Oxidative esterification of aldehydes to esters over anchored phosphotungstates

Article abstract of DOI:10.1007/s10562-014-1304-7

12-Tungstophosphoric acid and lacunary phosphotungstate anchored to MCM-41 and ZrO₂ were synthesized, characterized and used as bifunctional catalyst for oxidative esterification of benzaldehyde with methanol. The different aldehyde substrates study show excellent selectivity for esters, indicating the scope of the catalysts. A tentative reaction mechanism for oxidative esterification of aldehyde is also proposed. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1304-7

Catal Lett (2014) 144:1557–1567
DOI 10.1007/s10562-014-1304-7
Oxidative Esterification of Aldehydes to Esters over Anchored
Phosphotungstates
•
Sukriti Singh Anjali Patel
Received: 5 May 2014 / Accepted: 17 June 2014 / Published online: 16 July 2014
Ó Springer Science+Business Media New York 2014
Abstract 12-Tungstophosphoric acid and lacunary
phosphotungstate anchored to MCM-41 and ZrO2 were
synthesized, characterized and used as bifunctional catalyst
for oxidative esterification of benzaldehyde with methanol.
The different aldehyde substrates study show excellent
selectivity for esters, indicating the scope of the catalysts.
A tentative reaction mechanism for oxidative esterification
of aldehyde is also proposed.
allows oxidation as well as esterification in one pot. One-
step conventional methods reported require the use of
heavy-metal oxidants such as KMnO4 [5], CrO3 [6], oxone
[7], V2O5-SPC/SPB (SPC-sodium percarbonate, SPB
sodium perborate) [8] and 1,5-cyclooctadiene (cod) com-
plex of Iridium i.e. [IrCl(cod)]2/K2CO3 [9]. Most of the
reported methods are useful for direct transformation of
aldehydes with alcohols to corresponding esters. However,
many methods suffer from disadvantages such as use of
expensive and polluting reagents, an inert atmosphere and
lengthy reaction times. Due to the huge amount of toxic
wastes and by-products arising from chemical processes,
chemists have been constrained to develop cost-effective
and environmentally friendly catalytic routes that minimize
waste.
Keywords Phosphotungstates Á Anchored Á Oxidative
esterification Á Aldehydes Á Esters
1 Introduction
Methyl esters are traditionally prepared by the reaction of
carboxylic acid and methanol using acid catalysts such as
sulfuric, sulfonic, phosphoric, hydrochloric and p-toluene-
sulfonic acid [1, 2]. Alkoxides, such as sodium and
potassium alkoxides, have also been used to prepare methyl
esters from carboxylic acid precursors [3]. However, the
direct oxidative esterification of aldehyde, avoiding the use
of the corresponding carboxylic acid, is very attractive.
This reaction typically 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 acid intermediate to methyl ester.
Recently, various groups have reported oxidative ester-
ification of aldehydes based on supported Gold-Nickel
Oxide (AuNiOx) [10], TS-1 [11], supported gold nano-
particle Au/TiO2 [12], Pb and Mg doping in Al2O3-sup-
ported Pd [13], ionic liquid BmimBF4 [14], manganese
phthalocyanine immobilized on silica gel [15]. Conse-
quently from the viewpoint of demands as well as signifi-
cance of acid and oxidation reactions, it would be more
beneficial if a bifunctional catalyst could be developed for
oxidative esterification of aldehydes.
Catalysis by supported phosphotungstates [16, 17] have
greatly expanded during the last few years from the view
point of their variety in structures and compositions. It is
one of the most successful areas in contemporary catalysis,
where systematic studies of phosphotungstate catalysts at
the molecular level have led to a string of large-scale
industrial applications [18, 19]. The acidic as well as redox
properties are a function of the nature of the metal atoms
(addenda atoms) and of both the heteroatom and the
counter ions [20–25]. The replacement of one or more
Hence, oxidative esterification of aldehydes [4] has
received increasing attention during recent years, as it
S. Singh Á A. Patel (&)
Polyoxometalates and Catalysis Laboratory, Department of
Chemistry, Faculty of Science, The M.S. University of Baroda,
Vadodara 390002, India
e-mail: aupatel_chem@yahoo.com
123

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S. Singh, A. Patel
addenda atoms is expected to tune the acidic as well as
redox properties. These active species are called as lacu-
nary phosphotungstates (PW11O39)7- [26–28].
(CTAB), TEOS (tetraethyl orthosilicate), Sodium tungstate
dihydrate, anhydrous disodium hydrogen phosphate, ace-
tone, nitric acid, aldehydes, methanol were used as
received from Merck.
