A comparative study between transition-metal-substituted Keggin-type tungstosilicates supported on anatase leaf as catalyst for synthesis of symmetrical disulfides

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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.  
 
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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