Keggin type transition metal substituted phosphomolybdates: Heterogeneous catalysts for selective aerobic oxidation of alcohols and alkenes under solvent free condition

Article abstract of DOI:10.1039/c3cy00717k

Environmentally benign oxidation of alcohols and alkenes over transition metal substituted phosphomolybdates with molecular oxygen as an oxidant and TBHP as an initiator was carried out. The influence of different parameters on the conversion as well as the selectivity was investigated. All the catalysts showed good catalytic activity with excellent selectivity for the desired products as well as a higher TON. The system not only catalyzes the reaction but also avoids the use of organic solvents as it was carried out under solvent free conditions. Moreover, the catalysts could be recovered and reused four times without a significant loss in their activity and selectivity. A probable reaction mechanism was also proposed for the oxidation of alcohols and alkenes.

Full text of DOI:10.1039/c3cy00717k

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Keggin type transition metal substituted
phosphomolybdates: heterogeneous catalysts for
selective aerobic oxidation of alcohols and
alkenes under solvent free condition†
Cite this: Catal. Sci. Technol., 2014,
4, 648
*
Soyeb Pathan and Anjali Patel
Environmentally benign oxidation of alcohols and alkenes over transition metal substituted phosphomolybdates
with molecular oxygen as an oxidant and TBHP as an initiator was carried out. The influence of different
parameters on the conversion as well as the selectivity was investigated. All the catalysts showed good
catalytic activity with excellent selectivity for the desired products as well as a higher TON. The system not
only catalyzes the reaction but also avoids the use of organic solvents as it was carried out under solvent
free conditions. Moreover, the catalysts could be recovered and reused four times without a significant
loss in their activity and selectivity. A probable reaction mechanism was also proposed for the oxidation of
alcohols and alkenes.
Received 21st September 2013,
Accepted 23rd October 2013
DOI: 10.1039/c3cy00717k
www.rsc.org/catalysis
In this context, polyoxometalates (POMs) are excellent
candidates because of their redox properties.11–16 Recently,
1. Introduction
The catalytic oxidation of organic compounds is still a
challenge in modern chemistry and the industrial world1–3
as oxygenated derivatives, especially carbonyl compounds,
have extensive applications in perfumery, dyestuff and agro
chemical industries.4–6 Because of the unreactivity of present
catalysts with molecular oxygen, traditional procedures for
oxidation have employed more reactive forms of oxygen i.e. toxic
and hazardous oxidants based on chromates, hypochlorites,
and permanganates in stoichiometric amounts or a large
excess.7 However, these procedures have some drawbacks,
such as the use of relatively expensive oxidizing agents, lack
of selectivity, metal waste generation and the use of non-green
halogenated solvents, which are economically and environ-
mentally undesirable. Although reports are available on tran-
sition metal complex catalyzed oxidation reactions,8 these
catalytic systems suffer from poor recycling of the complexes
due to difficult separation and rapid loss of activity. Hence,
from the view point of green chemistry, the search for effi-
cient and heterogeneous catalytic systems exploiting clean
oxidants such as molecular oxygen is highly desirable.9,10
a subclass of polyoxometalates (POMs), transition metal
substituted POMs (TMSPOMs), have gained importance due
to their distinctive electrochemical, magnetic, medicinal and
catalytic properties.17–20 They can also be rationally modified
on the molecular level, including shape, size, charge density,
redox states and stability.20,21 It has been reported that the
addition of transition metals is expected to influence the
redox properties considerably, particularly when they are
incorporated in the primary structure of the Keggin ion.22,23
A literature survey shows that many reports on aerobic
oxidation catalyzed by transition metal silicotungstates and
phosphotungstates, i.e. [XW11M(L)O39]5− (X = P, Si, M = transition
metal, L = ligand) are available.24–29
It is well known that phosphomolybdates are better cata-
lysts for oxidation reactions than their tungstate counter-
parts.30 Among phosphomolybdates, mono vanadium substituted
phosphomolybdates, PVMo11O404−, have been widely investigated
in the selective aerobic oxidation of alkanes,31–33 alkenes,34,35
alcohols,14,36, isobutyric acid,37–41 and phenol.42 At the same
time, reports on aerobic oxidation catalyzed by Ru-substituted
phosphomolybdates43,44 and Fe-substituted phosphomolybdates45,46
are also available. However, in most applied protocols, cata-
lytic oxidation requires harsh temperature conditions, the use
of relatively expensive transition metals (V, Ru) as well as the
use of organic solvents.
