Synthesis, catalytic oxidation and oxidative bromination reaction of a reusable polymer anchored oxovanadium(IV) complex

Article abstract of DOI:10.1016/j.molcata.2012.02.009

Polymer anchored oxovanadium catalyst was synthesized and characterized. The solid catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). Its catalytic activity was evaluated for the oxidation of various alkenes, sulfides and aromatic alcohols with 30% H₂O₂ under mild reaction conditions. This catalyst was also effective for the oxidative bromination reaction of organic substrates with 80-100% selectivity of mono substituted products with H₂O₂/KBr at room temperature. The above reactions require minimum amount of H₂O₂, short time period and most importantly all the above reactions occur in aqueous medium. The developed catalyst can be facilely recovered and reused six times without significant decrease in activity and selectivity. This result confirms that the polymer anchored complex was not leached during the reaction, suggesting the true heterogeneous nature of the catalyst.

Full text of DOI:10.1016/j.molcata.2012.02.009

Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis, catalytic oxidation and oxidative bromination reaction of a reusable
polymer anchored oxovanadium(IV) complex
∗
Sk. Manirul Islam , Anupam Singha Roy, Paramita Mondal, Noor Salam
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, W.B., India
a r t i c l e i n f o
a b s t r a c t
Article history:
Polymer anchored oxovanadium catalyst was synthesized and characterized. The solid catalyst was char-
acterized by Fourier transform infrared spectroscopy (FT-IR), UV–vis diffuse reflectance spectroscopy
Received 19 October 2011
Received in revised form 3 January 2012
Accepted 18 February 2012
Available online 25 February 2012
(DRS), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). Its catalytic activity
was evaluated for the oxidation of various alkenes, sulfides and aromatic alcohols with 30% H2O2 under
mild reaction conditions. This catalyst was also effective for the oxidative bromination reaction of organic
substrates with 80–100% selectivity of mono substituted products with H2O2/KBr at room temperature.
The above reactions require minimum amount of H2O2, short time period and most importantly all the
above reactions occur in aqueous medium. The developed catalyst can be facilely recovered and reused
six times without significant decrease in activity and selectivity. This result confirms that the polymer
anchored complex was not leached during the reaction, suggesting the true heterogeneous nature of the
catalyst.
Keywords:
Polymer anchored
Oxovanadium(IV) complex
Oxidation
Oxidative bromination
Aqueous medium
H2O2
© 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Recently there has been an increasing interest in developing
catalysts for oxidation of alkane/alkene in the presence of hydrogen
peroxide, PhIO, tert-butyl hydroperoxide, NaIO , etc., as the oxi-
4
dants [7–10]. Apart from N O2 donor ligands, polymeric supports
2
environmental friendly greener processes which are also econom-
ically viable in industrial chemistry. A subject of concern is the
waste minimization by using alternative reagents and catalysts.
Also recovery and separation of products or catalysts from the reac-
tion mixture is one of the major contributions in a chemical process.
In this respect, the use of heterogeneous catalysts has been of great
interest due to easy separation and more stability of these catalysts,
and many attempts have been made on their preparation [1–6].
In recent years, polymers especially chloromethylated
polystyrene have opened new opportunities for application
in the field of supported catalysts due to (i) the fact that the
catalyst is easily separated from the reaction mixture by filtration,
with coordinating ligands such as ␤-diketones (O, O) [11], dipyridy-
lamine (N, N) [12], diphosphine (P, P) [13], etc., have frequently
been employed to form metal complexes for catalytic applications.
The chemical modification of a polymer incorporating bifunctional
ligand, such as N, O donor, has however received less attention.
In the chemical processes, traditional organic solvents are used
in large quantities, which have led to various environmental and
health concerns. As part of green chemistry efforts, a variety of
cleaner solvents have been used as replacements [14]. As an alter-
native solvent, water has been paid extraordinary attention. Apart
from being a chemically interesting solvent, water provides a cheap
(both in cost price and in ecotax) alternative for organic solvents,
making it environmentally and economically interesting as well.
However, for organic synthetic chemists, who are trained to put
compounds in solution and frequently approach organic reactions
from “like need like perspective”, it is less important, because water
is traditionally not a popular choice of solvent. It is commonly
accepted that a mixture of water and one or more nonpolar organic
reactants will usually have low reaction rates and low yields of
the desired products [15]. Sharpless and his colleagues report [16]
that the above assumption does not hold for a variety of organic
reactions (ene reactions, nucleophilic substitutions, Claisen rear-
rangements and Diels–Alder reactions) in the ‘on water’ approach
that they have pioneered [15]. Phase transfer catalysis is becom-
ing an interestingly important technique in organic synthesis. But
(
ii) and that the recovered catalyst can be reused. Surface func-
tionalization and incorporation of metal complexes is one of the
general methods for the preparation of supported catalysts.
Among the variety of catalytically active metal complexes, the
transition metals bearing N, O-donor ligands are of great interest
and several works have been devoted to investigate their catalytic
activity in various catalytic organic reactions. Transition metal
complexes of Mn(III), Fe(III), Cu(II) and Co(II) with polymer sup-
ported ligands (N O2 donor set) have been shown to be active
2
∗ Corresponding author. Tel.: +91 33 2582 8750; fax: +91 33 2582 8282.
E-mail address: manir65@rediffmail.com (Sk.M. Islam).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.02.009

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
39
there is one limitation in this method that many phase transfer
catalysts promote stable emulsion which render work-up difficult.
