Document Type : Research Paper
Authors
University of Zanjan
Abstract
Keywords
Online version is available on http://research.guilan.ac.ir/csm
CSM
Chemistry of Solid Materials
Vol. 2 No. 1 2014
[Research]
A comparative study between transition-metal-substituted Keggin-type
tungstosilicates supported on anatase leaf as catalyst for synthesis of
symmetrical disulfides
M. Ali Nia Asli
1*
, M. A. Rezvani
2*
, M. Oveisi
3
, L. Abdollahi
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: 29 Apr 2015, Accepted: 6 May 2015)
ABSTRACT
Transition-metal-substituted (TMS) polyoxometalates of the general formula [SiW9M3O39],
(where M = first row transition metal), has been synthesized and supported on anatase by sol–
gel method under oil-bath condition. The tetrabutylammonium (TBA) salts of the Keggin-type
polyoxotungstates [SiW9M3O39], (M = VII
, Cr
II
, MnII
, Fe
II
CoII
and Ni
II
), proved to be green,
reusable, and highly efficient catalysts for the oxidation of thiols and dithiols into the
corresponding disulfides using hydrogen peroxide as an oxidizing reagent. This article will be
focused on the discovery of other transition metal substituted silicotungstate structures with a
potential for homogeneous and heterogeneous oxidation catalysis. We will focus on red/ox
active 3d metals (e.g. Fe
2+
, Cr
2+
).The catalytic activity of these transition-metal-substituted
polyoxometalates (TMSPOMs) was strongly influenced by the type of transition metal in the
TMSPOMs. Among them, the (TBA7SiV3W9O40) catalytic system showed the highest activity.
Nanoparticle (TBA7SiV3W9O40-TiO2) has been synthesized by sol–gel method under oil-bath
condition at 100 °C. The materials were characterized by IR, XRD, TEM and UV–vis
techniques.
Keywords: Transition-metal, Polyoxometalate, Sol–gel method, Keggin, Thiol.
1. INTRODUCTION
The catalytic functions of transition-
metal-substituted (TMS) polyoxo-
metalates and related polyoxometalate
compounds have attracted much
attention, particularly over the last two
decades [1]. In this context, heteropoly
acids (HPAs) are promising catalysts.
A common and important class of these
acids and those used in the majority of
catalytic applications is the Keggin
compounds, with the general formula
HnXM12O40 (X = P, Si, As, Ge, B; M =
Mo, W) [2-4].
These solid acids are us-ually
insoluble in non-polar solvents but
highly soluble in polar ones. They can
be used in bulk or supported forms in
both homogeneous and hetero -geneous
systems. Furthermore, these POMs
have several advantages, including high
flexibility in modification of the acid
strength, ease of handling, environ-
mental compat-ibility, non-toxicity, and
experimental simplicity [3-5]. Keggin
type poly -oxoanions (Figure 1) have
widely been studied as homogeneous
and hetero-geneous catalyst for the oxi-
dation of organic compounds [4]. M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi
54
Fig. 1: Polyhedral representation of
for balls: Si (red), Mo (blue), O (brown)
Further catalytically important sub
classes of the Keggin compounds are
the mixed-addenda vanadium
substituted polyoxometalates with the
general formula of TBA
nVnO40 (M = Mo and W; n = 1
has been well known that one or three
addenda units are generally removed
from the Keggin-type polyoxometalate
to form so-called lacunary anions as
XM11O39
n-
and A-XM9O34
the other metal ions are incorporated to
the vacant sites of these lacunary
anions to form metal
polyoxometalates [6]. These com
pounds exhibit high activity in acid
base type catalytic reactions; hence
they are used in many catalytic areas as
homogeneous and hetero
catalysts. In continuation of our group
research on the syntheses and
application of polyoxometalates in
organic syntheses [5-13] and due to the
importance of derivatives of disulfides
in biological and chemical processes
we hereby report the applicability of
POM-TiO2 for efficient oxidation of
thiols to the corresponding disulfides.
Supporting the heteropolyacids on
solids with high surface areas improve
their catalytic performance in various
liquid–solid and solid–solid surface
M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.5
Polyhedral representation of SiMo3W9O40. Color coding for octahedra: WO
for balls: Si (red), Mo (blue), O (brown).
