Template-free sol–gel synthesis of high surface area mesoporous silica based catalysts for esterification of di-carboxylic acids

Article abstract of DOI:10.1016/j.crci.2016.02.001

High surface area mesoporous silica based catalysts have been prepared by a simple hydrolysis/sol–gel process without using any organic template and hydrothermal treatment. A controlled hydrolysis of ethyl silicate-40, an industrial bulk chemical, as a silica precursor, resulted in the formation of very high surface area (719?m²/g) mesoporous (pore size 67?? and pore volume 1.19?cc/g) silica. The formation of mesoporous silica has been correlated with the polymeric nature of the ethyl silicate-40 silica precursor which on hydrolysis and further condensation forms long chain silica species which hinders the formation of a close condensed structure thus creating larger pores resulting in the formation of high surface mesoporous silica. Ethyl silicate-40 was used further for preparing a solid acid catalyst by supporting molybdenum oxide nanoparticles on mesoporous silica by a simple hydrolysis sol–gel synthesis procedure. The catalysts showed very high acidity as determined by NH₃-TPD with the presence of Lewis as well as Br?nsted acidity. These catalysts showed very high catalytic activity for esterification; a typical acid catalyzed organic transformation of various mono- and di-carboxylic acids with a range of alcohols. The in situ formed silicomolybdic acid heteropoly-anion species during the catalytic reactions were found to be catalytically active species for these reactions. Ethyl silicate-40, an industrial bulk silica precursor, has shown a good potential for its use as a silica precursor for the preparation of mesoporous silica based heterogeneous catalysts on a larger scale at a lower cost.

Full text of DOI:10.1016/j.crci.2016.02.001

C. R. Chimie 19 (2016) 1247e1253
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ꢀ
Full paper/Memoire
Template-free solegel synthesis of high surface area
mesoporous silica based catalysts for esterification of di-
carboxylic acids
,
,
, *
Pavan M. More a b, Shubhangi B. Umbarkar a b, Mohan K. Dongare c
a CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
b Academy of Scientific and Innovative Research, CSIR, Anusandhan Bhawan, New Delhi 110 001, India
c Malati Fine Chemicals Pvt. Ltd., Durvankurdarshan Society-1, Panchwati Area, Pashan Road, Pune 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 August 2015
Accepted 1 February 2016
High surface area mesoporous silica based catalysts have been prepared by a simple hy-
drolysis/solegel process without using any organic template and hydrothermal treatment. A
controlled hydrolysis of ethyl silicate-40, an industrial bulk chemical, as a silica precursor,
resulted in the formation of very high surface area (719 m2/g) mesoporous (pore size 67 Å
and pore volume 1.19 cc/g) silica. The formation of mesoporous silica has been correlated
with the polymeric nature of the ethyl silicate-40 silica precursor which on hydrolysis and
further condensation forms long chain silica species which hinders the formation of a close
condensed structure thus creating larger pores resulting in the formation of high surface
mesoporous silica. Ethyl silicate-40 was used further for preparing a solid acid catalyst by
supporting molybdenum oxide nanoparticles on mesoporous silica by a simple hydrolysis
solegel synthesis procedure. The catalysts showed very high acidity as determined by NH3-
TPD with the presence of Lewis as well as Brønsted acidity. These catalysts showed very high
catalytic activity for esterification; a typical acid catalyzed organic transformation of various
mono- and di-carboxylic acids with a range of alcohols. The in situ formed silicomolybdic
acid heteropoly-anion species during the catalytic reactions were found to be catalytically
active species for these reactions. Ethyl silicate-40, an industrial bulk silica precursor, has
shown a good potential for its use as a silica precursor for the preparation of mesoporous
silica based heterogeneous catalysts on a larger scale at a lower cost.
This article is dedicated to Prof. Edmond
Payen on his 60th birthday
Keywords:
Ethyl silicate-40
Mesoporous silica
Esterification
Solid acid
Mono- and di-carboxylic acid
© 2016 Published by Elsevier Masson SAS on behalf of Académie des sciences.
1. Introduction
prepared using triblock copolymers as templates. Various
other structure directing templates have been used for the
The discovery of mesoporous silica materials by Beck
et al. in 1992 created tremendous interest in the synthesis
of these materials due to their great potential applications
in catalysis, separation technologies, drug delivery systems
and many other fields [1].
synthesis of mesoporous silica materials [2,3]. However, the
use of structure directing templates, hydrothermal treat-
ment of the silica precursor and requirement of slow tem-
plate removal for the preparation of mesoporous silica make
the synthesis process quite cumbersome. The synthesis of
mesoporous silica using such a tedious process makes the
final product quite costly. Hence, a simpler and cost effective
synthesis process for mesoporous silica is highly desired for
wider applications in catalysis and other applications.
Conventionally a tetraethyl orthosilicate [Si(OC2H5O)4]
monomer is used as a silica precursor which on hydrolysis
Mesoporous silica materials with good hydrothermal
stability such as SBA-15 with thicker pores have been
* Corresponding author.
E-mail address: dongare.mohan@gmail.com (M.K. Dongare).
http://dx.doi.org/10.1016/j.crci.2016.02.001
1631-0748/© 2016 Published by Elsevier Masson SAS on behalf of Académie des sciences.

