Online version is available on http://research.guilan.ac.ir/csm
CSM
Chemistry of Solid Materials
Vol. 2 No. 1 2014
[Research]
Nano composite mixed-addenda vanadium substituted polyoxo-
metalate-TiO2 as a green, reusable and efficient catalyst for rapid and
efficient synthesis of symmetric disulfides under ultrasound irradiation
M. A. Rezvani
*1
, M. Ali Nia Asli
2
, L. Abdollahi
3
, M. Oveisi
4
1,2
Assistant Professor, Department of Chemistry, Faculty of Science, University of
Zanjan, Zanjan, Iran
3,4
MSc Student, Department of Chemistry, Faculty of Science, University of Zanjan,
Zanjan, Iran
*Corresponding author; E-mail: marezvani@znu.ac.ir
Article history:
(Received: 5 Nov 2014, Revised: 21 Jan 2015, Accepted: 17 Feb 2015)
ABSTRACT
Mixed-addenda vanadium substituted polyoxometalate supported on anatase TiO2 crushed nano
leaf was synthesized by an unusual reaction with titanium tetraisopropoxide at 100 ºC via sol–
gel method under oil-bath condition. The materials characterized by XRD, TEM, IR and UV–
vis techniques. In the present work, efficient oxidative of thiols with polyoxometalate-
TiO2/hydrogen peroxide system using ultrasound irradiation is reported. The Keggin type
polyoxometalate-TiO2/H2O2, sandwich type POM-TiO2 and Wells Dowson type POM-
TiO2/H2O2 systems showed completely different reactivity ordering for the same oxidation of
thiols. Ultrasonic irradiation increased the catalytic activity of the catalyst, reduced the reaction
times and increased the products yields.
Keywords: Polyoxometales; Desulphurization; Anatase; Keggin; Dowson.
1. INTRODUCTION
Disulfides plays an important role in
synthetic organic chemistry as well as
biology, notably to control cellular
redox potential in biological systems in
which thiols are oxidized to prevent
oxidative damage [1-3]. Disulfides
have also found industrial applications
as vulcanizing agents and as important
synthetic intermediates in organic
synthesis [4]. Various reagents and
oxidants have been employed for
oxidation of thiols to homodisulfides
[4-6]. Some of these methods suffer
from obvious disadvantages such as
long reaction times, limited availability
of the oxidant, toxicity of reagents and
difficult isolation of products.
Consequently, the introduction of
readily available, safe and stable
reagents for the oxidation of thiols to
disulfide is still a necessity. The
application of heteropolyacids (HPAs)
as catalytic materials is growing
continuously in the catalytic field.
These compounds possess unique
properties such as: well-defined
structure, Brönsted acidity, possibility
to modify their acid–base and redox
properties by changing their chemical
composition (substituted HPAs), ability
to accept and release electrons, high
proton mobility, being environmentally
benign and presenting fewer disposal
problems [7, 8]. Supporting the
heteropolyacids on solids with high M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
42
surface areas improve their catalytic
performance in heterogeneous
reactions. In this article in continuation
of our group research [8-12], we
describe the synthesis and crystal
structure of a mixed-addenda
vanadium-containing heteropolyanion
supported on TiO2 by an unusual
reaction. Homogeneous catalysts
cannot be separated from the reaction
media and subsequently, cannot be
reused. Fixation of the homogeneous
catalysts onto a solid support may be a
strategy to overcome this problem. The
catalyst easily separated and reused at
the end of reaction without a significant
loss of its catalytic activity, which
suggests that the catalyst is stable under
different conditions. The ultrasound
irradiation is applicable to a broad
range of practical syntheses. Some
advantages of ultrasound procedure are
short reaction times and mild reaction
conditions, formation of purer products
and waste minimization. Ultrasound
irradiation can also be used to influence
selectivity and yields of reactions [13-
16]. Despite the vast advantages of this
technique, the use of ultra sound in
synthesis of organic compounds is not
fully developed. The reactions
proceeded smoothly under mild and
green ultrasound-accelerated conditions
to afford the products in high yields.
Application of ultrasound in a so-called
‘‘sonochemistry’’ has received enor-
mous interests since it offers a versatile
and challenging technique in organic
synthesis. Recently, ultrasonic
irradiation technique has been
employed not only to decrease reaction
times but also to improve yields in a
large variety of organic reactions. To
develop the applications of ultrasound
in organic reaction herein we wish to
report a very efficient and simple
method for oxidative of thiols under
ultrasound irradiation.
