Nickel salt of phosphomolybdic acid as a bi-functional homogeneous recyclable catalyst for base free transformation of aldehyde into ester

Article abstract of DOI:10.1039/d0ra04119j

A Ni salt of phosphomolybdic acid (NiHPMA) was synthesized and characterized by various physico-chemical techniques such as EDX, UV-Visible spectroscopy, FT-IR, Raman spectroscopy and XPS. FT-IR and Raman spectroscopy confirm the presence of Ni as a counter cation while UV-Visible and XPS studies to confirm the presence of Ni(ii) in the catalyst. The catalyst was evaluated for its bi-functional activity towards the tandem conversion of benzaldehyde to ethyl benzoate and it was found that very small amounts of Ni (2.64 × 10^?3mmol) enhance the selectivity towards benzoate. A detailed mechanistic study was carried out by UV-Visible and Raman spectroscopy to confirm that both intermediate species, Mo-peroxo and Ni-oxo, are responsible for higher selectivity towards esters. Further, a study to determine the effect of addenda atoms (heteropoly acid) was also carried out. The catalyst was also found to be viable for a number of aldehydes under optimized conditions.

Full text of DOI:10.1039/d0ra04119j

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Nickel salt of phosphomolybdic acid as a bi-
functional homogeneous recyclable catalyst for
Cite this: RSC Adv., 2020, 10, 22146
base free transformation of aldehyde into ester†
*
Anjali Patel
and Jay Patel
A Ni salt of phosphomolybdic acid (NiHPMA) was synthesized and characterized by various physico-
chemical techniques such as EDX, UV-Visible spectroscopy, FT-IR, Raman spectroscopy and XPS. FT-IR
and Raman spectroscopy confirm the presence of Ni as a counter cation while UV-Visible and XPS
studies to confirm the presence of Ni(II) in the catalyst. The catalyst was evaluated for its bi-functional
activity towards the tandem conversion of benzaldehyde to ethyl benzoate and it was found that very
small amounts of Ni (2.64 ꢀ 10ꢁ3 mmol) enhance the selectivity towards benzoate. A detailed
mechanistic study was carried out by UV-Visible and Raman spectroscopy to confirm that both
intermediate species, Mo–peroxo and Ni–oxo, are responsible for higher selectivity towards esters.
Further, a study to determine the effect of addenda atoms (heteropoly acid) was also carried out. The
catalyst was also found to be viable for a number of aldehydes under optimized conditions.
Received 7th May 2020
Accepted 29th May 2020
DOI: 10.1039/d0ra04119j
rsc.li/rsc-advances
reproducibility issues, and others.33,34 Towards the same, HPAs-
catalysed reactions have gained considerable interest for the
synthesis of few important compounds.20,35
Introduction
Keggin type heteropoly acids (HPAs) are an exclusive class of
inorganic compounds which have metal oxygen supra-molecular
clusters of early transition metal groups V (V, Nb, and Ta) and
VI (Cr, Mo, and W) in their higher oxidation states1–3 and have an
oxo-enriched surface, high thermal stability, and tunable acidic/
redox properties.4–9 They are useful acids and oxidation catalysts
in various reactions because their catalytic features can be varied
at a molecular level.10–16 In particular, the use of HPAs and their
related compounds as green catalysts is a eld of increasing
worldwide importance and numerous studies have been carried
out in basic and applied research in the last four decades.17–22 For
example, heteropoly acid-based ionic liquids (HPAILs) as novel
functionalized ionic liquids have attracted increasing atten-
tion,23,24 and have been reported as efficient and recyclable cata-
lysts for lipophilic alkene epoxidation and alcohol selective
oxidation with aqueous H2O2.25–28
Recently, the development of multicomponent reactions
(MCRs) and their applications for one-pot production of several
important compounds has fostered the synthetic toolbox expan-
sion.29–32 Although MCRs have been regarded as “advanced tools
for sustainable organic synthesis”, many have questioned their
potential due to several drawbacks such as low yields, long reac-
tion times, harsh conditions, requirement of reagents excesses,
More recently it was found that HPA derivatives embedded in
ionic liquids media have been successfully applied to catalyse
some MCRs.36–38 For example,
a polymeric heteropolyacid-
containing pyridinium IL catalyst proved to be a promising
system for the multicomponent Biginelli reaction.39 The Biginelli
reaction is believed by some to be the most important MCR.40 This
MCR (Scheme 1) allows the direct synthesis of bioactive DHMPs
(3,4-dihydropyrimidin-2(1H)-ones or -thiones) such as monastrol,
piperastrol, and enastron.41–46
It is well known that HPAs possess a very strong acidity in form
of protons, which can be changed by varying the chemical
composition of the primary structure of the heteropolyanion.17
These exchangeable protons can be easily replaced by other
cations, without affecting primary structure.47,48 Depending on the
nature of cations (i.e. salt of HPAs), acidic/redox properties can be
tuned, which make them important catalytic materials for various
organic transformations.49–51 Amongst, phosphotungstic acid is
studied in detailed and number of reports are available for the
same. At same time comparatively studied on phosphomolybdic
acid is very less.
