Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over Anchored Phosphotungstates: Effect of Supports as Well as Oxidants

Article abstract of DOI:10.1007/s10562-016-1732-7

Series of catalysts comprising of parent phosphotungstate (PW₁₂) and mono lacunary phosphotungstate (PW₁₁) anchored to different mesoporous materials (MCM-41 and MCM-48) were prepared. Environmentally benign oxidation of alkenes and alcohols were carried out with H₂O₂ and molecular oxygen as oxidants. The influence of different parameters on the conversion as well as the selectivity was investigated. Comparative study was ascertained over anchored parent, lacunary phosphotungstates as active species and the supports. The kinetic and thermodynamic studies were correlated with the effect of support as well as active species. Moreover, the catalysts could be recovered and reused four times without significant loss in their activity and selectivity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1732-7

Catal Lett
DOI 10.1007/s10562-016-1732-7
Environmentally Benign Oxidations of Alkenes and Alcohols
to Corresponding Aldehydes over Anchored Phosphotungstates:
Effect of Supports as Well as Oxidants
Sukriti Singh1 Anjali Patel1
•
Received: 5 March 2016 / Accepted: 13 March 2016
Ó Springer Science+Business Media New York 2016
Abstract Series of catalysts comprising of parent phos-
photungstate (PW12) and mono lacunary phosphotungstate
(PW11) anchored to different mesoporous materials
(MCM-41 and MCM-48) were prepared. Environmentally
benign oxidation of alkenes and alcohols were carried out
with H2O2 and molecular oxygen as oxidants. The influ-
ence of different parameters on the conversion as well as
the selectivity was investigated. Comparative study was
ascertained over anchored parent, lacunary phospho-
tungstates as active species and the supports. The kinetic
and thermodynamic studies were correlated with the effect
of support as well as active species. Moreover, the catalysts
could be recovered and reused four times without signifi-
cant loss in their activity and selectivity.
Graphical Abstract
Keywords 12-Tungstophosphoric acid Á Mono lacunary
phosphotungstate Á Mesoporous supports Á Environmental
benign Á Oxidation
1 Introduction
The products of oxidative cleavage of C–C bond to alde-
hydes or ketones, is widely recognized as one of the most
fundamental transformations in organic chemistry. Espe-
cially, benzaldehyde is very valuable and demanding
chemical widely used in laboratory, pharmaceutical, food,
agrochemical industries, cosmetics, dyestuffs as well as
intermediates for many important organic synthesis. It is
the second most important aromatic molecule (after
vanillin) used in the cosmetics and flavour industries.
Oxidizing reagents including permanganate, dichromate,
chromium reagents or activated dimethyl sulfoxide
(DMSO) reagents have conventionally been employed for
the oxidation of alcohols and alkenes, but these
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1732-7) contains supplementary
material, which is available to authorized users.
& Anjali Patel
aupatel_chem@yahoo.com
1
Polyoxometalates and Catalysis Laboratory, Department of
Chemistry, Faculty of Science, The M. S. University of
Baroda, Vadodara, Gujarat 390002, India
123

S. Singh, A. Patel
stoichiometric oxidants are expensive and/or toxic, suffer
from lack of selectivity. More seriously, produce a large
amount of heavy-metal wastes and use of non-green
halogenated solvents [1], which are economically and
environmentally hostile. Therefore, the development of
eco-sustainable catalytic systems would contribute in
launching green and sustainable chemical processes.
In this regard, since the past few years, heterogeneous
catalysts based on polyoxometalates (POMs) have gained
much attention in the field of acid as well as redox catalysis
from the perspective of their diversity of structures and
configurations [2, 3]. There are number of reports on oxi-
dation/epoxidation of different alcohol/alkene/olefin by
heterogeneous heteropolyacids with H2O2/Molecular oxy-
gen [4–9]. Their Lacunary counterpart (LPOMs), espe-
cially mono lacunary phosphotungstate (PW11) represent
an important class of compounds due to their unique
structural as well as chemical properties [10, 11]. During
the last decades, lacunary phosphotungstate have been
studied as effective homogeneous catalysts in the oxidation
of reactions with H2O2 [12–14]. Incorporation of POMs,
specifically 12-tungstophosphoric acid (PW12) or lacunary
PW11, into amorphous or structured silicates as well as
application of supported POMs in oxidative catalysis are
well-known [15–19]. More recently much work is focussed
in the heterogenization of PW11 and finding its catalytic
activity for oxidation reactions [20–22].
styrene and benzyl alcohol with H2O2 and O2. The con-
ditions for maximum conversion and selectivity for the
desired product were optimized by varying different reac-
tion parameters. To evaluate the role of mesoporous net-
work and active species, these two effects were correlated
with catalytic activity, kinetic and thermodynamic inves-
tigations. TON were obtained for each system. Scope and
limitation of synthesized materials as heterogeneous cata-
lysts was also investigated for oxidation of different alco-
hols and alkenes. Probable mechanism was proposed for
both the systems.
2 Experimental
2.1 Materials
All chemicals were received from Merck (A. R. grade).
12-Tungstophosphoric acid, CTAB (Cetyltrimethyl
ammonium bromide), TEOS (tetraethyl orthosilicate),
liquor ammonia, ethanol, sodium tungstate dihydrate,
anhydrous disodium hydrogen phosphate, nitric acid, ace-
tone, styrene, benzyl alcohol, and 30 % H2O2, were used as
received.
2.2 Synthesis of Supports and Catalysts
In a sequence of studies carried out by our group con-
cerning heterogeneous catalysts based on lacunary phos-
photungstate, promising result were obtained for oxidation
of alkenes and alcohols in aerobic as well as liquid phase
conditions [23–26]. In addition to these results from these
study we understood that it is important to investigate the
role of supports, as it is known that supports not only
provide mechanical stability to the active species but also
modify the catalytic properties of the catalyst. Incorpora-
tion of metal atom/active species onto the readily acces-
sible framework of ordered mesoporous molecular sieves
such as MCM-41 (hexagonal) and MCM-48 (cubic) is an
interesting topic of research.
The catalysts comprising 12-tungstophoshoric acid (PW12)
and MCM-41/MCM-48 has been synthesized and proce-
dure can be found in literature [9, 27]. The sodium salt of
mono lacunary phosphotungstate was effectively synthe-
sized by the same method reported by us [26]. The
extracted sodium salt was dried in air and designated as
PW11. The supports MCM-41 [9] and MCM-48 [27] were
synthesized by the earlier reported method. The catalyst
PW11/MCM-41 [26, 28] and PW11/MCM-48 [29] were
synthesized by impregnating 1 g of support MCM-41/
MCM-48 with an aqueous solution of PW11 (0.1–0.4 g in
10–40 mL of distilled water) and dried at 100 °C for 10 h.
The obtained materials were treated with 0.1 N HCl, fil-
tered, washed with distilled water and dried at 100 °C. The
obtained materials containing 10–40 % loading were
labelled as (PW11)1/MCM-41 [i.e. 10 % loading corre-
sponding to (PW11)1/MCM-41], (PW11)2/MCM-41,
(PW11)3/MCM-41, (PW11)4/MCM-41, respectively. Simi-
lar loading procedure was carried out for catalyst com-
prising of PW11 and MCM-48.
Also, till date no comparative study has been carried out
between: (1) Parent 12-tungstophosphoric acid with lacu-
nary phosphotungstate counterpart in heterogeneous con-
ditions, (2) Different mesoporous supports, (3) Effect of
different oxidants and (4) Kinetic and thermodynamic
studies, on the catalytic liquid phase oxidation of alkenes
and alcohols. The present article aims to develop a catalytic
system utilizing phosphotungstate PW12 and PW11 incor-
porated on pure mesoporous silica supports (MCM-41,
MCM-48). Much emphasis has been provided for the cat-
alytic activity. The 30 % loaded catalysts were character-
ized by various thermal, spectral, and surface techniques.
The catalytic activity was evaluated for oxidation of
2.3 Characterization
Adsorption-desorption isotherms of samples were recorded
on a Micromeritics ASAP 2010 surface area analyzer at
-196 °C. From adsorption-desorption isotherms surface
123

Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over…
area was calculated (BET method). 31P MAS NMR spectra
moles of product formed
moles of substrate consumed
Selectivity ð% Þ ¼
 100
were recorded at 121.49 MHz using a 7 mm rotor probe with
85 % phosphoric acid as an external standard. The spinning
rate was 5–7 kHz. Catalyst samplesafter treatment were kept
in a desiccator over P2O5 until the NMR measurement. The
29Si MAS NMR spectra were recorded at 121.49 MHz using
a 7 mm rotor probe with TMS as an external standard. The
spinning rate was 5–7 kHz. FT-IR spectra were obtained by
using the KBr wafer on the Perkin Elmer instrument. Ele-
mental analysis was carried out using JSM 5910 LV com-
bined with INCA instrument for EDS-SEM.
2.5 Leaching Test
Any leaching of the active species from the support makes
the catalyst unattractive and hence it is necessary to study
the stability as well as leaching of PW12/PW11 from the
supports. Heteropoly acids can be quantitatively charac-
terized by the heteropoly blue color, which is observed
when it reacted with a mild reducing agent such as ascorbic
acid. In the present study, reported method was used for
determining the leaching of PW12/PW11 from support [26,
27].
2.4 Oxidation of Alkene and Alcohol
2.4.1 Reaction with H2O2
Oxidation of styrene and benzyl alcohol was carried out in
a 50 mL batch reactor provided with a double-walled
condenser, styrene/benzyl alcohol (10 mmol), and 30 %
aq. H2O2 (30 mmol) with constant stirring at 80 °C (styr-
ene) and 90 °C (benzyl alcohol). After completion of
reaction, the products were analysed on a gas chro-
matograph (GC) using Rtx-5 capillary column after
extraction with dichloromethane and drying with magne-
sium sulphate.
3 Result and Discussion
3.1 Material Characterization
A detailed study on the characterizations of (PW12)3/
MCM-41 [9], (PW12)3/MCM-48 [27], (PW11)3/MCM-41
[26, 28] and (PW11)3/MCM-48 [29] can be found in our
earlier publications. However in the present article BET,
31P MAS NMR, 29Si MAS NMR and FT-IR are given for
reader’s convenience.
The BET surface area of the supports (MCM-41, MCM-
48) and catalysts (with 10–40 % loadings) are presented in
Table 1. The incorporation of PW12 or PW11 inside the
channels of support leads to decrease in the total surface
area of the catalysts. The decrease in surface area as well as
pore volume may be due to the fact that as the active
species enters the mesopore channels of support, there is
filling of the pores and surface coverage. The overall
decrease in the surface area gives the first indication of
chemical interaction between PW12 or PW11 with the
supports. On increasing the % loading for 10–30 %, con-
stant decrease in the surface area was observed which may
be due to the monolayer formation of the active species.
Further increase in loading up to 40 % leads to the drastic
decrease in the surface area as well as pore diameter due to
multilayer formation of PW12/PW11 on the surface of
MCM-41/MCM-48 and blocking of the channel frame-
work, among all the catalysts. As a result 30 % loaded
catalysts were selected for the further characterizations.
31P MAS NMR study was carried out for all the catalysts
(Table 2). MAS NMR of PW12 shows a single peak at
-15.626 ppm and that of (PW12)3/MCM-41 shows undis-
turbed peak at -15.5 ppm. (PW12)3/MCM-48 shows peaks
at -12.03 ppm can be assigned to partially dehydrated
intact Keggin structure of PW12 which is highly dispersed
inside the three dimensional structure of MCM-48 by
2.4.2 Reaction with Molecular Oxygen
The oxidation reactions were performed in a batch-type
reactor under atmospheric pressure. In a typical reaction
appropriate amount of catalyst was added in a 50 mL two-
necked flask containing substrates (100 mmol) and to that
0.2 mmol tertbutyl hydrogen peroxide (TBHP) was added
in order to initiate the reaction. The reaction mixture was
maintained at 80 °C (styrene) and 90 °C (benzyl alcohol)
on REMI magnetic stirrer equipped with preheated oil bath
at 600 rpm. The reaction was started by bubbling O2 (an-
alytical grade) into the liquid. After completion of reaction,
the catalyst was removed by simple centrifugation. The
products were analysed on a GC using Rtx-5 capillary
column by diluting with dichloromethane and drying with
magnesium sulphate. Identification of the product was
carried out by comparing it with authentic samples and
then by gas chromatography-mass spectroscopy (GC-MS).
The conversion and selectivity of both the reactions
were determined by following equation, where substrate-
styrene or benzyl alcohol.
Conversionð% Þ
ðInitial moles of substrate À Final moles of substrateÞ
¼
ðInitial moles of substrateÞ
 100
123

