216 (M+) (10%), 182 (16%), 165 (2%),
125 (5%), 91 (100%), 77 (11%), 65 (18%).
Di-n-butyl sulfone: EIMS, m/z (Relative intensity):178 (M+, 3%), 149 (2%), 123 (32%), 81
(4%), 57 (100%), 41 (52%), 29 (56%).
Ethyl phenyl sulfone: EIMS, m/z (Relative intensity): 170 (M+, 28%), 142 (18%), 141 (23%),
125 (8%), 94 (56%), 78 (71%), 77 (100%), 51 (55%).
Benzyl phenyl sulfone: EIMS, m/z (Relative intensity): 232 (M+, 4%), 167 (2%), 165 (2%), 152
(15%), 91 (100%), 77 (11%), 65 (17%), 51 (13%).
Di-benzyl sulfone: EIMS, m/z (Relative intensity): 182 (M+, 10%), 91 (100%), 65 (24%), 39
(11%).
Phenyl methyl sulfone: EIMS, m/z (Relative intensity): 156 (M+, 29%), 141 (27%), 94 (33%), 77
(100%), 51 (8%), 39 (9%).
Methyl p-tolyl sulfone: EIMS, m/z (Relative intensity): 170 (M+, 28%), 155 (30%), 107 (22%),
91 (100%), 77 (7%), 65 (22%), 39 (11%).
1,1-Dioxide-2,3,4,5-tetrahydrothiofene (sulfolane): EIMS, m/z (Relative intensity): 120 (M+,
44%), 92 (14%), 56 (85%), 55 (69%), 41 (100%), 28 (60%).
Phenyl vinyl sulfone: EIMS, m/z (Relative intensity): 168 (M+, 18%), 125 (100%), 77 (64%), 51
(27%), 27 (6%).

4,4’-Diaminodiphenyl sulfone (dapsone): EIMS, m/z (Relative intensity): 248 (M+, 83%), 184
(8%), 140 (62%), 108 (100%), 92 (37%), 65 (48%), 39 (18%).
2-(Methylthio)-benzothiazole sulfoxide: EIMS, m/z (Relative intensity): 197 (62%) M+.; 182
(78); 153 (60); 151 (100); 150 (64); 134 (42%); 123 (15); 108 (25); 90 (26); 50 (30); 39 (98).
2-(Methylthio)-benzothiazole sulfone: EIMS, m/z (Relative intensity): m/z (I %): 197 (18) M+.;
150 (100); 134 (42); 108 (20); 90 (25); 50 (24); 39 (74).
3. Results and discussion
3.1. Catalyst characterization
The characteristics of XRD patterns of SiW11MC samples were similar to those of the
original carbon, so neither the diffraction lines of the Na6[SiW11O39M] salts, nor those of other
crystalline phases were detected. On the other hand, according to the results obtained by energy
dispersive X-ray analysis (EDAX), a uniform distribution of the modified lacunary Keggin
anions along the width of the carbon particles was obtained. This indicates that the species
present onto the support surface are highly dispersed as a noncrystalline form, owing to the
interaction with the support.
The FT-IR spectrum of Na8[SiW11O39] showed the main bands of the lacunar anion placed
at 1008, 959, 882, 869, 794, and 720 cm-1 (Figure 2a). The spectrum of Na5[SiW11O39Mn]
(Figure 2b) displayed bands at 1002, 959, 899, 810, 701, and 541 cm-1. They are in agreement
with those reported by Rocchiccioli et al. [33] and are considered typical of the transition metal-
modified lacunary heteropolycompounds [33]. The FT-IR spectra of the other Na6[SiW11O39M]
salts (M = Fe2+, Cu2+, or Co2+) displayed the same features.
On the other hand, the FT-IR spectra of the SiW11MC (M = Mn2+, Fe2+, Cu2+, and Co2+)
showed the characteristic bands of the [SiW11O39M]6- anion superimposed to those of the carbon
used as support. These results confirm that the structure of the metal-modified lacunary

