Nickel exchanged supported 12-tungstophosphoric acid: synthesis, characterization and base free one-pot oxidative esterification of aldehyde and alcohol

Article abstract of DOI:10.1039/C8RA08419J

The present work describes for the first time, the synthesis and characterization of a bi-functional catalyst consisting of nickel and supported 12-tungstophosphoric acid as well as its application in one pot oxidative esterification of benzaldehyde and benzyl alcohol to benzoate ester. The said reactions were operated without any base at a low temperature and atmospheric pressure. The influence of reaction parameters such as nickel concentration, molar ratio of substrate to H₂O₂ as well as methanol, catalyst amount, reaction temperature and reaction time were investigated to optimize the conditions for maximum conversion with good selectivity towards the desired product. The superiority of the present work lies in obtaining higher conversion as well as higher selectivity of the desired product with a high TON for both the systems under sustainable reaction conditions. Moreover, the catalyst could be recovered and reused for up to three cycles without any significant loss in its selectivity. The obtained conversion as well as selectivity were discussed with a number of control experiments and based on the obtained results, mechanisms for both the reactions were proposed. Furthermore, the difference in activity towards oxidative esterification of benzaldehyde and benzyl alcohol was also correlated with the proposed mechanism.

Full text of DOI:10.1039/C8RA08419J

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PAPER
Nickel exchanged supported 12-tungstophosphoric
acid: synthesis, characterization and base free one-
pot oxidative esterification of aldehyde and
alcohol†
Cite this: RSC Adv., 2019, 9, 1460
*
Anish Patel
and Anjali Patel
The present work describes for the first time, the synthesis and characterization of a bi-functional catalyst
consisting of nickel and supported 12-tungstophosphoric acid as well as its application in one pot oxidative
esterification of benzaldehyde and benzyl alcohol to benzoate ester. The said reactions were operated
without any base at a low temperature and atmospheric pressure. The influence of reaction parameters
such as nickel concentration, molar ratio of substrate to H2O2 as well as methanol, catalyst amount,
reaction temperature and reaction time were investigated to optimize the conditions for maximum
conversion with good selectivity towards the desired product. The superiority of the present work lies in
obtaining higher conversion as well as higher selectivity of the desired product with a high TON for both
the systems under sustainable reaction conditions. Moreover, the catalyst could be recovered and
reused for up to three cycles without any significant loss in its selectivity. The obtained conversion as
well as selectivity were discussed with a number of control experiments and based on the obtained
results, mechanisms for both the reactions were proposed. Furthermore, the difference in activity
towards oxidative esterification of benzaldehyde and benzyl alcohol was also correlated with the
proposed mechanism.
Received 11th October 2018
Accepted 28th December 2018
DOI: 10.1039/c8ra08419j
rsc.li/rsc-advances
systems.18 Many methods suffer from disadvantages such as use
of expensive and polluting reagents, an inert atmosphere and
lengthy reaction times.7
Introduction
Oxidative esterication of aldehydes with alcohols has received
increasing attention during recent years because such raw
materials are abundantly available in industry.1 Various groups
have reported one step conventional methods for oxidative
esterication of aldehydes, based on TS-1,2 V2O5-SPC/SPB (SPC-
sodium percarbonate, SPB sodium perborate),3 supported gold
nanoparticle Au/TiO2,4 Pb and Mg doping in Al2O3-supported
Pd,5 palladium/NHC,6 manganese phthalocyanine immobilized
on silica gel,7 supported gold–nickel oxide nanoparticles
(AuNiOx),8 magnesia supported gold nanoparticles (Au/MgO),9
titanium superoxide,1 palladium complex immobilized on
magnetite–graphene oxide nanocomposite,10 graphite oxide
and oxone,11 iron oxalate capped iron–copper nanomaterial
(Fe(ox)Fe–CuOx),12 reduced graphene oxide supported Co3O4
nanoparticles (Co3O4/rGO),13 supported iron oxide nano parti-
cles (FeNP),14 gold nanoparticles supported on Ce–Zr oxide,15
polystyrene supported rhodium nanoparticles (Rh@PS),16 PTFE
supported gold nanoparticles17 and sulfate radical redox
Oxidative esterication of alcohols is highly desirable from
both economic and environmental points of view, as alcohols
are more readily available as bulk chemicals, more stable than
the carbonyl compounds, cheaper, less toxic and easier to
handle.19 Hence in this direction, to improve the conversion
and selectivity of the oxidative esterication of benzyl alcohol
reaction, tremendous efforts have been made to develop new
catalysts, including supported-monometallic catalysts such as
Au, Pt, Pd, Ag, etc. and supported-bimetallic catalysts such as
Au–Pd, Au–Cu, and Au–Ir catalysts.20 Various groups have re-
ported the same, based on Co3O4–N@C,21 palladium complex
immobilized on magnetite–graphene oxide nanocomposite,10
gold–palladium alloy nanoparticles on a phosphate-modied
hydrotalcite support,22 supported iron oxide nanoparticles,14
Co nanoparticles embedded in nitrogen doped graphite,23
polystyrene stabilized rhodium nanoparticles16 and Pd–Bi–Te/
C.24 On the other hand, gold catalysts have considerable interest
due to their unique catalytic properties and selectivity in
particular reactions9 such as Au/TiO2,25 Au/b-Ga2O3,26 Au
nanoparticles on porous coordination polymers,27 Au/SiO2,28
nanocrystalline gold supported on Fe–Ti–Ce-modied meso-
porous silica,29 Au/ZrO2,30 Au nanoparticles supported in the
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The Maharaja Sayajirao University of Baroda, Vadodara, India. E-mail:
anjali.patel-chem@msubaroda.ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra08419j
1460 | RSC Adv., 2019, 9, 1460–1471
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Scheme 1 Synthesis of Ni-TPA/ZrO2.
nanocages of SBA-16 31 magnesia-supported Au nanoparticles,9
Au/Al2O3 32 and iron doped graphene (Fe-Gr) supported gold.20
However, in most cases, liquid base additives (KOH and
K2CO3) are required to obtain a higher yield of the ester.9 It
would be interesting if the reaction could be carried out using
inexpensive metal as well as without base.
