Mono-copper substituted phosphotungstate supported on to neutral alumina: Synthesis, characterization and detailed studies for oxidation of styrene

Article abstract of DOI:10.1016/j.ica.2021.120357

Mono-copper substituted phosphotungstate (PW₁₁Cu) was supported on a neutral support (alumina) by wet impregnation method, which is the first instance of the use of alumina for supporting Transition metal substituted Polyoxometalates (TMSPOMs),

Full text of DOI:10.1016/j.ica.2021.120357

Inorganica Chimica Acta 522 (2021) 120357
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Research paper
Mono-copper substituted phosphotungstate supported on to neutral
alumina: Synthesis, characterization and detailed studies for oxidation
of styrene
Rajesh Sadasivan, Anjali Patel*
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat,
India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Mono-copper substituted phosphotungstate (PW11Cu) was supported on a neutral support (alumina) by wet
impregnation method, which is the first instance of the use of alumina for supporting Transition metal
substituted Polyoxometalates (TMSPOMs), characterized by various spectral techniques and the catalytic activity
was evaluated for the oxidation of styrene using tert-butyl hydroperoxide (TBHP) as oxidant. Various reaction
parameters were optimized and a detailed kinetic study was carried out to understand the role of each of the
components of the reaction. Finally, the activity of the present catalyst was compared with zirconia supported
PW11Cu, and the superiority was explained based on the presence of Lewis acidity of aluminum.
Copper polyoxometalate
Neutral alumina
Styrene oxidation
Kinetics
Thermodynamics
1. Introduction
substituted phosphotungstate over silica nano-sized particles
comprising amino-organosilicane surrounded by silica shell, and used
Polyoxometalates (POMs) are metal oxygen clusters with transition
metals in their highest oxidation state (WVI, VV, MoVI) and possess the
distinct ability to be modified at molecular level. Removal of one or
more transition metals gives rise to lacunary POMs (LPOMs) and sub-
stitution with other transition metal ions with lower oxidation states,
gives rise to transition metal substituted POMS (TMSPOMs). Both
LPOMs and TMSPOMs have been widely used as catalysts, however,
almost all of them suffer from traditional disadvantages such as high
solubility, low surface area, recovery and recycling. To overcome these,
various materials have been efficiently used as supports for LPOMs as
well as TMSPOMs [1–10]. Supported LPOMs/TMSPOMs possess high
thermal, chemical and structural stability [3] and also show advantages
over traditional homogeneous catalysts like high surface area, ease of
recovery and prevention of corrosion of the reactor.
the same as catalyst for oxidation of various olefins and also oxidative
desulfurization processes [12]. In a different set up in 2015, Romanelli
et al synthesized a number of transition metal substituted phospho-
tungstates using Co, Ni, Cu and Zn, supported them on activated carbon
and evaluated the same for the sulfoxidation of 2-(methylthio)-benzo-
thiazole [5]. Patel et al , in, 2016, synthesized zirconia supported mono-
nickel substituted phosphotungstate, which clearly proved to be a
cleaner alternative to traditional techniques for one-pot oxidative
esterification of benzaldehyde [13]. Very recently, zinc substituted
phosphotungstate was supported on to ionic-liquid modified MCM-41 by
Hajian et al and used for selective oxidation of alcohols [2]. Similarly,
di-vanadium substituted phosphotungstate was supported over
nitrogen-doped carbon nanomaterials by Kholdeeva et al [14] and used
for oxidation of various organic substrates.
Amongst, supported TMSPOMs have been of great interest in the last
decade, as they provide a two-fold advantage of a lower oxidation state
transition metal replacing a high oxidation state one, along with that of a
support. Guo et al, in 2009, supported various transition metal
substituted polyoxotungstates, K10-nXn+MW11O39 (X = P/Si, M = Co/
Ni/Cu/Mn), over modified SBA-15, and used the same for oxidation of
styrene [11]. In 2014, Balula et al successfully incorporated zinc-
Numerous reports are available on application of alumina supported
POMs as catalysts for various organic transformations [3,10,15]. How-
ever, it has never been used to support copper substituted POMs, nor has
such a material been used for alkene oxidation. It is well known that
supports not only help in providing mechanical hold to the active spe-
cies, but they also tend to alter the catalytic properties of the material
[7]. Hence, recently, as we have reported a detailed study on the flexible
* Corresponding author.