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The M. S. University of Baroda, Vadodara 390 002, India.
E-mail: aupatel_chem@yahoo.com
So, looking at the importance of oxidation reactions as well
as considering the economic and ecological aspects, it was
thought to be of interest to develop a sustainable catalytic
† Electronic supplementary information (ESI) available: A detailed study on
aerobic oxidation of styrene over PMo11M (M = Co, Mn, Ni), Scheme S1. See
DOI: 10.1039/c3cy00717k
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system for the aerobic oxidation of alkenes and alcohols
based on (relatively inexpensive) transition metal substituted
phosphomolybdates under solvent free conditions. Recently,
we have reported the synthesis and characterization47 of the
cesium salt of Keggin type mono transition metal(II)-substituted
phosphomolybdates (PMo11M; M = Co, Mn, Ni) and its use
as an efficient catalyst for the oxidation reaction with H2O2
as an oxidant.48,49 So, in order to extend the viability of these
catalysts, we here explore their use as sustainable catalysts
for solvent free oxidation of alcohols and alkenes with molecular
oxygen. The catalytic activity of the recycled catalysts was
evaluated under optimized conditions and a probable reac-
tion mechanism was also proposed.
ESR spectra were recorded on a Varian E-line Century series
X-band ESR spectrometer (room temperature and scanned
from 2000 to 3200 gauss).
2.4. Oxidation reactions
The catalytic activity was evaluated for the oxidation of
alcohols using molecular oxygen as an oxidant and TBHP as
an initiator. The oxidation reactions were carried out in a
batch type reactor operated under atmospheric pressure. In a
typical reaction, a measured amount of the catalyst was
added to a three necked flask containing alcohol at 90 °C.
The reaction was started by bubbling O2 into the liquid. A
similar procedure was followed for the alkene oxidations
with a reaction temperature of 80 °C. The reactions were
carried out by varying different parameters, such as the
amount of the catalyst, reaction time and reaction temperature.
After the 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 a BP-1 capillary column. The
product was identified by a comparison with the authentic
samples and finally by gas chromatography–mass spectros-
copy (GC–MS).
2. Experimental
2.1. Materials
All the chemicals used were of A.R. grade. 12-Molybdophosphoric
acid, H3PMo12O40, NaOH, Na2MoO4·2H2O, Co(CH3COO)2·4H2O,
Mn(CH3COO)2·4H2O, Ni(CH3COO)2·4H2O, styrene, α-methyl
styrene, benzyl alcohol, cyclopentanol, 1-hexanol, 1-octanol
and dichloromethane were obtained from Merck and used
as received.
2.2. Synthesis of the transition metal substituted
phosphomolybdates47
3. Results and discussion
3.1. Analytical techniques
H3PMo12O40 (PMo12, 1.825 g, 1 mmol) was dissolved in water
(10 mL) and the pH of the solution was adjusted to 4.3 using
an NaOH solution. Co(CH3COO)2·4H2O (0.249 g, 1 mmol)
dissolved in a minimum amount of water was mixed with the
above hot solution. The pH of the solution was adjusted to
4.5 (pH 4.3 for nickel substituted phosphomolybdate). The
solution was heated at 80 °C with stirring for 1 h and filtered
hot. A saturated solution of CsCl was added drop wise to the
hot filtrate and allowed to stand. The as-obtained crystals
were very poorly soluble in any solvent and no recrystallisation
was carried out. The obtained dark reddish brown crystals
were filtered, air dried and designated as PMo11Co. Other tran-
sition metal (M = Mn and Ni) substituted phosphomolybdates
were also synthesized according to a similar method by taking
the corresponding metal-acetate salt. The obtained materials
were designated as PMo11Mn and PMo11Ni respectively.
Based on the single crystal as well as the elemental analy-
sis,47,48 the chemical formula for the isolated complexes is
Cs5[PM(H2O)Mo11O39]·6H2O (M = Co, Mn, Ni).
