Indian Journal of Chemistry
Vol. 48A, July 2009, pp. 964-968
Liquid phase non-solvent selective oxidation
of styrene using aqueous hydrogen peroxide
with supported 12-tungstophosphoric acid
Pankaj Sharma & Anjali Patel*
Chemistry Department, Faculty of Science,
The MS University of Baroda, Vadodara, India
Email: [email protected]
Received 31 December 2008; revised and accepted 23 June 2009
Non-solvent oxidative cleavage of styrene has been carried out
over supported 12-tungstophosphoric acid with H2O2. The present
catalysts show very high conversion and selectivity towards
benzaldehyde, an important product used as intermediate in many
synthetic preparations. Further, the catalytic activity of calcinated
catalysts has also been evaluated under optimized conditions. The
present study shows the best catalysts to be PWA3/Z and PWA3/A
with PWA3/A thermally more stable than PWA3/Z.
Keywords: Catalysts,
Oxidations,
Supported
catalysts,
Non-solvent oxidations, Hydrocarbons, Styrene,
Tungstophosphoric acid
IPC Code: Int. Cl.8 B01J27/186; C07B31/00
Environmental concerns have forced the chemical
industry to re-evaluate many of its processes to reduce
or eliminate the formation of waste produced in the
synthesis of organic products1. This need is especially
required in oxidation technology and can be addressed
by the development of clean and safe oxidation
procedures. This is possible by establishment of green
catalytic processes by use of environmentally friendly
oxidants or green catalysts. H2O2 is an attractive
oxidant from both economic and environmental
perspective. In addition, H2O is the only theoretical
byproduct when H2O2 is used as an oxidant.
Catalytic oxidation is widely used in bulk
chemicals manufacturing and is becoming
increasingly important in synthesis of fine chemicals.
The oxidation of alkenes into carbonyl compounds
has many synthetic applications. Since simple
hydrocarbons are not miscible with aqueous H2O2,
reactions are either carried out in polar solvents such
as acetonitrile or acetic acid or in two phase systems
using chlorinated hydrocarbons and quaternary
ammonium catalyst ion pairs to maximize surface
contact and reactivity2. Therefore, an important goal
in the hydrocarbon oxidation is to carry out reactions
without addition of organic solvent with safe, clean,
economical and environmentally benign oxidant.
Ishii and co-workers, developed the process of
epoxidation of relatively electron poor terminal
olefins by utilizing the more environmentally and
economically significant oxidant, H2O2, and
heteropolyacids (HPAs, principally H3PW12O40), in
presence of a phase transfer catalyst (PTC) such as
cetyl pyridinium chloride (CPC) or Arquad3. Since
then, HPAs have attracted much attention and have
been used as a catalytic systems in presence of a PTC
for oxidation of a large number of organic
substrates4−8.
Alkylation
over
supported
heteropolyacids has also been reported9. Furthermore,
different salts of HPAs10-14 and transition metal
substituted HPAs15-18 have been synthesized and used
in biphasic medium with different organic solvents.
Some research groups have used supported HPAs19-22
for several organic conversions but they have carried
out reactions in organic solvents. Therefore, it was
thought of interest to use supported heteropolyacids
without any PTC or organic solvents for carrying
out oxidation reactions. Herein, we report results
for oxidation of styrene over supported
12-tungstophosphoric acid (PWA) in the absence of
PTC or organic solvent. H2O2 has been used as an
economical and environmentally benign oxidant
It is very interesting and important to note that the
support does not always play merely a mechanical
role but it can also modify the catalytic properties of
the HPAs. We have also tried to determine the role of
support on the redox behaviour, and hence, two
different supports, one acidic (zirconia) and another
neutral (neutral alumina), have been selected. The
oxidation of styrene using aqueous H2O2 has been
carried out over PWA supported onto zirconia (Z) and
neutral alumina (A). The oxidation of styrene has
been carried out by varying different parameters such
as % loading of PWA, mole ratio of styrene to H2O2,
amount of the catalyst and time to optimize the
conditions for maximum conversion as well as
selectivity of the products. Further, the best catalyst
was calcinated at 300 °C and 500 °C for 5 h and
characterized by differential scanning calorimetry
(DSC), diffuse reflectance spectroscopy (DRS) as
well as Raman spectroscopy to see any structural
NOTES
change, if any. The calcinated catalysts have been
evaluated for the same reaction under optimized
conditions.
