Supported heteropolyacids: Sytnhesis, characterization and effect of supports on esterification reactions

Article abstract of DOI:10.1134/S0023158410030092

12-Tungstophosphoric acid supported onto silica was synthesized by impregnation. The supports and synthesized catalysts were characterized for chemical stability, ion exchange capacity, thermogravimetric analysis, differential scanning calorimetry, FT-IR, and BET surface area. The catalytic activity was evaluated for liquid phase esterification reactions. The catalyst was regenerated and reused. The best catalyst was cal- cined at different temperatures and its catalytic activity was also evaluated for esterification reactions under optimized conditions. Further, obtained results are compared with l2-tungstophosphoric acid supported onto zirconia in order to see effect of acidic nature of support on catalytic activity as well as thermal stability of the catalyst. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0023158410030092

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 3, pp. 380–384. © Pleiades Publishing, Ltd., 2010.
Supported Heteropolyacids: Sytnhesis, Characterization
and Effect of Supports on Esterification Reactions1
V. Brahmkhatri and A. Patel
Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara, India
eꢀmail: aupatel_chem@yahoo.com
Received January 2, 2009
Abstract—12ꢀTungstophosphoric acid supported onto silica was synthesized by impregnation. The supports
and synthesized catalysts were characterized for chemical stability, ion exchange capacity, thermogravimetric
analysis, differential scanning calorimetry, FTꢀIR, and BET surface area. The catalytic activity was evaluated
for liquid phase esterification reactions. The catalyst was regenerated and reused. The best catalyst was calꢀ
cined at different temperatures and its catalytic activity was also evaluated for esterification reactions under
optimized conditions. Further, obtained results are compared with l2ꢀtungstophosphoric acid supported onto
zirconia in order to see effect of acidic nature of support on catalytic activity as well as thermal stability of the
catalyst.
DOI: 10.1134/S0023158410030092
1
In recent times supported heteropolyacids (HPAs) The best catalyst was calcined at different temperaꢀ
have been gaining importance as acid catalyst, espeꢀ tures (300, 400, and 500°C) and evaluated for esterifiꢀ
cially for reactions of esterification [1–6], alkylation cation reactions under optimized conditions. To see
[7–12] and acylation [8, 13–16], as they posses a the effect of support, the obtained results are comꢀ
number of advantages such as high catalytic activity pared with TPA supported onto ZrO2. The difference
and selectivity, high surface area, high thermal stability in the catalytic activity was correlated with the nature
over traditional corrosive and hazardous liquid acid of support.
catalyst, easy separation from reaction mixture and
possibility of their repeated use.
EXPERIMENTAL
Materials
In case of supported HPAs, it is known that support
does not play always merely a mechanical role but it
can also modify the catalytic properties of the HPAs.
Various supports (see [5, 6]) have been used for supꢀ
porting HPAs. As a contribution towards the same, we
have established the use of 12ꢀtungstophosphoric acid
(TPA) supported onto Al2O3 (neutral support) and
ZrO2 (acidic support) for some organic reactions such
as esterification of primary and secondary alcohols
[17], tertꢀbutylation of phenols and cresols [18, 19].
The obtained promising results encouraged us to
explore the catalytic activity of TPA supported onto
SiO2, more acidic support than ZrO2. The main aim of
present work is to see the effect of more acidic support
All chemicals used were of A.R. grade. H3PW12O40
⋅
n
H2O “Lobachemie,” Mumbai) and silica (60–
120 mesh size “SD Fine Chemicals,” Mumbai) was
used as received. Ethanol, propanol, ꢀbutanol, and
glacial acetic acid were obtained from “Merck” and
used as received.
n
Synthesis of the Catalyst
TPA was supported onto SiO2 by impregnation
method. A catalyst containing 30% TPA was syntheꢀ
sized by impregnating 1 g of SiO2 with an aqueous
solution of TPA (0.3 g/30 ml of bidistilled water) with
stirring for 35 h and dried at 100°C for 10 h. The
(
SiO2) on catalytic activity as well as on thermal stabilꢀ
ity of TPA.