1248
P.M. More et al. / C. R. Chimie 19 (2016) 1247e1253
forms a condensed silica product around the structure-
directing organic template during synthesis of meso-
porous silica materials. Ethyl silicate-40 a polymeric silica
[Si(OC2H5O)4]n (n ¼ 4e5) is used as an industrial bulk
chemical in the metallurgical industry as a binder in pre-
cision casting and other applications [4]. Ethyl silicate-40
being a polymeric silica precursor with moisture sensitive
terminal ethoxy groups has been used as a silica precursor
in our group for the synthesis of mesoporous silica based
catalysts by the simple hydrolysis process without the use
of any template and hydrothermal treatment which
showed a very high surface area as well as very high acidity.
The MoO3/SiO2 catalysts prepared using ES-40 as a silica
precursor by the solegel method have shown very high
activity for typically acid catalyzed transformations like
vapor/liquid phase nitration [5], transesterification [6],
Beckmann rearrangement [7], condensation-oxidation [8]
as well as protectionedeprotection of acetals [9], etc.
Esterification of carboxylic acids is one of the important
reactions in the preparation of key organic value added
intermediates which are widely used as organic solvents,
internal plasticizers, perfumeries, paints, foods and phar-
maceuticals [10]. Conventional esterification reactions are
catalyzed by sulfuric acid/p-toluene sulphonic acid, which
poses well-known environmental problems [11] due to
liberated hazardous and toxic waste products. To overcome
these problems and to satisfy the growing stringent global
environmental regulations, the use of solid acid catalysts is
highly desired [12]. Various solid acid catalysts have been
investigated to replace corrosive mineral acids for esterifi-
cation of carboxylic acids. Zeolite H-beta [13], ZSM-5 [14],
transition metal ion exchanged clays, Nafion functionalized
mesoporous MCM-41 silica [15], silicotungstic acid/zirconia
immobilized on SBA-15 [16], silica supported sulphonic
acids [17], iodine [18], HY-zeolite [19], and heteropolyacids
[20] have been investigated for esterification of carboxylic
acids with limited success. In the present paper, the MoO3/
SiO2 catalysts prepared by the solegel method have been
used as solid acid catalysts for esterification of mono- and
di-carboxylic acids.
Pure high surface area silica was prepared by adding
52 g ethyl silicate-40 to 100 g dry isopropyl alcohol; to this
mixture 0.02 g ammonium hydroxide solution (25%) was
added slowly with constant stirring. The transparent white
gel thus obtained was air dried and calcined in a muffle
ꢀ
furnace at 500 C for 5 h.
MoO3/SiO2 catalysts with varying molybdenum oxide
molar concentrations (1, 5, 10, 15, and 20) were prepared. In
a typical procedure 20 mol% MoO3/SiO2 catalyst was syn-
thesized by dissolving 14.11 g AHM in 40 ml water at 80 ꢀC.
This hot solution was added dropwise to the dry isopropyl
alcohol solution of ethyl silicate-40 (48.0 g ethyl silicate-40
in 100 ml of isopropyl alcohol) with constant stirring. The
resultant transparent greenish gel was air dried and
ꢀ
calcined at 500 C in the air in a muffle furnace for 5 h.
2.3. Characterization
2.3.1. Powder X-ray diffraction studies
The powder X-ray diffraction data of the samples were
collected on a Rigaku Miniflex diffractometer equipped
with a Ni filtered Cu K
a
radiation (
l
¼ 1.5406 Ǻ, 30 kV,
15 mA). The data were collected in the 2
q
range of ꢁ110ꢀe80ꢀ
with a step size of 0.02ꢀ and a scan rate of 4ꢀ min
.
2.3.2. Nitrogen adsorption studies
The BET surface area of the calcined samples was
determined by N2 sorption at 77 K using NOVA 1200
(Quanta Chrome) equipment. Prior to N2 adsorption, the
materials were evacuated at 573 K under vacuum. The
specific surface area, SBET, was determined according to the
BET equation.
2.3.3. NH3-temperature programmed desorption
Temperature programmed desorption of ammonia
(NH3-TPD) was carried out using a Micromeritics Autocue
2910 apparatus. The catalyst was cleaned up at 773 K in a
He flow and then cooled to 373 K prior to NH3 adsorption.
Then, desorption experiment was carried out at a rate of
10 K minꢁ1 till 873 K.
2. Experimental section
2.3.4. FTIR of adsorbed pyridine
2.1. Materials
The nature of the surface acid sites was studied by FTIR
of adsorbed pyridine at room temperature. The FTIR spectra
of chemisorbed pyridine (py-IR) were obtained in a high-
temperature cell (Spectra-Tech) fitted with a Zn-Se win-
dow (Shimadzu 8000 FTIR spectrophotometer). The tem-
perature in the cell was varied from 303 to 673 K. The
sample (30 mg) was finely crushed and placed in a sample
holder. Before pyridine adsorption, the sample was out-
gassed for 2 h at 673 K under N2 flow to eliminate the
adsorbed moisture. The cell was cooled to room tempera-
ture and the spectra of the neat catalyst were recorded (100
scans and resolution 4 cmꢁ1) at different temperatures. The
sample was dosed with two successive pulses of pyridine
All the reagents viz., ammonium heptamolybdate
(AHM), ethyl silicate-40 (CAS registry no. 18945-71-7),
isopropyl alcohol, 25% ammonia solution, absolute ethanol,
and acetic acid were of AR grade (99.8%); oxalic acid, maleic
acid, methanol, 1-butanol, 2-butanol, tert-butanol, hexanol,
benzyl alcohol were of AR grade (99.8%) and were obtained
from S. D. Fine, LOBA and Merck chemicals, India. The
chemicals were used without further purification.
2.2. Catalyst preparation
The catalysts were synthesized using the simple solegel
technique. Ethyl silicate-40 was used as a silica precursor
and ammonium heptamolybdate as a MoO3 precursor for
the synthesis of mesoporous silica and MoO3/SiO2 catalysts.
The typical synthesis procedures are discussed below:
(10 ml each). The spectrum was recorded at room temper-
ature after an equilibration time of 30 min. The
temperature-programmed desorption of pyridine was
studied at 373, 473, 573 and 673 K after equilibration for
30 min after attaining the temperature. The spectrum of

