Silicotungstic acid supported zirconia: An effective catalyst for esterification reaction

Article abstract of DOI:10.1016/j.molcata.2007.05.022

A series of solid acid catalysts were synthesized by incipient wetness impregnation method by varying the wt% of silicotungstic acid on hydrous zirconia (ZSTA). The prepared catalysts were characterized by PXRD, FTIR, UV-vis DRS, EPMA, BET surface area, acid sites, etc. FTIR and UV-vis DRS studies indicate that the material retain the Keggin-type structure of silicotungstic acid up to 500 °C. The suitability of the materials was studied for acid catalysed esterification reactions using formic, acetic, propionic, n-butyric acid and n-butyl alcohol (NBA), isobutyl alcohol (IBA) and sec-butyl alcohol (SBA). Material with 15 wt% STA on hydrous zirconia having high surface area and acid sites acts as better catalyst for esterification reactions. The esterification of acids with NBA was found to be higher than IBA and SBA. In all the cases, the selectivity for the corresponding esters is nearly100%. The straight-line plot of -ln(1 - conversion) versus reaction time for the reactions carried out at 98 °C supports that the esterification reaction obeys first order kinetics with respect to acid concentration. The reusability study justifies that the catalyst is stable and active.

Full text of DOI:10.1016/j.molcata.2007.05.022

Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
Silicotungstic acid supported zirconia: An effective
catalyst for esterification reaction
K.M. Parida∗, Sujata Mallick
Colloids and Material Chemistry Department, Regional Research Laboratory (CSIR), Bhubaneswar 751013, Orissa, India
Received 13 March 2007; received in revised form 8 May 2007; accepted 9 May 2007
Available online 21 May 2007
Abstract
A series of solid acid catalysts were synthesized by incipient wetness impregnation method by varying the wt% of silicotungstic acid on hydrous
zirconia (ZSTA). The prepared catalysts were characterized by PXRD, FTIR, UV–vis DRS, EPMA, BET surface area, acid sites, etc. FTIR and
UV–vis DRS studies indicate that the material retain the Keggin-type structure of silicotungstic acid up to 500 ◦C. The suitability of the materials
was studied for acid catalysed esterification reactions using formic, acetic, propionic, n-butyric acid and n-butyl alcohol (NBA), isobutyl alcohol
(IBA) and sec-butyl alcohol (SBA). Material with 15 wt% STA on hydrous zirconia having high surface area and acid sites acts as better catalyst
for esterification reactions. The esterification of acids with NBA was found to be higher than IBA and SBA. In all the cases, the selectivity for
the corresponding esters is nearly100%. The straight-line plot of −ln(1 − conversion) versus reaction time for the reactions carried out at 98 ◦C
supports that the esterification reaction obeys first order kinetics with respect to acid concentration. The reusability study justifies that the catalyst
is stable and active.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Silicotungstic acid; Esterification; N-butyl acetate; Isobutyl acetate; Sec-butyl acetate
1. Introduction
on solids with high surface area are also a useful method for
improving catalytic performance.
Heteropoly acids (HPA) have attracted attention as catalysts
for various industrial processes such as oxidation, hydration,
esterification and etheration [1,2]. These HPA are stronger acids
and they have higher catalytic activity compared to homoge-
neous and acid catalysts such as sulphuric acid, silica-alumina
or ion exchange resins [3–5]. The use of HPA as catalysts is
important in the development of clean technologies, since it
avoids the drawbacks of environmental pollution and corrosion
of the conventional technologies. Other advantage is the ability
of recovering and reusing them in liquid phase reactions com-
pared to the homogeneous catalysts and the possibility of their
use in continuous processes. The disadvantage of bulk HPA as
catalysts is their relatively low stability and also of their low
surface area (1–10 m2 g−1) and separation problem from reac-
tion mixtures [6,7]. To minimize these disadvantages the HPA
are usually supported on a carrier. Heteropoly acids supported
Esterification of carboxylic acids is one of the fundamental
reactions in organic chemistry [8]. Liquid phase esterification
is an important method for producing various esters. Esteri-
fication of acetic acid with n-butyl alcohol is commercially
important as the product n-butyl acetate is widely used in the
manufacture of lacquer, artificial perfumes, leather, flavoring
extract, photographic films, plastics, pharmaceuticals and safety
glasses [9–11]. It is also used as dehydrating agent. Conventional
method of esterification reaction involves the use of mineral
acids such as H2SO4, HCl, HF, H3PO4 and ClSO2OH, etc.