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The Maharaja Sayajirao University of Baroda, Vadodara-390002, India.
E-mail: anjali.patel-chem@msubaroda.ac.in
† Electronic supplementary information (ESI) available: Figure of TGA and
UV-Visible spectrum of NiHPMA. See DOI: 10.1039/d0ra04119j
Scheme 1 Oxidative esterification of benzaldehyde.
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In 1974 Tsigdinos,52 rst time, reported the synthesis of Ni3[- 0.5 mmol) was added drop wise into solution of H3PMo12O40
PMo12O40]2$34H2O and studied their thermal behavior, solubility (0.9126 g, 0.5 mmol) dissolved in a minimum amount of water.
ꢂ
and hydrolytic strength in different solvents mixture including The resulting mixture was aged for 1 h at 80 C and excess of
aqueous medium. Aer almost two decades, in 1994, Mizuno water was evaporated on water bath. The obtained material was
et al.53 reported synthesis of Cs2.5Ni0.08H0.34PMo12O40 and its use dried at 100 C for 10 hours and calcined at 300 C under air
for oxidation of isobutene to methacrylic acid and methacrolein atmosphere for 2 h in a muffle furnace. The resulting green
using molecular O2. In 1999, Demirel et al.54 reported liquefaction color, material was designated as NiHPMA.
ꢂ
ꢂ
of Wyodak coal using same catalyst, reported by Tsigdions.52
To see any decomposition, HPMA was also calcined under
Later, in 2010, Rabia et al.55 reported synthesis of (NH4)xMyHz- the same conditions and it was found that color remains yellow,
PMo12O40 (M ¼ Ni2+, Co2+ or Fe3+) using same method as Mizuno good agreement with the reported literature.61
et al. reported53 and catalytic activity for oxidation of propane. In
2015, the same group came up with the use of soluble H3ꢁ2xNix-
PMo12O40 and (NH4)3ꢁ2xNixPMo12O40 where x: 0.25–1.5 for
oxidation of cyclohexanone to adipic acid.56 Further in 2019, same
group57 synthesized a series of transition metal salt of phospho-
molybdic acid [HMPMo12O40 (M: Co, Ni, Mn, Cu or Zn)] by using
their own method and described their use as catalysts for the
synthesis of adipic acid from cyclohexanone.
Characterization
Elemental analysis was performed by EDX using JSM 5610 LV
combined with INCA instrument for EDX-SEM. Thermo-
gravimetric analysis (TGA) was performed under nitrogen atmo-
sphere conditions on the Mettler Toledo star SW 7.01 up to 500 ꢂC.
The UV-Vis spectra were recorded at room temperature on the
Perkin Elmer 35 LAMDA instrument using a quartz cell of 1 cm
within a range of 200–1100 nm. FT-IR spectra were carried out
using Shimadzu instrument (IRAffinity-1S) with KBr wafer pellets.
The Fourier Transform Raman (FT-Raman) spectra were per-
formed on a FT-Raman spectrophotometer (Model Bruker FRA
106). Measurement of X-ray photoelectron spectroscopy (XPS) was
performed with the PHI 5000Versa prob II device with Auger
electron spectroscopy (AES). Philips diffractometer instrument
(Model Pw-1830) was used to perform the powder X-ray diffraction
(Powder XRD).