A literature survey shows that even though, polyoxometalates
based catalysts are found to be excellent, sustainable and have
applications for number of oxidation as well as esterication
reactions, very few reports are available for the oxidative ester-
ication of the benzaldehyde. To name, anchored 12-tung-
stophosphoric acid and lacunary anchored phosphotungstate
(PW12/ZrO2, PW12/MCM-41, PW11/ZrO2 and PW11/MCM-41),33
12-tungstophosphoric acid immobilized on the surface of silica
encapsulated g-Fe2O3 nanoparticles34 and imidazolium poly-
oxometalate, [bmim]3[PW12O40].35 At the same time, it is very
surprising that not a single report is available on oxidative
esterication of benzyl alcohol using polyoxometalates.
Experimental
Materials
All chemicals used were of A. R. grade. Zirconium oxychloride
(ZrOCl2$8H2O) (Loba Chemie), 12-tungstophosphoric acid
(H3PW12O40), nickel acetate, benzaldehyde, benzylalcohol,
methanol, 30% hydrogen peroxide and dichloromethane ob-
tained from Merck were used as received.
Catalyst synthesis
Synthesis of zirconia supported 12-tungstophosphoric acid
(TPA/ZrO2). 30% of 12-tungstophosphoric (TPA) was supported
on hydrous zirconia (ZrO2)38 by incipient wet impregnation
method as reported by our group earlier.39 1 g of ZrO2 was
impregnated with aqueous solution of TPA (0.3/30 g mLꢀ1 of
double distilled water) and dried at 100 ꢁC for 10 h. The ob-
tained material was designated as TPA/ZrO2.
Synthesis of nickel exchanged supported 12-tungstophos-
phoric acid. A series of nickel exchanged supported 12-tung-
stophosphoric acid was synthesized by soaking 1 g of TPA/ZrO2
with 25 mL of 0.03–0.05 M solution of nickel acetate for 24 h
with stirring. The solution was ltered, removed solid was
washed with distilled water in order to remove the excess of
nickel and then dried in air at room temperature. The obtained
resulting materials were designated as (0.03 M) Ni-TPA/ZrO2,
(0.04 M) Ni-TPA/ZrO2 and (0.05 M) Ni-TPA/ZrO2 respectively.
The synthesis of Nickel exchanged supported 12-tung-
stophosphoric acid is shown in Scheme 1.
Recently our group has reported one pot oxidative esteri-
cation of benzaldehyde over two catalysts based on lacunary
phosphotungstate (i) Cs salt of mono nickel substituted phos-
photungstate (CsPW11Ni) in homogeneous medium36 (ii) sup-
ported Cs-salt of mono nickel substituted phosphotungstate
catalyst (CsPW11Ni/ZrO2).37 The obtained excellent results,
especially in supported nickel catalyst, encourage us to design
another catalyst based on nickel and phosphotungstate and
extend the work. To achieve same, new bi-functional catalyst
was designed by taking the advantage of available protons of
supported 12-tungstophosphoric acid (parent phosphotung-
state), characterized and evaluated for one pot oxidative ester-
ication of benzaldehyde as well as benzyl alcohol using H2O2
and methanol. Consequently, the various reaction parameters
such as nickel concentration, molar ratio of substrate to H2O2 as
well as methanol, catalyst amount, reaction temperature and
reaction time were optimized for the maximum conversion as
well as selectivity of the desired product. The catalyst was also
recycled and regenerated up to three cycles. Reaction mecha-
nisms for oxidative esterication of benzaldehyde as well as for
benzyl alcohol were also proposed and based on this the
difference in activity for the reactions were also discussed.
Characterization
The amount of nickel was determined by difference in the
standard solution as well as sample solution by volumetric
method.40 The acidity of the catalysts was determined by n-butyl
amine as well as by potentiometric titration. Thermo gravi-
metric analysis (TGA) was performed using Mettler Toledo Star
ꢁ
SW 7.01 up to 500 C. Adsorption–desorption isotherms were
done through Micromeritics ASAP 2010 surface area analyzer at
ꢁ
ꢀ196 C. Specic surface area was calculated using Brunauer–
Emmett–Teller (BET method). FT-IR spectrum of the material
was performed by using the KBr wafer on a Shimadzu instru-
ment (IRAffinity-1S). The Fourier Transform Raman (FT-Raman)
spectra were recorded on a FT-Raman Spectrophotometer
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Table 1 Potentiometric titration
Types of
acidic sites
(meq. gꢀ1
Total no. of
Strong Weak acidic sites
)
Acidic strength
Ei (mV)
Catalyst
ZrO2
TPA/ZrO2
(0.03 M) Ni-TPA/ZrO2 61
(0.04 M) Ni-TPA/ZrO2 87
(0.05 M) Ni-TPA/ZrO2 70
18
45
0.1
1.5
1.9
2.3
2.1
2.1
1.2
1.4
1.7
2.1
2.2
2.7
3.3
4.0
4.2
(Model Bruker FRA 106). Electron spin resonance (ESR) spectra
were recorded on a Varian E-line. Century series X-band ESR
spectrometer at liquid nitrogen temperature and scanned from
2500 to 5000 gauss. X-ray photoelectron spectroscopy (XPS)
measurements were performed with Auger Electron Spectros-
copy (AES) Module PHI 5000 Versa Prob II. Powder X-ray
Diffraction (Powder XRD) was carried out using Philips
Diffractometer (Model PW-1830). TEM analysis was carried out
on JEOL (JAPAN) TEM instrument (model-JEM 100CX II) with
accelerating voltage of 200 kV. The samples were dispersed in
ethanol and ultrasonicated for 5–10 min. A small drop of the
sample was then taken in a carbon coated copper grid and dried
before viewing.
Fig. 2 FT-IR spectra of (a) ZrO2 (b) TPA/ZrO2 and (c) Ni-TPA/ZrO2.