Further catalytically important sub-
classes of the Keggin compounds are
addenda vanadium (V)
substituted polyoxometalates with the
general formula of TBA3+nSiM12-
(M = Mo and W; n = 1–6). It
has been well known that one or three
addenda units are generally removed
type polyoxometalate
called lacunary anions as
34
n-
and that
r metal ions are incorporated to
the vacant sites of these lacunary
anions to form metal-substituted
polyoxometalates [6]. These com -
pounds exhibit high activity in acid-
base type catalytic reactions; hence
they are used in many catalytic areas as
eous and hetero-geneous
In continuation of our group
research on the syntheses and
application of polyoxometalates in
13] and due to the
importance of derivatives of disulfides
biological and chemical processes,
applicability of
for efficient oxidation of
thiols to the corresponding disulfides.
Supporting the heteropolyacids on
solids with high surface areas improve
their catalytic performance in various
solid surface
heterogeneous reactions.
dioxide is a wide band
conductor material that has received
intense scrutiny for a broad range of
applications, thanks to its intriguing
physical-chemical properties and
cheap, abundant, and reasonably
nontoxic nature [6-8]. TiO
widely used catalyst support as well as
a catalyst is known to enhance the
catalytic activity in many cases because
of the strong interaction between the
active phase and the support [6
Disulfide plays an important role in
biology and synthetic organic
chemistry [15-17]. In order to control
cellular redox potential in biological
systems, thiols are oxidized to prevent
oxidative damage. Disulfide is used as
a protecting group under oxidative
conditions for thiol, and can be
regenerated by S-S bond cleavage [18].
Disulfides have also found industrial
applications as vulcanizing agent and
are important synthetic intermediates in
organic synthesis. Thiol can also be
over-oxidized to sulphoxide and
sulphone, therefore controlled a
selective studies were carried out for
their oxidation [19]. Various reagents
and oxidants have been employed for
conversion of thiols to disulfides [17
53-64
Color coding for octahedra: WO6 (green),
heterogeneous reactions. Titanium
band-gap semi -
conductor material that has received
intense scrutiny for a broad range of
applications, thanks to its intriguing
chemical properties and
cheap, abundant, and reasonably
8]. TiO2, also a
widely used catalyst support as well as
a catalyst is known to enhance the
catalytic activity in many cases because
of the strong interaction between the
active phase and the support [6-8, 11].
Disulfide plays an important role in
biology and synthetic organic
17]. In order to control
cellular redox potential in biological
systems, thiols are oxidized to prevent
oxidative damage. Disulfide is used as
a protecting group under oxidative
conditions for thiol, and can be
S bond cleavage [18].
Disulfides have also found industrial
applications as vulcanizing agent and
are important synthetic intermediates in
organic synthesis. Thiol can also be
oxidized to sulphoxide and
sulphone, therefore controlled and
selective studies were carried out for
their oxidation [19]. Various reagents
and oxidants have been employed for
conversion of thiols to disulfides [17-M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
55
19]. 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.In
continuation of our group research on
the syntheses and application of
heteropolyacids in organic syntheses
and due to the importance of
derivatives of disulfides in biological
and chemical processes, we hereby
report the applicability of TBAPOMs
for efficient oxidation of thiols to the
corresponding disulfides. We wish to
report a very efficient and simple
method for oxidative of thiols into the
corresponding disulfides using
hydrogen peroxide as an oxidizing
reagent catalyzed by the TBA7Si-
V3W9O40-TiO2 nanocomposite under
mild conditions. Supporting the
heteropolyacids on solids with high
surface areas improves their catalytic
performance in various heterogeneous
reactions [6-9]. We designed anatase
TiO2 crushed nano leaf coupled by
mixed-addenda vanadium-containing
keggin type polyoxometalate at 100 °C
via sol–gel method under oil-bath
condition, as a nano catalyst for
oxidative of thiols.
2. EXPERIMENTAL
2.1 Materials
All solvents and reagents used in this
work are available commercially and
were used as received, unless otherwise
indicated. Previously reported methods
were used to purify the thiols [9].