Our group has been working in the field of supported
parent [29, 30] and lacunary phosphotungstates [31–33]
and has successfully established the use of these materials
for various organic transformations comprising of acid
catalyzed as well as oxidation reactions. Recently we have
reported bifunctional catalytic activity of 12-tungstophos-
phoric acid supported on metal oxide as well as mesopor-
ous material for esterification and oxidation of benzyl
alcohol [34]. We have also successfully evaluated the bi-
functional catalytic activity of lacunary phosphotungstates
for esterification as well as oxidation reactions [33]. Due to
the encouraging results of these reactions and also because
of the known industrial importance of esters as well as
carbonyl compounds, as an extension of our work, it was
thought of interest to evaluate the use of these materials as
bifunctional catalysts for oxidative esterification reaction,
where oxidation and esterification take place in one pot.
As per our knowledge only two reports are available on
oxidative esterification of aldehydes with H2O2 using
12-tungstophosphoric acid. One is 12-tungstophosphoric
acid immobilized on the surface of silica encapsulated
c-Fe2O3 nanoparticles [35] and the other being imidazoli-
um polyoxometalate [bmim]3[PW12O40] [36].
2.2 Synthesis of PW11 and the Supports
The sodium salt of PW11 was successfully synthesized and
characterized by the same method reported by us [31].
Sodium tungstate dihydrate (0.22 mol, 72.5 g) and anhy-
drous disodium hydrogen phosphate (0.02 mol, 2.84 g)
were dissolved in 150–200 mL of double distilled water
and heated to 80–90 °C followed by the addition of con-
centrated nitric acid in order to adjust the pH to 4.8. The
volume was then reduced to half by evaporation and the
heteropoly anion was separated by liquid–liquid extraction
with 80–100 mL of acetone. The extraction was repeated
until the acetone extract showed absence of nitrate ions.
The extracted sodium salt was dried in air.
MCM-41 was synthesized by following procedure
reported by us [37]. Surfactant (CTAB) was added to the
very dilute solution of NaOH with stirring at 60 °C. When
the solution became homogeneous, TEOS was added drop
wise, and the obtained gel was aged for 2 h. The resulting
product was filtered, washed with distilled water, and
then dried at room temperature. The obtained material
was calcined at 550 °C in air for 5 h and designated as
MCM-41.
In the present work, different catalysts based on parent
phosphotungstate (H3PW12O40) and lacunary phospho-
tungstate (PW11O39)7- anchored to MCM-41 and Zirconia
were synthesised and characterized. Consequently the
effect of various reaction parameters such as mole ratio,
catalyst amount, time, and temperature were studied to
optimize the conditions for maximum conversion by taking
benzaldehyde as model substrate for the reaction. The
versatility of using H2O2 as an efficient oxidant in the
transformation of benzaldehyde to methyl benzoate was
also studied. The work was extended by carrying out oxi-
dation of different aldehyde substrates. The main part of
the research highlights the possibility of catalyst recycling.
Effort was made to systematically evaluate the role of
supports and catalysts in the reaction as well as propose a
suitable mechanism. The final goal of the present paper is
therefore to obtain an active, selective and recyclable cat-
alytic system.
Hydrous zirconia was prepared by following method
reported by [38]. Aqueous ammonia solution was added to
aqueous solution of ZrOCl2Á8H2O upto pH 8.5. The pre-
cipitates were aged at 100 °C over a water bath for 1 h,
filtered, washed with conductivity water until chloride free
water was obtained and dried at 100 °C for 10 h. The
obtained material is designated as ZrO2.
2.3 Synthesis of the Catalysts
A series of catalysts containing 10–40 % of PW12 anchored
to MCM-41 and Zirconia were synthesized by impregnat-
ing an aqueous solution of PW12 (0.1/10–0.4/40 gmL-1 of
double distilled water) with MCM-41 (1 g) and dried at
100 °C for 10 h. The obtained materials were designated as
(PW12)1/MCM-41, (PW12)2/MCM-41, (PW12)3/MCM-41,
and (PW12)4/MCM-41, respectively. A series of catalysts
containing 10–40 % of PW12 supported on ZrO2 was
synthesized following the same method and the resulting
materials were designated as (PW12)1/ZrO2, (PW12)2/ZrO2,
(PW12)3/ZrO2, and (PW12)4/ZrO2, respectively.