Above literature survey shows (i) few reports are available on
MCRs using HPAs based compounds, in other words combi-
nation of HPAs and ionic liquids have been used ony for MCRs
(ii) oxidative esterication was not at all studied (iii) although
nickel salt of phosphomolybdic acid is excellent and sustain-
able for the oxidation reaction, full characterization of the same
is missing and no single report is available for any tandem
reaction to show the bi-functional effect of Ni as well as Mo.
Oxidative esterication is one of the best example of tandem
reaction, in which both Lewis acidity as well as redox ability of
the catalyst plays an important role. Oxidative esterication of
benzaldehyde is much more attracting due to the importance
and use of methyl benzoate for perfumery, food industry,
pharmaceutical, agrochemical and natural products.58,59
In the present work, for the rst time, we report detailed
characterization of Ni salt of phosphomolybdic acid by EDX,
TGA, UV-Vis, FT-IR, Raman spectroscopy, and XPS along with
catalytic evaluation for tandem oxidative esterication of
benzaldehyde to ester. The reaction was carried out using
methanol and H2O2 and various reaction parameters such as
catalyst amount, amount of H2O2, the volume of methanol,
reaction time and reaction temperature were optimized for
maximum conversion as well as selectivity towards ester. The
recycling and reusability of synthesized catalyst was carried out
up to three cycles. Detailed reaction mechanism was proposed
based on Raman and UV-Visible spectroscopy studies.
Catalytic evaluation
For oxidative esterication of benzaldehyde, 50 mL glass batch
reactor provided with a double walled air condenser, magnetic
stirrer and guard tube, was charged with benzaldehyde (10
mmol) with H2O2 (30 mmol, 30% H2O2 v/v), methanol (7 mL),
catalyst (5 mg) and stirred at 80 ꢂC for 6 h. Aer completion of
reaction, mixture was extracted with dichloromethane. The
products in obtained organic phase were analyzed by GC
keeping the following temperature progrꢂam: initiallyꢂ, the oven
and capillary temperature was set at 80 C and 250 C respec-
ꢂ
tively, then the temperature increased from 80 to 240 C, with
a rate of 10 ꢂC minꢁ1, compared with the reference standard
samples and no overlapping was found. For reader's conve-
nience, GC prole for the authentic samples as well as reaction
mixture are included in the ESI (Fig. S1).† TON over number
calculated by following formula:
mole of product
Experimental
TON ¼
:
mole of catalyst
Materials
All chemicals used were of A. R. grade. Phosphomolybdic acid
(H3PMo12O40), nickel acetate, benzaldehyde, methanol, 30%
hydrogen peroxide (H2O2) v/v and dichloromethane obtained
from Merck were used as received.
Procedure for recycling of the used catalyst
As recycling of 5 mg catalyst is difficult, we have kept three sets
of reaction under same optimized experimental conditions.
Aer completion of reaction, the reaction mixture was cooled to
room temperature and organic layer was extracted by
Synthesis of nickel salt of phosphomolybdic acid (NiHPMA)
The synthesis was carried out by the following reported method dichloromethane (DCM). The aqueous phase was dried at
in literature.60 A saturated solution of nickel acetate (0.1244 g, 100 ꢂC to recover the homogeneous solid catalyst. The obtained
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solid catalyst (recycled catalyst) was used for next cycle. The elements were in good agreement with the theoretical values.
similar procedure was followed up to three cycles.
EDX value (% wt): Ni, 2.89; Mo, 57.15; P, 1.39; O, 38.57 calcu-
lated value (% wt): Ni, 2.92; Mo, 57.34; P, 1.54; O, 38.20.
TGA of NiHPMA shows (Fig. S3†) initial weight loss of 2.46%
up to 100 ꢂC, due to the removal of adsorbed water. Further, it
shows 4.11% weight loss up to 280 ꢂC corresponding to removal
of water of crystallization. Further, no appreciable weight loss
was noticed, indicating the thermal stability of the synthesized
material up to 500 ꢂC. Based on the total weight loss, the
number of water molecule was calculated using the formula
Results and discussion
Characterization of catalyst
The EDX mapping of NiHPMA in Fig. 1 shows that presence of
all expected elements. The ascertained EDX values of all the
X½M þ 18nꢃ
18n ¼
100
where, n ¼ number of water molecules, X ¼ % loss from TGA
and M ¼ molecular weight of substance (without water of
crystallization) was found to be 6. From the EDX and TGA, the
chemical formula of the synthesized material is proposed NiH
[PMo12O40]$6H2O.
