Polyoxometalate coordinated transition metal complexes as catalysts: Oxidation of styrene to benzaldehyde/benzoic acid

Article abstract of DOI:10.1007/s12039-014-0719-4

Oxidation of styrene is carried out by using heptamolybdate coordinated transition metal (Co²⁺, Zn²⁺) complexes, [2-ampH]₄[{Co(H₂O)₅}Mo₇O₂₄] ·9H₂O (1), [3-ampH]_4<

Full text of DOI:10.1007/s12039-014-0719-4

c
J. Chem. Sci. Vol. 126, No. 6, November 2014, pp. 1641–1645. ꢀ Indian Academy of Sciences.
Polyoxometalate coordinated transition metal complexes as catalysts:
Oxidation of styrene to benzaldehyde/benzoic acid
SRINIVASA RAO AMANCHIa, ANJALI PATELb and SAMAR K DASa,∗
aSchool of Chemistry, University of Hyderabad, Hyderabad 500 046, India
bChemistry Department, Faculty of Science, M. S. University of Baroda, Vadodara 390 002, India
e-mail: skdsc@uohyd.ernet.in; samar439@gmail.com
MS received 16 January 2014; revised 5 April 2014; accepted 28 April 2014
Abstract. Oxidation of styrene is carried out by using heptamolybdate coordinated transition metal (Co2+
,
Zn2+) complexes, [2-ampH]4[{Co(H2O)5}Mo7O24]·9H2O (1), [3-ampH]4[{Co(H2O)5}Mo7O24]·9H2O (2), [2-
ampH]4[{Zn(H2O)5}Mo7O24]·4H2O (3) and [3-ampH]4[{Zn(3-ampy)(H2O)4}Mo7O24]·4H2O (4) as catalysts
and H2O2 as an oxidant at 80◦C. The leaching study has been carried out to check the quality of catalyst and it
has been reused for three times with good percentage of conversion. For the first two catalysts (compounds 1
and 2), the major product obtained is benzaldehyde, and benzoic acid is the major product for next two catalysts
(compounds 3 and 4). Stability of the catalysts has been analyzed by IR, UV-spectroscopy and powder X-ray
crystallography.
Keywords. Heptamolybdate type polyoxometalate cluster anion; transition metal coordination complexes;
oxidation of styrene; hydrogen peroxide as an oxidant; catalysis.
1. Introduction
the highly expensive materials. Thus we have chosen
hydrogen peroxide as a green oxidizing agent because,
in this case, water is the side product, which is benign to
our environment. And it is a very attractive oxidant for
liquid phase reactions.7 H2O2 can oxidize the organic
compounds with an efficiency of 47% (active oxidant=
47%) and it is very cheap (<0.7 US $ Kg−1). It is
essential to mention that H2O2 is an ideal and waste
free oxidant if it is used without organic solvents and
toxic materials. Safety reasons are not required very
much for its storage.8 Moreover, H2O2 is commonly
used in large scale reactions to synthesize caprolac-
tam, (CH2)5C(O)NH (Sumitomo Chemical Co.)9 and in
propylene oxidation (BASF and Dow Chemical Co.).10
Polyoxometalates are well known to act as cata-
lysts for oxidation of olefins and alcohols with good
selectivity.11Due to the availability of dual oxidation
states of metal ion, it offers the catalytic behaviour
towards oxidation of alcohols and olefins resulting in
aldehydes or ketones. Such properties are tuned by
changing the counter cation of the polyoxoanions, for
example, Co3[Si2W12O40] is a good catalyst in oxida-
tion of alcohols, whereas, other salts of Keggin cluster
anion, [Si2W12O40]6− are not that much efficient cat-
alyst for alcohol oxidation.12 In the present study, we
have examined the catalytic applications of four poly-
oxometalate coordinated transition metal complexes,
namely, [2-ampH]4[{Co(H2O)5}Mo7O24]·9H2O (1),
[3-ampH]4[{Co(H2O)5}Mo7O24]·9H2O (2), [2-ampH]4
Over the last few decades, the subject of polyoxomet-
alates (POMs) is discussed in the areas of catalysis,1
materials science2 and medicinal chemistry.3 The redox
properties of these POMs make them promising mate-
rials as good catalysts for a number of oxidation and
dehydrogenation reactions of organic substrates. The
main role of POM-cluster anion in catalysis is the acti-
vation of the oxidant, e.g., hydrogen peroxide, alkylhy-
droperoxide, etc. As a result of the activation of perox-
ide (oxidant), the inorganic peroxo, hydroperoxo, alkyl
peroxo, acyl peroxo, etc., intermediates are formed,
that are very reactive leading to the oxygenation of
concerned organic substrate.
The oxidation of olefins to corresponding aldehy-
des or ketones is currently a fascinating process in
industrial chemistry. This oxidation can be divided into
three categories: (i) the cleavage of the C=C bond by
using the surface of the metal oxide, e.g., osmium or
molybdenum oxides in stoichiometric amounts,4 (ii)
the ozonolysis of olefin bond leading to the forma-
tion of keto-functional group5 and (iii) olefin oxida-
tion by using hydrogen peroxide as an oxidant result-
ing in the formation of C=O functional group directly.6
Some problems, associated with these oxidation reac-
tions, are mainly wastage of the materials and usage of
∗For correspondence
1641