TGA of all of the catalysts show a weight loss of 4.2–4.83%
at 150 °C, corresponding to 7 H2O molecules. Similarly, DTA
of all the catalysts showed an endothermic peak at around
130 °C due to water of crystallization. An exothermic peak in
the region of 415–430 °C indicates crystallization of the
MoO3 phase after decomposition of the Keggin unit.
Table 1 shows the DRS spectra values for PMo11, PMo11Co,
PMo11Mn, and PMo11Ni. Absorption bands at ~200 and 285 nm
are observed for PMo11, due to O → Mo charge transfer. The
DRS spectra of PMo11Co, PMo11Mn, and PMo11Ni show two
peaks. The peaks in the region of 210–235 and 260–300 nm
correspond to O → Mo charge transfer, indicating the forma-
tion of the PMo11O39 lacuna in the synthesized compounds.
The observed shift compared to that of PMo11 may be due to
the substitution of the transition metal. A broad band in the
560–580, 390–430 and 550–600 nm regions, corresponding to
the presence of CoII, MnII, and NiII, was observed in PMo11Co,
PMo11Mn, and PMo11Ni respectively.
2.3. Characterization
A detailed study on the characterization of the synthesized
materials can be found in our earlier publications.47–50 The
main characterization of the catalysts, such as elemental
analysis, TG-DTA, DRS and ESR, is given for the readers' con-
venience. Elemental analysis was carried out using a JSM
5610 LV combined with INCA instrument for EDX-SEM. The
total weight loss was calculated by the TGA method on the
Mettler Toledo Star SW 7.01 up to 600 °C. FTIR spectra of the
samples were recorded as KBr pellets on a Perkin Elmer
instrument. The DR-UV-Visible spectrum was recorded at
room temperature on a Perkin Elmer LAMBDA instrument.
Table 1 DRS data of PMo11, PMo11Co, PMo11Mn, and PMo11Ni
Material
O → Mo charge transfer
d–d transition
PMo11
200, 285
235, 300
210, 260
270, 300
—
PMo11Co
PMo11Mn
PMo11Ni
560–580
390–430
550–600
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Furthermore, the presence of Co(II), and Mn(II) in the
Catalysis Science & Technology
synthesized complex was confirmed by ESR (Fig. 1). The
ESR spectra of PMo11Co show eight hyperfine signals
(Co2+; I = 7/2) with a g value of ~2.66, confirming the presence
of paramagnetic Co(II) in an octahedral or distorted octahe-
dral environment. Similarly, the ESR spectra of PMo11Mn
show six signals (Mn2+; s = 5/2), and the g value of ~2 indicates
the presence of Mn(II) with octahedral or distorted octahedral
symmetry and thus confirms the presence of paramagnetic
Mn(II).
3.2. Catalytic activity
All of the reactions were carried out without catalysts in order
to confirm that the catalytic activity is due to PMo11M. It was
found that no oxidation takes place without catalysts. A
detailed study was carried out on the oxidation of benzyl
alcohol and styrene using PMo11M (M = Co, Mn, Ni) as cata-
lysts, and a study on other alcohols and alkenes was carried
out under optimized conditions.
3.2.1. Oxidation of alcohols. A detailed study was carried
out on the oxidation of benzyl alcohol over PMo11M (M = Co,
Mn, Ni) by varying different parameters, such as the reaction
temperature, the amount of the catalyst and the reaction
time, to optimize the conditions for maximum conversion
and selectivity. Generally, oxidation of benzyl alcohol gives
benzaldehyde and benzoic acid. However, benzaldehyde
was characterized as the major oxidation product in the
present case.
3.2.1.1. Effect of temperature. In order to determine the
optimum temperature, the reaction was investigated at three
different temperatures, 70, 90 and 110 °C, keeping the other
parameters fixed (20 mg of catalyst, reaction time of 24 h).
The results are presented in Fig. 2. The results show that
conversion increased with increasing temperature from 70 to
110 °C for all the catalysts. At the same time, on increasing
the temperature from 90 to 110 °C, a drastic decrease
in the selectivity of benzaldehyde was observed. This is
due to over oxidation of benzaldehyde to benzoic acid at
elevated temperatures. So a temperature of 90 °C was used
for further studies.
Fig. 2 Effect of temperature, % conversion is based on benzyl alcohol;
benzyl alcohol = 100 mmol, TBHP = 0.2%, time = 24 h, amount of
catalyst = 20 mg.