These acids are corrosive and the excess acid has to be neu-
tralized after the reaction leaving considerable amount of salts
to be disposed off into the environment. So the replacement of
these conventional hazardous and polluting corrosive liquid cat-
alysts by solid acid catalysts is the demand of the day to create a
cleaner technology. Reduction of environmentally unacceptable
waste to minimize environmental pollution is the crucial factor
for developing environmentally friendly catalyst.
The study of hydrous zirconia is of interest due to its special
properties, which allow their use as catalyst or support. One of
∗
Corresponding author. Tel.: +91 674 2581636x425; fax: +91 674 2581637.
E-mail address: kmparida@yahoo.com (K.M. Parida).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.05.022

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K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
the main reasons that have drawn great attention to the use of
hydrated zirconia as a precursor of a catalyst carrier is due to
its large number of surface hydroxyl groups, which can undergo
chemical reaction or strong interaction with incorporated com-
ponents. The present study aims at synthesis, characterization
and evaluation of the catalytic activity of silicotungstic acid
supported on hydrous zirconia (ZSTA) towards esterification
of formic acid, acetic acid, propioinic acid and butyric acid
using various alkylating agents such as n-butyl alcohol (NBA),
isobutyl alcohol (IBA) and sec-butyl alcohol (SBA). Further
effect of various reaction parameters such as time, temperature,
molar ratio of the reactants and the amount of the catalyst on
n-butyl acetate formation was evaluated to optimize the reaction
conditions.
stirring followed by heating till complete evaporation of water.
Then it was dried in an oven at 120 ◦C for 24 h. The catalysts
will be termed: xZSTA (x varies from 3 to 20 wt%).
2.4. Physico-chemical characterization
The X-ray powdered diffraction pattern was recorded on a
Philips PW 1710 diffractometer with automatic control. The
patterns were run with a monochromatic Cu K␣ radiation with
a scan rate of 2◦ min−1
.
The UV–vis DRS spectra of the samples were recorded in
a Varian UV–vis spectrophotometer loaded with Carry 100
software. The spectra were recorded against the boric acid back-
ground.
DTA analysis of samples dried at 120 ◦C was carried out
under air atmosphere using a Thermal Analyzer (Perkin-Elmer
TG-DTA, Model: Diamond). The differential thermal analysis
(DTA)experimentswereperformedinN2-using4–5 mgsamples
at a heating of 10 ◦C/min.
2. Experimental
2.1. Preparation of support, hydrous zirconia
Zirconiumhydroxidegelwaspreparedfromaqueoussolution
of zirconium oxychloride (Fine Chemical) by drop wise addi-
tion of ammonium hydroxide solution (25% ammonia) (Merck,
India) till it attains pH 9.5. The hydrogel was refluxed at 100 ◦C
for 24 h, filtered, washed with deionized water and dried in an
oven at 120 ◦C for 24 h (named Z herein after).
The FTIR spectra were taken using Jasco FTIR 5300 in KBr
matrix in the range of 400–4000 cm−1
.
Micrographs showing X-ray image mapping of different ele-
ments of zirconia impregnated STA was taken using a Japanese
Model (JXA-8100) EPMA.
The ammonia-TPD of all the samples was carried out in a
CHEMBET-3000(Quantachrome)instrument. About0.1 gsam-
ple was taken inside quartz ‘U’ tube and degassed at 350 ◦C for
1 h with He gas flow. The sample was then cooled to 30 ◦C and
at this temperature the gas flow was changed to ammonia. It was
then heated at a heating rate of 10 ◦C/min up to 800 ◦C and the
spectra were recorded.