The UV-Visible spectrum of NiHPMA shows (Fig. S4†) the
absorption peak at 230 nm due to the charge transfer between
O2ꢁ / Mo6+.62 In Addition, it also shows an absorption at
396 nm, corresponding to the presence of Ni(II),58 rst indicates
the presence of Ni in the synthesized materials.
Fig. 1 EXD mapping of NiHPMA.
The FT-IR spectra of PMA shows (Fig. 2a) the characteristic
bands at 1070, 965, 870 and 790 cmꢁ1 corresponding to
stretching vibration of P–O, Mo]Ot, Mo–Ob–Mo and Mo–Oc–
Mo respectively.63,64 Whereas NiHPMA shows all characteristic
bands without any signicant shi, 1070, 960, 871 and
786 cmꢁ1 corresponding to stretching vibrations of P–O, M]Ot,
Mo–Ob–Mo and Mo–Oc–Mo respectively with an additional
band at 455 cmꢁ1, characteristic of (Ni–O) bond indicating the
presence of Ni.56,57 Moreover, the presence of P–O stretching is
at exactly same wavelength (1070 cmꢁ1) conrms the presence
of Ni as counter cation only, via exchange of available protons of
PMA.
The Raman spectra of PMA was recorded (Fig. 3a) and is in
good agreement with the reported one.65,66 The Raman spectra
of NiHPMA shows (Fig. 3b) peaks with slight shiing at 1004,
980, 973, 913, 609, and 295 cmꢁ1 for ns(Mo]Ot), nas(Mo–Ot), ns
(P–O), nas(Mo–Ob–Mo), ns(Mo–Ob–Mo) and ns(Mo–Oa) vibration
respectively.67 The observed shiing in all the characteristic
Fig. 2 FT-IR spectra of (a) PMA and (b) NiHPMA.
Fig. 3 Raman spectra of (a) PMA (b) NiHPMA.
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Fig. 4 X-ray photoelectron spectrum of NiHPMA in which (a) Mo(VI) and (b) Ni(II).
bands may be due to change in the environment because of the amount of H2O2. Concurrently, the selectivity towards desired
exchange of available protons of PMA by Ni. product, methyl benzoate, decreases because the higher
The XPS of NiHPMA shows (Fig. 4.) that intense peaks at 233, amount of oxidant will tolerate more oxidation of benzaldehyde
397 and 416 eV corresponds to 3d3/2, 3p3/2 and 3p1/2 respectively to convert into benzoic acid. Hence 1 : 3 ratio of substrate to
and low intense peak at 235 eV corresponds to 3d3/2 energy level H2O2 was optimized.
of Mo(VI).68,69 Furthermore, the peak at 871 eV with lower
The effect of catalyst amount was studied by varying the
intensity corresponds to 2p1/2 energy level of Ni(II) in good amount from 5 to 20 mg. The obtained results are shown in
agreement with reported ones.70–72 However, low intensity of Fig. 5a. On increasing the amount of catalyst from 5 to 20 mg,
peak of Ni can be attributed to low concentration of Ni (2.64 ꢀ decrease in % conversion as well as % selectivity of ester was
10ꢁ3 mmol) in the synthesized material.
observed due to the unproductive decomposition of H2O2 in the
presence of excess catalyst which generates additional water,
shiing the equilibrium towards le and hence the decrease in
conversion, which is in good agreement with the reported one.59
Therefore 5 mg of catalyst amount was optimized for maximum
68% conversion with highest selectivity (86%) towards ester.
The effect of methanol amount was investigated in the range
of 4 to 8 mL (Fig. 5b). Obtained results showed that on
increasing volume of methanol, % conversion decreased and
selectivity of ester increased up to 7 mL, which may be due to
dilution of substrate with methanol. Hence, 7 mL of methanol
was optimized from the obtained results.
