One pot oxidative esterification of aldehyde over recyclable cesium salt of nickel substituted phosphotungstate

Article abstract of DOI:10.1016/j.apcata.2015.07.032

First time, one-pot oxidative esterification of aldehydes with methanol and hydrogen peroxide as a green oxidant was carried out over Keggin-type cesium salt of nickel substituted phosphotungstate. This protocol affords a novel and efficient approach for achieving high conversion (87%), selectivity for ester (80%) with high turnover numbers (TONs). The catalyst proved to be a bi-functional one, showing synergic effect of lacunary phosphotungstate and nickel by combining their acidic and redox properties. Mechanistic study carried out by UV-Visible spectrophotometer shows that tungsten and nickel both take part in the reaction. The scope of various aldehyde substrates were also evaluated and it was found that the reactions are facile, easy to work up and provide a mild oxidative alternative for organic chemists.

Full text of DOI:10.1016/j.apcata.2015.07.032

Accepted Manuscript
Title: One pot oxidative esterification of aldehyde over
recyclable Cesium salt of Nickel substituted Phosphotungstate
Author: Sukriti Singh Anjali Patel Pravya Prakashan
PII:
S0926-860X(15)30084-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.07.032
APCATA 15482
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
25-3-2015
3-7-2015
22-7-2015
Please cite this article as: S. Singh, A. Patel, P. Prakashan, One pot
oxidative esterification of aldehyde over recyclable Cesium salt of
Nickel substituted Phosphotungstate, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.07.032
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*Graphical Abstract
One pot oxidative esterification of aldehyde over recyclable Cesium salt of Nickel
substituted Phosphotungstate
Sukriti Singh, Anjali Patel* and Pravya Prakashan
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The M.S. University of Baroda, Vadodara 390002, Gujarat, India.
E-mail: aupatel_chem@yahoo.com,
Telephone: +91- 265 2795552.
Graphical Abstract
Page 1 of 27

*Highlights
Highlights:

Cesium salt of Nickel substituted Phosphotungstate synthesised by one pot method.
Application of catalyst for one pot oxidative esterification of aldehydes.
The superiority lies in obtaining higher selectivity for the desired ester product.
High Turn Over Number under sustainable reaction conditions.




Advantage of using nickel as transition metal instead of expensive metal.
Page 2 of 27

*Revised manuscript
One pot oxidative esterification of aldehyde over recyclable Cesium salt of Nickel
substituted Phosphotungstate
Sukriti Singh, Anjali Patel* and Pravya Prakashan
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science,
The M.S. University of Baroda, Vadodara 390002, Gujarat, India
*
Telephone: +91- 265 2795552,
*E-mail: aupatel_chem@yahoo.com
ABSTRACT
First time, one-pot oxidative esterification of aldehydes with methanol and hydrogen
peroxide as a green oxidant was carried out over Keggin-type cesium salt of nickel
substituted phosphotungstate. This protocol affords a novel and efficient approach for
achieving high conversion (87%), selectivity for ester (80%) with high turnover numbers
(TONs). The catalyst proved to be a bi-functional one, showing synergic effect of lacunary
phosphotungstate and nickel by combining their acidic and redox properties. Mechanistic
study carried out by UV-Visible spectrophotometer shows that tungsten and nickel both take
part in the reaction. The scope of various aldehyde substrates were also evaluated and it was
found that the reactions are facile, easy to work up and provide a mild oxidative alternative
for organic chemists.
KEYWORDS: Oxidative esterification, one pot reaction, Mono nickel substituted,
phosphotungstate, bi-functional catalyst
1. INTRODUCTION
Methyl esters are important products in the chemical industry, such as in fragrance
industry, flavoring agents, solvent extractants, diluents and intermediates. Traditionally the
synthesis of ester takes place in a two-step procedure, first the oxidation of the aldehyde with
manganese dioxide or sodium dichromate, and the subsequent conversion of the carboxylic
1
Page 3 of 27

