Cobalt (II) exchanged supported 12-tungstophosphoric acid: Synthesis, characterization and non-solvent liquid phase aerobic oxidation of alkenes

Article abstract of DOI:10.1016/j.molcata.2010.01.014

Co²⁺ exchanged supported 12-tungstophosphoric acid was synthesized and characterized. Its catalytic activity was evaluated for the oxidation of alkenes such as styrene, cyclohexene and cis-cyclooctene under mild conditions. The superiority of the catalyst lies efficiently in catalyzing the non-solvent liquid phase oxidation of alkenes, especially oxidation of cyclohexene with 98% conversion and 57% selectivity for cyclohexene oxide, with molecular oxygen at 1 atm pressure. The active intermediate, responsible for the oxidation of alkenes, was also isolated and characterized by ESR. Its catalytic activity was evaluated for the oxidation of styrene under optimized conditions. Based on the above study a probable reaction mechanism was also proposed.

Full text of DOI:10.1016/j.molcata.2010.01.014

Journal of Molecular Catalysis A: Chemical 321 (2010) 22–26
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Cobalt (II) exchanged supported 12-tungstophosphoric acid: Synthesis,
characterization and non-solvent liquid phase aerobic
oxidation of alkenes
∗
Pragati Shringarpure, Anjali Patel
Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara 390002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Co2+ exchanged supported 12-tungstophosphoric acid was synthesized and characterized. Its catalytic
activity was evaluated for the oxidation of alkenes such as styrene, cyclohexene and cis-cyclooctene
under mild conditions. The superiority of the catalyst lies efficiently in catalyzing the non-solvent liquid
phase oxidation of alkenes, especially oxidation of cyclohexene with 98% conversion and 57% selectivity
for cyclohexene oxide, with molecular oxygen at 1 atm pressure. The active intermediate, responsible for
the oxidation of alkenes, was also isolated and characterized by ESR. Its catalytic activity was evaluated
for the oxidation of styrene under optimized conditions. Based on the above study a probable reaction
mechanism was also proposed.
Article history:
Received 18 May 2009
Received in revised form
2
6 December 2009
Accepted 21 January 2010
Available online 28 January 2010
Keywords:
Cobalt
©
2010 Elsevier B.V. All rights reserved.
1
2-Tungstophosphoric acid
Oxidation
Alkenes
Epoxides
1
. Introduction
carried out without organic solvents with molecular oxygen under
mild conditions. This could be possible by designing a catalyst
possessing advantages of supported HPAs as well as Co. In the
present paper, we report the results for solvent-free oxidation
of alkenes with molecular oxygen under mild conditions over a
new catalyst. We have made use of supported HPAs as exchanging
agent. Cobalt was exchanged with available protons of supported
In recent years, supported heteropolyacids (HPAs) have received
much attention as solid acid catalysts [1–9]. The presence of
replaceable H+ is known to be responsible for acid catalyzed
reactions [10]. The protons in the secondary structure of the het-
eropolyacids can be easily exchanged, completely or partially, with
different cations without affecting the primary Keggin structure
12-tungstophosphoric acid (TPA/ZrO ). The resulting amorphous
2
[
11].
Due to the known advantages, use of Co containing catalytic
catalyst, Co exchanged supported 12-tungstophosphoric acid
(CoTPA/ZrO ), was characterized by various physico-chemical
2
systems for the oxidation of organic substrates has attracted much
attention [12]; however, reports using molecular oxygen are very
scanty. Use of Co complexes for the oxidation of terminal olefins
using molecular oxygen [13–15] with sacrificial co-reductant,
isobutyraldehyde, has been reported. Oxidation of styrene using
Co exchanged faujasite type zeolites [16] and oxidation of cyclic
olefins as well as terminal olefins over Co exchanged zeolites have
been reported by Jasra et al. [17,18]. In both cases, DMF was used as
solvent and reactions were carried out at 60 psi (4 atm) pressure.
Thus, almost all reported oxidation reactions with molecular
oxygen are carried out using solvent under high-pressure con-
ditions. It would be interesting if oxidation reactions could be
techniques such as elemental analysis, thermogravimetric analy-
sis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray
diffraction (XRD) and surface area measurement (BET method). The
oxidation state of Co was confirmed by magnetic moment studies
and ESR. The catalytic activity was evaluated for the non-solvent
liquid phase oxidation of alkenes with molecular oxygen as the oxi-
dant and tert-butyl hydroperoxide (TBHP) as an initiator under mild
conditions.
