Polyoxotungstate ([PW11O39]7-) immobilized on mesoporous polymer for selective liquid-phase oxidation of alcohols using H2O2

Article abstract of DOI:10.1039/d0ra07178a

Selective oxidation of alcohols is an attractive organic transformation and has received tremendous attention from the scientific community over the years. Herein, a mesoporous polymer (MP) was synthesized by a template-free solvothermal approach. The surface of the MP was functionalized with quaternary ammonium groups and polyoxotungstate anion (PW11O397-) was subsequently supported on the MP as a counter anion to the ammonium cation by a simple ion-exchange procedure. The structure of PW11 and PW4 complexes was confirmed by 31P NMR and FTIR analysis. The surface properties of all the catalysts synthesized were explored by various characterization techniques such as nitrogen sorption, TGA, contact angle measurement, and ICP-OES analysis. The synthesized PW11/MP catalysts were employed for selective oxidation of alcohols. Among the various PW11 supported catalysts, PW11/MP (80 : 20) demonstrated excellent catalytic activity for the oxidation of alcohols using aqueous H2O2. The PW11/MP (80 : 20) catalyst showed good catalytic activity for oxidation of a wide range of alcohols including substituted, heterocyclic and secondary alcohols. The superior catalytic activity of PW11/MP (80 : 20) is attributed to an optimum balance in the hydrophilicity/hydrophobicity in the mesoporous environment, better catalyst wettability, and enrichment of reactants in the catalytic active sites.

Full text of DOI:10.1039/d0ra07178a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Polyoxotungstate ([PW11O39]7ꢀ) immobilized on
mesoporous polymer for selective liquid-phase
oxidation of alcohols using H2O2†
Cite this: RSC Adv., 2020, 10, 35988
ab
ab
Sathyapal R. Churipard,
and Sanjeev P. Maradur
Kempanna S. Kanakikodi,
Jyoti Roy Choudhuric
a
*
Selective oxidation of alcohols is an attractive organic transformation and has received tremendous
attention from the scientific community over the years. Herein, a mesoporous polymer (MP) was
synthesized by a template-free solvothermal approach. The surface of the MP was functionalized with
quaternary ammonium groups and polyoxotungstate anion (PW11O397ꢀ) was subsequently supported on
the MP as a counter anion to the ammonium cation by a simple ion-exchange procedure. The structure
of PW11 and PW4 complexes was confirmed by 31P NMR and FTIR analysis. The surface properties of all
the catalysts synthesized were explored by various characterization techniques such as nitrogen
sorption, TGA, contact angle measurement, and ICP-OES analysis. The synthesized PW11/MP catalysts
were employed for selective oxidation of alcohols. Among the various PW11 supported catalysts, PW11/
MP (80 : 20) demonstrated excellent catalytic activity for the oxidation of alcohols using aqueous H2O2.
The PW11/MP (80 : 20) catalyst showed good catalytic activity for oxidation of a wide range of alcohols
including substituted, heterocyclic and secondary alcohols. The superior catalytic activity of PW11/MP
(80 : 20) is attributed to an optimum balance in the hydrophilicity/hydrophobicity in the mesoporous
environment, better catalyst wettability, and enrichment of reactants in the catalytic active sites.
Received 20th August 2020
Accepted 18th September 2020
DOI: 10.1039/d0ra07178a
rsc.li/rsc-advances
easy separation, recyclability, and reusability of the catalysts. In
this context, many noble metal-supported catalysts were devel-
1 Introduction
oped which exhibit superior catalytic activity for aerobic
oxidation of alcohols.15–22 But many of these catalysts are prone
to deactivation, complex synthetic procedures, and are expen-
sive which curb their use for industrial applications.23 There-
fore, it was obligatory to design non-noble metal-based catalysts
for oxidation reactions. To replace the noble-metals catalysts,
several transition metal-based catalysts are reported for efficient
oxidation of various substrates. Among them, tungsten-based
Oxidation reactions are among the most signicant reactions in
organic synthesis. However, they are amongst the most
hazardous and problematic processes which oen entail a high
E-factor and environmental risks.1–5 Oxidation of alcohols,
particularly benzyl alcohol to benzaldehyde is one of the key
transformations in organic synthesis due to immense applica-
tions of benzaldehyde in perfumery, dye, pharmaceuticals, and
agricultural industries.6,7 Previously, alcohol oxidations were
performed using stoichiometric amounts of toxic oxidants
which inevitably produce harmful by-products and pose severe
threats to the environment.8–13 As a result, researchers focused
to explore new heterogeneous catalysts that can utilize envi-
ronmentally friendly oxidants like molecular oxygen or H2O2 for
oxidation reactions.14 Thereby, it offers several advantages like
catalyst systems occupy
a special place because of its
resourceful versatility and overwhelming ability in oxidation
reactions. Jacobson, Ishii, Venturello, and Noyori's pioneering
work on tungsten-based catalyst systems revealed the superi-
ority of tungsten-based catalysts for oxidation of various
substrates including alcohols, suldes, and olens.24–27 There-
aer several efficient protocols for oxidation reactions were
developed using tungsten-based catalysts.28–34 But many of these
catalysts are homogeneous and suffer from lack of catalysts
recovery and reusability which still needs to be addressed. The
use of costly phase transfer catalyst in some of these systems
hinders its commercial exploitation.1,35 Therefore it is impera-
tive to immobilize the tungsten-based catalysts on solid
supports, which is one of the promising ways to solve the
aforementioned problems.36,37
aMaterials Science and Catalysis Division, Poornaprajna Institute of Scientic,
Research (PPISR), Bidalur Post, Devanahalli, Bangalore-562164, Karnataka State,
India. E-mail: sanjeevpm@poornaprajna.org; Fax: +91 23619034; Tel: +91 80
27408552
bGraduate Studies, Manipal Academy of Higher Education, Manipal – 576104,
Karnataka, India
cBMS Institute of Technology and Management, Doddaballapur Main Road,
Avalahalli, Yelahanka, Bangalore, Karnataka – 560064, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra07178a
35988 | RSC Adv., 2020, 10, 35988–35997
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
On the other hand, polyoxometalates (POM) stands as
RSC Advances
In the present study, we have synthesized polyoxotungstate
a versatile catalyst owing to its redox and acidic properties. In immobilized mesoporous polymer (PW11/MP) by template-free
particular, Keggin heteropolyacids (HPA) have received signi- solvothermal approach. The catalytic activity of the PW11/MP
cant attention in catalysis which stems from the fact that the catalyst was evaluated for the green oxidation benzyl alcohol to
acidic and redox properties in this material can be easily tuned benzaldehyde using H2O2. Additionally, the catalytic applica-
by changing their composition and structures. Among the tion of the PW11/MP (80 : 20) catalyst was extended to a wide
various Keggin heteropolyacids, HPAs with tungsten atoms are range of alcohol substrates. The concentration of quaternary
extensively studied for catalysis. Nevertheless, the POMs still functional groups in the material was varied to investigate the
suffer from limitations such as low surface area, high solubility effect of hydrophilic environment on the substrate wettability
in polar solvents, lack of recyclability, and reusability which and catalytic activity. The optimum hydrophobic–hydrophilic
needs to be addressed.38–40
environment helps to provide better catalyst wettability for the
In this context, the conversion of heteropolyacids to neutral oxidation of hydrophobic benzyl alcohol with aqueous H2O2.
