One pot oxidative esterification of benzaldehyde over a supported Cs-salt of mono nickel substituted phosphotungstate

Article abstract of DOI:10.1039/c6ra04362c

As a cleaner alternative to traditional two steps procedures, the one pot oxidative esterification of benzaldehyde to methyl ester was carried out over a supported Cs salt of mono nickel substituted phosphotungstate (CsPW₁₁Ni). CsPW₁₁Ni was supported on ZrO₂ and characterized using various physico-chemical techniques. The influence of reaction parameters, such as molar ratio of substrate to H₂O₂, amount of catalyst, reaction time, and reaction temperature, on the oxidative esterification reaction was investigated. Moreover, the catalyst could be recovered and reused up to three cycles without significant loss in selectivity. The present heterogeneous catalytic system was found to be efficient not only in terms of activity (63%) but also in selectivity (79%) of the desired product.

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One pot oxidative esterification of benzaldehyde over supported
Cs-salt of mono nickel substituted phosphotungstate
Anjali Patel*a, Soyeb Pathana and Pravya Prakashana
Received 00th January 20xx,
Accepted 00th January 20xx
As a cleaner alternative to traditional two steps procedures, one pot oxidative esterification of benzaldehyde to methyl
ester was carried out over supported Cs salt of mono nickel substituted phosphotungstates (CsPW11Ni). CsPW11Ni was
supported on to ZrO2 and characterized by various physico-chemical techniques. The influence of reaction parameters
such as molar ratio of substrate to H2O2, amount of the catalyst, reaction time, and reaction temperature on oxidative
esterification reaction was investigated. Moreover, the catalyst could be recovered and reused upto three cycles without a
significant loss in their selectivity. The present heterogeneous catalytic system was found efficient not only in terms of
activity (63%) but also in selectivity (79%) of the desire product.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Zn2+) supported onto carbon was carried out by Pizzio and co-
workers in 2007.19 The catalytic activity of the synthesized
materials was also evaluated for isopropyl alcohol dehydration
Introduction
Esters are valuable chemical products those have widespread
applications such as in fragrance industry, flavoring agents,
solvent extract-ants, diluents and intermediates. Traditionally
two steps synthesis of ester from aldehydes generally suffer
from generation of vast amount wastes, by-products and use
of toxic reagents.1 From the viewpoint of demands as well as
significance of acid and oxidation reactions, it would be more
beneficial to develop bifunctional catalytic systems for single
step oxidative esterification reactions. Till, considerable efforts
have been devoted to develop process for direct synthesis of
esters from the oxidative esterification of aldehydes with
alcohols using different catalysts based on Gold-Nickel Oxide
(AuNiOx),2 TS-1,3 gold nanoparticle Au/TiO2,4 Pb and Mg
doping in Al2O3-supported Pd,5 ionic liquid BmimBF4,6
manganese phthalocyanine immobilized on silica gel.7 In the
same context, polyoxometalates (POMs) based materials have
been well explored for the said reaction.8-10 Recently, a
by them. Synthesis and photocatalytic activity of amine-
functionalized mesoporous silica/anatase titania impregnated
with TMSPOMs ([M(H2O)PW11O39]5−-APS-TiO2, M=Co/Ni) was
reported by Guo et al. in 2007.20 Hu and co-workers in 2009,
carried out study of K10-nXn+MW11O39-Schiff-SBA-15, (X = P/Si,
M = Co/Ni/Cu/Mn) and its catalytic behaviour was evaluated
for oxidation of styrene to benzaldehyde.21 In 2012, Li et al.
prepared MCM-41 incorporated (PW11O39M1)5-(M1–POM, M1
=
Ni2+, Co2+ or Cu2+) and its catalytic performance in
esterification was investigated.22 Recently, R. A. Frenzel et al.