2. EXPERIMENTAL
2.1 Materials
All the chemicals were obtained from
Merck Company and used as received.
All reagents and solvents used in this
work are available commercially and
were used as received, unless otherwise
indicated. Hydrogen peroxide (30
vol%) were obtained from Aldrich
Chemical Company. Na5[PV2Mo10-
O40]–TiO2 and other polyoxometalate
were prepared according to our
previous work [8-11]. The compound
A-β-Na8HPW9O34. 24H2O (abbre-
viated as A-PW9) and other catalysts
were prepared as previously described
[10, 11]. Ultra sound apparatus was
Wiseclear (Seol, Korea), with a
frequency of 40 kHz, nominal power of
770W and output of 200 W.
2.2 Preparation of H5[PMo10V2O40]
(VPOM )
Sodium metavanadate (12.2 g, 100
mmol) was dissolved by boiling in 50
mL of water and then mixed with (3.55
g, 25 mmol) of Na2HP04 in 50 mL of
water. After the solution was cooled, (5
mL, 17 M, 85 mmol) of concentrated
sulfuric acid was added, and the
solution developed a red color. An
addition of (60.5 g, 250 mmol) of
Na2MoO4.2H2O dissolved in 100 mL
of water and then was added to the red
solution with vigorous stirring,
followed by slow addition of
concentrated sulfuric acid (42 mL, 17
M, 714 mmol). The hot solution was
allowed to cool to room temperature.
The 10-molybdo-2-vanadophosphoric
acid was then extracted with 500 mL of
ethyl ether. Air was passed through the
heteropoly etherate (bottom layer) to
free it of ether. The solid remaining
behind was dissolved in water,
concentrated to first crystal formation,
as already described, and then allowed
to crystallize further. The large red
crystals that formed were filtered,
washed with water, and air-dried [9].
M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
43
2.3 Preparation of nano catalyst VPOM-
TiO2
The VPOM–TiO2 nanoparticle was
prepared as following: First, titanium
tetraisopropoxide was added into
glacial acetic acid with stirring. Next, a
solution of VPOM in water was drop
wised in it. The mixture was stirred to
dissolve any solid. Then, the sol was
heated to 100 °C under oil bath
condition until a homogenous VPOM –
TiO2 hydrogel was formed. Finally, the
gel was filtered, washed with deionized
water-acetone and dried in oven at
50 ºC overnight (Scheme 1).
Scheme 1. Chart of synthesis of nanocomposite.
2.4. General procedure for oxidation
reactions with H2O2 under ultrasonic
irradiation
To a mixture of thiol (0.5 mmol) and
nano catalysts (16 mg, containing 2.20
μmol of VPOM –TiO2) in EtOH (8
mL) was added 2 mL of 30% hydrogen
peroxide and the mixture was exposed
to ultrasonic irradiation. The reaction
was monitored by TLC. After the
reaction was completed, the reaction
mixture was diluted with CH2Cl2 (30
mL) and filtered. The nano catalyst was
thoroughly washed with CH2Cl2 and
combined washings and filtrates were
purified on a silica gel plates or a
silica- gel column.
2.5. Characterization methods
X-ray diffraction (XRD) patterns were
recorded by a D8 Bruker Advanced, X-
ray diffractometer using Cu Kα
radiation (α=1.54 A). The patterns were
collected in the range 2θ = 20–70° and
continuous scan mode. Transmission
electron microscope (TEM) images
were obtained on a Philips CM10
transmission electron microscope with
an accelerating voltage of 100 kV. The
electronic spectra of the synthesized
catalysts were taken on a RAYLEIGH
(UV-1800) ultraviolet–visible (UV–
vis) scanning spectrometer. Infrared
spectra were recorded as KBr disks on
a Buck 500 scientific spectrometer.
M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
44
2.6. Recycling of the nano catalyst
At the end of the oxidation of the
thiols, the catalyst was filtered, washed
with dichloromethane. In order to know
whether the catalyst would succumb to
poisoning and lose its catalytic activity
during the reaction, we investigated the
reusability of the catalyst. For this
purpose we carried out the oxidation
reaction of 4-Chlorothiophenol in the
presence of catalyst. Even after three
runs for the reaction, the catalytic
activity of (VPOM-TiO2) was almost
the same as that freshly used catalyst.