salt by the replacement of protons in heteropolyacids by large Herein, we have demonstrated the superior role of catalyst
cations makes it insoluble in many solvents and serves as an wettability than other physicochemical properties of catalyst in
efficient heterogeneous POM catalyst. Furthermore, the enhancing the catalytic activity.
removal of addenda atom (metal) from Keggin heteropolyanion
creates vacancy and generates lacunary Keggin structure which
is more potential catalyst in activating various organic
substrates than its precursor with saturated anion.41,42
2 Results and discussion
Mesoporous polymers (MP) were synthesized by free radical
polymerization of divinylbenzene and vinylbenzyl chloride. It
There are several reports on the heterogenization of
tungsten-based POMs on high surface area supports.43 Van
Bekkum et al. have done pioneering work on MCM-41-
supported heteropoly acids.44 Kholdheeva et al. have worked
extensively on supported POMs and also reviewed recent trends
in the eld.45–48 Then several supports such as MOF,49 SBA-15,50
zeolites,51 were used to increase the dispersion, atomic utiliza-
tion, and the number of accessible active sites of POM.52,53
However, many of the heterogeneous POM catalysts entail
complex synthetic procedures and have limitations such as low
catalytic activity, tungsten leaching, and low accessibility of
active sites to the substrates. Hence, it is obligatory to design
the tungsten-based heterogeneous catalyst with a simple
synthetic procedure, which offers higher catalytic activity, better
catalyst wettability and accessibility of active sites to the
reactants.
In this context, mesoporous polymers are potential candi-
dates for the immobilization of various active sites. It shares the
property of mesoporous silica and organic framework of
a polymer. The pore diameters can be tailored and the organic
frameworks can be functionalized through co-polymerization
and/or post-synthesis organic reactions.54–59 The hydropho-
bicity–hydrophilicity in the material can be easily tuned by
varying the composition of monomers or by post-synthetic
modications. The effect of substrate wettability on catalytic
activity is an important aspect that is overlooked for a long time
in heterogeneous catalysis. There are very few reports on the
effect of catalyst wettability on catalytic activity. Unlike other
catalysts, mesoporous polymers offer better control over tuning
the catalyst wettability. Catalyst wettability is one of the key
factors in catalysis since it greatly inuences the adsorption and
desorption of reactants and products and plays a decisive role in
enhancing the catalytic activity.60–67 Furthermore, mesoporous
polymer have diverse synthetic routes that facilitate the incor-
poration of several chemical functionalities and offer signi-
cant advantages than conventional porous materials. These
promising features of mesoporous polymer make it a potential
candidate for heterogeneous catalysis.68–71
was then functionalized with trimethylamine. The poly-
7ꢀ
oxotungstate anion [PW11O39
]
abbreviated as (PW11) was
supported on the polymer by electrostatic interaction to get
polyoxotungstate supported mesoporous polymer (PW11/MP).
For comparison, we have synthesized polyoxotungstate sup-
ported KIT-6 (PW11/KIT-6) and polyoxotungstate supported
Amberlite (PW11/Amberlite). The detailed synthesis procedure
of the catalysts is given in the experimental section. The cata-
lytic activity of PW11/MP was evaluated for the oxidation of
benzyl alcohol and various other alcohols. Its catalytic activity
was compared with PW11/KIT-6 and PW11/Amberlite catalysts.
2.1 Physicochemical characterization
FT-IR spectra of MP, PW11, and PW11 supported on different
supports are shown in Fig. 1. The PW11 complex exhibited
a characteristic peak at 975 cmꢀ1 which corresponds to the
W]O bond, this characteristic peak was also observed in PW11
Fig. 1 FTIR spectra of polyoxotungstate supported catalysts and
unmodified polymer (MP).
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 35988–35997 | 35989

View Article Online
RSC Advances
Paper
supported catalysts which conrms the successful incorpora- 20 mol% of vinylbenzyl chloride is the threshold limit for
tion of polyoxotungstate complex in the mesoporous polymer as retaining the mesoporosity in the polymer, beyond which the
well as in other supports.72 The peaks observed at 1605, 2960 porosity of the polymer is lost and the material becomes
and 2850 cmꢀ1 in the MP and PW11/MP correspond to C]C, nonporous. All the polymers showed a BJH pore size of 13.9 nm
CH2, and C–H bonds of the polymeric support respectively. The except PW11/MP (70 : 30) which was nonporous (ESI Fig. S6†).
band around 676 cmꢀ1 in unmodied MP polymer is attributed The recycled PW11/MP catalyst was also characterized to check
to the stretching vibration of C–Cl bond which is found to be the stability of the porous structure in the reaction conditions.
absent in the polyoxotungstate supported mesoporous poly- The analysis showed that the material retained its mesoporosity
mers which conrms the successful quaternization of meso- with a slight decrease in the surface area even aer ve recycles
porous polymers by trimethylamine. For clarity we have also (ESI Table S1†). Polyoxotungstate supported catalysts were also
subtracted the FTIR spectrum of PW11/MP (80 : 20) with the characterized by ICP-OES to determine the amount of tungsten
quarternary ammonium functionalised mesoporous polymer in loaded in each catalyst (Table 1). The tungsten loading was in
the NO3ꢀ form without PW11 loading to distinguish the peaks the range of 0.0867 to 0.1556 mmol gꢀ1 of mesoporous polymer
corresponding to PW11 complex in the PW11 supported MP catalyst. The data suggests that with an increase in the amount
catalyst (ESI Fig. S1 and S2†).
of vinylbenzyl chloride in polymer, there is an increase in
FT-IR spectrum of the PW4 complex revealed the presence of tungsten loading in the catalyst.
a characteristic peak at 870 cmꢀ1 which is attributed to O–O
The structural integrity of the prepared PW11 complex and
bond of the peroxo group (ESI Fig. S3†). The peak at 591 and PW11 supported mesoporous polymer were conrmed by 31P
523 cmꢀ1 is assigned to W–O2 which is a vibration of a typical MAS NMR analysis which is the most reliable technique so far in
peroxotungstate complex (PW4).26 These characteristic peaks of elucidating the structure of polyoxotungstates (Fig. 2). The PW11
peroxotungstate (PW4) complex were found absent in the lacu- complex showed a characteristic peak at ꢀ12.7 ppm which is
7-
nary Keggin polyoxotungstate (PW11) complex which distin- ascribed to the lacunary Keggin-type structure of [PW11O39
guishes the structural difference between the two complexes.
]
ꢀcomplex.75 The PW11/MP (80 : 20) catalyst exhibited a strong
The thermal stability of the polymeric catalyst was investi- resonance peak at ꢀ14.7 ppm, which is attributed to the PW11
gated by thermogravimetric analysis (TGA) (ESI Fig. Sꢀ41†). TGA complex in the new environment of the quaternary ammonium
analysis was performed at a ramp rate of 10 ꢁC min under functionalized mesoporous polymer.52,76 The shi in 31P MAS
a ow of nitrogen. The PW11/MP (80 : 20) catalyst showed
a multistage decomposition from 30 to 800 ꢁC. The initial
weight loss of 3 wt% in PW11/MP (80 : 20) polymer below 100 ꢁC
is attributed to the desorption of physisorbed water molecules.