Catalytic evaluation
Oxidative esterication reaction of benzaldehyde/benzylalcohol
(10 mmol) with H2O2 (30 mmol), methanol (5/7.5 mL respec-
tively) and catalyst (10 mg) were charged in a 50 mL batch reactor
provided with a double walled air condenser, magnetic stirrer,
and a guard tube. The reaction mixture was reuxed at 80 ꢁC for 6
h/24 h respectively. In both the cases, the obtained products were
extracted with dichloromethane and analyzed on a gas chro-
matograph (Shimadzu-2014) using a capillary column (RTX-5).
Acidity measurement
Total acidity by n-butyl amine titration. A 0.025 M solution of
n-butyl amine in toluene was prepared for assessment of total
acidity of the catalyst.41 The 0.25 g catalyst was suspended in
0.025 M n-butyl amine solution for 24 h and the excess base was
titrated against trichloroacetic acid in toluene using neutral red
as an indicator.
Results and discussion
Characterization of catalyst
The volumetric analysis of nickel in standard solution and
ltrate show the presence of 0.24 wt%, 0.3 wt% and 0.4 wt% of
Ni in (0.03 M) Ni-TPA/ZrO2, (0.04 M) Ni-TPA/ZrO2, and (0.05 M)
Potentiometric titration. The type of acidic sites as well as
acidic strength was investigated by employing potentiometric
titration with 0.05 N n-butylamine which helps in computing
different acid sites.42 0.5 g of catalyst sample was suspended in
50 mL acetonitrile and the mixture was aged at 25 ꢁC. 0.05 N n-
butyl amine in acetonitrile solution was added in equal time
periods and the potential (mV) was recorded.
Fig. 1 Nitrogen adsorption desorption isotherms.
1462 | RSC Adv., 2019, 9, 1460–1471
Fig. 3 X-ray photoelectron spectrum of Ni-TPA/ZrO2.
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adsorbed water molecules. Further, there was no weight loss
ꢁ
observed up to 500 C indicating the stability of the supported
catalyst. TGA of Ni-TPA/ZrO2 shows initial weight loss of 8.5%
ꢁ
up to 180 C indicating the loss of adsorbed water molecules.
Besides this, also no signicant weight loss was observed up to
500 ꢁC, suggests higher thermal stability of synthesized catalyst.
The BET surface area of ZrO2, TPA/ZrO2 and Ni-TPA/ZrO2
were measured. Specic surface area decreased for supported
catalysts TPA/ZrO2 (146 m2 gꢀ1) as compared to that of support
ZrO2 (170 m2 gꢀ1) and in good agreement with reported fact43
that there may be decrease in surface area in case of the sup-
ported catalyst in which oxides are used as supports. This is
because of strong interaction of TPA with the oxide support. The
surface area of Ni-TPA/ZrO2 (221 m2 gꢀ1) is higher as compare to
that of TPA/ZrO2, indicating the exchange of available surface
protons of the TPA/ZrO2 with Ni as well as high dispersion of Ni
on the surface of TPA/ZrO2, which is also seen in nitrogen
adsorption desorption isotherms (Fig. 1). It also conrms no
change in the basic structure.
The FT-IR spectra of ZrO2 (Fig. 2) shows broad bands in the
region of 3400, 1600 and 1370, and 600 cmꢀ1 attributed to O–H
asymmetric stretches, H–O–H and O–H–O bending, and Zr–OH
bending respectively.38 FT-IR spectrum of TPA/ZrO2 (Table S1†)
exhibits bands at 1070; 964; and 812 cmꢀ1 corresponding to P–
O, W]O, and W–O–W stretching vibration frequencies
respectively. The FT-IR spectrum of Ni-TPA/ZrO2 also shows all
the characteristic bands for TPA but with signicant shi which
may be due to the exchange of Ni with available protons of TPA,
with an additional band at 495 cmꢀ1 is observed attributing to
Ni–O vibration.
Fig. 4 XRD spectra of (a) ZrO2, (b) TPA (c) TPA/ZrO2 and (d) Ni-TPA/
ZrO2.
Raman spectra of ZrO2 (Fig. S2†) shows broad peaks in the
region from 200 to 800 cmꢀ1, which are associated with long-
range disordering arrangement in amorphous state.44 Raman
spectra of TPA/ZrO2 shows bands at 1010, 990, 900, 550, and
217 cmꢀ1, which are assigned to ys (W–Od), yas (W–Od), yas (W–
Ob–W), ys (W–Oc–W), and ys (W–Oa), respectively where Oa, Ob,
Oc, and Od correspond to the oxygen atoms linked to phos-
phorus, to oxygen atoms bridging two tungsten (from two
different triads for Ob and from the same triad for Oc), and to
the terminal oxygen W]O, respectively. Raman spectra of Ni-
TPA/ZrO2 shows bands at 947, 929, 859, 532 and 232, corre-
sponding to ys (W–Od), yas (W–Od), yas (W–Ob–W), ys (W–Oc–W),
and ys (W–Oa) respectively. The decrease in intensities of the
bands as well as signicant shiing45 in all the characteristic
bands may be due to change in the environment because of the
exchange of surface protons of TPA by nickel.
The full range (5000–2500 G) X-band liquid nitrogen
temperature ESR spectrum for Ni-TPA/ZrO2 (Fig. S3†) was
recorded. The calculated g value (g ꢂ 2.05) conrms the pres-
ence of Ni(II) species, in good agreement with reported one.46
Further, the presence of Ni(II) is conrmed by XPS.
The XPS of Ni-TPA/ZrO2 is displayed in Fig. 3. A low intense
peak at 854.5 eV (2p3/2) is in good agreement with reported one47
conrming the presence of Ni(II) on the surface. However, the
obtained poor-quality spectra with low peak intensity of 2p3/2
may be due to the presence of very low concentration of Ni (0.3
%wt) in the synthesized catalyst.
Scheme 2 Oxidative esterification of benzaldehyde.