P.M. More et al. / C. R. Chimie 19 (2016) 1247e1253
1249
nature of the material at lower Mo loading (Fig. 1b and c)
indicating the formation of solid solution or high dispersion
of amorphous MoO3 on the amorphous silica support.
Catalysts with 20% Mo loading (Fig. 1d) showed a highly
crystalline nature with intense peaks at 2
25.8 and 27.4ꢀ corresponding to
-MoO3 in the ortho-
rhombic phase. No -MoO3 phase was observed in the
structure because the samples were calcined at 500 ꢀC, a
temperature at which the -MoO3 phase is not stable [21].
q
¼ 12.9, 23.4,
a
b
b
It is interesting to note that even though the MoO3 is in the
crystalline form at higher Mo loading, the silica support still
retains its amorphous nature leading to the high surface
area of the catalysts [6].
The surface areas of all the catalysts determined using
the BET method are given in Table 1. A very high surface
area of 719 m2/g was obtained for pure silica prepared by
solegel synthesis using ethyl silicate-40 as the silica source.
A typical H4 type isotherm (Fig. 2a) for mesoporous ma-
terials was observed for silica with a broad pore size dis-
tribution curve (Fig. 2b) ranging from 10 to 200 Å with an
average pore size of 67.3 Å. Ethyl silicate-40 is a polymeric
form (trimeric and tetrameric) of tetraethyl orthosilicate
monomer, which on controlled hydrolysis yields the silica
of a very high surface area [22]. On controlled hydrolysis of
ethyl silicate-40, the trimeric and tetrameric species form
corresponding trimeric and tetrameric silica species. The
trimeric and tetrameric silica species restrict the growth of
large particles. The surface area was found to decrease with
increase in MoO3 loading. During the solegel synthesis, an
aqueous solution of ammonium heptamolybdate is added
to ethyl silicate-40, which hydrolyzes ethyl silicate-40.
However, the control of the rate of hydrolysis is difficult,
because of the use of excess water for dissolving ammo-
nium heptamolybdate, which leads to a decrease in the
surface area. It is expected that, as MoO3 loading is
increased, the crystalline molybdenum oxide clusters are
formed that cover the amorphous silica support, reducing
the total surface area of the catalyst. However, the catalysts
prepared by solegel techniques showed a very high surface
area as compared to the catalysts prepared by the
impregnation method. For 1% Mo loading, the catalyst
prepared by the solegel technique showed a surface area of
583 m2/g whereas the catalyst prepared by the impregna-
tion method shows a surface area of only 155 m2/g [23].
Fig. 1. XRD of a ¼ silica and MoO3/SiO2 with b ¼ 1, c ¼ 10, d ¼ 20% MoO3/
SiO2 loading.
the neat sample was subtracted from that of the pyridine
adsorbed sample.
2.4. Catalytic activity
A typical esterification of carboxylic acids with alcohols
was carried out in a 50 ml two-necked round bottom flask
fitted with a reflux condenser. The flask was charged with
mono- or di-carboxylic acid, alcohol, and catalyst. The re-
action mixture was refluxed for 6e8 h (up to maximum
conversion) at desired temperature. The reaction was
monitored by GC. Samples were withdrawn at regular time
intervals and analyzed using a HewlettePackard GC model
HP6890 with HP (5% cross-linked polyethylene siloxane)
column (diameter: 0.53 mm, film thickness: 1 mm, length:
60 m). After completion of the reaction, the reaction
mixture was cooled and filtered. The products were
confirmed by GCeMS (model GC Agilent 6890N with HP-5
and MS Agilent 5973 network MSD, fitted with a 30 m
capillary column and GC-IR model PerkineElmer Spectrum
2001, column DB and 25m-length 0.32 mm internal
diameter).
3. Results and discussion
3.1. XRD
The XRD patterns of 1, 10 and 20% MoO3/SiO2 catalysts
prepared by the solegel technique are shown in Fig. 1. For
comparison, the XRD pattern of pure silica is also included
in the figure (Fig. 1a). The patterns showed the amorphous
3.2. FTIR studies
The FTIR spectra of all the catalyst samples prepared by
the solegel technique are presented in Fig. 3 also confirmed
Table 1
Textural data and acidity of the catalysts.
Catalyst
Surface area,
m2/g
Pore volume,
cm3/g
Pore diameter, Å
NH3 desorbed,
mmol/g
Surface density of Mo
on support (Dm), nmꢁ2
SiO2
719
583
432
284
275
180
1.19
0.59
0.63
0.57
0.51
0.23
67.3
40.7
58.2
79.6
74.2
71.64
0.03
0.18
0.56
0.71
0.86
0.94
00
1% MoO3/SiO2
5% MoO3/SiO2
10% MoO3/SiO2
15% MoO3/SiO2
20% MoO3/SiO2
0.0716
0.483
1.472
2.280
7.891

