Study of ethylbenzene oxidation over polymer-silica hybrid supported Co (II) and Cu (II) complexes

Article abstract of DOI:10.1016/j.cattod.2020.02.043

Polymer-silica hybrid support (SA-2-AMPS-SiO₂) was synthesized and Co (II), Cu (II) metals were loaded over hybrid support by incipient impregnation technique. Oxidation of ethylbenzene was carried out for testing catalytic activity of catalyst

Full text of DOI:10.1016/j.cattod.2020.02.043

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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Study of ethylbenzene oxidation over polymer-silica hybrid supported Co
(II) and Cu (II) complexes
Vasu Chaudhary, Sweta Sharma*
Department of Chemical Engineering and Technology, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, 221005, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Polymer
Impregnation
Oxidation
Reusability
Activity
Polymer-silica hybrid support (SA-2-AMPS-SiO2) was synthesized and Co (II), Cu (II) metals were loaded over
hybrid support by incipient impregnation technique. Oxidation of ethylbenzene was carried out for testing
catalytic activity of catalysts at different reaction conditions i.e. the mole ratio of ethylbenzene to TBHP and
reaction temperature. The main oxidation product was acetophenone along with benzaldehyde as the minor
product and the highest ethylbenzene conversion (83.65 %) was attained at a 1:4 mol ratio of ethylbenzene to
TBHP and temperature at 100 °C. The reusability of the catalysts was also determined and results showed that
they can be easily reused three times without much loss in their structural and catalytic properties.
1. Introduction
great interest and extremely concerned matter to synthesize highly
selective, efficient and environmentally friendly heterogeneous catalyst
Oxidation is recognized as one of the most important processes in
the organic chemical industry [1–4]. For the production of various bulk
and fine chemicals, development in the synthesis of highly active and
selective catalysts for the oxidation of aldehydes, ketone, alkenes, al-
cohols, and hydrocarbons is now of great interest from academic as well
as industrial viewpoint [5–9].
Liquid phase catalytic oxidation of ethylbenzene is a commercially
important reaction. Acetophenone and benzaldehyde are obtained as
major products in the presence of oxidizing agent and homogeneous/
heterogeneous catalysts, of these two, former has higher commercial
value [10–17]. Hence, it is desirable to increase the selectivity of
acetophenone from ethylbenzene oxidation. Acetophenone is a viscous
colorless liquid, used as an intermediate for the manufacturing of
pharmaceuticals, resins, drugs, alcohols, perfumes, esters, aldehydes
and flavoring agents in many drinks and solvents [18–23].
Initially, aromatic ketones were produced by the oxidation of aro-
matic compounds with homogeneous acid catalysts and inorganic oxi-
dizing agents (KMnO4 and K2Cr2O7), famously known as Friedel-Crafts
acylation process. However, this method generates highly toxic and
corrosive waste with poor selectivity of acetophenone and therefore
needs disposal [24–29]. Consequently, increasing concern for the green
environment has compelled the scientists and researchers to synthesize
effective catalysts for ethylbenzene oxidation using air or oxygen as an
oxidizing agent. Homogeneous catalysts require energy extensive pro-
cess which increases the overall production cost [30]. Therefore, it is of
for the production of aromatic ketones.
In heterogeneous catalysts, support facilitates the mechanical
strength to the catalyst along with an increase in an effective surface
area. A wide range of organic (polymer resins, ceramic materials,
clays), as well as inorganic (molecular sieves, silica, alumina, zeolites)
catalyst supports, have been used so far [31]. Nowadays, organic sup-
ports are widely used due to their promising features such as non-vo-
latility, non-toxicity, recyclability, insolubility and easy separation of
products. Also, polymer properties can be easily modified during its
synthesis by varying several parameters i.e. initiator, diluent and cross-
linker amount, agitation speed and polymerization temperature.
Therefore in the last few years; organic supports are attaining great
attention in the organic transformations (oxidation, epoxidation, and
hydrogenation) as they generate less waste than non-supported cata-
lysts, which makes the whole process more economical and eco-friendly
[32–37].
On the other hand, functionalized polymer-supported catalysts have
disadvantages like low chemical and thermal stability, swelling and
shrinking under the influence of temperature and several solvents,
whereas inorganic supported catalysts have the advantage of high sta-
bility and durability [38–40]. Immobilizing organic materials on to the
surface of inorganic supports (two or more integrating components
combine at the molecular level) has become the most popular way to
synthesize heterogeneous catalysts, famously known as organic-in-
organic hybrid materials. These materials were tailored specially to give
⁎ Corresponding author.
E-mail address: shweta.che@iitbhu.ac.in (S. Sharma).
https://doi.org/10.1016/j.cattod.2020.02.043
Received 15 November 2019; Received in revised form 14 February 2020; Accepted 29 February 2020
0920-5861/©2020PublishedbyElsevierB.V.
Pleasecitethisarticleas:VasuChaudharyandSwetaSharma,CatalysisToday,https://doi.org/10.1016/j.cattod.2020.02.043

V. Chaudhary and S. Sharma
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Fig. 1. Synthesis mechanism of the hybrid support.
desirable properties of the polymer as well as inorganic materials and
suppress the undesirable ones, finally leading to an advanced frame-
work with a wide range of applications [41–45]. The desired properties
of organic-inorganic hybrid materials are superior chemical, mechan-
ical, thermal and biological stability compared to their pure organic
and inorganic materials [46–49]. The commonly used inorganic mate-
rials include silica [50], layer silicate [51,52], carbon nanotube [53,54]
and CaCO3 [55–57].
Trimethylpentane (99 % purity, S. D. Fine Chemical); Copper acetate
(99 % purity, Sisco Research Laboratories Pvt. Ltd.); Cobalt acetate (99
% purity, S. D. Fine Chemical); Merrifield’s Peptide Resin (99 % purity,
Sigma Aldrich); Tert-Butyl hydroperoxide (TBHP; 99
% purity,
Spectrochem Pvt Ltd) were used as obtained. Other solvents were of
standard grade.