heteropolycompounds remains unaltered after the interaction with carbon that takes place during
the impregnation.
According to the DTA studies, the thermal decomposition of sodium [SiW11O39M]6- takes
place at temperatures in the range 450–490 °C. The exothermic peaks assigned to
Na6[SiW11O39Mn] and Na6[SiW11O39Fe] decomposition were observed at 485 and 478 ºC,
respectively. These salts are thermally slightly more stable than Na6[SiW11O39Cu] and
Na6[SiW11O39Co], which showed the exothermic peak at 469 and 458 ºC, respectively. The
dehydration of the salts occurs at temperatures below 150 °C. Their DTA diagrams showed a
broad endothermic peak with a maximum at 60–70 ºC, which was reflected in the TGA diagram
as the appearance of continuous weight loss.
In the DTA diagrams of the SiW11MC samples, an endothermic peak with a maximum at
around 70 °C was observed. It was similar to that displayed by the support and ascribed to water
evolution. A wide exothermic signal with a maximum in the range 470–500 °C, assigned to
[SiW11O39M]6- decomposition, was observed. The decomposition temperature of transition
metal-modified lacunary tungstosilicic polyoxometallates was slightly enhanced as a result of
their interaction with the functional groups present on the support surface [34].
The TPR pattern of Na8SiW11O39 showed two peaks with maxima at 691 and 917 °C
(Figure 3). They appeared at temperatures higher than the decomposition temperature of the
[SiW11O39]8- anion and were assigned to the reduction of the species resulting from lacunary
anion decomposition. The same peaks, shifted to higher temperatures, were present in the
patterns of Na6[SiW11O39Mn] (711 and 917 ºC), Na6[SiW11O39Fe] (722 and 935 ºC),
Na6[SiW11O39Cu] (731 and 948 ºC), and Na6[SiW11O39Co] (742 and 972 ºC). Due to the
introduction of M in the octahedral lacuna, the reduction of the species resulting from
N6[SiW11O39M] decomposition takes place at slightly higher temperatures. These species may be

the constituent oxides of the decomposed salts. Gamelas et al. [35] have found WO3 as the main
component of the decomposition products of potassium salts of transition metal-modified
lacunary heteropolycompounds.
On the other hand, the TPR patterns of the SiW11MnC, SiW11FeC, SiW11CuC, and
SiW11CoC samples (Figure 4) showed a strong and wide peak with a maximum in the range
660–720 °C, overlapping the wide band of the support (480–750 ºC). Taking into account the
DTA results, they appeared at temperature values higher than the decomposition temperature of
the [SiW11O39M]6- anion in the SiW11MC samples. We can suggest that as result of the
interaction of the transition metal-modified lacunary tungstosilicic polyoxometallates with the
functional groups present on the support surface, the reduction of [SiW11O39M]6- anion
decomposition products takes place almost at the same temperature and only reduction peak in
the TGR patterns was displayed. For these samples, the reduction temperature (TPRSiW11MC
)
slightly increased according to the following order: SiW11MnC (662 ºC) < SiW11FeC (681 ºC)
< SiW11CuC (697 ºC) < SiW11CoC (711 ºC).
Isopropanol dehydration proceeds quickly on acidic sites [36] and has been used [37, 38]
as a test reaction for comparing the acidity of different catalysts. At temperatures lower than 300
ºC, isopropanol decomposition can form either propene and water (dehydration) or acetone and
hydrogen (dehydrogenation).
The results of isopropanol dehydration (CSPROP) catalyzed by SiW11MC samples are
shown in Table 1. We found that isopropanol decomposes to propene and only traces of
diisopropylether were detected by means of GCMS. According to these results, the acidy of the
samples decreases in the following order: SiW11MnC > SiW11FeC > SiW11CuC > SiW11CoC.
3.2. Catalytic activity