E-mail address: anjali.patel-chem@msubaroda.ac.in (A. Patel).
https://doi.org/10.1016/j.ica.2021.120357
Received 30 September 2020; Received in revised form 25 February 2021; Accepted 19 March 2021
Available online 23 March 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Fig. 1. EDX mapping of PW11Cu/Al2O3.
against trichloroacetic acid using neutral red indicator.
oxidation of styrene over mono-copper substituted phosphotungstate
(PW11Cu) supported on zirconia, it was thought to study the effect of
different supports on the activity of PW11Cu. It is well known that het-
eropoly compounds cannot be supported over basic supports, as this
may lead to decomposition of the same, we have selected neutral Al2O3
as a support for PW11Cu, thereby synthesizing a novel heterogeneous
catalyst for alkene oxidation. The synthesized material was character-
ized using various physico-chemical techniques and investigated for the
oxidation of styrene using THBP as oxidant, because of the industrial
importance of benzaldehyde as well as styrene oxide. Studies for
regeneration and recycling were also carried out and a detailed kinetic
study was also carried out to understand the role of each component of
the reaction. Finally, thermodynamic parameters were determined in
order to understand the effect of nature of support.
2.5. Acid sites determination by potentiometry
The different types of acidic sites were determined by potentiometric
titrations using n-butylamine [18]. 0.25 g of the synthesized material
was suspended in 50 mL acetonitrile and aged at 25 ◦C. To this, 0.1 mL of
0.05 N n-butylamine in acetonitrile was added at regular time intervals
and the potential (mV) after each addition was recorded.
2.6. Characterization
The synthesized material was characterized for its acidic strength,
and also by thermo gravimetric-differential thermal analysis (TG-DTA),
EDX, Brauner-Emmett-Teller (BET) surface area analysis, Temperature
Programmed Reduction (TPR), Fourier Transform–Infrared (FT-IR)
spectroscopy, FT–Raman spectroscopy, 31P Magic Angle Spin Nuclear
Magnetic Resonance (MAS NMR) Spectroscopy and Powder X-Ray
Diffractometry (XRD). Elemental analysis of the solid catalyst was car-
ried out by Hitachi Regulus8100. Adsorption-desorption analysis for
specific surface area calculations was carried out in the Micrometrics
ASAP 2010 instrument at ꢀ 196 ◦C. TGA was carried out using Mettler
Toledo Star SW 7.01 instrument up to 550 ◦C. The TPR studies were
investigated in a self-made reactor set-up with a quartz reactor vessel.
50 mg of sample was taken and heated up to 800 ◦C and the linear
ramping rate was 10 ◦Cminꢀ 1 with 5% (35 mLminꢀ 1) H2/Ar flow for 60
min. the consumption of H2 gas was monitored using GC instrument
equipped with TCD (m/s, CIC Instruments, India). FT-IR spectra of the
sample were obtained by using the KBr pellet on the Perkin Elmer in-
strument. 31P MAS NMR was recorded in JOEL ECX 400 MHz High
Resolution Multinuclear FT-NMR spectrometer for solids. Powder XRD
2. Experimental
2.1. Materials
All chemicals used were of A.R. grade. 12-tungstophosphoric acid,
Copper chloride dihydrate, Cesium chloride, styrene, dichloromethane,
70% tert-butyl hydroperoxide and neutral aluminium oxide (γ-Alumina)
were obtained from Merck and were used as received.
2.2. Synthesis of undecatungstophospho(aqua) cuprate(II) [16]
10 mL solution of 12-tungstophosphoric acid (PW12) (2.88 g; 1
mmol) was adjusted to 4.8 pH using supersaturated NaOH solution (~5
g in 20 mL water) and heated to 90 ◦C with stirring. To this POM so-
lution, CuCl2⋅2H2O (0.17 g; 1 mmol) dissolved in 5 mL water was added
drop-wise followed by air-refluxing for 1.5 h at 90 ◦C. This solution was
then filtered hot and solid CsCl (0.5 g) was immediately added. The
resulting greenish blue precipitates were filtered, dried at room tem-
perature and designated as PW11Cu.
analysis was carried out in a Phillips PW-1830 instrument (Cu K
α source;
scanning range 50 to 800)
2.7. Catalytic reaction
2.3. Supporting of PW11Cu on to neutral alumina.
Oxidation of styrene was carried out using PW11Cu/Al2O3 as cata-
lyst, in a 50 mL batch reactor attached to a double-walled air condenser
on a magnetic stirrer and heating plate. 1 mL (10 mmol) of styrene was
taken, to which adequate quantities of the catalyst and TBHP were
added. Dichloromethane was used to extract the products after the re-
action, which were then analyzed in Shimadzu-2014 Gas Chromato-
graph, using an RTX-5 capillary column. The products were identified by
comparison with authentic samples.
PW11Cu was supported on to neutral alumina by wet impregnation
method. 1 g γ-Al2O3 was added to aqueous solution of PW11Cu (0.3 g/30
mL) and left to age for 35 h at room temperature with continuous stir-
ring. This mixture was then dried at 100 ◦C for 10 h and the resultant
material was designated 30% PW11Cu/Al2O3. On similar lines, 10%,
20% and 40% PW11Cu/Al2O3 were prepared and as designated.