2.2. Synthesis of the polymer-silica hybrid support
L. Zhang et al. have presented the catalytic activity of polystyrene-
silica SBA-15 supported Ti metal catalysts in the epoxidation reactions
using TBHP or aqueous H2O2 as the oxidant and revealed that the use of
both inorganic silica and organic polymer were very active (catalyti-
cally) for the epoxidation of ethylbenzene with TBHP [58]. For the li-
quid-phase oxidation reactions, M. Islam et al. have synthesized
polymer anchored Cu (II) azo complex and described that this catalyst
can be reused more than 5 times with no significant loss in its catalytic
activity [59]. R. Wang et al. have defined an innovative way for im-
pregnation of Co tetraphenylporphyrins on Poly (4-vinyl-pyridine-co-
styrene)/SiO2 for ethylbenzene oxidation into acetophenone and the
catalyst showed the satisfactory catalytic activity [60]. Y. Zhang et al.
have synthesized Cu (II)-poly-4-vinyl-pyridine/SiO2 catalyst for ethyl-
benzene oxidation into acetophenone with molecular oxygen and at-
tained high catalytic activity with selectivity more than 98 % for the
transformation of ethylbenzene to acetophenone [61]. Therefore,
polymer-silica hybrid supported catalysts have presented the vast
considerations of the researchers for the acetophenone production from
ethylbenzene oxidation. As a result, it will be very promising to syn-
thesize recyclable polymer-silica hybrid supported metal catalysts for
the production of acetophenone with excellent activity and selectivity.
In the present investigation, polymer-silica hybrid supported metal
catalysts have been synthesized and characterized by BET, CHNS, SEM-
EDX, FT-IR and TGA techniques. Further, these catalysts were in-
vestigated for the liquid phase oxidation of ethylbenzene (solvent-free)
using TBHP as an effective oxidizing agent. All the catalysts have shown
excellent catalytic activity and selectivity for the production of acet-
ophenone as desired product. These catalysts combine the catalytic
property of crosslinked polystyrene polymer such as low cost, smaller
particle size with uniform size distribution, high porosity, high surface
area, high activity and selectivity and the characteristics of silica gel
(SiO2), such as high specific area, excellent mechanical property and
thermal stability and resistance to sintering of metal particles [62–64].
Merrifield’s peptide resin (chloromethylated polystyrene cross-
linked with 2% divinylbenzene; 50–100 mesh; Sigma Aldrich) was used
as the polymer support. In the initial step, this polymer was dissolved in
a mixture of tetraethyl ortho-silicate (TEOS), HCl and distilled water
[TEOS: HCl: H2O = 1: 6: 0.02 (volume ratio)] in presence of acetoni-
trile and stirred at 300 rpm for 20 h at 70 ºC followed by filtration and
drying to give chloromethylated polystyrene impregnated over SiO2
silica (denoted as CMPS-SiO2).
The functionalization was carried out with 2-aminomethyl pyridine
(2-AMP; Dipicolylamine) in presence of distilled water, followed by
stirring (at 70 ºC and 300 rpm for 18 h), filtration and drying. The
product synthesized was aminomethylated polystyrene embedded over
SiO2 (represented as 2-AMPS-SiO2). The final reaction was Schiff base
functionalization for improving catalytic activity. For this purpose,
salicylaldehyde was added to the 2-AMPS-SiO2 in the presence of
acetonitrile. After 20 h of stirring at 70 ºC, filtration and drying, the
final obtained product is salicylaldehyde-2-amino-methylated-poly-
styrene impregnated over SiO2 silica (called SA-2-AMPS-SiO2). Fig. 1
shows the synthesis mechanism of hybrid support. In the following
section of the paper, SAP-Si notation has been used in place of SA-2-
AMPS-SiO2 for the synthesized polymer-silica hybrid support.
2.3. Loading of Co (II) and Cu (II) complexes on hybrid support
For the synthesis of Co-Cu/SAP-Si catalyst via incipient impregna-
tion technique, loading of 5 wt% of cobalt acetate quadrahydrate and
5 wt% of copper acetate tetrahydrate were dissolved in 1 ml distilled
water separately. Next, 1 g of the hybrid support was added to this
solution and mixed thoroughly and the final product was obtained in
powder form, after drying in an oven for 6 h at 80 °C. In the same way,
monometallic Co/SAP-Si and Cu/SAP-Si catalysts (10 wt. %; 2 mmol)
were also synthesized. Fig. 2 shows the synthesis of polymer-silica hy-
brid supported metal catalysts.
2. Experimental studies
2.4. Catalyst characterization
2.1. Materials
Nitrogen adsorption and desorption isotherms at −196 K using
Ethylbenzene (99
%
purity, Avra synthesis Pvt Ltd); 2,2,4-
ASAP 2020 (Micromeritics, USA) were determined for the Brunauer
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Fig. 2. Schematic illustration of the synthesis of hybrid supported metal catalysts.
Emmett Teller (BET) surface area followed by degassing at 120 °C for
6 h before adsorption measurements. CHNS elemental analyzer (EURO
EA Elemental Analyzer) was used for the elemental analysis. The
structural morphology and characteristics of the samples were detected
by Scanning Electron Microscopy (SEM) method using SEM: EVO 18-
20-45 system.
In order to recognize the functional groups attached during the
support synthesis, a NICOLET-5700 (Thermo Electron Corporation)
Fourier-Transform Infrared (FT-IR) Spectrometer unit operating at a
resolution of 6 and 32 scans and between the wavenumber range of
300–4000 was used. Thermal analysis of the samples was done by
Thermal Gravimetric Analysis (TGA-50, Shimadzu; Thermogravimetric
Analyzer). Samples were dynamically heated from room temperature to
700 °C with a ramp rate of 10 °C/min.
2.5. Evaluation of catalytic performance
A small scale batch reactor was used for carrying out liquid phase
catalytic oxidation of ethylbenzene (Fig. 3). A 50 ml three-neck flask
was put in a temperature-controlled oil bath (silicone oil) along with a
reflux water condenser. In the following reaction, 30 mg catalyst and
ethylbenzene (1.0620 g, 10 mmol) were added to this reaction mixture
followed by the addition of TBHP (0.9012 g, 10 mmol). The reaction
mixture was heated 80 °C for 6 h with continuous stirring. The product
samples were collected at every half-an-hour time interval for analysis.
The products were analyzed by gas chromatography (GC; Thermo Sci-
entific, USA). The GC, equipped with a capillary column of length 30 m,
ID 0.25 mm and a flame ionization detector with N2 as the carrier gas,
was used for the separation of benzaldehyde and acetophenone. For
calculating the conversion of reactant and selectivity of the products,
iso-octane was used as the internal standard.