reported use of transition metal-modified polyoxometalates
[PW11O39M(H2O)]5−, where M= Ni2+, Co2+, Cu2+ or Zn2+
supported on carbon as
a catalyst in 2-(methylthio)-
benzothiazole sulfoxidation.23 Thus, the literature survey
shows that there are no reports on oxidative esterification of
aldehyde, an important industrial organic transformation,
using supported Ni substituted phosphotungstate.
subgroup of
POMs, transition
metal substituted
Recently we have reported one pot oxidative esterification
over Cs salt of mono nickel substituted phosphotungstate
(CsPW11Ni), although the catalyst was homogeneous it could
be recycled upto two cycles without any degradation and
modification of the structure, so as an extension of that work
we explored heterogeneous behavior of the catalyst by
supporting it onto zirconia. As we mentioned in our report,24
we have been working for the same since last six months and
succeeded to develop heterogeneous catalyst comprising
CsPW11Ni and hydrous Zirconia.
polyoxometalates (TMSPOMs), are gaining remarkable
attention in field of catalysis.11–14 Among the various
TMSPOMs ([XW11M(L)O39]5− ; X = P, Si; M = transition metal),
the nickel-substituted POMs are of considerable interest
because of their redox properties and variable oxidation
states, which makes them important for various catalytic
processes.15-17
In 2004, Y. Yang and group prepared TiO2-APS-PW11M (M =
Ni/Co) and explored its catalytic performance for
organochlorine pesticide and dyes.18 Preparation and detailed
characterization of Na5[PW11O39M], (M= Ni2+, Co2+, Cu2+, or
Thus, in the present paper, we report synthesis of Cs salt of
mono nickel substituted phosphotungstate supported onto
Zirconia, its characterization as well as catalytic activity for
oxidative esterification of benzaldehyde to methyl benzoate
was evaluated. Different reaction parameters including %
a.Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat,
India.
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loading of catalyst, mole ratio, amount of catalyst, time, Electron spin resonance (ESR) spectra were recorded on a
temperature, and quantity of methanol were optimized for Varian E-line Century series X-band ESR spectrometer at low
better result. The catalyst was recycled and regenerated upto temperature and scanned from 2000 to 3200 gauss.
three cycles.
Catalyst acidity
n-Butyl amine acidity by titration
Experimental
A 0.025 M solution of n-butyl amine in toluene was prepared
for assessment of total acidity of the catalyst.26 The 0.25 g
Materials
catalyst was suspended in 0.025 M n-butyl amine solution for
24 h and the excess base was titrated against trichloroacetic
acid using neutral red as an indicator. This test gives the total
acidity of the material.
All chemicals used were of A. R. grade. ZrOCl2.8H2O (Loba
Chemie), 12-Tungstophosphoric acid (H3PW12O40), NaOH,
NiCl2·6H2O, CsCl, liq. NH3, C6H5CHO, CH3OH, 30% H2O2, CH2Cl2
were obtained from Merck and used as received.
Acid sites determination using potentiometric titration
Synthesis of the support, ZrO2
The type of acid sites was investigated by employing
potentiometric titration with 0.05 N n-butylamine which helps
in computing different acid sites.27 0.5 g of catalyst sample was
suspended in 50 mL acetonitrile and the mixture was aged at
25 oC. 0.05 N n-butylamine (0.5 mL) in acetonitrile solution was
added in equal time periods and the potential (mV) was
recorded.
Hydrous zirconia was synthesized by the same method
reported by us earlier.25 In the typical method, aqueous
ammonia solution was added to an aqueous solution of
ZrOCl2.8H2O up to pH 8.5, aged at 100 oC over a water bath for
1 h, filtered, washed with conductivity water until chloride free
water was obtained and dried at 100 oC for 10 h. The obtained
material is designated as ZrO2.