2.2. Preparation of silicotungstic acid;
H4SiW12O40·nH2O(STA)
Ist step: Preparation of silicic acid: A small amount of sodium
silicate (meta) was dissolved in minimum amount of water. Then
6N HCl was added drop wise with constant stirring till the solu-
tion is neutral to litmus paper. Then it was stirred for 15 min and
then a small amount of 6N HCl was added to precipitate silicic
acid. It was then filtered, washed with double distilled water.
2nd step: About 6 g of sodium tungstate was dissolved in
water. Then 6N HCl was added drop wise with constant stirring
to dissolve the white precipitate formed. The addition of HCl
continues till it is neutral to the litmus paper. Then to this solution
the freshly prepared sillicic acid was added. The whole solution
is boiled for 2 h. The solution is kept acidic by adding little HCl
to the solution during the above process. Then it was filtered and
the filtrate is evaporated to minimize the volume.
The above solution is shaken with ether and HCl vigorously.
A adduct was formed with ether which settled down at the bot-
tom. This is separated from the aqueous solution. The adduct
is white in colour. It was exposed to air to remove ether and
then dried at around 45 ◦C. The content of Si and W in STA was
analyzedbyICPandfoundtobe1.05and76.5 wt%, respectively.
The surface area measurement was carried out by BET
method using Quantasorb instrument (Quantachrome, USA)
by nitrogen adsorption-desorption measurements. The samples
were degassed at 120 ◦C at 10−3 Torr vacuum.
Surface acidity was determined spectrophotometrically on
the basis of irreversible adsorption of organic bases such as
pyridine (PY, pKb = 3.5) and 2,6-dimethyl pyridine (DYPY,
pKb = 8.7) [12]. In this method, adsorption experiment was
carried out in a 50 ml stoppered conical flask taking 10 ml of
each freshly prepared adsorbate (Fluka, Switzerland) solution,
along with 0.05 g of sample preheated at 393 K with constant
shaking. The concentration range for each adsorbate was
varied from 0.005 to 0.01 M in cyclohexane (Merck). After
2 h the contents were filtered and absorbance of the filtrate
was measured at preset wavelengths. For all cases, the sorption
experiments were carried out in the adsorbate concentration
range where Beer–Lambert’s law was valid. The time required
to reach equilibrium at room temperature was checked for all
of the samples and was never more than 1 h. In other words, the
time required for all the solute to adsorb on the active sites of
the catalyst is optimum in 1 h. All the absorbance measurements
were recorded in a spectrophotometer (Varian, Cary 1E) using
10 mm matched quartz cells.
2.3. Preparation of catalyst
The catalysts were prepared by wet impregnation method.
A series of catalysts having different loading ranging from
3 to 20 wt% were synthesized by impregnating 2 g of neat
Z with an aqueous solution of silicotungstic acid (STA)
(0.06–0.30 g/10–50 ml of conductivity water) under constant
The chemical interaction between the adsorbate and the
catalyst may be described by the linear transferred Langmuir
adsorption isotherm.

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
79
C
1
bXm
C
=
+
X
Xm
where C is the concentration of organic substrate in solution in
equilibrium with the adsorbate substrate, b the constant and X
is the monolayer coverage, which corresponds to the theoretical
amount of solute required to cover all the active sites for base
adsorption.
2.5. Catalytic reaction
The esterification reaction was carried out taking 0.025 g of
the calcined sample (dried at 120 ◦C for 6 h in an oven), 2 mmol
of acetic acid (Merck, 99.8%), 32 mmol of alcohol (Merck, 98%)
and 0.20 mmole of n-heptane (Merck, 99%) as an internal stan-
dard in a 100 ml round-bottomed flask equipped with a reflux
condenser. Thecontentswerethenrefluxedgentlyat98 ◦C. Then
the reaction mixture was filtered and the products were analysed
by means of gas chromatograph (GC-17A Shimadzu).
Fig. 2. DTA spectra of (a) Z (b) 6 wt% ZSTA, (c) 9 wt% ZSTA (d) 12 wt%
ZSTA, (e) 15 wt% ZSTA and (f) 20 wt% ZSTA.