Catalytic activity
Oxidative esterication was selected as a model reaction for
evaluation of catalyst efficiency and for this benzaldehyde and
methanol were used as test substrates in the presence of
hydrogen peroxide as an oxidant (Scheme 1). The effect of
various reaction parameters such as catalyst amount, methanol
volume, substrate to H2O2 ratio, time and temperature were
studied to optimize the conditions for maximum conversion as
well as selectivity towards the ester. Each experiment was
carried out thrice and the results obtained were reproducible
with an error of ꢄ1–1.5%.
The effect of temperature was screened between the range of
60 to 90 ꢂC. The obtained results show (Fig. 5c) that on
increasing reaction temperature up to 90 ꢂC, conversion
increased and the selectivity towards ester decreased because of
thermal and catalytic decomposition of H2O2 at high tempera-
To evaluate the effect of H2O2, the reaction was carried out
with different mole ratio of benzaldehyde to H2O2 by keeping all
other parameters constant. Obtained result shows (Table 1) that
on increasing the amount of oxidant from 10 mmol (1 : 1) to
40 mmol (1 : 4), conversion also increases which is in good
agreement with chemical dynamics,72,73 indicate that the
oxidative esterication could be improved by increasing the
ture.59,72 Therefore, 80 C was optimized.
ꢂ
The effect of reaction time was monitored between range of 4
to 8 h (Fig. 5d). Obtained results display that as reaction time
increases, conversion also increases but the selectivity of ester
decreases which is due to decomposition of H2O2 generated
water molecule, which leads to hydrolysis of ester to aldehyde.
The optimized condition for maximum 68% conversion with
highest 86% selectivity are: benzaldehyde, 10 mmol; 30% H2O2
v/v, 30 mmol; catalyst, 5 mg (active amount of Ni: 0.155 mg;
catalyst/substrate ratio, 2.64 ꢀ 10ꢁ4); methanol, 7 mL; temper-
Table 1 Effect of mole ratioa
% Selectivity
Substrate : H2O2
% Conversion
Ester
Acid
1 : 1
1 : 2
1 : 3
1 : 4
51
62
68
74
96
88
86
64
4
ꢂ
ature, 80 C; and time, 6 hours, TON: 2576.
12
14
36
Control experiment and investigation of mechanism
a
Reaction conditions: catalyst (5 mg), catalyst/substrate ratio (2.64 ꢀ
10ꢁ4) benzaldehyde (10 mmol), oxidant (30% H2O2 v/v), methanol (7
mL), temp. (80 ꢂC) and time (6 h).
In control experiments, the reaction was carried out with
Ni(CH3COO)2 and PMA in optimized experimental conditions
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Fig. 5 Optimization of parameters for oxidative esterification of benzaldehyde. Reaction condition: (a) effect of catalyst amount: benzaldehyde
(10 mmol), H2O2 (30 mmol, 30% H2O2 v/v), methanol (7 mL), temperature (80 ꢂC) and time (6 h); (b) effect of methanol: catalyst (5 mg), catalyst/
substrate ratio (2.64 ꢀ 10ꢁ4), benzaldehyde (10 mmol), H2O2 (30 mmol, 30% H2O2 v/v), temperature (80 ꢂC) and time (6 h); (c) effect of temp.:
catalyst (5 mg), catalyst/substrate ratio (2.64 ꢀ 10ꢁ4), benzaldehyde (10 mmol), H2O2 (30 mmol, 30% H2O2 v/v), methanol (7 mL) and time (6 h);
(d) effect of time: catalyst (5 mg), catalyst/substrate ratio (2.64 ꢀ 10ꢁ4), benzaldehyde (10 mmol), H2O2 (30 mmol, 30% H2O2 v/v), methanol (7
mL) and temperature (80 ꢂC).
and obtained results are shown in Table 2. In case of Ni(CH3- evaluated for the said reaction under same experimental
COO)2, 49% conversion was obtained with 64% selectivity of conditions and obtained results are presented in table. It is very
ester because of the Lewis acidity of Ni, whereas in case of PMA, interesting to note down that the trend is exactly follows the
46% conversion was achieved with 67% ester selectivity. know order of acidity and redox ability of different
Furthermore, when the reaction was carried out with NiHPMA, heteropolyacids.