acid intermediate to methyl ester [1]. But these methods are limited due to the generation of
huge amount of toxic wastes and by-products arising from chemical processes. In recent
years, considerable effort has been devoted to develop the direct synthesis of esters from the
oxidative esterification of aldehydes with alcohols where two reaction steps occur in single
step. In comparison with the conventional methods, this route can represent a big step
forward toward green, economic, and sustainable chemistry because the whole process
consists of only one step of reaction and yields no byproducts.
Recently, various groups have reported oxidative esterification of aldehydes based on
supported Gold-Nickel Oxide (AuNiOx) [2], TS-1 [3], V
nanoparticle Au/TiO [5], Pb and Mg doping in Al -supported Pd [6], ionic liquid
BmimBF [7], manganese phthalocyanine immobilized on silica gel [8]. Reports are also
available using 12-tungstophosphoric acid immobilized on the surface of silica encapsulated
γ-Fe nanoparticles [9], imidazolium polyoxometalate [bmim] 40] [10], 12-
tungstophosphoric acid and lacunary phosphotungstate anchored to MCM-41 and ZrO
2 5 2 2
O -H O [4], supported gold
2
2 3
O
4
2
O
3
3
[PW12O
2
[11].
As in most of the reported literature costly metals such as gold and palladium has been used,
it would be interesting to observe the effect of cheaper metal such as nickel. A literature
survey shows no report for oxidative esterification using Transition metal substituted
polyoxometalates (TMSPOMs). So it was thought of interest to use Ni substituted POM for
this study.
TMSPOMs comprises a new sub class of polyoxometalates in which the transition
metals are incorporated into the lacuna created in the parent polyoxometalate, gaining
tremendous attention in field of catalysis [12-15]. Among the various TMSPOMs, the nickel
substituted species are of considerable interest because of their unique redox and catalytic
properties. The guanidinium salt of Keggin type mono nickel substituted silico and
phosphotungstate were synthesized by Weakley and Malik in 1967 [16].
2
Page 4 of 27

Then after almost 22 years, studies began in 1989. There are number of reports
available on synthesis and characterization of mono nickel substituted phosphotungstate,
consisting of different salts such as TBA, potassium and Octadecyl trimethylammonium
chloride following two step synthesis [17-26]. A literature survey also shows that only
Mansouri group [27], had reported synthesis of Keggin type Cs salt of PW11Ni following the
same procedure [16] with a modification. The modification includes the treatment of
potassium salt of K PW .xH O with an aqueous solution of M(NO) , M =Ni and addition of
7
11
2
3
cesium chloride under refluxing condition followed by filtration and washing with an
ethanol/H O (1/1) mixture and dried at 353 K. They also reported the catalytic application of
2
the synthesized material for partial oxidation of methane. Thus, a literature survey shows that
there is only one report on the synthesis of CsPW11Ni. Further, all the reports explore the
applications of Ni substituted silico/phosphotungstates for oxidation reactions only.
In the present paper, first time we report one pot synthesis of cesium salt of nickel
substituted phosphotungstate (CsPW11Ni) starting from commercially available 12-
tungstophosphoric acid as well as its application as a homogeneous catalyst for oxidative
esterification of aldehydes. The effect of various reaction parameters were studied to
optimize the conditions for maximum conversion by taking benzaldehyde as model substrate
for the reaction. The scope of reaction was extended to various other aldehyde substrates. The
catalyst was regenerated and recycled under optimized conditions. Mechanistic aspects were
studied by UV-Visible technique and probable mechanism was proposed.
2
2
. EXPERIMENTAL
.1. Materials
All chemicals used were of A. R. grade. 12-Tungstophosphoric acid (H
Nickel chloride hexahydrate (NiCl .6H O), cesium chloride (CsCl), benzaldehyde, 30%
, and methanol, were obtained from Merck and used as received.
3
PW12O40), NaOH,
2
2
2 2
H O
3
Page 5 of 27

2.2. Synthesis of CsPW11Ni
H PW O (PW , 2.88 g, 1 mmol) was dissolved in water (10 mL) and pH of the solution
3
12 40
12
2 2
was adjusted to 4.8 using NaOH solution. NiCl .6H O (0.237 g, 1 mmol) dissolved in a
minimum amount of water was mixed with the above hot solution. The final pH of the
resultant solution was adjusted to 4.8 and it was heated at 80 °C with stirring for 2 h and
filtered hot. A saturated solution of CsCl was added drop wise to the hot filtrate and allowed
to stand. The as-obtained crystals were re-crystalized in water. The obtained light green
crystals were filtered, air dried and designated as CsPW11Ni.
2.3. Catalytic Reaction
The reaction of benzaldehyde (0.01 mol) with H
2 2
O (0.03 mol) and methanol was carried out
in a 100 mL batch reactor provided with a double walled air condenser, Dean-Stark
apparatus, magnetic stirrer, and a guard tube. Appropriate amount of catalyst was added and
it is seen that it dissolves in the reaction medium providing a homogeneous conditions. The
o
reaction mixture was refluxed at 80 C for 6 h. The product was extracted with
dichloromethane by repeated extractions. The obtained products were analyzed on a gas
chromatograph (Shimatzu-2014) using a capillary column (RTX-5). The obtained products
were identified by comparison with the authentic samples and finally by gas chromatography
mass spectroscopy (GC-MS).
ʚInitialꢀmol%ʛ Ǝ ʚFinalꢀmol%ʛ
Conversionꢀ% Ɣ
ʚInitialꢀmol%ʛ
molesꢀofꢀproductꢀformed
Selectivityꢀ% Ɣ
ꢀƐ 100
molesꢀofꢀsubstrateꢀconsumed
2.4. Characterization
4
Page 6 of 27