2 Method
2.1 Materials
Catalyst was synthesized by impregnating pure MCM-
41(1 g) with an aqueous solution of PW11 (0.1–0.4 g in
10–40 mL of double distilled water) and dried at 100 °C
for 10 h. The obtained materials were treated with 0.1 N
All chemicals used were of A.R. grade. 12-tungstophos-
phoric acid (PW12), ZrOCl2.8H2O (Zirconium oxychlo-
ride), ammonia, Cetyltrimethyl ammonium bromide
123

Oxidative Esterification of Aldehydes
1559
HCl, filtered, washed with double distilled water and dried
at 100 °C. The obtained materials were designated as
(PW11)1/MCM-41, (PW11)2/MCM-41, (PW11)3/MCM-41,
stretching, respectively. The FT-IR spectrum of MCM-41
shows a broad band around 1,300–1,000 cm-1 corre-
sponding to asymmetric stretching of Si–O–Si. The band at
801 and 458 cm-1 are due to symmetric stretching and
bending vibration of Si–O–Si, respectively. The band at
966 cm-1 corresponds to symmetric stretching vibration of
Si–OH. FT-IR spectra of (PW12)3/MCM-41 is almost same
as that of MCM-41. The absence of respective FT-IR bands
for PW12 in (PW12)3/MCM-41 may be due to the over-
lapping of PW12 bands with that of support. (PW12)3/ZrO2
shows bands at 812, 964 and 1,070 cm-1 corresponding to
the symmetric stretching of W–O–W, W=O and P–O,
respectively, indicating the retainment of the Keggin unit
even after supporting on ZrO2 [38].
(PW11)4/MCM-41.
A series of catalysts containing
10–40 % of PW11 supported on ZrO2 was synthesized
following the same method. The obtained materials were
designated as (PW11)1/ZrO2, (PW11)2/ZrO2, (PW11)3/ZrO2,
and (PW11)4/ZrO2, respectively.
2.4 Characterization
A detailed study on the characterizations of all the syn-
thesized catalysts can be found in our earlier publications
[31, 37, 38]. However in the present article FT-IR, BET,
total acidity measurements, FT-Raman, and XRD are given
for reader’s convenience.
The formation of lacunary PW11 can be confirmed by
splitting of P–O bond. The P–O stretching band
(1,080 cm-1) for PW12 splits into two new bands 1,085 and
1,043 cm-1 in PW11. This is due to the loss of one W–O
unit and lowering of the symmetry from Td (PW12) to Cs
(PW11) around the central heteroatom, phosphorus [40].
The W–O–W and W=O stretching bands for PW11 appear
at 808, 863 and 952 cm-1 respectively. The reported bands
for PW11, at 1,085, 1043, 808 cm-1 corresponding to P–O,
W–O–W, respectively, are absent in (PW11)3/MCM-41 due
to overlapping with the bands of MCM-41. But bands of
PW11 at 952 and 863 cm-1 attributed to W=O and W–O–
W, respectively shifts to a higher wave number (962 and
896 cm-1) in (PW11)3/MCM-41, indicating a strong
interaction between the Keggin unit and the silanol group
of support. The FT-IR spectrum of (PW11)3/ZrO2 shows
bands at 870, 958, 1097 and 1048 cm-1 corresponding to
the symmetric stretching of W–O–W, W=O and P–O bonds
respectively. The positions are in good agreement with
those of PW11 confirming the presence of these groups in
the synthesized materials.
FT-IR spectra were obtained by using the KBr wafer on the
PerkinElmerinstrument. Adsorption–desorptionisotherms of
samples were recorded on a Micromeritics ASAP 2010 sur-
face area analyzer at -196 °C. From adsorption–desorption
isotherms surface area was calculated using the BET method.
FT-Raman spectra were recorded on spectrophotometer
Model Bruker FRA 106. The XRD pattern was obtained by
using PHILIPS PW-1830. The conditions used were as fol-
˚
lows: Cu Ka radiation (1.5417 A), scanning angle (2h) from
0o to 60o. The total surface acidity was determined by
n-butylamine titration [39]. A 0.025 M solution of n-butyl-
amine in toluene was used for estimation. A 0.5 g catalyst
mass was suspended in this solution for 24 h and excess base
was titrated against trichloroacetic acid using neutral red as an
indicator. This gives the total acidity of the material.