68% conversion with 86% selectivity towards ester was achieved
Order of acidity: phosphotungstic acid (PW12) > silico-
due to Ni increased Lewis acidity and total acidity of catalyst tungstic acid (SiW12) > phosphomolybdic acid (PMo12) > sili-
which enhance selectivity of ester. The obtained result indicates comolybdic acid (SiMo12).
the bi-functional nature of catalyst where Mo contributes for
oxidation and Ni for esterication due to its Lewis acidity.
Order of redox ability: phosphomolybdic acid (PMo12) > sil-
icomolybdic acid (SiMo12) > silicotungstic acid (SiW12) > phos-
In order to see the effect of an addenda atom, the catalytic photungstic acid (PW12).
activity of Ni salt of phosphotungstic acid (NiHTPA) was also
Table 3 Role of substratesa
Table 2 Control experiment for oxidative esterification of benzaldehyde
% Selectivity
% Selectivity
Catalyst
% Conversion Ester
Acid
100
24
Catalyst
% Conversion
Ester
Acid
Without catalyst
4
—
76
—
aNi(CH3COO)2
PMA
49
46
68
45
64
67
86
97
36
33
14
03
(benzaldehyde + methanol + H2O2)
Without H2O2
(catalyst + methanol + benzaldehyde)
Without methanol
15
45
aNiHPMA
aNiHTPA
100
(catalyst + benzaldehyde + H2O2)
a
Reaction conditions: catalyst (5 mg), catalyst/substrate ratio (2.64 ꢀ
10ꢁ4), benzaldehyde (10 mmol), oxidant (30 mmol, 30% H2O2 v/v),
methanol (7 mL), temp. (80 ꢂC) and time (6 h). Active amount of
nickel (0.155 mg).
a
Reaction conditions: catalyst (5 mg), catalyst/substrate ratio (2.64 ꢀ
10ꢁ4), benzaldehyde (10 mmol), oxidant (30 mmol, 30% H2O2 v/v),
methanol (7 mL), temp. (80 ꢂC) and time (6 h).
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Fig. 6 UV-Vis spectra of (a) reaction starting point (b) after 3 h (c) after
6 h (d) after 8 h.
Fig. 7 Raman spectra of (a) fresh NiHPMA (b) reaction starting (c) after
3 h (d) after 6 h (e) after 8 h and (f) after 8 h + H2O2.
As the PMo12 has maximum oxidation ability, it will give
more conversion (as per the proposed mechanism, Mo–peroxo
is responsible for oxidation), while being a most acidic PW12
will contribute more towards the formation of the ester in
addition to Ni (Lewis acidity).
proceeds through in situ generation of benzoic acid rather than
an acetal intermediates.
Here, in this paper, rst time we attempted for more insight
to understand the mechanistic aspects and for that the said
reaction was monitored at different time intervals by UV-Vis and
Raman spectroscopy (Fig. 6 and 7). UV-Vis spectra of fresh
NiHPMA shows (Fig. S2†) two absorbance peaks at 230 and
396 nm due to the ligand to metal charge transfer (O2ꢁ / Mo6+)
and d–d transition of Ni respectively.58,75 It is interesting to note
that the formation of new absorbance peaks at 363 and 309 nm
(Fig. 6a) aer the addition of H2O2 indicates presence of oxo
species of Ni(O2)58 and peroxo species of {PO4[MoO(O2)2]4}3–75
respectively, which are responsible for catalytic activity and are
in good agreement with the reported one. UV-Vis spectrum were
taken aer 3 h (Fig. 6b) as well as aer 6 h (Fig. 6c) and was
found similar conrming the presence of active species. While
aer the completion of 8 hours the disappearance of absor-
bance bands at 365 and 309 nm and reappearance of original
peaks (Fig. 6d) indicate that both, Ni and Mo play role for the
mentioned reaction.
To know the role of each substrate (Table 3), various exper-
iments were carried out: (i) without catalyst (benzaldehyde +
methanol + H2O2) the reaction proceed to give negligible
conversion, which indicates the requirement of efficient catalyst
for the fast reaction (ii) without H2O2 (catalyst + methanol +
benzaldehyde) the reaction did not show signicant conversion
(iii) without methanol (catalyst + benzaldehyde + H2O2), gives
100% conversion with single selective product benzoic acid.