Effect of the nature of the transition metal on the catalytic activity of SiW11MC samples
Table 1 shows the obtained results for methyl p-tolyl sulfide conversion and selectivity to
the corresponding sulfoxide and sulfone using the prepared transition metal-modified lacunary
tungstosilicic polyoxometallates supported on carbon (SiW11MnC, SiW11FeC, SiW11CuC and
SiW11CoC). The experimental conditions were: 100 mg of catalyst, 1 mmol of substrate, 10
mmol of H2O2 (35 % (w/v)) and 9 mL of acetonitrile; reaction time and temperature of 60 min
and 40ºC, respectively. In these conditions, the corresponding sulfoxide and sulfone were
obtained with a high selectivity (98%–99%), free of other secondary products. The blank
experiment was performed in the absence of the catalyst using an excess of aqueous H2O2 (35%
(w/v), 10 mmol), and acetonitrile (9 mL) as reaction solvent. Under these conditions, the reaction
conversion was very low (10% at 60 min and 40 °C, Table 1, entry 1).
The complete conversion of methyl p-tolyl sulfide was obtained in 60 min using
SiW11MnC as catalyst (Table 1, entry 2). The conversion values reached using the other
SiW11MC samples were slightly lower (Table 1, entries 2–5) and decreased in the following
order: SiW11MnC > SiW11FeC > SiW11CuC > SiW11CoC. This order was opposite to that
found for the TPRSiW11MC values. In other words, the activity of the catalysts decreases in parallel
with the increment of the TPRSiW11MC values. So, the most easily reducible catalyst displayed the
highest conversion values.
Palermo et al. reported [39] that the oxidation of diphenyl sulfide catalyzed by Mo-based
Keggin heteropolyacids takes place through the formation of peroxo-molybdate species and the
subsequent nucleophilic attack of the sulfur atom in the sulfide on the peroxo species. Maciuca
et al. suggested that the electrophilicity of the peroxide oxygen of H2O2 is increased by an
oxometal group (M=O) in the heteropolycompound [40].

Additionally, the influence of acidity on the formation rate of peroxo complexes has been
reported [41] as well as the fact that the selective oxidation of sulfides to sulfoxides or sulfones
in the presence of aqueous H2O2 is promoted by Brönsted acidity [42]. Taking into account that
the acidity of the SiW11MC samples decreases in the same way as the catalytic activity and
previous reports [41, 42], we can suggest that both the easiness of reduction and the acidity
strongly affect the catalytic activity of the SiW11MC samples.
On the other hand, the selectivity of the catalysts to sulfone was sensitive to the type of
cation used in the modification of the lacunary tungstosilicic polyoxometallate and it decreased
in parallel with the conversion decrease (80%, 75%, 70 and 65%, respectively, for sulfone
production).
The possible mechanism for the oxidation of methyl p-tolyl sulfide to the corresponding
sulfoxide or sulfone using H2O2 in the presence of SIW11MC catalyts is outlined in Scheme 3.
The interaction of hydrogen peroxide with transition metal-modified lacunary tungstosilicic
polyoxometallate generates an electrophilic intermediate (peroxo oxygen/TMPOM), which
produces an electrophilic attack on the sulfur atom in the sulfide, generating the corresponding
sulfoxide (Scheme 2). The mechanism for the oxidation of the sulfoxide to the corresponding
sulfone is different. First, it involves the formation of a TMPOM-sulfoxide intermediate through
the nucleophilic attack on the tungsten atom in the TMPOM by the oxygen of the sulfoxide, and
then the nucleophilic attack of the sulfur atom in TMPOM-sulfoxide by H2O2 via an SN2
mechanism [43-45] (Scheme 2).
Scheme 2. Proposed mechanism for the oxidation of methyl p-tolyl sulfide to the corresponding
sulfoxide and sulfone with aqueous hydrogen peroxide in the presence of polyoxometallates