2. Experimental
2.1. Materials
All chemicals used were of A.R. grade. 12-Tungstophosphoric
∗ Corresponding author. Tel.: +91 265 2795552; fax: +91 265 2795552.
E-mail address: aupatel chem@yahoo.com (A. Patel).
acid and zirconium oxychloride were obtained from Loba Chemie,
Mumbai. Cobalt acetate tetrahydrate and styrene were obtained
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.01.014

P. Shringarpure, A. Patel / Journal of Molecular Catalysis A: Chemical 321 (2010) 22–26
23
Table 1
from Merck and used as received. Cyclohexene and cis-cyclooctene
were obtained from Spectrochem, Mumbai.
FT-IR frequencies of the materials.
Materials
ZrO2
FT-IR band frequencies (cm−1)
2.2. Synthesis of catalyst
H–OH
3400
Zr–O–H
600
P–O
W
O
W–O–W
–
Co–O
–
–
–
The Co exchanged supported 12-tungstophosphoric acid was
1600
synthesized in two-step process. The first step involves the synthe-
sis of the supported 12-tungstophosphoric acid while the second
step involves the synthesis of cobalt exchanged supported 12-
tungstophosphoric acid.
1370
3400
3306
TPA/ZrO2
600
–
1070
1058
964
943
812
851
–
CoTPA/ZrO2
475
1622
2
.2.1. Synthesis of supported 12-tungstophosphoric acid
TPA/ZrO2)
0% of 12-tungstophosphoric acid (TPA) supported onto ZrO2
was synthesized by impregnation as reported by us earlier [19].
(
HP-1 capillary column (30 m, 0.5 mm id) with EI (70 eV). The con-
version was calculated on the basis of mole percent of alkenes:
3
2
1
.2.2. Synthesis of cobalt exchanged supported
2-tungstophosphoric acid (Co TPA/ZrO2)
(initial mol %) − (final mol %)
conversion (%) =
× 100
initial mol %
This was followed by the wet impregnation of 1 g of TPA/ZrO2
with 25 mL 1% aqueous solution of Co(CH COO) ·4H O for 24 h with
3
2
2
stirring. The solution was then filtered, washed with distilled water
3. Results and discussion
◦
in order to remove the excess of cobalt and dried at 100 C for 1 h.
The resulting material was designed as CoTPA/ZrO2.
The amount of exchanged Co was 72 mg/g of TPA/ZrO . The FT-IR
2
bands for zirconia (ZrO ), TPA/ZrO2 and CoTPA/ZrO2 are shown in
2
−
Table 1. The FT-IR spectrum for ZrO shows the bands at 3400 cm
2
1
2.3. Characterization
corresponding to symmetric aquo stretching, and two bending
−
1
The filtrate and solid CoTPA/ZrO2 were analyzed for Co as well
vibrations at 1600 and 1370 cm for the O–H–O and H–O–H vibra-
−
1
as acetate anion. The analysis of filtrate showed the presence of
acetate anions and excess of Co ions, which were determined by
UV–vis spectroscopy. The analysis of the product mixtures for any
leaching of Co was carried by using atomic absorption spectrometer
AAS GBC-902 instrument. Elemental analysis for cobalt estimation
was carried out by using a JSM 5610 LV EDX-SEM analyzer. FT-IR
spectrum of the sample was obtained by using the KBr wafer on a
PerkinElmer instrument. TGA of the samples was carried out on a
tions. It also shows a weak bending vibration at 600 cm attributed
to the Zr–O–H) vibration. The FT-IR spectrum of TPA/ZrO2 shows
additional stretching vibrational bands for W–O–W, W O and P–O
respectively. The FT-IR spectrum of
CoTPA/ZrO shows bands at 851, 943 and 1058 cm corresponding
to the symmetric stretching for W–O–W, W O and P–O. The shift
in the band positions as compared to those of TPA/ZrO may be due
to the change in the environment. An additional bending vibration
at 475 cm corresponds to the Co–O vibration and indicates the
−
1
at 812, 964 and 1070 cm
−
1
2
2
◦
−1
Mettler Toledo Star SW 7.01 in the temperature range of 50–600 C
under nitrogen atmosphere with a flow rate of 2 mL/min and a
presence of Co in the synthesized material.