The weight loss from 200 ꢁC to 250 ꢁC is possibly due to the
decomposition of quaternary ammonium groups. The third
step of weight loss is observed from 370–470 ꢁC which is
attributed to the destruction of the polymeric framework in the
catalysts.73,74 This conrms that the catalyst is stable up to
ꢁ
200 C.
The physicochemical properties of various catalysts are
summarized in Tables 1 and S1.† The porous properties of all
the catalysts were determined by nitrogen sorption measure-
ments. All the MP polymers and PW11/MP catalysts displayed
typical type IV isotherm (ESI Fig. S5†) which is a characteristic
property of mesoporous materials. The total surface area
reduced drastically upon increasing the molar ratio of vinyl-
Fig. 2 31P NMR spectra of PW11complex, fresh and recycled PW11/MP
polymer.
benzyl chloride in the polymer (Table 1). The MP polymer with
Table 1 Physicochemical properties of MP and PW11/MP
Sample
SBET (m2 gꢀ1
)
Vta (ccgꢀ1
)
Pore size (nm)a
Tungstenb (mmol gꢀ1
)
MP (90 : 10)
MP (80 : 20)
MP (70 : 30)
PW11/MP (90 : 10)
PW11/MP (80 : 20)
PW11/MP (70 : 30)
384
225
46
358
157
7
0.57
0.43
0.11
0.49
0.46
—
13.9
13.9
13.9
13.9
13.9
—
—
—
—
0.0867
0.130
0.1556
a
By BJH method. b Measured by ICP-OES analysis.
35990 | RSC Adv., 2020, 10, 35988–35997
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 3 Contact angle of H2O2 droplet on: (a) PW11/MP (90 : 10), (b)
PW11/MP (80 : 20) and (c) PW11/MP (70 : 30).
Scheme 1 Reaction scheme for the oxidation of benzyl alcohol using
H2O2.
NMR signal from ꢀ12.7 to ꢀ14.7 ppm of PW11/MP (80 : 20)
could be due to the strong interaction between the PW11
complex and the polymeric support or due to the hydration or
protonation of the PW11 anion.49,77
The 5 times recycled PW11/MP (80 : 20) catalyst also exhibi-
ted the characteristic resonance peak at ꢀ14.7 ppm similar to
that of fresh PW11/MP (80 : 20) catalyst which conrms that the
catalyst is stable in the reaction conditions. It is rather unusual
that PW11 complex retaining the structure aer catalysis. The
PW11 complex will undergo decomposition in the presence of
excess of H2O2.
For comparison of the structure of the PW11 complex, we also
synthesized the tetranuclear peroxotungstate complex (PW4)
and conrmed its structure by characterization. The structure
of the tetranuclear peroxotungstate (PW4) complex is conrmed
by 31P NMR analysis. The 31P NMR analysis of the PW4 complex
showed a resonance peak at +7.1 ppm which suggests that the
synthesized complex has a tetranuclear peroxotungstate (PW4)
structure (ESI Fig. S7†).
quaternary ammonium salt, pH, and the synthesis conditions
used will signicantly affect the structure and stability of the
nal polyoxotungstate synthesized. However, tungsten-based
POMs are less labile than molybdenum, thus higher pH
values are required to remove the WO unit and generate the
Keggin structure.38,78–80
In the current synthetic approach of the PW11 complex, the
pH of the solution during the synthesis was found to be high
(pH ¼ 7) and the quaternary ammonium salt used was tetra-
butylammonium bromide which has favoured the formation of
monolacunary Keggin structure (PW11). Whereas the pH during
the synthesis of tetranuclear peroxotungstate complex was low
(pH ¼ 1) and we have employed methyltrioctylammonium
chloride which has resulted in the tetranuclear peroxotungstate
(PW4) complex. The structures of both complexes were
conrmed by 31P NMR and FTIR analysis.
2.3 Catalytic activity studies
To understand the role of catalyst wettability of PW11/MP
catalysts, contact angle measurement was performed (Fig. 3).
PW11/MP (90 : 10) displayed a contact angle of 120ꢁ for 30%
H2O2 whereas PW11/MP (80 : 20) and PW11/MP (70 : 30) showed
a lower contact angle of 107ꢁ and 82ꢁ respectively. This indicates
that PW11/MP (70 : 30) exhibits higher wettability for H2O2 than
PW11/MP (90 : 10) and PW11/MP (80 : 20). Since benzyl alcohol
was passing through the pellet of PW11/MP catalysts, the contact
angle of benzyl alcohol on the PW11/MP catalyst could not be
measured.
All the polyoxotungstate (PW11) supported catalysts were
screened for the oxidation of benzyl alcohol to benzaldehyde
using H2O2 as a green oxidant (Scheme 1). Three blank reac-
tions were performed, (1) in the absence of catalyst, (2) in the
absence of H2O2, and (3) with unmodied MP polymer and
H2O2. Blank 1 and 2 experiments ensured that both H2O2 and
the catalyst are required for the oxidation of benzyl alcohol. The
blank 3 experiment proved the innocuous nature of meso-
porous polymer (MP) support for the immobilization of poly-
oxotungstate complex (PW11) and as such MP is not active in the
However, the catalyst wettability of PW11/MP catalysts for
benzyl alcohol was measured in terms of its adsorption capacity
(ESI Table S2†). The adsorption capacity of benzyl alcohol was
measured for all PW11/MP catalysts. PW11/MP (90 : 10) catalyst
showed the highest adsorption capacity of 4.2 times, whereas
PW11/MP (80 : 20) and PW11/MP (70 : 30) showed adsorption
capacity of 2.2 and 1.7 times respectively. This conrms that
PW11/MP (90 : 10) exhibits higher wettability for benzyl alcohol.
On the other hand, the PW11/MP (80 : 20) offers a good wetta-
bility for both the reactants thereby leading to enhanced cata-
lytic activity without any competitive diffusion of reactants to
the catalytic active sites in the polymeric matrix.
Table 2 Comparison of catalytic activitya
Benzyl alcohol
Benzaldehyde
selectivity (wt%)
Catalyst
conversion (wt%)
Blank 1b
Traces
Traces
Traces
95.0
Traces
Traces
Traces
78.2
67.5
69.8
Blank 2c
Blank 3d
PW11
PW11/MP (90 : 10)
PW11/MP (80 : 20)
PW11/MP (70 : 30)
PW11/KIT-6
85.5
94.2
85.5
10.7
67.6
37.6
2.2 Effect of pH and reaction conditions on the structure of
PW11/Amberlite IRA 900
66.3
74.8
polyoxotungstate complex synthesized
a
Reaction conditions: benzyl alcohol ¼ 20 mmol, H2O2 ¼ 30 mmol,
catalyst ¼ 10 wt% w.r.t alcohol, temperature ¼ 90 ꢁC, reaction time ¼
Typically, Keggin HPAs are treated with alkali to remove the
protons and generate a lacunary Keggin structure. The type of
addenda atom (tungsten), hetero atom (phosphorous),
b
c
d
6 h. In the absence catalyst. In the absence ofH2O2. In the
presence of unmodied MP and H2O2.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 35988–35997 | 35991

View Article Online
RSC Advances
Paper
reaction (Table 2). The blank reactions showed negligible
conversion (<1%) which conrms that this reaction proceeds
only with the aid of a catalyst. All the polyoxotungstate sup-
ported catalysts were screened for benzyl alcohol oxidation
under identical conditions.