The selective oxidation of methyl-p-tolyl sulfide to the corresponding sulfoxide, in
homogeneous and heterogeneous system, using aqueous hydrogen peroxide, catalyzed by
heteropolyacids or related compounds such as: H4PMo11VO40 [46], H5PMo11Al0.5V0.5O40 [8],
PyHPMo11VO40 [47], [TBA]4[Al4(H2O)10(β-AsW9O33H2)2]4 [48], [TBA]4[-SiW10O36(PhPO)2]
[49], [TBA]4[-HPW10V2O40] [50], H3[PW12O40]-polyacrylamide based ammonium salt [51],
and [-1,2-H2SiW10V2O40] [52], were reported in the literature.
In all the above mentioned examples the reaction yields were high and very similar to
those obtained using the methodology reported in this work. However our SIW11MC catalysts
have some advantages for example: easy preparation, no moisture sensitive and very low toxicity
of the catalyst. Additionally, they could be used and easily recycled several times, without
appreciable loss of their catalytic activity.
Effect of the solvent on the reaction activity
Then, with the most active catalyst, SiW11MnC, we decided to study the effect of solvent
and catalyst reusability for the oxidation of methyl p-tolyl sulfide (Table 2). The reaction was
attempted with different solvents, such as ethanol, acetonitrile, and toluene. Complete subtract
conversion was obtained using ethanol and acetonitrile (Table 2, entries 1and 2). However, when
toluene was used as solvent, two phases were observed due to aqueous hydrogen peroxide
insolubility in toluene, and methyl p-tolyl sulfide conversion was significantly lower (Table 2,
entry 3).
The catalyst was recovered by filtration, washed with the reaction solvent, dried in
vacuum at 40 ºC and reused. The conversion remained practically constant when acetonitrile or
toluene was used as solvent. However, in the case of ethanol the conversion was remarkably

lower (Table 2, entry 1). This was in agreement with the leaching experiments that showed that
only when the catalysts were extracted with ethanol, an important loss of the [SiW11O39Mn]6−
lacunary anion (almost 22% of the original amount) took place.
For the SiW11MnC catalyst, an additional test was performed in order to evaluate the
possible catalyst solubilization. The sample was refluxed in acetonitrile at 40 ºC for 2 h, filtered
and dried in vacuum till constant weight. The activity of the so-treated catalyst was the same as
that of the fresh catalyst. The refluxed acetonitrile was used as solvent for attempting the
reaction without adding the catalyst. After 1 h a conversion similar to that of the blank
experiment (10%), was obtained.
Effect of the reaction conditions on the selectivity
The reaction conditions were optimized using methyl p-tolyl sulfide as substrate at 20°C.
The results of the study of the oxidation reaction of methyl p-tolyl sulfide with the use of
SiW11MnC as catalyst are summarized in Table 3. A conversion of 15% was obtained in the
absence of the catalyst at 20 ºC, in a time period of 10 h using 10 mmol of 35% (w/v) hydrogen
peroxide (Table 3, entry 1). However, when SiW11MnC catalyst was added (Table 3, entry 2),
the reaction took place efficiently obtaining a conversion of 75% with a 95% selectivity of the
corresponding sulfoxide in 10 h. Then, the effect of H2O2 amount on conversion and selectivity
was studied using four different H2O2 amounts: 1, 2, 5 and 10 mmol (Table 3, entries 2–5).
When 2, 5 and 10 mmol were used, a conversion of 100 % was obtained in 7, 5 and 4 h,
respectively; however, sulfoxide selectivity was very good only when 2 mmol of H2O2 was used
(conversion 100%, sulfoxide selectivity 91%), (Table 3, entry 3). This condition was considered
as optimal for the selective synthesis of sulfoxides.
In order to continue with the study of the reaction optimization, the effect of temperature
on the conversion and selectivity of methyl p-tolyl sulfide was analyzed using H2O2 in excess