Elemental analysis was carried out using a JSM 5610 LV EDX-SEM analyzer. TG-DTA was
o
31
carried out on the Mettler Toledo Star SW 7.01 up to 600 C. P solution NMR was carried
out in D O solvent on a Bruker AVANCE 161.97-Mhz instrument. Fourier Transform
2
Infrared Spectroscopy (FT-IR) spectra of the sample were obtained by using the KBr pellet
on the Perkin Elmer instrument. ESR spectra were recorded on a Varian E-line Century series
X-band ESR spectrometer (Liquid nitrogen temperature and scanned from 2000 to 3000
Gauss). The UV-Vis spectrum was recorded at room temperature on Perkin Elmer 35
LAMDA instrument using the 1 cm quartz cell in range of 200-900 nm. The XRD pattern
was obtained by using PHILIPS PW-1830. The conditions were: Cu Kα radiation (1.5427 Å),
o
o
scanning angle from 0 to 60 . Cyclic voltammetry was performed on CHI660E
Electrochemical Workstation, Model 600E Series. A glassy carbon working electrode and an
Ag/AgCl reference electrode was used.
2. 5. Determination of acidic strength (Potentiometric titration method)
A small quantity of 0.05 N n-butylamine in acetonitrile was added to a suspension of 0.25 g
catalyst in 25 mL acetonitrile and stirred for 3 h [28, 29]. Then, the suspension was
potentiometrically titrated with the same solution of n-butylamine in acetonitrile. The
electrode potential variation was measured with a digital pH meter.
3
3
. RESULTS AND DISCUSSIONS
.1 Catalyst characterization
The complex CsPW11Ni was isolated as the cesium salt after completion of the reaction and
the remaining solution was filtered. The filtrate was analyzed to estimate the amount of non-
reacted tungsten gravimetrically (1.12 %), corresponding to loss of one equivalent of W from
H
3
PW12
O40. The % composition of W (58.54%) and Ni (1.69%) in CsPW11Ni was also
estimated gravimetrically [30] and volumetrically [31], respectively. The % composition of
5
Page 7 of 27

W in CsPW11 was also estimated by gravimetric analysis following the same procedure
(Theoretical W-52.33%, Obtained W-52.1%).
The elemental analysis (EDX) result for CsPW11Ni shows that the found values are in
good agreement with analytical values (theoretically calculated). Anal calc. %: Cs, 18.84; W,
57.35; P, 0.87; Ni, 1.66; O, 20.87. Found %: Cs, 17.74; W, 57.37; P, 0.83; Ni, 1.70; O, 19.18.
o
The TGA curve (Supplementary Fig. 1), shows total weight loss (3.5%) up to 600 C,
corresponding to 7 H O molecules. From the elemental as well as the thermal analysis, the
2
chemical formula of the isolated complex obtained is Cs
5
[PW11Ni (H
2 2
O)O39]ꢀ 6H O.
3
1
P NMR is the most important method to study the chemical environment around
3
1
phosphorus in heteropoly compounds. The solution P NMR for CsPW11 and CsPW11Ni is
shown in Fig. 1. It is well known that the removal of one WO from the PW12 Keggin unit has
a larger effect on the chemical shift value rather than the successive removal of other W from
the parent. As a result, δ shift of 4.5 ppm [from PW12 (-14.99 ppm)] is expected for mono
lacunary CsPW11 i.e. -10.9 ppm [32]. The δ value for CsPW11Ni is -11.86 ppm. The
downfield shift in the δ value is attributed to the change in the chemical environment around
the central P atom due to incorporation of Ni(II). It is important to note that single peak with
no fragmentation/species confirms the presence of only CsPW Ni without any isomers. Thus
11
the NMR spectrum supports the fact that nickel ion is incorporated into lacuna of PW11
.
6
Page 8 of 27