Conversion and selectivity of each product was calculated as follows:
2.4. Acidity determination by n-butylamine titration
(initialmol%) ꢀ (finalmol%)
Conversion =
x100
Total acidity of the material was determined by technique reported
in literature [17]. 0.25 g of material was suspended in a 0.025 M solu-
tion of n-butylamine in toluene for 24 h. The excess base was titrated
(initialmol%)
2

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Table 1
n-Butyl amine acidity values.
Material
Acidity (mmol n-butyl amine/g)
Al2O3
0.2
PW11Cu
0.5
10% PW11Cu/Al2O3
20% PW11Cu/Al2O3
30% PW11Cu/Al2O3
40% PW11Cu/Al2O3
0.43
0.61
0.83
0.72
Table 2
Acidic strength and acidic sites determined by potentiometry.
Material
Acidic
Types of acidic sites
Total no. of
acidic sites
strength (mV)
Very
Strong
Weak
strong
Al2O3
ꢀ 30
20
0
0
0
0
0.1
0.3
0.4
0.1
0.4
0.5
PW11Cu
0.1
0.1
Fig. 2. Effect of % loading (Catalyst amount ꢀ 25 mg; Time – 16 h; TBHP ꢀ 2
mL; Temperature ꢀ 60 ◦C).
10% PW11Cu/
Al2O3
62
20% PW11Cu/
Al2O3
88
0
0.1
0.1
0.1
0.9
1.3
1.7
1.0
1.5
1.8
30% PW11Cu/
Al2O3
113
83
0.1
0
40% PW11Cu/
Al2O3
moles of product formed
moles of substrate consumed
Selectivity =
x100
Further, Turn Over Number (TON) was calculated using the equa-
tion:
moles of product
moles of catalyst
TON =
2.8. Leaching test
Polyoxometalates can be easily characterized by a clear heteropoly
blue colour when reacted with a mild reducing agent like ascorbic acid.
This method was used to check for leaching of PW11Cu from the support
[19].
Fig. 3. TGA of (a) PW11Cu and (b) PW11Cu/Al2O3.
well as total number of acidic sites are presented in table 2. Neutral
alumina shows no significant acidic strength as well as acidic sites.
Similarly, the acidic strength of PW11Cu is also very small. Supporting
increases the acidity as well as acidic strength of the catalyst to a great
extent and as it can be seen from table 2, increase in % loading shows
corresponding rise in strength as well as acidic sites. However, the acidic
strength for 40% loaded catalyst is significantly lesser than that of 30%
loaded one, despite its higher acidic sites. This is attributed to blocking
of catalytic sites and is in good agreement with the results obtained from
n-butylamine titration. Thus, from the acidity determination by n-
butylamine titration as well as by potentiometry, 30% loading was
found to be optimum for carrying out detail characterization.
3. Results and discussion
3.1. Catalyst characterization
The EDX mapping of PW11Cu/Al2O3 (Fig. 1) shows the presence of all
elements. The ascertained EDX values (% wt): P, 0.21; W, 13.96; Cu,
0.39; Cs; 4.64 are in good agreement with the theoretical calculated
value (% wt): P, 0.20; W, 13.36; Cu, 0.41; Cs; 4.38.
Initially the catalyst was subjected to treatment with ascorbic acid
solution. 1 g of the catalyst was refluxed for 24 h in 10 mL distilled
water, after which, 10% ascorbic acid solution was added. Absence of
blue colour under warm conditions indicated that leaching of PW11Cu
does not take place from the support. This also shows existence of strong
interactions between the active species and support. Further, the total
acidity of the synthesized materials was determined by n-butyl amine
titrations and the results are presented in table 1. It can be noted that the
acidity of the material increases with increase in %loading. This may be
attributed to two reasons. The acidity of unsupported PW11Cu itself is
very less, and with increase in loading on alumina there is steady in-
crease in overall acidity of the material, which is as expected. Decrease
in acidity value from 30% to 40% loading may be due to blocking of
acidic sites as a result of higher loading.
In order to confirm the same, preliminary reaction was carried out
for the oxidation of styrene varying the % loading of PW11Cu (Fig. 2). A
steady increase in conversion along with high selectivity for benzalde-
hyde is seen from 10% to 30% loading which is expected because of the
increasing acidity of the catalyst. With further increase in loading there
is drastic decrease in the conversion, attributed to blocking of catalytic
sites and also confirming the observations of acidity experiments.
Hence, 30% loading was considered optimum and further character-
ization and catalytic activity were done with 30% PW11Cu/Al2O3,
designated as PW11Cu/Al2O3.