Experimental
All the chemicals used were of AR grade.
H3PW12O40.nH2O
(Lobachemie,
Mumbai),
ZrOCl2.nH2O (SD Fine Chemicals, Mumbai), neutral
active Al2O3, styrene and 30 % aqueous H2O2 were
used as received from Merck. The support has been
synthesized by the method reported earlier23.
A series of catalyst was synthesized by
impregnation method. Z (1 g) was impregnated with
an aqueous solution of PWA (0.1-0.70 g/10-70 ml of
conductivity water) at 100 ºC with stirring for 10 h.
The materials thus obtained were designated as
PWA1/Z, PWA2/Z, PWA3/Z, PWA4/Z, PWA5/Z and
PWA7/Z respectively. The best catalyst of the series,
PWA3/Z, was calcinated at 300 ºC and 500 ºC for 5 h
and designated as PWA33/Z and PWA35/Z
respectively.
A series of catalyst containing 10-70% of
12-tungstophosphoric acid (PWA) supported onto
neutral alumina (A) was synthesized similarly by wet
impregnation. The support, A, (1 g) was impregnated
with an aqueous solution of PWA (0.1/10 – 0.7/70 ml
of conductivity water) with stirring for 35 h and dried
at 100 ºC for 10 h. The obtained materials were
designated as PWA1/A, PWA2/A, PWA3/A, PWA4/A,
PWA5/A and PWA7/A. The best catalyst of the series,
PWA3/A3, was calcinated at 300 ºC and 500 ºC for 5 h
and designated as PWA33/A and PWA35/A
respectively.
The thermal stability of the catalysts was studied by
TGA and DSC. TGA of the samples were carried out
on a TA instrument (Q 50) under nitrogen atmosphere
with a flow rate of 2ml/min and a heating rate of
10 °C/min in the range of 50-600 ºC. DSC
measurements were made on a TA Instrument,
(DSC-2010) under nitrogen atmosphere with a flow
rate of 2 ml/min and a heating rate of 10 °C/min in the
range of 50-600 ºC. The DRS spectra of samples were
recorded on a Jasco DR-UV-vis spectrophotometer
(model V-560) using barium sulphate as a reference.
Raman spectra were recorded on a spectrophotometer
(model Bruker FRA 106).
The oxidation was carried out in a round bottom
flask provided with a double walled condenser
containing the catalyst, styrene and hydrogen
965
peroxide at 80 °C with constant stirring for 48 h. After
completion of the reaction, the catalyst was removed
and the product was extracted with dichloromethane.
The product was dried with magnesium sulphate and
analyzed on a gas chromatograph using SE-30
column. The products were identified by comparison
with the authentic samples as well as by gas
chromatography-mass spectroscopy.
Results and discussion
TGA of Z shows 3-5 % weight loss within a
temperature range of 100-180 ºC, which is due to loss
of adsorbed water molecules. TGA of PWA shows
4-7 % weight loss within a temperature range of
100-180 ºC and 1-4 % weight loss at 250-280 ºC
which is due to loss of adsorbed water molecules and
loss of crystallization water molecules, respectively.
Further, it shows 1-4 % weight loss at 474 ºC which
may be due to decomposition of PWA which is in
good agreement with results reported earlier24. TGA
of PWA3/Z shows weight loss in the 400-410 ºC
temperature range which may be due to phase change
and is in good agreement with our earlier reported
results25. TGA of PWA3/A shows 6-8 % weight loss
in the 100-180 ºC temperature range due to loss of
adsorbed water. There is no appreciable change in
weight upto 600 ºC indicating an increase in the
stability of PWA. The decrease in percentage weight
loss indicates the interaction between the support and
PWA.