Regen introduced the new concept of triphase catalysis, in which
the catalyst and each of a pair of reagents are located in sepa-
rated phase [17,18]. The major advantage of triphase catalysis over
phase transfer catalysis is that the catalyst can be removed from
the reaction mixture by simple filtration. So, based on the above
two concepts, we tried to synthesize a polymer anchored metal
complex which catalyze the organic reactions in aqueous medium
without any phase transfer catalyst.
catalyst in which 10 mmol of H O2 (30% in aq.) was added. After
2
the reaction, the organic products were separated from the reac-
tion mixture by extraction with dichloromethane (5 ml ×2). The
combined organic portions were dried and concentrated. Prod-
uct analysis was performed by Varian 3400 gas chromatograph
equipped with a 30 m CP-SIL8CB capillary column and a Flame
Ionization Detector using cyclohexanone as internal standard. All
reaction products were identified by using Trace DSQ II GC–MS.
2.4. General procedure for oxidative bromination reaction of
organic substrates catalyzed by PS-[VO-An]
Here, preparation, characterization and investigation of cat-
alytic activities for oxidation and oxidative bromination reaction of
a polymer anchored oxovanadium complex were made using H O2
as oxygen source. The catalytic activities were also tested with the
recycled catalyst.
In a typical reaction, aqueous 30% H2O2 (20 mmol) was added
to the mixture of substrates (10 mmol) and KBr (20 mmol) taken in
10 ml of water. Catalyst (50 mg) and H2SO4 (5 mmol) were added
to it and the reaction mixture was stirred at room temperature. An
additional 15 mmol H2SO4 was added to the reaction mixture in
three equal portions at 30 min intervals under continuous stirring.
After specified time of the reaction, the catalyst was filtered and the
solid was washed with ether. The combined filtrates were washed
with saturated sodium bicarbonate solution and then shaken with
ether in a separating funnel. The organic extract was dried over
anhydrous sodium sulfate. The products were analyzed by Varian
3400 gas chromatograph equipped with a 30 m CP-SIL8CB capillary
column and a Flame Ionization Detector. Identity of the products
was also confirmed by using Trace DSQ II GC–MS.
2
2
. Experimental
2.1. Materials and instruments
All the reagents used were chemically pure and were of ana-
lytical reagent grade. The solvents were dried and distilled before
use following the standard procedures [19]. Chloromethylated
polystyrene was supplied by Sigma–Aldrich chemicals Company,
USA. Other reagents were obtained from Merck or Fluka.
A Perkin-Elmer 2400C elemental analyzer was used to collect
micro analytical data (C, H and N). Vanadium content of the sample
was measured by Varian AA240 atomic absorption spectropho-
tometer (AAS). The FT-IR spectra of the samples were recorded
on a Perkin-Elmer FT-IR 783 spectrophotometer using KBr pellets.
Diffuse reflectance spectra (DRS) were registered on a Shimadzu
UV/3101 PC spectrophotometer. Mettler Toledo TGA/SDTA 851
instrument was used for the thermogravimertric analysis (TGA).
Morphology of functionalized polystyrene and complex was ana-
lyzed using a scanning electron microscope (SEM) (ZEISS EVO40,
England) equipped with EDX facility.
3. Results and discussion
3.1. Characterization of polymer anchored oxovanadium complex
The reaction of chloromethylated polystyrene, cross-linked
with 5.5% divinylbenzene, with anthranilic acid in DMF leads to
the formation of polymer anchored ligand. During this process
CH2Cl group of the polymer reacts with amine nitrogen of the
anthranilic acid moiety as shown in Scheme 1. The synthesis of
the ligand has been carried out earlier [20]. The polymer anchored
ligand, on reaction with VO(acac)2, gave an oxovanadium(IV) com-
plex which we designate as PS-[VO-An]. The physicochemical data
of the isolated complex are presented in Table 1. These data show
that metal to ligand loading in polymer complex is close to 1:2.
Scheme 1 presents the proposed structure of the anchored complex.
Due to insolubilities of the polymer anchored oxovanadium(IV)
complex in all common organic solvents, its structure has been
established on the basis of elemental analyses, spectroscopic (FT-IR
and electronic) studies, scanning electron micrographs (SEM) and
thermogravimetric analysis (TGA).
2.2. Synthesis of catalyst
The outline for the preparation of polymer anchored VO(IV)
complex is given in Scheme 1.
2.2.1. Synthesis of polymer anchored ligand PS-An
Polymer anchored ligand was prepared according to the liter-
ature [20]. To 1.00 g of the chloromethylated beads suspended in
0 ml of DMF was added 0.750 g of anthranilic acid. This was stirred
for 20 h, filtered, and washed with 200 ml of absolute ethanol.
5
2
.2.2. Loading of metal ions on to the polymer anchored ligand
PS-[VO-An]
The loading of metal ions on the polymeric support was carried
3.1.1. IR spectral study
Various frameworks bonding present in the polymer supported
metal catalyst were obtained from the FT-IR spectrum (Fig. 1). The
sharp C Cl peak due to CH2Cl group in polymer at 1264 cm
was seen as weak after loading of anthranilic acid on the support
[21]. A strong broad band around 3455 cm in polymeric support
is observed due to NH (secondary amine) vibration. Above two IR
data confirm the loading of anthranilic acid on the polymer matrix.
The ꢀ(C O), ꢁasym (COO) and ꢁsym (COO) stretching vibrations are
−
1
out as follows: the polymer anchored ligand (1.00 g) was stirred for
2
At the end of this reaction the metal loaded polymer was filtered,
washed thoroughly with ethanol, dioxane and methanol to ensure
the removal of any unreacted metal ions and dried in vacuum for
◦
4 h with 0.100 g of VO(acac) in 20 ml of absolute ethanol at 70 C.
2
−
1
◦
6
h at 90 C.