1642
Srinivasa Rao Amanchi et al.
Scheme 1. Oxidation of styrene leading to benzaldehyde and benzoic acid using
heptamolybdate coordinated transition metal complex as a catalyst.
[{Zn(H2O)5}Mo7O24]·4H2O (3) and [3-ampH]4[{Zn(3- ZB-1 column (30 m × 0.25 mm, pressure = 20.0 k Pa,
ampy)(H2O)4}Mo7O24]·4H2O (4) in the conversion detector = EI, 300◦C) with helium gas as carrier.
of styrene to benzaldehyde/benzoic acid as shown in
scheme 1. These catalysts are efficient to be reused for
three times with similar percentage of conversion.
3. Results and Discussions
3.1 Catalysis
2. Experimental
The oxidation of styrene with 30% of H2O2, catalyzed
2.1 Materials
by four synthesized materials (compounds 1-4), pro-
duces benzaldehyde/benzoic acid as the major prod-
uct; the minor products are styrene oxide and acetophe-
none. The advantages of these catalysts lie in the fact
that they are recycled in the same oxidation for at least
three times with similar percentage of conversion. The
complete results of the styrene oxidation by using four
catalysts are shown in table 1. When the same oxida-
tion was performed with ammonium heptamolybdate as
a catalyst (instead of compounds 1–4: heptamolybdate
cluster anion coordinated transition metal complexes),
we found only 5% conversion. On the other hand, use
of compounds 1-4 (as catalysts) results in a good per-
[2-ampH]4[{Co(H2O)5}Mo7O24]·9H2O (1), [3-ampH]4
[{Co(H2O)5}Mo7O24]·9H2O (2), [2-ampH]4[{ Zn(H2O)5}
Mo7O24]·4H2O (3) and [3-ampH]4[{Zn(3-ampy)(H2O)4}
Mo7O24]·4H2O (4) were synthesized according to liter-
ature procedures, reported by us and others.13 Styrene
and H2O2 (30%) were purchased from Hi-Chem.
Schematic representation for synthesis of four com-
pounds 1-4 is shown in scheme 2.
2.2 Physical measurements
Infrared (IR) spectra were recorded on KBr pellets centage (>95%) of conversion under similar condition.
with a JASCO FT/IR-5300 spectrometer in the region This demonstrates that, in the present study, transi-
of 400–4000 cm−1. Electronic absorption spectra were tion metal complex, supported on the surface of POM
recorded on a Cary 100 Bio UV–Vis spectrophotometer. cluster anion by coordinate-covalent bonds, acts as a
G.C. analysis was performed on GCMS equipped with catalyst. It is notable from table 1 that, isopolyanion
Scheme 2. Schematic representation for synthesis of compounds [2-ampH]4[{Co(H2O)5}Mo7O24]·9H2O
(1), [3-ampH]4[{Co(H2O)5}Mo7O24]·9H2O (2), [2-ampH]4[{Zn(H2O)5}Mo7O24]·4H2O (3) and [3-ampH]4
[{Zn(3-ampy)(H2O)4}Mo7O24]·4H2O (4) (catalysts). Colour codes: Mo, yellow; Co/ Zn, Cyan; C, brown;
O, red; N, green.