2.5 Catalytic Reaction
The reaction of benzaldehyde (0.01 mol) with H2O2
(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. The
reaction mixture was refluxed at 80 °C for 6 h. The product
was extracted with dichloromethane by repeated extrac-
tions. The obtained products were analyzed on a gas
chromatograph (Shimatzu-2014) using a capillary column
(RTX-5). The obtained products were identified by com-
parison with the authentic samples and finally by gas
chromatography mass spectroscopy.
The data for surface area and total acidity are presented
in Table 1. The surface area decreases for catalysts as
compared to the supports, except for (PW11)3/ZrO2. In case
of (PW11)3/ZrO2 the size of the active species (PW11) is
smaller as compared to that of PW12. The small size may
lead to close packing of particles and strong interaction
with the support surface. This forms a surface overlayer
that reduces the surface diffusion of zirconia and inhibits
sintering, which results an increase in surface area. The
decrease in surface area is the first evidence of chemical
interaction of PW12/PW11 with the supports. N-Butyl
amine acidity values indicate that MCM-41 is fairly acidic
as compared to ZrO2 (Table 1). Acidity of all the catalysts
increases as compared to the supports which is as expected.
Raman spectra of PW12, (PW12)3/MCM-41 and
(PW12)3/ZrO2 are shown in Fig. 1. The Raman spectrum of
3 Results and Discussion
3.1 Catalyst Characterization
The typical FT-IR bands for PW12 are seen at 1080, 987, 893
and 800 cm-1 corresponding to P–O, W=O and W–O–W
PW12 shows bands at 1010, 990, 900, 550, and 217 cm-1
,
which are assigned to ms (W–Od), mas (W–Od), mas (W–Ob–
123

1560
S. Singh, A. Patel
bands for PW12, which clearly indicates that PW12 has been
successfully incorporated on the support.
Table 1 Textural and Acidity Properties of all the Catalysts
Catalyst
n-Butyl amine acidity
(mmol/g) (Total acidity)
Surface area
(m2/g)
Raman spectra of PW11 (Fig. 1d) show bands at 990,
985, 909, 513, and 225 cm-1, corresponding to ms
(W = Od), mas (W-Od), mas (W–Ob–W), ms (W–Oc–W), and
ms (W–Oa), respectively, which is identical to the reported
literature [42]. The spectra of (PW11)3/MCM-41 (Fig. 1e)
displays all the bands of mas (W–Od), mas (W–Ob–W), ms
(W–Oc–W), and ms (W–Oa), except ms (W=Od) band. The
presence of all reported bands of lacunary PW11 in catalyst
indicates that the Keggin structure of PW11 is intact even
after supporting. Raman spectra (Fig. 1f) of (PW11)3/ZrO2
also shows identical bands for PW11, which clearly indi-
cates that PW11 has been successfully incorporated on the
support.
ZrO2
0.62
0.82
1.10
1.41
0.95
1.01
170
659
146
360
226
253
MCM-41
(PW12)3/ZrO2
(PW12)3/MCM-41
(PW11)3/ZrO2
(PW11)3/MCM-41
W), ms (W–Oc–W), and ms (W–Oa), respectively (Fig. 1a)
where Oa, Ob, Oc, and Od correspond to the oxygen atoms
linked to phosphorus, 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 [41]. In case of (PW12)3/MCM-41 (Fig. 1b),
the absence of any significant band shifts in the spectra
indicates that the environment of the Keggin unit (PW12) is
retainment even after supporting to MCM-41. Raman
spectra (Fig. 1c) of (PW12)3/ZrO2 also shows identical
The XRD patterns of the supports and catalysts are
shown in Fig. 2. The XRD pattern of MCM-41 shows a
sharp peak at around 2h = 2° and a few weak peaks in
2h = 3°–5°, which indicated a well-ordered hexagonal
structure of MCM-41 (Fig. 2b). Further, the absence of
characteristic peaks of crystalline phase of PW12 (Fig. 2a)
Fig. 1 Raman spectra of
a PW12, b (PW12)3/MCM-41,
c (PW12)3/ZrO2 d PW11
e (PW11)3/MCM-41 and
f (PW11)3/ZrO2
123

Oxidative Esterification of Aldehydes
1561
Fig. 2 XRD pattern of a PW12, b MCM-41, c (PW12)3/MCM-41, d PW11, e (PW11)3/MCM-41, f ZrO2, g (PW12)3/ZrO2, h (PW11)3/ZrO2
3.2 Catalytic Performance
indicates that PW12 is finely dispersed inside the hexagonal
channels of MCM-41 (Fig. 2c). The large band 2h = 20°–
30° (Fig. 2c, e) is characteristic of the distribution of Si–O
bond lengths and is ascribed to the diffraction peak of
amorphous silica, which is typical of MCM-41 mate-
rial.The XRD pattern of (PW12)3/ZrO2 (Fig. 2g) shows no
crystalline peak corresponding to PW12 which indicates
that PW12 is finely dispersed on to the surface of ZrO2.
