Polymer-anchored oxovanadium(IV) complex for the oxidation of thioanisole, styrene and ethylbenzene

Article abstract of DOI:10.1016/j.molcata.2009.11.004

The ligand 2-(2′-hydroxyphenyl)-1H-imidazoline (Hpimin) was covalently linked to chloromethylated polystyrene (PS-Cl). This resin was then allowed to react with vanadyl sulfate to afford the polymer-anchored oxovanadium(IV) complex, PS-[VO(pimin)_x], which was characterised using elemental analysis, IR and AFM. This catalyst was shown to catalyse the hydrogen peroxide-based oxidation of styrene, ethylbenzene and thioanisole. A maximum conversion of ethylbenzene (30.6%) and styrene (99.9%) was obtained by using 0.025 g of PS-[VO(pimin)_x] and 4 equiv. of hydrogen peroxide at 80 °C after 6 h. The oxidation of thioanisole proceeded quantitatively (99.6%) within 4 h, with 0.025 g of PS-[VO(pimin)_x], at room temperature with 2 equiv. of hydrogen peroxide. At the steady state, the main products from the oxidation of the various substrates were benzaldehyde (57.6%) > 1-phenylethane-1,2-diol (16.2%) > benzoic acid (7.1%) > styrene oxide (3.1%) for styrene; acetophenone (20.5%) > benzaldehyde (2.7%) for ethylbenzene; and methyl phenyl sulfoxide (67.4%) > methyl phenyl sulfone (31.9%) for thioanisole.

Full text of DOI:10.1016/j.molcata.2009.11.004

Journal of Molecular Catalysis A: Chemical 318 (2010) 30–35
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Polymer-anchored oxovanadium(IV) complex for the oxidation of thioanisole,
styrene and ethylbenzene
Zenixole R. Tshentua,∗, Chamunorwa Togob, Ryan S. Walmsleya
a Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
b Department of Biochemistry, Microbiology and Biotechnology, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
The ligand 2-(2ꢀ-hydroxyphenyl)-1H-imidazoline (Hpimin) was covalently linked to chloromethylated
polystyrene (PS-Cl). This resin was then allowed to react with vanadyl sulfate to afford the polymer-
anchored oxovanadium(IV) complex, PS-[VO(pimin)x], which was characterised using elemental analysis,
IR and AFM. This catalyst was shown to catalyse the hydrogen peroxide-based oxidation of styrene,
ethylbenzene and thioanisole. A maximum conversion of ethylbenzene (30.6%) and styrene (99.9%) was
obtained by using 0.025 g of PS-[VO(pimin)x] and 4 equiv. of hydrogen peroxide at 80 ◦C after 6 h. The oxi-
dation of thioanisole proceeded quantitatively (99.6%) within 4 h, with 0.025 g of PS-[VO(pimin)x], at room
temperature with 2 equiv. of hydrogen peroxide. At the steady state, the main products from the oxidation
of the various substrates were benzaldehyde (57.6%) > 1-phenylethane-1,2-diol (16.2%) > benzoic acid
(7.1%) > styrene oxide (3.1%) for styrene; acetophenone (20.5%) > benzaldehyde (2.7%) for ethylbenzene;
and methyl phenyl sulfoxide (67.4%) > methyl phenyl sulfone (31.9%) for thioanisole.
Article history:
Received 5 June 2009
Received in revised form 21 October 2009
Accepted 4 November 2009
Available online 13 November 2009
Keywords:
Polymer-anchored complex
Oxovanadium
Imidazoline
Oxidation
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The ligand 2-(2ꢀ-hydroxyphenyl)-1H-imidazoline was covalently
linked to chloromethylated polystyrene, and the resultant cream-
The simple separation and recyclability [1] of heterogeneous
catalysts provides an attractive option for laboratory scale synthe-
sis, while the ability to easily apply these catalysts to industry in the
form of packed or fluidised bed columns [2] provides yet another
incentive. The use of polymeric supports for the immobilisation of
metal catalysts has been thoroughly studied [1,3,4] and remains an
exciting area of research with potential for further discoveries [5].