The present work consists of synthesis and characꢀ
terization of TPA supported onto SiO2. The support
and catalysts were characterized for chemical stability,
ion exchange capacity, thermogravimetric analysis
(TGA), differential scanning calorimetry (DSC), FTꢀ
IR, and BET surface area. The catalytic property was
evaluated for the esterification of primary alcohols
obtained material was designated as TPA3/SiO2
.
The same process was followed for the synthesis of
series of supported TPA containing 20 and 40% of
TPA (0.2 or 0.4 g, 20 or 40 ml of bidistilled water). The
obtained materials were designated as TPA2/SiO2 and
TPA4/SiO2, respectively. The best catalyst TPA3/SiO2
was calcined at 300, 400, and 500°C in air for 5 h
and the resulting materials were designated as
(ethanol, nꢀpropanol, nꢀbutanol) with acetic acid.
1
The article is published in the original.
380

SUPPORTED HETEROPOLYACIDS: SYTNHESIS, CHARACTERIZATION
381
Table 1. Ion exchange capacity (IEC) values of the catalysts
C3ꢀTPA3/SiO2
respectively.
,
C4ꢀTPA3/SiO2, and C5ꢀTPA3/SiO2,
Catalyst
SiO2
IEC, meq/g
0.21
0.57
0.62
0.69
Characterization
TPA2/SiO2
TPA3/SiO2
TPA4/SiO2
Detail characterization of TPA3/ZrO2 can be found
in our earlier publications [17, 20]. The present study
includes detail characterization of TPA supported
onto SiO2. Different mineral acids and bases were used
for checking the chemical stability of the material.
The ion exchange capacity (IEC) was determined
by using the following formula:
Therefore the chemical stability of the material has
been checked in different mineral acids (HCl, H2SO4,
HNO3) and bases (NaOH, Na2CO3) up to 4 M conꢀ
centration. The present catalyst TPA3/SiO2 shows no
change in color or form indicating their stability.
IEC (in meq/g) =
(normality of NaOH) × (volume of NaOH)
.
weight of material
Table 1 shows values of ion exchange capacity.
These values give an idea about the acidity of materiꢀ
als. It is an indirect way to determine the Brønsted
acidity of materials. It is observed from the Table 1 that
value of IEC increases as the amount of TPA supꢀ
ported onto SiO2 increases.
Thermogravimetric analysis of the samples was
carried out on METTLER TOLEDO STAR SW 7.01
instrument. Differential scanning calorimetry of the
samples was carried out on METTLER S RSW810.
The FTꢀIR spectra of the samples (fresh and calcined)
were obtained by using KBr wafer on Perkin–Elmer
spectrometer. Adsorption–desorption isotherms of
samples were recorded on a Micromatries ASAp 2010
surface area analyzer at –196°C. From adsorption–
desorption isotherms surface area was calculated using
BET method.
In order to check the stability, the catalyst was studꢀ
ied for thermogravimetric analysis. The TGA of TPA
(Fig. 1) shows initial weight loss up to 210°C is due to
loss of adsorbed water and water of crystallization.
After that there is no weight loss up to 475°C. The
weight loss between 475 to 500°C may be due to the
decomposition of TPA. The TGA of TPA3/SiO2 shows
6.6% weight loss within 50–100°C temperature range
which is due to loss of adsorbed water and there is no
appreciable change in weight till 500°C indicating
increase in the stability of the TPA. This decrease in
percentage weight loss indicates the presence of chemꢀ
ical interaction between the support, SiO2 and TPA.