No separate characteristic peak of crystalline phase of
PW11 was observed in the (PW11)3/MCM-41 (Fig. 2e), which
indicates that PW11 is finely dispersed inside the hexagonal
channels of MCM-41. The XRD pattern of (PW11)3/ZrO2
(Fig. 2h) shows the amorphous nature of the materials indi-
cating that the crystallinity of PW11 is lost on supporting it
onto ZrO2. Further, it does not show any diffraction lines of
lacunary PW11 indicating a very high dispersion of solute as a
non-crystalline form on the support surface.
In order to examine the effects of catalyst on the direct
transformation of aldehydes to corresponding esters,
benzaldehyde and methanol were selected as test substrates
in presence of hydrogen peroxide as oxidant (Scheme 1).
The effect of different reaction variables such as benzal-
dehyde/H2O2 mol ratio, amount of catalyst, reaction time
and temperature were studied to optimize the conditions for
maximum conversion.
To see the effect of the mole ratio, the reaction was per-
formed by varying the mole ratio of benzaldehyde/H2O2,
with 0.1 g of the catalyst for 6 h at 80 °C (Fig. 3). According
to the chemical dynamics, the oxidative-esterification could
be improved by increasing the amount of H2O2. It was
observed from Fig. 3 that the conversion increases with an
increase in the molar ratio and reaches above 80 % for all the
catalysts at mole ratio of 1:3. With a further increase in the
mole ratio, there is no significant increase in conversion,
however slight increase in selectivity of ester is observed.
In all the cases major selectivity of methyl ester and
benzoic acid as minor product was obtained at 1:3 mol
FT-IR and FT-Raman spectra indicate that PW12 and
PW11 are retained in the catalysts. XRD and BET studies
show the uniform dispersion of PW12 and PW11 inside the
channels of the support (MCM-41) as well as on the sur-
face in case of ZrO2.
123

1562
S. Singh, A. Patel
Scheme 1 Direct synthesis of
Methyl benzoate from
Benzaldehyde
increase in % conversion with temperature. A maximum of
98, 95, 90, 95 % conversion was achieved respectively for
(PW12)3/MCM-41, (PW12)3/ZrO2, (PW11)3/ZrO2, (PW11)3/
MCM-41, at 80 °C.
It is necessary to study the effect of methanol in the
reaction medium as the methanol quantity greatly affects
the selectivity toward the methyl ester. Reactions were
carried out by varying the methanol quantity [using
(PW12)3/MCM-41] and it was observed that with increase
in the methanol the selectivity to methyl benzoate
increased. The selectivity of the ester was 60 % by using
2 mL of methanol as the reactant (Table 3). In contrast,
91 % of methyl benzoate (with 98 % conversion) was
achieved with 4 mL of methanol. The effect of methanol
was identical with all the other catalysts.
The optimum conditions are, mole ratio to benzaldehyde
to H2O2 = 1:3; catalyst amount 100 mg, temperature
80 °C, time 6 h, methanol 4 mL.
Fig. 3 Effect of molar ratio. Reaction conditions: amount of catalyst,
100 mg; reaction temperature, 80 °C; reaction time, 6 h; methanol,
5 mL
Under the optimised conditions, Table 4 shows that all
the catalysts give conversion in the range 90–98 % as well
as selectivity in the range 85–89 %. From the results it is
seen that present catalytic system is not a surface type
heterogeneous catalyst in which the catalytic activity is
directly proportional to surface area but it is a pseudoliquid
type heterogeneous catalyst in which catalytic activity is
proportional to the total acidity (Table 1).
ratio. Hence, the mole ratio of 1:3 is optimum for obtaining
high conversion as well as selectivity.
The effect of the amount of catalyst on conversion was
studied by varying the catalyst amount in the range of
50–200 mg. Oxidative esterification of aldehyde is signif-
icantly affected by acidity as well as oxidizing property of
the catalyst. The increase in the conversion can be attrib-
uted to an increase in the number of available catalytically
active sites. Hence, in the present case all the catalysts give
very good conversion. It is observed from Fig. 4, that the
conversion increases with an increase in the amount of
catalysts and reaches a maximum of 98, 95, 90 and 95 %
for (PW12)3/MCM-41, (PW12)3/ZrO2, (PW11)3/ZrO2,
(PW11)3/MCM-41, at 100 mg, respectively. 100 mg of the
catalyst was considered to be optimum for the maximum
conversion. The selectivity to ester product increased on
increasing the amount of the catalyst and major ester
product was observed using 100 mg of the catalyst in rel-
atively short reaction time (Fig. 4).