The catalytic properties of oxovanadium compounds lead them to
be of particular interest in this regard [6].
coloured beads were allowed to react with vanadyl sulfate to form
the light green polymer-anchored complex. The beads were char-
acterised by microanalysis, infrared spectroscopy and atomic force
microscopy. The catalytic activity of the oxovanadium(IV) complex
was investigated for the oxidation of thioanisole, styrene and ethyl-
benzene by using hydrogen peroxide as the oxidant. The reaction
progress was monitored by gas chromatography.
2. Experimental
The ever increasing environmental concerns present a motive
for replacing toxic oxidising agents such as chromium and
permanganate-based agents with ‘environmentally friendly’ vari-
eties such as molecular oxygen and hydrogen peroxide, which
generate water as the only by-product [7,8]. Alkenes, sulfides and
alcohols have been oxidised with both air oxygen [9,10] and excited
singlet oxygen [11,12] through metal-based activations. Hydrogen
peroxide has also become increasingly popular due to its low cost
and availability [5] and has been used for a wide range of reactions
including the oxidation of sulfides, alkenes and alkanes [13].
Herein, we describe the preparation, characterisation and cat-
alytic evaluation of a heterogeneous oxovanadium(IV) catalyst.
2.1. Materials
Thioanisole, ethylbenzene, styrene, ethylenediamine, vanadyl
sulfate hydrate and Merrifield polymer crosslinked with divinyl-
benzene (5.5% crosslinked, ∼5.5 mmol Cl/g resin and 16–50 mesh)
were purchased from Aldrich Chemical Company and used with-
out further purification. All other chemicals and solvents were of
reagent grade.
2.2. Instrumentation
The infrared spectra were recorded on a PerkinElmer 2000 FTIR
spectrometer in the mid-IR range (4000–400 cm−1) as KBr pellets.
1H and 13C NMR spectra were recorded on a Bruker AMX 400
NMR MHz spectrometer and reported relative to tetramethylsi-
∗
Corresponding author. Tel.: +27 46603 8846; fax: +27 46622 5109.
E-mail address: z.tshentu@ru.ac.za (Z.R. Tshentu).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.11.004

Z.R. Tshentu et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 30–35
31
lane (ı 0.00). Electronic spectra were recorded on a Varian Cary
2.4. Catalytic activity studies
500 Scan UV–vis spectrophotometer using 1 cm quartz cells and
dimethylsulfoxide as the solvent. The vanadium content of beads
was determined by using a Thermo Electron (iCAP 6000 Series)
inductively coupled plasma (ICP) spectrometer equipped with an
OES detector. The wavelengths 290.88 nm, 292.40 nm, 309.31 nm
and 311.07 nm were chosen and triplicate analyses were per-
formed at each wavelength. Catalysed reactions were monitored
by using an Agilent 6820 gas chromatograph (GC), fitted with a
flame ionization detector (FID) and a ZB-5MSi capillary column
(30 m × 0.25 mm × 0.25 ␮m). Mass spectra were obtained by using
a Thermo-Finnigan GC–MS fitted with an electron impact ion-
ization mass selective detector. Atomic force microscopy (AFM)
imaging was performed using a CP-11 scanning probe microscope
from Veeco Instruments in non-contact mode at a scan rate of 2 Hz
by using an MP11123 cantilever. Microanalysis was carried out by
using a Vario Elementar Microcube ELIII.
In a typical catalytic oxidation experiment, 20 mL of acetoni-
trile was added to a 100 mL round bottom flask fitted with a glass
condenser. The temperature of the oil bath was regulated to 1 ◦C
by using an external temperature probe. The stirring rate was
kept constant at 300 rpm for all reactions. Then, styrene (1.04 g,
10 mmol), ethylbenzene (1.06 g, 10 mmol) or thioanisole (1.24 g,
10 mmol) was added followed by the required moles of hydrogen
peroxide. Immediately after the addition of hydrogen peroxide, the
catalyst was added and this was considered the start of the reac-
tion. The reaction progress was monitored by gas chromatography
at specific time intervals.
