Host (aluminum incorporated mesocellulous silica foam (Al-MCF))-guest (tungsten polyoxometalate) nanocomposite material: An efficient and reusable catalyst for selective oxidation of sulfides to sulfoxides and sulfones

Article abstract of DOI:10.1016/j.catcom.2012.09.018

A green, efficient and selective approach for the oxidation of sulfides to sulfoxides and sulfones with H₂O₂ at room temperature is reported. The reaction is performed in the presence of tungstophosphoric acid (PW₁₂) supported on an aluminum incorporated mesocellulous silica foam (Al-MCF) (via both: the impregnation and encapsulation methods), as heterogeneous and reusable catalysts. The structural and textural characterizations of these catalysts were done using FTIR, X-ray diffraction, N₂ adsorption-desorption and TEM.

Full text of DOI:10.1016/j.catcom.2012.09.018

Catalysis Communications 29 (2012) 48–52
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Host (aluminum incorporated mesocellulous silica foam (Al-MCF))-guest (tungsten
polyoxometalate) nanocomposite material: an efficient and reusable catalyst for
selective oxidation of sulfides to sulfoxides and sulfones
b
c
c
Razieh Fazaeli a, , Hamid Aliyan , Mohammad Ali Ahmadi , Saeedeh Hashemian
⁎
a
Department of Chemistry, Shahreza Branch, Islamic Azad University, 86145‐311, Iran
Department of Chemistry, Mobarakeh Branch, Islamic Azad University, 84815‐119, Iran
Department of Chemistry, Yazd Branch, Islamic Azad University, 89195‐155, Iran
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2012
Received in revised form 11 September 2012
Accepted 12 September 2012
Available online 19 September 2012
A green, efficient and selective approach for the oxidation of sulfides to sulfoxides and sulfones with H2O2 at
room temperature is reported. The reaction is performed in the presence of tungstophosphoric acid (PW12
supported on an aluminum incorporated mesocellulous silica foam (Al-MCF) (via both: the impregnation
and encapsulation methods), as heterogeneous and reusable catalysts. The structural and textural character-
izations of these catalysts were done using FTIR, X-ray diffraction, N2 adsorption–desorption and TEM.
© 2012 Elsevier B.V. All rights reserved.
)
Keywords:
Tungstophosphoric acid
Mesocellular silica foams
Sulfides
Sulfoxides
Sulfones
1. Introduction
encapsulated and impregnated methods) for the selective oxidation
of sulfides to sulfoxides and sulfones (Scheme 1). The series of sul-
Oxidation of sulfides is the most straightforward method for the
synthesis of sulfoxides and sulfones [1], some examples of which
are important as commodity chemicals and, in some cases, as phar-
maceuticals [2]. This transformation has been performed in various
ways [3–6]. However, the reported methods rarely offer the ideal
combination of simplicity of method, rapid and selective reactions,
and high yields of products and often suffer from a lack of generality
and economical applicability.
fides selected for these studies is shown in Scheme 2. The series in-
cludes aliphatic and aromatic sulfides (a–h), olefinic sulfides (i),
ester (j), ketosulfides (k), alcohol (l) and olefinic sulfides (m, n).
2. Experimental
2.1. Preparation of Al-MCF
Mesocellular silica foam (MCF) is a newly reported aerogel-like
mesoporous material composed of large uniform spherical cells (up
to 50 nm) interconnected with uniformly sized windows featuring
a continuous three dimensional (3D) mesopore system [7]. Especial-
ly given its continuous 3D mesopore system with ultra‐large pore di-
ameters and interconnected windows, MCF materials have the
advantage over their more ordered counterparts such as MCM-41
or SBA-15 having better diffusion of reactants and products and
thus overcome internal mass transfer limitations [8].
The synthesis of Al-MCF using the pH adjusting method was car-
ried out as follows: 2 g of triblock copolymer P123 (EO20PO70EO20
)
was dissolved in 75 ml of HCl solution (pH=1.5) with 1 g 1,3,5-
trimethylbenzene (TMB) was stirred at 40 °C for 4 h, followed by
the addition a requisite amount of Al (NO3)3. 9H2O to get solution
A. Then, 4.25 g of TEOS and a certain amount of Al(i-PrO)3 were
O
O
(i)
(ii)
R S R'
In continuation of our work on the catalytic properties of
heteropoly acids (HPAs) [9–12], for the first time, we report on the
catalytic behavior of PW12@Al-MCF catalysts (fabricated via the
R S R'
R S R'
O
2
1
3
⁎
Corresponding author. Tel.:+98 321 3232706‐7; fax: +98 321 3232701.
E-mail address: fazaeli@iaush.ac.ir (R. Fazaeli).
Scheme 1. Reagents and conditions: (i) H2O2 (8 mmol), PW12 @Al-MCF (3 mol%) and
CH3OH, rt, and (ii) H2O2 (8 mmol), PW12 @Al-MCF (4 mol%) and CH3CN, rt.
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.09.018