S. Singh, A. Patel
Table 1 Textural properties of
support and catalysts
Surface area (m2/g)
Pore volume (cm3/g)
˚
Pore diameter (A)
Materials
MCM-41
659
400
372
360
139
352
305
252
119
755
381
339
286
262
400
356
319
272
47.9
30.69
30.53
30.13
11.1
35.2
31.5
30.1
15.5
30.0
29.1
28.5
28.0
19.5
28.4
28.0
27.5
20.2
0.79
0.55
0.50
0.50
0.33
0.54
0.45
0.42
0.10
0.56
0.43
0.43
0.38
0.35
0.43
0.43
0.40
0.35
(PW12)1/MCM-41
(PW12)2/MCM-41
(PW12)3/MCM-41
(PW12)4/MCM-41
(PW11)1/MCM-41
(PW11)2/MCM-41
(PW11)3/MCM-41
(PW11)4/MCM-41
MCM-48
(PW12)1/MCM-48
(PW12)2/MCM-48
(PW12)3/MCM-48
(PW12)4/MCM-48
(PW11)1/MCM-48
(PW11)2/MCM-48
(PW11)3/MCM-48
(PW11)4/MCM-48
Table 2 Textural, chemical and compositional properties of supports and catalysts
Catalyst
Elemental
analysis
(wt%)
31P MAS NMR (ppm)
29Si MAS NMR ppm (Q2/Q3/Q4) FT-IR wavenumber (cm-1
)
W
P
P–O
W=O W–O–W
PW12
–
–
–
–
–
–
-15.6
-11.3
–
–
1080
987
893, 800
863, 808
–
PW11
–
1080, 1043 952
MCM-41
(PW12)3/MCM-41
-93/-103/-110
-96/-101/–109
–
–
–
–
–
–
–
–
–
–
–
–
18.0 0.30
-15.2
–
987
990
962
970
–
897
R-(PW12)3/MCM-41 17.8 0.30
(PW11)3/MCM-41 16.2 0.18
R-(PW11)3/MCM-41 16.0 0.17
898
-2.9 (major), -8.4 (major), -12.7 -99/-100/-109.6
896
–
–
890
MCM-48
–
–
–
-96/-102.3/-110.5
–
(PW12)3/MCM-48
17.2 0.3
-4.6 (minor), -12.0 (major)
-96.5/-99.7/-110.2
952
956
967
965
896
R-(PW12)3/MCM-48 17.0 0.29
(PW11)3/MCM-48 15.9 0.18
R-(PW11)3/MCM-48 15.0 0.164
–
–
895
-10.5 (minor), -11.8 (major)
-99/-101.3/-110.3
875
–
–
872
strong interaction with the support [27]. However, shift
from -15.6 to -12.03 ppm was due to the strong inter-
action with the support as explained above. An additional,
very low intensity peak at -4.6 ppm was due to adsorbed
phosphorus species derived from the highly fragmented
Keggin species.
31P MAS NMR spectrum for PW11 shows an intense
peak at -11.30 ppm (Table 2), indicating the formation of
the mono-lacunary species [26]. The 31P MAS NMR for
the (PW11)3/MCM-41 reveals peak at -2.98 ppm due to
the presence of strongly adsorbed highly fragmented
Keggin unit. The peak at -8 ppm indicates presence of
phosphate character due to the partial fragmentation of the
Keggin unit to produce, ‘‘11-defect’’ Keggin species and
the peak at -12.7 ppm was due to intact Keggin PW11 unit
interacting with surface hydroxyl groups of the support
[26]. The slight broadening and shifting in the peak may be
due to interaction of PW11 and MCM-41. Similarly,
(PW11)3/MCM-48 reveals two resonances: at -10.5 due to
presence of strongly adsorbed highly fragmented Keggin
unit and intense peak at -11.8 ppm due to intact PW11
anion interacting with surface hydroxyl groups. The slight
123

Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over…
3.2.1 Oxidation of Styrene and Benzyl Alcohol with H2O2
broadening and shifting in the peak may be due to inter-
action of PW11 with the silanol (-OH) groups of MCM-48.
29Si MAS NMR is the most important method to study
chemical environment around the silicon nuclei in meso-
porous silica materials. The peak positions of all the cat-
alysts are shown in Table 2. Broad peaks between -90
and-125 ppm was observed in MCM-41 and MCM-48
which can be attributed to three main components with
chemical shifts. These signals resulted from Q2, Q3 and Q4
silicon nuclei, where Qx corresponds to a silicon nuclei
with x siloxane linkages, i.e., Q2 to disilanol Si–(O–Si)2
(–O–X)2, where X is H or PW12/PW11, Q3 to silanol (X–
O)–Si–(O–Si)3, and Q4 to Si–(O–Si)4 in the framework.
The presence of resonance originating from Q3 and Q4 in
the catalyst indicates that MCM-41/MCM-48 structures are
retained. A small chemical shift (Table 2) in (PW11)3/
MCM-41 [28] arises from strong hydrogen bonding
between PW11 and Q2 (surface silanol groups) (d
-99 ppm) of MCM-41. In addition to this the intensity of
Q2, Q3 peaks declines in catalyst and Q4 peak (d
-109.6 ppm) remains intact which suggests that there is
interaction of PW11 with MCM-41 and the mesoporous
structure of MCM-41 was intact even after anchoring. The
spectrum of (PW11)3/MCM-48 shows three main compo-
nents with slight change in chemical shifts values at -99
(Q2), -101.3 (Q3), and -110.3 ppm (Q4) (Table 2). The
observed peaks for catalyst were relatively broad as com-
pared to MCM-48, which may be due to the presence of
PW11. The Q4 peak is not disturbed which indicates that the
mesoporous structure of the support is retained even after
incorporation of PW11. Also the observed decrease in the
intensity of Q2 and Q3 peaks in catalyst is an indication of
bonding/chemical interaction between the silanol groups of
MCM-48 and PW11 species.
Detail study was carried out for oxidation of styrene and
benzyl alcohol using (PW12)3/MCM-41, (PW12)3/MCM-
48, (PW11)3/MCM-41 and (PW11)3/MCM-48 as catalysts.
Generally, styrene on oxidation gives styrene oxide, ben-
zaldehyde, acetophenone, diol, phenyl acetaldehyde, and
benzoic acid. However in the present study much emphasis
is made on selectivity towards benzaldehyde by two dif-
ferent ways (1) Oxidation of styrene with H2O2 and O2, and
(2) Oxidation of benzyl alcohol with H2O2 and O2.
The effect of % loading (10–40 %) of PW12/PW11 on
MCM-41 as well as MCM-48 was carried out for oxidation
of styrene using H2O2 (1:3 molar ratio, 25 mg Catalyst,
80 °C for 18 h). The results are presented in Fig. 1a, b,
which shows an increase in the conversion with increase in
the % loading of active species from 10 to 20 %. A sig-
nificant increase in the conversion was observed on
increasing the loading from 20 to 30 %. The 30 % loaded
catalyst showed highest conversion, however further
increasing the loading to 40 % did not show significant
change. Hence 30 % loaded catalysts were used for further
studies for all the catalysts.
Change in the molar ratio (styrene to H2O2) also sig-
nificantly affects the conversion as well as selectivity
towards the products (Fig. 1c, d). The study shows linear
increase in the conversion for all the catalysts on increasing
the molar ratio from 1:1 to 1:3. Increasing the molar ratio
of styrene (ie. 2:1) also did not improve the reaction sig-
nificantly. It was observed that 1:3 molar ratio gives
maximum conversion.
The amount of catalyst has a significant effect on the
oxidation reaction hence, the catalyst amount was
increased from 15 mg (0.138 wt%) to 30 mg (0.2772 wt%)
(Fig. 1e–f). Initially a sharp increase in conversion was
observed on increasing the catalyst amount from 15 to
25 mg. On further, increasing the catalyst amount the
conversion does not increase significantly. However,
selectivity for benzaldehyde decreased on increasing the
catalyst amount from 25 to 30 mg. This may be due to
further oxidation of benzaldehyde to benzoic acid in
presence of higher loading of active species. Therefore,
25 mg amount catalyst (0.231 wt%), was found optimum
for obtaining maximum conversion in the case of anchored
PW12 based catalysts. However, a marked difference is
observed in the catalytic study with 20 mg anchored PW11,
which demonstrated best activity. This effect can be well
explained by better oxidative property of the lacunary
species due to formation of lacuna. Of all the catalysts
(PW11)3/MCM-48 showed highest conversion for styrene.
The effect of reaction time was carried out by observing
change in % conversion at different time intervals. It was
observed on increasing the reaction time, the % conversion
FT-IR band positions (Table 2) of active species (PW12/
PW11) in the catalysts clearly indicates that the structure of
PW12/PW11 is retained on incorporation inside the support.
EDS analyses of all the catalysts suggests presence of
PW12/PW11 inside the pores of supports (Table 2). 31P
MAS NMR and 29Si MAS NMR clearly indicated inter-
action of PW11 with the silanol groups of supports and
structure of the MCM-41/MCM-48 remains intact.
3.2 Oxidation Reactions
Detailed catalytic activity of all the catalysts were carried
out for oxidation of styrene and benzyl alcohol in presence
of H2O2 as well as molecular oxygen. To ensure the cat-
alytic activity, all reactions were carried out without cata-
lyst. It was found that no oxidation takes place. The
supports, MCM-41 and MCM-48, were also used as cata-
lyst for the oxidation of styrene and benzyl alcohol and no
conversion was found.
123

S. Singh, A. Patel
Fig. 1 Reaction optimization of a (PW12)3/MCM-41, b (PW12)3/MCM-48, c (PW11)3/MCM-41 and d (PW11)3/MCM-48, for effect of (A, B) %
loading; (C, D) Molar ratio of styrene to H2O2; and (E, F) Catalyst amount (mg). Selectivity towards benzaldehyde
also increases significantly (Fig. 2). It is known that more
reaction time is necessary to form a reactive intermediate
(substrate ? catalyst) which subsequently leads to the
products. Maximum 98 % conversion for (PW11)3/MCM-
48 was observed at 18 h. However, when the reaction was
allowed to continue till 18 h for (PW12)3/MCM-48 and
(PW12)3/MCM-41 no significant conversion was observed,
but selectivity of benzaldehyde decreases drastically. Also
reaction time of 18 h was observed for (PW11)3/MCM-48
and (PW11)3/MCM-41 (Fig. 2b). Further explanation can
be explained by kinetic and thermodynamic effect as well
as the effect of lacunary species on the reaction, which will
be explained in the following sections.
The effect of temperature was investigated in the range
70–100 °C (not shown). At 80 °C an optimum of 92, 98 %
conversion was achieved for (PW11)3/MCM-41 and
(PW11)3/MCM-48, respectively. On increasing the tem-
perature further to 100 °C there was no significant increase
123

Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over…
Fig. 2 Effect of reaction time of styrene conversion, a (PW12)3/MCM-41, b (PW12)3/MCM-48, c (PW11)3/MCM-41 and d (PW11)3/MCM-48.
Selectivity for benzaldehyde
in the conversion. Thus, 80 °C was considered optimised
temperature for obtaining maximum conversion as well as
selectivity for benzaldehyde.
amount, reaction time and temperature to optimize the
conditions for the maximum conversion of styrene (Sup-
plementary Fig. S2) and benzyl alcohol (Supplementary
Fig. S3). Under the present reaction conditions, benzalde-
hyde was the major oxidation product, may be due to
subsequent direct oxidative cleavage of C=C of styrene and
fast conversion of styrene oxide to Benzaldehyde.
Following are the optimised conditions for obtaining
maximum % conversion of styrene to benzaldehyde:
loading of PW11—30 %, amount of catalyst—20 mg
(0.231 wt%) for (PW11)3/MCM-41 and (PW11)3/MCM-48,
time—18 h, temperature—80 °C.
The oxidation of styrene was carried out by varying
catalysts amount (10–30 mg). Initially, on increasing cat-
alyst amount from 10 to 25/20 mg, the conversion
increases sharply. Further increase in the catalyst amount
does not increase the conversion very significantly.
Therefore, 20 mg amount of catalyst for styrene oxidation
(Supplementary Fig. S2 a, b) and 25 mg for benzyl alcohol
(Supplementary Fig. S3 a, b) was considered optimum for
obtaining maximum conversion.
Optimization of reaction conditions for oxidation of
benzyl alcohol was also carried out with H2O2 for all the
catalysts. Special attention was given to (PW11)3/MCM-41
and (PW11)3/MCM-48 as these two catalysts provided
promising results for styrene oxidation with H2O2. It is
important to mention that oxidation reaction of benzyl
alcohol over (PW11)3/MCM-41 has been carried out by our
group [26], but for comparison purpose it has been inclu-
ded here. In addition, effect of catalyst amount, reaction
time and temperature were studied in similar manner for
oxidation of benzyl alcohol with H2O2 and details have
been provided in Supplementary Fig. S1 [for (PW11)3/
MCM-48 as catalyst]. It was observed that 25 mg catalyst
at 90 °C and 20 h reaction time gives 30, and 40 % con-
version respectively, for (PW11)3/MCM-41, and (PW11)3/
MCM-48 (Table 3).
The effect of reaction time on the selective oxidation of
styrene was carried out by monitoring % conversion at
different time intervals (Supplementary Fig. S2 c, d). Ini-
tially with increase in reaction time up to 6 h, the % con-
version increases slowly. The catalyst initially takes an
induction period to accelerate the reaction. The maximum
conversion was achieved at 8 h of reaction time. On further
increase in the reaction time, polymer formation is initi-
ated. 20 h reaction time was optimised for benzyl alcohol
reaction after thorough investigation (Supplementary
Fig. S3 c, d).
3.2.2 Oxidation of Styrene and Benzyl Alcohol
with Molecular Oxygen (O2)
To determine the optimum aerobic oxidation tempera-
ture, the reaction was investigated at temperatures
60–90 °C for styrene (Supplementary Fig. S2 e, f) and
60–100 °C for benzyl alcohol oxidation (Supplementary
Fig. S3 e, f), keeping other parameters fixed (25 mg cata-
lyst amount, reaction time 24 h). The results show that
conversion increases with increase in temperature which is
as expected. Similarly, on increasing temperature from 90
For carrying out oxidation reaction with O2 it is necessary
to check whether the reaction is just auto-oxidation or not,
we have carried out control experiment without catalysts
and no conversion of substrates indicates that the reaction
does not proceed through auto-oxidation. A detailed cat-
alytic study for (PW11)3/MCM-41, and (PW11)3/MCM-48
was carried out by varying different parameters: catalyst
123

S. Singh, A. Patel
Table 3 Comparison of
Catalyst
Substrates
Styrene
Oxidant
Conv. (%)
Sele. (%)
TON
4250
catalytic activity of all the
catalysts for oxidation reactions
of styrene and benzyl alcohol
(PW12)3/MCM-41
H2Oa2
O2b
H2Oc2
O2d
H2Oa2
O2b
H2Oc2
O2d
H2Oa2
O2b
H2Oc2
O2d
H2Oa2
O2b
H2Oc2
O2d
85
48
24
20
90
49
25
20
95
55
30
30
98
59
40
32
86
73
82
85
85
71
85
90
89
74
90
90
88
70
60
89
24,000
1200
Benzyl alcohol
Styrene
10000
4500
(PW12)3/MCM-48
(PW11)3/MCM-41
(PW11)3/MCM-48
24500
1250
Benzyl alcohol
Styrene
10,000
4522
30914
1673
Benzyl alcohol
Styrene
16,862
4384
31,476
2000
Benzyl alcohol
a
% Conversion based on substrate: Reaction with H2O2, substrate: H2O2 1:3, catalyst amount—20 mg,
time—18 h, temp.—80 °C; b Reaction with O2, substrate 100 mmol, catalyst amount—20 mg, time—8 h,
temp.—80 °C; time—20 h; time—24 h. Selectivity for Benzaldehyde. Turnover number (TON) =
moles of product obtained/moles of catalyst (active species)
17,986
c
d
to 100 °C, conversion increases but at the same time
selectivity of benzaldehyde decreases. This result is
attributed to further oxidation of benzaldehyde to benzoic
acid at higher temperature. So temperature of 80 °C was
found optimal for the maximum conversion of benzyl
alcohol and 80 °C for styrene.
The optimum conditions for styrene oxidation are: cat-
alyst amount = 20 mg; time = 8 h; temperature = 80 °C.
For
benzyl
alcohol:
catalyst
amount = 25 mg;
time = 24 h; temperature = 80 °C. Overall summary of
all the optimised conditions with different substrate as well
as oxidants has been provided in Supplementary Table 1.
3.3 Kinetics and Thermodynamic Investigations
A detailed kinetic study was carried out for oxidation
reaction of styrene with H2O2 over all the catalysts. In all
the experiments, reaction mixtures were analysed at fixed
interval of time using GC.
3.3.1 Determination of Order as Well as Rate of Reaction
The oxidation of styrene with H2O2 was carried in 1:3
molar ratio. Since one of the reactant was taken in large
excess, the rate law is expected to follow first order
dependence over all the catalysts. The plot of log Co/C
versus time (Fig. 3, Top) shows a linear relationship of
Fig. 3 (Top) Plot of log (Co/C) versus time, (Bottom) plot of rate of
reaction versus catalyst quantity for oxidation of styrene with H2O2:
a (PW12)3/MCM-41, b (PW12)3/MCM-48, c (PW11)3/MCM-41 and
d (PW11)3/MCM-48
123

Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over…
styrene consumption with respect to time. With increase in
reaction time there is a gradual and linear decrease in the
styrene concentration.
it could be determined that PW11 based catalysts showed
lower Ea, especially (PW11)3/MCM-48. Hence it indicates
that the rate is truly governed by chemical step and there
is no diffusion limitation due to three dimensional pore
network of MCM-48.
This was further supported by the study of the effect of
catalyst concentration (mmol) on the rate of oxidation
reaction. The catalyst concentration for oxidation was
varied from 1 9 10-3 to 2.4 9 10-3 mmol at 80 °C tem-
perature. It can be observed from Fig. 3 (Bottom) that the
rate of the reaction increases linearly with an increase in
the catalyst concentration.
3.3.3 Thermodynamic Investigation
The influence of various reaction conditions plays signifi-
cant role for obtaining a better catalytic activity, but it is
also inevitable to determine the thermodynamics effect.
Thermodynamics parameters were evaluated for all the
reactions (Table 4). From the equilibrium constant Eq. 2, it
was possible to build the linear plots of -RlnKeq vs.1/T
(Fig. 5). The slope of the curve provides the value of DH,
and the Y axis intercept identifies DS [31]. In the present
case, Gibbs free energy of activation DG (Table 4) was
calculated using Eq. 3.
3.3.2 Estimation of Activation Energy (Ea)
The graph of ln k versus 1/T was plotted (Fig. 4) and the
value of activation energy (Ea) was determined from the
plot using Arrhenius equation Eq. (1).
k ¼ AeÀEa=RT
ð1Þ
It is important to recognize whether the reaction rate is
diffusion/mass transfer limited or it is truly governed by
the chemical step, where the catalyst is being used to its
maximum capacity. It is reported that the activation
energy for diffusion limited reactions is as low as
10–15 kJ mol-1, and reactions whose rate is governed by
a truly chemical step usually show activation energy
excess of 25 kJ mol-1 [30]. From Kinetic study (Table 4)
lnKeq ¼ DS=R À DH=RT
DG ¼ DH À TDS
Under the optimized conditions, (PW11)3/MCM-48
presented more negative DG value as compared to all other
catalysts indicating a spontaneous oxidation process
(Table 4) for styrene oxidation with H2O2.
ð2Þ
ð3Þ
Fig. 4 Arrhenius plot for determination of activation energy:
a (PW12)3/MCM-41, b (PW12)3/MCM-48, c (PW11)3/MCM-41 and
d (PW11)3/MCM-48
Fig. 5 Thermodynamic study for styrene oxidation with H2O2
Table 4 Thermodynamic and
Catalysts
Thermodynamic parameters
Kinetics parameters
Kinetic investigation for styrene
oxidation with H2O2 over all the
catalysts
Temp
DG (kJ/mol)
R2
k 9 10-3 min-1
Ea (kJ/mol)
R2
(PW12)3/MCM-41
(PW12)3/MCM-48
(PW11)3/MCM-41
(PW11)3/MCM-48
353
353
353
353
-6.5
-6.3
-7.5
-9.8
0.96
0.96
0.93
0.97
6.5
5.7
8.4
9.9
53.5
50.8
46.9
44.1
0.98
0.99
0.93
0.99
123

S. Singh, A. Patel
3.4 Comparative Study: Effect of Support and Role
of Oxidant
rendering these oxygen atoms more reactive toward elec-
trophilic groups. Overall, this leads to an increase and
localization of the anionic charge, the resulting lacunary
anion becomes highly nucleophilic and reacts easily with
electrophilic groups [34]. Also, it has been reported by
Yoshida et al. [35] that in the lacunary species the surface
W(=O)2 atoms are active enough to catalyse the reaction.
Hence, in the present work comparatively higher activity
for lacunary PW11 anchored to MCM-41/48, higher TONs,
low activation energy as well as negative DG, goes well in
agreement with the above explanations.
Tuning the catalyst support is an important parameter to
build highly active catalysts for liquid phase oxidation.
Modifying the nature, texture and structure of the catalyst
support can yield an improvement in the catalytic perfor-
mances. Hence, a comparative study was carried out to
study the effect of support by studying kinetic and ther-
modynamic parameters and having a correlation between
them.
Among all the catalysts (Table 3), (PW11)3/MCM-48
proved to be the best catalyst for oxidation of styrene and
benzyl alcohol in terms of conversion as well as high TON.
The reason for this effect can be derived from the fact that
the support allows high degree of dispersion of active
species (PW11) and relatively higher surface area of
(PW11)3/MCM-48 (319 m2/g) as compared to (PW11)3/
MCM-41 (252 m2/g) (Table 1). It is known that MCM-48
has well ordered three dimensional pore network which
allows better dispersion of the active species inside the
channels and provide feasible catalytic active sites for the
reaction. This high dispersion leads to better mass transfer
which allows the product and reactant molecules to move
in the channels of the support. It was observed that for
oxidation of styrene in H2O2, (PW11)3/MCM-48 gives
lower activation energy-Ea (44.1 kJ mol-1) as compared to
(PW11)3/MCM-41 (46.9 kJ mol-1) (Table 4). These results
prove that, rate is truly governed by chemical step and
there is no diffusion limitation. The thermodynamic
parameters suggests that the styrene oxidation over
(PW11)3/MCM-48 is more favoured and spontaneous as the
Gibbs free energy is more negative. The selection of
MCM-48 was thus performed, taking into account the
surface area (as high as possible to increase the metal
dispersion) and porosity (mesopores for diffusional prob-
lems) and the obtained results go well in agreement with
the above correlation between kinetic and thermodynamic
studies.
It is seen from Table 3 that for all the catalysts order of
activity of oxidant was O2 [ H2O2 i.e. high mole conver-
sion of substrate and TON was observed with O2 as com-
pared to that of H2O2. The reason for difference in
conversion can be explained as follows. In case of H2O2
the activation of the metal center takes place through the
formation of [XO4{MO(O2)2}4]3- intermediate in situ
from XM11O39 [36]. The formed active peroxo intermedi-
ate oxidizes the alkenes as well as alcohols. In the case of
TBHP as an initiator and O2 as a main oxidant, initiation of
TBHP forms species BuO*/*OH and hence activates O2 to
form the superoxo-species [37]. Thus, the activation of the
catalyst requires more time in the case of H2O2 as com-
pared to that of direct involvement of O2. Hence, more
conversion is expected in the case of molecular oxygen.
The obtained results are in good agreement with the pro-
posed explanation. The difference in the selectivity of
products can be explained on the basis of the oxidant
nature.
3.5 Heterogeneity Tests and Control Experiments
For the proof of heterogeneity, hot filtration method [38]
was carried for oxidation of styrene over all the catalysts
(Fig. 6). The test was initiated by filtering the catalyst from
the reaction mixture at 80 °C after 10 h and further
allowing the filtrate to react up to 18 h. The reaction
mixture of 10 h and the filtrate were analysed by GC. No
change in the % conversion (between reaction mixture and
filtrate) of styrene indicates that the catalysts falls into
category C [38]. Category C means the metal does not
leach in any form and observed catalysis is truly hetero-
geneous in nature.
The effect of lacunary PW11 was more pronounced for
oxidation reaction on comparing the difference in the cat-
alytic activity between (PW11)3/MCM-48 and (PW12)3/
MCM-48. (PW11)3/MCM-48 showed higher oxidative
conversion (Table 3) as compared to (PW12)3/MCM-48
due to higher redox activity of PW11 than the parent PW12.
It is well known that lacunary PW11 have commendable
catalytic activity because of removal of a tungsten-oxygen
octahedral moiety from a saturated PW12 framework (as
lacunary core has a greater negative charge than the parent
PW12) [32–34]. In PW12 the negative charge is delocalized
over the entire structure. However, in PW11 the nucle-
ophilic properties of the oxygen atoms increases and
become localized at the surface of the lacuna, ultimately
In order to ascertain the influence of active species
(PW11) and the supports (MCM-41 and MCM-48) control
experiments were also carried out under optimized condi-
tions. It was observed that supports are not very active
toward the oxidation reaction (\1 % conversion). The
same reaction was carried out by taking the active amount
of PW11 as well as the parent PW12 which gives excellent
conversions 92 and 85 %, respectively. The properties of
the active species PW11 is retained in the catalyst on
123

Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over…
3.6 Characterisation of Regenerated Catalysts
No appreciable shift in the FT-IR band position of the
regenerated catalyst compared to fresh catalyst indicates
the retention of Keggin structure of PW12 on MCM-41
(Table 2 and Supplementary Fig. 5a). FT-IR spectra of the
used catalyst (PW11)3/MCM-41 (Table 2 and Supplemen-
tary Fig. 5b) shows retention of bands of W=O and W–O–
W, respectively at 962 and 896 cm-1. No appreciable shift
in the FT-IR band position of the regenerated catalyst
compared to fresh one indicates retention of PW11 structure
into MCM-41. No appreciable shift in the FT-IR band
position of the regenerated catalysts compared to fresh
catalyst indicates the retention of Keggin structure of
PW12/PW11 on MCM-48 (Table 2; Supplementary Fig. 5c,
d).
The Raman spectrum of PW12 in the catalyst shows
bands at 1012, 992, 906, 550, and 219 cm-1, which have
already been assigned to ms(W–Od), mas(W–Od), mas(W–Ob–
W), ms(W–Oc–W), and ms(W–Oa), respectively (Supple-
mentary Fig. 5 a) where Oa, Ob, Oc, and Od are the oxygen
atoms linked to phosphorous, oxygen atoms bridging two
tungsten (from two different triads for Ob and from the
same triad for Oc), and to the terminal oxygen W=O,
respectively [27]. Absence of any significant band shifts in
the spectra of recycled catalyst (Supplementary Fig. 5b)
indicates retention of the Keggin structure even after
anchoring to support MCM-41. Similar observation was
found for other catalyst (PW12)3/MCM-48. The Raman
spectra of (PW11)3/MCM-41 shows typical bands at 987,
912, 520 and 216 cm-1 corresponding to mas (W=Od), mas
(W–Ob–W), ms (W–Oc–W), and ms (W–Oa), respectively,
except mas (W=Od) band [28]. The presence of all reported
bands of lacunary PW11 in catalyst indicates that the
structure of PW11 is intact even after anchoring. The
Raman spectrum of recycled catalysts (Supplementary
Fig. 6a–d) shows retention of typical bands of PW12/PW11
even after recycling. The spectrum is slightly different
from the fresh one in terms of intensity. However, no
appreciable shift suggests retention of structure after
recycling. Similar observation was found for other catalyst
(PW11)3/MCM-48.
Fig. 6 Heterogeneity study (A, B) for styrene oxidation with H2O2
over a (PW12)3/MCM-41, b (PW12)3/MCM-48, c (PW11)3/MCM-41
and d (PW11)3/MCM-48
anchoring, which is justified by increased activity in case
of (PW11)3/MCM-41 as well as (PW11)3/MCM-48. This
result indicates that the catalytic activity is mainly due to
real active species and the dispersion of PW11 inside
MCM-41/MCM-48 causes a significant change. Thus, we
were successful in supporting PW11 on mesoporous sup-
ports without any significant loss in activity.
Recycling study for oxidation of styrene (PW11)3/MCM-
41 and (PW11)3/MCM-48 under optimized conditions with
H2O2 was carried out (Supplementary Table 2). Regener-
ated catalysts after fourth cycle was also subjected to EDS
analysis and no significant difference was observed for the
recycled catalyst as compared to the fresh one (Table 2).
Absence of any leaching of W and P from catalyst was
confirmed by carrying out an analysis of the product
mixtures after the reaction by AAS. Analysis of the product
mixtures shows that if any W was present it was below the
detection limit, which corresponds to less than 1 ppm.
These observations strongly suggest that there was no
leaching of any active species from the support.
3.7 Comparison with Reported Catalysts
Comparative data for the aerobic oxidation of benzyl
alcohol and styrene over the present catalysts and reported
catalysts was carried out. 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 [39].
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, it
123