3.2.1.2. Effect of catalyst amount. The effect of the amount
of the catalyst on the conversion was studied and the
obtained results are shown in Fig. 3. With an increase in the
amount of the catalysts i.e. concentration of metal contains,
the % conversion also increases. This suggests that the
transition metal functions as an active site for oxidation. It is
very interesting to observe the difference in the selectivity of
the products with an increase in the concentration of the
catalyst.
It is observed from Fig. 3 that with a lower amount of the
catalysts, >96% selectivity of benzaldehyde is obtained. On
increasing the amount of the catalysts (more than 15 mg),
the selectivity for benzaldehyde decreases. This may be due
to the fact that with an increase in the amount of the active
species the reaction becomes fast, which favours the conver-
sion of the formed benzaldehyde to benzoic acid. Thus, the
Fig. 3 Effect of catalyst amount, % conversion is based on benzyl
alcohol; benzyl alcohol = 100 mmol, TBHP = 0.2%, temp. = 90 °C,
time = 24 h.
Fig. 1 ESR spectra of (a) PMo11Co and (b) PMo11Mn.
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amount of the catalysts was optimized to 20 mg for optimum
conversion and selectivity.
Effect of reaction time. The percentage of conversion was
monitored at different reaction times and the result is
presented in Fig. 4. It can be seen from Fig. 4, that with an
increase in the reaction time, the
% conversion also
increases. The initial conversion of benzyl alcohol increased
with the reaction time. This is due to the fact that more time
is required for the formation of the reactive intermediate
(substrate + catalyst) which is finally converted into the
products. For, PMo11Co,
a 32.6% conversion of benzyl
alcohol with 92.3% selectivity for benzaldehyde was observed
at 24 h. When the reaction was allowed to continue after 24
h, no significant change in the conversion was observed, but
the selectivity for benzaldehyde was decreased. This is
because of over oxidation of benzaldehyde to benzoic acid. A
similar trend in activity was observed for PMo11M (M = Mn,
Ni). So, the reaction time was optimized as 24 h.
Fig. 5 % Conversion is based on benzyl alcohol; benzyl alcohol = 100
mmol, oxidant = O2, TBHP = 0.2%, temp. = 90 °C, catalyst = PMo11Co.
It is known that in a kinetically consecutive reaction, con-
version is a function of both time and the catalyst amount,
and the selectivity is itself a function of conversion. There-
fore, the optimal catalyst amount depends on the reaction
time chosen, and vice versa. Hence, to study the above, the
oxidation of benzyl alcohol was carried out by varying the
time with two different amounts (15 mg and 20 mg) of
PMo11Co as the catalyst and the results are presented in
Fig. 5. From Fig. 5 it is clear that for each time interval, the
observed % conversion is less with 15 mg compared to that
of 20 mg. However, at the same time, no significant differ-
ence (<1.5%) in the selectivity for benzaldehyde was
observed. Thus, 20 mg of the catalyst and a 24 h reaction
time was selected for the optimum conversion and selectivity.
The optimum conditions for the optimum conversion of
alcohol and selectivity for benzaldehyde over PMo11M (M =
Co, Mn, Ni) are: 20 mg of the catalyst, a 24 h reaction time
and a reaction temperature of 90 °C.
So, in order to explore the scope and limitations of the
present catalytic system, oxidation of different alcohols was
also carried out using PMo11M (M = Co, Mn, Ni) under the
optimized conditions and the results are presented in
Table 2.
It can be seen from Table 2 that PMo11Co and PMo11Ni
show almost the same conversion of benzyl alcohol whereas a
lower conversion of benzyl alcohol was observed for PMo11Mn.