3. Result and discussion
The XRD patterns of different weight percentage of silico-
tungstic acid impregnated on hydrous zirconia show similar
pattern to that of the support. This may be probably because,
the STA impregnated on the support is highly dispersed and
exhibits a non-crystalline to amorphous form and hence do not
show any additional X-ray reflection peak.
due to the crystalliazation to zirconia. The DTA of all sam-
ples show an endothermic peak at around 80–135 ◦C associated
with the loss of water molecule. The 6 wt% ZSTA showed an
exothermic peak at 460 ◦C, where as other catalysts such as
9, 12, 15 wt% ZSTA showed an additional exothermic peak
at around 570–596 ◦C. The 20 wt% ZSTA exhibited a broad
exothermic effect at around 690–735 ◦C, which could be due
to the crystallization of WO3.
The FTIR spectra of hydrous zirconia and silicotungstic acid
impregnatedonhydrouszirconiadriedat120 ◦C(15 wt%ZSTA)
are presented in Fig. 3a and b. The FTIR spectrum of Z (Fig. 3a)
shows broadband in the region of 3410 cm−1 due to asymmetric
Fig. 1 shows the background subtracted UV–vis DRS spec-
tra of (a) hydrous zirconia, (b) 15 wt% ZSTA and (c) 15 wt%
ZSTA calcined at 500 ◦C. The hydrous zirconia (Fig. 1a) exhibits
a strong absorption band at 230 nm, which may be attributed
to the charge transfer from oxide species to zirconium cation
(O− → Zr4+). In contrast the spectra of both 15 wt% ZSTA
dried at 120 ◦C and 15 wt% ZSTA calcined at 500 ◦C sample,
show broad band at 260 nm, which match well with the litera-
ture value [13], suggesting thereby the presence of undegraded
H3SiW12O40 species. This also indicates that the Keggin phase
remains unaltered up to 500 ◦C.
The catalysts with different STA loadings dried at 120 ◦C
were characterized by differential thermal analysis (Fig. 2). The
DTA of hydrous zirconia showed exothermic peaks at 450 ◦C
Fig. 1. DRS spectra of (a) Z (b) 15 wt% ZSTA and (c) 15 wt% ZSTA calcined
at 500 ◦C.
Fig. 3. FTIR spectra of (a) Z (b) STA, (c) 15 wt% ZSTA and (d) 15 wt% ZSTA
calcined at 500 ◦C.

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K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
stretching of OH group and two bands at 1621 and 1386 cm−1
are due to bending vibration of –(H–O–H)– and –(O–H–O)–
bond. The band at 600 cm−1 can be attributed to the presence of
Zr–O–H bond. The fingerprint bands of the STA Keggin anion
(Fig. 3b) appeared at 978, 915, 885 and 798 cm−1, which could
be assigned to the typical antisymmetric stretching vibrations of
W O, Si–O, W–Oc–W and W–Oe–W, respectively, where c for
the corner sharing oxygen atom connecting W3O13 units and e
for the edge sharing oxygen connecting W’s. [14]. This was also
observed in case of 15 wt% ZSTA (Fig. 3c).
Table 1
Surface area and acid sites of various STA impregnated hydrous zirconia
Catalysts
Surface area (m2 g−1
)
Acid sites (␮mol g−1
)
PY
2,6 DMPY
Z
412.3
413.8
434.6
467.4
475.9
505.9
456.1
156.5
165.2
192.7
211.6
238.1
285.3
247.5
58.20
95.58
106.1
154.7
198.3
202.7
179.9
ZSTA-3
ZSTA-6
ZSTA-9
ZSTA-12
ZSTA-15
ZSTA-20
The FTIR spectra of 15 wt% ZSTA sample calcined at 500 ◦C
is shown in Fig. 3d. There is slight shifting of bands, indicating
that the Keggin phase remains unaltered up to 500 ◦C.
the surface area increases from 412 to 505 m2 g−1. However,
from 15 wt% onwards, the surface area decreases gradually.
This implies that the presence of STA play a role in making
the material more porous up to the amount required to form
monolayer. However, when the STA content increases beyond
15 wt%, pore blocking takes place due to the presence of an
excess amount of silicotungstic acid.