The results are summarized in Table 1.
Table 1. Reuse of the catalyst for oxidation of 4-Chlorothiophenol (Table 2, entry 4)
Entry Isolated yield (%)
1 96
2 94
3 94
3. RESULTS AND DISCUSSION
3.1 Characterization of synthesized
catalysts
XRD patterns of TiO2, VPOM and
VPOM-TiO2 are shown in Figure 1.
XRD patterns (a) and (b) in Figure 1
are corresponded to pristine TiO2 and
VPOM, respectively. The XRD pattern
corresponding to pure TiO2 was found
to match with that of fully anatase
phase. No peaks from any else
impurities or rutile phase were
observed, which indicates the high
purity of the obtained powders. The
sharp diffraction peaks manifest that
the obtained TiO2 have high
crystallinity. When VPOM is bound to
the TiO2 surface, (VPOM -TiO2), all of
signals corresponding to VPOM is
disappeared and the final pattern
matched to fully anatase phase of TiO2
(JCPDS No. 21-1272), which is most
likely due to VPOM forming only a
thin coating on the TiO2 surface and
thus the majority of the observed
signals are due to the crystal phases of
anatase TiO2. Using the Scherrer
equation, the crystallite diameter of
VPOM -TiO2 is about 9 nm.
Fig. 1. XRD pattern of (a) TiO2, (b) VPOM and (c) VPOM-TiO2. M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
45
Figure 2 depicts the transmission
electron micrographs of obtained
powders. Figure 2(a) shows TEM
image of obtained fully anatase phase
of TiO2 as crushed nano leaf with
average size of about 20 nm. It is
observed from the TEM image, after
modification of anatase with VPOM a
significant change in morphology and
size was occurred. It can be seen that in
the TEM image, most of the obtained
powders are nano particles with
average size about 10 nm and there are
some nano rods.
Fig. 2. TEM image of (a) TiO2 and (b) VPOM-TiO2.
Also UV-visible spectroscopy of
obtained powders was studied. UV-vis
spectra of TiO2, VPOM and VPOM -
TiO2 nanocomposite are shown in
Figure 3. In ultraviolet light regions,
which are shorter than 350 nm, pure
nano TiO2 whose band gap energy
equivalent to around 275 nm (3.70 eV)
shows the highest absorbance due to
charge-transfer from the valence band
(mainly formed by 2p orbitals of the
oxide anions) to the conduction band
(mainly formed by 3d t2g orbitals of the
Ti
4+
cations) [12]. In addition, some
hyper fine structure in the range from
280 to 330 nm observed in VPOM
spectrum. The inset of the figure shows
the UV-vis spectrum of the VPOM -
TiO2 indicating there are two peaks
around 220 and 260 nm. The above
UV–vis results indicate that
introduction of VPOM into TiO2
framework has an influence on
coordination environment of TiO2
crystalline [7].
M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
46
Fig. 3. UV-vis spectra of (a) TiO2, (b) VPOM and (c) VPOM-TiO2.
IR spectrum of the prepared catalyst in
the range 700–1100 cm-1
showed
absorption bands at 1078, 968, 879 and
763 cm-1
, corresponding to the four
typical skeletal vibrations of the
Keggin polyoxoanions, which indicated
that VPOM has been supported on
TiO2 (Fig. 4). These peaks could be
attributed to ν(P–O), ν(Mo–O), ν(Mo–
Ob–Mo) and ν(Mo–Oc–Mo) (Ob:
corner-sharing oxygen, Oc: edge-
sharing oxygen), respectively [8, 17].
Fig. 4. IR spectrum of (a) VPOM and (b) VPOM-TiO2.
3.2. Catalytic results
3.2.1. Effect of the substituent
The effects of various substituents on
the yields of produced disulfides have
been examined in the presence of
VPOM-TiO2 as a nano catalyst. The
results are given in Table 2. Halogens
were chosen as electron-withdrawing
groups (entries 3–5), while methyl,
phenolic hydroxyl and methylthiol
groups (entries 1, 6 and 7, respectively) M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
47
were chosen as electron-donating
substituents. One heteroaromatic thiol,
i.e., pyridine-2-thiol, was successfully
oxidized in good yield (entry 9) as well
as benzylthiol (entry 8) as a benzylic
aliphatic representative. The yields
were generally very good (>75 %) to
excellent (>90 %) with no obvious
relationship between the aromatic
substituent and yield (compare entries 4
with 5 and 2 with 10). A highlight of
the method is the ease by which the
product may be isolated via simple
filtration followed by removal of the
solvent.