Esterification Reactions
The esterification reaction was carried out in a
50 ml glass reactor provided with a double walled air
condenser, Dean–Stark apparatus, magnetic stirrer
and a guard tube. Dean–Stark apparatus was attached
to a round bottom flask to separate the water formed
during the reaction. The reaction mixture was heated
at 80°C for 4 h. The obtained esters were analyzed on
a gas chromatograph (Nuconꢀ5700) using a Carbowax
20 column. Turnover number (TON) is defined as
mole substrate reacted per mole of the catalyst and was
calculated using following formula:
This can be further supported by DSC. DSC of
TPA3/SiO2 (Fig. 2) shows an endothermic peak in the
region of 80–100°C indicating the loss of adsorbed
water molecule. The DSC of TPA3/SiO2 did not show
any endothermic peak up to 500°C indicating no
decomposition of supported material. TGA and DSC
numbers of moles of substrate reacted
TON =
.
number of moles of catalyst
Weight, %
100
RESULTS AND DISCUSSION
Characterization of Catalysts
98
2
The catalyst comprising TPA supported onto ZrO2
was synthesized in our laboratory and well characterꢀ
ized by us earlier. From our earlier study, it was found
that catalyst containing 30% loading of TPA onto
ZrO2 was the best. The main characterization of
TPA3/SiO2 and TPA3/ZrO2 like ion exchange capacꢀ
ity, surface area measurement and thermal stability are
presented in Table 3.
96
94
1
92
0
100
200
300
400
500
Temperature, °C
600
Chemical stability of material plays an important
role. The material having solubility in water and/or in
acidic media may not be very useful as a catalyst.
Fig. 1. TGA of TPA (1) and TPA /SiO (2).
3 2
KINETICS AND CATALYSIS Vol. 51
No. 3
2010

382
Heat flow, mW
BRAHMKHATRI, PATEL
(a)
−
1
−
−
−
2
TPA3/SiO2
SiO2
3
4
4000 3200 2400 1800 1400 1000 600
(b)
50
100
5
200
15
300
25
400 500
Temperature, °C
C4ꢀTPA3/SiO2
C5ꢀTPA3/SiO2
0
35 45
Time, min
Fig. 2. DSC of TPA /SiO
.
3
2
C3ꢀTPA3/SiO2
indicate that TPA is thermally stable up to 500°C
when supported onto silica.
FTꢀIR spectrum of SiO2 (Fig. 3a) shows broad
band at of 3464 cm–1 attributed to asymmetric
hydroxo (–OH) stretches and two bending vibrations
at 1636 and 1370 cm–1 corresponding to H–O–H and
O–H–O, respectively. In addition to these bands, FTꢀ
IR spectrum of TPA3/SiO2 (Fig. 3a) shows bands at
1088, 987 and 800 cm–1 which correspond to W–O–W
bending, W–O and P–O symmetric stretching,
respectively. The positions are in good agreement with
those reported earlier [21, 22]. Further, no appreciable
shifting in the band positions (Fig. 3b) of calcined catꢀ
4000 3200 2400 1800 1400 1000 600
1
Wavenumber, cm−
Fig. 3. (a) FTꢀlR spectra of SiO and TPA /SiO ; (b) FTꢀ
2
3
2
IR spectra of C ꢀTPA /SiO , C ꢀTPA /SiO , and C ꢀ
3
3
2
4
3
2
5
TPA /SiO
.
3
2
Esterification of nꢀButanol
Effect of loading of TPA onto SiO2. The esterificaꢀ
tion reaction was carried out with TPA2/SiO2,
TPA3/SiO2, and TPA4/SiO2. The obtained results are
shown in Fig. 4. It is observed from the Fig. 4 that with
increase in the loading of TPA onto SiO2, the yield also
increases. For 30 and 40% loading, the difference in
yields is not that much appreciable. This may be due to
the blocking of active sites. Hence, the catalyst conꢀ
taining 30% of TPA was used for the detail study.
alyst (C3ꢀTPA3/SiO2, C4ꢀTPA3/SiO2, C5ꢀTPA3/SiO2
)
were observed as compared to TPA3/SiO2, indicating
that the TPA keeps its Keggin type structure undeꢀ
graded up to 500°C when supported onto SiO2
.