Polyoxometalate supported coordination complexes as catalysts
1643
Table 1. Oxidation of styrene by using the four catalysts (compounds 1–4 as catalysts).
Selectivity (%)
Catalysts**
Conversion (%)
Benzaldehyde
Epoxide
Benzyl alcohol
Benzoic acid
Acetophenone
Compound 1
Compound 2
Compound 3
Compound 4
99.3
96
100
100
76
76
16
15
–
3
–
–
5
6
24
15
58
69
–
1
20
11
3.8
1.2
**A typical catalysis reaction includes 25 mg of the catalyst, 1:2 ratio of styrene and H2O2 for catalysts 1 and 2 and 1:3 ratio
of styrene and H2O2 for catalysts 3 and 4, heating the reaction mixture at 80◦C for 24 h.
supported cobalt complexes (catalysts 1 and 2) pro- the conversion of benzoic acid increases with increase
duce benzaldehyde as maximum percentage of conver- in concentration of H2O2. In all the cases, benzalde-
sion, whereas, the isopolyanion supported zinc com- hyde is the product with major selectivity, especially
plexes (catalysts 3 and 4) bring about benzoic acid as under the condition of 1: 2 ratio of styrene and hydro-
the major product. The probable explanation for this gen peroxide; other products are epoxide and ben-
diversity is the difference in stability of cobalt perox- zophenone with minor selectivity. We have run sev-
ide and zinc peroxide species, formed as intermediates eral reactions for styrene oxidation with catalyst 1
during respective catalysis.14
([2-ampH]4[{Co(H2O)5}Mo7O24]·9H2O) under various
The comparison of the IR spectrum of parent cat- conditions and finally a particular reaction condition
alyst 1 with that of regenerated catalyst 1 suggests was optimized with 1: 2 ratio of styrene and hydrogen
that compound [2-ampH]4[{Co(H2O)5}Mo7O24]·9H2O peroxide at 80◦C for 24 h for the maximum selectivity
(1) remains identical even after several cycles of catal- of benzaldehyde.
ysis. Infrared spectra of both parent 1 and regenerated
During catalysis, after 2 hours of reaction, the colour
1 are shown in the figures S1 and S2 respectively. Solid of the reaction mixture became yellow; subsequently
state (diffuse reflectance) electronic absorption spectra this colour disappeared (after 10 h) and finally the reac-
of the parent compound 1 and regenerated 1, shown in tion mixture became bluish in colour. This may be due
figures S3 and S4 respectively, also suggest that a cat- to the change in oxidation state of molybdenum in the
alyst does not leach during catalytic cycles. Powder- reaction mixture. The catalyst is not soluble in any indi-
XRD studies of parent compound 1 and regenerated 1 vidual solvent (for example, in the starting substrates
also indicate that a catalyst is not destroyed after the and in hydrogen peroxide separately) but it is com-
catalysis is over, as shown in figure S5. We have carried pletely soluble in reaction mixture (substrate + hydro-
out the same reaction by a varying amount of hydrogen gen peroxide). Thus the regeneration of the catalyst was
peroxide (oxidant) as well as by varying amount of the done by evaporating the reaction mixture after separa-
catalyst; the relevant observations have been described tion of the organic part (extraction with DCM layer).
in tables 2 and 3 respectively.
The percentage of conversion increases with increasing
As shown in table 2, increasing the concentration the amount of the catalyst 1 (figure 1). Similarly, the
of H2O2 increases the percentage of conversion but selectivity of benzaldehyde increases with an increase
the conversion of benzaldehyde is decreased. This is in the amount of catalyst as shown in figure 1. The iden-
because the additional hydrogen peroxide oxidizes the tical trend was observed with a second catalyst for the
generated benzaldehyde further to benzoic acid. So same conversion.
Table 2. Results for the reaction of oxidation of styrene by varying the concentration of H2O2 (reaction condition is same
as described in the caption of table 1).
Reaction condition
(Sty:H2O2)
Percentage
of Conversion
Selectivity (%)
Benzyl alcohol
Benzaldehyde
Epoxide
Benzoic acid
Acetophenone
1:1
1:2
1:3
36
96
99.3
88
76
53
2
–
3.5
–
–
–
10
24
31
–
–
12.5