TGA of PW11Cu and PW11Cu/Al2O3 are shown in fig. 3. The TGA of
PW11Cu/Al2O3 shows an initial weight loss of 2.1% up to 150 ◦C
attributed to loss of adsorbed water molecules. Further, weight loss of
The various acidic sites were calculated by potentiometric titration
method and the acidic strength in terms of initial electrode potential as
3

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Table 3
FT - IR frequencies.
FT-IR Frequencies (cmꢀ 1
–
–
–
P O
–
– –
O
– –
O H
)
W
O
W–O–W
Cu–O
O
H
H
H
O
Al2O3
–
–
–
–
–
–
–
3365
3437
1625
1627
1396
PW11Cu/Al2O3
1099
1064
1103
1061
–
PW11Cu
964
887
810
489
–
–
–
Table 4
FT - Raman frequencies.
Raman Frequencies (cmꢀ 1
PW11Cu
–
–
)
W
O
W – O
W–O–W
Cu – O
996
985
215
980
232
967
946
972
483
PW11Cu/Al2O3
1004
485
Fig. 5. 31P MAS NMR of (a) PW11Cu and (b) PW11Cu/Al2O3.
996 cmꢀ 1, 985 and 215 cmꢀ 1, 967 and 946 cmꢀ 1 corresponding to W
O
–
–
symmetric stretch, W–O symmetric stretch and W–O–W symmetric
stretch respectively are observed in case of PW11Cu along with addi-
tional band at 483 cmꢀ 1 corresponding to Cu–O symmetric stretch [22].
Similar bands are observed in case of PW11Cu/Al2O3 indicating that the
Keggin unit remains intact even after supporting. A slight shift in case of
supported material indicates the successful supporting of PW11Cu on to
Al2O3 via chemical interactions.
Fig. 4. H2-TPR curve of (a) PW11Cu and (b) PW11Cu/Al2O3.
5.3% between 180 and 500 ◦C may be due to loss of crystallization water
molecules, as well as condensation of surface hydroxyl groups of
alumina, which is generally observed in all metal oxide supports. The
higher temperature recorded for the loss of adsorbed water in supported
catalyst compared to unsupported one, is attributed to strong chemical
interactions between the PW11Cu and support. This indicates that the
stability of the material is further enhanced on supporting.
Fig. 4 shows the H2-TPR spectra of PW11Cu and PW11Cu/Al2O3.
PW11Cu shows maxima at 592 ◦C and 819 ◦C corresponding to formation
of WO3 species after decomposition of anion which is as expected [23].
However, the decrease in reduction temperature compared to reported
ones is attributed to presence of Cs counter-cation, which leads to in-
crease in consumption of H2 gas [24].
The H2-TPR curve of PW11Cu/Al2O3 shows two maxima at 522 ◦C
and 750 ◦C corresponding to PW11Cu. Along with this, broad peaks are
observed at temperatures above 800 ◦C analogous to the reduction of
alumina. The decrease in the intensity as well as reduction temperatures
of PW11Cu in the supported material is attributed to strong chemical
interactions between the active species and the support [25].
The FT–IR frequencies of Al2O3, PW11Cu/Al2O3 and PW11Cu are
given in table 3 and their spectra are presented in figure S1. Alumina
shows characteristic bands at 3365, 1625 and 1396 cmꢀ 1 corresponding
– – –
– –
H O stretching vibrations respectively.
to O H, H
O
H and O
Further, broad bands between 1000 and 400 cmꢀ 1 correspond to the
various Al–O vibrations. Band around 900 cmꢀ 1 corresponds to Al–O–Al
stretching vibrations while a band at 763 cmꢀ 1 corresponds to Al–O
stretching vibrations and the peaks between 700 and 500 cmꢀ 1 are
assigned to AlO6 vibrations [20,21]. PW11Cu shows characteristic bands
31P MAS NMR studies were carried out to understand the changes in
the environment around phosphorus after supporting and also the in-
teractions of the POM with support. PW11Cu/Al2O3 shows a single peak
at 6.01 ppm, which is downfield compared to unsupported PW11Cu
(-11.82 ppm) (Fig. 5). This downfield shift is attributed to two reasons.
Firstly, some water molecules of PW11Cu are lost when they are
immobilized on to the support during impregnation [26]. Secondly, the
existence of strong chemical interactions between PW11Cu and Al2O3, as
indicated in previous studies. The acidity of the medium caused by the
at 1103 and 1061, 964, 887 and 810 cmꢀ 1 corresponding to P O, W
O
–
–
–
and W–O–W stretching vibrations respectively. Also, the band obtained
at 489 cmꢀ 1 corresponds to Cu–O frequency [16]. PW11Cu/Al2O3 shows
bands at 1099 and 1064 cmꢀ 1 corresponding to P O stretching vibra-
–
tions of PW11Cu. Apart from this, bands at 3437 and 1627 cmꢀ 1 corre-
–
– –
O H vibrations of Al2O3 are also observed.
sponding to O H and H
The bands of PW11Cu between 900 and 500 cmꢀ 1 are not visible due to
overlapping with bands belonging to Al2O3. Slight shift in the bands
along with absence of some bands in case of PW11Cu/Al2O3 may be due
to chemical interactions between PW11Cu and the support.