The values of BET surface area of SiO2 and
TPA3/SiO2 were found to be 500 and 234 m2/g,
respectively. It is known [23] that there may be
decrease in the surface area in case of the supported
catalyst in which oxides are used as supports. Decrease
in surface area [24] could be explained due to support
pore blocking by active phase.
Effect of molar ratio of
reaction was carried out by varying molar ratio of
ꢀbutanol to acetic acid, with 0.25 g of the catalyst for
nꢀbutanol to acetic acid. The
n
4 h at 80°C. It is observed that with increase in conꢀ
centration of acid the yield of the product increases.
With 1 : 2 molar ratio of nꢀbutanol to acetic acid the
yield of butyl acetate was found to be 52% while with
1 : 4.4 molar ratio was found to be 70%. Hence, further
reactions were carried out in 1 : 4.4 molar ratio of alcoꢀ
hol to acid.
Esterification Reactions
Esterification is a straight forward reaction subꢀ
jected to general Brønsted acid catalysis. The yields
can be increased by increasing the concentration of
either alcohol or acid. In a practical means, it is desirꢀ
able to obtain maximum yield for economic reasons,
the reactant that is usually less expensive is taken in
excess. In present study corresponding acid is taken in
excess.
Effect of amount of catalyst (TPA3/SiO2). To study
the effect of the amount of the catalyst, the reaction
was carried out with different amounts of the catalyst
keeping the molar ratio of alcohol to acid 1 : 4.4 for 4 h
at 80°C. The yield of butyl acetate is reported in Fig. 5.
It is seen that initially the activity increases with an
KINETICS AND CATALYSIS Vol. 51
No. 3
2010

SUPPORTED HETEROPOLYACIDS: SYTNHESIS, CHARACTERIZATION
383
Yield, %
100
Yield, %
100
90
80
70
90
80
70
60
50
60
50
0.1
0.2
0.3
0.4
0.5
0.6
Amount of catalyst, g
0.7
0.8
20
30
40
Loading, %
Fig. 5. The yield of butyl acetate with TPA /SiO catalyst
Fig. 4. The yield of butyl acetate with different catalysts
(molar ratio of alcohol to acid is 1 : 4.4).
3
2
(molar ratio of alcohol to acid is 1 : 4.4).
terms of turnover number, it is also seen from Table 2
that TPA3/SiO2 was proved to be more efficient cataꢀ
lyst for long chain alcohols.
increase in the amount of the catalyst. On further
increase with the amount from 0.25 to 0.75 g of the
catalyst, almost 71–72% yield of ester was obtained.
The effect of the amount of the catalyst was studied
keeping all other parameters (temperature, reaction
time, molar ratio of alcohol to acid) constant. Under
these conditions, no increase in the yield with increase
in amount of catalyst indicates the attainment of the
maximum equilibrium. The obtained results are as
expected. Hence, 0.25 g is the optimized amount of
the catalyst.
Effect of the Support
TPA was supported onto two different supports,
SiO2 and ZrO2. As the active species, TPA is the same,
same catalytic activity is expected. But it is seen from
Table 3 that TPA supported onto SiO2 is more active as
compared to that of ZrO2. The obtained difference in
catalytic activity may be due to the nature of support.
It is known that IEC is an indirect method of measurꢀ
The optimum conditions for esterification of
ꢀbutanol using TPA3/SiO2 are: molar ratio of alcohol
n
to acid is 1 : 4.4; amount of catalyst, 0.25 g; reaction
temperature, 80°C and reaction time, 4 h.
ing Brønsted acidity and it involves exchange of ions
on the surface. The IEC values for SiO2 and ZrO2 were
found to be 0.21 and 0.16, respectively. It is interesting
to note that even though SiO2 is more acidic than ZrO2
the IEC for TPA3/SiO2 and TPA3/ZrO2 is the same
(Table 3) at equal concentration of TPA. This may be
due to the fact that exchange of ions occurs only from
the same surface species, i.e., from TPA for both supꢀ
ports.