Ni-TPA/ZrO2 respectively. Here, low exchanged amount of nickel
in each catalyst, indicates that only the protons of TPA were
exchanged by the Ni. EDX value of W (16.45 wt%) and Ni
(0.32 wt%) in (0.04 M) Ni-TPA/ZrO2 is in good agreement with
calculated one (17.17 wt% of W, 0.30 wt% of Ni). Very low % of
Ni indicates that only the protons of TPA were exchanged.
The types and strength of the acidic sites were determined by
potentiometric titration. The strength of acidic sites in terms of
initial electrode potential is shown in Table 1. The drastic
increase in acidic sites as well as acidic strength of all nickel-
based catalysts compare to that of TPA/ZrO2, may be due to
the exchange of available protons of TPA/ZrO2 by nickel. It is
interesting to note down that, as increase in nickel concentra-
tion, the total numbers of acidic sites are also increases and this
may be due to the Lewis acidity of Ni. The total acidity of (0.05
M) Ni-TPA/ZrO2 is higher compare to (0.04 M) Ni-TPA/ZrO2 but
the acidic strength and strong acidic sites are less, this may be
due to blocking of strong acidic sites at higher exchange. Hence,
(0.04 M) Ni-TPA/ZrO2 was selected for detail characterization
and re-designated as Ni-TPA/ZrO2.
TGA curve (Fig. S1†) of TPA/ZrO2 shows 12.6% weight loss in
the temperature range of 70–100 ꢁC indicating the loss of
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Fig. 5 Optimization of parameters for oxidative esterification of benzaldehyde. Optimization of parameters for oxidative esterification of
benzaldehyde. Reaction conditions: (A) effect of the catalyst amount: benzaldehyde (10 mmol), H2O2 (30 mmol), methanol (5 mL), catalyst/
substrate ratio (5.11 ꢃ 10ꢀ5), time (6 h), temp. (80 ꢁC); (B) effect of the temperature: benzaldehyde (10 mmol), H2O2 (30 mmol), methanol (5 mL),
catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5), time (6 h); (C) effect of the methanol amount: benzaldehyde (10 mmol), H2O2 (30 mmol),
catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5), time (6 h), temp. (80 ꢁC) and (D) effect of the time: benzaldehyde (10 mmol), H2O2 (30
mmol), methanol (5 mL), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5), temp. (80 ꢁC).
The XRD patterns of ZrO2, TPA, TPA/ZrO2 and Ni-TPA/ZrO2
TEM images of the fresh catalyst Ni-TPA/ZrO2 are presented
catalysts are shown in Fig. 4. The XRD patterns of ZrO2 (Fig. 4a) in (Fig. S4†) at various magnications. SAED image (a) indicates
shows the amorphous nature of the support. Absence of char- the non-crystalline and uniform distribution of nickel in the
acteristic diffraction lines corresponding to TPA, in the XRD synthesized catalyst. Images (b and c) show the uniform
pattern of TPA/ZrO2 (Fig. 4c), indicates a high dispersion of TPA dispersion of Ni on the surface of TPA/ZrO2.
on the surface of ZrO2. The XRD patterns of Ni-TPA/ZrO2
In summary, FT-IR and Raman spectrum indicate the
(Fig. 4d) are very similar to TPA/ZrO2, suggesting very high retention of TPA structure in the catalyst as well as exchange of
dispersion of nickel on the non-crystalline surface of TPA/ZrO2, nickel with available surface protons of TPA/ZrO2. ESR and XPS
which is also observed in TEM.
studies conrm the presence of Ni(II) in the catalyst. While XRD
and TEM reveals the uniform dispersion of Ni over TPA/ZrO2.
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Table 2 Effect of mole ratioa
Catalytic activity
Oxidative esterication of benzaldehyde. To evaluate the
efficiency of the catalyst, for the oxidative esterication of
benzaldehyde into their corresponding esters, benzaldehyde
and methanol were selected as test substrates in the presence of
hydrogen peroxide as an oxidant (Scheme 2). Effect of different
reaction parameters such as nickel concentration, molar ratio
of substrate to H2O2 as well as methanol, catalyst amount,
reaction temperature and reaction time were studied to opti-
mize the conditions for maximum conversion.
% Selectivity
Substrate : H2O2
% Conversion
Ester
Acid
1 : 1
1 : 2
1 : 3
1 : 4
34
54
65
51
74
70
63
56
26
30
37
44
a
Reaction conditions: benzaldehyde (10 mmol), oxidant (H2O2),
methanol (5 mL), catalyst (10 mg), time (6 h), temp. (80 ꢁC).
The reaction was carried out in presence of different
concentration of nickel and the results are presented in Table
S2.† It can be seen that initially, % conversion increases with
increase in nickel concentration without change in % selectivity
of the desired product. This can be explained on the bases of the
acidity of the catalyst (Table 1). With increase in concentration
of nickel, the % conversion also increases due to the increase in
total number of acidic sites of the catalyst. With further increase
in nickel concentration (0.05 M) the % conversion decreases
with increase in % selectivity of the desired product. This may
be due to the lower acidic strength of the (0.05 M) Ni-TPA/ZrO2
compare to Ni-TPA/ZrO2 (Table 1). Conversion was achieved
highest in case of Ni-TPA/ZrO2 compared to (0.03 M) Ni-TPA/
ZrO2 and (0.05 M) Ni-TPA/ZrO2. Hence, further optimization
was carried out using Ni-TPA/ZrO2.
chemical dynamics34 according to which the oxidative esteri-
cation could be improved by increasing the amount of H2O2.
Simultaneously, the selectivity of the desired product, ester
decreases while that of acid increases because higher amount of
oxidant will tolerate more benzaldehyde to oxidize in benzoic
acid. Further with increase in oxidant amount (1 : 4) the drastic
decrease in conversion was found and hence substrate to
oxidant ratio (1 : 3) was optimized.
From the above study, the conditions optimized for the
maximum conversion are: benzaldehyde (10 mmol), H2O2 (30
mmol), methanol (5 mL), conc. of Ni (5.11 ꢃ 10ꢀ4 mmol),
catalyst/substrate ratio (5.11 ꢃ 10ꢀ5), time (6 h), temp. (80 C).