From the results, it is clear that the activity of the catalyst is
governed by the reduction potential of the metals used for the
substitution, which is in good agreement with reported results
(Fig. S4, ESI†).48 It can be seen from Table 2 that, as with
benzyl alcohol oxidation, a similar trend in the activity of
PMo11M (M = Co, Ni, Mn) was observed for the oxidation
of other alcohols, such as cyclopentanol, 1-hexanol and
1-octanol, i.e. PMo11Co shows better activity compared to
PMo11Mn and PMo11Ni. The order of the activity of the
Table 2 Oxidation of alcohols with O2 using PMo11M (M = Co, Mn,
Ni)a
Catalyst
Alcohols
% Conv. Products (% selectivity) TON
PMo11Co Benzyl alcohol 32.6
Cyclopentanol 29.8
Benzaldehyde (92.3)
Cyclopentanone (100)
Cyclohexanone (100)
1-Hexanal (100)
—
4281
3913
3834
2705
—
Cyclohexanol
1-Hexanol
1-Octanol
29.2
20.6
NC
PMo11Mn Benzyl alcohol 28.3
Cyclopentanol 24.3
Benzaldehyde (93.6)
Cyclopentanone (100)
Cyclohexanone (100)
1-Hexanal (100)
—
3628
3115
3038
2038
—
Cyclohexanol
1-Hexanol
1-Octanol
23.7
15.9
NC
PMo11Ni
Benzyl alcohol 31.9
Cyclopentanol 27.6
Benzaldehyde (92.3)
Cyclopentanone (100)
Cyclohexanone (100)
1-Hexanal (100)
—
4075
3526
3078
2261
—
Cyclohexanol
1-Hexanol
1-Octanol
24.1
17.7
NC
a
Fig.
4 Effect of reaction time, % conversion is based on benzyl
Conversion is based on the alcohols; alcohol, 100 mmol; oxidant,
O2: 0.2% TBHP; amount of catalyst = 20 mg; temp. = 90 °C, time = 24 h,
NC = no conversion.
alcohol; benzyl alcohol = 100 mmol, oxidant = O2, TBHP = 0.2%,
temp. = 90 °C, amount of catalyst = 20 mg.
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Catalysis Science & Technology
catalysts for oxidation is PMo11Co > PMo11Ni > PMo11Mn. It
can be observed from Table 2 that oxidation of a secondary
alcohol is easier compared to primary aliphatic alcohols. This
observed trend is in good agreement with other reports.51 It is
also observed that, in all cases, very good selectivity for the
desired product with a high TON is obtained. It is known that
oxidation of long chain alcohols (C8 and onwards) is still a
challenging task because of lower reactivity52 and thus, the
present catalytic system is also not applicable to less reactive
long chain primary alcohols, such as 1-octanol.
3.2.2. Oxidation of alkenes. A detailed study, similar to the
benzyl alcohol oxidation, was carried out on the oxidation of
styrene using PMo11M (M = Co, Mn, Ni) by varying different
parameters, such as the reaction temperature, catalyst amount
and reaction time, in order to optimize the conditions using
the environmentally benign oxidant O2 (Fig. S1–S3, ESI†).
Generally, oxidation of styrene gives styrene oxide, benzaldehyde,
acetophenone, diol, phenyl acetaldehyde, and benzoic acid.
However, benzaldehyde was characterized as the major (>87%)
oxidation product along with styrene oxide, and benzoic acid
as the minor product (13%) in the present case.
complexes, corresponding to an oxidative addition and reduc-
tive elimination, are responsible for effective catalysis. Hence,
the activated catalysts attack the CC site in the case of sty-
rene and preferentially follow an oxidative cleavage route rather
than epoxide formation.54 Thus, the obtained results are in
good agreement with the reported explanation. In all cases,
excellent selectivity (88–93%) for the desired product with a
very high TON is obtained. As the present catalytic system
followed oxidative cleavage rather than epoxide formation,
cyclic olefins do not undergo epoxidation. Also, CC bond
cleavage in cyclic olefins is a difficult task and therefore no
ring opening product was obtained. Thus, the present catalytic
system is not applicable for the oxidation of cyclic olefins.
3.3. Control experiment
A controlled experiment for the oxidation of benzyl alcohol
with CsPMo11, CsPMo12, and PMo11M (M = Co, Mn, Ni) was
carried out under optimized conditions and the results are
presented in Table 5. It can be seen from Table 5 that in the
CsPMo11 and CsPMo12 catalyzed oxidation reactions, very low
conversion of the substrates with 80–90% selectivity for benz-
aldehyde was obtained. However, PMo11M, showed good con-
version, i.e. >18 % more conversion, and excellent selectivity
for benzaldehyde was observed. The controlled experiment
shows that PMo11M (M = Co, Mn, Ni) are better catalysts
The optimum conditions for the maximum % conversion
of styrene (>36%) with >87% selectivity for benzaldehyde
with PMo11M are: 20 mg of the catalyst, an 8 h reaction time
and a reaction temperature of 80 °C.