The butyl acetate yield over the calcined catalysts
C3ꢀTPA3/SiO2, C4ꢀTPA3/SiO2, and C5ꢀTPA3/SiO2
under optimized conditions was 74, 76, and 78%,
respectively.
Recycling of the catalyst. The catalyst was recycled
in order to test its activity as well as stability. The cataꢀ
lyst was separated from the reaction mixture only by
simple filtration, washed with conductivity water till
the filtrate is free from the acid, dried at 100°C and the
recovered catalyst was charged for the further run.
There is no appreciable change in the yield of butyl
acetate using regenerated catalyst up to four cycles.
Hence, the catalyst can be reused and regenerated.
Esterification reactions are purely Brønsted acid
catalyzed reactions. As mentioned earlier, support
may also play a role in modifying the catalytic activity
of HPAs. As SiO2 is more acidic than ZrO2, higher catꢀ
alytic activity is expected in case of TPA3/SiO2. The
results are as expected. The turnover numbers meaꢀ
sured also suggested that TPA3/SiO2 was more active
Esterification of Various Alcohols
Using TPA3/SiO2 As Catalyst
*
Table 2. The yields of different esters using TPA3/SiO2
The esterification of other alcohols such as nꢀproꢀ
panol and ethanol with acetic acid was also carried out
under optimized conditions and the yields of correꢀ
sponding esters are shown in the Table 2.
Name
of alcohol
Yield
of ester, %
Name of ester
TON
The order of catalytic activity is ethyl acetate < proꢀ
pyl acetate < butyl acetate. The activities of alcohols
were increased as the carbon chain of the alcohols
grew longer. This could be explained due to the effect
of OHꢀgroup location in long chain alcohols which is
more exposed towards active sites of the catalyst. In
Ethanol
Ethyl acetate
Propyl acetate
Butyl acetate
60
66
71
2985
3280
3545
n
n
ꢀPropanol
ꢀButanol
* Amount of catalyst 0.25 g, molar ratio 1 : 4.4, reaction temperaꢀ
ture 80 C, reaction time 4 h.
°
KINETICS AND CATALYSIS Vol. 51
No. 3
2010

384
BRAHMKHATRI, PATEL
Table 3. Main characteristics of TPA3/SiO2 and TPA3/ZrO2
Thermal stability
(TGA), °C
Catalyst
IEC, meq/g
Surface area, m2/g Yield of butyl acetate, %
TON
TPA3/SiO2
TPA3/ZrO2
500
430
0.62
0.62
234
146
71
53
3545
2650
4. Yang, L., Qi, Y., Yuan, X., et al., J. Mol. Catal. A: Chem.
2005, vol. 229, p. 199.
5. Shanmugam, S., Viswanathan, B., and Varadarajan, T.K.,
J. Mol. Catal. A: Chem., 2004, vol. 223, p. 143.
6. Pizzio, L.R., Vázquez, P.G., Cáceres, C.V., and
Blanco, M.N., Appl. Catal., A, 2003, vol. 256, p. 125.
7. Janik, M.J., Davis, R.J., and Neurock, M., J. Catal.
2006, vol. 244, p. 65.
8. Kamiya, Y., Ooka, Y., Obara, C., et al., J. Mol. Catal. A:
Chem., 2007, vol. 262, p. 77.
,
catalyst than TPA3/ZrO2 since higher TON are
expected for more efficient catalyst.
Surface area of catalyst also plays an important role
in catalytic reactions. The surface area of SiO2 is
higher than that of ZrO2. Increase in surface area may
be due to the nature of support. The higher value of
surface area for TPA3/SiO2 as compared to that of
TPA3/ZrO2 is also responsible for higher catalytic
activity.