31
31
Fig. 1. (Top) P solution NMR, (
BB ottom) P MAS NMR of a) CsPW11 and b) CsP WW 11Ni.
The FT-IR spectra for PW , CsPW and CsPW Ni are shown in Fig. 2. The
F
T-IR
893
nds,
hing vibrations of PW12 of the central phosphorou
group are known to result in splitting due to decrease in symmetry. Hence similar obs rvation
is observed for CsPW which show bands at 1084 and 1041 (P-O stretching), 952 (W-Od),
and 856 and 806 cm attributed W-O-W, respectively (Fig. 2b).
The reported complex CsPW Ni exhibits bands at 1062, 961, and 885 and 81
corresponding to P-O, W=O, and W O-W stretching frequencies (Fig. 2c) respectivel
filling of the hole in the octahedral lacuna by a metal cation restores the symmetry of
central tetrahedron, owing to the int raction between metal and the available oxygen
central PO group (O ) [33]. In the p esent case CsPW Ni, the ꢂν splitting for P-O w
12
11
11
of PW12 shows bands at 1080 (P-O s
t
t
retching), 982 (W=O
d
symmetric stretching), an
dd
-1
and 812 cm corresponding to the as
ymmetric stretching of W-O
b
-W and W-O -W boo
c
respectively (Fig. 2a). The P-O stret
c
s
e
ss
1
1
-1
-1
11 1
00
cm
--
yy . The
the
ee
oo f the
r
aa s
4
a
11
-
1
observed to be 0 cm which is in go
provide an indirect measure of the i
lower ꢁν. In addition to this the pres
which is due to Ni-O vibration, indi
o
d agreement with the literature [34]. Also the ꢁν values
teraction strength of the metal M with the O ato , being
nce of an additional band is also observed at 49
ating the substitution and a strong interaction of
n
a
mm
-
1
ee
00
cm
cc
mm
etal
Page 9 of 27

ion with the lacuna of PW11. Thus, FT-IR spectra clearly indicates the incorporation of Ni
into the lacunary PW11
.
Fig. 2. Comparison between FT-IR spectra of (a) PW12 (b) CsPW11 and (c) CsPW11Ni. Inset
is the enlarged figure of b) and c).
The full range (3000-2000 G) X-band liquid nitrogen temperature ESR spectra (Fig.
3
) for CsPW Ni was recorded. The g value obtained (g~2.07) is in good agreement with
11
reported g value (g~2.25) of Ni(II) in ammonium nickel(II) sulfate hexahydrate crystal [35]
and indicates the presence of Ni(II) in octahedral or distorted octahedral environment. Thus,
31
from FT-IR, P NMR and ESR spectrum it was confirmed that Ni(II) was incorporated into
the lacunary PW11
.
Page 10 of 27

Fig. 3. ESR spectra of CsPW11Ni at liquid nitrogen temperature.
The UV-Visible spectrum for CsPW11Ni was recorded in aqueous medium
Supplementary Fig. 2).The CsPW Ni spectrum shows an intense peak at 290 nm indicating
(
11
7
-
the formation of [PW11O39] in the synthesized complex which results from the O→W
charge transfer transition [36]. A broad peak at 419 nm (inset Supplementary Fig. 2), and a
weak broad peak in the range of 700-800 nm was obtained in CsPW11Ni which arises due to
the d-d transitions for Ni(II) [37]. Thus, the incorporation of Ni(II) into the vacant cavity of
lacunary PW is confirmed from the UV-Visible spectrum.
11
Since, the obtained crystals were not suitable for single crystal, they were subjected to
Powder XRD. Powder XRD patterns of PW12, PW11, and CsPW11Ni are shown in Fig. 4. The
o
o
peaks ranging from 15 to 30 2θ value indicate the presence of characteristic peaks of
Keggin ion in PW12. The shift in the 2θ value for CsPW11Ni compared to PW11 may be due to
substitution of the transition metal into the lacuna. Further, the Powder XRD pattern of
CsPW11Ni is entirely different from PW12, but similar to that of PW11. Thus, the Powder
XRD clearly suggests the presence of PW11 unit and formation of crystalline product
CsPW11Ni. This powder pattern was indexed using Powder X5 software [38]. Further, the
o
analysis shows that the unit cell has cell parameters, a = 20.82; b = 20.82; c = 10.46; γ = 90 .
Almost the same cell parameters was reported by Mansouri et al., [28] however the procedure
of synthesizing CsPW11Ni is different.
9
Page 11 of 27