In order to see the scope of the present catalytic system,
oxidation of alkenes was also carried out over PMo11M under
optimized conditions and the results are presented in Table 3.
From the results it is clear that, similar to alcohol oxida-
tion, styrene and substituted styrene was efficiently oxidised to
the desired product by PMo11M (M = Co, Mn, Ni) and the order
of activity for oxidation is PMo11Co > PMo11Ni > PMo11Mn.
Generally, the oxidation of styrene gives styrene oxide, benzal-
dehyde, diol, benzoic acid and acetophenone. However, in the
present case, a CC cleaved carbonyl compound was charac-
terized as a major product. It is well known that the stable
product benzaldehyde was observed as the major product via
i) direct oxidative cleavage of CC of styrene and ii) fast con-
version of styrene oxide to benzaldehyde. It has been reported
by Vancheesan et al.53 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
Table 4 Controlled experiment for the oxidation of benzyl alcohol
and styrene with CsPMo11, CsPMo12, PMo11M (M = Co, Mn, Ni) under
optimized conditionsa
% Selectivity
Entry
Catalyst
% Conversion
Benzaldehyde
Other
1
2
3
CsPMo11
CsPMo12
13.4a/16.5b
6.2a/11.3b
91.2a/90.1b
8.8a/9.9b
87.2a/82.2b
12.8a/17.8b
7–8a/8–13b
PMo11M
28–33a/39–41b
92–93a/87–92b
% Conversion is based on the substrate (abenzyl alcohol, bstyrene);
substrate = 100 mmol, O2, TBHP = 0.2%, temp. = a90, b80 °C, time =
124 h, 28 h, amount of catalyst = 25 mg (PMo11M), 18 mg (CsPMo12),
a
27 mg PMo11
.
Table
5 Oxidation of benzyl alcohol and styrene with fresh and
regenerated catalystsa
Selectivity (%)
Benzaldehyde
Table 3 Oxidation of alkenes with O2 using PMo11M (M = Co, Mn, Ni)a
Substrate
Cycle
Conversion (%)
1Benzyl alcohol
Fresh
1
2
3
32.6a/28.3b/31.9c
32.6a/28.3b/31.9c
32.6a/28.1b/31.8c
32.4a/28.1b/31.8c
32.3a/28.0b/31.7c
41.1a/35.9b/39.9c
41.1a/35.7b/39.9c
41.0a/35.6b/39.7c
41.0a/35.6b/39.7c
41.0a/35.6b/39.5c
92.3a/93.6b/92.3c
92.0a/93.4b/92.2c
92.2a/93.4b/92.0c
92.2a/93.4b/92.1c
92.1a/93.3b/92.1c
92.2a/90.7b/87.2c
92.2a/90.5b/87.1c
92.1a/90.4b/87.4c
92.0a/90.4b/87.0c
92.0a/90.5b/86.9c
Catalyst
Alkenes
% Conv. Products (% selectivity) TON
PMo11Co Styrene
41.1
α-Methyl styrene 36.2
Benzaldehyde (92.2)
Acetophenone (93.1)
—
5253
4627
—
aCyclohexene
PMo11Mn Styrene
NC
4
35.9
Benzaldehyde (90.7)
Acetophenone (91.1)
—
4602
3885
—
2Styrene
Fresh
α-Methyl styrene 30.3
1
2
3
4
aCyclohexene
PMo11Ni Styrene
NC
39.9
Benzaldehyde (87.2)
Acetophenone (87.1)
—
5074
4362
—
α-Methyl styrene 34.3
aCyclohexene
NC
a
% Conversion is based on the substrate; substrate = 100 mmol,
TBHP = 0.2%, temp. = 190, 280 °C, time = 124 h, 28 h amount of
catalyst = 20 mg, catalysts = PMo11M (M = aCo, bMn, cNi).
a
Conversion is based on the alkenes; alkene, 100 mmol; oxidant,
O2: 0.2% TBHP; amount of catalyst, 20 mg; 80 °C, a60 °C time, 8 h.
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compared to their parent (CsPMo12) and lacunary counter-
part (CsPMo11). From this experiment it can also be con-
cluded that although the transition metal can act as an active
centre for oxidation, the possibility of the involvement of the
Mo species cannot be ruled out.