(10 mmol) (Table 3, entries 5–8). As can be observed, at 20 °C, the reaction reached 100% of
methyl p-tolyl sulfide conversion, but the selectivity to sulfone was only 68% (Table 3, entry 5).
An increase of the reaction temperature improved the selectivity to sulfone values. For example,
at 50 °C and for 3 h, 100% selectivity to methyl p-tolyl sulfone was achieved (Table 3, entry 8).
Once the reaction conditions for the selective oxidation of sulfides to sulfoxides and
sulfones had been optimized, the reaction was extended to other different substrates. Diaryl
(Table 4, entries 1, 2 and 12), aryl-alkyl (Table 4, entries 3–6 and 10), dialkyl (Table 4, entries 7
and 8) and more complex sulfides (Table 4, entries 9, 11) were used as starting materials. In
Table 4, the third column shows the results obtained for the selective oxidation of these sulfides
to the corresponding sulfoxides. All the reactions were run within a reasonable short reaction
time and the sulfoxides were obtained in excellent yields (92%–59%), and only traces of
sulfones and other secondary products were detected. Therefore, the reaction condition is
selective to the transformation of sulfides to sulfoxides. For example, in Table 4 (entries 5 and
10), it can be seen that the hydroxyl and the formyl groups were not significantly affected under
these reaction conditions. On the other hand, the sulfide 4,4´-diamino-diphenyl sulfide (Table 4,
entry 12) can be oxidized to the corresponding sulfoxide or sulfone under the suitable reaction
conditions with good yields. For this substrate, low amounts of several secondary unidentified
compounds were detected by TLC. The optimized conditions were: 1 mmol of sulfide, 0.2 mL
of 35 % p/V H2O2, 100 mg of SiW11MnC catalyst, 9 mL of acetonitrile, 20 ºC, during a time
period depending on the substrate structure. The procedure allows obtaining dapsone (a leader
drug for leprae) with good yields (71%, Table 4, entry 12). Finally, the selective oxidation of
sulfide to the corresponding sulfone was obtained with excellent yields (Table 4), in short time
periods (2–4 h) by increasing the reaction temperature from 20 to 50 ºC.

Reaction chemoselectivity
In order to determine the chemoselectivity of the SiW11MnC catalyst under the
previously mentioned reaction conditions, some competitive reactions were studied. Thus, the
competitive oxidation of sulfides in the presence of benzylic alcohol, benzaldehyde, benzonitrile,
cyclohexene and methyl- p-tolyl sulfoxide were checked (Scheme 3). These experiment show
that the method is applicable for the chemoselective oxidation of sulfides to sulfoxides in the
presence of the previously mentioned functional groups and can be considered as a useful
practical method for the oxidation of sulfides to sulfoxides without appreciable oxidation of
other functional groups. The optimal reaction conditions for the chemoselective oxidation of
sulfides were: 1 mmol of sulfide, 1 mmol of the other substrate, 0.2 mL of 35 % p/V H2O2, 100
mg of SiW11MnC catalyst, 9 mL of acetonitrile, 20 ºC, after 7 h under continuous stirring.
Sulfide reactivity study
Recently, we found that there is a relationship between sulfide oxidation facility and the
sulfur atom charge obtained by a computational method [46]. For this reason, we explored again
the reactivity of the different sulfides tested in the present work, using a semi-empirical method.
The relationship between the electron densities of the sulfur atoms, which were estimated by
Hyperchem release 5, and the oxidative facility of the sulfur-containing compounds was
analyzed. The electron densities on the sulfur atom of the 12 sulfides, which varied between -
0.078 and 0.168, are shown in Figure 4.
Figure 4 shows a relationship between the substrate reactivity, evaluated as the reaction time
for a 100% conversion of sulfide, and the electron densities on the sulfur atom. From the results
obtained, it could be observed that dialkyl sulfides (-0.031, -0.047 and -0.078) are more reactive
than alkyl-aryl sulfides (0.076–0.097), and alkyl-aryl sulfides are more reactive than diaryl
sulfides (0.165 and 0.220). This different reactivity is associated with the electronic density on