Preparation of mixed heteropolyacids
and salts were based on a literature
procedure with the following
modifications [6, 8]. Titanium (IV)
tetraisopropoxide was obtained from
Merck Chemical Company. All
chemicals were purchased from Merck
and used without purification.
2.2 Catalyst Synthesis
2.2.1 Preparation of TBA7SiCr3W9O40
First, in 40 ml of distilled water (10
o
C), Na9H[SiW9O34].16H2O, (5 g) was
dissolved. An aqueous solution of
Cr(NO3)3.9H2O was added stoichio-
metrically to the above suspension.The
solution was heated for 30 min on
awater bath. KCl (12.5 g) was then
added to thesolution. The green crystals
were isolated and recrystallized in
water. The potassium salt wasfiltered
and washed with a dilute solution of
KCl,then EtOH and Et2O-dried,
respectively. Tetrabutylammonium
(TBA) salts were obtained by adding n-
Bu4NCl in solution, to the solution of
K10SiCr3W9O40.
2.2.2 Preparation of TBA7SiFe3W9O40
In 25 ml of distilled water,
Na9H[SiW9O34]. 16H2O (6.5 g) was
stirred, and an aqueous solution of
Fe(NO3)3.9H20 (3.2 g) was added
dropwise to the resulting suspension.
The yellow-brown clear solution which
formed was heated (30 min.) on a
water-bath after adjusting the pH =4
with 1 M NaOH. The insoluble solid
resulting was filtered off, and the pH of
the filtrate was readjusted to4. Solid
KCl was then added to the filtrate
toprecipitate a yellow-brown salt,
which was recrystallized thrice from a
buffer solution of HOAc-NaOAc.
Tetrabutylammonium (TBA) salts were
obtained by adding n-Bu4NCl in
solution (pH = 2), to the solution of
K10SiFe3W9O40.
2.2.3 Preparation of TBA7SiV3W9O40
First, in 30 ml of distilled water,
sodium vanadate (0.05 g; 0.43 mmol) is
dissolved. To the stirred solution is
added Na10[SiW9O34].18H2O (4.03 g;
1.4 mmol), followed by 20 mL of 6 M
sulfuric acid. Then, for 30 min, the
solution is maintained under stirring. M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi
56
By addition of solid potassium
carbonate, the pH is adjusted between 6
and 7. By addition of solid potassium
chloride (2.2g) an orange potassium
salt (2.5g) is precipitated and
recrystallized in water.
ammonium (TBA) salts were obtained
by adding n-Bu4NCl in solution (pH =
2), to the solution of K10SiV
2.3 Preparation of catalyst
The TBASiW9V3–TiO2
was prepared as following: First,
titanium tetraisopropoxide was added
into glacial acetic acid with stirring.
Scheme 1.
M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.5
By addition of solid potassium
carbonate, the pH is adjusted between 6
and 7. By addition of solid potassium
an orange potassium
salt (2.5g) is precipitated and
Tetrabutyl-
salts were obtained
NCl in solution (pH =
SiV3W9O40.
nanoparticle
was prepared as following: First,
ropoxide was added
into glacial acetic acid with stirring.
Next, a solution of TBA
[10] 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 TBASiW9V
gel was formed. Finally, the gel was
filtered, washed with deionized wa
acetone and dried in oven at 50 ºC
overnight (Scheme 1).
Scheme 1. Chart of synthesis of nanocatalyst.
53-64
Next, a solution of TBA7SiV3W9O40
[10] 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
V3–TiO2 hydro-
gel was formed. Finally, the gel was
filtered, washed with deionized water-
acetone and dried in oven at 50 ºC
M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
57
2.4 General procedure for oxidation of
thiols
The TBA7SiW9V3–TiO2 (0.3 g, 0.1
mmol) was dissolved in the mixture of
17 ml of ethanol and 3 ml of H2O. The
substrate, (thiol) (4 mmol) and 5 mL
H2O2 were added to solution. The
reaction mixture was stirred at room
temperature until thin layer
chromatography, TLC, indicated the
reaction was complete. The solvent was
then removed and the resulting residue
was then washed with CH2Cl2. After
completion of the reaction, the solid
product was filtered off and
recrystallized. The products were
isolated and identified by comparison
of their physical and spectral data with
authentic samples prepared according
to a previous method [12].