Fig. 3. Schematic representation of ethylbenzene oxidation experimental setup.
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Fig. 4. (a) Nitrogen sorption isotherm of the catalysts; (b) pore volume and pore size distribution of synthesized samples.
Table 1
Physicochemical properties of support as well as catalysts.
Catalysts
Surface area(m2/g)
Pore size (nm)
Pore volume(cc/g)
Elements (wt. %)
C
H
N
SA-2-AMPS-SiO2 (SAP-Si)
Cu-Co/SAP-Si (Cat-A)
Cu/SAP-Si (Cat-B)
61.315
55.340
48.862
44.119
9.610
6.438
6.830
6.951
0.368
0.089
0.083
0.079
79.981
39.639
36.917
34.557
4.010
4.433
3.887
3.288
3.107
1.432
1.811
0.981
Co/SAP-Si (Cat-C)
Fig. 5. (a) Scanning Electron Micrograph (SEM) of the synthesized SAP-Si support; (b) particle surface morphology at high resolution by SEM, and (c) elemental
analysis by EDX method.
3. Results and discussion
BET study proved that a reduction in surface area was observed in the
catalysts when metals were dispersed onto the support, which may be
due to the blocking of surface pores by metal ions. Monometallic cat-
alysts (Cat-B and Cat-C) have lower surface area than bimetallic catalyst
(Cat-A), as there is a synergistic effect between two metals. The size of
pores was reported in the range of 6.951-9.610 nm and pore volumes
were 0.079-0.368 cc·g−1 (Fig. 4(b)). The textural and surface properties
of support and catalysts are concised in Table 1. And showed that with
bimetallic loading, the pore size (6.438 nm) decreases gradually, which
provides an opportunity for tuning the pore size of mesoporous mate-
rials and the surface properties at the same time. Thus, the BET study
confirmed the presence of mesopores in the samples with very little
pore blocking and suggesting the bimetallic loading.
The textural properties of the hybrid supported catalysts were re-
presented by nitrogen adsorption-desorption measurements (Fig. 4(a)).
All the catalysts showed type IV isotherms with steep H-1 hysteresis
loops at relative pressure p/po around 0.4-0.9, which is the distinctive
property of material with uniform mesopore diameter and shows there
is no pore blocking in the samples. As we move from bottom to top in
the graph, type IV isotherm moves from 0.4-0.9 (for Cat-C), 0.43-0.9
(Cat-B) and 0.46-0.9 (Cat-A), which may be due to different metal
loadings, suggesting that a considerable proportion of the mesopores
were filled with the metals.
The support exposes a good surface area of 61.315 m2·g−1 with a
pore size and pore volume of 9.610 nm and 0.368 cc·g−1, respectively.
The C, H and N content of all the samples was calculated by the
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Fig. 6. Physical properties and surface topology of the (A) Cat-A; (B) Cat-B; (C) Cat-C by SEM and EDX method; (a) Scanning Electron Micrograph (SEM) of the
synthesized catalyst particles; (b) particle surface morphology at high resolution by SEM, and (c) elemental analysis by EDX method.
CHNS analyzer, which signifies the elemental composition of the syn-
thesized samples (Table 1). Also, it has been indicated that C, H and N
content is present in all the catalysts, as in the support, ever after the
impregnation of different metals, therefore, suggesting the metal
loading over the synthesized hybrid support.
shows a surface image of the support particles surface at lower mag-
nification value.
Though, when we analyzed the SEM images of all the catalysts, we
found that catalytic particles have been distributed over the surface of
hybrid support (Fig. 6). Cat-C has more particles of irregular-shaped
than Cat-A and Cat-B, while, Cat-B has smaller particles than Cat-A and
Cat-C as all three were analyzed at the same magnification (Fig. 6(b),
(c) and (d)). All the catalysts have particles of spherical-shape, very
fewer particles are of curvy nature with the broken surface. Also, it is
clear from the SEM images that Cat-B has more uniform particle dis-
tribution than Cat-A and Cat-C. Fig. 6(A (b)), (B (b)) and (C (b)) are
representing the surface of the catalyst particles at higher magnifica-
tions.
Energy Dispersive X-Ray Analysis (EDX) was used for the analysis of
the composition of synthesized SAP-Si support and catalysts. EDX re-
sults of SAP-Si (Fig. 6(a)) successfully confirmed the existence of silica,
carbon, nitrogen, and oxygen in the synthesized polymer-silica matrix.
The EDX spectra of all the catalyst materials Cat-A, Cat-B and Cat-C
3.1. Surface morphology (SEM)
Particle surface structure, morphology, and topology of support, as
well as all catalysts synthesized by the incipient impregnation, were
revealed by scanning electron microscopy (SEM). Fig. 5(a) shows the
random configuration of polymer particles and irregular large size
particle agglomeration. Although, it is interesting to observe evident
changes in the morphology of the support which is due to the im-
mobilization of Co (II) and Cu (II) metals on the support, but it is
verified that physical structure and texture of hybrid support SAP-Si has
been preserved after impregnation, hence, proposing the immobiliza-
tion of these metals onto the surface of the hybrid support. Fig. 5(b)
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Table 3
Thermogravimetric studies and weight loss calculations for all synthesized
samples.
Samples Weight loss (wt. %)
200−350 °C 350−450 °C 450−550 °C 550−700 °C Total
weight
loss (wt.
%)
SAP-Si
Cat-A
Cat-B
Cat-C
21.06
13.22
17.26
6.57
59.17
33.33
28.88
27.16
12.95
13.19
8.43
14.66
26.28
12.82
28.91
107.84
88.02
67.39
96.01
33.37
Fig. 7. Analysis of functional groups of (a) SAP-Si support; (b) Cat-A; (c) Cat-B
and (d) Cat-C by FT-IR method.
Table 2
Spectroscopic data for support as well as catalysts.
FT-IR bands (cm−1
)
SAP-Si
Cat-A
Cat-B
Cat-C
υ (C = N)
υ (C = C)
υ (C–H)
1550
1450
2900
3450
1590
1425
2900
—
1590
1425
2900
—
1590
1425
2900
—
υ (O-H)
Fig. 9. (a) Conversion vs time for ethylbenzene and (b) products selectivity; at a
different mole ratio of ethylbenzene to TBHP for Cat-A (30 mg) at 100 °C
temperature.