The catalyst was washed with dichloromethane and dried at
100 °C. Oxidation of benzyl alcohol and styrene was carried
out with the recycled catalysts under the optimized condi-
tions. The obtained results are presented in Table 4. As can
be seen from the table, there was no appreciable change
observed in the conversion as well as the selectivity, which
shows that the catalysts are stable and can be regenerated for
repeated use (up to four cycles).
3.4. Heterogeneity test and recyclability
The decomposition or leaching of the metal content from
PMo11M was confirmed by carrying out an analysis of the
used catalyst (EDX) as well as the product mixtures (AAS). For
all the catalysts, the analysis of the used catalyst did not
show an appreciable loss in the metal content compared to
the fresh catalyst. Analysis of the product mixtures shows
that if any metal was present, it was below the detection
limit, which corresponded to less than 1 ppm.
3.5. Comparison with reported catalysts
Comparative data for the aerobic oxidation of benzyl alcohol
and styrene over the present catalysts and reported catalysts
is given in Tables 5 and 6 respectively.
Mizuno et al. reported the aerobic oxidation of alcohol
catalyzed by [(n-C4H9)4N]4H[SiW11RuIII(H2O)O39]·2H2O.55 36%
conversion of benzyl alcohol was obtained with 65% selectivity
for benzaldehyde using O2 as the sole oxidant. However, this
reaction required a very long time (120 h), relatively high
temperature (110 °C) and isobutyl acetate solvent. Similarly,
Ueda et al. carried out the aerobic oxidation of benzyl alcohol
catalyzed by Mo–V–O oxide as a heterogeneous catalyst using
toluene as a solvent and 22% conversion was observed with
excellent selectivity (>99) for benzaldehyde.56 However, the
reaction was carried out in toluene as a solvent. On other
hand, PMo11M (M = Co, Mn, Ni) gives 28–33% conversion
and excellent selectivity (92–93%) towards benzaldehyde
(TON 3628–4281) in 24 h under solvent free mild conditions.
Xu et al. reported the use of Mo-exchanged zeolite (Mo–Y)
as a catalyst for the oxidation of styrene with air + TBHP
using DMF as a solvent.57 15.7% conversion of styrene with
34.8% and 65.2% selectivity for benzaldehyde and the epoxide
was found in 5 h for Mo–Y. In comparison with the present
catalysts PMo11M (M = Co, Mn, Ni), it was found that Mo–Y is
better in terms of selectivity for the epoxide (Table 7).
Furthermore, a heterogeneity test was carried out for the
oxidation of benzyl alcohol (Fig. 6a) and styrene (Fig. 6b) over
PMo11Co as examples. For rigorous proof of heterogeneity,
the test54 was carried out by filtering the catalyst from the
reaction mixture at 90 °C after 12 h (for styrene; 80 °C after 4 h)
and the filtrate was allowed to react for up to 28 h (for styrene; 8 h).
The reaction mixture after 12 h and the filtrate were analyzed
by gas chromatography. A similar test was carried for PMo11M
(M = Mn, Ni). No change in the % conversion as well as
% selectivity was found, indicating the present catalysts fall
into category C,54 i.e., the active species does not leach and
the observed catalysis is truly heterogeneous in nature.
The catalysts are heterogeneous in nature and hence, can
be easily separated by simple filtration followed by washing.
However, at the same time, an excellent selectivity for
benzaldehyde was observed for PMo11M (M = Co, Mn, Ni).
Moreover, the reported reactions were carried out with a lower
concentration of styrene (3 mmol) and a higher amount of the
catalyst (300 mg) at a higher temperature (90 °C) in DMF as a
solvent, while the present catalytic system offered solvent free
oxidation reactions with a relatively higher concentration of
styrene (100 mmol) and a lower amount of the catalyst (20 mg)
at a lower temperature (80 °C).
3.6. Probable reaction mechanism
In order to study the reaction mechanism, the same set of
reactions were carried under two different conditions: (i) sty-
rene + oxidant + TBHP and (ii) styrene + oxidant + PMo11M.
In both cases, the reaction did not progress significantly.
These observations indicate that the liberation of O2 from
TBHP was not sufficient for the reaction to proceed nor acti-
vate M2+ to M3+, necessary for provoking the reaction under
the optimized conditions. Hence, it may be concluded that in
the present study, TBHP acts as a radical initiator only.