Among all the catalysts screened, PW11 complex (homoge-
neous), and the PW11/MP (80 : 20) catalyst showed the best
catalytic activities. PW11/MP (80 : 20) catalyst retained the
catalytic activity of the PW11 complex even aer heterogeniza-
tion and exhibited the highest conversion (94.2%) with 69.8%
benzaldehyde selectivity (Table 2). The superior catalytic activity
of the PW11/MP (80 : 20) catalyst is attributed to the optimum
amount of active sites distributed in the mesoporous environ-
ment and better wettability of catalyst for both the reactants.34,62
Despite higher tungsten loading (0.1556 mmol gꢀ1), the
PW11/MP (70 : 30) catalyst showed lower conversion and selec-
tivity. This may be due to the non-porous nature of the catalyst
and lower wettability for the hydrophobic alcohol substrate on
the catalyst surface. PW11/Amberlite showed a lower yield of
benzaldehyde which is possibly due to the lower surface area of
the catalyst (ESI Table S1†). PW11/KIT-6 catalyst has higher
surface area but showed lower catalytic activity, which could be
attributed to the lower wettability and lower accessibility of
active sites to the reactants. Hence, PW11/MP (80 : 20) was
selected as the best catalyst for further screening and optimi-
zation of reaction conditions.
To identify the unique properties of the PW11/MP (80 : 20)
catalyst, benzyl alcohol oxidation was done with all PW11/MP
catalysts with identical amounts of tungsten (ESI Table S3†).
Despite having high surface area PW11/MP (90 : 10) catalyst was
less active as compared to PW11/MP (80 : 20). Similarly, PW11/
MP (70 : 30) catalyst also gave a low yield of benzaldehyde.
Hence, these ndings suggest that catalyst wettability is playing
a major role in this catalyst than high surface area and the
presence of optimum active sites.
Fig. 4 Effect of catalyst concentration on benzyl alcohol conversion
and selectivity. Reaction conditions: benzyl alcohol ¼ 20 mmol, H2O2
¼ 20 mmol, reaction temperature ¼ 90 ꢁC, reaction time ¼ 6 h,
catalyst ¼ PW11/MP (80 : 20).
Hence, the H2O2 to benzaldehyde mole ratio of 1 : 1 was found
to be optimum and selected for further studies.
2.5 Effect of catalyst concentration
Effect of catalyst concentration on benzyl alcohol oxidation was
studied by varying the catalyst amount from 5 to 15 wt% w.r.t.
benzyl alcohol (Fig. 4). As the catalyst concentration was
increased from 5 to 10 wt%, there was an increase in benzyl
alcohol conversion and benzaldehyde selectivity. The catalyst
concentration above 10 wt% did not have any signicant
inuence on the catalytic activity. Hence 10 wt% of catalyst was
selected as optimum catalyst concentration for further studies.
2.6 Effect of reaction time
The inuence of reaction time on catalytic activity was studied
using PW11/MP (80 : 20) catalyst (ESI Fig. S8†). The results
indicate that the catalyst is highly active and showed more than
74% benzyl alcohol conversion with 86% benzaldehyde selec-
tivity in the initial 3 h. With the increase in time, the benzyl
alcohol conversion increased to 87%. However, the selectivity to
benzaldehyde slightly decreased aer 6 h. This is possibly due
to the over oxidation of benzaldehyde to benzoic acid during
prolonged reaction time.81
2.4 Effect of the substrate to oxidant mole ratio
The effect of the substrate to oxidant mole ratio was studied by
varying the moles of H2O2 taken. When the mole ratio of H2O2
to benzaldehyde was increased from 1 to 1.5 the benzyl alcohol
conversion increased from 87.9% to 94% (Table 3). However,
there was a 12% decrease in benzaldehyde selectivity upon an
increase in mole ratio because of the formation of by-products.
2.7 Effect of reaction temperature
Effect of temperature on benzyl alcohol oxidation was investi-
gated in the range of 70–110 C using benzyl alcohol to H2O2
Table 3 Effect of mole ratioa
ꢁ
mole ratio of 1 and 10 wt% of PW11/MP (80 : 20) ꢁcatalyst (Fig. 5).
As the temperature was increased from 70–90 C, the yield of
benzaldehyde has signicantly improved but further increasing
Benzyl alcohol
conv. (wt%)
Benzaldehyde
selec. (wt%)
Catalyst
H2O2/BzOH
ꢁ
PW11/MP (80 : 20)
PW11/MP (80 : 20)
PW11/MP (80 : 20)
1.5
1.2
1.0
94.2
94.2
87.9
69.8
74.1
81.6
the temperature beyond 90 C shows less benzaldehyde yield.
This could be possibly due to the fast decomposition of H2O2 at
higher temperatures. Hence, 90 ꢁC was selected as the optimum
temperature for further studies. This suggests that temperature
can increase the benzyl alcohol conversion but it does not
inuence the benzaldehyde selectivity.82
a
Reaction conditions: benzyl alcohol ¼ 20 mmol, catalyst ¼ PW11/MP
(80 : 20) 10wt% w.r.t. benzyl alcohol, reaction temperature ¼ 90 ꢁC,
reaction time ¼ 6 h.
35992 | RSC Adv., 2020, 10, 35988–35997
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 7 Catalyst reusability test. Reaction conditions: benzyl alcohol ¼
20 mmol, H2O2 ¼ 20 mmol, catalyst ¼ PW11/MP (80 : 20) 10 wt% (w.r.t
benzyl alcohol), temperature ¼ 90 ꢁC, reaction time ¼ 6 h.
Fig. 5 Effect of reaction temperature. Reaction conditions: benzyl
alcohol ¼ 20 mmol, H2O2 ¼ 20 mmol, catalyst ¼ PW11/MP (80 : 20)
10 wt% (w.r.t benzyl alcohol), reaction time ¼ 6 h,others ¼ benzoic
acid and benzyl benzoate.
heterogeneous catalysts for triphasic oxidation of benzyl
alcohol on water.
2.8 Effect of solvents
The effect of solvent on catalytic oxidation of benzyl alcohol was
studied using various solvents like water, toluene, acetonitrile,
benzonitrile, and acetone under optimized reaction conditions
(Fig. 6). Interestingly it was observed that water acts as the best
solvent for the oxidation of benzyl alcohol. A higher yield of
benzaldehyde was obtained when water was used as a solvent.