The control experiments with supports, PW12 and PW11
were also carried out under optimized conditions. In absence
of catalyst, no ester was formed and only benzoic acid was
detected. It is seen from Table 5 that the supports are not
very active towards the reaction and benzoic acid was only
detected as major product. Bulk PW12 and PW11 as catalyst
showed lower activity in comparison with supported ones
pointing out the importance of the concentration of surface
acid sites (Table 5). In most of the runs, methyl benzoate
was a main product at the end of the reaction. High TON
values was obtained for active species as well as catalysts.
The catalytic activity of PW12 as well as PW11 is retained
after heterogenization on ZrO2 and into the mesoporous
channels of MCM-41. Thus, we were successful in synthe-
sizing heterogeneous catalysts and overcoming the tradi-
tional problems of homogeneous catalysts.
The effect of reaction time (Table 2) shows that the
conversion increases with an increase in reaction time
along with the selectivity toward methyl benzoate. After
6 h, no significant increase in the conversion was observed.
The effect of reaction temperature (Table 2) indicates an
For the rigorous proof of heterogeneity, a test [43] was
carried out by filtering catalyst from the reaction mixture at
123

Oxidative Esterification of Aldehydes
1563
Table 3 Effect of methanol quantity
Methanol quantity
(mL)
% conversion
% selectivity
Methyl
benzoate
Benzoic
acid
1
2
3
4
5
6
80
30
70
85
60
30
90
85
15
98.5
99
91.5
92
8.5
8.0
7.0
99
93
Reaction conditions: mole ratio 1:3; reaction temperature, 80 °C;
reaction time, 6 h
Table 4 Conversion and selectivity of all the catalysts in optimized
conditions
Fig. 4 Effect of catalyst amount. Reaction conditions: mole ratio 1:3;
reaction temperature, 80 °C; reaction time, 6 h; methanol, 5 mL
Catalysts
% conversion
% selectivity
methyl benzoate
TON
(PW12)3/MCM-41
(PW12)3/ZrO2
98
95
95
90
91
85
89
89
1,224
1,186
1,179
1,166
Table 2 Effect of reaction temperature and time
Reaction time and
temperature
% conversiona/b/c/d
% selectivitya/b/c/d
methyl benzoate
(PW11)3/MCM-41
(PW11)3/ZrO2
2 h
50/45/60/66
88/85/95/88
98/95/95/90
98/95/95/90
50/50/60/55
80/87/80/77
98/95/95/90
98/95/95/91
30/21/40/40
90/82/85/84
91/85/89/89
92/87/89/89
50/20/40/30
89/50/66/60
91/85/89/89
80/85/90/89
Reaction conditions: mole ratio 1:3; catalyst amount 100 mg, reaction
temperature, 80 °C; reaction time, 6 h; methanol, 4 mL. Turnover
number (TON) = moles of product obtained/moles of catalyst (active
species)
4 h
6 h
8 h
60 °C
70 °C
80 °C
90 °C
catalyst with 10 mL of conductive water was refluxed for
24 h. Then, 1 mL of the supernatant solution was treated
with 10 % ascorbic acid. Development of blue colour was
not observed, indicating that there was no leaching. The
same procedure was repeated with alcohols and the filtrate
of the reaction mixture after completion of the reaction in
order to check the presence of any leached PW12 or PW11.
The absence of blue colour indicates no leaching of active
species from the supports.
a
(PW12)3/MCM-41
b
(PW12)3/ZrO2
c
(PW11)3/MCM-41
d
(PW11)3/ZrO2
80 °C after 4 h, and the filtrate was allowed to react up to
the completion of the reaction (6 h). The reaction mixture
of 4 h and the filtrate were analyzed by gas chromatogra-
phy. No change in the % conversion or the % selectivity
was found indicating the present catalyst falls into category
C [43].
3.3 Recycling of Catalysts
The catalyst was recycled to test its activity as well as
stability (Table 6). The catalyst was separated from the
reaction mixture by filtration, washed with methanol till the
filtrate is free from the un-reacted substrate (if any), fol-
lowed by washing with double distilled water and then
dried at 100 °C. Catalyst from different reactions was
recovered following the above procedure. The recovered
catalyst was charged for the subsequent run. After each
run, catalyst was collected quantitatively and the same
procedure was carried out to recover the catalyst after each
cycle. There was no appreciable change in % conversion
using regenerated catalyst up to four cycles.