ꢁ
Conversion ¼ 65% with TON ¼ 12 712.
The effect of the catalyst amount was evaluated by varying it
from 5 to 25 mg (Fig. 5A). By increasing the catalyst amount up
to 10 mg conversion was decreases with increase in % selectivity
of desired product, with further increase in catalyst amount, %
conversion as well as selectivity decreases. This is due to
unproductive decomposition of H2O2 which generates addi-
tional water and results in decrease in selectivity of ester, is in
good agreement with the reported one.37 Maximum % conver-
sion with highest % selectivity of ester was obtained for 10 mg
Control experiments and investigation of mechanism.
Number of experiments were carried out in order to study the
role of each substrate (Table S3†) as well as catalyst: (i) without
catalyst (benzaldehyde + methanol + H2O2) the reaction gives
negligible conversion (ii) without H2O2 (benzaldehyde + meth-
anol + catalyst) no signicant reaction occurs, (iii) without
methanol (benzaldehyde + H2O2 + catalyst), oxidation of alde-
hyde to benzoic acid takes place. In all three cases, benzoic acid
was obtained as a single product. This study shows that both
H2O2 and methanol as well as catalyst are essential for the
oxidative esterication reaction to take place.
The control experiments were also carried out with Ni(CH3-
COO)2, ZrO2, TPA/ZrO2 and Ni-TPA/ZrO2 in identical experi-
mental conditions and results are presented in Table 3. ZrO2
and TPA/ZrO2 both were moderately active with low selectivity of
the desired product benzoate ester. However, Ni(CH3COO)2
gives 54% conversion and 52% ester selectivity as expected. The
ꢁ
of catalyst. The temperature effect screened from 60 to 90 C,
with increase in temperature conversion also increases with
decrease in ester selectivity (Fig. 5B), may be due to the thermal
decomposition of H2O2.37 80 C temperature was optimized to
ꢁ
achieve maximum conversion as well as ester selectivity. Then,
effect of methanol volume was carried out from 1 to 6 mL
(Fig. 5C). Results show that with enlarging methanol amount,
conversion decreases with increase in ester selectivity. Here,
decrease in conversion is because of dilution of the substrate
concentration.37 From obtained results, 5 mL of methanol was
optimized. Finally, the time effect was performed with in 4 h to
12 h range (Fig. 5D). Results indicate that with rise in time the
conversion increases whereas selectivity of ester decreases. This
is because of decomposition of H2O2 which results in the
formation of the water molecule to hydrolyze the ester to alde-
hyde. From the results, 6 h time was optimized for the reaction.
The obtained results are summarized in Fig. 5.
Table
3 Control experiments for oxidative esterification of
benzaldehydea
% Selectivity
Ester
Catalyst
% Conversion
Acid
aNi(CH3COO)2
ZrO2
TPA/ZrO2
54
27
52
65
52
41
43
63
48
59
57
37
Further to see the effect of H2O2, the reaction was carried out
with different mole ratio of substrate : H2O2 keeping all other aNi-TPA/ZrO2
parameters constant (Table 2). From the results it can be seen
that the conversion increases with increasing oxidant amount
up to mole ratio (1 : 3) and is in good agreement with the
a
Reaction conditions: benzaldehyde (10 mmol), H2O2 (30 mmol),
methanol (5 mL), catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ
10ꢀ5), time (6 h), temp. (80 ꢁC). Active amount of nickel (5.11 ꢃ 10ꢀ4
a
mmol).
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Scheme 4 Oxidative esterification of benzyl alcohol.
as an oxidant (Scheme 4). The inuence of different reaction
parameters such as nickel concentration, molar ratio of
substrate to H2O2 as well as methanol, catalyst amount, reaction
temperature and reaction time was studied to optimize the
conditions for maximum conversion.
To determine the effect of nickel concentration, the reaction
was carried using (0.03 M) Ni-TPA/ZrO2, Ni-TPA/ZrO2 and (0.05
M) Ni-TPA/ZrO2. The obtained results are shown in Table S4.†
The trend in the % conversion as well as % selectivity was found
to be the same as earlier (Table S2†). And hence, further opti-
mization was carried out using Ni-TPA/ZrO2.
Scheme 3 Proposed reaction mechanism for oxidative esterification
of benzaldehyde.
To optimize the catalyst amount, the reaction was performed
by varying amount (i.e. from 5 to 25 mg) by keeping other
parameters constant. Oxidative esterication is signicantly
affected by acidity as well as oxidizing property of the catalyst.
The increase in the conversion can be attributed to an increase
in the number of available catalytically active sites. From Fig. 6
it can be seen that initially the conversion increases with
increasing the amount from 5 to 10 mg. Further, with
increasing the amount, the conversion as well as selectivity of
ester decreases as observed earlier (Fig. 5A). This may be due to
the rapid catalytic decomposition of H2O2 in presence of excess
amount of catalyst which generates the water molecules.37 As
esterication is reversible process in presence of water, the
selectivity of the ester product decreases with increase in cata-
lyst amount.
increase in the conversion as well as selectivity in case of Ni-
TPA/ZrO2 shows the synergic effect due to the presence of
nickel, which is also conrmed by acidity measurement. Total
number of acidic sites as well as acidic strength (Table 1) is
almost double and responsible for increase in activity.
