Nano composite mixed-addenda vanadium substituted polyoxometalate-TiO2 as a green, reusable and efficient catalyst for rapid and efficient synthesis of symmetric disulfides under ultrasound irradiation

 
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.  
 
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