,
Thermal stability of the catalyst at higher temperaꢀ
ture decides its applicability in extreme conditions.
The thermal stability of TPA increases from
TPA3/ZrO2 to TPA3/SiO2 as observed from TGA and
DSC results. This increased thermal stability of TPA
on SiO2 could be explained due to the nature of supꢀ
port.
9. Pizzio, L.R., Vázquez, P.G., Cáceres, C.V., et al., Appl.
Catal., A, 2005, vol. 287, p. 1.
10. Nandhini, K.U., Arabindoo, B., Palanichamy, M., and
Murugesan, V., J. Mol. Catal. A: Chem., 2004, vol. 223,
p. 201.
11. Yadav, G.D. and Doshi, N.S., J. Mol. Catal. A: Chem.
2003, vol. 194, p. 195.
,
Thermal study shows that the TPA keeps its Keggin 12. Mao, J., Kamiya, Y., and Okuhara, T., Appl. Catal., A
2003, vol. 255, p. 337.
SiO2. The present catalyst is proved to be successful in 13. Sarsani, V.R., Lyon, C.J., Hutchenson, K.W., et al.,
J. Catal., 2007, vol. 245, p. 184.
tions as compared with the traditional liquid acid catꢀ 14. Yadav, G.D. and Bhagat, R.D., J. Mol. Catal. A: Chem.
2005, vol. 235, p. 98.
,
structure undegraded up to 500°C when supported on
esterification of primary alcohols under mild condiꢀ
alyst. The TPA3/SiO2 was found to be more active as
,
compared to TPA3/ZrO2. The difference in the cataꢀ 15. Cardoso, L.A.M., Alves, W., Gonzaga, A.R.E., et al.,
J. Mol. Catal. A: Chem., 2004, vol. 209, p. 189.
lytic activity may be due to the more acidic nature of
SiO2 than ZrO2.
16. Castro, C.D., Primo, J., and Corma, A., J. Mol. Catal.
A: Chem., 1998, vol. 134, p. 215.
17. Sharma, P. and Patel, A., Bull. Mater. Sci., 2006,
ACKNOWLEDGMENTS
vol. 29, p. 439.
18. Bhatt, N., Shah, C., and Patel, A., Catal. Lett., 2007,
vol. 117, p. 146.
19. Bhatt, N. and Patel, A., Catal. Lett., 2007, vol. 113,
One of the authors, Ms. Varsha Brahmkhatri, is
thankful to BRNS (Project no. 2007/37/20/
BRNS/917, Mumbai) for the financial assistance.
p. 99.
20. Patel, S., Purohit, N., and Patel, A., J. Mol. Catal.
2003, vol. 192, p. 195.
21. Misono, M., Mizuno, N., Katamura, K., et al., Bull.
Chem. Jpn., 1985, vol. 55, p. 400.
22. Vázquez, P., Pizzio, L., Cáceres, C., et al., J. Mol.
Catal. A: Chem., 2000, vol. 161, p. 223.
,
REFERENCES
1. Peters, T.A., Benes, N.E., Holmen, A., and
Keurentjes, J.T.F., Appl. Catal., A, 2006, vol. 297,
p. 182.
2. Sejidov, F.T., Mansoori, Y., and Goodarzi, N., J. Mol.
Catal. A: Chem., 2005, vol. 240, p. 186.
23. Varga, G.M., Papaconstantious, E., and Pope, M.T.,
Inorg. Chem., 1970, vol. 9, p. 667.
3. Sepúlveda, J.H., Yori, J.C., and Vera, C.R., Appl. 24. Popa, A., Sasca, V., Kiš, E.E., et al., J. Optoelectron.
Catal., A, 2005, vol. 288, p. 18.
Adv. Mater., 2005, vol. 7, p. 3169.
KINETICS AND CATALYSIS Vol. 51
No. 3
2010