3. Results and discussion
3.1. Synthesis and general considerations
The synthesis of the ligand (Hpimin) has been carried out before
[14]. Our synthesis, however, employs an efficient microwave
method with a short reaction time of 7 min and improved yield of
78%. The alkylation of Hpimin with chloromethylated polystyrene
under the specified conditions might occur at different positions
(see Scheme 1) as also acknowledged recently by Maurya et al. [15].
2.3. Preparative work
2.3.1. 2-(2’-Hydroxyphenyl)-1H-imidazoline (Hpimin)
Methyl salicylate (18.4 g, 0.12 mol) was added to an excess
of ethylenediamine (32 g, 0.53 mol) in a conical flask and placed
in a conventional microwave (7 min, 180 W) with intermittent
stirring. Starting materials were removed by vacuum distillation
(7 × 10−3 mbar at 160 ◦C) to afford a crude solid product which was
then purified by digestion from a large volume of chloroform. Yield:
78.1%. 1H NMR (400 MHz, DMSO-d6): ı 3.71 (s, 4H, CH2CH2), 6.69 (t,
1H, Ar-H) 6.77, (d, 1H, Ar-H), 7.27 (t, 1H, Ar-H), 7.56 (d, 1H, Ar-H).
13C NMR (400 MHz, DMSO-d6): ı 46.5, 110.2, 115.5, 118.4, 127.2,
132.7, 163.5, 166.1; Anal. Calcd for C9H10N2O (Found): C, 66.65%
(66.68%); H, 6.21% (6.20%); N, 17.27% (16.98%).
Scheme 1. Expected products for the alkylation of Hpimin.
As a result of the above, the subsequent complex formation
step may be expected to yield an oxovanadium(IV) complex with
a different stereochemistry to that of the neat square pyramidal
complex, [VO(pimin)2]. In [VO(pimin)2], Hpimin coordinates as a
bidentate phenolate ligand via the imidazoline nitrogen and the
deprotonated phenolic group to form the five coordinate complex.
2.3.2. [VO(pimin)2]
To a solution of Hpimin (0.25 g, 1.5 mmol) in 5 mL methanol was
added vanadyl sulfate (0.15 g, 0.7 mmol) dissolved in 5 mL water.
The mixture was stirred at room temperature to afford a green pre-
cipitate within 10 min. The reaction was allowed to proceed for
further 2 h to reach completion. The solid was collected by filtration
and washed with water followed by cold methanol, and then finally
dried in an oven at 100 ◦C. Yield: 57%. Anal. Calcd for C18H18N4O3V
(Found): C, 55.53% (55.43%); H, 4.66% (4.74%); N, 14.39% (14.21%).
3.2. Spectral characterisation
The infrared spectrum of chloromethylated polystyrene (PS-Cl)
shows a band at 670 cm−1 corresponding to the ꢀ(C–Cl) vibration
[16], and the attachment of Hpimin to polystyrene was con-
firmed by its disappearance and the appearance of a new band at
1615 cm−1 corresponding to the ꢀ(C N) vibration. In the spectrum
of the free ligand, a medium and broad band for the stretching fre-
quency of the hydrogen bonded O–H appears at 2695 cm−1 [17] but
disappears in the spectrum of PS-pimin. This along with the appear-
ance of the ꢀ(C–O–C) band at 1233 cm−1 suggests that alkylation
occurs predominantly via the hydroxyl group [14]. The selected
infrared data is listed in Table 1.
2.3.3. Polymer-anchored ligand (PS-pimin)
Chloromethylated polystyrene (1.0 g, 5.5 mmol Cl) was allowed
to swell in DMF (15 mL) overnight. To this was added Hpimin
(0.93 g, 5.7 mmol) followed by a solution of triethlyamine (1.25 g)
in ethylacetate (30 mL). The reaction mixture was then heated and
stirred under reflux for 9 h. Following this, the cream-coloured
beads were allowed to cool and then filtered and washed with hot
DMF followed by ethanol. The beads were dried in the oven at 60 ◦C.