the sulfur, since a higher electronic density on this atom facilitates the electrophilic attack of the
electrophilic intermediate peroxo oxygen/TMPOM.
Finally, in order to explore a possible reaction mechanism, an additional experiment was
performed using a radical inhibitor such as t-butanol. The experimental conditions were: 1 mmol
of methyl p-tolyl sulfide, 0.2 mL of H2O2 (35% w/v), 100 mg of SiW11MnC, 8 mL of
acetonitrile and 1 mL of t-butanol; reaction for 7 h at 20ºC. In these conditions, the conversion of
methyl p-tolyl sulfide (100%) and the selectivity of sulfoxide (91%) obtained were similar to
those reached without a radical inhibitor. This result indicates that the reaction does not take
place via a free radical path and it is in agreement with the suggested mechanism.
4. Conclusions
The sodium salts of metal-modified lacunary tungstosilicic polyoxometallates
[SiW11O39M(H2O)]6-, where M= Mn2+, Fe2+, Co2+or Cu2+, were successfully prepared and
supported on activated carbon to obtain the SiW11MC catalysts.
The [SiW11O39M]6- anions are the main species present in SiW11MC materials and they
are uniformly distributed along the width of the carbon particles. As a result of their interaction
with the support surface groups, the [SiW11O39M]6- anion temperature decomposition was
slightly enhanced.
The activity and selectivity of the catalysts were evaluated in the selective oxidation of
methyl p-tolyl sulfide to the corresponding sulfoxides or sulfones. The reaction was carried out
in acetonitrile as solvent using H2O2 35% p/v as a clean oxidant. The conversion values
decreased in the following order: SiW11MnC > SiW11FeC > SiW11CuC > SiW11CoC, and it
was found that the activity of the catalysts decreases in parallel with the increment in the
reduction temperature (TPRSiW11MC) values. The most easily reducible catalyst displayed the
highest conversion values.

In this research we found a convenient and selective procedure for oxidizing sulfides to
sulfoxides, using 35% (w/v) aqueous hydrogen peroxide and a catalytic amount of lacunary
tungstosilicic polyoxometallates supported on carbon. The reaction for sulfoxide preparation
was carried out at room temperature (20 °C), in a reasonably short reaction time.
Meanwhile, the selective oxidation of sulfides to sulfones required a temperature of 50 ºC
and aqueous hydrogen peroxide in excess. Under this condition, excellent yields of sulfone were
obtained.
The different reactivity of sulfides is associated with the electronic density on the sulfur
atom, since a higher electronic density on this atom facilitates the attack by the electrophilic
form of the hydrogen peroxide/catalyst system.
Using carbon-supported lacunary tungstosilicic polyoxometallates as catalysts, the
heterogeneous liquid-phase sulfoxidation of methyl p-tolyl sulfide takes place without significant
loss of the active phase, facilitating the separation and reuse. The use of aqueous hydrogen
peroxide gives only water as a byproduct, contributing to the field of clean technologies.
Acknowledgements
The authors thank Agencia Nacional de Promoción Científica y Tecnológica, CONICET
and Universidad Nacional de La Plata (Argentina) for the financial support and L. Osiglio, D.
Peña and N. Firpo for their collaboration in the experimental measurements.
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Figures
Figure 1.Bioactives sulfoxides and sulfones
Omeprazole
Dapsone

Figure 2.
FT-IR spectra of the Na8SiW11O39 (a) Transition metal-modified lacunary
tungstosilicic polyoxometallates salt Na6[SiW11O39M]6-, with M = Mn2+ (b), Fe2+
(c), Cu2+ (d), or Co2+ (e).
a
b
c
d
e
1200
1000
800
600
400
Wavenumber (cm-1)

Figure 3.
TPR patterns of Na8SiW11O39 (a) and of lacunary tungstosilicic
polyoxometallatessalt Na6[SiW11O39M]6-, with M = Mn2+ (b), Fe2+ (c), Cu2+ (d), or
Co2+ (e).

Figure 4.
TPR patterns of SiW11MnC (a), SiW11FeC (b), SiW11CuC (c), SiW11CoC (d),
and the carbon support.

Figure 5. Reaction time for 100% conversion of sulfide vs. Qs (charge on sulfur atom in the
corresponding sulfide)a.
aNumber 1-12 corresponding to the different sulfides (see Table 4).
Scheme1. Selective oxidation de sulfide to sulfoxide or sulfone
Scheme 2. Proposed mechanism for the oxidation of metil p-tolyl sulfide to the
corresponding sulfoxide and sulfone with aqueous hydrogen peroxide in
the presence of polyoxometalates