In case of the surface acid sites, adsorption of pyridine
(PY) measures the total acidity where as 2,6-dimethyl pyri-
dine (DMPY) can be adsorbed on the Br␾nsted acid sites. It
Fig. 4 illustrates the X-ray image maps of W, P, Zr elements
in a selected area (SA) showing their level of concentration
through clustering of pixels. It can be seen from the maps that
W is adsorbed in Zr grain more than Si. This is obvious since in
original STA, tungsten content is 12 times more than silicon.
Table 1shows the BET surface area and acid sites of zirconia
and different weight percentage of ZSTA samples. The pure
hydrous zirconia dried at 120 ◦C showed a surface area of
412 m2 g−1. With the increase in STA content from 0 to 15 wt%,
Fig. 4. Scanning electron micrograph of 15 wt% ZSTA sample. (A) X-ray image map of Si, (B) secondary electron image of morphological view of 15 wt% ZSTA,
(C) X-ray image map of Zr and (D) X-ray image map of W.

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
81
Table 2
Catalytic activity of various catalysts for esterification of acetic acid with various
alcohols
Catalysts
Conversion (%)
with NBA
Conversion (%)
with IBA
Conversion (%)
with SBA
Z
50.28
65.12
72.53
79.25
86.21
91.45
88.68
48.5
38.25
50.54
64.56
69.87
70.63
75.26
71.24
3 ZSTA
6 ZSTA
9 ZSTA
12 ZSTA
15 ZSTA
20 ZSTA
63.89
70.42
77.36
83.64
89.1
85.91
Time = 4 h, reaction temperature = 98 ◦C, catalyst amount = 0.025 g,
acid:alcohol = 1:16.
Such an esterification reaction can be catalysed by strong
Br␾nsted acid sites. The reaction following the Eley–Rideal
mechanism takes place between alcohol chemisorbed on the
active Br␾nsted acid sites of the catalyst surface, forming a car-
bocation. Then nucleophilic attack by acetic acid on the stable
carbocation takes place resulting the formation of the n-butyl
acetate or isobutyl acetate. The role of an acid catalyst here is to
facilitate the formation of the carbocation, and to help remove
OH− from the alcohol [17].
Fig. 5. Temperature programmed desorption of ammonia (a) Z (b) 15 wt%
ZSTA.
The mechanism involved in the reaction is as follows.
is observed that by increasing the STA loading up to 15 wt% the
acidity gradually increases (from 165 to 285 ␮mol g−1), there-
after it decreases to 247 ␮mol g−1 for 20 wt% ZSTA. The same
trend is also found for Br␾nsted acid sites. The increase in sur-
face acidity, with an increase in STA loading may be due to the
formation of patchy monolayer of STA on zirconia. The decrease
in the surface acidity at high STA concentration is probably due
to the formation of polylayer coverage of STA on zirconia, which
decreases the number of Br␾nsted acid sites and consequently
that of total acid sites.
Temperature programmed reduction desorption (TPD) of
ammonia is a common method for investigating both the strength
and number of acid sites present on the surface of an acidic
solid. The TPD profile of zirconia and 15 wt% ZSTA are shown
in Fig. 5. The zirconia sample showed only one maxima in the
low temperature region, where as for the 15 wt% ZSTA samples,
ammonia could still be desorbed at 500 ◦C. This corroborates the
observation that 15 wt% ZSTA contains highest Bronsted acids
as well as total acid sites (Table 1) determined by spectroscopic
method.
Table 2exhibits the data on the conversion of acetic acid
with various alcohol over neat and STA promoted zirconia.
The product analysis by GC showed that the selectivity n-butyl
acetate, iso-butyl acetate and sec-butyl acetate is 100%. When
the esterification reaction was carried on hydrous zirconia,
50% of conversion of acetic acid to n-butyl acetate takes place.
The conversion of acetic acid increases with silicotungstic acid
loading on zirconia, reaching a maximum at 15 wt% ZSTA and
decreasing thereafter. This may be due to increase of Br␾nsted
acid sites and surface area with STA loading over zirconia.