Due to some degradation from mechanical stirring the beads were
first passed through sieves of various mesh sizes such that only
the largest beads remained (>15 mesh). Anal. Found: C, 69.80%; H,
6.93%; N, 5.79%.
A medium intensity band at 992 cm−1 confirms the presence of
the oxovanadium unit [15]. Interestingly, this stretching frequency
is markedly different from that of the non-polymer-bound com-
plex, [VO(pimin)2], once again suggesting that the stereochemistry
is different, which agrees well with our proposal that alkylation
occurs via the hydroxyl group of the ligand. This was again corrobo-
rated by the microanalysis results which show a significant amount
of sulfur (2.11% from sulfate) as well as a V/S ratio of approximately
1:1.3, indicating that a cationic species of oxovanadium(IV) forms
within the polymer. Furthermore, a V/N ratio in the polymer of
1:3.6 indicates the possibility of vanadyl bound to two imidazoline
units. It is, however, not possible to characterise the exact nature
2.3.4. Polymer-anchored vanadium complex (PS-[VO(pimin)x])
PS-pimin (0.5 g) was allowed to swell in DMF for 2 h. A DMF
solution of vanadyl sulfate (0.6 g, 2.75 mmol) was added to the
above mixture, which was stirred and heated at 90 ◦C for 8 h. The
green beads were collected by filtration, rinsed with warm DMF
and methanol and then dried in the oven at 60 ◦C. Anal. Found: C,
62.12%; H, 5.99%; N, 4.63%; S, 2.11%; V, 4.70%.

32
Z.R. Tshentu et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 30–35
Table 1
Selected infrared bands and the colours of the compounds.
Compound
Colour
IR bands (cm−1
)
ꢀ(C–Cl)
ꢀ(C N)
ꢀ(O–H)
ꢀ(V O)
Hpimin
PS-Cl
PS-pimin
PS-[VO(pimin)x]
[VO(pimin)2]
Cream
White
Cream
Green
Green
–
670
–
–
–
1618
–
1615
1610
1609
2695
–
–
–
992
932
–
–
–
–
of the complex species within the polymer. The coordination of
pimin to oxovanadium(IV) is shown by the decrease in ꢀ(C N)
from 1618 cm−1 in the free ligand to 1609 cm−1 and 1610 cm−1
in PS-[VO(pimin)x] and [VO(pimin)2], respectively. The ꢀ(C–O) for
Hpimin was observed at 1269 cm−1 and shifted to 1255 cm−1 upon
coordination in the non-polymer-bound complex, [VO(pimin)2].
The electronic spectrum of the neat complex shows three bands
at 411 nm (a * ← b ), 549 nm (b * ← b ) and 617 nm (e * ← b )
Fig. 1. The effect of catalyst amount on thioanisole oxidation. Reaction conditions:
thioanisole (1.24 g, 10 mmol), 30% H2O2 (20 mmol), acetonitrile (20 mL) and room
temperature (RT).
␲
1
2
1
2
2
typical of the five-coordinate, square pyramidal geometry corre-
sponding to the neutral [VO(pimin)2] [18,19].
3.3. Atomic force microscopy
The atomic force micrographs for the single beads of PS-Cl, PS-
pimin, PS-[VO(pimin)x] and the recovered PS-[VO(pimin)x] showed
the morphological changes of the surfaces of the beads after the
various synthetic steps. Additionally, the extent of degradation of
the polymer after a catalytic cycle was investigated.
The results show an increase in surface roughness from 4.47 nm
in PS-Cl to 6.14 nm in PS-pimin, and a decrease to 5.06 nm upon
forming the PS-[VO(pimin)x] signifying perhaps the rigidity of the
ligands at the surface upon coordination to vanadyl. The surface
roughness of the beads recovered from a room temperature reac-
tion for the oxidation of thioanisole with peroxide is 104.7 nm. It
is apparent that the peroxide plays a significant role in the degra-
dation of the beads at the surfaces despite the fact that they still
exhibit catalytic activity upon recycling. The AFM images are shown
in Appendix A.