The observed phenomenon could be attributed to the proper
balance of hydrophobicity–hydrophilicity in the catalyst surface
that offers excellent wettability for both benzyl alcohol and
aqueous H2O2.62 The proper balance of the hydrophobic
support and hydrophilic quaternary ammonium groups would
lead to enhanced catalytic activity when water is used as
a solvent.83 Hence, the PW11/MP (80 : 20) catalyst acts as green
2.9 Catalyst reusability test and leaching studies
The catalyst reusability test was performed for PW11/MP
(80 : 20) catalyst under optimized reaction conditions to check
the stability of catalyst under the reaction conditions (Fig. 7).
Aer each run, the catalyst was ltered, washed with an excess
of methanol and acetone to remove the adsorbed reactants or
products.
It was dried at 70 ꢁC for 3 h and used for the next run. PW11/
MP (80 : 20) showed good recyclability and retained its catalytic
activity up to 5 recycles without signicant loss in its catalytic
activity. This proves that the catalyst is truly heterogeneous and
can be used for several recycles without loss in its catalytic
activity.
Leaching study was performed to check the stability of the
PW11/MP (80 : 20) catalyst towards the leaching of the active
species (PW11). The reaction was stopped aer 1 h and the
Fig. 6 Effect of solvents. Reaction conditions: benzyl alcohol ¼
20 mmol, catalyst ¼ PW11/MP (80 : 20) 10 wt% (w.r.t benzyl alcohol), Fig.
8
Leaching studies. Reaction conditions: benzyl alcohol ¼
solvent ¼ 6 ml, benzyl alcohol: H2O2 ¼ 1 : 1, temperature ¼ reflux 20 mmol, H2O2 ¼ 20 mmol, catalyst ¼ PW11/MP (80 : 20) 10 wt% (w.r.t
temperature of solvent, reaction time ¼ 6 h.
benzyl alcohol), temperature ¼ 90 ꢁC.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 35988–35997 | 35993

View Article Online
RSC Advances
Paper
Table 4 Substrate scope for alcohol oxidationa
Alcohol substrate
Alcohol:H2O2
Alcohol conv. (wt%)
Product selec. (wt%)
Benzyl alcohol
1 : 1
1 : 1
1 : 2
1 : 2
1 : 2
1 : 2
1 : 1
1 : 1
1 : 1
87.9
81.1
74.2
72.9
82.0
67.6
6.3
81.6
68.0
100.0
86.2
67.2
89.2
100.0
100.0
89.4
4-Hydroxy benzyl alcoholb
Cyclohexanol
4-Chlorobenzyl alcoholb
4-Methoxy benzyl alcoholb
3-Phenoxy benzyl alcoholb,c
1-Hexanolb,c
1-Octanolb,c
7.5
49.1
Pyridine methanolb
a
Reaction conditions: alcohol ¼ 20 mmol, catalyst ¼ PW11/MP (80 : 20) 10 wt% w.r.t alcohol, temperature ¼ 90 ꢁC, reaction time ¼ 6 h. b 6 ml of
acetonitrile was used as solvent. c Reaction time 18 h.
catalyst was separated from the reaction mixture by ltration. group substituted alcohol like phenoxy benzyl alcohol took
The reaction was then allowed to proceed further without the longer reaction time (18 h) to get a high yield. Heterocyclic
catalyst. There was almost no change in the benzyl alcohol alcohol (pyridine methanol) and aliphatic alcohols showed
conversion aer removal of the catalyst from the reaction lower yield compared to other alcohol substrates.
medium which conrms the heterogeneous nature of the PW11/
MP (80 : 20) catalyst (Fig. 8). Further, the reaction mixture was
subjected to ICP-OES analysis to estimate the amount of tung-
2.11 Characterization of spent catalyst
The FT-IR spectrum of the 5 times recycled PW11/MP (80 : 20)
catalyst was compared with that of the fresh catalyst (Fig. 9). The
recycled catalyst showed a characteristic peak at 975 cmꢀ1
which corresponds to the W]O bond. Polyoxotungstate func-
tionalized polymer has retained all the characteristic peaks of
the polyoxotungstate complex. The 5 times recycled catalyst was
also characterized by ICP-OES analysis. It showed almost
similar amount of tungsten as the fresh catalyst, this conrms
that there is no leaching of tungsten in the reaction medium
(ESI Table S1†). The results of FTIR and ICP-OES reveal that the
catalyst is stable under the reaction conditions and the poly-
oxotungstate complex is intact in the mesoporous polymer even
aer 5 recycles.
sten leached in the solution. The ICP-OES data showed that
there is no detectable amount of tungsten in the ltered reac-
tion solution which conrms that there is no leaching of PW11
active species from the catalyst (ESI Table S1†).
2.10 Substrate scope for alcohol oxidation
To investigate the general applicability of present methodology,
PW11/MP (80 : 20) catalyst was employed for the oxidation of
various alcohols including primary, secondary, and heterocyclic
alcohols. PW11/MP (80 : 20) showed high catalytic activity for
oxidation of primary, secondary, and substituted alcohols
(Table 4).
High yield of corresponding carbonyl compounds was ob-
tained in 6 h for the oxidation of benzyl alcohol, 4-hydroxy
benzyl alcohol, 4-chloro benzyl alcohol, and 4-methoxy benzyl
alcohol. Whereas for non-activated cyclohexanol and bulky
3 Conclusion
Polyoxotungstate supported mesoporous polymers (PW11/MP)
were hydrothermally synthesized by free radical polymerization
technique using THF and water as porogen. The 31P NMR and
FTIR analysis conrmed that the synthesized polyoxotungstate
complex has a PW11 structure. The change in chemical shi
value from ꢀ12.7 ppm to ꢀ14.7 ppm in 31P NMR analysis
indicates the strong interaction of the PW11 complex with the
polymeric support upon immobilization. The catalytic activity
of PW11/MP catalysts was evaluated for the selective oxidation of
various alcohols. PW11/MP (80 : 20) showed high catalytic
activity compared to all other PW11 supported catalysts
screened. The reaction conditions were optimized to get a high
yield of targeted products. High yield of benzaldehyde was ob-
tained for the oxidation of benzyl alcohol when water was used
as a solvent, this shed light on better wettability of the reactants
in PW11/MP (80 : 20) catalyst. The high catalytic performance of
PW11/MP (80 : 20) is attributed to the better dispersion of the
optimum number of active sites (PW11) in mesopores and
Fig. 9 FTIR spectra of fresh & 5 times recycled PW11/MP (80 : 20).