2.5 General procedure for oxidation of
dithiols to cyclic disulfides
TBA7SiW9V3–TiO2 (1.5 g, 0.5 mmol)
was dissolved in the mixture of 26 mlof
ethanol and 4 ml of H2O in a small
beaker. Then dithiol (4 ml, 40 mmol)
and H2O2 (10 ml, 330 mmol) was
added. The reaction mixture was stirred
at room temperature until TLC
indicated the reaction was complete.
The solvent was then removed and the
resulting residue (white precipitate) was
then washed with CH2Cl2.
2.6 Characterization methods
By a D8 Bruker Advanced, X-ray
diffractometer using Cu Kα radiation
(α=1.54 A), X-ray diffraction (XRD)
patterns were recorded. The patterns
were collected in the range 2θ = 20–70°
and continuous scan mode. On a
Philips CM10 transmission electron
microscope with an accelerating
voltage of 100 kV, transmission
electron microscope (TEM) images
were obtained. 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.
3. RESULTS AND DISCUSSION
3.1 Characterization of synthesized nano-
catalysts
As distinct from other M-O vibrations,
the antisymmetricstretching between
terminal oxygen (unsharedoxygen) and
tungsten [υas(W-Oa)] is a pure
stretchingvibration mode. Its frequency
increases as the cation-size increases,
show that the W-Oa distance shortens
with theincrease in cation-size. In the
K7SiW9M3,theantisymmetric vibra-
tional frequences, between tungs-
tenatoms and corner-sharing oxygen
atoms linking the twoW3O13groups [υas
(W--Ob--W)], shift to higher
wavenumberas the cation-size
increases. The [υas (W--Ob--W)], of the
intergroup bridging oxygen atoms is
related to theW--Ob--W angle; the
bigger the cations, the larger theangle.
As the cation-size increases, the
tungsten and edgesharingoxygen
vibrations in the W3O13 group
decreaseto some extent, showing that
the W-Oc-W angle decrease. In the
tetrabutylammonium (TBA) salts of the
Keggin-type polyoxotungstates
[SiW9M3O39], M = VII
, Cr
II
, MnII
, Fe
II
CoII
and Ni
II
), the vibrational
frequences of the SiO4 tetrahedron
increase as cation-size increases. The
main IR spectral bands are
characteristic of the Keggin structure. It
was confirmed that the various salts of
SiW9M3 have an identical Keggin
structure. The band at ~960 cm-1
, the
broader band at ~ 900 cm-1
, and the
very broad band at ~ 800 cm-1
were
assigned to the vibrations ofW-Od, Si-
Oa, overlapping of corner-sharing
octahedral W-O-W, and edge-sharing
octahedral W-O-W, respectively. The
main vibrational frequencies increased
with the cation-size increase (Figure 2).
M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi
58
Fig 2: IR spectra of (a) TBA
TBA7SiCr3W9O40.
XRD patterns of TBA
TiO2,TBA7SiV3W9O40, TBA
O40, TBA7SiCr3W9O40,
W9O40 and TiO2 are shown in Figure 3.
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
crystallinity. When TBA7SiV
M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.5
TBA7SiV3W9O40 (b) TBA7SiV3W9O40-TiO2(c) TBA
TBA7SiV3W9O40-
TBA7SiFe3W9 -
, TBA7SiNi3-
are shown in Figure 3.
The XRD pattern corresponding to pure
was found to match with that of
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
have high
SiV3W9O40 is
bound to the TiO
TBA7SiV3W9O40-TiO2, approximately
all of signals corresponding to
TBA7SiV3W9O40 is disappeared
(Figure 3(f)) and the final pattern
matched to fully anatase phase of TiO
(JCPDS No. 21-1272), which is most
likely due to TBA7SiV3
only a thin coating on the TiO
and thus the majority of the observed
signals are due to the crystal phases of
anatase TiO2.