◦
heating rate of 10 C/min. Magnetic susceptibility measurements
The TGA curve of TPA/ZrO2 shows 12.6% weight loss in the tem-
◦
◦
were carried out with the Gouy Balance method at 25 C by apply-
perature range of 70–100 C indicating the loss of adsorbed water.
◦
ing Pascal corrections. The ESR spectra were recorded on a Varian
E-line Century series X-band ESR spectrometer (at room temper-
ature and scanned from 2000 to 4000 G). The XRD pattern was
obtained by using a Philips PW-1830 diffractometer. The condi-
Further, there is no weight loss observed up to 450 C indicating the
stabilization of the supported material. TGA of CoTPA/ZrO2 shows
10.2% weight loss in the temperature range of 100–150 C due to
◦
the loss of adsorbed water. No appreciable weight loss is observed
◦
◦
◦
tions were: Cu K␣ radiation (1.54 Å), scanning angle from 0 to 60 .
up to 400 C indicating that the synthesized catalyst is stable up to
◦
Adsorption–desorption isotherms of samples were recorded on a
400 C.
The magnetic susceptibility value for TPA/ZrO2 is 0.2 ␮B
◦
Micromeritics ASAP 2010 surface area analyser at −196 C.
indicating the diamagnetic nature of TPA/ZrO . The magnetic sus-
2
ceptibility value for CoTPA/ZrO2 is 1.5 ␮B (after applying Pascal
corrections), which corresponds to one unpaired electron. The mag-
netic study indicates the presence of paramagnetic Co species
2
.4. Oxidation of alkenes
The catalytic activity was evaluated for the oxidation of alkenes
6
1
(low spin t2g eg ; S = 1/2), which was further confirmed by ESR
using molecular oxygen as oxidant and TBHP as co-oxidant. Oxida-
tion reaction was carried out in a batch type reactor operated under
atmospheric pressure. In a typical reaction, a measured amount
of catalyst was added to a three-necked flask containing alkene
spectra. TPA/ZrO2 is ESR silent indicating the presence of dia-
magnetic TPA/ZrO2 system. The room temperature X-band ESR of
CoTPA/ZrO2 shows a typical anisotropic spectrum for Co(II), S = 1/2
◦
◦
spin system revealing the presence of paramagnetic Co with g
⊥
and initiator TBHP (0.15 mmol) at 80 C (for styrene) and 50 C (for
cyclic alkenes). The reaction was started by bubbling O2 into the
liquid. The reaction was carried out by varying different param-
eters such as the amount of the catalyst and reaction time. After
completion of the reaction, the liquid product was extracted with
dichloromethane, dried with magnesium sulphate and analyzed on
a gas chromatograph (Nucon 5700 model) having a flame ioniza-
tion detector and an SE-30 packed column (2 m length and 10%
silica based stationary phase. Product identification was done by
comparison with authentic samples and finally by a combined gas
chromatography mass spectrometer (Hewlett-Packard) using an
2
.706 and g|| 2.723.
The XRD patterns of TPA/ZrO2 and CoTPA/ZrO2 are presented in
Fig. S1. XRD pattern of CoTPA/ZrO2 shows the amorphous nature
of the catalyst. It does not show any characteristic diffraction line
indicating a very high dispersion of solute in a non-crystalline
form on the surface of TPA/ZrO . This observation was further sup-
2
ported by BET surface area values. The increase in surface area
2
f
o
r
t
h
e
C
o
T
P
A
/
Z
r
O
(
3
0
7
m
/
g
)
a
s
c
o
m
p
a
r
e
d
t
o
t
h
a
t
o
f
T
P
A
/
Z
r
O
2
2
2
(
146 m /g) indicates high uniform dispersion of the Co on the sur-
face of TPA/ZrO2.

2
4
P. Shringarpure, A. Patel / Journal of Molecular Catalysis A: Chemical 321 (2010) 22–26
Table 2
Oxidation of styrene using different concentrations of catalyst with molecular oxygen.