Fig. 2. Effect of temperature on oxidation of thioanisole (1.24 g) using 0.025 g of
catalyst, and the comparison with neat complex (0.0090 g) at 60 ◦C.
3.4. Catalytic activity
The polymer-anchored complex, PS-[VO(pimin)x], and its
stant. The effect of the amount of catalyst, in the range 0.0–0.035 g,
on the rate of oxidation was studied as a function of time and the
results are shown in Fig. 1. For a fixed amount of thioanisole (1.24 g,
10 mmol) in 20 mL of acetonitrile and 30% H2O2 (20 mmol) at room
temperature, the rate of the oxidation reaction is fast for the high-
est catalyst amount of 0.035 g reaching 98.9% conversion within
1 h. However, the 0.025 g amount of catalyst results in maximum
conversion of 99.6% within 3 h at room temperature, making this a
catalytically relevant amount.
The rate of reaction improves drastically at higher tempera-
tures, reaching maximum conversion of 98.3% within 20 min when
0.025 g of catalyst is used, and compares well with its homoge-
neous counterpart at 60 ◦C (see Fig. 2). The amount of [VO(pimin)2]
(0.0090 g) was reconciled by determining the amount of vanadium
in 0.025 g of the beads. This was done by placing 0.025 g of catalyst
in 10 mL of TraceSelect nitric acid (69%) at 40 ◦C overnight to leach
out vanadium. This acid-leached solution was then diluted with
deionized, distilled water to 100 mL, and analysed by ICP-OES.
The initial rate for the oxidation of thioanisole increases rapidly
due to the formation of methyl phenyl sulfoxide, with little con-
version of the sulfoxide to sulfone. The kinetics follows those of
consecutive reactions, with the sulfoxide being converted to the
sulfone form. Fig. 3 shows the selectivities or the progress of the
consecutive reactions with time.
geometrically
non-equivalent
homogeneous
counterpart,
[VO(pimin)2], were investigated for their catalytic efficiency
for the oxidation of thioanisole, styrene and alkylbenzene using
acetonitrile as a solvent and 30% hydrogen peroxide as the oxidant.
3.4.1. Oxidation of thioanisole
In this study thioanisole is oxidised to give the corresponding
methyl phenyl sulfoxide and methyl phenyl sulfone, which are
identified by GC and GC–MS data, as shown in Scheme 2.
Scheme 2. Oxidation products for thioanisole.
To optimise the reaction conditions, the effects of the amount
of catalyst as well as the effect of temperature were investigated.
According to the literature [20], the oxidation of sulfides invariably
reaches a maximum conversion at a 2:1 peroxide-to-sulfide ratio
and even less, and therefore in our study this ratio was kept con-

Z.R. Tshentu et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 30–35
33
under the experimental conditions with further loss of activity on
the third cycle possibly due to the leaching of the vanadium at the
surface of the beads, which is in agreement with the AFM data. The
results from the third cycle are comparable to those obtained with
the 0.011 g amount of catalyst (see Fig. 1), once again supporting
our argument about the loss of vanadium at the surfaces.
3.4.2. Oxidation of styrene
Fig. 5 shows the percentage conversions in the oxidation of
styrene at different catalyst amounts using 2 equiv. of 30% H2O2.
The reaction products that were identified by GC–MS are shown in
Scheme 3.
Fig. 3. Product selectivity for the oxidation of thioanisole. Reaction conditions:
1.24 g of thioanisole, 0.035 g of catalyst, 2 equiv. of H2O2, room temperature and
20 mL of acetonitrile.
Scheme 3. Oxidation products from styrene.
The rates of conversion were very high at the beginning when
the catalyst of 0.025 g or 0.035 g was used and rather slow when
the catalyst of 0.011 g was used; however, the higher conversion of
83.9% was reached after 4 h compared with the former. It was there-
fore necessary to investigate the effect of oxidant since a maximum
of 73% was reached within 2.5 h with 0.025 g of catalyst.