53-64
TBA7SiFe3W9O40 (d)
bound to the TiO2 surface,
, approximately
all of signals corresponding to
is disappeared
(Figure 3(f)) and the final pattern
matched to fully anatase phase of TiO2
, which is most
3W9O40 forming
g on the TiO2 surface
and thus the majority of the observed
signals are due to the crystal phases of M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
59
Fig. 3. XRD pattern of (a) TiO2, (b) TBA7SiV3W9O40, (c) TBA7SiFe3W9O40, (d)
TBA7SiCr3W9O40, (e) TBA7SiMn3W9O40 and (f)TBA7SiV3W9O40-TiO2.
The UV spectra were characteristic of
12-heteropoly tungstosilicate anions
with Keggin structure and were
assigned to O W charge-transfer
bands. In the heteropolyanion Fe
3+
is
located in an octahedral field and has a
high-spin d5
configuration. The d-d
transition is both spin and orbital
forbidden and hence very weak. In
(K7SiW9Fe3) at 267 cm-1
, an intense
absorption band is characteristic of the
9-heteropoly tungstosilicate anion. For
K7SiW9Cr3, an intense absorption band
at 250 nm was observed. It was
assigned as d-d transition arising from
the d3
configuration in a near-
octahedral crystal field. UV-vis spectra
of TBA7SiV3W9O40-TiO2 nanocompo-
site, TBA7SiV3W9O40 and TiO2 are
shown in (Figure 4). UV-vis spectra
showed broad and strong absorption in
range of 200-400 nm for
TBA7SiV3W9O40-TiO2 crystallite,
which was different from original
TBA7SiV3W9O40 and anatase TiO2.
The TBA7SiV3W9O40-TiO2 nano-
composite shows a red shift compared
with the parent anatase, and a blue shift M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
60
compared with TBA7SiV3W9O40. The
inset of the figure shows the UV-vis
spectrum of the TBA7SiV3W9O40-TiO2
indicating there is one peak around 320
nm. The above UV–vis results indicate
that introduction of TBA7SiV3W9O40
into TiO2 framework has an influence
on coordination environment of TiO2
crystalline [11]. In ultraviolet light
regions, which are shorter than 340 nm,
pure nano TiO2 whose band gap energy
equivalent to around 335nm (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) [11].
Fig. 4. UV-vis spectra of obtained catalysts.
3.2 Effects of the catalyst structure
In this article we focused on the
discovery of other transition metal
substituted silicotungstate structures
with a potential for homogeneous and
heterogeneous oxidation catalysis. We
focused on red/ox active 3d metals (e.g.
Fe
2+
, Cr
2+
, V2+
and Ni
2+
). Transition-
metal-substituted (TMS) polyoxo-
metalates of the general formula
[SiW9M3O40], where (M = first row
transition metal), has been synthesized
and comparative catalytic activity of
them. The tetrabutylammonium (TBA)
salts of the Keggin-type polyoxo-
tungstates [SiW9M3O40], M = VII
, Cr
II
,
MnII
, Fe
II
CoII
and Ni
II
), proved to be
green, reusable, and highly efficient
catalysts for the oxidation of thiols and
dithiols into the corresponding
disulfides using hydrogen peroxide as
an oxidizing reagent. Table 1 was
shown effect of catalyst on oxidation of
thiols by H2O2. 4-chlorothiophenol was
taken as a model compound. The
TBA7Si-V3W9O40-TiO2 nanoparticle
was very active catalyst system for M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
61
oxidative of thiols, while other
polyoxometalates systems were much
less active. The amount of each catalyst
was constant throughout the series. The
Keggin-type polyoxometalates led to
more effective reactions in comparison
with the Wells–Dawson type
polyoxometalates [12, 13].
3.3 Effect of temperature
The reaction was carried out at
different temperatures under the same
conditions by TBA7SiV3W9O40-TiO2 as
a nanocatalysts and H2O2 system.
4-chlorothiophenol was taken as a
model compound. The results are
shown in Table 1, 2.