Catalytic Evaluation
Synthesis of Cs salt of mono nickel substituted phosphotungstate24
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, magnetic stirrer,
H3PW12O40 (2.88 g, 1 mmol) was dissolved in water (10 mL)
and pH of the solution was adjusted to 4.8 using NaOH
solution. NiCl2·6H2O (0.237 g, 1 mmol) dissolved in a minimum
amount of water was mixed with the above hot solution. The
final pH was adjusted to 4.8 and heated at 80oCwith stirring for
2 h and filtered hot to which a saturated solution of CsCl was
added. The obtained light green crystals were filtered, air dried
and designated as CsPW11Ni.
o
and a guard tube. The reaction mixture was refluxed at 80 C
for 6 h. The product was extracted with dichloromethane. The
obtained products were analyzed on a gas chromatograph
(Shimadzu-2014) using
a capillary column (RTX-5). Each
analysis was carried out three times and the associated %
conversion values were found in the range of ±1.5 % which
was calculated using the following eqn (1) of standard
deviation for error analysis.
Synthesis of Cs salt of mono nickel substituted phosphotungstate
supported to zirconia
ꢀꢁ = ꢂꢄꢃ (ꢅꢆ − ꢅ
̅
)
(1)
A series of catalysts containing 10–40% of CsPW11Ni supported
onto ZrO2 was synthesized using impregnation method. ZrO2 (1
g) was impregnated with an aqueous solution of CsPW11Ni
(0.1/10–0.4/40 g mL-1 of double distilled water) and dried at
100 °C for 10 h. The resulting materials were designated as
10% PW11Ni/ZrO2, 20% PW11Ni/ZrO2, 30% PW11Ni/ZrO2, and
40% PW11Ni/ZrO2 respectively.
ꢇ
∑
Leaching Test
The leaching of active species from the support makes the
catalyst unattractive and unproductive, thus it is necessary to
examine the stability as well as leaching of PW11Ni from the
support. Polyoxometalates are quantitatively characterized by
the heteropoly blue color, which is visible when it reacts with a
mild reducing agent such as ascorbic acid. In the present study,
reported method was used for determining the leaching of
PW11Ni from the support.28 The obtained results were further
confirmed by atomic absorption spectroscopy (AAS).
Characterization
The acidity of catalyst was determined from n-butyl amine and
potentiometric test. Thermo gravimetric analysis (TGA) was
done using Mettler Toledo Star SW 7.01 up to 600oC.
Adsorption–desorption isotherms were done through
Micromeritics ASAP 2010 Surface area analyzer at -196 oC.
Specific surface area was calculated using Brunauer-Emmett-
Teller (BET method). Fourier transform infrared (FT-IR)
Result and discussion
spectroscopy was done using KBr pellets on a Perkin Elmer Characterization of Catalyst
instrument. The Fourier Transform Raman (FT-Raman) spectra
Leaching test shows absence of blue color indicates no
were recorded on a FT-Raman Spectrophotometer (Model
Bruker FRA 106). Powder X-ray Diffraction (Powder XRD) was
carried out using Philips Diffractometer (Model PW-1830).
leaching of PW11Ni from support into reaction medium.
Further, AAS analysis did not show presence of any metal
content of PW11Ni suggested that absence of leaching or if any
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metal was present, it was below the detection limit, which The TGA of 30% PW11Ni/ZrO2 (Fig. 1) shows initial weight loss
corresponded to less than 1 ppm.
of 8.8 % upto 180 oC indicating the loss of adsorbed water
The total acidity of catalysts, 10-40% PW11Ni loaded ZrO2 was molecule. Further, about 2.2 % weight loss upto 300 oC was
evaluated by n-butyl amine titration method and the results observed due to the loss of water of crystallization. Besides
are presented in Table 1. It can be seen from Table 1 that with this, no significant weight loss was observed upto 550 oC,
increase in % loading, total acidity increases up to 30% loading. suggests higher thermal stability of synthesized catalyst.
Further increase in % loading leads to decrease in the acidity. A The BET surface area of 30% PW11Ni/ZrO2 was found to be 229
decrease in the acidic value can be attributed to the blocking m2/g whereas of zirconia is 170 m2/g.25 The surface area of
of acidic sites for higher % loading.