–
POM results in the dehydroxylation of surface –O H groups of alumina
to give rise to electron donor and acceptor sites. These interact with the
protons of the water molecules of PW11Cu, thereby resulting in an
overall negative charge over the Keggin unit. The negatively charged
Keggin anions bind with the protonated support via electrostatic
Table 4 shows the FT-Raman frequencies of PW11Cu and PW11Cu/
Al2O3 while the spectra are shown in figure S2. Characteristic bands at
4

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
attributed to the fact that oxide supported catalysts tend to show
decrease because of strong chemical interactions between the support
and active species [29]. From the pore size distribution curve, the
average pore diameter of the supported material was found to be 88 A0.
3.2. Catalytic evaluation
The synthesized material was evaluated for its catalytic activity for
the oxidation of styrene using tert-butyl hydroperoxide (TBHP) as
oxidant. Initial experiments without catalyst and also using H2O2 (3 mL;
Mole ratio of styrene:H2O2 – 10:30) as oxidant yielded negligible results,
indicating that catalyst is essential for the reaction and that H2O2 would
not suffice as oxidant for the reaction. Various reaction parameters like
catalyst amount, reaction time, amount of TBHP and temperature were
optimized for best possible conversion and selectivity of desired product
(Fig. 8). Benzaldehyde and styrene-oxide were obtained as major
products, while small quantities of acetophenone (selectivity ~ 8–10%)
and benzoic acid (selectivity ~ 2–3%) were also obtained (Scheme 1).
The effect of catalyst amount was first studied keeping other pa-
rameters constant (Time –16 h; temp – 60 ◦C; TBHP – 2 mL). On increase
of catalyst amount from 10 mg to 25 mg, there is a steady increase in the
conversion as well as selectivity of benzaldehyde, with a decrease in
selectivity of styrene-oxide. This is expected because overall acidity of
the system will increase with increase in catalyst amount. On further
increase of catalyst amount, there is a decrease in the conversion and
this may be explained as follows. At constant rate of agitation (700 rpm),
increase in catalyst amount results in clumping of catalyst and as a
result, blocking of catalytic sites occurs. Hence, 25 mg was considered
optimum amount for the reaction.
Fig. 6. Powder XRD of (a) Al2O3, (b) PW11Cu/Al2O3 and (c) PW11Cu.
Table 5
BET Surface area of Al2O3 (support) and PW11Cu/Al2O3 (supported catalyst).
Material
Surface area (m2/g)
Pore diameter (A0)
Al2O3
83
59
105
88
PW11Cu/Al2O3
interactions and form strong chemical bonds also ensuring that PW11Cu
does not leach out from the support [27]. Similar type of interactions are
also observed in case of other supports like SiO2 and ZrO2 [1,22,28].
Fig. 6 shows the wide angle powder XRD patterns of alumina,
PW11Cu/Al2O3 and PW11Cu. Alumina shows reflection peaks at 370, 480
and 660 2θ corresponding to 311, 400 and 440 reflection planes and are
in good agreement with reported work [27]. Powder XRD patterns of
PW11Cu show sharp crystalline peaks between 200-300 2θ which are
characteristic of Keggin unit, with a slight shift due to presence of copper
[16]. In case of PW11Cu/Al2O3, the sharp peaks disappear indicating
homogeneous dispersion of PW11Cu over the support.
Next, the reaction time was varied keeping other parameters con-
stant (Catalyst amount – 25 mg; temp – 60 ◦C; TBHP – 2 mL). With in-
crease in time from 4 h to 16 h, conversion increases. However, initially
the selectivity of benzaldehyde decreases with increase in styrene-oxide
selectivity and then both become constant. This may be explained as
follows. It is well-known that styrene-oxide is the intermediate formed
during the synthesis of benzaldehyde. In the present case, initially the
selectivity for benzaldehyde is high. But as the reaction proceeds, an
equilibrium is created between the intermediate and the final product,
and hence the selectivity for benzaldehyde and styrene-oxide remain
constant for the rest of the reaction. Beyond 16 h, there is negligible
increase in the conversion and hence 16 h was considered optimum.
In order to optimize amount of oxidant, the reaction was carried out
by adding different quantities of TBHP (catalyst amount – 25 mg; temp –
BET surface area of Al2O3 and PW11Cu/Al2O3 are shown in table 5
and the isotherms are shown in fig. 7. There is decrease in surface area of
the supported material because of incorporation of PW11Cu on to the
support which is in agreement with the reported results [3]. This is
Fig. 7. N2 sorption curves and (inset) average pore diameters of (a) Al2O3 and (b) PW11Cu/Al2O3.