Table 2. Oxidation of different thiols using H5PV2Mo10O40-TiO2 as a catalyst under ultrasound
irradiation.
Entr
y
Thiol Disulfide Time
(min)
Yield
a,b
(%)
M.P
(
o
C)
found
M.P(
o
C)
Literature
9, 12
1
10 96
43-
44
44-45
2
15 96 60-61 61
3
20 94 90-92 91-93
4
20 97 72-73 70-71
5
30 84 49-51 --
6
30 92
Liqui
d29
--
7
30 83 40-43 40-43
8
40 78 69-71 69-70
9
30 81 55-56 55-57
a
Isolated yield on the basis of the weight of the pure product obtained.
b
The products were identified by comparison of physical and spectroscopic properties with authentic
compounds.
SH CH3 S S CH3
C H3
SH S S
SH Br S S Br Br
SH Cl S S Cl Cl
SH F S S F F
OH
SH
S S
OH O H
SH CH3S S S SCH3 CH3S
CH2SH CH2S SCH2
N
SH
N
S
N
SM. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
48
3.2.2. Effect of the catalyst structure
The effect of the structure of the
catalyst on the oxidation of 4-
chlorothiophenol, as a model
compound, is presented in Table 3. It
was studied using Keggin, Wells
Dowson and sandwich type polyoxo-
metalate-anatase nanoparticle as a
catalyst and hydrogen peroxide as an
oxidant. POMs-TiO2 nanocomposite
has presented higher catalytic activity
than that of the unsupported
polyoxometalates. The VPOM -TiO2
nano particle was very active catalyst
systems for the model compound
oxidation, while other polyoxo-
metalates systems were less active. In
the Keggin-type polyoxometalates
series, H5PV2Mo10-O40-TiO2 showed
the highest catalytic activity. The
results of Table 3 show that the
heteropoly salt type catalysts were less
efficient than the heteropolyacids. The
Keggin-type polyoxometalates led to
more effective reactions in comparison
with the sandwich and Wells–Dawson
type polyoxometalates. However,
H6P2Mo18-O62 was more effective than
H6P2W18O62 in the oxidation of thiols,
possibly due to the difference in
tungsten and molybdenum reduction
potentials. The compression of
efficiency of TiO2-supported mixed
addenda heteropolyacid (VPOM-TiO2)
with mixed addenda heteropolyacid
(VPOM) has been carried out. The
results are shown in Table 3. It is clear
that Nano composite VPOM-TiO2 gave
the better yields than VPOM.
Table 3 . Effect of different catalyst in oxidation of 4-Chlorothiophenol (Table
2, entry 4)
a
Entry Catalyst Time (min.) Yeild (%)
1 H5PV2Mo10O40-TiO2 20 97
2 (Bu4N)7H3[P2W18Cd4]-TiO2 25 97
3 H5PV2Mo10O40 20 95
4 H4PVMo11O40
20 93
5 (Bu4N)7H3[P2W18Cd4] 20 90
6 (NH4)10[P2W18Cd4] 20 87
7 K5PV2Mo10O40 30 86
8 K4PVMo11O40 35 84
9 K10[P2W18Zn4]
35 83
10 H6P2Mo18O62 20 82
11 H6P2W18O62 20 80
a
Condition for oxidation: 2 ml H2O2 as an oxidant, 2.20 μmol mmol catalyst, 30 ml
CH2Cl2 as an extraction solvent and ultrasonic irradiation. M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
49
3. 2. 3. Effect of ultrasound irradiation
To investigate the role of ultrasound
irradiation in this method, the reactions
were carried out in the presence of the
same amount of nanocomposite VPOM
-TiO2 under stirring condition in EtOH
at room temperature. The results are
summarized in Table 4. It is clear that
in the same reaction condition reactions
under ultrasound irradiation led to
relatively higher yields and shorter
reaction times (Table 2 and 3). The
power of ultrasound is a very important
parameter and also has a great
influence on the phenomena of acoustic
cavitation and efficiency of ultrasound
treatment.