35994 | RSC Adv., 2020, 10, 35988–35997
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
proper balance between the hydrophobic and hydrophilic with 100 ml of 0.2 M sodium nitrate to get nitrate form of tri-
surface in the catalyst. PW11/MP (80 : 20) catalyst was stable and methylamine functionalized mesoporous polymer.
retained its catalytic activity up to 5 recycles. These promising
features of PW11/MP (80 : 20) make it an efficient catalyst for the polyoxotungstate [TBA4H3]7+[PW11O39
selective oxidation of a wide range of alcohols using green nium salt of polyoxotungstate (TBAPW11) was synthesized by
4.1.3. Preparation of tetrabutylammonium salt of lacunary
]
7ꢀ. Quaternary ammo-
oxidant (H2O2).
using sodium tungstate as a precursor and tetrabutylammo-
nium (TBA) bromide as quaternizing agent (ESI Scheme S1†). In
a typical synthesis, 9.89 g of sodium tungstate dihydrate was
taken in a 250 ml RB ask tted with a reux condenser and
a stirrer. Then 26 ml of 30% H2O2 was carefully added to it
dropwise at 60 ꢁC. An exothermic reaction takes place and it was
stirred until a colorless solution was obtained. The solution was
cooled to room temperature, then 2.1 ml 40% H3PO4 was added
and stirred. The pH of the solution was found to be 7. Then
4.836 g of tetrabutylammonium bromide was added to the
above mixture along with 66 ml of distilled water. It was stirred
for another 15 minutes to get a white-colored tetrabuty-
lammonium salt of lacunary polyoxotungstate complex (PW11).
The obtained product was ltered and washed with an excess of
4. Experimental section
Tetrahydrofuran (THF), acetone, acetonitrile, hydrogen
peroxide (H2O2), toluene, alcohols, and product standards were
purchased from Merck India Ltd, vinylbenzyl chloride (VBC)
was purchased from Sigma Aldrich, sodium tungstate, trime-
thylamine, phosphoric acid, sodium nitrate was obtained from
SD ne chemicals Ltd, tetrabutylammonium bromide was
purchased from Sisco Research Laboratories Pvt. Ltd (SRL),
divinylbenzene (DVB) was purchased from TCI chemicals, azo-
bisisobutyronitrile (AIBN) was obtained from Paras Polymer
and Chemical, India, Amberlite IRA 900 resin was obtained
from Alfa Aesar.
ꢁ
distilled water (1.5 l) and dried at 60 C for 2 h. The obtained
polyoxotungstate complex is designated as PW11.
For comparison, we have synthesized tetranuclear perox-
otungstate complex (PW4) following the previously reported
procedure (ESI Section 1†).26
4.1 Catalyst preparation
4.1.1. Synthesis of mesoporous divinylbenzene-vinylbenzyl
4.1.4. Synthesis of polyoxotungstate supported meso-
chloride copolymer (MP). Mesoporous divinylbenzene- porous polymer (PW11/MP). The polyoxotungstate anion
vinylbenzyl chloride copolymer of varying mole ratios (90 : 10, (PW11O397ꢀ) is supported on mesoporous polymer by the elec-
80 : 20, and 70 : 30) was synthesized solvothermally by free trostatic interaction. In a typical procedure, 50 ml acetone
radical polymerization of divinylbenzene and vinylbenzyl chlo- containing 1.0 g of PW11 complex was added to 2.5 ml of 30wt%
ride monomers. Wherein, (90 : 10, 80 : 20, and 70 : 30) indi- H2O2 and 2 g of amine-functionalized DVB/VBC copolymer in its
cates the molar ratio between the divinylbenzene and NO3ꢀ form (ion-exchanged with 0.2 M NaNO3). The resulting
vinylbenzyl chloride in the co-polymer synthesised. In a typical mixture was stirred for 16 h at room temperature for exchanging
synthesis, 3.125 g of DVB and 0.9157 g of VBC were added to the NO3ꢀ with PW11complex to get the PW11/MP catalyst (ESI
solution containing 40 ml THF, 0.1 g of AIBN radical initiator, Scheme S2†).
and 4 g of water. The above solution as stirred at room
For comparison, KIT-6 was synthesized according to the re-
temperature for 3 h, then transferred to autoclave and hydro- ported procedure.85 Prior to the functionalization of KIT-6 with
thermally treated at 100 ꢁC for 48 h to get mesoporous polymer trimethylamine, it was silylated using chloropropyl trimethoxy
MP (80 : 20), where 80 : 20 is the molar ratio of PDVB to VBC. silane following the reported procedure.86 The amine-
Similarly, mesoporous polymers MP (70 : 30) and MP (90 : 10) functionalized KIT-6 and Amberlite IRA 900 were supported
were synthesized by varying mole ratios of DVB to VBC.
with PW11complex using the same ion-exchange procedure as
In this synthetic approach, the solvent used in the synthesis the PW11/MP catalyst.
itself acts as a porogen and creates mesoporosity in the polymer
without the aid of external templates.84 This unique feature of
solvothermal polymerization is more desirable as it eliminates
4.2. Catalysts characterization
the use of templates and subsequent template removing The FTIR spectra of all the samples were recorded in the range
process.
of 4000–500 cmꢀ1 using alpha T-Bruker spectrometer in trans-
4.1.2. Synthesis of trimethylamine functionalized meso- mission mode by KBr pellet technique to examine the formation
porous polymer. The obtained mesoporous polymer (MP) was of polyoxotungstate complex and also to conrm the successful
ꢁ
degassed at 100 C for 3 h to remove the trapped solvents. The anchoring of polyoxotungstate complex on to the mesoporous
quaternization of MP was carried out using trimethylamine. In polymer.31P MAS NMR was recorded in ECX-JEOL 400(S),
a typical synthesis, 3.0 g of degassed MP was dispersed in AVIII400(L) NMR spectrometer. The thermal stability of the
a mixture of 8 ml of trimethylamine and 40 ml of acetonitrile. polymeric catalyst is determined by thermogravimetric analysis
The solution was stirred at 60 ꢁC for 24 h and the resulting (TGA). TGA analysis was performed using a Discovery TꢀG1A by TA
ꢁ
product was ltered, washed thoroughly with an excess of Instruments-Waters Lab at a ramp rate of 10 C min under
distilled water and acetonitrile. The obtained material was dried a ow of nitrogen. The nitrogen sorption measurements were
ꢁ
under vacuum at 60 C for 8 h. The chloride ions of trimethyl- performed using the BELSORP-mini instrument at 77 K
amine functionalized mesoporous polymer were exchanged temperature. Before the analysis, the samples were degassed at
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 35988–35997 | 35995

View Article Online
RSC Advances
Paper
ꢁ
100 C for 2 h under a high vacuum. The amount of tungsten 10 P. Sudarsanam, R. Zhong, S. Van den Bosch, S. M. Coman,
loaded on catalysts is estimated by Inductively Coupled Plasma
Optical Emission Spectrometer (ICP-OES) by PerkinElmer-
V. I. Parvulescu and B. F. Sels, Chem. Soc. Rev., 2018, 47,
8349–8402.
Optima 7000V instrument. Contact angle measurement was 11 P. Sudarsanam, E. Peeters, E. V. Makshina, V. I. Parvulescu
done by making a pellet of PW11/MP catalysts and placing and B. F. Sels, Chem. Soc. Rev., 2019, 48, 2366–2421.
a drop of 30% H2O2 on the pellet. The contact angle of H2O2 was 12 V. R. Bakuru, S. R. Churipard, S. P. Maradur and
captured using a digital camera and the image was analyzed.
S. B. Kalidindi, Dalton Trans., 2019, 48, 843–847.
´
´
´
13 R. Yepez, S. Garcıa, P. Schachat, M. Sanchez-Sanchez,
´
´
J. H. Gonzalez-Estefan, E. Gonzalez-Zamora, I. A. Ibarra
and J. Aguilar-Pliego, New J. Chem., 2015, 39, 5112–5115.