5

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Fig. 8. Optimization of parameters for oxidation of styrene (a) Effect of catalyst amount (Time –16 h; temp – 60 ◦C; TBHP – 2 mL; styrene-1 mL); (b) Effect of time
(Catalyst amount – 25 mg; temp – 60 ◦C; TBHP – 2 mL; styrene-1 mL); (c) Effect of amount of TBHP (catalyst amount – 25 mg; temp – 60 ◦C; time – 16 h; styrene-1
mL); (d) Effect of temperature (catalyst amount – 25 mg; time – 16 h; TBHP – 2 mL; styrene-1 mL).
Scheme 1. Oxidation of styrene.
60 ◦C; time – 16 h). With increase in TBHP amount from 1 mL to 2 mL,
there is a steady increase in conversion of styrene while the selectivity
for benzaldehyde remains more or less constant. On further increase in
TBHP amount to 2.7 mL, there is a drastic decrease in conversion as well
as selectivity of desired products. This is because excess of TBHP results
in higher concentration of oxygen which is used up in the oxidation of
benzaldehyde to benzoic acid. Hence, 2 mL TBHP was considered op-
timum for the reaction. Varying results under varying amounts of TBHP
also indicates that reaction rate depends on concentration of the
oxidant.
But on increasing the temperature beyond 60 ◦C, there is a significant
decrease in the selectivity due to the degradation of TBHP at higher
temperatures, thereby resulting in polymerization of styrene. Hence,
temperature was optimized at 60 ◦C.
3.3. Test for leaching and heterogeneity
The reaction mixture was subjected to treatment with 10% ascorbic
acid solution at 60 ◦C after the completion of the reaction and separation
of catalyst. Absence of blue color confirmed that PW11Cu does not leach
out from the support.
Finally, the reaction was carried at different temperatures (catalyst
amount – 25 mg; time – 16 h; TBHP – 2 mL) and the results are presented
in fig. 8d. When the temperature was increased from 50 ◦C to 60 ◦C,
there is an increase in conversion as well as selectivity of benzaldehyde.
To confirm the heterogeneous nature of the catalyst, the reaction was
run for 12 h, after which, the catalyst was filtered out. Then the reaction
was allowed to continue for further 4 h. No change was observed in the
6

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Table 6
Heterogeinity test.
Reaction time
% conversion
% selectivity
Styrene-oxide
Benzaldehyde
12 h
16 h
49
49
28
28
58
59
Catalyst amount: 25 mg; Time: 16 h; Temp: 60 ◦C; TBHP: 2 mL; Styrene-1 mL
Table 7
Role of support.
Catalyst
%
% selectivity
Styrene-
oxide
Turn Over
conversion
Benzaldehyde
Number (TON)
[a]
Al2O3
NA
21
63
NA
20
19
NA
67
60
NA
PW11Cu[b]
PW11Cu/
1102
3306
[c]
Al2O3
Fig. 9. Plot of 1/(a-x) versus time.
PW11Cu/
44
44
37
2309
ZrO2[c] [22]
[a]
Catalyst amount- 19.75 mg; TBHP – 2 mL; Temperature – 60 ◦C; Time–16 h;
Styrene-1 mL.
[b]
Catalyst amount- 6.25 mg; TBHP – 2 mL; Temperature – 60 ◦C; Time–16 h;
Styrene-1 mL.
[c]
Catalyst amount-25 mg (active amount of PW11Cu ꢀ 6.25 mg); TBHP – 2
mL; Temperature – 60 ◦C; Time–16 h; Styrene-1 mL.
conversion as well as selectivity of the products, indicating that the
catalyst is truly heterogeneous in nature Table 6.
3.4. Role of support
In order to understand the role of support, the reaction was carried
out using the active amount of PW11Cu present in the catalyst under
optimized conditions, and the results are presented in table 7. It can be
seen that the supported catalyst gives three times the conversion as that
of unsupported one. This is attributed to two reasons: higher acidity of
supported catalyst and homogeneous dispersion of active species on the
support, due to which more number of active species are available for
the reaction. Further, a comparison of the catalytic activity with that of
PW11Cu/ZrO2 [22] shows that alumina is better support than zirconia,
which may be due to the nature of support. Alumina contains Lewis
acidic sites in the form of aluminium. The less nucleophilic intermediate
formed, is able to desorb faster from PW11Cu/Al2O3 as compared to
PW11Cu/ZrO2, and hence gives higher conversion as well as higher
selectivity for benzaldehyde, the complete oxidation product. This is
further supported by thermodynamic studies.
Fig. 10. Plot of log[(b-x)/(a-x)] versus time.