−
1
observed at 1705, 1589 and 1435 cm for polymer anchored ligand
[22]. The amino acid was found to be bidentate ligands and bound
to the central metal ion through the carboxylic OH and the sec-
2.3. General procedure for oxidation reaction catalyzed by
PS-[VO-An]
−
1
ondary amino group; NH . The bands at 1589 and 1435 cm , due
The liquid phase oxidation reactions were carried out in a two-
to ꢁasym (COO ) and ꢁsym (COO ) of the amino acids, appear in the
−
1
necked round bottom flask fitted with a water condenser and
placed in an oil bath at different temperatures under vigorous stir-
ring for a certain period of time. Substrates (5 mmol) were taken
in water (5 ml) for different sets of reactions together with 50 mg
complex at 1555 and 1420 cm . The shift of these two bands sug-
gests the involvement of the carboxylic groups of the amino acid in
complex formation [23,24]. The participation of OH group in bond-
ing was confirmed from the shift in the position of the ı OH of the

4
0
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
P
CH2
NH
NH2
DMF
-HCl
P
CH Cl
2
+
OH
OH
C
O
C
O
(1)
(2)
VO(acac)2
0
7
0 C
Ethanol
P
O
CH2
NH
O
V
C
O
O
HN
C
O
CH2
(3)
P
Scheme 1. Synthesis of polymer anchored VO(IV) complex.
free ligand (1410 cm 1) by 8–10 cm−1 in the spectra of the com-
−
PS-[VO-An] is given in Fig. 2. The reflectance spectrum of the PS-
[VO-An] catalyst shows a broad band from 300 nm to 400 nm which
may be assigned to charge transfer transition, while the shoulder
around 580 nm may be attributed to d-d electronic transition of
vanadium present in the complex [29]. In the recycled catalyst all
types of transition bands were present which indicates its hetero-
geneous nature.
plex. Also, it was evidenced from the phenolic C O stretch band
1242 cm ) which was found to be shifted into the higher range
25]. The decrease in the intensity of N H stretching frequency
of the secondary amine group in the complex indicates that the
N’ of amino group may be coordinated to the metal [26]. Polymer
anchored oxovanadium complex showed two new bands at 534 and
56 cm assigned to ꢁ(V O) and ꢁ(V N), respectively [27]. Again
a band appeared around 970 cm due to stretching vibration of
O [28] in polymer anchored catalyst. In Fig. 1(d), the stretching
vibrations do not change significantly, suggesting existence of all
the properties in the recycled catalyst.
−1
(
[
‘
−1
4
−1
3.1.3. Scanning electron micrographs (SEM) and energy
V
dispersive X-ray analyses (EDAX)
Field emission-scanning electron micrographs of single bead of
pure chloromethylated polystyrene, polymer anchored ligand (PS-
An) and its complex PS-[VO-An] were recorded and it was observed
that the morphological changes occurred on the polystyrene beads
at various stages of the synthesis. The SEM images of polymer
anchored ligand (A) and the immobilized vanadium complex
(B) on functionalized polymer are shown in Fig. 3. The pure
3
.1.2. DRS–UV spectroscopy
The electronic spectrum of the polymer anchored complex was
recorded in diffuse reflectance spectrum mode as MgCO /BaSO4
disc. The UV–vis spectrum (DRS) of the polymer anchored complex
3
Table 1
Chemical composition polymer anchored oxovanadium(IV) complex.
Compound
C (%)
H (%)
N (%)
Cl (%)
Metal (%)
Ligand loading
Metal ion loading
(mmol equiv. g of
Ligand: metal
−
1
−1
(mmol equiv. g of
resin)
resin)
PS-An
PS-[VO-An]
75.01
68.98
5.90
5.11
3.81
4.05
6.39
3.77
–
6.61
–
2.89
–
1.29
–
2.24:1

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
41
Fig. 1. FT-IR spectra of chloromethylated polystyrene (a), polymer anchored ligand (b), polymer anchored oxovanadium catalyst (c) and reused catalyst (d).
chloromethylated polystyrene bead has a smooth surface (not
shown) while polymer anchored ligand and complex show slight
roughening of the top layer of polymer beads. This roughening is
relatively more in complex. Also the presence of vanadium metal
can be further proved by energy dispersive spectroscopy analysis
of X-rays (EDAX) (Fig. 4) which suggests the formation of metal
complex with the polymer anchored ligand.
3.2. Catalytic activity
Since polymer anchored metal systems exhibit catalytic activ-
ity in a wide range of the industrially important processes and have
been extensively studied, we have decided to investigate the cat-
alytic activity of polymer anchored oxovanadium complex in the
oxidation of alkenes, sulfides and aromatic alcohols and oxidative
bromination reaction with H O2 as oxygen source.
2
3.1.4. TGA studies
Thermal stability of complex was investigated using TGA–DTA
3
.2.1. Oxidation of alkenes with H O catalyzed by PS-[VO-An]
To test the catalytic activity of the present catalyst, first oxi-
2 2
◦
at a heating rate of 10 C/min in air over a temperature range
◦
of 30–600 C. TGA curves of polymer anchored ligand and oxo-
dation of olefins was tested with hydrogen peroxide in water. For
optimization of reaction conditions, we choose oxidation of styrene
as a probe reaction. In this case styrene is selectively converted to
benzaldehyde (Scheme 2). Effects of various oxidants on the con-
version of styrene and selectivity of benzaldehyde using polymer
anchored VO(IV) catalyst are summarized in Table 2. We investi-
vanadium complex are shown in Fig. 5. Polymer anchored ligand
◦
decomposed in the temperature range 380–390 C. After complex-
ation of vanadium on polymer anchored ligand, thermal stability of
◦
the immobilized complex improved by 30–50 C. Polymer anchored
◦
oxovanadium complex decomposed at 410–440 C. So from the
thermal stability, it concludes that polymer anchored metal com-
plex degraded at considerably higher temperature.
gated the ability of various oxidants like TBHP, H O , NaIO , PhIO,
KHSO5 and molecular oxygen in styrene oxidation (Table 2, entries
2
2
4

4
2
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
Fig. 2. DRS–UV–vis absorption spectra of polymer anchored VO(IV) complex.
Fig. 3. FE SEM images of polymer anchored ligand (A) and polymer anchored VO(IV) complex (B).