R. Fazaeli et al. / Catalysis Communications 29 (2012) 48–52
49
Br
S
S
S
(a)
S
(d)
Br
(c)
(b)
S
S
S
S
O2N
(f)
(g)
S
(h)
(e)
H3C
H3C
O
O
S
S
CH3
OH
O
(i)
(j)
(k)
S
(l)
H
N
H
N
S
S
NO2
N
N
(n)
(m)
Scheme 2. The structures of the used sulfides.
filtration
Teflon container
493 K
PW12 + Al-MCF
PW12@ Al-MCFENCAPSULATED
Scheme 3. Preparation of PW12@Al-MCFEncapsulated
.
were calcined
for 4 h at 400 oC
were dried
assembly
H2O, r.t
PW12@ Al-MCF
PW12 + Al-MCF
IMPREGNATED
for 4 h at 100 oC
Scheme 4. Preparation of PW12@Al-MCFImpregnated
.
added to 5 ml of HCl aqueous solution at pH=1.5 to obtain solution
B, which was stirred at room temperature for about 0.5–3 h then
combined with solution A. The resultant solution mixture was stirred
vigorously for 20 h at 40 °C. After that, a small amount of NH3. H2O
was added drop by drop to adjust the pH value equal to 7.0. Then it
was transferred into an autoclave to age for 24 h at 100 °C. The
final solid was collected by filtration and dried at 100 °C. The surfac-
tants were removed through calcinations [13].
H2O2 (8 mmol) was added in one portion. The slurry was stirred at
room temperature for 20 min. The catalyst was filtered off and
washed with methanol (5 ml). Ethyl acetate (5 ml) was added and
resulting solution was dried with anhydrous sodium sulfate and
evaporated in vacuo to afford the crude product which was purified
2.2. PW12@Al-MCFen
For preparation of PW12@Al-MCFEncapsulated, PW12 was added to
the Teflon container with the rest of the mixture under static condi-
tions at 493 K for 8 h. According to elemental analysis, the loadings
was obtained 50 wt.% (Scheme 3).
2.3. PW12@Al-MCFim
PW12@Al-MCFImpregnated was prepared by method of incipient
wetness: mixing 50 mg of PW12 and 100 mg of the Al-MCF sample
with 10 ml of distilled water in an Erlenmeyer flask at room temper-
ature and under stirring for 12 h. The resulting pastes were dried for
4 h at 100 °C and calcined for 4 h at 400 °C (Scheme 4).
2.4. Synthesis of sulfoxides and sulfones
To a stirred suspension of the selected sulfide (1 mmol) and the
Fig. 1. FTIR spectra of (a) PW12; (b) PW12@Al-MCFen; (c) PW12@Al-MCFim; and
(d) Al-MCF.
heterogeneous catalyst PW12@Al-MCF (3 mol%) in methanol (5 ml),