3.5. Kinetics
Reaction kinetics plays an important role to help understand the role
of each component of the reaction. In the present case, various experi-
ments were carried out to obtain the rate and determine the order of
reaction, and also the activation energy of the reaction.
It has already been determined that the reaction rate depends on
concentration of both, styrene and TBHP (Fig. 8c and subsequent
explanation). Further confirmation of the same can be obtained by
carrying out the experiments under two different conditions.
First, when same concentration of styrene and TBHP is taken, a
relation between the concentration and time can be established by Eq.
(1), where ‘a’ is the initial concentrations of styrene and TBHP and ‘x’ is
the conversion of styrene at time t
Fig. 11. Plot of rate of reaction versus catalyst concentration.
taken, a relation between individual concentrations and time is estab-
lished by Eq. (2), where ‘a’ is the initial concentration of styrene, ‘b’ is
the initial concentration of TBHP and ‘x’ is the concentration at time t.
1
= kt + c
(1)
(a ꢀ x)
Similarly, when different concentrations of substrate and TBHP are
7

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Table 9
Inhibition experiment using radical scavenger.
Reaction
Condition
%
% selectivity
Styrene-
oxide
conversion
Benzaldehyde
Styrene
12 h
49
50
28
27
58
60
Oxidation
16 h (after
addition)
Catalyst amount: 25 mg; Temp: 60 ◦C; TBHP: 2 mL; Styrene-1 mL
Table 10
Regeneration study.
Catalyst
% Conversion
% Selectivity
Benzaldehyde
Styrene-oxide
PW11Cu/Al2O3
63
63
62
61
60
60
60
61
19
20
20
20
Rec1-PW11Cu/Al2O3
Rec2-PW11Cu/Al2O3
Rec3-PW11Cu/Al2O3
Fig. 12. Plot for determination of activation energy.
Catalyst amount ꢀ 25 mg; Time ꢀ 16 h; Temperature ꢀ 60 ◦C; TBHP ꢀ 2 mL;
Styrene-1 mL
Table 8
Polanyi equation given by eq. (4). A plot of ln(k/T) versus 1/T
showed linearity and the enthalpy, ΔH, was calculated from the slope
while the entropy, ΔS, was calculated from the intercept. These were
further used to calculate the Gibbs’ free energy using Eq. (5).
Kinetic and thermodynamic parameters of styrene oxidation over PW11Cu/
Al2O3 as well as PW11Cu/ZrO2.
Catalyst
Activation energy
Enthalpy (ΔH,
Entropy
Gibbs’ free
energy (ΔG,
J)
(Ea, kJmolꢀ 1
)
kJmolꢀ 1
)
(ΔS, JKꢀ 1
)
κT
h
ΔS
ꢀ ΔH
R
RT
k =
e .e
(4)
PW11Cu/
Al2O3
69.3
76.8
31.1
37.5
64,313
PW11Cu/
ZrO2
40.3 [22]
ꢀ 104
65,717
ΔG = ΔH ꢀ TΔS
(5)
The positive value for Gibbs’ free energy confirms that the reaction is
non-spontaneous. The values of ΔH, ΔS and ΔG are shown in table 8.
Table 8 shows a comparison of kinetic as well as thermodynamic
parameters of styrene oxidation over PW11Cu/Al2O3 as well as PW11Cu/
ZrO2. Despite significantly higher activation energy and lower acidity
(as mentioned in previous sections) compared to PW11Cu/ZrO2,
PW11Cu/Al2O3 shows higher conversion and higher selectivity for
benzaldehyde. This is attributed to the positive entropy, which indicates
greater favorability of the reaction. Positive entropy along with lower
Gibbs’ free energy illustrates that alumina as a support is more superior
compared to zirconia for the present system.
2.303
a(b ꢀ x)
log
= kt
(2)
(b ꢀ a)
b(a ꢀ x)
From Eq. (1), a plot of 1/(a-x) versus time (Fig. 9) shows a linear
relationship indicating that the reaction follows second order kinetics
with respect to concentration of styrene and TBHP overall.
Similarly, from Eq. (2), a plot of log[(b-x)/(a-x)] versus time (Fig. 10)
gave a straight line indicating that, the reaction follows first order ki-
netics with respect to styrene and TBHP, individually [30].
The effect of rate of reaction with respect to concentration of catalyst
was studied by carrying out the reaction at different catalyst concen-
trations. A plot of catalyst concentration versus rate constant gave a
straight line, thereby indicating that the reaction follows first order with
respect to catalyst concentration as well Fig. 11.
3.7. Inhibition experiment using radical scavenger
It is well known that transition metal catalyzed oxidation reactions
follow either peroxo-intermediate formation or proceed via radical
mechanism [7]. In order to gain an idea on the same, a radical scavenger
approach was used. 2, 6-di-tertbutyl-4-methyl phenol was used as a
radical scavenger and was added into the system after 12 h of the re-
action. The results presented in table 9 showed that there was no sig-
nificant increase in conversion as well as selectivity of epoxide,
indicating that the reaction followed radical mechanism as expected
[22].