Table 1. Effect of different catalyst in Oxidation of 4-Chlorothiophenol
a
Entry Catalyst Time (min) Temperature
(
o
C)
Yield (%)
1 TBA7SiV3W9O40-TiO2 15 25 98
2 TBA7SiV3W9O40 20 25 94
3 TBA7SiFe3W9O40 20 25 94
4 TBA7SiCr3W9O40 20 25 93
5 TBA7SiMn3W9O40 20 25 92
6 TBA7SiCo3W9O40 20 25 91
7 TBA7SiNi3W9O40 20 25 91
8 Na4SiW12O40 25 30 85
9 Na3PMo12O40 25 30 84
10 Na3PW12O40 25 30 83
11 Na6P2Mo18O62 25 30 79
12 Na6P2W18O62 30 30 78
a
Condition for oxidation: 4 mmol substrate, 5 ml H2O2 as an oxidant, 1.0 mmol
catalyst, 20 ml solvent 25 ml CH2Cl2 as an extraction solvent.
M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
62
The results in Table 3 showed that
yields of products are a function of
temperature. The results show that
yield increased as the reaction
temperature was raised. Table 2 and 3
show % conversion of model
compound increased as the temperature
and time raised. In Table 4, %
conversion of 1, 8-Octanedithiol at 60
ºC is higher than that at 50 ºC. 98%
conversion of 1, 8-Octanedithiol
(SHCH2(CH2)6CH2SH) was obtained at
60 ºC. The catalytic activities of the
TBA7SiV3W9O40-TiO2 nanocatalysts in
the oxidation of 1, 8-octanedithiol at
different temperatures, 10 - 60 °C were
compared.
Table 2. Oxidation of dithiols using TBA7SiV3W9O40-TiO2 and TBA7SiV3W9O40as catalysts
Catalyst ( TBA7SiV3W9O40)
NanoCatalyst ( TBA7SiV3W9O40-TiO2)
Entry Yield
%
Conversion
%
Time
(min.)
Substrate Product Time
(min.)
Conversion
%
Yield
%
1 92 94 30 1,2-Dithiolane 1,3-Propanedithiol 20 99 98
2 91 93 30 1,2-Dithiane 1,4-Butanedithiol 30 98 96
3 93 94 40 1,2-Dithiepane 1,5-Pentanedithiol 30 97 96
4 95 97 40 1,2-Dithiacyclooctane 1,6-Hexanedithiol 20 99 98
5 90 93 40 1,2-Dithiacyclodecane 1,8-Octanedithiol 20 96 95
Table 3. Effect of temperature on oxidation of different thiol and dithiol using TBA7SiV3W9O40-TiO2
catalyst
a
Entry
Temperature
(°C)
Conversion %
4-florothiophenol
4-
methylethiophenol
1,2-
Dithiolane
1,2-
Dithiane
1,2-
Dithiacyclodecane
1 10 55 46 44 36 31
2 20 79 79 75 60 52
3 30 91 98 89 76 66
4 40 92 -- 98 89 74
5 50 -- -- -- 96 87
6 60 -- -- -- 95
a
Condition for oxidation: 4 mmol substrate, 5 ml H2O2 as an oxidant, 1.0 mmol catalyst, 20 ml solvent 25 ml
CH2Cl2 as an extraction solvent.
M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
63
Entry Dithiol Cyclic Disulfide Disulfide
Time
(min)
Temperature
(°C)
Yield
(%)
a
1
1,3-Propanedithiol
(SHCH2CH2CH2SH)
1,2-Dithiolane
20 40 98
2
1,4-Butanedithiol
(SHCH2CH2CH2CH2SH)
1,2-Dithiane
30 50 96
3
1,5-Pentanedithiol
(SHCH2CH2CH2CH2CH2SH)
1,2-Dithiepane
30 50 96
4
1,6-Hexanedithiol
(SHCH2(CH2)4CH2SH)
1,2-
Dithiacyclooctane
20 30 98
5
1,8-Octanedithiol
(SHCH2(CH2)6CH2SH)
1,2-
Dithiacyclodecan
e
20 60 95
a
Isolated yield on the basis of the weight of the pure product obtained.
3.4 Effect of dithiols substituent
The effects of various substituents on the
yields of produced cyclic disulfides have
been examined in the presence of
TBA7SiV3W9O40-TiO2catalyst. The
structural formulas of different dithiols
are shown in Table 4. The first of
dithiols was oxidized with great speed
than others. The most notable feature is
that we have been able to apply this
procedure successfully in the oxidation
of dithiols to cyclic disulfides. Large
ring disulfides are difficult tosynthesize
due to competing intermolecular
reaction.