30% PW11Ni/ZrO2 was observed to be higher as compared to
that of the support zirconia because of supporting of PW11Ni
as expected. The pore size distribution curve (Fig. 2) exhibits
average pore diameter of 30.9 Å (3.09 nm).
Table 1 n-Butyl amine acidity values
Catalyst
Acidic sites mmol. of
n- Butyl amine/g
ZrO2
0.62
0.68
0.72
10% PW11Ni/ZrO2
20% PW11Ni/ZrO2
30% PW11Ni/ZrO2
40% PW11Ni/ZrO2
0.80
0.75
The types and strength of the acidic sites were further
obtained by potentiometric titration method. The strength of
acidic sites in terms of initial electrode potential is shown in
Table 2.
Table 2 Acidity determined by potentiometric titration
Catalyst
Acidic
stength
Ei(mV)
Types of acidic sites
(meq.g-1)
Total
no. of
acidic
sites
1.3
Very
Strong Weak
strong
ZrO2
53
50
58
55
70
0
0
0
0
0
0.5
0.7
0.5
1.2
1.2
0.8
1.7
1.0
1.6
1.8
Fig. 2 Nitrogen adsorption–desorption isotherm and pore size distribution of
PW11Ni
2.4
30% PW11Ni/ZrO2.
30% PW11/ZrO2
10% PW11Ni/ZrO2
20% PW11Ni/ZrO2
1.5
2.8
The FT-IR spectra of ZrO2 (Table 3) shows broadband in the
region of 3400, 1600 and 1370, and 600 cm−1 attributed to O-H
asymmetric stretches, H–O–H and O–H–O bending, and Zr-O-
H bending respectively.25 FT-IR data of CsPW11Ni (Table 3)
exhibits bands at 1062; 961; and 885, 810 cm-1 corresponding
to P-O, W=O, and W-O-W asymmetric stretching frequencies
respectively, also a band at 490cm-1 is observed which is
attributed to Ni-O vibration, indicating the inclusion of metal
ion into the Keggin framework. In case of CsPW11Ni the
splitting value (∆υ) for P–O is 0cm-1 which is in good agreement
with the literature.29
3.0
30% PW11Ni/ZrO2
40% PW11Ni/ZrO2
110
75
0.2
0
1.3
1.4
2.2
2.5
3.7
3.9
It is seen from the Table 2 that the incorporation of PW11Ni
increases the strength of the acid sites of catalysts to a great
extent. It is interesting to note that 30% PW11Ni/ZrO2 shows
highest acidic strength. For 40% loaded catalyst the total
number of acidic sites is higher, however, acidic strength
decreases due to blocking of acidic sited at higher loading.
Hence 30% PW11Ni/ZrO2 was selected for detailed
characterization studies.
Table 3 FT-IR data of ZrO2, CsPW11Ni and 30% PW11Ni/ZrO2
Catalyst
Frequencies (cm-1)
O-H
3400
3447
3255
H–O–H
O–H–O
1600
Zr–O–H
600
-
P-O
W=O W-O- Ni-O
W
ZrO2
-
-
-
-
1370
CsPW11Ni
1624
1062
1064
961
938
885,
810
852
490
453
30%
1624
1375
682
PW11Ni/ZrO2
FT-IR data of 30% PW11Ni/ZrO2 shows similar bands as
CsPW11Ni at 1064; 938; 852 453; cm-1 indicating P-O; W=O; W-
O-W and Ni-O respectively (Fig. 4). In addition, peaks at 3255;
1624 and 1375; and 682 cm-1 attributed to O-H stretches, H–
Fig. 1. TGA of 30% PW11Ni/ZrO2.