Table 2
Oxidation of Styrene catalyzed by polymer anchored VO(IV) catalyst.a
Entry
Oxidant
Temperature
Conversion (%)b
Selectivity (%)b
Benzaldehyde
TON
–
Styrene oxide
Formaldehyde
◦
1
2
3
4
5
6
7
8
None
O2
60
60
60
60
60
60
60
50
70
60
60
60
C
C
C
C
C
C
C
C
C
C
C
C
0
5
–
73
85
80
58
43
53
85
75
81
88
73
–
25
12
18
40
55
45
14
23
18
10
25
–
2
1
2
3
2
2
1
2
1
2
2
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
3.88
H2O2
TBHP
PhIO
NaIO4
KHSO5
H2O2
H2O2
H2O2
H2O2
H2O2
96
57
21
37
29
57
96
68
96.4
8
74.42
44.19
16.28
28.68
22.48
44.19
74.42
52.72
74.73
–
9
c
1
1
1
0
1
2
d
e
a
b
c
Reaction conditions: catalyst (0.05 g), styrene (5 mmol), water (5 ml), oxidant (10 mmol), time (8 h).
Determined by GC.
mmol ratio of styrene and TBHP = 1:1.
mmol ratio of styrene and TBHP = 1:3.
Reaction carried out without catalyst.
d
e

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
43
Fig. 4. EDX images of polymer anchored ligand (A) and polymer anchored VO(IV) complex (B).
2
–7). Without any oxidant, styrene oxidation did not occur (Table 2,
olefins were carried out at this temperature. The activity of the
polymer anchored VO(IV) catalyst in the oxidation of styrene was
entry 1). The reaction shows almost no conversion when molecular
oxygen is used as oxidant (Table 2, entry 2). As shown in Table 2,
H O is the best oxidant among the six oxidants that are used in
also evaluated using three different molar ratios of styrene to H O2
2
(Table 2, entries 3, 10 and 11). The conversion of styrene increased
2
2
styrene oxidation reaction (Table 2, entry 3). Oxidants like TBHP,
from 68% to 96% on increasing the styrene: H O2 molar ratio from
2
PhIO and NaIO4 are clearly less efficient than H O2 as is evident
1:1 to 1:2. On further increasing this ratio to 1:3, the conversion was
hardly affected. From the results, it was suggested that 1:2 ratio of
2
from the percentage of conversion of styrene (Table 2, entries 4–6).
The use of KHSO5 is not favored because a buffer is needed (Table 2,
entry 7). The effect of temperature on the performance of the cat-
alyst was also studied at three different temperatures, viz. 50, 60
styrene to H O2 is sufficient for the good conversion of styrene.
2
Blank experiment (Table 2, entry 12) in the presence of oxidant
under the same reaction conditions in the absence of catalyst was
also investigated in the oxidation of styrene. The obtained result
◦
and 70 C at fixed amount of styrene (5 mmol), H O (10 mmol),
2
2
VO(IV) catalyst (50 mg) in 5 ml water (Table 2, entries 3, 8 and 9).
A maximum of 96% conversion was achieved on carrying out the
showed that only H O2 has a poor ability to oxidize the styrene.
2
The above optimized reaction conditions can be applied to the
oxidation reaction of other olefins catalyzed by polymer anchored
VO(IV) catalyst and the results are shown in Table 3. This catalyst
efficiently converts olefins to their corresponding allylic products
with H O in aqueous medium. Styrene was converted to benzalde-
◦
◦
◦
reaction at 60 C. At 50 C, the conversion was low. At 70 C, the ini-
◦
tial conversion of styrene was higher than that at 60 C but when
reaction continues the conversion of styrene was almost same with
◦
◦
conversion at 60 C at 8 h of reaction time. Therefore, 60 C was
the optimum temperature and all catalytic oxidation reactions of
2
2
hyde in high yields. In the oxidation of cyclohexene, allylic products
O
CHO
Polymer anchored VO(IV) catalyst
H O / water
+
2
2
0
6
0 C
Benzaldehyde
Styrene oxide
Styrene
Scheme 2. Oxidation product of styrene.

4
4
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
Table 3
Olefins oxidation using H2O2 catalyzed by polymer anchored VO(IV) catalyst.
a
Entry
1
Substrate
Conversion (%)b
83
Product (selectivity %)b
TON
Cyclohexene
2-Cyclohexene-1-one (64)
64.34
2-Cyclohexene-1-ol (29)
Cyclohexene epoxide (7)
Benzaldehyde (78)
Benzil (16)
trans-Stilbene oxide (6)
Verbenol (32)
2
4
trans-Stilbene
␣-Pinene
65
49
50.39
37.98
Verbenone (55)
␣
-Pinene oxide (13)
4-Methyl benzaldehyde (84)
-Methylstyrene epoxide (16)
4-Chloro benzaldehyde (86)
-Chlorostyrene epoxide (14)
4-Nitro benzaldehyde (83)
-Nitro styrene oxide (17)
5
6
7
4-Methylstyrene
4-Chlorostyrene
4-Nitrostyrene
84
86
83
79
65.12
66.67
64.34
61.24
4
4
4
8
␣-Methyl styrene
Acetophenone (100)
a
◦
Reaction conditions: catalyst (0.05 g), substrate (5 mmol), water (5 ml), H2O2 (10 mmol), temperature (60 C), time (8 h).
Determined by GC.
b
2
-cyclohexene-1-one and 2-cyclohexene-1-ol were produced. Sub-
significant effect on the conversion and selectivity of methyl phenyl
sulfide oxidation reaction. Conversion increased when amount of
H O increased from 5 mmol (run 4) to 10 mmol (run 1) but there
stituted styrene produced selectively corresponding aldehydes.