A maximum conversion of 99.5% was obtained with 4 equiv.
of peroxide after only 2.5 h of reaction time, and surpassed the
non-polymer-bound counterpart in initial rates (see Fig. 6). At
two-equivalents of peroxide the initial rate of conversion for the
homogeneous complex, [VO(pimin)2], was comparable to that of
the polymer-bound complex.
Fig. 4. The recyclability of PS-[VO(pimin)x] in the oxidation of thioanisole. Reac-
tion conditions: 1.24 g of thioanisole, 0.025 g of catalyst, 2 equiv. of H2O2, room
temperature and 20 mL of acetonitrile.
After 2.5 h, when the reaction reaches maximum conversion, the
yield order is as follows: benzaldehyde (57.6%) > phenylethane-1,2-
diol (16.2%) > benzoic acid (7.1%) and > styrene oxide (3.1%); clearly,
styrene oxide converts readily to benzaldehyde and phenylethane-
1,2-diol within that reaction time. It is only when benzaldehyde
The recyclability of the catalyst was investigated by subjecting
the beads to further 2 cycles under the same conditions. The results
shown in Fig. 4 clearly show that the rate slows down after the first
cycle, possibly indicating a change in coordination environment
Fig. 6. Effect of the amount of oxidant on the oxidation of styrene (1.04 g) using
0.025 g of catalyst, and the comparison with neat complex (0.0090 g) at 80 ◦C and in
20 mL of acetonitrile.
Fig. 5. The effect of catalyst amount on styrene oxidation. Reaction conditions:
styrene (1.04 g, 10 mmol), 30% H2O2 (20 mmol), acetonitrile (20 mL) and 80 ◦C.

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Z.R. Tshentu et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 30–35
Fig. 9. Product selectivity for the oxidation of ethylbenzene. Reaction conditions:
1.06 g of ethylbenzene, 0.025 g of catalyst, 4 equiv. of H2O2, 80 ◦C and 20 mL of
acetonitrile.
Fig. 7. Product selectivity for the oxidation of styrene. Reaction conditions: 1.04 g
of styrene, 0.025 g of catalyst, 4 equiv. of H2O2, at 80 ◦C and 20 mL of acetonitrile.
Table 2
Percentage conversion and turnover frequency (TOF).
b
Substrate
% conversiona TOF (h−1
)
Catalyst
amount (g) amount
(mmol)
H2O2
Temperature
(◦C)
Ethylbenzene 30.3
53
173
169
121
860
31
0.025c
0.025c
0.025c
0.025c
0.025c
0.009e
0.009e
0.009e
40
40
20
20
20
40
40
20
80
80
RT
RT
60
80
80
60
Styrene
99.4
97.4
Thioanisole
Thioanisoled 97.0
Thioanisole
98.8
Ethylbenzene 25.3
Styrene
Thioanisole
99.8
98.6
78
613
RT = room temperature.
a
Maximum conversion of 10 mmol of substrate.
Determined as moles of substrate converted/moles of vanadium in catalyst/time
b
(h).
c
Heterogeneous.
Second cycle of catalyst.
Homogeneous.
d
e
Fig. 8. Effect of the amount of oxidant on the oxidation of ethylbenzene (1.06 g)
using 0.025 g of catalyst, and the comparison with neat complex (0.0090 g) at 80 ◦C
and 20 mL of acetonitrile.
Fig. 9 shows that the conversion tends towards the forma-
tion of acetophenone with no trace of phenylacetic acid which
was found earlier by other researchers [15]. At 3 h when a max-
imum is reached, the product selectivity order is acetophenone
(20.5%) > benzaldehyde (2.7%).
The summary of the results of the oxidation reactions is pre-
sented in Table 2. The percentage conversions refer to the point
of reaching a steady state and likewise for the turnover frequency.
It is evident from these results that PS-[VO(pimin)x] and to some
extent its homogeneous analogue is most efficient in catalysing the
oxidation of thioanisole.
converts to benzoic acid that a steady state is reached for the con-
version of styrene oxide (see Fig. 7).