For oxidative esterication of benzaldehyde, we are expect-
ing the same mechanism as reported by us earlier,36 contra-
dictory to general mechanism. The general mechanism
demonstrates the formation of acetal as intermediate.48 To
explain the contradiction, following set of experiments was
carried out. (1) When benzoic acid is used as a substrate instead
of benzaldehyde under identical conditions (in absence of
H2O2), 34% conversion with 100% selectivity of methyl benzoate
was obtained. (2) In another reaction, benzaldehyde was reacted
with methanol in presence of catalyst and absence of H2O2 gives
11% conversion with 100% selectivity for benzoic acid in 6 h. On
prolonging the reaction for 3 h aer addition of H2O2, 46%
conversion was achieved with 60% ester selectivity. The study
shows that the oxidation of benzaldehyde does not proceed
through an acetal intermediate, but through in situ formation of
benzoic acid. The proposed reaction mechanism is shown in
Scheme 3. Benzaldehyde is rst converted into benzoic acid in
presence of H2O2 and the formed benzoic acid is esteried in
presence of methanol to desire ester. It must be noted that both
the steps take place in situ. Here, it might be possible that in the
beginning, the formation of metal-peroxo active intermediate
(here, metal can be both, W as well as Ni) takes place.49
However, as reported by us earlier, the detail mechanism of the
synthesis of esters involving POM and transition metal is not
clear.49
Oxidative esterication of benzyl alcohol. To evaluate the
competence of the catalyst, for the direct conversion of into
their corresponding esters, benzyl alcohol and methanol were
selected as test substrates in the presence of hydrogen peroxide
Fig. 6 Effect of catalyst amount. Reaction conditions: benzyl alcohol
(10 mmol), H2O2 (30 mmol), methanol (7.5 mL), time (24 h), temp.
(80 ꢁC).
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Table 4 Effect of methanol amounta
% Selectivity
Benzaldehyde
Methanol (mL)
% Conversion
Ester
Acid
26
12
10
9
7
4
4
5
1
2
3
4
5
6
7
7.5
8
38
45
49
48
47
46
45
43
35
61
77
75
75
72
69
65
61
55
13
11
15
16
21
27
31
34
40
5
a
Reaction condition: benzyl alcohol (10 mmol), H2O2 (30 mmol),
catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5), temp. (80 ꢁC),
time (24 h).
Effect of methanol amount was studied in the range of 1 mL
to 8 mL and obtained results are presented in Table 4. Initially,
the amount of methanol increased up to 7.5 mL, the %
conversion remains almost same with increase in the % selec-
tivity of the desired product. With further increase in methanol
quantity (8 mL), % conversion decreases; this is because of
higher the volume of methanol, which dilute the concentration
of the substrates. Hence, 7.5 mL volume of methanol was
optimized for the reaction for further study.
Fig. 7 Effect of temperature. Reaction condition: benzyl alcohol (10
mmol), H2O2 (30 mmol), methanol (7.5 mL), catalyst (10 mg), catalyst/
substrate ratio (5.11 ꢃ 10ꢀ5), time (24 h).
Ni (5.11 ꢃ 10ꢀ4 mmol), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5),
ꢁ
time (24 h) and temp. (80 C).
Control experiments and investigation of mechanism.
Number of experiments (Table S5†) were carried out in order to
study the role of each substrate as well as catalyst: (i) without
catalyst (benzyl alcohol + methanol + H2O2), the reaction did not
progress signicantly (ii) without oxidant (benzyl alcohol +
methanol + catalyst) reaction did not occur, (iii) without
methanol (benzyl alcohol + H2O2 + catalyst), oxidation of benzyl
alcohol to benzaldehyde takes place. Two things can be
concluded from the above studies (i) H2O2, methanol as well as
catalyst are essential for the feasibility of the oxidative esteri-
cation reaction and (ii) W and Ni both are responsible (due to
the synergic effect) for the oxidation as in later case the found
conversion was almost double.
The control experiments were also carried out with Ni(CH3-
COO)2, ZrO2, TPA/ZrO2 and Ni-TPA/ZrO2 in identical experi-
mental conditions and results are presented in Table 6. ZrO2
and TPA/ZrO2 both were moderately active with low selectivity of
the desired product, benzoate ester. In presence of nickel salt
Ni(CH3COO)2, only 5% conversion with 32% ester selectivity
was obtained. The increase in the conversion as well as selec-
tivity in case of Ni-TPA/ZrO2 shows the synergic effect due to the
The effect of temperature was carried out and obtained
results were presented in Fig. 7, it shows that conversion as well
selectivity of ester increases with increase in the temperature
ꢁ
ꢁ
from 50 C to 80 C. Further, with increase in temperature to
ꢁ
90 C, conversion also increases but ester selectivity decreases.
This may be due to the thermal decomposition of the H2O2.
Decomposition of H2O2 produces water molecule and hence
ester hydrolysed to benzaldehyde.37
The effect of time was investigated by varying the reaction
time. As shown in Fig. S5,† the reaction was performed at three
different time intervals. Initially, increase in the reaction time
from 18 h to 24 h, conversion as well as ester selectivity
increases. Further, increasing the reaction time shows no
notable change in the results because of attainment of reaction
equilibrium.37 So, 24 h is optimized for the maximum reaction
conversion and ester selectivity.
Finally, to see the effect of H2O2, the reaction was carried out
with different mole ratio of substrate : H2O2 keeping all other
parameters constant (Table 5). From the results it can be seen
that the conversion increases with increasing the oxidant
amount with the selectivity of desired product ester up to mole
ratio (1 : 3). Because it is well known that the oxidative esteri-
cation could be improved by increasing the amount of H2O2,
according to the chemical dynamics.34 Further with increase in
oxidant amount (1 : 4) the slight decrease in conversion was
found. Hence, (1 : 3) substrate to oxidant ratio was optimized
for further catalytic optimization.
Table 5 Effect of mole ratioa
% Selectivity
Substrate : H2O2
% Conversion
Aldehyde
Ester
Acid
1 : 1
1 : 2
1 : 3
1 : 4
10
34
43
41
89
80
61
41
11
20
34
33
—
—
5
From the above study, the conditions optimized for the
maximum conversion (43%) with TON ¼ 8415 are: benzyl
alcohol (10 mmol), H2O2 (30 mmol), methanol (7.5 mL), conc. of
4
a
Reaction conditions: benzyl alcohol (10 mmol), oxidant (H2O2),
methanol (7.5 mL), catalyst (10 mg), time (24 h), temp. (80 ꢁC).