1644
Srinivasa Rao Amanchi et al.
Table 3. Results of percentages of conversion as a function of amount of catalyst 1 for the oxidation of styrene (reaction
condition is same as described in the caption of table 1).
Amount of
Catalyst 1
Percentage of
Conversion
Selectivity (%)
Benzyl alcohol
Benzaldehyde
Epoxide
Benzoic acid
Acetophenone
5 mg
74
91
98
99
99.6
56.6
61.6
65.4
71
3
1
4.6
1.6
0
4.4
2
3.8
3.4
0
28
27
18
18
24
8
10 mg
15 mg
20 mg
25 mg
8.4
8.2
6
76
0
Figure 1. Left: graphical view of percentage of conversion in the reaction of oxidation of styrene
with amount of catalyst. Right: selectivity (%) of benzaldehyde by changing the amount of catalyst
1.
3.2 Leaching property of the catalyst
The percentage-conversion of the substrate and the
percentage-selectivity of the products in the reaction are
calculated as shown below.
All the catalysts (compounds 1-4) are regenerated by
evaporation and reused for minimum of three times with
similar percentage of conversion. Catalysts do not lose
their catalytic activity until three times in the above said
reaction, which has been established based on infrared
spectroscopy, solid state UV spectroscopy and powder
X-ray crystallography as shown in the figures S1–S16
for catalysts 1-4.
Percentage of conversion = {[initial mol% –final
mol%]/initial mol%} ×100.
Percentage of selectivity = [product mol% /substrate
mol%] ×100.
4. Conclusion
We have demonstrated the catalytic activity of four
different heptamolybdate cluster coordinated/supported
3.3 GC-MS calculations
The oxidation of styrene is analyzed by gas transition metal coordination complexes in the oxida-
chromatography-mass spectral studies. After comple- tion of styrene using H2O2 as an oxidant. All the cat-
tion of the reaction, DCM is added to the reaction alysts have been reused for three cycles with simi-
mixture whereby the organic part comes to the DCM lar yields. Benzaldehyde is produced as a major prod-
layer and the aqueous layer is evaporated to dryness to uct with catalysts 1 and 2, whereas benzoic acid is
regenerate the catalyst. Subsequently 0.8 µ Lt. of the the major product with catalysts 3 and 4. The leach-
reaction mixture was injected into the column, from ing properties of the catalysts have been studied suc-
where it entered into the programmed oven (injector cessfully and the relevant studies have been supported
temperature: 200◦C and detector temperature: 250◦C, by infrared spectroscopy, solid state UV-visible spec-
ramping temperature: 2.5◦C).
troscopy and powder X-ray crystallography.

Polyoxometalate supported coordination complexes as catalysts
1645
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Supplementary Information
FT-IR spectra, solid state UV spectra, powder X-
ray crystallographic data of parent catalysts and used/
regenerated catalysts (compounds 1–4) and the results
for the reactions of styrene oxidation with catalysts 1–4
are available in supplementary information at www.ias.
ac.in/chemsci.
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Acknowledgement
We thank Science & Engineering Research Board
(a statutory body under the Department of Science
and Technology), Government of India, for finan-
cial support (Project No. SB/SI/IC/034/2013). We also
thank the Centre for Nano Technology, University of
Hyderabad. The National X-ray Diffractometer facility
at the University of Hyderabad by the Department of
Science and Technology, Government of India, is grate-
fully acknowledged. ASR thanks CSIR, Government of
India for a fellowship.
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