Finally, the effect of reaction temperature was studied as most
oxidation reactions show very high sensitivity to temperature. A gradual
increase in conversion of styrene is seen with increase in temperature
from 333 K to 353 K and a plot of 1/T versus lnk shows linearity
(Fig. 12). Based on this, the activation energy of the reaction was ob-
tained using Arrhenius equation
k = Aeꢀ Ea/RT
In a two phase reaction system like the present one, it is necessary to
know that the reaction is governed by a truly chemical step and is not
diffusion/mass transfer limited. Generally, the Ea of a mass transfer
limited reaction is known to be about 10–15 kJ/mol, while a truly
chemical reaction has Ea value greater than 25 kJ/mol [31]. Signifi-
cantly higher activation energy of 69.3 kJ/mol indicates that the reac-
tion is truly governed by a chemical step and also that the catalyst has
been exploited to its maximum capacity.
3.8. Regeneration study
Regeneration of the catalyst was carried out by simple centrifugation
method followed by washing with methanol and drying. The regener-
ated catalyst was evaluated for its activity for the same reaction under
optimized conditions and the results are presented in Table 10. No
changes in the conversion as well as selectivity of products even after
three cycles indicate that the catalyst can be reused multiple times.
3.6. Thermodynamic studies
Thermodynamic parameters were calculated using the Eyring-
8

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
Fig. 13. EDX of EDX values of Regenerated PW11Cu/Al2O3.
Table 11
FT-IR frequencies of fresh and regenerated PW11Cu/Al2O3.
FT-IR Frequencies (cmꢀ 1
)
W
O
W–O–W
Cu–O
O
H
H
O
–
–
–
P O
–
– –
O H
– –
H O
PW11Cu/Al2O3
1099
1064
1101
1067
–
–
–
3437
1627
–
Rec. PW11Cu/Al2O3
–
–
–
3443
1604
–
evaluation, which showed higher conversion and higher selectivity for
benzaldehyde compared to zirconia supported PW11Cu, which is more
acidic in nature. This behaviour was attribuuted to presence of Lewis
acidic sites in the form of aluminum, and was further established by
thermodynamic studies, which gave positive entropy along with lower
Gibbs’ free energy compared to zirconia supported one.
Author contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
CRediT authorship contribution statement
Rajesh Sadasivan: Conceptualization, Methodology, Writing - re-
view & editing, Supervision. Anjali Patel: Visualization, Investigation,
Data curation, Validation, Writing - original draft.
Fig. 14. Powder XRD of (a) Fresh and (b) recycled PW11Cu/ZrO2.
Declaration of Competing Interest
3.9. Characterization of regenerated catalyst
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
The regenerated catalyst was characterized by EDX, FT – IR as well as
powder XRD. EDX values of Regenerated PW11Cu/Al2O3 value (% wt):
P, 0.21; W, 13.96; Cu, 0.39; Cs; 4.64 are in good agreement with that of
fresh catalyst, indicating no leaching of Cu during the reaction (Fig. 13).
Regenerated PW11Cu/Al2O3 shows Table 11 the same FT – IR bands
as that of the fresh one, indicating that the structural morphology of the
catalyst remains intact even after the completion of the reaction.
Powder XRD of fresh and recycled PW11Cu/Al2O3 is shown in Fig. 14.
The dispersed spectra is retained in case of recycled catalyst indicating
that the catalyst remains dispersed during the course of the reaction an
also after recycling
Acknowledgments
The authors would like to acknowledge Department of Science and
Technology (DST), Govt. of India, Project No. SR/S1/IC-36/2012 for
financial support. Sincere thanks to Department of Chemistry, The
Maharaja Sayajirao University of Baroda for BET, Department of Phys-
ics, The Maharaja Sayajirao University of Baroda for FT-Raman analyses
and Department of Geology, The Maharaja Sayajirao University of
Baroda for EDX analysis We also acknowledge Prof. N. Lingaiah, CSIR-
IICT, Hyderabad, for H2-TPR studies. One of the authors, RS is thank-
ful to Council of Scientific and Industrial Research (CSIR), Govt. of India,
for award of Senior Research Fellowship (SRF).
4. Conclusion
Intact structure of PW11Cu after supporting on alumina was
confirmed by IR and Raman, while uniform dispersion was confirmed by
XRD and TPR, and NMR studies established the chemical interactions
between support and active species. Despite its neutral nature, alumina
increased the activity of PW11Cu, clearly observed in its catalytic
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120357.
9

R. Sadasivan and A. Patel
Inorganica Chimica Acta 522 (2021) 120357
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