Fig. 8. Thermogravimetric analysis of (a) SAP-Si support; (b) Cat-A; (c) Cat-B
and (d) Cat-C by TGA method.
IR measurement of hybrid support (SA-2-AMPS-SiO2) depicts some
characteristic groups which are directly related to the functionalized
copolymers (SA-2-AMPS) on silica (Fig. 7(a)); the band at 1550 cm−1 is
correlated with the skeletal vibration of benzene and the vibration of
C]N in pyridine groups. A motion of type C]C vibration appears
above 1600 cm−1 [65], on the other hand, vibrations between 1400
and 1500 cm-1 are due to methylene scissor deformations [66,67]. The
bands at 2900 cm−1 and 1502 cm−1 are the absorptions of CeH in
benzene, the band at 700 cm−1 represent the mono-substituted benzene
and broadband at about 3450 cm−1 is a characteristic absorption of the
stretching phenolic OeH unit of the reacted salicylaldehyde (Fig. 7(a)).
These characteristic groups are also present in all the synthesized
presented the presence of cobalt and copper metals, thus confirming the
metal dispersion (Fig. 6(b), (c) and (d)). The elemental analysis re-
ported in the EDX analysis was made on a small portion of the sample,
supposing an even composition throughout the sample. Furthermore,
silica and carbon were preserved in catalysts even after the impregna-
tion.
3.2. Spectroscopic studies (FT-IR)
Fig. 7 shows the FT-IR spectrum of all the synthesized samples. FT-
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and this is due to the rupture of the organic moiety. For support, less
weight loss observed in the temperature range of 100−200 °C, after
that it decreases up to 450 °C, again very less weight loss occurred from
450 °C to 700 °C, which maybe because of the degradation of the
polymer network.
However, there are considerable changes between the TGA curves of
support and catalysts. After the addition of transition metals, shifting in
the TGA curves of catalysts was observed (Fig. 8(b), (c) and (d)) and
this is due to the decomposition of the polymer phase. Variations in the
TGA curves between support and catalysts showed something about the
interactions. It is highly probable that the presence of catalysts will
change the decomposition of the polymer too, which is clearly re-
presented in the TGA diagram.
The results clearly showed that supported catalysts have higher
thermal stability and a slower degradation rate than support. Among all
the catalysts, Cat-C (Fig. 8(c)) has higher thermal stability than Cat-A
and Cat-B. However, the degradation rate is higher in the case of Cat-C
however it is very smooth for Cat-A and Cat-B, thus, we would optimize
that catalyst which will have higher thermal stability with even de-
gradation rate i.e. Cat-A. Hence, after metal loading, thermal stability is
enhanced significantly and this is very essential for the catalyst appli-
cation.
4. Solvent-free oxidation of ethylbenzene with TBHP
The polymer-silica hybrid anchored metal catalysts are the most
proficient catalysts for ethylbenzene oxidation. Acetophenone (main
product) and benzaldehyde were obtained during the oxidation reac-
tion. Variables such as reactant to oxidant mole ratio and reaction
temperature were selected to optimize the reaction conditions for the
maximum oxidation of ethylbenzene and selectivity of desired product
acetophenone. Initially, solvent-free catalytic oxidation was carried out
using Cat-A and TBHP as the oxidant at 100 °C. Oxidation reaction
analysis was calculated in terms of ethylbenzene conversion, the se-
lectivity of acetophenone and benzaldehyde, activation energy of the
reaction and reusability. The conversion and selectivity are calculated
as follows:
Fig. 10. (a) Conversion vs time for ethylbenzene and (b) products selectivity; at
different reaction temperature for Cat-A (30 mg) at 1:4 mol ratio of ethylben-
zene to TBHP.
initial moles − final moles
Conversion (% )=
SelectivityA(% )=
× 100
initial moles
samples (Fig. 7(a), (b), (c) and (d)).
moles of product A
In all the catalysts, a new peak observes at 1357 cm−1 which may be
due to the variation in CeO stretching and coordinating of Cu+2
(Fig. 7(b) and 7 (c)) and Co+2 ions (Fig. 7(d)) with the phenolic oxygen,
however, the sharpness of this peak is decreasing as we move towards
up in the spectra. The weak absorption peak at 675 cm−1 recognized for
Co-N bond interaction with Co+2 ions (Fig. 7(b) and 7 (d)), the bands at
970 are because of the Si-O-Si vibrations in Cu metal incorporated
support [68]. One absorption band at regions 1050 cm−1 ascribed to
the asymmetric stretching of Si-O-Si of the silicate network in all the
samples [65,66]. Table 2 shows the spectroscopic data of all the sam-
ples analyzed by FT-IR spectroscopy.
× 100
total moles of products
4.1. Effect of mole ratio of ethylbenzene to TBHP
The effect of the mole ratio of ethylbenzene to TBHP on the ethyl-
benzene conversion and product selectivity was calculated by changing
the mole ratios from 1:1 to 1:5 at 100 °C for Cat-A. The experiments
revealed that for all the mole ratios of ethylbenzene to TBHP, ethyl-
benzene conversion increases with time. Overall ethylbenzene conver-
sion was increased from 58.92 % (1:1) to 83.65 % (1:4) and then de-
creases to 80.56 % (1:5) after 6 h of reaction (Fig. 9(a)). There was a
slight change in the final conversion from 1:3 (78.57 %) to 1:4 (83.65
%) and 1:5 (80.56 %), though; there was a major change when the ratio
was changed from 1:1 (58.92) to 1:2 (63.73 %) and further1:3 (78.57
%).
As TBHP acts as an oxidizing agent, it activates the catalyst by
providing oxygen atoms. At the same time, when ethylbenzene to TBHP
mole ratios increased from 1:1 to 1:3, more oxidant density (number of
oxygen atoms per unit volume of the reaction mixture) was provided
and conversion increased. However, when this ratio was increased from
1:3 to 1:4, there was a slight increment in conversion, this may be due
to the reason that less oxidant density (thus oxidant amount) is needed
for further conversion (after 1:3). However, when this ratio was in-
creased further from 1:4 (83.65 %) to 1:5 (80.56 %), conversion again
All the above explanations reveals that the coordination between N
atoms on the pyridine ring of Schiff-base functional group in support
with Co2+and Cu+2 ions has been occurred (Fig. 2). Therefore, from IR
spectral studies, the presence of characteristic bands of the support in
the catalysts confirmed the impregnation of Co (II) and Cu (II) metals
over hybrid support.