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O–H and O–H–O bending, and Zr-OH bending respectively. The CsPW11Ni due to the substitution of metal ion into the lacuna.
shift in the band positions of supported catalyst as compared The diffraction peaks corresponding to crystalline phase
to CsPW11Ni is observed due to the interaction of terminal CsPW11Ni are absent in XRD pattern of 30% PW11Ni/ZrO2
oxygen of CsPW11Ni with hydrogen from surface -OH of confirming the well dispersion of PW11Ni onto the support.
zirconia. The Thus, it can be seen that the characteristic bands
of active species in supported catalyst are retained even after
the heterogenization.
Fig. 4 Powder XRD of a) CsPW11Ni b) ZrO2 and c) 30% PW11Ni/ZrO2.
The full range (3200–2000 G) X-band liquid nitrogen
temperature ESR spectra for CsPW11Ni (Fig. 5a) and 30%
PW11Ni/ZrO2 (Fig. 5b) was recorded. The obtained g value for
CsPW11Ni (g
∼
2.07) is in good agreement with reported one,31
indicates the presence of Ni(II) in octahedral or distorted
octahedral environment. Similarly, ESR spectra of 30%
PW11Ni/ZrO2 shows signal at g ~ 2.0 confirm undegraded
CsPMo11Ni on the surface of ZrO2. In other words, the Keggin
structure remains unaltered after supporting it on ZrO2.
Fig. 3 Raman Spectra of (a) ZrO2, (b) CsPW11Ni and (c) 30% PW11Ni/ZrO2
Raman spectra of ZrO2 (Fig. 3) shows broad peaks in the region
from 100 to 800 cm-1, which are associated with long-range
disordering arrangement in amorphous state.30 Raman spectra
of CsPW11Ni (Fig. 3) shows bands at 993, 976, 895, 506, and
225 cm-1, corresponding to υs (W=Od), υas (W-Od), υas (W-Ob-
W), υs (W-Oc-W), and υs (W-Oa) respectively. An additional
band in the range of 400-500 cm-1 is attributed to Ni-O
stretching confirming the presence of nickel in the CsPW11Ni.
The spectrum of 30% PW11Ni/ZrO2 displays bands at 992, 975,
942, 522, 213 corresponding to υs (W=Od), υas (W-Od), υas (W-
Ob-W), υs (W-Oc-W), and υs (W-Oa) respectively. The presence
of all the bands of PW11Ni in 30% PW11Ni/ZrO2 confirms that
structure of CsPW11Ni remains intact even after supporting on
to ZrO2. Observed significant shift in the bands is due to the
interaction of CsPW11Ni with surface hydroxyl groups of ZrO2.
The Powder XRD of PW11Ni is shown in Fig. 4. The peaks
ranging from15o to 30o 2Theta value indicating the presence of
characteristic peaks of parent Keggin ion are shifted in case of
Fig. 5 ESR Spectra of a) CsPW11Ni b) 30% PW11Ni/ZrO2.
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Catalytic Activity
ARTICLE
seen from Fig. 7 with an increase in the mole ratio from 1:1 to
1:3, conversion and selectivity increased simultaneously.
Scheme 1 Oxidative Esterification of Benzaldehyde.
In order to evaluate the efficiency of the catalyst, for the direct
conversion of aldehydes into their corresponding esters
reaction of benzaldehyde with methanol in the presence of
hydrogen peroxide as an oxidant was carried out as shown in
Scheme 1. The effect of different reaction variables such as
benzaldehyde/H2O2 mole ratio, amount of catalyst, reaction
time and temperature were studied to optimize the conditions
for maximum conversion.
Fig. 7 Amount of catalyst (10 mg), temperature (80 °C), time (6 h), amount of methanol
Effect of % loading
(5 ml).
The reaction was carried out by varying % loading of the active
species (10-40%) onto the support with 10 mg of catalysts for
The increase in conversion may be due to rise in concentration
of H2O2. A decrease in the conversion and selectivity is
observed in case of 1:4. Therefore, 1:3 mole ratio of
benzaldehyde: H2O2 was optimized which gave 63% conversion
and 79% ester.
o
6 h at 80 C. The results are presented in Fig. 6, which shows,
conversion and selectivity increases with increase in % loading
from 10% to 30% PW11Ni/ZrO2.