50
R. Fazaeli et al. / Catalysis Communications 29 (2012) 48–52
Table 1
The texture parameters of Al-MCF and PW12/Al-MCF materials in comparison with the
bulk PW12 materials.
Materials
Texture parameters (N2 adsorption)
Surface area
Pore volume
Pore diameter
(nm)
a
a
(m2g−1
)
(cm3 g−1
)
Al-MCF
371
258
233
245
8
1.48
1.22
1.21
1.39
–
7.9
6.4
6.6
6.9
–
PW12@Al-MCFen
PW12@Al-MCFen (recovered)
PW12@Al-MCFim
PW12 (bulk)
a
Total pore volume measured at p/p0=0.99.
3. Results and discussion
3.1. Physico-chemical characterization
Fig.
1 presents the FTIR spectra in the skeletal region of
4000–400 cm−1 for the Al-MCF materials. The primary structure of
bulk PW12 (Fig. 1a) could be identified by four characteristic IR bands
appearing at 1080 cm−1 (P\O band), 982 cm−1 (W_O band), 892
and 791 cm−1 (W-O-W bands) [14]. The characteristic IR bands of
PW12 in the PW12@Al-MCF silica (Fig. 1b and c) were somewhat differ-
Fig. 2. XRD patterns of (a) Al-MCF, (b) PW12@Al-MCFen, and (c) PW12@Al-MCFim
.
by column chromatography on silica gel (10% EtOAc in hexane) to af-
ford the pure sulfoxide.
Similar method was utilized to produce sulfones. In this case
4 mol% of PW12@Al-MCF in CH3CN was utilized.
ent from those of unsupported PW12. The P_O band in the PW12
@
Al-MCFen and PW12 @Al-MCFim was not clearly identified due to the
overlap by the broad Si\O\Si band. However, W_O and W\O\W
bands of PW12 in the PW12@Al-MCFen and PW12@Al-MCFim appeared
at shifted positions compared to those of unsupported PW12, indicating
a strong chemical interaction between PW12 and PW12@Al-MCF [9]. The
Si\O\Si stretching bands originated from Al-MCF silica were observed
at around 1250–1000 and 807 cm−1 (Fig. 1d).
Fig. 2 shows the XRD patterns of Al-MCF, PW12@Al-MCFen, and
PW12@Al-MCFim, respectively. Unsupported PW12 catalyst showed a
characteristic XRD pattern of the HPA. On the other hand, Al-MCF
showed no characteristic XRD patterns due to their amorphous na-
ture. It is noticeable that PW12@Al-MCF materials also showed no
characteristic XRD pattern, even though 50 wt.% PW12 was loaded
on the Al-MCF.
Fig. 3 shows the N2 adsorption–desorption isotherms and pore size
distributions of Al-MCF and PW12@Al-MCF materials. All the samples
exhibited typical IV type isotherms and H1 type hysteresis loops at
high relative pressures. This indicates that Al-MCF silica with large
pore size distribution was successfully prepared. Interestingly, the
PW12@Al-MCF showed very similar isotherm patterns compared to
those of Al-MCF, indicating that the mesopore structure of Al-MCF silica
was still maintained even after the encapsulation or immobilization
step of PW12. Physical properties of Al-MCF and PW12@Al-MCF mate-
rials are listed in Table 1. As expected, the BET surface areas, total pore
volumes and mesopore sizes of Al-MCF material are decreased from
371 to 258 (245)m2 g−1, from 1.48 to 1.22 (1.39)cm3 g−1, and from
7.9 to 6.4 (6.9)nm, respectively, after the encapsulation (impregnation)
with PW12. These changes reflect that part of the mesopore volume in
the Al-MCF matrix is filled with PW12, resulting in pore diameters that
are less to that of silica channels.
Fig. 4 shows the TEM images of Al-MCF, PW12@Al-MCFen, and
PW12@Al-MCFim, respectively. Disordered pore arrays can be clearly
seen in PW12@Al-MCF samples.
3.2. Catalytic activity
3.2.1. Oxidation to sulfides
In order to check the feasibility of H2O2-PW12@Al-MCF system in an
organic medium, we performed the oxidation in methanol, n-hexane,
dichloromethane, acetonitrile, ethanol, H2O and ethanol:H2O (1:1). As
Fig. 3. N2 adsorption–desorption isotherms and pore size distributions of (a) Al-MCF
silica (b) PW12@Al-MCFen, (c) PW12@Al-MCFim and (d) recovered PW12@Al-MCFen
catalyst.