S. Singh, A. Patel
was found that Mo-Y is better in terms of selectivity for the
epoxide. Ueda et al. carried out the aerobic oxidation of
benzyl alcohol catalysed by Mo–V–O oxide as a hetero-
geneous catalyst using toluene as a solvent and 22 %
conversion was observed with excellent selectivity ([99)
for benzaldehyde [40]. However, the reaction was carried
out in toluene as a solvent.
difference in the selectivity of product and whatever dif-
ference is observed, will only be due to the acidity dif-
ference of the support. It was also observed from Table 5
that, oxidation of secondary alcohol is easier as compared
to primary alcohols. The obtained results suggest that the
catalyst and reaction system can withstand variety of sub-
strates under mild reaction conditions.
Jian et al. [41] reported the use of phase transfer mod-
ified PW12O40 in presence of solvent for liquid phase
oxidation of benzyl alcohol. 14.3 % conversion was
obtained at 0.08 mmol catalyst amount and 80 °C.
Oxoperoxo molybdenum (VI) and tungsten (VI) complexes
with 1-(20-hydroxyphenyl) ethanone oxime [42] have been
used as catalyst in presence of solvent (Acetonitrile) for
oxidation reaction. 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/benzyl
alcohol (100 mmol) and a lower amount of the catalyst
(20 mg) at a lower temperature (80/90 °C).
3.9 Reaction Mechanism
In order to study the reaction mechanism, sets of reactions
were carried out at different stages of the reactions. It is
known for oxidation reactions with transition metal atoms,
especially polyoxometalates, that H2O2 first binds to the
metal centre and then transfers an oxygen atom to the
olefin/alcohol. Thus, the activation of the metal center
results via. the generation of the active species which may
be hydroperoxo or bridging peroxo species [36]. In the
present case, the catalysts are also expected to follow the
same mechanism (for oxidation of styrene and benzyl
alcohol with H2O2) via. formation of an active tungsten-
peroxo or hydroperoxo intermediate [36] (Scheme 1a, b).
Three different oxidation processes were studied to
establish the role of TBHP and O2 for the reaction of
styrene and benzyl alcohol with the best catalyst so far
(PW11)3/MCM-48. In the Reaction (1) TBHP as the sole
oxidant showed 38 % conversion with only formation of
polymerized products for oxidation of styrene, whereas no
conversion was obtained for benzyl alcohol, Reaction (2)
using O2 as the sole oxidant gives only 30 % conversion of
styrene and (3) employing O2 as the oxidant with 0.2 mmol
TBHP as the initiator (Table 3). On comparing the activi-
ties of Reactions (2) and (3), the conversions increase
significantly in the case of Reaction (3). This can also be
explained on the bases of kinetic curve (% conversion vs
time) (Supplementary Fig. S4) where O2 as the only oxi-
dant requires a longer time for activation, i.e. a larger
induction period, whereas in the case of TBHP ? O2,
TBHP acts as an radical initiator by reducing the induction
3.8 Scope of Oxidation of Different Substrates
with H2O2
The scope of oxidation reaction were extended over dif-
ferent substrates of alkenes and alcohols. It is seen from
Table 5 that, for (PW11)3/MCM-48, higher conversion of
styrene as well as substituted styrene is obtained, but at the
same time product selectivity was found to be similar. This
can be due to better mass transport efficiency of MCM-48
as discussed in the earlier section. Generally, oxidation of
styrene gives styrene oxide as an intermediate product.
However, in most of the cases, stable product benzalde-
hyde was observed as major product via (1) direct oxida-
tive cleavage of C=C of styrene and (2) fast conversion of
styrene oxide to benzaldehyde. However, as the active
species is same it is expected that there will not be much
Table 5 Oxidation of various
Catalyst
Substrate
Conv. (%)
Products
Selec. (%)
alkenes and alcohols over
supported catalysts, under
optimized conditions
(PW11)3/MCM-41
4-Methyl styrene
a-Methyl styrene
4-chloro benzyl alcohol
Cyclohexanol
90
85
34
30
30
95
92
37
32
32
4-Methyl Benzaldehyde
Acetophenone
95
100
82
90
90
92
100
85
90
89
4-chloro benzaldehyde
Cyclohexanone
1-Hexanol
Hexanal
(PW11)3/MCM-48
4-Methyl styrene
a-Methyl styrene
4-chloro benzyl alcohol
Cyclohexanol
4-Methyl Benzaldehyde
Acetophenone
4-chloro benzaldehyde
Cyclohexanone
1-Hexanol
Hexanal
123

Environmentally Benign Oxidations of Alkenes and Alcohols to Corresponding Aldehydes over…
Scheme 1 Probable reaction mechanisms for liquid phase oxidation of a styrene and b benzyl alcohol
Scheme 2 Probable reaction mechanisms for aerobic oxidation of styrene and benzyl alcohol
period. O2 proved to be better oxidant as compared to H2O2
based on the high TONs.
4 Conclusion
Also for investigations of mechanism with O2 different
sets of reactions proposed that the reaction appears to be a
radical mechanism and activation of catalyst was necessary
for initiating the reaction (Scheme 2), where TBHP acts as
a radical initiator only. Our group had earlier proposed a
mechanism for aerobic oxidation of alkenes and alcohols
over anchored phosphomolybdates [43], similarly in the
present case also we expect the same mechanism.
Detailed kinetic studies reveal that the reactions follow first
order kinetics and low Ea values indicates that the reaction
rate is truly governed by the chemical step with no mass/
diffusion limitation. The thermodynamic parameters sug-
gests the same, where oxidation of styrene with H2O2 was
found to be thermodynamically favoured in presence of
catalyst. A comparative kinetic and thermodynamic study
shows that PW11 was the most active species as compared
123

S. Singh, A. Patel
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to PW12 and MCM-48 was the best support due to high
dispersion leading to better mass transfer which allows the
product and reactant molecules to move in the channels of
the support. Hence overall, (PW11)3/MCM-48 proved to be
the best catalyst for oxidation of styrene and benzyl alcohol
in terms of conversion as well as high TON. Order of
activity of oxidant was O2 [ H2O2. Experimental results
helped us to propose plausible mechanism for both the
reactions. The catalysts were recycled for four cycles after
simple regeneration, without any significant loss in the
conversion.
Acknowledgments We are thankful to Department of Science and
technology (DST), Project No. SR/S5/GC-01/2009, New Delhi, for
the financial support. One of the authors Ms. Sukriti Singh is thankful
to the same for the grant of research fellowship.
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