14 D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu,
A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely,
D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362–
365.
4.3. Catalytic activity studies
The catalytic reactions were performed using 20 mmol of benzyl
alcohol, the requisite amount of 30% hydrogen peroxide and
catalyst (wt% w.r.t to benzyl alcohol) in a 25 ml RB ask
equipped with a reux condenser. The reaction mixture was
magnetically stirred using a magnetic stirrer at the desired
temperature. Aer the completion of the reaction, methanol
was added to the reaction mixture to make it homogenous. The
catalyst was separated by centrifugation and products were
analyzed by gas chromatography (Shimadzu-2014 equipped
with FID detector) using DB-wax column.
15 Y.-H. Kim, S.-K. Hwang, J. W. Kim and Y.-S. Lee, Ind. Eng.
Chem. Res., 2014, 53, 12548–12552.
16 V. R. Choudhary, A. Dhar, P. Jana, R. Jha and B. S. Uphade,
Green Chem., 2005, 7, 768–770.
17 H. Liu, Y. Liu, Y. Li, Z. Tang and H. Jiang, J. Phys. Chem. C,
2010, 114, 13362–13369.
18 S. Venkatesan, A. S. Kumar, J.-F. Lee, T.-S. Chan and
J.-M. Zen, Chem. Commun., 2009, 1912–1914.
19 N. Kakiuchi, Y. Maeda, T. Nishimura and S. Uemura, J. Org.
Chem., 2001, 66, 6620–6625.
Conflicts of interest
There are no conicts to declare.
20 L. Liotta, A. Venezia, G. Deganello, A. Longo, A. Martorana,
Z. Schay and L. Guczi, Catal. Today, 2001, 66, 271–276.
21 T. Matsushita, K. Ebitani and K. Kaneda, Chem. Commun.,
1999, 265–266.
22 V. R. Choudhary, R. Jha and P. Jana, Green Chem., 2007, 9,
267–272.
23 F. Arena, B. Gumina, A. F. Lombardo, C. Espro, A. Patti,
L. Spadaro and L. Spiccia, Appl. Catal., B, 2015, 162, 260–267.
24 S. Jacobson, D. Muccigrosso and F. Mares, J. Org. Chem.,
1979, 44, 921–924.
25 Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida and
M. Ogawa, J. Org. Chem., 1988, 53, 3587–3593.
26 C. Venturello and R. D'Aloisio, J. Org. Chem., 1988, 53, 1553–
1557.
Acknowledgements
SRC acknowledges Admar Mutt Education Foundation (AMEF),
Bangalore for providing research facilities. SRC thanks CSIR,
India for Senior Research Fellowship [File Number: 09/
1052(0007)2K19 EMR-Z]. SRC acknowledges Manipal Univer-
sity for permitting this research as a part of the PhD program.
The authors are grateful to Dr A. B. Halgeri, Director, Poor-
naprajna Institute of Scientic Research (PPISR) for his
constant support and encouragement.
References
27 K. Sato, M. Aoki and R. Noyori, Science, 1998, 281, 1646–
1647.
28 S. Parihar, S. Pathan, R. Jadeja, A. Patel and V. K. Gupta,
Inorg. Chem., 2011, 51, 1152–1161.
1 R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003, 1977–
1986.
2 R. Noyori, Chem. Commun., 2005, 1807–1811.
3 C. Parmeggiani and F. Cardona, Green Chem., 2012, 14, 547– 29 P. Shringarpure and A. Patel, J. Mol. Catal. A: Chem., 2010,
564.
321, 22–26.
4 S. Pathan and A. Patel, Chem. Eng. J., 2014, 243, 183–191.
30 S. Nojima, K. Kamata, K. Suzuki, K. Yamaguchi and
N. Mizuno, ChemCatChem, 2015, 7, 1097–1104.
´
´
´
5 J. G. Flores, E. Sanchez-Gonzalez, A. Gutierrez-Alejandre,
´
´
J. Aguilar-Pliego, A. Martınez, T. Jurado-Vazquez, E. Lima, 31 C. Venturello, E. Alneri and M. Ricci, J. Org. Chem., 1983, 48,
´
´
´
´
E. Gonzalez-Zamora, M. Dıaz-Garcıa and M. Sanchez-
3831–3833.
´
Sanchez, Dalton Trans., 2018, 47, 4639–4645.
32 C. Venturello, R. D'Aloisio, J. C. Bart and M. Ricci, J. Mol.
Catal., 1985, 32, 107–110.
33 R. Neumann and H. Miller, Chem. Commun., 1995, 2277–
2278.
34 V. Y. Evtushok, I. D. Ivanchikova, O. Y. Podyacheva,
O. A. Stonkus, A. N. Suboch, Y. A. Chesalov,
O. V. Zalomaeva and O. A. Kholdeeva, Front. Chem., 2019,
7, 858.
6 A. Savara, I. Rossetti, C. E. Chan-Thaw, L. Prati and A. Villa,
ChemCatChem, 2016, 8, 2482–2491.
7 A. Patel and S. Pathan, Ind. Eng. Chem. Res., 2011, 51, 732–
740.
8 A. Sakthivel, S. E. Dapurkar and P. Selvam, Catal. Lett., 2001,
77, 155–158.
9 A. Sakthivel, S. K. Badamali and P. Selvam, Catal. Lett., 2002,
80, 73–76.
35996 | RSC Adv., 2020, 10, 35988–35997
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
35 K. Yamaguchi, C. Yoshida, S. Uchida and N. Mizuno, J. Am. 59 S. R. Churipard, K. S. Kanakikodi, D. A. Rambhia,
Chem. Soc., 2005, 127, 530–531.
36 Y. Liu, K. Murata and M. Inaba, Green Chem., 2004, 6, 510–
C. S. K. Raju, A. Halgeri, N. V. Choudary, G. S. Ganesh,
R. Ravishankar and S. P. Maradur, Chem. Eng. J., 2020, 380,
122481.
515.
37 Y. Liu, K. Murata, T. Hanaoka, M. Inaba and K. Sakanishi, J. 60 L. Wang and F. S. Xiao, ChemCatChem, 2014, 6, 3048–3052.
Catal., 2007, 248, 277–287.
38 N. C. Coronel and M. J. da Silva, J. Cluster Sci., 2018, 29, 195–
205.
39 C. B. Vilanculo, M. J. Da Silva, M. G. Teixeira and
J. A. Villarreal, RSC Adv., 2020, 10, 7691–7697.
40 O. A. Kholdeeva, T. A. Trubitsina, M. N. Timofeeva,
G. M. Maksimov, R. I. Maksimovskaya and V. A. Rogov, J.
Mol. Catal. A: Chem., 2005, 232, 173–178.
61 M. Wang, F. Wang, J. Ma, C. Chen, S. Shi and J. Xu, Chem.
Commun., 2013, 49, 6623–6625.
62 G. Chen, Y. Zhou, Z. Long, X. Wang, J. Li and J. Wang, ACS
Appl. Mater. Interfaces, 2014, 6, 4438–4446.