Acetophenone was detected in the oxidation of ␣-methyl styrene
as major product. trans-Stilbene was also oxidized by this hetero-
geneous catalyst in high yields. trans-Stilbene gives benzaldehyde
as a major product with benzil. This catalytic system shows a good
activity in the case of ␣-pinene. In the oxidation of ␣-pinene, the
major products were verbenone and verbenol.
2
2
was no significant change in conversion when amount changed to
15 mmol (run 5) at the same reaction conditions. The selectivity of
sulfoxide decreased with the increase of the amount of H O . This
2
2
is due to further oxidation of sulfoxide. The reaction was also mon-
itored with different catalyst amount. At a lower catalyst amount
(40 mg, run 6) the reaction took longer time and was not complete.
While with 50 mg of catalyst (run 1) the reaction was complete
in 4 h, yielding 92% sulfoxide with a small amount of sulfone (8%).
It was also observed that by increasing the amount of catalyst to
3
.2.2. Oxidation of sulfides with H O2 catalyzed by PS-[VO-An]
In continuation of our interest in oxidation reactions, we were
2
in search of a high-yielding, catalytic, cheap and environmentally
benign reagent for sulfide oxidation and considered PS-[VO-An] as a
choice. Thus when methyl phenyl sulfide was treated with PS-[VO-
6
0 mg (run 7), the reaction led to more sulfone along with sulfoxide.
Therefore, an optimum amount of catalyst and oxidant with a suit-
able solvent are necessary for the above oxidation and based on the
above observation we come to a conclusion that 50 mg of polymer
An] and 30% H O2 in water at room temperature, methyl phenyl
2
sulfoxide was obtained. The general reaction is shown in Scheme 3.
Considering methyl phenyl sulfide as a model substrate, it was sub-
jected to different reaction conditions as shown in Table 4. The
effect of temperature was investigated for the oxidation of methyl
phenyl sulfide and results are shown in Table 4. While reaction
temperature increased from room (run 1) to 50 C (run 2) there
was a slight increase in conversion but conversion kept constant
on further increase in temperature (run 3). Amount of H O2 has a
anchored catalyst, 10 mmol 30% H O2 and water as a solvent make
2
for the best condition for the oxidation of sulfides to sulfoxides with
good selectivity. It was also found that without H O , PS-[VO-An]
2
2
alone cannot oxidize sulfides (run 8).
◦
To examine the reactivity of the catalyst further, oxidation of
other sulfides also has been investigated (Table 5). Substrate scope
is extended to diphenyl sulfide, ethyl phenyl sulfide, dibutyl sulfide,
etc. The sulfoxides were selectively obtained in all cases. A series of
substrates, aryl alkyl, aryl allyl and dialkyl sulfides, can be oxidized
to the corresponding sulfoxides. The reactivity and conversion were
dependent on the nature of the substituent. In the case of allyl sul-
fides, small oxidation was observed at the carbon–carbon double
bond. Similarly, benzylic sulfides can be oxidized to the correspond-
2
ing sulfoxides with small oxidation products of the benzylic C
bond.
H
3
.2.3. Oxidation of aromatic alcohols with H O catalyzed by
2 2
PS-[VO-An]
Although several excellent catalysts for the oxidation of alco-
hols to the corresponding aldehydes or ketones have been reported,
most of them required organic solvents and addition of base, which
violate the requirements of the “green chemistry”. Water is the
most ideal solvent because it is not only a green solvent but also can
avoid the hazards associated with the use of oxidisable organic sol-
vents. Base-free systems are also highly desired in that the presence
of base probably leads to the unwanted by-products. In line with
the requirements of “green chemistry”, we chose water as a reac-
tion medium to conduct the oxidation of alcohols in the absence of
base (Scheme 4).
PS-[VO-An]
PS-An
1
00
200
300
400
0
500
600
Temperature ( C)
Fig. 5. Thermogravimetric weight loss plots for polymer anchored ligand and poly-
mer anchored VO(IV) complex.

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
45
O
O
S
S
S
PS-[VO-An]
H O /water
CH3
CH3
CH3
+
2
2
O
RT
methyl phenyl sulfide
methyl phenyl sulfoxide
methyl phenyl sulfone
Scheme 3. Oxidation product of methyl phenyl sulfide.
Table 4
Oxidation of methyl phenyl sulfide with polymer anchored VO(IV) catalyst.
a
◦
Conversion (%)b
Run
Temperature ( C)
Amount of H2O2 (mmol)
Selectivity of sulfoxide/sulfone
TON
b
(
%)
1
2
3
4
5
Room
10
10
10
5
15
10
10
–
94
92/8
85/15
79/21
95/5
71/29
89/11
69/31
–
72.87
73.64
73.64
49.61
74.42
75.58
61.37
–
◦
50
60
C
C
95
95
64
96
78
95
–
◦
Room
Room
Room
Room
Room
c
6
7
d
8
a
Conditions: methyl phenyl sulfide (5 mmol), Solvent (5 ml); 50 mg catalyst.
Conversion and selectivity were determined by GC.
Catalyst amount = 40 mg.
b
c
d
Catalyst amount = 60 mg.
Table 5
a
Oxidation of sulfides using 30% H2O2 catalyzed by polymer anchored VO(IV) catalyst.
b
Entry
Substrates
Conversion (%) /time (h)
Selectivity of
sulfoxide/sulfone (%)
TON
b
1
2
3
4
5
6
7
8
C4H9SC4H9
PhSPh
PhSC2H5
PhSC6H13
PhSCH2Ph
PhSCH2CH CH2
p-ClC6H4SCH3
p-CH3C6H4SCH3
88/5
95/4
91/4
85/5
87/4
69/5
89/4
92/4
77/23
92/8
68.22
73.64
70.54
65.89
67.44
53.49
68.99
71.32
88/12
89/11
86/14
87/13
73/27
88/12
a
Conditions: sulfide (5 mmol); 30% aq H2O2 (10 mmol); water (5 ml); 50 mg catalyst at room temperature.