3.3. Thermal analysis (TGA)
Thermal analysis of the synthesized support and catalysts is estab-
lished via TGA curves (Fig. 8). Four separate steps can be distinguished:
at 200–350, 350–450, 450–550 and above 550 °C (Table 3). From the
TGA curve, it can be seen that support, as well as all the catalysts,
expose dramatic weight loss in the temperature range of 100−700 °C,
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Fig. 11. (a) Ethylbenzene conversion; (b) Acetophenone and (c) Benzaldehyde selectivity for all the three catalysts (30 mg) at 100 °C reaction temperature and
1:4 mol ratio of ethylbenzene to TBHP.
4.2. Effect of reaction temperature
Table 4
Catalytic activity of polymer-silica hybrid supported catalysts for ethylbenzene
oxidation at optimized reaction conditions.
The reaction temperature is an important influencing factor due to
which there may be considerable changes in catalytic performance and
structural properties of a catalyst. The temperature effect on ethyl-
benzene oxidation was considered in a range of 80−120 °C at ethyl-
benzene to TBHP mole ratio 1:4 for Cat-A. The results revealed that as
temperature increases the conversion of ethylbenzene increased with
time. Only 61.02 % conversion was obtained at 80 °C temperature
however it was increased to 73.13 % (120 °C) as shown in Fig. 10(a). At
100 °C, the maximum conversion of ethylbenzene and maximum se-
lectivity of acetophenone were 83.65 % and 87.23 %, respectively.
This may be explained by the following reason, as at lower tem-
peratures, the concentration of ethylbenzene molecules is constant at
the surface of a heterogeneous catalyst and when the temperature is
increased, ethylbenzene molecules vibrate faster and hence successful
collisions take place between the molecules. As a result, acetophenone
production is found to be in a greater quantity in 6 h. This occurs when
the temperature is raised from 80−100 °C, but when the temperature is
increased from 100−120 °C, acetophenone selectivity decreased
(Fig. 10(b)). This may happen due to more increment in temperature,
collisions of reactant molecules increased in a greater quantity. But
there may be more ineffective collisions in reactant molecules than
effective collisions, which then leads to lower production of acet-
ophenone. And, we need higher selectivity of acetophenone as the de-
sired product, thus, 100 °C temperature was selected for further studies.
Catalytic evaluation
Catalysts
Cat-A
Cat-B
Cat-C
Ethylbenzene conversion (%)
Acetophenone selectivity (%)
Benzaldehyde selectivity (%)
83.65
87.23
12.77
80.01
81.27
18.73
73.82
75.14
24.86
diminished, which means no more oxidant amount was needed and
therefore, we can say this is the saturated oxidant amount for this
catalyst at 100 °C and conversion has been reached to its maximum
value i.e. 83.65 % at a 1:4 ratio of ethylbenzene to TBHP (Fig. 9(a)).
Initially, benzaldehyde was attained as the major product and its
selectivity decreases and of acetophenone increases with an increase in
ethylbenzene to TBHP mole ratios and reaction time. This may be due
to the selective nature of this heterogeneous catalyst towards acet-
ophenone. After 6 h, acetophenone, with a significant selectivity of
83.19 %, was reported as a major reaction product at a 1:3 mol ratio
(Fig. 9(b)). At 1:4 mol ratio, selectivity (87.23 %) was increased to a
small extent which shows a lesser amount of oxidant is required at
100 °C. Next, at 1:5 mol ratio again acetophenone selectivity hampered,
thus the catalyst (selective for acetophenone at 1:4 mol ratio) didn’t
require more oxidizing agent. As maximum ethylbenzene conversion
and acetophenone selectivity was achieved at 1:4 mol ratio of ethyl-
benzene to TBHP, therefore, all other catalytic parameters were esti-
mated at this ratio.
8

V. Chaudhary and S. Sharma
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Fig. 12. (a) First order liner fit for the reaction over catalyst Cat-A at different temperatures 80 ºC (353 K), 100 ºC (373 K), and 120 ºC (393 K); (b) Arrhenius plot and
calculation of activation energy for Cat-A.
TBHP = 40 mmol (3.6048 g), temperature = 100 °C, ethylbenzene:
TBHP = 1:4, catalyst amount = 30 mg, reaction time = 6 h
Table 5
Reaction rate constant (K) for ethylbenzene oxidation.
This extent of ethylbenzene conversion may be due to these two
major reasons; first is the synergistic effect of bimetallic catalyst Cat-A,
as synergistic effect between two metals increase the catalytic rate than
a monometallic catalyst (Cat-B and Cat-C) and structural changes in
catalysts morphology after metal loading in the case of Cat-B and Cat-C.
Also, every metal exposed a different outcome in the ethylbenzene
oxidation, due to variable oxidation states of Cu (+1, +2), Co (+2,
+3) and of variable valences, the electronegativity of these two metals
is in the increasing order is as follows: Co < Cu. Thus, there is a strong
synergistic effect between Cu and Co metals in comparison to Co and Cu
in the case of ethylbenzene oxidation [71]. As a result, it can be con-
cluded that the synergistic effect between Cu and Co greatly enhances
the catalytic efficiency of Cat-A. Hence from the characterization
methods and performance of the oxidation reaction, Cat-A is found to
be the more proficient, effective and selective catalyst for the produc-
tion of acetophenone from ethylbenzene oxidation (Fig. 11).
Catalyst
Cat-A
Temperature (K)
Rate constant (min−1
)
R2
353
373
393
373
373
284.29
526.58
867.76
393.04
351.19
0.95
0.99
0.98
0.96
0.95
Cat-B
Cat-C
Table 6
Kinetic parameters of oxidation reaction at optimized reaction conditions for
Cat-A.