Any leaching of the active species from the support
makes the catalyst unattractive, and hence, it is necessary
to study the leaching of PW12 or PW11 from the support.
Polyoxometalates can be quantitatively characterized by
the heteropoly blue colour, which is observed when it
reacts with a mild reducing agent, such as ascorbic acid. In
the present study, this method was used for determining the
leaching of active species from the support. One gram of
123

1564
S. Singh, A. Patel
The regenerated catalyst was characterized by FT-IR
and n-butyl acidity measurements (as explained earlier) in
order to confirm the retention of the catalyst structure, after
the completion of the reaction as discussed earlier. The FT-
IR data for the fresh as well as the regenerated catalysts are
presented in Fig. 5. No appreciable shift in the FT-IR band
positions of the regenerated catalysts compared to the fresh
catalysts indicates the retention of structure of PW12, PW11
on MCM-41 and ZrO2. Further, the acidity value for fresh
(PW12)3/ZrO2, (PW12)3/MCM-41, (PW11)3/ZrO2, (PW11)3/
MCM-41 (1.10, 1.41, 0.95, 1.01 mmol/g, respectively) and
reused catalysts also (1.09, 1.40, 0.95, 1.01 mmol/g,
respectively) did not show any appreciable change in
acidity. Hence there was no deactivation of the catalyst.
Table 5 Control experiments
Catalysts
% conversion
% selectivity
methyl benzoate
TON
MCM-41
ZrO2
PWa12
30
25
88
83
–
–
–
–
80
75
1,013
830
PWa11
Reaction conditions: mole ratio 1:3; catalyst amount
a
23 mg, reaction temperature, 80 °C; reaction time, 6 h; methanol,
4 mL
Table 6 Recycling of the catalysts
Catalyst
% conversion
Fresh
1st run
2nd run
3rd run
3.4 Comparison with the Reported Catalysts
(PW12)3/MCM-41
(PW12)3/ZrO2
98
95
95
90
97
93
93
88
96
93
93
88
96
92
91
87
Comparison of the present catalysts with reported one is
shown in Table 7. Keggin-type H3PW12O40 immobilized
on the surface of silica encapsulated Fe2O3 nanoparticles
(PW/Fe@Si) [35] showed 98 % conversion at room
(PW11)3/MCM-41
(PW11)3/ZrO2
Fig. 5 FT-IR spectra of a (PW12)3/MCM-41, b R-(PW12)3/MCM-41, c (PW12)3/ZrO2, d R-(PW12)3/ZrO2, e (PW11)3/MCM-41, f R-(PW11)3/
MCM-41, g (PW11)3/ZrO2 and h R-(PW11)3/ZrO2
123

Oxidative Esterification of Aldehydes
1565
Table 7 Comparison of present
catalysts with reported literature
Catalyst
Reaction Parameters
Molar Catalyst
Conversion % selectivity Reference
(%)
for methyl
benzoate
T (°C) Time
(h)
ratio
amount (g)
(PW12)3/MCM-41
(PW12)3/ZrO2
(PW11)3/MCM-41
(PW11)3/ZrO2
PW/Fe@Si
1:3
1:3
1:3
1:3
1:6
1:4
1:4
0.10
0.10
0.10
0.10
0.25a
0.1
80
80
80
80
R.T
80
–
6
6
98
91
Present work
Present work
Present work
Present work
[35]
a
mol%
95
85
b
0.3 mol aldehyde, and
1.5 mol alcohol, O2-3 kg/cm2
6
95
89
c
6
90
89
Reaction conditions: a
8
98
100
70.2
100
66.4
–
mixture of 1a (0.5 mmol),
oxidant agent (MnO2) and
methanol was added to 0.5 mL
of BmimBF4 containing the
base
POM-IL
8
86.4
100
80.1
86
[36]
V2O5–H2O2
0.04
2.0
3
[44]
b
–
Pd5Pb5Mg2/Al2O3
80
25
100
60
2
[13]
c
–
d
BmimBF4
Cu(ClO4)Á26H2O, InBr3
MnPc-APSG
0.30
–
24
16
3
[14]
Aldehyde (1.0 equiv), alcohol
d
–
(1.5 equiv), TBHP (Oxidant),
Cu(ClO4)2.6H2O (5.0 mol%),
and InBr3 (5.0 mol%)
83
–
[45]
1:2
0.02
100
100
[15]
Table 8 Oxidative
Substrate
Product
% conversion
98
% selectivity
91
TON
1,224
esterification of aldehydes to
corresponding esters over
(PW12)3/MCM-41
82
80
86
75
1,024
999
78
85
87
86
88
70
949
1,062
1,087
78
76
80
85
974
949
temperature, but the molar ratio of H2O2 to benzaldehyde
and catalyst amount was high. H3PW12O40 based Ionic
liquid (POM-IL) [36] showed good conversion of 86 %
and high selectivity, but the reaction time was higher.