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Table 6 Control experiments for oxidative esterification of benzyl
alcohola
% Selectivity
Catalyst
% Conversion
Benzaldehyde
Ester
Acid
aNi(CH3COO)2
ZrO2
5
10
22
43
68
77
61
61
32
23
27
34
—
—
12
5
TPA/ZrO2
aNi-TPA/ZrO2
a
Reaction conditions: benzyl alcohol (10 mmol), H2O2 (30 mmol),
methanol (7.5 mL), catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ
10ꢀ5), time (24 h), temp. (80 ꢁC). aActive amount of nickel (5.11 ꢃ
10ꢀ4 mmol).
presence of nickel, which is also conrmed by acidity
measurement. Total number of acidic sites as well as acidic
strength (Table 1) is almost double and responsible for increase
in activity.
For oxidative esterication of benzyl alcohol, two possible
mechanisms are known (1) two step mechanism involving
oxidation of alcohol to benzaldehyde and further oxidative
esterication of benzaldehyde to ester via benzoic acid forma-
tion and (2) formation of hemiacetal as an intermediate,25,26
whose further dehydration gives the ester. In order to know
about the possible mechanism, the following experiments have Scheme 5 Proposed reaction mechanism for oxidative esterification
of benzyl alcohol.
been carried out. (1) Benzyl alcohol was reacted with H2O2 in
presence of catalyst, without adding methanol, under the
out for 12 h, centrifuged to remove the catalyst and then ltrate
was allowed to react up to 24 h. Aer that obtained reaction
mixtures were extracted by using dichloromethane and
analyzed by Gas chromatogram. Results showed that there was
no signicant change in conversion of the reaction aer
removing the catalyst, indicates no leaching of Ni during the
reaction. This study indicates that TPA plays an important role
to bind Ni very strongly and thus does not allow the leaching of
Ni into the reaction mixture, making it a true heterogeneous
catalyst to recycle and reuse. Here, heterogeneous nature of the
catalyst and withholding of the active species on the support
during the reaction shows that present catalyst is of category
C.50
identical conditions, 23% conversion with 98% selectivity of
benzaldehyde was obtained. Benzoic acid was obtained with
poor selectivity (2%). (2) Benzyl alcohol was reacted with
methanol in presence of catalyst, without adding H2O2, no
conversion was found. However, on prolonging the same reac-
tion with addition of H2O2 up to 24 h, 41% conversion was
achieved with % selectivity 68, 26 and 8 for benzaldehyde, ester
and acid respectively. The obtained results are consistent with
two step mechanism i.e. oxidation of benzyl alcohol to benzal-
dehyde (also conrmed from Table S5†) and further oxidative
esterication of benzaldehyde to ester formation. Hence, we are
assuming same mechanism, as proposed for oxidative esteri-
cation of aldehyde. Only the difference is that in present case,
rst alcohol is converted in to aldehyde and then it follows the
same mechanism. Based on the above study, the following
reaction mechanism is proposed (Scheme 5).
The recovery and reusability of the catalyst is a key issue for
the sustainability of any catalytic process. Results of recycla-
bility for oxidative esterication of benzaldehyde and oxidative
esterication of benzyl alcohol are shown in Fig. 8 (a and b)
respectively for Ni-TPA/ZrO2. Aer completion of reaction, the
It is interesting to note that activity of the catalyst for
oxidative esterication of benzaldehyde is greater than benzyl
alcohol. This is in good agreement with the proposed mecha-
nism, which shows that the latter involves one more additional
step. i.e. oxidation of benzyl alcohol to benzaldehyde.
Table 7 Heterogeneity test for oxidative esterification of benzalde-
hyde reactiona
% Selectivity
Heterogeneity test
Catalyst
% Conversion
Ester
Acid
In both the cases, heterogeneity test was carried out by centri-
fuging the catalyst from the reaction mixture. In the case of
oxidative esterication of benzaldehyde (Table 7) reaction was
carried out up to 4 h, then reaction mixture was centrifuged and
ltrate was allowed to react up to 6 h. Similarly, oxidative
esterication of benzyl alcohol (Table 8), reaction was carried
Ni-TPA/ZrO2
50 (aer 4 h)
50 (aer 6 h)
62
61
38
39
a
Reaction conditions: benzaldehyde (10 mmol), H2O2 (30 mmol),
methanol (5 mL), catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ
10ꢀ5), temp. (80 ꢁC).
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Table
8 Heterogeneity test for oxidative esterification of benzyl
alcohol reactiona
% Selectivity
Benzaldehyde
Catalyst
% Conversion
Ester
Acid
Ni-TPA/ZrO2
20 (aer 12 h)
19 (aer 24 h)
71
72
25
24
4
4
a
Reaction conditions: benzyl alcohol (10 mmol), H2O2 (30 mmol),
catalyst (10 mg), methanol (7.5 mL), catalyst/substrate ratio (5.11 ꢃ
10ꢀ5), temp. (80 ꢁC).
organic layer was extracted by dichloromethane. The catalyst
was recovered by simple centrifugation, washed with dichloro-
methane followed by water and then dried in oven at 100 ꢁC for
an hour and nally used for the next cycle. Present catalyst was
examined up to three cycles only, due to the small scale of
reaction with very low concentration of catalyst. In both the
cases obtained results show that there was no signicant
change in conversion as well as in desired product selectivity.
The constant results indicate the activity of the catalyst towards
its repetitive use and its true heterogeneity nature during the
reaction.
Fig. 9 FTIR spectra of (a) Ni-TPA/ZrO2 and (b) R-Ni-TPA/ZrO2.
Characterization of regenerated catalyst
Regenerated catalyst was characterized by elemental analysis
(EDX), n-butyl amine acidity, potentiometric acidity, FT-IR and
TEM analysis for the conrmation of the catalyst structure
retention.
EDX values of Ni (0.34 wt%) and W (16.52 wt%) of regen-
erated Ni-TPA/ZrO2 is in good agreement with values of fresh
catalyst (16.45 wt% of Ni, 0.32 wt% of Ni) conrming no emis-
sion of Ni as well as TPA from ZrO2 during the reaction.
The acidity and total number of acidic sites of the fresh and
regenerated catalyst were determined by volumetric titration
Fig. 10 XPS of regenerated Ni-TPA/ZrO2.