Kinetic parameters
Cat-A
Rate constant (K)
526.58 min−1
37.02 kJ mol−1
33.92 kJ.mol−1
−90.94 J.mol-1 K-1
Activation energy (Ea) (K = Ko*exp(−Ea/RT)
)
Enthalpy of activation (ΔH)(ΔH = Ea − nRT)
Entropy of reaction (ΔS)(ΔS = R[ln A − ln(KT ) − n]
)
h
4.4. Kinetic study
4.3. Optimized reaction conditions
The kinetic study of the reaction was done for Cat-A at the favorable
reaction conditions in the temperature ranges of 80–100 ºC. A plot of -ln
(1-XA) versus reaction time (Fig. 12(a)) signifies the pseudo-first-order
of the reaction. Next, the rate constant (K) at different temperatures (T)
was calculated from the slope of the linear curve (Table 5). The acti-
vation energy was obtained from the Arrhenius law for Cat-A. The ex-
pected energy was calculated from the graph of ln (K) vs (1/T)
(Fig. 12(b)) and was found to be 37.02 kJ mol−1 i.e. the reaction is of
endothermic nature (+ve value of Ea) and the rate is controlled by the
chemical step. Other kinetic parameters were calculated by carrying out
the reaction at 100 ºC for 6 h with 30 mg of Cat-A catalyst.
Thus, most favorable reaction conditions for ethylbenzene oxidation
have been achieved at ethylbenzene to TBHP mole ratio of 1:4, reaction
temperature 100 °C with 30 mg catalyst, when 10 mmol of ethylbenzene
and 10 mmol of TBHP has been used in the beginning. At these opti-
mized reaction parameters, maximum ethylbenzene conversion was
83.65 % and selectivity of acetophenone and benzaldehyde were 87.23
% and 12.77 % respectively in 6 h. At these conditions, the selectivity of
products is shown in Fig. 11(b). At 100 °C, 30 mg catalyst and 1:4 mol
ratio of ethylbenzene to TBHP, acetophenone selectivity increased
whereas benzaldehyde selectivity decreased with time.
Under these favorable reaction conditions, Cat-B and Cat-C were
also investigated for the same reaction and their results are summarized
in Table 4. The results showed that each catalyst exposed a very good
catalytic activity and selectivity towards acetophenone. In the first
three hours of reaction time, Cat-B and Cat-C give almost the same
conversion. After 3 h, as temperature increased, Cat-A shows higher
conversion than others, which is increased continuously up to 6 h. Thus,
Cat-A showed higher catalytic activity for ethylbenzene conversion
(83.65 %) than Cat-B (80.01 %) and Cat-C (73.82 %). Also, the acet-
ophenone selectivity (87.23 %) is greater and benzaldehyde selectivity
(12.77 %) is lower in the case of Cat-A than the other two catalysts
(Fig. 11).
The enthalpy of activation (ΔH) was calculated as 33.92 kJ.mol−1
i.e. it represents the difference in energy between the ground state and
the transition state in the following oxidation reaction. The higher the
activation enthalpy, the more energy is required for the products to
form. The entropy of the reaction (ΔS) was calculated as -90.94 J mol-
1 K-1. All the calculated kinetic parameters were summarized in Table 6
for optimized catalyst Cat-A.
4.5. Proposed reaction mechanism
Ethylbenzene oxidation generally follows the free radical me-
chanism [72,73]. During the synthesis, adsorption of TBHP is done by
electron-deficient M+2 species and thus OeO bond in TBHP will be
activated and due to which t-butyl oxygen radicals and hydroxyl
Reaction
conditions:
ethylbenzene = 10 mmol
(1.0620 g),
9

V. Chaudhary and S. Sharma
CatalysisTodayxxx(xxxx)xxx–xxx
Scheme 1. Proposed free radical reaction mechanism of the ethylbenzene oxidation with TBHP (Cat-A).
radicals will generate. When these radicals will react with ethylbenzene
molecules α-ethylbenzene radicals will be formed, which then give
acetophenone and benzaldehyde after reacting with t-butyl oxygen
radicals. Scheme 1 represents the proposed reaction mechanism of
recovery and reusability, all catalysts after the experiment was sepa-
rated from the reaction mixture by simple filtration and continuously
washed with hot acetonitrile to remove adsorbed organic compounds
followed by drying at 70 °C. By performing three consecutive runs, the
recyclability of the catalyst was assessed (Fig. 13) and found that the
catalyst could be recycled three times with a slight decrease in con-
version and selectivity.
Fig. 14 shows the surface morphology and structure of the catalysts
which are recycled three times for ethylbenzene oxidation. When we
compare SEM images and elemental analysis by EDX of both fresh
(Fig. 6) and used catalysts (Fig. 14), we found that there are not many
changes and deformations in the structures of catalytic particles. Also,
from the EDX analysis table, it is confirmed that all elements are present
in the sample. Thus the preservation of the original structure is there
ethylbenzene oxidation over a hybrid-supported metal catalyst (M+2
-
Cu/SAP-Si; where, M+2=Co+2) in the presence of TBHP. Further, the
reusability of all the catalysts was tested for the same oxidation reaction
and found that Cat-A gives the best result.
4.6. Activity of spent catalysts
For the estimation of the catalytic performance of a heterogeneous
catalyst, reusability is a very significant parameter and it can be
achieved by studying the recyclability of the catalyst. To determine the
10

V. Chaudhary and S. Sharma
CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 13. (a) Ethylbenzene conversion; (b) Acetophenone and (c) Benzaldehyde selectivity for the reusability test of all the catalysts over 3 runs.
Fig. 14. Scanning electron microscopy (SEM) and EDX analysis of used catalysts; (a) Cat-A; (b) Cat-B and (c) Cat-C.
when reusing three times, which is usually required. The reusability
5. Conclusions
study confirmed the stability of the catalyst and shown that catalysts
could be used several times without any major loss in catalytic activity.
After recycling the study of all the three catalysts, it is found that Cat-A
gives better results when we compare them in all aspects. In addition,
compared with other reported catalysts in the literature (Table 7), Cat-
A behaves in a comparable way or even better in most of the cases,
under mild reaction conditions, which is the novelty of the present
research work [74,75].
All catalysts were synthesized by incipient impregnation technique
and characterized by BET, CHNS, SEM-EDX, FT-IR and TGA methods.
Further, these catalysts were studied for the ethylbenzene oxidation
(solvent-free) at different reaction conditions with formation of acet-
ophenone as major reaction product.