Fig. 4. Comparison of Powder XRD patterns of a) PW12, b) PW11 and c) CsPW11Ni.
The Cyclic Voltammogram of 1 mM solution of CsPW11Ni was recorded in buffer
medium of pH 2.2 (perchlorate buffer) using glassy carbon electrode as working electrode,
Ag/AgCl as reference electrode and Platinum wire as the counter electrode. Cyclic
voltammogram of CsPW11Ni (Supplementary Fig. 3) shows Epc (i.e. cathodic peak potential)
at -0.450 V and -0.650 V and Epa (i.e. anodic peak potential) at -0.550 V and -0.700 V. The
observed peak potential values are in good agreement with the reported values for potassium
salt of mono nickel substituted phosphotungstate [39]. However, the slight change in E and
pc
Epa values observed may be due to change in the counter cation.
The acidity measurement by potentiometric titration permits us to estimate the acidic
strength of the catalyst. The initial electrode potential (Ei) indicates maximum acid strength
of the surface sites [29] and range where the plateau is reached indicates the total number of
acid sites (meq/g solid). The acidic strength of surface sites can be assigned as follows: very
1
0
Page 12 of 27

strong acidic site, Ei > 100 mV; strong acidic site, 0 < Ei < 100 mV; weak acidic site, -100<
Ei < 0 mV. The plots of the electrode potential as a function of mequiv. amine/g for CsPW11
and CsPW11Ni are shown in Supplementary Fig. 4. The strength of acidic sites in terms of
initial electrode potential is shown in Table 1. It is observed that, Acidic strength, total
number of acidic sites as well as strong acidic sites for CsPW11Ni is much higher than
CsPW11. This may be due to incorporation of Nickel which is Lewis acid in nature. The
obtained values are in good agreement with the expected one.
Table 1. n-butyl amine acidity by Potentiometric titration.
-1
Acidic
Type of acidic sites (mequiv. g ) Total number of
Materials
Strength
Ei (mV)
acidic sites
(mequiv. g )
Very
strong
0
-1
Strong
weak
CsPW11
10
50
0.1
0.7
1.0
1.7
1.1
2.4
CsPW Ni
0
11
3
.2. Catalytic performance
Benzaldehyde and methanol were selected as test substrates in presence of hydrogen
peroxide as oxidant. The effect of different reaction parameters such as benzaldehyde/H
2
O
2
mole ratio, amount of catalyst, reaction time and temperature were studied to enhance the
conditions for highest conversion. The overall conversion of the substrate benzaldehyde to
methyl benzoate is dependent on the formation of benzoic acid. But the formation of benzoic
acid is dependent on the mole ratio of aldehyde/H O and can be improved by increasing the
2
2
amount of oxidant.
2 2
To see the effect of mole ratio (benzaldehyde/H O ) the reaction was performed with
o
1
0 mg catalyst for 6 h at 80 C (Fig. 5a). It is known that the oxidative-esterification could be
2 2
improved by increasing the amount of H O , according to the chemical dynamics [9]. The
conversion was improved by increasing the amount of oxidant from 10 (1:1) to 20 mmol
1
1
Page 13 of 27

(1:2). On increasing the mole ratio to 30 mmol (1:3), 87 % conversion and 80% selectivity
for ester was obtained. Increasing the mole ratio above 1:3, showed no significant increase in
the conversion, however slight increase in selectivity of ester was observed.
The effect of the amount of catalyst (from 5-20 mg under homogeneous conditions)
on conversion was studied and the obtained results are presented in Fig. 5b. Oxidative
esterification of aldehyde is significantly affected by acidity as well as oxidizing property of
the catalyst. The transition metal (in the present case Ni) functions in synergism with PW as
11
an active site for the present reaction. With an increase in the amount of the catalyst (from 5
to 10 mg) there was an increase in the number of available catalytically active sites which
increases the kinetics of the reaction hence leading to higher conversion. Acidity of the
catalyst determined by potentiometric titration (having both strong and weak acidic sites),
favours the selectivity of ester by subsequent esterification reactions. After 10 mg no
substantial change in conversion and selectivity has been obtained. On increasing the amount
to 15 mg, no significant increase in conversion was obtained. Thus, the amount of the catalyst
was optimized to 10 mg for highest conversion and 80% selectivity for ester.
The percentage conversion was monitored at different reaction times (Fig. 5c). It can
be seen from Fig. 5c, that with an increase in the reaction time, the initial conversion of
benzaldehyde also increases. This is due to the fact that more time is required for the
formation of the reactive intermediate (substrate + catalyst) which is finally converted into
the products. Also the selectivity for ester progressively increases with the reaction time. It
was observed that at 6 h, maximum conversion (87%) was obtained with 80% selectivity for
ester. On further increase in time the conversion remains constant however, slight increase in
the selectivity of ester (85%) was observed. Hence, reaction time of 6 h was optimized.
1
2
Page 14 of 27