Effect of amount of 30% PW11Ni/ZrO2
In order to determine the optimum amount of catalyst, the
reaction was investigated at five different amounts of
catalysts, keeping other parameters fixed (1: 3 mole ratio of
o
benzaldehyde to H2O2 for 6 h at 80 C and 5 ml of methanol)
and result is presented in Fig. 8.
Fig. 6 Mole ratio (benzaldehyde/ H2O2) (1:3), amount of catalyst (10 mg), temperature
(80 °C), time (6 h), amount of methanol (5 ml).
However, further increase in loading from 30% to 40% results
decrease in conversion and selectivity. This may be due to
faster decomposition of H2O2 in presence of excess of active
centers in case of 40% PW11Ni/ZrO2, results decrease in
conversion of benzaldehyde. The obtained results may be due
to the blocking of acidic sites in the catalysts (Table 1 and 2).
Thus, 30% PW11Ni/ZrO2 catalyst was selected for detailed
catalytic study.
Fig. 8 Mole ratio (benzaldehyde/ H2O2) (1:3), temperature (80 oC), time (6 h), amount
of methanol (5 ml).
It is seen from the Fig. 8 that the conversion initially increases
with an increase in the amount of 30% PW11Ni/ZrO2 from 7.5
mg to 10 mg. However, on increasing catalysts amount from
10mg, decrease in conversion as well as selectivity for ester
was observed. This drop in conversion with increase in
Effect of Mole Ratio
The effect of H2O2 on the catalytic activity was studied by
varying mole ratio from 1:1 to 1:4 for benzaldehyde: H2O2. As
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amount of catalyst is due to rapid unproductive decomposition
of H2O2 in presence of excess amount of catalyst. In order to
confirm the obtained results, decomposition of H2O2 (SI-1)
with different amount of catalysts were studied and results are
presented in Table S1. From Table S1, it is clear that on
increasing catalyst amount, percentage decomposition of H2O2
increases under identical time. The obtained result suggests
that the best utilization of H2O2 was obtained in the case of 10
mg of catalysts whereas amount greater than 10 mg results in
unproductive decomposition of H2O2. Also, rapid
decomposition of H2O2 generates relatively excess moles of
water results decrease in selectivity of esterification process
which is a reversible process in presence of water. The
reaction carried out using 10 mg of catalyst gave 63 %
conversion and 79 % selectivity for ester, which is quite good
with such a minimal amount of catalyst.
Fig. 10 Mole ratio (benzaldehyde/ H2O2) (1:3), amount of catalyst (10 mg), temp. (80
°C), amount of methanol (5 ml).
Effect of Temperature
Effect of methanol quantity
The effect of volume of methanol was also studied for
optimizing the reaction (Table 4). With 2 ml methanol it gave
67% with 35% selection of ester was observed.
Table 4 Effect of Methanol Quantity
Methanol quantity
% Conv.
%Sel.
Ester
35
Acid
65
2 ml
5 ml
8 ml
67
63
79
21
56.3
74.5
25.5
Mole ratio benzaldehyde to H2O2 1:3; reaction temperature, 80 oC; reaction time,
6 h.
But, as volume of methanol was increased to 5 ml, better
conversion and selectivity was obtained. Further, it was
observed that with an increase in the volume of methanol
there was a decrease in conversion which may be due to less
availability of hydrogen peroxide for the reaction and dilution
of the reaction with increasing volume of methanol. Thus, 5 ml
methanol was selected as an optimized parameter for the
reaction.
Fig. 9 Mole ratio (benzaldehyde/ H2O2) (1:3), amount of catalyst (10 mg), time (6 h),
amount of methanol (5 ml).