The result of DSC analysis shows that Z has an
endothermic peak at 126 ºC and an exothermic peak
at 400 ºC which is due to loss of adsorbed water
molecules and a phase change of hydrous zirconia to
tetragonal zirconia respectively. These results are in
good agreement with those reported earlier26. DSC of
PWA shows an endothermic peak at 127 ºC and an
exothermic peak at 474 ºC due to loss of adsorbed
molecules and decomposition of PWA respectively,
which are in good agreement with those reported
earlier24. DSC of PWA3/Z shows an endothermic
peak at 126 ºC and an exothermic peak at 400 ºC
indicating the loss of adsorbed water molecules and
phase change of zirconia to tetragonal phase. DSC of
PWA3/A shows only an endothermic peak at 126 ºC
indicating the loss of adsorbed water molecules.
Absence of any endothermic and exothermic peak up
to 600 ºC indicates no decomposition of the supported
species.
966
INDIAN J CHEM, SEC A, JULY 2009
The above results indicate that PWA is thermally
stable upto 400 ºC and 600 ºC respectively when
supported onto hydrous zirconia and neutral alumina
respectively. PWA3/A catalyst is thermally more
stable than PWA3/Z and alumina is a better support as
compared to hydrous zirconia.
The DRS gives information about the non-reduced
heteropolyanion due to oxygen to metal charge
transfer. The DRS spectra of PWA3/Z, PWA33/Z,
PWA35/Z, PWA3/A, PWA33/A and PWA34/A show λmax
at 260 nm, which is in good agreement with the
results reported earlier25-27confirming the presence of
the undegraded PWA species. In other words, the
Keggin phase remain unaltered upto 500 °C.
The Raman spectra of PWA, PWA3/Z, PWA33/Z,
PWA35/Z, PWA3/A, PWA33/A and PWA34/A were
recorded. It is important to note that in PWAs, among
the W-O bonds, W - Od is Raman active. The Raman
spectra of PWA shows bands at 1011, 996, 993, 925,
900, 536, 217 cm-1 corresponding to νsW=Od,
νasW=Od, νsP-Oa, νasW-Ob-W, νas W-Ob-W, νs W-O-W
and νsW-Oa respectively. The Raman spectra of all
these catalysts (fresh as well as calcinated) show the
presence of all these bands at their corresponding
frequencies and are in good agreement with earlier
reports28. This confirms the presence of active PWA
species on the surface of the supports.
Oxidation of styrene involves formation of epoxide,
diol, benzaldehyde, acetophenone and formaldehyde.
To check the catalytic activity, the same reaction was
carried out without catalyst also. It was found that no
oxidation takes place in the absence of the catalyst.
Both the supports alone, Z and A, were found to be
inactive for oxidation of styrene.
The oxidation of styrene was carried out with H2O2
in 1:3 molar ratio by using 25 mg of fresh catalysts
for 48 h at 80˚ C. The obtained results are presented
in Fig 1. Figure 1 shows a sharp increase in the
conversion with increase in the % loading of PWA
from 10% to 30% on the support. Further, on
increasing the % loading from 30% to 40%, the
percent conversion starts decreasing. This may be due
to the blocking of the active sites.
From the above study, the optimum % loading of
PWA has been fixed at 30% and all further studies
were carried out with the catalyst containing 30%
loading of PWA.
The reaction was carried out by varying molar ratio
of styrene: H2O2 with 25 mg of the catalyst for 48 h at
80 ºC (Table 1). Result shows that in both the cases,
with increase in the concentration of H2O2, there is a
drastic change in the % conversion with change in
% selectivity of the products. The increase in
% conversion may be due to higher concentration of
oxidizing agent, H2O2, in the reaction medium. Due to
the higher concentration of the H2O2, more oxidant
molecules are available on the surface of the catalyst
to react with styrene molecules and as a result more
styrene molecules undergo reaction and as a result
% conversion increases drastically. The % conversion
of styrene is 96 % and the product selectivity to
benzaldehyde is 91 % and 89 % for PWA3/A and
PWA3/Z respectively when molar ratio of styrene to
H2O2 is 1: 3.