Catalysta
Amount of catalyst (mg)
Concentration of the active
catalyst (mmol) on TPA/ZrO2
Conversion (%)
Selectivity (%)
Benzaldehyde
Styrene oxide
CoTPA/ZrO2
25
0.03
0.06
0.09
0.122
0.244
23
49
62
76/95
77/95
52
65
>99
>99
>99
48
35
–
–
–
5
7
0
5
b
1
2
00
00
b
a
◦
◦
Conversion is based on styrene; styrene, 100 mmol; oxidant, O2 1 atm; TBHP 0.15 mmol; temperature, 80 C; time, 4 h.
b
Conversion is based on styrene; styrene, 100 mmol; oxidant, O2 1 atm; TBHP 0.15 mmol; temperature, 80 C; time 24 h.
3
.1. Oxidation of alkenes
In the case of oxidation of cyclohexene, besides cyclohexene
oxide, cyclohexanol was also obtained which is unusual as com-
pared to the reported results. Cyclic alkenes generally follow allylic
oxidation resulting in 2-cyclohexe-1-none or 2-cyclohexe-1-nol
as products rather than the epoxidation or bond cleavage mech-
anism. Oxidation of cyclohexene has been reported by a number of
groups [18,22], when a peroxide type of oxidants is used, resulting
in allylic oxidation products. In the present study bond cleavage
was observed giving rise to cyclohexanol. This could be explained
on the basis that the oxidant used contains the highest content
of active oxygen. Also the catalyst used is highly acidic with acid-
ity equivalent to superacids. Under these extreme acidic oxidation
conditions the bond cleavage mechanism is preferred to the typical
allylic oxidation.
A detailed study was carried out on the oxidation of styrene
to optimize the conditions. TPA/ZrO2 was almost inactive for the
oxidation of alkenes under the present reaction conditions. The
effect of concentration of the catalyst on the conversion is shown
in Table 2.
With an increase in the amount of Co, % conversion also
increases. This suggests that Co functions as active sites for oxi-
dation. The obtained results are in good agreement with a reported
one [16]. It is very interesting to observe the difference in the selec-
tivity of the products with an increase in the concentration of the
catalyst. As shown in Table 2, with 0.03 mmol concentration of
active catalyst, epoxide is selectively obtained. On increasing the
concentration of the catalyst, the product selectivity shifts from the
less stable intermediate (epoxide) towards the more stable prod-
uct (benzaldehyde). This may be due to the fact that with increase
in the amount of the active species the reaction becomes very fast
which favours the conversion of the formed styrene oxide to ben-
zaldehyde.
The catalytic performances of the catalyst for the oxidation of
cyclic alkenes are shown in Table 3. It has been reported by Kanmani
and Vancheesan [20] that the use of TBHP with transition metal
based catalysts activates the metal centre. The inter-conversion of
the two oxidation states for the metal complexes, corresponding
to an oxidative addition and reductive elimination, are responsi-
ble for effective catalysis. Hence, the activated catalysts, attack the
3.2. Test for heterogeneity
The leaching of Co from CoTPA/ZrO2 was confirmed by carrying
out an analysis of the used catalyst (EDX) as well as the product mix-
tures (AAS). Analysis of the used catalyst did not show appreciable
loss in the cobalt content as compared to the fresh catalyst. Analysis
of the product mixtures showed that if any cobalt was present it was
below the detection limit, which corresponded to less than 1 ppm.
These observations strongly suggest that the present catalyst is
truly heterogeneous in nature.
Heterogeneity test was carried out for the oxidation of styrene as
an example. For the rigorous proof of heterogeneity, a test [23] was
◦
C
C site of styrene and preferentially lead to an oxidative cleav-
carried out by filtering catalyst from the reaction mixture at 80 C
age rather than epoxide formation [21]. While in the case of cyclic
olefins, an allylic attack is preferred, giving rise to epoxide which in
turn rearranges by reductive elimination of the catalyst resulting in
further oxygenated products. Thus the obtained results are in good
agreement with the reported explanation.
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 indicating the present catalyst fall
into category C [23]. The results are presented in Table 4.
The observed order for the reactivity of cyclic olefins is cyclo-
hexene > cis-cyclooctene. The lower conversion for cis-cyclooctene
is mainly due to two factors: (i) the bulkiness of the cyclic ring as
well as ring strain makes it difficult to coordinate and thus partially
prevents the oxidation process (ii) initially the rate of desorption
of the product (i.e. cyclooctene oxide) from the catalyst surface is
high but as the concentration of the bulky product increases in the
reaction mixture, the rate becomes slow.