Conversion and selectivity were determined by GC.
b
Table 6
a
Oxidation of aromatic alcohols with 30% H2O2 catalyzed by PS-[VO-An] catalyst.
Entry
1c
Aromatic alcohol
Benzyl alcohol
Conversion (%)b
64
Product selectivity (%)b
TON
Benzaldehyde (91)
Benzoic acid (9)
49.61
2d
3e
4f
5g
6
Benzyl alcohol
96
96
58
96.4
82
84
78
77
Benzaldehyde (97)
Benzoic acid (3)
74.42
74.42
44.96
74.73
63.57
65.12
60.47
59.69
Benzyl alcohol
Benzaldehyde (73)
Benzoic acid (27)
Benzaldehyde (87)
Benzoic acid (13)
Benzaldehyde (79)
Benzoic acid (21)
Benzyl alcohol
Benzyl alcohol
4-Methoxybenzyl alcohol
4-Methylbenzyl alcohol
4-Nitrobenzyl alcohol
1-Phenyl ethanol
4-Methoxy benzaldehyde (83)
-Methoxy benzoic acid (17)
4-Methyl benzaldehyde (90)
-Methyl benzoic acid (10)
4-Nitro benzaldehyde (88)
-Nitro benzoic acid (12)
4
7
4
8
4
9
Acetophenone (100)
a
◦
Reaction condition: aromatic alcohol (5 mmol), 30% H2O2 (10 mmol), water (5 ml), catalyst (50 mg), temperature (50 C), time (6 h).
b
c
d
e
f
Determined by GC.
Benzyl alcohol:H2O2 = 1:1.
Benzyl alcohol:H2O2 = 1:2, temperature (50 C).
◦
Benzyl alcohol:H2O2 = 1:3.
◦
◦
Temperature (40 C).
g
Temperature (60 C).

4
6
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
CH OH
OH
OH
2
CHO
COOH
PS-[VO-An]
H O /KBr/H SO
4
PS-[VO-An]
H O / water
2
2
2
CHO
CHO
Br
water
+
2
2
0
Scheme 5. Oxidative bromination reaction of salicylaldehyde.
5
0 C
benzyl alcohol
benzaldehyde
benzoic acid
Scheme 4. Oxidation product of benzyl alcohol.
58%, 96% and 96.4% conversion was recorded corresponding to 40,
◦
◦
5
0 and 60 C. Therefore, 50 C was selected as the optimum tem-
perature. At lower temperature, the conversion of benzyl alcohol
into benzaldehyde was considerably decreased.
We examined the catalytic activity of the polymer anchored
VO(IV) catalyst for the oxidation of benzyl alcohol by varying the
In order to study the further catalytic activity of the catalyst
for the oxidation of aromatic alcohols, a series of benzylic alco-
hols were used. Substituted benzyl alcohols are also oxidized to
corresponding aldehydes. In all above cases trace amount of acids
were observed. On the otherhand, 1-phenyl ethanol is selectively
converted to acetophenone on oxidation with this catalyst.
various reaction conditions. The effect of the amount of H O2 on
2
the oxidation of benzyl alcohol into benzaldehyde was studied and
the results are shown in Table 6. The percent conversion of benzyl
alcohol into benzaldehyde is dependent upon H O up to a consid-
2
2
erable amount. The amount of H O2 was varied in terms of benzyl
2
alcohol to H O molar ratio. Benzyl alcohol to H O2 molar ratio of
2
2
2
1
5
:1 resulted in 64% conversion when the amount of catalyst was
0 mg, temperature was 50 C and reaction time was 6 h. When
◦
3.2.4. Oxidative bromination catalyzed by PS-[VO-An]
Oxidative bromination reaction has been optimized taking sali-
cylaldehyde as a substrate (Scheme 5). Two different parameters
benzyl alcohol to H O2 molar ratio was raised to 1:2, the conver-
sion increased to 96% under all other similar conditions. However,
the conversion was found almost constant when benzyl alcohol to
H O molar ratio was further increased up to 1:3. Therefore 1:2
molar ratio of benzyl alcohol and H O2 was found to be optimum.
The results for the oxidation of benzyl alcohol in the presence of dif-
ferent amounts of catalyst are presented in Fig. 6. All the tests were
conducted with 5 mmol substrate, 10 mmol H O2 and at 50 C in
5
alyst afforded a conversion of 23% within 6 h. No by-products such
as benzoic acid were detected under this reaction condition. When
the amount of the catalyst increased up to 0.035 g, the conversion
had an apparent increase and a 78% conversion was achieved. But in
this condition, a little amount of benzoic acid was detected. A fur-
ther increase in catalyst amount up to 0.05 g led to an increase in
the conversion (96%). However, when the catalyst amount contin-
uously increased up to 0.065 g and 0.075 g, the conversion remains
almost constant but the selectivity of benzaldehyde reduces. These
findings indicate that the oxidation of benzyl alcohol is highly
dependent on the amount of the solid catalyst. The influence of
temperature on the oxidation of benzyl alcohol was investigated
2
such as volume of sulfuric acid and H O2 were varied at room
2
temperature. Experimental results showed that for a fixed amount
of salicylaldehyde (10 mmol), catalyst (0.050 g), KBr (20 mmol) in
2
2
2
1
0 ml of water at least 20 mmol of H SO4 and 20 mmol of H O2
2
2
were found to be sufficient to give maximum conversion. Con-
version of salicylaldehyde significantly improves from 56% to 98%
◦
2
with the increment of H SO4 from 10 mmol to 20 mmol. The pres-
2
ml water under base free condition. 0.02 g polymer anchored cat-
ence of excess acid may decompose the catalyst under the above
reaction conditions. H SO4 was added to the reaction mixture in
2
definite time interval to prevent decomposition of catalyst. Under
optimized reaction conditions, the performance of the polymer
anchored catalyst was tested and the results are summarized in
Table 7. Approximately 98% conversion of salicylaldehyde with ca.