Fig. 8 Oxidative esterification of (a) benzaldehyde and (b) benzyl alcohol with fresh and regenerated catalyst. Reaction conditions: benzalde-
hyde (10 mmol), H2O2 (30 mmol), methanol (5 mL), time (6 h). Reaction conditions: benzyl alcohol (10 mmol), H2O2 (30 mmol), methanol (7.5
mL), time (24 h). Catalyst (10 mg), temp. (80 ꢁC), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5).
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Table 9 Comparison with reported catalysta
Types of acidic sites (meq. gꢀ1
)
% Selectivity
Acidic strength
Ei (mV)
Total no. of
acidic sites
Catalyst
Very Strong
Strong
Weak
% Conversion
Ester
Acid
a
Ni-TPA/ZrO2
87
50
110
0
0
0.2
2.3
0.7
1.3
1.7
1.7
2.2
4.0
2.4
3.7
65
65
40
63
55
75
37
45
25
b CsPW11Ni36
37
c CsPW11Ni/ZrO2
a
Reaction conditions: benzaldehyde (10 mmol), H2O2 (30 mmol), methanol (5 mL), catalyst (10 mg), catalyst/substrate ratio (5.11 ꢃ 10ꢀ5), time (6
h), temp. (80 ꢁC). Active amount of nickel (5.11 ꢃ 10ꢀ4 mmol). bActive amount of nickel (1.5 ꢃ 10ꢀ3 mmol). Active amount of nickel (5 ꢃ 10ꢀ4
a
c
mmol).
with n-butyl amine as well as by potentiometric titration, as ZrO2).37 Comparison shows that present catalyst gives higher %
shown in Table S6† respectively. From the tables it can be seen conversion compared to CsPW11Ni/ZrO2, which may be due to
that acidic sites of regenerated catalyst slightly decrease presence of higher total number of acidic sites whereas selec-
compared to fresh catalyst whereas acidic strength in terms of tivity of ester was low due to lower acidic strength. However,
initial electrode potential also decreases. This obtained result homogeneous catalyst CsPW11Ni gives the identical % conver-
indicates that few strong acidic sites get blocked during the sion compare to present catalyst, but it must be noted that in
reaction. Here, blocking of strong acidic sites did not alter the case of homogeneous catalyst the active amount of nickel was 3
activity of the regenerated catalyst, which can be seen from the times more compared to present catalyst.
catalytic activity of the regenerated catalyst as shown in Fig. 8.
At the same time, not a single report is available on oxidative
The FTIR spectra of the fresh catalyst and regenerated cata- esterication of benzyl alcohol using polyoxometalate.
lyst are shown in Fig. 9. From the gure it can be seen that
almost identical spectrum was obtained without any signicant
shi in the bands of regenerated catalyst (R-Ni-TPA/ZrO2)
Conclusion
New versatile bi-functional heterogeneous catalyst comprising
nickel and supported 12-tungstophosphoric acid was intro-
duced for base free one pot oxidative esterication of benzal-
dehyde as well as benzyl alcohol. The catalyst proved to be a bi-
functional one by combining their acidic and redox properties.
The superiority of present work lies in obtaining higher
conversion as well as higher selectivity of the desired product
with high TON (>8000 in both the cases) under sustainable
reaction conditions. The activity of the catalyst for oxidative
esterication of benzyl alcohol was lower than benzaldehyde
because the former reaction involves one more additional step.
i.e. oxidation of benzyl alcohol to benzaldehyde, which
undergoes oxidative esterication. The catalyst can be recycled
up to three cycles without any degradation. In addition, the
advantages of using Ni as an alternative of more expensive
metals as heterogeneous catalyst makes this methodology
interesting from an economic as well as an ecological point of
view.
compare to the fresh catalyst (Ni-TPA/ZrO2), indicates that
catalyst structure remains unaltered even aer the regenera-
tion. However, the spectrum is slightly different from the fresh
one in terms of intensity. This might be due to the sticking of
the substrates on the surface, although this might not be
signicant in the reutilization of the catalyst.51
The X-ray photoelectron spectrum of regenerated Ni-TPA/
ZrO2 is displayed in Fig. 10. A low intense peak at 854.6 eV (2p3/
2) is in agreement with reported one47 and conrms the pres-
ence of Ni(II) on the surface. Obtained spectrum is identical with
the fresh catalyst Ni-TPA/ZrO2, conrms that catalyst is
sustainable during the reaction.
TEM images of regenerated catalyst Ni-TPA/ZrO2 are dis-
played in Fig. S6† at various magnications. SAED image (a)
indicates the non-crystalline and uniform distribution of nickel
in the regenerated catalyst. Images (b and c) show the dark
uniform suspension in the amorphous nature of the catalyst.
This indicates the uniform dispersion of the catalyst over the
surface of the support. Obtained images were identical with the
fresh catalyst, show the stability and sustainability of the cata-
lyst during the reaction.
Conflicts of interest
There are no conicts to declare.
Comparison with reported catalyst
Acknowledgements
Catalytic activity of the present catalyst is also compared with
reported one (Table 9). Literature survey shows that only two We are thankful to Department of Atomic Energy (DAE) and
reports are available, and that are by our group only, in which Board of Research in Nuclear Science (BRNS), Project No. 37(2)/
polyoxometalate and nickel based catalysts are used for the 14/34/2014-BRNS, Mumbai, for the nancial support. One of the
oxidative esterication of benzaldehyde: (i) Cs salt of mono authors Mr Anish Patel is thankful to the same for the grant of
nickel substituted phosphotungstate (CsPW11Ni) in homoge- JRF. We are also thankful to Department of Chemistry, The
neous medium36 and (ii) heterogeneous catalyst, supported Cs Maharaja Sayajirao University of Baroda for BET surface area
salt of mono nickel substituted phosphotungstate (CsPW11Ni/ analysis.
1470 | RSC Adv., 2019, 9, 1460–1471
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Eur. J., 2008, 14, 7131.
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