The conversion of acetic acid with various alcohols decrease
in the order NBA > IBA > SBA. The difference in conversion
may be due to the degree of positive charge of the carbonium
ion. Since each alcohol after chemisorption on the Br␾nsted acid
sites give a carbonium ion for nucleophilic reaction with acetic
acid. NBA can give a carbonium ion of high degree of positive
charge compared to that of IBA and SBA, so the conversion for
esterification of acetic acid with NBA is higher than IBA and
SBA. The variation of different reaction parameters was studied
on 15 wt% of ZSTA catalyst using n-butyl alcohol.
3.1. Catalytic activity
The esterification of acetic acid with various alcohols is an
electrophilic substitution reaction. The reaction is relatively
slow and need activation either by higher temperature or by
catalyst to achieve higher conversion to a reasonable amount.
Samantray and Parida [15] and Namba et al. [16] reported
that the reaction of acetic acid and n-butanol in liquid phase
catalysed by solid acids proceeds according to a rate equation,
which are first order with respect to acetic acids and zero order
with respect to n-butyl alcohol.
3.1.1. Influence of reaction time
The influence of reaction time on the acetic acid conversion
was given in Fig. 6 using 15 wt% ZSTA as catalyst (0.025 g)
under other identical reaction conditions. A gradual rise in the

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K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
Fig. 6. Effect of reaction time on esterification of acetic acid using 15 wt%
ZSTA as catalyst, catalyst amount = 0.025 g, reaction temperature = 98 ◦C, acetic
acid:n-butyl alcohol = 1:16.
Fig. 8. Effect of reaction temperature on esterification of acetic acid using
15 wt% of ZSTA as catalyst; catalyst amount = 0.025 g, acetic acid:n-butyl alco-
hol = 1:16, time = 4 h.
conversion was seen with increase in duration of the reaction
period. As seen from Fig. 6, in 4 h of reaction time, 91.5% of
conversion is obtained, where as at the end of 5 h only 92% of
the reaction is complete. The selectivity towards n-butyl acetate
on the other hand remains same, i.e. 100% in 5 h. This suggests
that 4 h is sufficient to optimize the reaction parameters.
of carbonium ion which react with acetic acid to produce n-butyl
acetate.
3.1.4. Influence of molar ratio of reactants
Mole ratios of acetic acid to n-butanol were varied from 1:10
to 1:18 and the result was shown in Fig. 9. In all cases and in
all mole ratios of the reactants, n-butyl acetate was observed as
the main product. The conversion increased from 68.7 to 91.5%
with a change in mole ratio of acetic acid to n-butanol from
1:10 to 1:16. With further increase in mole ratio of acetic acid
to n-butanol, a slightly decrease in conversion was observed.
The reaction rate at different alcohol concentration increases
linearly with low alcohol concentration, but it is nearly inde-
pendent of alcohol concentration at high level. Decrease in
conversion in higher molar ratio of acid to alcohol (1:18) may
be due to the saturation of the catalytic surface with the alcohol
or prevention of nucleophilic attack by shielding the protonated
alcohol by its own excess. This analysis confirms that the reac-
tion mechanism can be represented by Eley–Rideal mechanism,
that is, the reaction takes place with chemisorption of alco-
hol on the Br␾nsted acid sites of the catalyst. This result is
very similar to Kirumakki and co-workers [18] who used zeo-
lite as catalyst over esterification of benzyl alcohol with acetic
acid and Jermy and Pandurangan [9], who used Al-MCM-41
catalyst over esterification of acetic acid with n-butyl alcohol,
3.1.2. Influence of catalyst amount
The amount of catalyst was varied from 0.01 to 0.03 g using
15 wt% ZSTA while keeping the molar ratio of acid:alcohol at
1:16 and reaction temperature at 98 ◦C. The reaction was car-
ried out for 4 h and the products were analyzed. The results are
represented in Fig. 7. With the increase in catalyst amount from
0.01 to 0.03 g, the conversion of acetic acid increases from 62.5
to 93.7%. This is due to the availability of large surface area and
acid sites, which favors the dispersion of more active species.
Therefore, the accessibility of the large number of molecules of
the reactants to the catalyst surface is favored. The selectivity
towards butyl acetate is nearly 100% in all cases.