This study indicates that all three components; catalyst, meth-
anol and H2O2 are essential for the oxidative esterication
reaction.
To conrm either the reaction proceeds via formation of
acetal as intermediate according to a reported general reaction
mechanism37 or through benzoic acid as intermediate, earlier
reported by our group58,72 the following set of reactions were
carried out: (i) when benzoic acid was used as a substrate under
optimized condition (in absence of H2O2) yielding 45%
conversion with 100% selectivity of methyl benzoate. (ii) in
another set of reaction, benzaldehyde was allowed to react with
methanol in presence of catalyst and absence of H2O2 yielding
15% conversion with 76% selectivity of ester and 24% selectivity
of benzoic acid. The reaction was prolonged for 3 h with addi-
tion of H2O2, yielding 50% conversion as well as 69% selectivity
of ester. This study shows that oxidative esterication reaction
Further to conrm the above results, same reaction mixtures
were studied by Raman spectroscopy. Unfortunately, we could
not get any information about Ni oxo species due to very less
concentration of active Ni (2.64 ꢀ 10ꢁ3 mmol). However, the
study clearly shows that peroxo species of Mo played role for in
situ oxidation of benzaldehyde to benzoic acid. Fig. 7. Shows the
Raman spectra of NiHPMA at different time interval during the
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Fig. 8 XPS of recycle NiHPMA in which (a) Mo(VI) and (b) Ni(II).
Scheme 2 Proposed reaction mechanism for oxidative esterification of benzaldehyde.
reaction. Aer the addition of H2O2, a new peak appears at the substrate. Thus, the activation of the metal centre results via
881 cmꢁ1 (Fig. 8b), which is a characteristic of Mo(O2)62,75 with the generation of the active species, peroxo species.76 In the
a less intense and slight shiing showing the formation of present case also, the formation of active metal–peroxo inter-
peroxo species immediately aer addition of H2O2. With the mediate (where, metal is both, Mo as well as Ni) (Fig. 6 and 7) is
progress of the reaction the intensity of the Mo]O (1004 cmꢁ1
)
peak decreases (Fig. 7c and d) with increase in the intensity of
the Mo(O2) band due to the more formation of peroxo species.
Aer completion of the reaction (Fig. 7e), the original band of
1004 cmꢁ1 (Mo]O) reappears. Further, in order to conrm the
formation of peroxo species, H2O2 was once again added into
the reaction mixture and we found the formation of a new band
at 881 cmꢁ1 (Fig. 7f). Thus, the study conrms the formation of
peroxo species during the reaction is because of H2O2. The
following mechanism (Scheme 2), all most same as reported by
our group earlier.58,72 It is known that for oxidation reactions
involving polyoxometalates and transition metals, H2O2, rst
binds to the metal centre and then transfers an oxygen atom to
Table 4 Recycling studya
% Selectivity
Ester
Catalyst
% Conversion
Acid
Fresh
68
67
67
67
86
84
84
84
14
16
16
16
Cycle – 1
Cycle – 2
Cycle – 3
a
Reaction conditions: catalyst (5 mg), catalyst/substrate ratio (2.64 ꢀ
10ꢁ4), benzaldehyde (10 mmol), oxidant (30 mmol, 30% H2O2 v/v),
methanol (7 mL), temp. (80 ꢂC) and time (6 h).
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The X-ray photoelectron spectra of recycled NiHPMA is
shown in Fig. 8. The obtained spectrum show intense peaks at
234, 236, 397 and 416 eV which correspond to 3d5/2, 3d3/2, 3p3/2
and 3p1/2 energy level of Mo(VI)68,69 and a low intense peak at
856 eV which corresponds to 2p1/2 of Ni(II)70–72 on the surface
and are identical with fresh catalyst NiHPMA. This shows that
the catalyst structure remains intact even aer regeneration.
The FT-IR spectra of regenerated NiHPMA show (Fig. 9)
bands at 1065, 964, 879, 787 and 456 cmꢁ1 corresponding to
nas(P–O), nas(M]Ot), nas(Mo–Ob–Mo), nas(Mo–Oc–Mo) and
nas(Ni–O) respectively, which is similar to the fresh catalyst
without exhibiting any signicant shi. This indicates that the
catalyst structure is intact even aer regeneration. However, in
terms of intensity, spectrum differs slightly from the fresh one,
which may be due to the sticking of the substrates on the
surface of regenerated catalyst.