The reaction was carried out with different amounts
of catalyst with molar ratio of 1: 3 for 48 h. It is seen
that with increase in the amount of catalyst above
Fig. 1―The conversion of styrene (%) and selectivity (%)
with different % loading of PWA. [Temp. = 80° C; Time =
48 h; Mole ratio of styrene to H2O2: 1:3; Curve ♦ 1,
% conversion of styrene (PWA3/A); ■ 2, % selectivity of
benzaldehyde (PWA3/Z); ● 3, % selectivity of
acetophenone (PWA3/Z); − 4, % conversion of styrene
(PWA3/Z; * 5, % selectivity of acetophenone (PWA3/A);
▲ 6, % selectivity of benzaldehyde (PWA3/A)].
Table 1―The conversion of styrene (%) and selectivity (%) with
calcined catalysts under optimum conditions.
[Amt. of catalyst = 25 mg; Temp. = 80 °C; Time = 48h;
Mole ratio of styrene to H2O2: 1: 3]
Catalyst
Conversion
(%)
PWA3/Z
PWA33/Z
PWA35/Z
PWA3/A
PWA33/A
PWA35/A
96
92
88
96
94
90
Selectivity (%)
Benzaldehyde
Acetophenone
89
87
94
91
90
93
11
13
6
9
10
7
NOTES
25 mg, the % conversion becomes constant, i.e., the
change is not appreciable. This may be attributed to
the blocking of the catalytically active sites which
indicates that the rate is controlled by the surface
reaction over the surface of the catalyst.
Oxidation of styrene with H2O2 was carried out at
different time periods at a molar ratio of 1:3 using
25 mg of catalyst. It is seen that with increase in
reaction time, the % conversion also increases. This is
because more time is required for the formation of
reactive intermediate (substrate + catalyst) which is
finally converted into the products.
The optimum conditions for 96 % conversion with
maximum selectivity towards benzaldehyde are as
follows; 25 mg of the catalyst, mole ratio of
styrene to H2O2 is 1:3, reaction time 48 h and
reaction temperature 80 °C. The % conversion and
% selectivity of different products for calcinated
catalysts under optimized conditions are presented in
Table 1.
The % conversion and % selectivity of different
products with calcinated catalysts under optimized
conditions are presented in Table 1. It is seen that
there is not much difference in the % conversion as
well as the % selectivity of the products especially
when A is used as support. This also indicates thermal
stability of the present catalysts upto 600 ºC. It is very
important and interesting to note that both the
catalysts (PWA3/A and PWA3/Z) show same activity
towards oxidation reaction. This may be due to the
presence of the same active species, PWA, in both the
catalysts. The very small change in the selectivity of
products may be attributed to the difference in nature
of supports.
Both the catalysts are equally active for the reaction
with the difference in the stability. PWA supported
onto Z (PWA3/Z) is stable only upto 400 ºC, whereas
PWA supported onto A (PWA3/A) is stable upto
600 ºC, which shows that PWA3/A is a better catalyst
than PWA3/Z.
Since the amount of catalyst used is very small
(25 mg), there is some loss during filtration and also
due to difficulty in collection. Hence, the recycling of
the catalyst was not very efficient. Here, both the
catalysts are equally active for the reaction with a
difference in the stability. PWA3/Z is only stable up to
400 ºC whereas PWA3/A is stable upto 500 ºC which
shows that neutral alumina can be a better support
than hydrous zirconia.
967
The oxidation of styrene was carried out in
presence of catalysts PWA3/A and PWA3/Z and the
analysis of the product reveals that double bond of
styrene had been completely cleaved to give
benzaldehyde in high yield and selectivity. The
obtained results are in good agreement with the
known observations and the reaction follows the
same mechanistic pathways as proposed by
Johnstone et al28.
The present study reports non-solvent liquid phase
oxidation of styrene at low temperature using
economical and environmentally friendly 30%
aqueous hydrogen peroxide. The studied catalyst
gives 96% conversion with 91% selectivity towards
benzaldehyde and can be used successfully to replace
the conventional catalysts and a cleaner technology
can be established for oxidation of styrene. This
catalyst can be used as a green catalyst to develop
benign and sustainable methods for oxidation
reactions.
Acknowledgement
PS is thankful to UGC, New Delhi, for financial
assistance.
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