Optimized reaction conditions are as follows; ethylbenzene to TBHP
•
mole ratio is 1:4, the reaction temperature is 100 °C with 30 mg of
catalyst amount
11

V. Chaudhary and S. Sharma
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Cat-A was reported to be highly capable among all the catalysts with
83.65 % conversion of ethylbenzene and 87.23 % acetophenone
selectivity at these optimum values of reaction conditions.
•
•
In addition, the activation energy, enthalpy of activation and en-
tropy of the oxidation reaction were calculated i.e. 37.02 kJ mol−1
33.92 kJ.mol-1 and -90.94 J mol-1 K-1 respectively.
,
CRediT authorship contribution statement
Vasu Chaudhary: Writing original draft, Conceptualization,
-
Methodology, Software. Sweta Sharma: Supervision, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
The authors thank Indian Institute of Technology (Banaras Hindu
University), Varanasi for the financial support, instrumentation and
characterization facilities. Although this research did not receive any
specific grant from funding agencies in the public, commercial, or not-
for-profit sectors.
References
[1] G.I. Panov, K.A. Dubkov, E.V. Starokon, Catal. Today 117 (2006) 148–155.
[2] K. Herbst, G. Mogensen, F. Huber, M. Ostberg, M.S.S. Rasmussen, Catal. Today 157
(2010) 297–302.
[3] P.L. Mills, R.V. Chaudhari, Catal. Today 48 (1999) 17–29.
[4] J.S. Rafelt, J.H. Clark, Catal. Today 57 (2000) 33–44.
[5] Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang, Y. Yang, Chem. Soc. Rev. 43 (2014)
3480–3524.
[6] M.S. Kamal, S.A. Razzak, M.M. Hossain, Atmos. Environ. 140 (2016) 117–134.
[7] V. Chaudhary, Sweta, Chin. J. Chem. Eng. 26 (2018) 1300–1306.
[8] V. Chaudhary, Sweta, IJSER 10 (2016) 1743–1748.
[9] I.A. Ike, T. Karanfil, J. Cho, J. Hur, Water Res. 164 (2019) 114929.
[10] Y. Yang, W. Zhong, B. Nie, J. Chen, Z. Wei, X. Liu, J. Organomet. Chem. 853 (2017)
136–142.
[11] S. Jie, X. Lin, Z. Chao, Z. Liu, Mater. Chem. Phys. 214 (2018) 239–246.
[12] L. Liu, J. Li, H. Zhang, L. Li, P. Zhou, X. Meng, M. Guo, J. Jia, T. Sun, J. Hazard.
Mater. 362 (2019) 178–186.
[13] X. Lin, S. Zhao, Y. Chen, L. Fu, R. Zhu, Z. Liu, J. Mol. Catal. A Chem. 420 (2016)
11–17.
[14] R. Xie, G. Fan, L. Yang, F. Li, Chem. Eng. J. 288 (2016) 169–178.
[15] A.P.S. Oliveira, I.S. Gomes, A.S.B. Neto, A.C. Oliveira, J.M. Filho, G.D. Saraiva,
J.M. Soares, S.T. Cuapa, Mol. Catal. 436 (2017) 29–42.
[16] B.R. Jermy, S.Y. Kim, D.K. Kim, D.W. Park, J. Ind, Eng. Chem. 17 (2011) 130–137.
[17] L. Yang, B. Fan, X. Cui, X. Shi, R. Li, J. Ind, Eng. Chem. 21 (2015) 689–695.
[18] E.A. Sujandi, D.S. Prasetyanto, S.C. Han, S.E. Lee, Park, Catal. Today 141 (2009)
374–377.
[19] M. Arshadi, M. Ghiaci, A. Rahmanian, H. Ghaziaskar, A. Gil, Appl. Catal. B: Environ.
119 (2012) 81–90.
[20] V.R. Choudhary, J.R. Indurkar, V.S. Narkhede, R. Jha, J. Catal. 227 (2004)
257–261.
[21] B. Gutmann, P. Elsner, D. Roberge, C.O. Kappe, ACS Catal. 3 (2013) 2669–2676.
[22] C. Yang, L. Fu, R. Zhub, Z. Liu, Phys. Chem. Chem. Phys. 18 (2016) 4635–4642.
[23] R. Xie, G. Fan, L. Yang, F. Li, Catal. Sci. Technol. 5 (2015) 540–548.
[24] C. Bernardon, M.B. Osman, G. Laugel, B. Louis, P. Pale, C. R, Chimie 20 (2017)
20–29.
[25] J.C. Kim, K. Cho, S. Lee, R. Ryoo, Catal. Today 243 (2015) 103–108.
[26] T. Yamazaki, M. Makihara, K. Komura, J. Mol. Catal. A Chem. 426 (2017) 170–176.
[27] E.M. Seo, L.S. Tan, J.B. Baek, Adv. Mater. 29 (2017) 1606317–1606324.
[28] X. Jin, A. Wang, H. Cao, S. Zhang, L. Wang, X. Zheng, X. Zheng, Res. Chem.
Intermed. 44 (2018) 5521–5530.
[29] R. Aleixo, R.E. Leitao, F. Martins, A.P. Carvalho, A. Brigas, A. Martins, N. Nunes,
Mol. Catal. 434 (2017) 175–183.
[30] Q.H. Xia, H.Q. Ge, C.P. Ye, Z.M. Liu, K.X. Su, Chem. Rev. 105 (2005) 1603–1662.
[31] I. Fechete, Y. Wang, J.C. Vedrine, Catal. Today 189 (2012) 2–27.
[32] V. Chaudhary, S. Sharma, J. Porous Mater. 24 (2017) 741–749.
[33] D.R. Godhani, H.D. Nakum, D.K. Parmar, J.P. Mehta, N.C. Desai, J. Mol. Catal. A
Chem. 415 (2016) 37–55.
[34] C. Hulubei, C.D. Vlad, I. Stoica, D. Popovici, G. Lisa, S.L. Nica, A.I. Barzic, J. Polym.
Res. 21 (2014) 514–529.
[35] V. Chaudhary, S. Sharma, J. Polym. Res. 26 (2019) 102–113.
12

V. Chaudhary and S. Sharma
CatalysisTodayxxx(xxxx)xxx–xxx
[36] V. Chaudhary, S. Sharma, J. Porous. Mater. 6 (2019) 1559–1571.
[37] Q. Liu, L. Wang, A. Xiao, H. Yu, Des. Monomers Polym. 13 (2010) 369–375.