Fig. 9 FT-IR spectra of (a) fresh NiHPMA (b) recycle NiHPMA.
Table 5 Substrate studya
conrmed by UV-Vis and Raman spectroscopy studies. In the
beginning of the reaction, NiHPMA reacts with 30% H2O2 (v/v)
to produce the active metal–peroxo intermediate-II, which is
an active species for the insertion of benzaldehyde to form
intermediate-III. Finally, MeOH directly reacts with
intermediate-III and form product (methyl benzoate) as shown
in Scheme 2 (step-IV). It can be concluded that both Mo and Ni
contributes simultaneously in terms of Mo–peroxo species for
oxidation while Ni–oxo species for esterication reaction.
% Conversion/
Sr. Substrate
Product
% selectivity
TON
2576
Recycling of catalyst
1
68/86
The obtained results, with the regenerated catalyst in optimized
identical conditions, show (Table 4) no signicant change in %
conversion as well as % selectivity of ester up to three cycles.
Obtained result indicates that during the reaction the catalyst
remains stable, and can be reused over multiple cycles.
2
3
72/88
69/87
2711
2590
Characterization of regenerated catalyst
Regenerated catalyst was characterized by EDX, XPS, FT-IR and
UV-Visible for the conrmation of prevention of catalyst
structure.
EDX values of regenerated NiHPMA (% wt): Ni, 2.87; Mo,
57.16; P, 1.37; O, 38.60 is in good agreement with EDX values of
fresh catalyst, indicating no leaching of Ni during the reaction.
4
65/86
2447
5
6
67/89
61/81
59/77
2522
2296
2221
7
a
Reaction condition: catalyst amount (5 mg), catalyst/substrate ratio
(2.64 ꢀ 10ꢁ4), substrate (10 mmol), H2O2 (30 mmol, 30% H2O2 v/v),
methanol (7 mL), temperature (80 ꢂC) and time (6 h).
Fig. 10 UV-vis spectra of the (a) Fresh NiHPMA and (b) recycle
NiHPMA.
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The UV-Vis spectra of regenerated catalyst was carried in
aqueous phase and shows (Fig. 10) d–d transition at 397 nm
characteristic peaks for Ni(II).58 Also LMCT charge transfer
between O and Mo at 230 nm was observed,62 which is identical
with fresh catalyst and conrms that the Keggin unit remains
intact.
7 Y. Ding, B. Ma, Q. Gao, G. Li, L. Yan and J. Suo, J. Mol. Catal.
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Effect of different aldehyde substrate
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To nd reactivity of synthesized catalyst towards different
aromatic aldehyde, the catalytic activity was carried out under
optimized reaction condition and obtained results are shown in
Table 5. Study shows that the strong electron withdrawing
group –Cl facilitates the reaction more as compared to –Br as
expected. Aliphatic aldehyde also gives satisfactory conversions.
This achieved result displays that the activity of synthesized
catalyst with various substrates under mild reaction condition
is outstanding.
´
17 V. Palermo, A. G. Sathicq, T. Constantieux, J. Rodrıguez,
´
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First time we have come up with the bi-functional catalytic
activity of Ni (Lewis acid) and Mo (redox property) for conver-
sion of benzaldehyde to benzoate. Very small amount of Ni
(0.155 mg) can enhance TON from 1731 to 2560. The catalyst is
found to be effective for number of aldehydes, giving % selec-
tivity of 77 to 89% of ester with maximum 2711 TON. Even
though the catalyst is homogeneous, we can recycle by simple
method and reuse for a number of cycles. Formation as well as
role of intermediates, peroxo and oxo species, were conrmed
by UV-Visible and Raman spectroscopy.
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Conflicts of interest
There are no conicts to declare.
27 Y. Leng, J. Liu, P. P. Jiang and J. Wang, RSC Adv., 2012, 2,
11653.
Acknowledgements
AP is thankful to Science and Engineering Research Board
(SERB), Project No. EMR/2016/005718, New Delhi for the
nancial support. Department of Physics, The Maharaja Saya-
jirao University of Baroda for FT-Raman analyses.
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