1
00
1
00
%
%
%
Conversion
Selectivity (Ester)
Selectivity (Acid)
b)
a)
8
0
0
8
6
4
2
0
0
0
0
0
6
40
20
0
1
:1
1:2
1:3
Mole Ratio
1:4
5
10
15
20
Catalyst Amount (mg)
d)
100
c)
1
00
80
60
40
20
0
80
60
40
20
0
60
70
80
90
2
3
4
5
6
7
8
o
Reaction Time (h)
Reaction Temp. ( C)
Fig. 5. Optimization of parameters for oxidative esterification of benzaldehyde over
2 2
CsPW11Ni catalyst: a) Effect of mole ratio (aldehyde to H O ): amount of catalyst (10 mg),
o
temp. (80 C), time (6 h); b) Effect of catalyst amount: mole ratio (aldehyde to H
2
O
2
) 1:3,
o
temp. (80 C), time (6 h); c) Effect of time: mole ratio (aldehyde to H
2
O
2
) 1:3, amount of
) 1:3,
o
catalyst (10 mg), temp. (80 C); d) Effect of temperature: mole ratio (aldehyde to H
amount of catalyst (10 mg), time (6 h).
2 2
O
The effect of reaction temperature was also investigated (Fig. 5d). The present
esterification was very sensitive to the reaction temperature, and drastic increase in
o
conversion was observed along with very high selectivity for ester product till 80 C. The best
o
result was obtained at 80 °C. Further increasing the temperature to 90 C does not improve
the conversion indicating attainment of equilibrium, also at such high temperature methanol
will be present in vapour phase and will be less available to the reaction environment.
1
3
Page 15 of 27

Similar to peroxide, solvent also played an important role in controlling the catalytic
activity. Reactions were carried out by varying the methanol quantity and it was observed that
with increase in the methanol the selectivity to methyl benzoate increased. The selectivity of
the ester was 52% by using 2 mL of methanol as the reactant (Table 2). In contrast, 80% of
methyl benzoate (with 87% conversion) was achieved with 5 mL of methanol. On further
increasing the methanol content the conversion decreases, this surprising result might be
explained by the fact that the hydrogen peroxide might be less available for the reaction and
dilution of the reaction with 8 and 10 mL of methanol.
Table 2. Effect of Methanol Quantity
Methanol
Quantity
% Selectivity
% Conversion Methyl
Benzoic
acid
Benzoate
(mL)
2
5
8
1
52
87
80
76
30
80
75
77
70
20
25
23
0
Reaction conditions over CsPW11Ni catalyst: mole ratio 1:3; catalyst amount 10 mg, reaction
temperature, 80 °C; reaction time, 6 h.
The optimum conditions for homogeneous catalyst are, mole ratio to benzaldehyde to
o
H
2
O
2
= 1:3; catalyst amount 10 mg, temperature 80 C, time 6 h, methanol 5 mL.
3
.3. Effect of different aldehyde substrates
Notably, the system also showed excellent activity toward the oxidative esterification
of other representative aromatic aldehydes. The present catalyst catalysed reaction of
benzaldehyde (Table 3, entry 1) with methanol giving methyl benzoate in good yield. As
shown in Table 2, strong electron-withdrawing groups facilitate the oxidative esterification,
Page 16 of 27

regardless of its position on benzaldehyde. However, ortho-substituted benzaldehyde may
suffer from a certain steric effect that retards their ester conversion (Table 3, entry 3). These
conditions led to excellent conversion of only electron-poor aromatic aldehydes and saturated
aldehydes (entry 5). We next examined conditions to improve the conversion of electron-rich
aromatic aldehydes and chose 4-methyl benzaldehyde and 4-nitro benzaldehyde (Entry 4, 5)
for this purpose. The conversion of electron-rich aromatic aldehydes was significantly lower,
possibly due to a lowering of the reactivity of the carbonyl group due to resonance effects of
the electron-donating substituents. The reaction was extended to aliphatic aldehydes giving
satisfactory conversion. These results suggest that the catalyst and reaction system can
withstand variety of substrates under mild reaction conditions.
Table 3. Oxidative esterification of Aldehydes to corresponding esters over homogeneous
catalyst.
Where R=Cl, NO
%
2 3
, CH
Entry Substrate
Product
%
TON
3068
Conversion Selectivity
1.
87
80
2
.
.
78
72
80
75
2751
2539
3
4.
78
86
2751
1
5
Page 17 of 27