63 S. Doherty, J. G. Knight, J. R. Ellison, D. Weekes,
R. W. Harrington, C. Hardacre and H. Manyar, Green
Chem., 2012, 14, 925–929.
64 Q. Han, C. He, M. Zhao, B. Qi, J. Niu and C. Duan, J. Am.
Chem. Soc., 2013, 135, 10186–10189.
41 M. J. da Silva, P. H. da Silva Andrade, S. O. Ferreira,
C. B. Vilanculo and C. M. Oliveira, Catal. Lett., 2018, 148, 65 D. Li, P. Yin and T. Liu, Dalton Trans., 2012, 41, 2853–2861.
2516–2527.
66 L. Jing, J. Shi, F. Zhang, Y. Zhong and W. Zhu, Ind. Eng.
Chem. Res., 2013, 52, 10095–10104.
67 A. Nisar, Y. Lu, J. Zhuang and X. Wang, Angew. Chem., 2011,
123, 3245–3250.
42 X. Sheng, Y. Zhou, Y. Zhang, M. Xue and Y. Duan, Chem. Eng.
J., 2012, 179, 295–301.
43 L. Wu, in Encapsulated Catalysts, Elsevier, 2017, pp. 1–33.
44 M. J. Verhoef, P. J. Kooyman, J. A. Peters and H. Van Bekkum, 68 Y. Zhang and S. N. Riduan, Chem. Soc. Rev., 2012, 41, 2083–
Microporous Mesoporous Mater., 1999, 27, 365–371. 2094.
45 N. Maksimchuk, M. Timofeeva, M. Melgunov, A. Shmakov, 69 L. Wang, H. Wang, F. Liu, A. Zheng, J. Zhang, Q. Sun,
Y. A. Chesalov, D. Dybtsev, V. Fedin and O. Kholdeeva, J.
Catal., 2008, 257, 315–323.
J. P. Lewis, L. Zhu, X. Meng and F. S. Xiao, ChemSusChem,
2014, 7, 402–406.
46 M. G. Clerici and O. A. Kholdeeva, Liquid phase oxidation via 70 F. Liu, X. Meng, Y. Zhang, L. Ren, F. Nawaz and F.-S. Xiao, J.
heterogeneous catalysis: organic synthesis and industrial
applications, John Wiley & Sons, 2013.
47 O. A. Kholdeeva, in Frontiers of Green Catalytic Selective
Oxidations, Springer, 2019, pp. 61–91.
Catal., 2010, 271, 52–58.
71 S. R. Churipard, K. S. Kanakikodi, N. Jose and S. P. Maradur,
ChemistrySelect, 2020, 5, 293–299.
72 B. Tamami and H. Yeganeh, React. Funct. Polym., 2002, 50,
101–106.
48 V. Y. Evtushok, O. Y. Podyacheva, A. N. Suboch,
N. V. Maksimchuk, O. A. Stonkus, L. S. Kibis and 73 H. Wang, L. Fang, Y. Yang, R. Hu and Y. Wang, Appl. Catal.,
O. A. Kholdeeva, Catal. Today, 2020, 354, 196–203. A, 2016, 520, 35–43.
49 N. V. Maksimchuk, K. A. Kovalenko, S. S. Arzumanov, 74 S. P. Das, S. R. Ankireddy, J. J. Boruah and N. S. Islam, RSC
Y. A. Chesalov, M. S. Melgunov, A. G. Stepanov, V. P. Fedin
and O. A. Kholdeeva, Inorg. Chem., 2010, 49, 2920–2930.
50 R. Villanneau, A. Marzouk, Y. Wang, A. B. Djamaa, G. Laugel,
Adv., 2012, 2, 7248–7261.
75 W. Zhao, B. Ma, Y. Ding and W. Qiu, React. Kinet. Mech.
Catal., 2010, 102, 459–472.
A. Proust and F. Launay, Inorg. Chem., 2013, 52, 2958–2965. 76 F. Lefebvre, Chem. Commun., 1992, 756–757.
51 C. L. Marchena, R. A. Frenzel, S. Gomez, L. B. Pierella and 77 I. Kozhevnikov, K. Kloetstra, A. Sinnema, H. Zandbergen and
L. R. Pizzio, Appl. Catal., B, 2013, 130, 187–196.
H. v. van Bekkum, J. Mol. Catal. A: Chem., 1996, 114, 287–298.
52 Z. E. A. Abdalla and B. Li, Chem. Eng. J., 2012, 200, 113–121. 78 N. Narkhede, S. Singh and A. Patel, Green Chem., 2015, 17,
53 R. J. White, R. Luque, V. L. Budarin, J. H. Clark and
D. J. Macquarrie, Chem. Soc. Rev., 2009, 38, 481–494.
54 S. R. Churipard, P. Manjunathan, P. Chandra,
89–107.
79 M. Misono, in Studies in Surface Science and Catalysis,
Elsevier, 1993, vol. 75, pp. 69–101.
G. V. Shanbhag, R. Ravishankar, P. V. Rao, G. S. Ganesh, 80 T. M. Anderson and C. L. Hill, Inorg. Chem., 2002, 41, 4252–
A. Halgeri and S. P. Maradur, New J. Chem., 2017, 5745. 4258.
55 P. Manjunathan, M. Kumar, S. R. Churipard, 81 R. H. Adnan, G. G. Andersson, M. I. Polson, G. F. Metha and
S. Sivasankaran, G. V. Shanbhag and S. P. Maradur, RSC
Adv., 2016, 6, 82654–82660.
56 R. Xing, H. Wu, X. Li, Z. Zhao, Y. Liu, L. Chen and P. Wu, J.
Mater. Chem., 2009, 19, 4004–4011.
V. B. Golovko, Catal. Sci. Technol., 2015, 5, 1323–1333.
82 Y. Yu, B. Lu, X. Wang, J. Zhao, X. Wang and Q. Cai, Chem.
Eng. J., 2010, 162, 738–742.
83 S. P. Maradur, C. Jo, D. H. Choi, K. Kim and R. Ryoo,
ChemCatChem, 2011, 3, 1435–1438.
57 K. S. Kanakikodi, S. R. Churipard, A. Halgeri and
S. P. Maradur, Microporous Mesoporous Mater., 2019, 286, 84 Y. Zhang, S. Wei, F. Liu, Y. Du, S. Liu, Y. Ji, T. Yokoi,
133–140. T. Tatsumi and F.-S. Xiao, Nano Today, 2009, 4, 135–142.
58 S. R. Churipard, P. Manjunathan, P. Chandra, 85 C. Pirez, J.-M. Caderon, J.-P. Dacquin, A. F. Lee and
G. V. Shanbhag, R. Ravishankar, P. V. Rao, G. S. Ganesh, K. Wilson, ACS Catal., 2012, 2, 1607–1614.
A. Halgeri and S. P. Maradur, New J. Chem., 2017, 41, 86 D.-H. Lee, M. Choi, B.-W. Yu and R. Ryoo, Chem. Commun.,
5745–5751.
2009, 74–76.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 35988–35997 | 35997