Increase in temperature resulted increase in conversion as
seen from the case of 60 oC and 80 oC. As observed from Fig. 9,
at 60 oC good selectivity is observed. In case of 80 oC
temperature, it gave better conversion and selectivity when
compared with that of other varied temperatures. In case of
Control Experiment
o
90 C, conversion and selectivity for ester was decreased. This
Table 5 Control experiments.
may be due fast thermal and catalytic decomposition of H2O2
o
at elevated temperature. Thus, as a result 80 C was selected
Entry
% Conv.
%Sel.
for the optimum reaction temperature.
Ester
-
Acid
100
Without catalyst
30% PW11/ZrO2
PW11Nia
3.0
56.3
59.4
63.0
61.9
62.3
79.0
38.1
37.7
21.0
Effect of time
For the optimization of reaction time (Fig. 10), reaction was
carried out at various time duration, resulting good conversion
in case of 6 h as compared to other cases. As shown in the Fig.
10 with increase in time of reaction, conversion was increased
proficiently along with selectivity. But as the reaction time was
increased no significant change in conversion and selectivity of
ester was observed, due to attainment of equilibrium in the
reaction. As a result, reaction was optimized for 6 h for better
conversion and selectivity.
30% PW11Ni/ZrO2
Mole ratio, 1:3; reaction temperature, 80 oC; catalyst amount, 10 mg (a=2.3 mg);
reaction time, 6 h; amount of methanol, 5ml.
The control experiment for present catalytic system was
carried under optimized condition and results are shown in
Table 5. Oxidative esterification of benzaldehyde was done
without employing any of the catalyst which gave very
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negligible amount of conversion (3%) proving the need of an
efficient catalyst to speed up the reaction. When supported
lacunary counterpart 30% PW11/ZrO2 was employed as a
catalyst, 56.3% conversion with 61.9% selectivity for ester was
obtained. On comparing this result with present catalytic
system, it clearly shows the 30% PW11Ni/ZrO2 is better
catalysts in terms of both activity and selectivity. The control
experiment was also carried out with unsupported CsPW11Ni,
which shows 59.4% conversion with 62.3% selectivity of ester.
Further, by comparing the conversion and selectivity for
homogeneous as well as supported systems, it can be
observed that the 30% PW11Ni/ZrO2 shows relatively proficient
amount of ester. The obtained data from the control
experiments revealed the catalytic activity of the supported
catalyst to be a good source to accelerate and precede the
reaction to the desired level. On comparison the activity and
selectivity of all catalysts, 30% PW11Ni/ZrO2 was found to be
best catalyst, gives excellent conversion and selectivity with a
very low catalyst amount.
Fig. 11 Powder XRD of a) 30% PW11Ni/ZrO2 b) R- 30% PW11Ni/ZrO2.
The reused catalyst was characterized by Powder XRD as
shown in Fig. 11. There is no appreciable change in the XRD
pattern of fresh 30% PW11Ni/ZrO2 and recycled R- 30%
PW11Ni/ZrO2 indicating that the material remains unchanged
even after regeneration.
Thus, the optimum conditions are, mole ratio to benzaldehyde
to H2O2 (1:3); catalyst amount 10 mg, temperature 80 oC, time
6 h, methanol 5 mL, resulting Turnover number (TON) to be
9690.
Catalytic activity of recycled catalyst
Table 7 Recycling study of 30% PW11Ni/ZrO2
Cycle
Total no.
of acidic
sites
%
% Selectivity
Turn over
number
(TON)
Turn over
frequencies
(TOFs) h-1
Heterogeneity Test
Conv.
Accurate evidence of heterogeneity can be gained only by
filtering the catalysts before completion of the reaction and
analyzing the filtrate for % conversion.32 A test was performed
by filtering the catalyst from the reaction mixture at 80 °C after
3h of the reaction, and the filtrate was allowed to react
further.