3.3. 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
◦
at 100 C. Oxidation of styrene was carried out with the recycled
catalyst under the optimized conditions. The catalyst was recy-
Table 3
Oxidation of alkenes using molecular oxygen.
Catalysta
Alkene
Conversion (%)
Products
Selectivity (%)
>99
TON
623
803
CoTPA/ZrO2
Styrene
76
98
Benzaldehyde
Cyclohexene
Cyclohexene oxide
Cyclohexanol
57
43
cis-Cyclooctene
21
Cyclooctene oxide
>99
172
a
◦
Conversion is based on substrate; substrate, 100 mmol; oxidant, O2 1 atm; TBHP 0.15 mmol; catalyst 0.12 mmol; temperature, 80 C; reaction time 4 h (for styrene), 24 h
for cyclic olefins).
(

P. Shringarpure, A. Patel / Journal of Molecular Catalysis A: Chemical 321 (2010) 22–26
25
Table 4
cled in order to test its activity as well as stability. The obtained
%
Conversion and % selectivity for the oxidation of styrene (with and without
results are presented in Table 5. As seen from Table 5, the recycled
catalyst did not show any appreciable change in the activity, indi-
cating that the catalyst is stable and can be regenerated for repeated
use.
catalyst).
Catalyst
% Conversion
% Selectivity
Benzaldehyde
Similarly, recycling of the catalyst was carried out for the oxida-
tion of cyclic alkenes. No appreciable change in conversion as well
as selectivity indicates that the catalyst can be reused.
CoTPA/ZrO2 (4 h)
Filtrate (6 h)
76
75.6
>99
>99
% Conversion is based on alkene; catalyst, 0.122 mmol; oxidant, O2; TBHP 0.15 mmol;
reaction temperature 80 C.
◦
3.4. Reaction mechanism
Table 5
In order to study the reaction mechanism the same sets
of reactions were carried under two different conditions: (i)
Oxidation of styrene with fresh and regenerated catalyst.
aCatalyst
% Conversion
% Selectivity
alkene + oxidant + TBHP and (ii) alkene + oxidant + CoTPA/ZrO . In
2
Benzaldehyde
both the cases the reaction did not progress significantly. These
observations indicate that the liberation of O2 from TBHP was not
CoTPA/ZrO2
R1-CoTPA/ZrO2
R2-CoTPA/ZrO2
76
72
72
>99
>99
>99
2+
sufficient to induce the reaction as well as the activation of Co
to Co3+ being necessary for provoking the reaction under the opti-
a
mized conditions. Hence it may be concluded that in the present
study TBHP acts as an initiator only.
Styrene, 100 mmol; oxidant, 1 atm O2; TBHP 0.15 mmol; reaction time, 4 h; cat-
◦
alyst, 0.122 mmol; temperature 80 C.
Based on the above observations a tentative reaction mecha-
nism for styrene, as an example, is proposed in Scheme 1. It has
been reported that the catalyst containing metal cations in low
valency states and involving O2 as an oxidant always follow the
radical chain mechanism induced by M-O2 intermediate [24,25].
In the present catalytic system, the mechanism involving the Co
species is expected to follow the same path. As described in the
proposed reaction mechanism, for the catalytic reaction to pro-
was also evaluated for the oxidation of alkenes, especially styrene.
•
3+
The room temperature X-band ESR of ( OCo TPA/ZrO ) shows
2
a typical anisotropic ESR with splitting of the ESR signal which
may be due to the presence of a radical electron (Fig. S2). The
broadening of the signal may be due to the presence of a transi-
tion metal ion (i.e. Co3+). The anisotropic ESR signal is indicative
of the presence of a free radical. The free radical resides on O-
atom and it is strongly coupled with Co3+. The reported, g-value
of organic free radicals is 2.0023 [26]. The average g-value calcu-
lated for the present system is 2.01. The observed higher g-value
may be due to the nature of the formed radical intermediate. In the
present system the nature of the radical is inorganic and hence the
result is as expected. The ESR studies strongly support the proposed
mechanism.
ceed activation of Co2 is required. The activation of Co species
+
2+
takes place through attack of TBHP followed by the formation of an
•
3+
active intermediate, OCo TPA/ZrO . It is expected that TBHP gets
2
2+
3+
3+
reduced to form tert-butyl alcohol and in turn oxidizes Co → Co
in situ. The liberated oxygen from TBHP attacks Co3 to form OCo
+
•
TPA/ZrO . This formed intermediate is responsible for the activa-
2
tion of alkenes.