1
00% selectivity towards the formation of 5-bromosalicylaldehyde
was achieved with this catalyst under optimized conditions. The
salt effect was also studied for the oxidative bromination reac-
tion of salicylaldehyde. The catalyst was employed using H O2 as
2
an oxidant and MBr (M = Li, Na, K) as a bromine source (Table 7).
Among the three salts, KBr has been found to be the most efficient
bromine source and a monoselective product is obtained. The use
of sodium bromide did not produce regioselective products. LiBr,
though efficient, is less selective than KBr. Influence of tempera-
ture was also studied but there was no effect on the conversion
of salicylaldehyde. With increase of temperature the conversion
of salicylaldehyde was almost same. The role of hydrogen per-
oxide was confirmed by conducting a blank experiment where
the formation of bromo compound was not observed Oxidative
bromination reaction of other organic substrates has been stud-
ied under optimized reaction conditions and the results are shown
in Table 7. All substrates are selectively converted to their mono-
brominated products. Substrates like phenol, resorcinol, anisole
and N,N-dimethylaniline showed high conversion and excellent
para-selectivity. The inactive substrates, like benzene, have shown
low conversion and required longer reaction time. On the other
hand, deactivated aromatic ring did not show any conversion at
this reaction conditions. 4-Substituted aromatics are selectively
converted to 2-bromo derivatives and vice versa.
◦
by performing the reaction at a temperature range from 40 to 60 C
with all other parameters fixed. The results are given in Table 6
which reveals that the conversion is dependent on temperature.
1
00
1
9
8
00
0
9
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
0
Conversion of benzyl alcohol
Selectivity of benzaldehyde
70
6
0.08
0
4. Test for heterogeneity
0
.02
0.03
0.04
0.05
0.06
0.07
Amount of catalyst (g)
The leaching of vanadium from polymer anchored vanadium
complex was confirmed by carrying out analysis of the used catalyst
EDX, IR) as well as the product mixtures (AAS, UV–vis). Analysis
of the used catalyst did not show appreciable loss in the vanadium
Fig. 6. Effect of amount of catalyst on oxidation reaction of benzyl alcohol
(
catalyzed by polymer anchored VO(IV) complex. Reaction condition: [benzyl alco-
◦
hol] = 5 mmol, H2O2 (10 mmol), water (10 ml), temperature (50 C).

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
47
Table 7
Oxidative bromination of various organic substrates catalyzed by polymer anchored VO(IV) catalyst.
a
Entry
1c
Substrate
Conversion (%)b (time/h)
Product selectivityb,c
TON
Salicylaldehyde
Salicylaldehyde
Salicylaldehyde
Salicylaldehyde
Phenol
58(3.0)
98(3.0)
89(3.0)
78(3.0)
96(2.5)
95(2.5)
93(2.5)
80 (6)
93 (3)
96(2.5)
93 (3)
86 (3)
88 (3)
92 (3)
92 (3)
10 (6.0)
–
5-Bromo-2-hydroxy benzaldehyde (100)
5-Bromo-2-hydroxy benzaldehyde (100)
5-Bromo-2-hydroxy benzaldehyde (79)
5-Bromo-2-hydroxy benzaldehyde (64)
4-Bromophenol (100)
4-Bromo-1,3-dihydroxybenzene (100)
2-Bromo-4-methylphenol (100)
2-Bromo-4-nitrophenol (100)
4-Bromoanisole (100)
44.96
75.97
68.99
60.47
74.42
73.64
72.09
62.02
72.09
74.42
72.09
66.67
68.22
71.32
71.32
7.75
d
2
3
4
e
f
5
6
7
8
9
Resorcinol
4-Methylphenol
4-Nitrophenol
Anisole
10
11
12
13
14
15
16
17
Aniline
4-Bromoaniline (83)
4-Methylaniline
4-Chloroaniline
2-Methylaniline
2-Chloroaniline
N,N-Dimethylaniline
Benzene
2-Bromo-4-methylaniline (85)
2-Bromo-4-chloroaniline (100)
4-Bromo-2-methylaniline (80)
4-Bromo-2-chloroaniline (91)
4-Bromo-N,N-dimethylaniline (100)
Bromobenzene (100)
Nitrobenzene
–
–
a
b
c
d
e
f
Conditions: substrate (10 mmol); KBr (20 mmol); H2SO4 (20 mmol); 30% aq. H2O2 (20 mmol), water (10 ml), 50 mg catalyst at room temperature.
Conversion and selectivity were determined by GC.
H2SO4 (10 mmol).
KBr as bromine source.
LiBr as bromine source.
NaBr as bromine source.
content as compared to the fresh catalyst. IR spectrum of the recy-
cled catalyst was quite similar to that of fresh sample indicating
the heterogeneous nature of this complex. Analysis of the product
mixtures showed that if any vanadium was present it was below
the detection limit. The UV–vis spectroscopy was also used to deter-
mine the stability of this catalyst. The UV–vis spectra of the reaction
solution, at the first run, did not show any absorption peaks char-
acteristic of vanadium metal, which indicates that the leaching of
metal did not take place during the course of the reaction. The metal
content of the recycled catalyst was determined with the help of
AAS and it was found that vanadium content of the recycled catalyst
remained almost unaltered. These observations strongly suggest
that the present catalyst is truly heterogeneous in nature.
after 4 h and the filtrate was allowed to react up to the completion of
the reaction (6 h). The reaction mixture of 4 h and the filtrate were
analyzed by gas chromatography. No change in the conversion (%)
as well as selectivity (%) was found which indicates that the present
catalyst was heterogeneous in nature.