V2O5–H2O2 [44] mediated reaction yielded high ester
product but the reaction required addition of HClO4.
123

1566
S. Singh, A. Patel
O
Scheme 2 Proposed reaction
mechanism for Oxidative
esterification of aldehyde with
methanol
H+
+
Ar
H
H
O
H2O
O
O
O
Ar
W
H
H2O2
O
O
O
Ar
W
O
W
H
O
O
O
Ar
OMe
Ar
O
O
W
O
OH
H
MeOH
H2O
O
Ar
OH
3.5 Effect of Different Aldehyde Substrates
and Proposed Reaction Mechanism
Pd5Pb5Mg2/Al2O3 showed 80 % conversion with molecu-
lar oxygen as oxidant and 2.0 g catalyst in stainless steel
reactor [13].
The scope of one-pot oxidative esterification was extended
over different substrates of aldehyde (Table 8). The
obtained results suggest that the catalyst and reaction sys-
tem can withstand variety of substrates under mild reaction
conditions.
Even though high conversion was achieved, selectivity
for methyl benzoate was 66.4 % and 1.0 mL 2 wt% solu-
tion of NaOH and 0.1 g Mg(OH)2 were also added into the
reactor to neutralize the produced carboxylic acid as an
additional step. Ionic liquid BmimBF4 (1-butyl-3-methy-
limidazolium tetrafluoroborate) [14] showed 86 % yield in
24 h, but the reaction procedure required the use of organic
base and oxidant-MnO2. Cu(ClO4).26H2O required InBr3 for
oxidative esterification of benzaldehyde using TBHP as
oxidant [45], but the yield obtained was less, even though
the temperature and reaction time was high. The use of
Manganese phthalocyanine immobilized on silica gel
(MnPc-APSG) [15] was extensively studied with high
conversion (100 %) and selectivity to methyl benzoate
(100 %) under mild conditions. From the extensive litera-
ture survey it was observed that reports on oxidative
esterification of benzaldehyde using H2O2 was less. In the
present work all the catalysts show higher conversions
under mild reaction conditions and attained good selec-
tivity towards methyl benzoate.
In order to study the reaction mechanism, the set of
reactions were carried out under two different conditions
(i) in the absence of catalysts (ii) in the absence of oxidant,
i.e. H2O2. Under both conditions, the reaction did not
progress significantly, indicating that no auto-oxidation
reaction takes place. From control experiments (Table 4) it
was seen that support alone i.e. MCM-41 and ZrO2 did not
show any significant conversion and only benzoic acid was
obtained as product. From these study a tentative reaction
mechanism for the present catalytic reaction is proposed in
Scheme 2.
It has been reported by Chavan et al. [11] that the cat-
alyst titanium-silicate with H2O2 forms hydroperoxo or
peroxo species which reacts with aldehyde to give titanium
derived trioxolane species as intermediate. It is known for
123

Oxidative Esterification of Aldehydes
1567
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parent as well as lacunary phosphotungstate. We have
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selectivity for ester. The order of activity of all the catalysts
was (PW12)3/MCM-41 [ (PW12)3/ZrO2 [ (PW11)3/MCM-
41 [ (PW11)3/ZrO2.The advantages of present catalytic
system include high substrate conversion, short reaction
time, ambient temperature and TON [ 1,000. The scope of
one pot reaction was extended over different aldehyde
substrates. The catalysts show potential of being used as a
recyclable catalytic material after simple regeneration
without any significant loss in the conversion. A probable
reaction mechanism was proposed which suggests that
acidity of the catalyst is responsible for difference in
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Acknowledgments We are thankful to Department of Science and
technology (DST), Project. No.SR/S5/GC-01/2009, New Delhi, for
the financial support. One of the authors Ms. Sukriti Singh is thankful
to the same for fellowship.
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