3.5 Recycling of the catalyst
At the end of the oxidation of thiols to
disulfides, 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. All products are soluble in
dichloromethane but the catalyst is not.
Thus, it could be separated by a simple
filtration and washed with dichloro-
methane and dried at 90 °C for 1 h, and
reused in another reaction with the same
substrate. Even after five runs for the
reaction, the catalytic activity of
TBA7SiV3W9O40 was almost the same as
that freshly used catalyst. The results are
summarized in Table 5.
S S
S S
S
S
S
S
S
S
Table 4. Oxidation of thiols with different substituents by TBA7SiV3W9O40-TiO2 as catalyst with H2O2 as
oxidant M. Ali Nia Asli, M. A. Rezvani, M. Oveisi, L. Abdollahi /CSM Vol.2 No.1, 2014 pp.53-64
64
4. CONCLUSION
The TBA7SiV3W9O40-TiO2 nanoarticle
was very active catalyst system for the
models compound oxidation, while
unmodified TBA7SiV3W9O40 showed
much lower activity. This TiO2/ poly-
oxometalates/H2O2 system provides an
efficient, convenient and practical
method for the syntheses of
symmetrical disulfides.
REFERENCES
[1] T. Ueda, J. Nambu, H. Yokota, M.
Hojo, Polyhedron. 28, 43 (2009).
[2] Y. G. Chen, J. Gong, L.Y. Qu, Coord.
Chem. Rev. 248, 245 (2003).
[3] M. M. Heravi, T. Benmorad, K.
Bakhtiari, F. F. Bamoharram, H. H.
Oskooie, J. Mol. Catal. A: Chem.
264, 318 (2007).
[4] E. Coronado, J. Carlos, G.Garcia
Chem. Rev. 98, 273 (1998).
[5] M. M. Heravi, L. Ranjbar, F.
Derikvand , H. A. Oskooie, F. F.
Bamoharram, J. Mol. Catal. A:
Chemical 265, 186 (2007).
[6] A. F. Shojaie, M. A. Rezvani, M. H.
Loghmani, Fuel Process. Technol.
118, 1(2014).
[7] M. A. Rezvani, A. F. Shojaie, M. H.
Loghmani, Catal. Commun. 25, 36
(2012).
[8] A. Fallah Shojaei, M. A. Rezvani, F.
M. Zonoz, J. Serb. Chem. Soc. 78,
129 (2013).
[9] A. Fallah Shojaei, M. A. Rezvani, M.
Heravi, J. Serb. Chem. Soc. 76, 955
(2011).
[10] A. Fallah Shojaei, M. A. Rezvani, M.
Heravi, J. Serb. Chem. Soc.76, 1513
(2011).
[11] A. Fallah Shojaei, M.H. Loghmani,
Chem. Eng. J. 157, 263 (2010).
[12] R. Harutyunyan, M. A. Rezvani, M.
Heravi, Synth. React. Inorg. Met.-
Org. Chem., 41: 94 (2011).
[13] M. A. Rezvani, R. Harutyunyan, M.
M. Heravi, Synth. React. Inorg. Met.-
Org. Chem.,42, 1232 (2012) .
[14] S. Tangestaninejad, V. Mirkhani, M.
Moghadam, I. Mohammadpoor-
Baltork, E. Shams, Ultrason.
Sonochem. 15, 438 (2008).
[15] Y. Yang, Q. Wuc, Y. Guoa, C. Hu, J.
Mol. Catal. A. 225: 203 (2005).
[16] N. Mizuno, M. Misono, Chem. Rev.
98, 199 (1998).
[17] T. Okuhara, N. Mizuno, M. Misono,
Adv. Catal. 41, 113 (1996).
[18] T. Okuhara, T. Nishimura, H.
Watanabe, J. Mol. Catal. A:
Chem.74, 247 (1992).
[19] W. Zhu, H. Li, X. He, Q. Zhang, H.
Shu, Y. Yan, Catal. Commun. 9, 551
(2008).
Table5. Reuse of the catalyst for oxidation of 4-Chlorothiophenol
(Table 2, entry 4)
Isolated yield (%) Entry
96 1
94 2
94 3
92 4
91 5
)