(
meqg-1)
Ester Acid
Fresh
3.7
3.2
3.2
3.1
63
63
63
61
79
62
64
63
21
38
36
37
9690
9636
9587
9364
1615
1606
1598
1560
1
2
3
Table 6 Heterogeneity test
Reaction conditions: mole ratio, 1:3; reaction temperature, 80 oC; catalyst
amount, 10 mg; reaction time, 6 h; amount of methanol, 5ml.
Reaction time
% Conv.
49.8
% Ester
74.5
% Acid
25.4
3h
6h
Recycling data (Table 7) shows almost same conversion for all
the catalysts. However, remarkable decrease in selectivity of
ester was observed, especially for the first cycle. After that, no
significant change in selectivity was observed for subsequent
cycles. The observed trend in selectivity can be explained on
the basis of acidity of catalysts. From the Table 7; It is clearly
seen that, for first cycle, total number of acidic sites decreased
considerably as compare to fresh one which is responsible for
drastic decrease in selectivity of ester. However, total number
of acidic sites of successive recycled catalysts remained
constant; as a result no significant change in selectivity of ester
was observed.
49.6
73.9
26.0
The reaction mixture and the filtrate (after 6h) were analysed
by a gas chromatogram (Table 6). No significant change in the
% conversion indicates role of catalyst in obtaining good yield
and selectivity of desired product. The present catalyst falls
into the category C,32 i.e. the active species does not leach and
the observed catalysis is truly heterogeneous in nature.
Regeneration of the Catalyst
The catalyst was regenerated so as to examine its stability and
recycled for investigating its activity. The catalyst after the
reaction was separated, from the reaction mixture by simple
centrifugation; washed using dichloromethane and then dried
at 100 °C.
Conclusion
The present contribution reports one pot oxidative
esterification for the direct conversion of benzaldehyde to
The regenerated catalyst was further characterized by powder methyl benzoate over heterogeneous catalyst. The superiority
Characterization of regenerated catalyst
XRD for confirmation of retention of the structure of catalyst.
of the catalyst lies in better conversion and excellent
selectivity of the methyl benzoate with high TON (9690). The
advantages of using a recyclable Ni based catalyst under mild
reaction conditions instead of more expensive metals makes
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DOI: 10.1039/C6RA04362C
ARTICLE
Journal Name
27 P. Vasquez, L. Pizzio, C. Caceres, M. Blanco, H. Thomas, and
E. Alesso, J. Mol. Catal. A., 2000, 161, 223–32.
28 S. Pathan and A. Patel, Dalton Trans., 2011, 40, 348–355.
29 K. Nowinska, A. Waclaw, W. Masierak and A. Gutsze, Catal.
Lett., 2004, 92, 157–162.
30 H. Wang, G. Li, Y. Xue and L. Li, J. Solid State Chem., 2007,
180, 2790–2797
31 W. L. Feng, T. H. Chen, L. J. Wang and J. J. Chen, Optik., 2010,
121, 362–365.
this methodology interesting from an economic and an
ecological point of view.
Acknowledgement
We are thankful to Department of Science and Technology
(DST), Project. No. SR/S1/IC-36/2012, New Delhi, for the
financial assistance. Ms. Pravya Prakashan is thankful to the
same for the award of a research fellowship. We are also
thankful to Department of Chemistry, The M. S. University of
Baroda, for N2 adsorption-desorption analysis.
32 R. A. Sheldon, M. Walau, I. W. C. E. Arends and U.
Schuchurdt, Acc. Chem. Res., 1998, 31, 485–493.
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DOI: 10.1039/C6RA04362C
One pot oxidative esterification of benzaldehyde over supported Cs-salt of
mono nickel substituted phosphotungstate
Anjali Patel*a, Soyeb Pathana and Pravya Prakashana
• Table of Content Graphics:
•
Synopsis:
The synthesized mono Ni(II)-substituted phosphotungstate supported on to zirconia was used as
a catalyst for one pot oxidative esterification of benzaldehyde.
1