Table 4. Oxidation of different thiols using H5PV2Mo10O40-TiO2 as catalyst under refluxing
condition.
Entry Disulfide
Time (h) Yield
a
(%)
1
2 98
2
2 94
3
2 96
4
2 98
5
3 95
6
3 91
7
3 84
8
3 83
9
3 80
a
Isolated yield on the basis of the weight of the pure product obtained.
S S CH3
C H3
S S
S S Br Br
S S Cl Cl
S S F F
S S
OH O H
S S SCH3
CH3
S
CH2S SCH2
N
S
N
SM. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
50
Figure 5 shows the effect of irradiation
power on the oxidation of thiols, which
indicates that increasing of ultrasound
power will improve the extent of
oxidation and the highest conversion,
was observed at a power of 400 W.
Fig. 5. Effect of ultrasound irradiation intensity on the oxidation of thiol with H2O2 catalyzed
by VPOM–TiO2.
4. CONCLUSION
VPOM-TiO2 nanocomposite has been
synthesized at low temperature via sol–
gel method under oil-bath condition.
Fixing of VPOM into TiO2 decreases
the particle size of crushed nano leaf of
anatase phase. The VPOM-TiO2 nano
composite was very active catalyst
systems for the model compound
oxidation, while unmodified VPOM
showed much lower activity.
REFERENCES
[1] J. Langer, R. Fischer, H. Gorls, Am. J.
Physiol. Renal Physiol. 278, 2952
(2000).
[2] D.C. Jocelyn, Biochemistry of the
Thiol Groups, Academic press, New
York, (1992).
[3] R.J. Cremlyn, An Introduction to
Organosulfur Chemistry, Wiley and
Sons, New York, (1996).
[4] S. Oae, Organic Sulfur Chemistry:
Structure and Mechanism, CRC
Press, Boca Raton, FL, (1991).
[5] G. Ledung, M. Bergkvist, J. Carlsson,
S. Oscarsson, Langmuir 17, 6056
(2001).
[6] Y. Nishiyama, K. Maehira, J. Nakase,
N. Sonoda, Tetrahedron Lett. 46,
7415 (2005).
[7] F.M. Collins, A.R. Lucy, C. Sharp, J.
Mol. Catal. A: Chem. 117, 397
(1997).
[8] M. A. Rezvani, A. F. Shojaie, M. H.
Loghmani, Catal. Commun. 25, 36
(2012).
[9] A. Fallah Shojaei, M.A. Rezvani, M.
Heravi, J. Serb. Chem. Soc. 76, 955
(2011).
[10] A. F. Shojaie, M. A. Rezvani, M. H.
Loghmani, Fuel Process.
Technol.,118, 1 (2014).
[11] A. Fallah Shojaei, M.A. Rezvani, M.
Heravi, J. Serb. Chem. Soc. 76, 1513
(2011). M. A. Rezvani, M. Ali Nia Asli, L. Abdollahi, M. Oveisi/CSM Vol.2 No.1, 2014 pp.41-51
51
[12] R. Harutyunyan; M. A. Rezvani;
Majid M. Heravi, Synth. React.
Inorg. Met.-Org. Chem., 41, 94
(2011).
[13] W. Bonrath, Ultrason. Sonochem. 10,
55 (2003).
[14] K.S. Suslick, Ultrasound: its
Chemical, Physical and Biological
Effect, VCH, Weinheim, (1988).
[15] M. M. Heravi, S. Sadjadi, Ultrason.
Sonochem., 16, 708 (2009).
[16] M. M. Heravi, S. Sadjadi, Ultrason.
Sonochem., 16, 718 (2009).
[17] Y. Yang, Q. Wu, Y. Guo, C. Hu, E.
Wang, J. Mol. Catal. A: Chem. 225,
203 (2005).
[18] M. M. Khodaei, I. Mohammadpoor,
Bull. Korean Chem. Soc. 24, 885
(2003).
[19] A. McKillop, D. Koyuncu, and M.
Trabelsi, Tetrahedron Lett. 31, 5007
(1990).
[20] H. Golchoubian, F. Hosseinpoor,
Catal. Commun. 8, 697 (2007).