3.4.3. Oxidation of ethylbenzene
Fig. 8 shows the effect of the peroxide-to-substrate ratio on
the oxidation of ethylbenzene. A maximum conversion of 30.6%
is obtained within 3 h of reaction time which is rather impressive
for the oxidation of alkanes. The higher oxidant amount is crucial
in achieving the highest conversion since the 2 equiv. amount of
peroxide only yields a maximum of 13.1% within the same reac-
tion time. The homogeneous analogue is once again inferior to
PS-[VO(pimin)x] in terms of activity under the same conditions for
the oxidation of ethylbenzene.
3.4.4. Possible reaction pathways
When the beads are introduced to the reaction mixture, there
is an immediate colour change from green to yellow, and this is
attributed to the oxidation of the vanadium(IV) center. To provide
insight into this first step of the mechanism, we have performed
a titration of a solution of the complex (ca. 10−4 M), [VO(pimin)2],
with 30% hydrogen peroxide in dimethyl sulfoxide (DMSO). This
reaction was monitored by electronic absorption spectroscopy (see
Fig. 10). The disappearance of the three d–d transitions and the
appearance of charge transfer band at 372 nm which appears as a
shoulder of the n → ␲* band at 352 nm confirm this oxidation.
This spectral change is indicative of the formation of the oxoper-
oxovanadium(V) complex, which is the active intermediate species
that participates in the oxidation reactions by transferring one
of its oxygens to the substrate [15,21]. The induction periods
The evidence of GC–MS and 1H NMR data indicates that the
main oxidation products for the oxidation of ethylbenzene were
benzaldehyde and acetophenone (see Scheme 4).
Scheme 4. The oxidation products from ethylbenzene.

Z.R. Tshentu et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 30–35
35
(0.025 g of catalyst, RT and 2 equiv. of H2O2); and 99.9% and 30.6%
respectively for styrene and ethylbenzene (0.025 g of catalyst,
80 ◦C and 2 equiv. of H2O2).
The resultant stereochemistry of the vanadium complex within
the polymer was influenced by the alkylation of Hpimin to PS-Cl via
the hydroxyl group yielding an ethereal connection rather than a
nitrogen-bound ligand. This resulted in a cationic oxovanadium(IV)
species forming within the polymer. The net result is that this cat-
alyst degrades in the presence of high peroxide amount and at
high temperatures. However, PS-[VO(pimin)x] should be suitable
for room temperature oxidation of sulfides as shown by our AFM
results and recyclability studies.
Acknowledgements
The authors would like to thank Mr M. Mtyopo from
InnoVenton-Technology Support Unit (NMMU) for the GC–MS data,
Dr. E. Antunes from DST/Mintek-NIC (Rhodes) for the microanalysis
data and Dr. R. Nel from Sasol for supplying a fine temperature-
regulating hotplate. We are also grateful for financial support from
Sasol and NRF.
Fig. 10. Spectrophotometric titration of [VO(pimin)2] with H2O2 in DMSO.
observed in styrene and ethylbenzene oxidation reactions with PS-
[VO(pimin)x] can be explained by this first step of the catalytic
process. Generally, the induction periods for the homogeneous
catalyst are longer than those for the heterogeneous catalyst but
improve with a high oxidant amount. This delay is due to the
activity of the different vanadyl centers which influence the rate
of formation of oxoperoxovanadium(V) complex. This trend is not
observed for thioanisole oxidation since sulfide oxidation with per-
oxide proceeds without delay in the absence of the catalyst (see
Fig. 1).
The formation of styrene oxide from styrene has been dis-
cussed in detail by Mimoun et al. [22]. The dominant product for
the oxidation of styrene in our case is benzaldehyde followed by
phenylethane-1,2-diol and benzoic acid. This is not surprising since
benzaldehyde can form from styrene oxide under acidic conditions
by oxidative cleavage of the ring via a radical mechanism [21] while
the water present in peroxide is responsible for the formation of
phenylethane-1,2-diol [23]. The decrease in the benzaldehyde yield
(Fig. 7) with reaction time seems to be influenced by the oxidation
to benzoic acid in the presence of peroxide.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2009.11.004
References
[1] R. Ando, T. Yagyu, M. Maeda, Inorg. Chim. Acta 357 (2004) 2237–2244.