5.
85
88
2998
6
.
75
67
70
70
2645
2363
7.
Reaction conditions: mole ratio 1:3; catalyst amount 10 mg, reaction temperature, 80 °C;
reaction time, 6 h. TON= moles of product/moles of catalyst.
3
.4. Control experiments and Mechanistic investigation
A set of experiments were carried out in order to ensure the role of each substrate as
well as catalyst. (i) in the absence of catalyst (i.e. only Benzaldehyde+Methnaol+H
Supplementary Table 1) the reaction did not progress significantly (ii) in the absence of
oxidant (i.e. H ) no significant reaction occurs, (iii) in presence of H and absence of
2
2
O )
(
2
O
2
2 2
O
methanol only auto-oxidation reaction of aldehyde to benzoic acid takes place. This study
shows that all three substrates as well as catalyst are essential for the feasibility of the
oxidative esterification reaction.
2
The control experiments were also carried out with CsPW11, NiCl and CsPW11Ni
under optimized conditions and results are presented in Supplementary Table 1. In absence of
catalyst, no ester was formed and benzoic acid was detected as the only product. It was
observed that with CsPW , 78% conversion and high selectivity for ester (70%) was
1
1
obtained. However, in metal salt catalysed reaction (NiCl
72%) with only 69% selectivity for ester was obtained. As Nickel is a good Lewis acid, good
yield of ester is as expected. CsPW11Ni, shows 87% conversion with excellent selectivity
80%) for ester. The observed increase in the selectivity of ester may be due to the presence
of Nickel and this is further confirmed by potentiometric titration (Supplementary Fig. 4). It
2
), good conversion of benzaldehyde
(
(
1
6
Page 18 of 27

is seen from Table 1 that total number of acidic sites in CsPW11Ni is almost double (2.4
mequiv/g) as compared to CsPW (1.1 mequiv/g). Also higher number of strong and weak
11
acidic sites in CsPW11Ni as compared to CsPW11 indicates increase in the acidity after
incorporation of Ni, responsible for higher selectivity.
Catalytic activity of CsPW11Ni shows synergic effect between PW11 and the metal salt
by combining their redox and acidic properties. Mechanistically, it seems plausible that the
benzaldehyde is oxidized with CsPW Ni-H O to the corresponding acid, which is esterified
11
2
2
immediately with alcohol. However, contradictory to this, it has been demonstrated in many
cases that there is formation of acetal as intermediate [4, 40]. To explain, following set of
experiments were carried out (Supplementary data Scheme 1) (1) when Benzoic acid is used
2 2
as substrate instead of benzaldehyde under identical conditions (except H O ) 65%
conversion with 100% methyl benzoate is obtained. (2) In another reaction benzaldehyde was
reacted with methanol in presence of catalyst and absence of oxidant, to give 52% conversion
with major selectivity for benzoic acid and poor selectivity towards ester in 6 h. On
2 2
prolonging the reaction for 3 h after addition of H O , 89 % conversion was achieved with
80% ester selectivity. The study shows that the oxidation of benzaldehyde in an alcoholic
medium does not proceed through an acetal intermediate as reported [4, 40], but through
insitu formation of benzoic acid.
Although, the detailed reaction mechanism of the synthesis of esters from aldehydes
and methanol over TMSPOM is not clear at this stage, in the present paper an attempt was
made to investigate the mechanistic aspect, of the said reaction at different stages by UV-
visible spectrophotometer and shown in Fig. 6. (a) Spectra of fresh catalyst in aqueous
solution, (b) Spectra of reaction after 3 h and (c) Spectra after completion of the reaction. It is
seen from Fig. 6b, that absorbance band at 250 nm (PW11) get split into two bands at 216 and
252 nm. This may be due to formation of peroxo intermediate with the lacunary species
1
7
Page 19 of 27

4 2 2 4
{PO [W(O)(O ) ] } and is in good agreement with the reported one [41]. It also shows
2
+
disappearance of absorbance band at 419 nm (due to the d-d transition of Ni ) and formation
of a new absorbance band at 343 nm. This new peak can be assigned to the formation of
2
peroxo species, [Ni(O )] (right, Fig. 6b). It is also very interesting to note that on prolonging
2+
the reaction upto 6 h, the original peaks of PW11 as well as Ni reappear (right, Fig. 6c). This
study clearly shows the involvement of both W as well as Ni for the reaction, in agreement
with the reported literature [42]. Based on the above UV-visible as well as control
experiments, following probable mechanism was deduced.
Fig. 6. UV-visible study for mechanistic evaluation, left- full range, right- Nickel absorbance
band, a) Fresh catalyst, b) reaction intermediate after 3 h, and c) reaction completion after 6
h.
It is known for oxidation reactions with transition metal atoms, especially
2 2
polyoxometalates, that H O first binds to the metal center and then transfers an oxygen atom
to the substrate. Thus, the activation of the metal center results via the generation of the
active species which may be bridging peroxo species [43]. In the present case also, it may be
1
8
Page 20 of 27