3.1.3. Influence of reaction temperature
Fig. 8 illustrates the effect of reaction temperature on the
esterification of acetic acid with n-butanol. The reaction was
carried out in the temperature region 80–110 ◦C taking 15 wt%
ZSTA as catalyst without altering other reaction parameters. The
conversion of acetic acid increases (57–93%) with increase in
reaction temperature having nearly 100% selectivity. This sug-
gests that increase in reaction temperature favors the formation
Fig. 9. Effect of molar ratio of acid/alcohol on esterification of acetic acid using
15 wt% ZSTA as catalyst, catalyst amount 0.025 g, reaction temperature = 98 ◦C,
time = 4 h.
Fig. 7. Effect of catalyst amount on esterification of acetic acid carried out at
reaction temperature = 98 ◦C, acetic acid:n-butyl alcohol = 1:16, time = 4 h.

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 275 (2007) 77–83
83
water several times, dried and calcined at 120 ◦C and used in
the esterification reaction with a fresh reaction mixture. In the
regenerated sample after five cycles, the yield decreases by 5%.
4. Conclusion
Silicotungstic acid (15 wt%) supported on hydrous zirconia
acts as an efficient and stable solid acid catalyst for esterifica-
tion of acids using alcohols. Probably the high surface area and
Br␾nsted acid sites will play a key role in the high activity of
the catalyst. FTIR spectra and UV–vis spectra confirm that the
silicotungstic acid keeps its Keggin type structure even at 500 ◦C
calcination when supported on hydrous zirconia. EPMA studies
support that the STA is well dispersed on the surface of hydrous
zirconia. The highly dispersed STA on hydrous zirconia possibly
provide active sites for the esterification of acids using alcohols.
The percentage of conversion of formic, acetic, propionic, n-
butyric acid with various alcohols follows the order: n-butyl
alcohol > iso-butyl alcohol > sec-butyl alcohol. The esterifica-
tion of different acids with the above mentioned alcohols over
15 wt% ZSTA follows the order: formic > acetic > propionic > n-
butyric. In all the cases the selectivity towards the formation of
ester is 100%. The catalyst can be regenerated easily and reused
at least five times.
Fig. 10. Plot of −ln(1 − conversion) vs. reaction time for the reactions carried
out at 98 ◦C.
although in both the cases the experiments were performed in a
lower molar ratio of acid and alcohol compared to this present
work.
3.1.5. Kinetics of esterification of acetic acid with n-butanol
Fig. 10 shows the first order nature of the esterification reac-
tion, which gives a straight-line plot of −ln(1 − conversion)
versusreactiontimeforthereactionscarriedoutat98 ◦C. Namba
et al. [16] showed the reaction between acetic acid and n-butanol
proceeds according to a rate equation, which is first order with
respect to acetic acid and zeroth order with respect to n-butanol.
Acknowledgements
3.1.6. Reaction with other acids and alcohol
Formic acid, proponoic acid and n-butyric acid were sub-
jected to esterification reaction with n-butanol, iso-butanol,
sec-butanol under identical condition using 15 wt% ZSTA
as catalyst to test the generality of this method and
the results are summarized in Table 3. The percentage
of conversion of these acids follows the following order:
formic > acetic > proponoic > n-butyric which can be explained
on the basis of the strength and size of the reacting acids. The
strength of these aliphatic acids follows the same order as that
of conversion, i.e. formic > acetic > proponoic > n-butyric. This
may be due to the +I effect of the alkyl group of the acids. As
the bulkiness of the acids increases the steric hindrance also
decreases which results increase in acid conversion.
The authors are thankful to Prof. B.K. Mishra Director,
Regional Research Laboratory (CSIR), Bhubaneswar for his
constant encouragement and permission to publish this paper.
One of the authors Mrs. Sujata Mallick is highly obliged to
CSIR for SRF fellowship.
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Table 3
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1.
Acids
Conversion (%)
with NBA
Conversion (%)
with IBA
Conversion (%)
with IBA
Formic
Acetic
Proponoic
n-butyric
95.1
91.5
86.0
78.5
87.3
89.1
84.6
75.6
73.9
75.2
69.4
62.8
Time = 4 h, reaction temperature = 98 ◦C, catalyst amount = 0.025 g,
acid:alcohol = 1:16.