[2] D.C. Sherrington, Catal. Today 57 (2000) 87–104.
[3] S. Leinonen, D.C. Sherrington, A. Sneddon, D. McLoughlin, J. Corker, C. Canevali,
F. Morazzoni, J. Reedijk, S.B.D. Spratt, J. Catal. 183 (1999) 251–266.
[4] M.M. Miller, D.C. Sherrington, J. Catal. 152 (1995) 368–376.
[5] D.C. Sherrington, Pure Appl. Chem. 60 (1987) 401–414.
[6] V. Conte, F. Di Furia, G. Licini, Appl. Catal. A 157 (1997) 335–361.
[7] S. Velusamy, A. Srinivasan, T. Punniyamurthy, Tetrahedron Lett. 47 (2006)
923–926.
[8] D. Bagchi, S.J. Stohs, B.W. Downs, M. Bagchi, H.G. Preuss, Toxicology 186 (2003)
175–177.
[9] C.L. Hill, T.M. Anderson, J.W. Han, D.A. Hillesheim, Y.V. Geletii, N.M. Okun, R.
Cao, B. Botar, D.G. Musaev, K. Morokuma, J. Mol. Catal. A: Chem. 251 (2006)
234–238.
[10] S. Mohebbi, D.M. Boghaei, A.H. Sarvestani, A. Salimi, Appl. Catal. A 278 (2005)
263–267.
The oxidation of ethylbenzene yielded both benzaldehyde and
acetophenone, with the latter being the major product. The mech-
anism of formation of acetophenone proposed by Hoshino et al.
involves the abstraction of an ␣-hydrogen from ethylbenzene to
form 1-phenylethyl radicals, followed by a reaction with oxygen to
produce acetophenone. The formation of benzaldehyde is believed
to occur via the decomposition of 1-phenylethoxyl radicals [24].
[11] P. Tau, T. Nyokong, J. Mol. Catal. A: Chem. 273 (2007) 149–155.
[12] V. Chauke, T. Nyokong, J. Mol. Catal. A: Chem. 289 (2008) 9–13.
[13] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 274 (2007)
192–201.
[14] P. Parík, S. Senauerová, V. Liková, K. Handlír, M. Ludwig, J. Heterocycl. Chem. 43
(2006) 835–841.
[15] M.R. Maurya, A. Arya, P. Adão, J.C. Pessoa, Appl. Catal. A 351 (2008) 239–252.
[16] C. Lü, B. Gao, Q. Liu, C. Qi, Colloid. Polym. Sci. 286 (2008) 553–561.
[17] T.J. Lane, I. Nakagawa, J.L. Walter, A.J. Kandathil, Inorg. Chem. 1 (1962) 267–276.
[18] J. Selbin, Chem. Rev. 65 (1965) 153–175.
[19] K.S. Patel, G.A. Kolawole, A. Earnshaw, J. Inorg. Nucl. Chem. 43 (1981)
3107–3112.
4. Conclusions
[20] M.R. Maurya, M. Kumar, A. Arya, Catal. Commun. 10 (2008) 187–191.
[21] M.R. Maurya, S. Sikarwar, M. Kumar, Catal. Commun. 8 (2007) 2017–2024.
[22] H. Mimoun, M. Mignard, P. Brechot, L. Saussine, J. Am. Chem. Soc. 108 (1986)
3711–3718.
[23] Z. Wang, Y.-T. Cui, Z.-B. Xu, J. Qu, J. Org. Chem. 73 (2008) 2270–2274.
[24] M. Hoshino, H. Akimoto, M. Okuda, Bull. Chem. Soc. Jpn. 51 (1978) 718–724.
The polymer-anchored oxovanadium(IV) catalyst, PS-
[VO(pimin)x], has been prepared. The catalytic activity for the
oxidation of thioanisole, styrene and ethylbenzene has been
evaluated by using 30% H2O2 as the oxidant. The catalyst showed
good potential for the oxidation of the substrates, yielding the
following conversions at the steady state; 99.6% for thioanisole