•
3+
This activated species ( OCo TPA/ZrO ) gets attached with
2
•
3+
Oxidation of styrene was carried out by using OCo3+TPA/ZrO2
•
O2 species and forms OOCo TPA/ZrO2 radical (i.e. Co–O2
metal–superoxo intermediate) which then attacks the substrate.
The metal–superoxo intermediate reversibly binds to the alkene
attacking the reaction site, resulting in oxidation of substrate to
form products. Further, oxidation of alkenes via radical chain mech-
anism is a known process [24,25].
under optimized conditions (100mmol styrene; 0.15 mmol TBHP;
◦
1 atm O ; conc. of catalyst 100 mg; temperature 80 C; reaction
2
time 4 h). 76% conversion with >99% selectivity for benzalde-
hyde was obtained with CoTPA/ZrO2 for the oxidation of styrene.
If oxidation reaction follows the proposed reaction mechanism,
•
3+
In order to confirm the role of formed active intermediate, it
was isolated and characterized by ESR. Further, its catalytic activity
OCo TPA/ZrO2 must give almost the same conversion as well as
selectivity. The results are as expected. 73% conversion with >99%
Scheme 1. Proposed reaction mechanism for the oxidation of styrene.

2
6
P. Shringarpure, A. Patel / Journal of Molecular Catalysis A: Chemical 321 (2010) 22–26
Table 6
Comparison of conversion and selectivity values for the oxidation of alkenes.
Catalyst
Alkene
aReaction conditions
Conversion (%)
Products/selectivity
TON
623
CoTPA/ZrO2
NaCoX96 [17]
Styrene
100;4;1;0;100
10;4;4;20;200
76
100
BA/ > 99
BA/67
16.0
StyO/33
StyO/60
Co2+X [16]
44
13.0
803
CoTPA/ZrO2
NaCoX96 [18]
Cyclohexene
8;24;1;0;100
2;8;4;40;200
98
26
Cy6O/57
Cy6O/48
–
CoTPA/ZrO2
NaCoX96 [18]
cis-Cyclooctene
8;24;1;0;100
2;8;4;40;200
21
47
cis-Cy8O/>99
cis-Cy8O/100
172
a
Substrate (mmol): reaction time (h): pressure (atm): solvent (mL): amount of catalyst (mg). 1 atm = 14.5 psi.
selectivity for benzaldehyde was obtained. Thus the ESR studies
and catalytic study strongly support the proposed mechanism con-
firming that the formed intermediate is only responsible for higher
conversion.
Acknowledgments
One of the authors, Ms Pragati A Shringarpure is thankful to Uni-
versity Grants Commission (UGC), New Delhi for providing financial
assistance.
3.5. Comparison with reported catalysts
Appendix A. Supplementary data
The superiority of the present catalyst lies in obtaining higher
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.01.014.
conversion as well as selectivity, especially in the case of cyclo-
hexene, than NaCoX96. It is seen from the Table 6 that in the case
of styrene, 100% conversion was obtained with NaCoX96 but the
selectivity for benzaldehyde is 67%. In the case of Co2 –X, styrene
oxide and benzaldehyde were the two main products along with
minor quantities of styrene glycol, benzoic acid and mandelic acid,
only 44% conversion being obtained. The present catalyst gives 76%
conversion with >99% selectivity for benzaldehyde. Other products
obtained were less than 1%. In the case of oxidation of cyclic alkenes,
especially for cyclohexene, results are very unique and outstand-
ing. The present catalyst gives 98% conversion and 57% selectivity
for cyclohexene oxide.
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[
[
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4
. Conclusion
[
4
In conclusion, we have come up with a new oxidation catalyst;
[
[
[
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Co exchanged supported 12-tungstophosphoric acid. The present
contribution reports solvent-free liquid phase aerobic oxidation of
alkenes at lower temperature and ambient pressure. The superi-
ority of the present catalyst lies, in obtaining 98% conversion for
cyclohexene with 57% selectivity for cyclohexene oxide as well as
in obtaining very high turnover number (TON) for all oxidation
reactions. The active intermediate, responsible for oxidation, was
isolated and studied for ESR as well as catalytic activity. Based on
the results, a probable mechanism for the oxidation of alkenes was
also proposed.
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[
[
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4
[
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