2 2
possible that at the beginning of the reaction CsPW11Ni reacts with 30% H O to produce
active metal-peroxo intermediate (where metal can be both W as well as Ni) (Fig. 6) [42],
which may be the active intermediate for the oxidative esterification of aldehydes to ester.
Also, for oxidative esterification of aldehydes it has been reported by Chavan et.al., that the
2 2
catalyst titanium-silicate with H O forms hydroperoxo or peroxo species which reacts with
aldehyde to give titanium derived trioxolane species as intermediate [4]. In the present case
also similar mechanism can be expected (Scheme 1).
Scheme 1. Probable mechanism of oxidative esterification of aldehydes over CsPW11Ni.
3.5. Recycling and regeneration of catalyst
In the present case, CsPW Ni behaves as homogeneous catalyst. Even though the
11
catalyst was soluble in the aqueous phase, it was successfully regenerated after a simple
workup and reused. After the completion of the reaction, the reaction mixture was cooled to
room temperature, the organic products were extracted from the aqueous layer (containing
catalyst) by dichloromethane and the conversion was determined by GC. The aqueous phase
o
was dried at 100 C to collect the solid catalyst. The catalytic activity of the regenerated
catalyst was evaluated under the optimized conditions. It was observed that on recycling
1
9
Page 21 of 27

(cycle 1) 85% conversion and 75% selectivity for ester was obtained showing very good
conversion and selectivity. After repeating the procedure, (cycle 2) significant decrease in
conversion (80.5%) was observed with ester selectivity 72%.
The decrease in conversion as well as selectivity may be due to the loss of catalyst
during the recycling. In order to investigate the same, the conversion and selectivity of fresh
and recycled catalyst was compared with reaction time (Supplementary Fig. 5) and iso-
conversion and iso-selectivity (for ester) was found at higher reaction time of 8 h as
compared to 6 h for fresh catalyst.
The regenerated catalyst was characterised by UV-Visible spectrophotometer (Top,
Fig. 7b) and FT-IR. The UV-visible study was also carried out for regenerated solid catalyst
2+
and it was observed that the characteristic peaks for Ni is intact at 420 nm. This suggests
that the Nickel does not leach into the solution and the kegging structure of CsPW Ni
1
1
remains intact even after regeneration.
The regenerated catalyst R-CsPW11Ni shows corresponding bands at 1071, 956, and
-1
8
81 and 815 cm for P-O, W=O, and W-O-W stretching frequencies at, respectively and Ni-
-1
O vibration band at 495 cm (Bottom, Fig. 7b). No significant shifting in all bands as
compared to the fresh catalyst suggests that the catalyst does not degrade as well as there is
no change in the structural morphology even after recycling, also, nickel does not leach from
the catalyst and the structure is intact.
2
0
Page 22 of 27

Fig. 7. Top: UV-visible spectra of the a) Fresh CsPW11Ni and b) Regenerated R-CsPW11Ni;
Bottom: FT-IR spectra of a) fresh CsPW Ni and b) Regenerated R-CsPW Ni.
11
11
31
In order to demonstrate and prove that the structure is intact, P solution NMR of the
regenerated catalyst (Fig. 8) was carried out. Sharp single peak with no shift confirms that the
structure remain undegraded in the recycled catalyst and there is no modification.
Page 23 of 27

3
1
Fig. 8. P solution NMR of Regenerated CsPW11Ni after two cycles.
The catalyst separation are present concerns that can be solved by the immobilization
of the active species on solid support. We are therefore currently working towards the
heterogenization of CsPW11Ni onto suitable support.
4
. CONCLUSION
We have introduced a one-pot synthesis procedure for Keggin-type Cs-salt of mono nickel-
substituted phosphotungstate (CsPW11Ni) starting from parent 12-tungstophosphoric acid.
31
The presence of Ni was confirmed by P solution NMR, FT-IR, ESR, UV-visible and cyclic
voltammetry. For the first time the bi-functional properties of CsPW11Ni was evaluated for
green, environmentally benign liquid phase oxidative esterification of aldehydes with
methanol. The superiority of present work lies in obtaining higher selectivity of the desired
product as well as high TON under sustainable reaction conditions. The catalyst can be
recycled upto two cycles without any degradation which was confirmed by UV-visible, FT-
31
IR and P NMR. In addition, the advantages of using Ni instead of more expensive metals
and using aqueous medium makes this methodology interesting from an economic and an
ecological point of view.
Page 24 of 27

ACKNOWLEDGMENT
We are thankful to Department of Science and Technology (DST), Project. No. SR/S1/IC-
36/2012, New Delhi, for the financial support. Ms. Pravya Prakashan is thankful to the same
for the award of a research fellowship. We are also thankful to Department of Chemistry, The
31
M. S. University of Baroda, for solution P NMR study.
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