Fig.
6 %
Conversion is based on the substrate (abenzyl alcohol/
bstyrene); amount of PMo11Co = 20 mg; molar ratio of the substrate
(100 mmol), O2, TBHP = 0.2%, temperature a90 °C, b80 °C.
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Table 6 Comparison of the catalytic activity for the aerobic oxidation of benzyl alcohol with reported catalysts
% Selectivity
Reaction
%
Conv.
Catalyst
conditionsa
Benzaldehyde
TON
Ref.
55
56
aTBA–SiW11Ru
1 : 2 : 120 : 110 : 3
0.7 : 30 : 24 : 80 : 1.6
100 : 25 : 24 : 90 : 0
36
22
28–33
65
>99
92–93
540
—
3628–4281
bMo–V–O oxide
cPMo11M (M = Co, Mn, Ni)
Present work
% Conversion is based on benzyl alcohol; solvent = aisobutyl acetate/btoluene; cTBHP was used as an initiator.a Benzyl alcohol: amount of
catalysts (mg): time (h): temperature (°C): solvent (g).
Table 7 Comparison of the catalytic activity for the aerobic oxidation of styrene with reported catalysts
% Selectivity
Reaction
%
Conv
Catalyst
conditionsa
Benzaldehyde
Epoxide other
Ref.
57
aMo-exchanged zeolite
3 : 300 : 5 : 90 : 10
100 : 25 : 8 : 80 : 0
15.7
39–41
34.8
87–92
65.2
3–6
bPMo11M (M = Co, Mn, Ni)
Present work
a
% Conversion is based on styrene; asolvent = DMF; air as the oxidant, bO2 as the oxidant and TBHP was used as an initiator.a Styrene: amount
of catalysts (mg): time (h): temperature (°C): solvent (g).
In order to confirm the radical mechanism, an experiment
was carried out using an excess of i-PrOH as a radical termina-
tor and the result is presented in Fig. 7. When the free-radical
terminator was added to the oxidation of styrene (at 4 h), the
reaction stopped and no significant increase in the conversion
of styrene was observed.
Based on the above observations, a tentative reaction
mechanism for the aerobic oxidation of styrene with TBHP as
an initiator is proposed in Scheme 1. It has been reported that,
in the case of transition metal substituted polyoxotungstates,
transition metals behave as active centres for catalysis.58,59
It has also been reported that for TMSPOMs catalysts
containing metal cations in low valency states and involving O2
as an oxidant always follow the radical chain mechanism
induced by an M–O2 intermediate.60,61 In the present catalytic
system, the mechanism is expected to follow the same path. It
is expected that the reaction of M2+ with TBHP causes the
Scheme 1 Proposed reaction mechanism for the oxidation of styrene
using O2.
oxidization of M2+ to M3+ in situ. The activation of this spe-
cies takes place with a radical (tert-BuO˙) generated during
the decomposition of TBHP and the attack of O2 simulta-
neously, which results in the formation of the activated
species OOM3+PMo11.
PMo11Mn + tert-BuOOH → PMo11Mn+1 + tert-BuO˙ + OH−
Fig. 7 Inhibition experiment with i-PrOH as a radical terminator (blue),
This activated species (˙OOM3+PMo11) then attacks the
without i-PrOH under optimized conditions (black), catalyst
=
PMo11Co.
substrate. The metal-superoxo intermediate reversibly binds
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styrene, attacking the reaction site which results in the oxida-
tion of the substrate to form the products. Thus, it is believed
that the ˙OOM3+PMo11 intermediate may be responsible for
the oxidation of styrene. However, activation of the catalyst
may also be possible with different radicals (tert-BuOO˙,
tert-BuOOO˙, OH˙), generated during the decomposition of
TBHP. Although attempts to isolate the activated catalyst have
been made, it was not possible to isolate and characterize the
active catalyst as it is highly reactive in nature. Similarly, a
mechanism for alcohol oxidation was also proposed in which
the active species ˙OOM3+PMo11 is responsible for the oxida-
tion of alcohols to the corresponding carbonyl compounds
(Scheme S1, ESI†). However, at the same time, as described in
the controlled experiment, addenda atoms (Mo species) may
also be involved in the catalytic cycle.
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