5. Recycling of catalyst
The catalyst remains insoluble in the present reaction conditions
and hence can be easily separated by simple filtration followed by
washing. The catalyst was washed with dichloromethane and dried
◦
Heterogeneity test was carried out for the oxidation of styrene
as an example. For the rigorous proof of heterogeneity, a test was
at 100 C. Oxidation of styrene, methyl phenyl sulfide and benzyl
alcohol was carried out with the recycled catalyst under the opti-
mized reaction conditions. The catalyst was recycled in order to test
its activity as well as stability. The obtained results are presented
in Fig. 7. As seen from Fig. 7, the recycled catalyst did not show
any appreciable change in the activity which indicates that the cat-
alyst is stable and can be regenerated for repeated use. Similarly,
recycling of the catalyst was tested for the oxidative bromination
reaction of salicylaldehyde. No appreciable change in conversion as
well as selectivity indicates that the catalyst can be reused.
◦
carried out by filtering catalyst from the reaction mixture at 60 C
Oxidation of styrene
Oxidation of methyl phenyl sulfide
Oxidation of benzyl alcohol
bromination of salicylaldehyde
100
8
0
0
0
0
6. Conclusions
6
4
2
Chloromethylated polystyrene anchored oxovanadium com-
plex was prepared and characterized. This complex was proved to
be active and reusable catalyst for the oxidation of alkenes, sulfides
and alcohols. This catalyst also shows excellent catalytic activity
in oxidative bromination reaction. Another important factor is the
stability and recyclability of the catalyst under the reaction condi-
tions used. This heterogeneous catalyst shows no significant loss of
activity in the recycling experiments. The active sites do not leach
out from the support and thus can be reused without appreciable
loss of activity, indicating that the anchoring procedure was effec-
tive. The reusability of this catalyst is high and can be reused six
times without significant decrease in its initial activity. We hope
that the present catalytic system has a bright future in industrial
application.
0
1
2
3
4
5
6
No of cycles
Fig. 7. Recycling efficiency for the oxidation of styrene, methyl phenyl sulfide, ben-
zyl alcohol and oxidative bromination of salicylaldehyde with PS-[VO-An] complex.

4
8
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 38–48
Acknowledgements
[9] K.C. Gupta, A.K. Sutar, J. Appl. Polym. Sci. 108 (2008) 3927.
10] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, P. Iyer, React. Funct. Polym. 34 (1997) 153.
11] I. Cauz, C. Caze, Eur. Polym. J. 29 (1993) 1615.
12] M.R. Kartz, D.G. Hendicker, Polymer 27 (1986) 1641.
[13] M.J. Sundell, J.H. Nasman, Chem. Tech. 23 (1993) 16.
14] C.J. Li, Green Chem. 10 (2008) 151.
15] J.E. Klijin, J.B.F.N. Engberts, Nature 435 (2005) 746.
16] S. Narayan, J. Muldoon, M.G. Finn, V.V. Fokin, H.C. Kolb, K.B. Sharpless, Angew.
Chem. Int. Ed. 44 (2005) 3275.
17] S.L. Regen, Angew. Chem. Int. Ed. 18 (1979) 421.
18] S. Quici, S.L. Regen, J. Org. Chem. 44 (1979) 3436.
19] A.I. Vogel, Test Book of Practical Organic Chemistry, 5th ed., Longman, London,
[
[
[
We acknowledge Department of Science and Technology (DST),
Council of Scientific and Industrial Research (CSIR) and University
Grant Commission (UGC), New Delhi, India for funding.
[
[
[
References
[
[
[
[1] J.H. Clark, A.P. Kybett, D.J. Macquarrie, Supported Reagents: Preparation, Anal-
ysis and Application, VCH, Weinheim, 1992.
1989.
[
[
2] J.H. Clark, Supported Reagents in Organic Reactions, VCH, Weinheim, 1994.
3] K. Smith (Ed.), Solid Supports and Catalysts in Organic Reactions, Ellis Horwood
Ltd., Chichester, 1992.
[
[
20] N.L. Holy, J. Org. Chem. 44 (1979) 239.
21] V.B. Valodkar, G.L. Tembe, M. Ravindranathan, R.N. Ram, H.S. Rama, J. Mol. Catal.
A: Chem. 208 (2004) 21.
22] G.G. Mohamed, M.M. Omar, A.H.H. Hindy, Spectrochim. Acta. A 62 (2005) 1140.
23] G.K. Sandhu, S.P. Verma, Polyhedron 6 (1987) 587.
24] N. Rao, P.V. Rao, V. Raju, M.C. Ganorkar, Indian J. Chem. 24A (1985) 877.
25] Z.M. Zaki, S.S. Haggag, A.A. Soayed, Spectrosc. Lett. 31 (1998) 757.
26] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Com-
pounds, Wiley, New York, 1997.
27] M.R. Maurya, M. Kumar, U. Kumar, J. Mol. Catal. A: Chem. 273 (2007) 133.
28] R.A. Shiel, K. Venkatasubbaiah, C.W. Jones, Adv. Synth. Catal. 350 (2008) 2823.
29] K.M. Parida, S. Singha, P.C. Sahoo, J. Mol. Catal. A: Chem. 325 (2010) 40.
[
[
[
[
[
4] D.C. Sherrington, A.P. Kybett (Eds.), Supported Catalysts and Their Applications,
RSC, Cambridge, 2001.
5] S. Mukherjee, S. Samanta, B.C. Roy, A. Bhaumik, Appl. Catal. A: Gen. 301 (2006)
[
[
[
[
[
7
9.
6] S.K. Maiti, S. Dinda, M. Nandi, A. Bhaumik, R.G. Bhattacharyya, J. Mol. Catal. A:
Chem. 287 (2008) 135.
7] V. Mirkhani, M. Moghadam, S. Tangestaninejad, B. Bahramian, Appl. Catal. A:
Gen. 311 (2006) 43.
[
[
[
8] R. Antony, G.L. Tembe, M. Ravindranathan, R.N. Ram, J. Mol. Catal. A: Chem. 171
(
2001) 159.