R. Fazaeli et al. / Catalysis Communications 29 (2012) 48–52
51
Fig. 4. TEM images of (a) Al-MCF, (b) PW12@Al-MCFen and (c) PW12@Al-MCFim
.
Table 2
low yield of sulfoxide was obtained in the presence of 8 equiv of H2O2
after 6 h.
Effect of different conditions in the oxidation of sulfides to sulfoxides after 45 min.a
Entry
Solvent
Catalyst (mol%)
Yield (%)b
Several functionalized sulfides were selected to examine the scope
and limitations of this procedure (Table 3). Aryl alkyl, aryl allyl, diaryl,
dialkyl, cyclic and heterocyclic sulfides could be oxidized to the corre-
sponding sulfoxides in excellent yields. Sulfides carrying either
electron-withdrawing (Table 3, entries d and e) or electron-donating
(Table 3, entry f) substituent gave the corresponding sulfoxides in ex-
cellent yields with high purity. Importantly, this method was compati-
ble with other functional groups such as ketone (Table 3, entry l),
alkene (Table 3, entry i), ester (Table 3, entry j), and alcohol (Table 3,
entry k). Typically, these oxidations could usually be stopped at the sulf-
oxide stage without over-oxidation to the sulfone.
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
CH3CN
EtOH
3c
2
3
4
3
3
3
3
3
3
23
35
92
92
85
80
80
74
57
36
H2O
EtOH:H2O
n-hexane
CH2Cl2
10
a
Reaction conditions: benzyl phenyl sulfide (1 mmol), H2O2 (8 mmol), and catalyst:
PW12@Al-MCFen, solvent (5 ml), r.t.
b
Isolated yield.
3.2.2. Oxidation to sulfones
c
Al-MCF was used as catalyst in the oxidation of sulfides to sulfoxides after 6 h.
In order to demonstrate the efficiency and applicability of the PW12
@
Al-MCF system further, the chemoselective oxidation of sulfide to sul-
fones was also investigated (Table 3). The reaction conditions specified
in Table 3 was optimized for carrying out the reaction in CH3CN at
room temperature (sulfide/H2O2/PW12@Al-MCF ratio: 1:8:4). It was
found that a wide variety of sulfides were oxidized to their correspond-
ing sulfones in excellent yields in CH3CN at room temperature.
The recovery and reusability of the catalyst has been investigated.
We have noticed that after the addition of CHCl3 to the reaction mix-
ture, this catalyst can be easily recovered quantitatively by simple fil-
tration. The wet catalyst was recycled (the nature of the recovered
catalysts has been followed by NAA, XRD and FT-IR spectra) and no
appreciable change in activity was noticed after three cycles
(Table 4). The N2 isotherm of the recovered catalyst is shown in
Fig. 3d. The surface area of the recovered PW12@Al-MCF decreased
from 258 to 233 m2 g−1 (Table 1, entry 3).
clearly shown in Table 2 and in agreement with our studies, solvents of
high hydrogen bonding capacity, such as methanol and water, favor the
formation of sulfide with high chemoselectivity [15]. We next
performed a set of preliminary experiments on benzyl phenyl sulfide
to examine the effects of different amounts of PW12@Al-MCFen in meth-
anol at 25 °C (Table 2).
It is noteworthy that in a blank experiment (Table 2, entry 1), no sig-
nificant oxidation was observed in the absence of Al-MCF, and only a
Table 3
Oxidation of sulfides to sulfoxides and sulfones using the PW12@Al-MCF systems.
Sulfide 1
Sulfoxide 2a
Sulfone 3b
Yieldc (%):time (min)
Yieldc (%):time (min)
PW12
@Al-MCFen
PW12
Al-MCFim
@
PW12
Al-MCFen
@
PW12@
Al-MCFim
4. Conclusions
a
b
c
d
e
f
g
h
i
j
k
l
m
n
90:120
92:120
90:35
97:10
98:40
97:120
98:20
92:45
95:45
85:120
76:85
90:120
70:15
67:105
90:120
92:100
90:25
97:10
98:45
97:35
98:35
92:45
95:75
87:100
86:75
90:65
67:120
75:120
90:120
92:20
90:105
97:10
98:35
97:120
98:65
92:100
95:120
92:35
85:85
90:60
90:145
95:35
90:70
95:15
98:95
97:30
98:35
92:60
92:70
92:75
87:35
87:120
70:30
75:130
A series of PW12@Al-MCF catalysts was prepared by the encapsu-
lation and wet impregnation methods. PW12@Al-MCF is an excellent
Table 4
Investigation of the feasibility of reusing PW12
@
Al-MCFen in the oxidation of sulfides to sulfones
with H2O2 in CH3CN after 70 min.a
Run
Yield (%)b
1
2
3
4
92
92
90
57
72:120
70:120
a
Reaction conditions: sulfide (1 mmol), H2O2 (8 mmol), catalyst (3 mol%), and
a
CH3OH (5 ml), r.t.
Reaction conditions: diphenyl sulfide (1 mmol),
b
Reaction conditions: sulfide (1 mmol), H2O2 (8 mmol), catalyst (4 mol%), and
H2O2 (8 mmol), PW12@Al-MCFen(4 mol%), and
CH3CN (5 ml), r.t.
CH3CN (5 ml), r.t.
c
b
Isolated yields.
Isolated yields.

52
R. Fazaeli et al. / Catalysis Communications 29 (2012) 48–52
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Acknowledgments
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We gratefully thank Yazd Branch, Islamic Azad University, for fi-
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