[38] A.M.J. Siouffi, J. Chromatogr. A 1000 (2003) 801–818.
[39] M. Kralik, A. Biffis, J. Mol. Catal. A Chem. 177 (2001) 113–138.
[40] Z. Mehraban, F. Farzaneh, A. Shafiekhani, Opt. Mater. 29 (2007) 927–931.
[41] L. He, C.S. Toh, Anal. Chim. Acta 556 (2006) 1–15.
[42] J. Mu, Y. Liu, S. Zheng, Polymer 48 (2007) 1176–1184.
[43] J. Zheng, G. Li, X. Ma, Y. Wang, G. Wu, Y. Cheng, Sens. Actuators B Chem. 133
(2008) 374–380.
J. Inorg, J. Inorg. Organomet. Polym. 20 (2010) 87–96.
[60] R. Wang, B. Gao, W. Jiao, Appl. Surf. Sci. 255 (2009) 4109–4113.
[61] B. Gao, D. Kong, Y. Zhang, J. Mol. Catal. A Chem. 286 (2008) 143–148.
[62] S.D. Hua, J.L. Tao, L.Z. Gang, S.W. Bin, G.C. Cheng, J. Mol. Catal. A Chem. 379
(2013) 15–20.
[63] K.A.D.F. Castro, S.M.G. Pires, M.A. Ribeiro, M.M.Q. Simoes, M.G.P.M.S. Neves,
W.H. Schreiner, F. Wypych, J.A.S. Cavaleiro, S. Nakagaki, J Colloid Interf. Sci. 450
(2015) 339–352.
[64] K.B. Vu, L.G. Bach, T.V. Tran, N.T. Thuong, H.N. Giang, Q.T.P. Bui, S.T. Ngo, Chem.
[44] N. Ostapenko, G. Dovbeshko, N. Kozlova, S. Suto, A. Watanabe, Thin Solid Films
516 (2008) 8944–8948.
[45] J.E. Mark, Acc. Chem. Res. 39 (2006) 881–888.
[46] Y.Y. Yu, C.Y. Chen, W.C. Chen, Polymer 44 (2003) 593–601.
[47] P.S. Calefi, E.J. Nassar, C.R. Neri, O.A. Serra, J. Non-Cryst. Solids 247 (1999)
120–123.
[48] S. Hofacker, M. Mechtel, M. Mager, H. Krans, Prog. Org. Coat. 45 (2002) 159–164.
[49] T. Ogashi, H. Itoh, K.M. Jim, Y. Chujo, Macromol 35 (2002) 334–338.
[50] H. Zou, S.S. Wu, Shen. J. Chem. Rev. 108 (2008) 3893–3957.
[51] L.A. Goettler, K.Y. Lee, H. Thakkar, Polym. Rev. 47 (2007) 291–317.
[52] A. Okada, A. Usuki, Macromol. Mater. Eng. 291 (2006) 1449–1476.
[53] S.L. Shi, L.Z. Zhang, J.S. Li, J. Polym. Res. 16 (2009) 395–399.
[54] P. Garg, B. Singh, G. Kumar, T. Gupta, I. Pandey, R. Seth, R. Tandon, R. Mathur, J.
Polym. Res. 18 (2010) 1397–1407.
Phys. Lett. 728 (2019) 80–86.
[65] J.R. Beattie, S.E.J. Bell, B.W. Moss, Lipids 39 (2004) 407–419.
[66] L.F.L. Silva, W.P. Jr, G.S. Pinheiro, J.G. da, S. Filho, P.T.C. Freire, F.F. de Sousa,
S.G.C. Moreira, Cryst. Eng. Comm. 21 (2019) 297–309.
[67] L.F.L. da Silva, T.A. Filho, P.T.C. Freire, J.M. Filho, J.G. da Silva Filho, G.D. Saraiva,
S.G.C. Moreira, F.F. de Sousa, J. Phys. Chem. A 12 (2017) 4830–4842.
[68] M. Grochowicz, A. Kierys, Polym. Degrad. Stab. 138 (2017) 151–160.
[71] F.C. Buciuman, F. Patcas, T. Hahn, Stud. Surf. Sci. Catal. 138 (2001) 315–322.
[72] K. George, S. Sugunan, Catal. Comm. 9 (2008) 2149–2153.
[73] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, J. Catal. 267 (2009) 1–4.
[74] Wang R, B. Gao, W. Jiao, Appl. Surf. Sci. 255 (2009) 4109–4113.
[75] B. Gao, D. Kong, Y. Zhang, J. Mol, Catal. A: Chem. 286 (2008) 143–148.
[76] M.R. Maurya, A. Arya, P. Adaob, J.C. Pesso, Appl. Catal. A Gen. 351 (2008)
239–252.
[55] M.E.E. Maurizio Avella, G. Gentile, Macromol. Symp. 247 (2007) 140–146.
[56] X.K. Ma, B. Zhou, Y.H. Deng, Y. Sheng, C.Y. Wang, Y. Pan, Z.C. Wang, Colloids Surf.
A. 312 (2008) 190–194.
[57] X. Chen, J. Yu, M. He, S. Guo, Z. Luo, S. Lu, J. Polym. Res. 16 (2009) 357–362.
[58] L. Zhang, H.C.L. Abbenhuis, G. Gerritsen, N.N. Bhriain, P.C.M.M. Magusin,
B. Mezari, W. Han, R.A. Santen, Q. Yang, C. Li, Chem. Eur. J. 13 (2007) 1210–1221.
[59] M. Islam, P. Mondal, S. Mondal, S. Mukherjee, A.S. Roy, M. Mubarak, M. Paul,
[77] M. Islam, P. Mondal, S. Mondal, S. Mukherjee, A.S. Roy, M. Mubarak, M. Paul,
J. Inorg, J. Inorg. Organomet. Polym. Mater. 20 (2010) 87–96.
[78] M.R. Maurya, M. Kumar, U. Kumar, J. Mol. Catal. A Chem. 273 (2007) 133–143.
[79] R. Wang, H. Zhu, B. Gao, Reac. Kinet. Mech. Cat. 103 (2011) 431–441.
[80] A.P. Singh, N. Torita, S. Shylesh, N. Iwasa, M. Arai, Catal. Lett. 132 (2009)
492–499.
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