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P.M. More et al. / C. R. Chimie 19 (2016) 1247e1253
Fig. 2. (a) Adsorption desorption isotherm and (b) pore size distribution of silica.
the formation of the
a
MoO3 at higher loading (main bands
NH3 desorbed is given in Table 1. The pure silica catalyst
at 993, 872, 570 cmꢁ1) whereas the spectra of the 1e5 mol%
loading (3 bee) showed mainly the bands of the silica with
those of oxo molybdenum species (908, 951 cmꢁ1) [24].
showed very poor acidity (0.0317 mmol/g NH3) With the
increase in MoO3 loading the acidity gradually increased
reaching maximum for 20% MoO3/SiO2 (NH3 desorbed:
0.937 mmol/g). Ma et al. [23] have prepared a series of MoO3/
SiO2 catalysts (1e16 wt%) by impregnation and the NH3, TPD
showed desorption at much lower temperature (<250 ꢀC)
indicating the presence of weak acidity. The amount of
ammonia desorbed for 1% Mo/SiO2 is reported to be only
0.056 mmol/g which increased to 0.102 mmol/g for 16%
MoO3/SiO2. This indicated the significantly low acidity of
catalysts prepared by impregnation compared to the cata-
lysts prepared by solegel (present work), which is in agree-
mentwiththeaforementioned increaseindispersion(Fig. 4).
FTIR of adsorbed pyridine as described before [25]
revealed the presence of Lewis acidity in all the catalysts.
However at higher MoO3 loading the presence of Brønsted
acidity was also evident. The generation of Brønsted acidity
could be due to the formation of polymolybdate Keggin
structures [20] which were evident in FTIR analysis (peaks
at 959 and 914 cmꢁ1). All the catalysts were calcined at
500 ꢀC, the temperature at which transformation of hex-
agonal form of molybdenum trioxide into its orthorhombic
form takes place. The oxide becomes hydrated and subse-
quently converted into polymolybdic trimers [26,27]. Such
structural units are expected to show Brønsted acidity.
With the increase in Mo loading the ratio of Brønsted to
Lewis (B/L) acidity increased (Fig. 5).
3.3. Acidity measurements
Ammonia-TPD experiments were carried out to deter-
mine the acid strength of the catalysts (Fig. 3). The amount of
Fig. 3. FTIR spectra of silica (a) and MoO3/SiO2 with 1% (b), 5% (c), 10% (d),
15% (e), 20% (f) MoO3 loading by the solegel technique.

P.M. More et al. / C. R. Chimie 19 (2016) 1247e1253
1251
3.4. Catalytic reaction
The esterification of mono- and di-carboxylic acid with
alcohol is shown in Scheme 1.
Esterification of acetic acid with methanol was carried
out using a series of MoO3/SiO2 with different loadings of
MoO3 on silica, and the results are given in Table 2. The
results clearly showed an increase in the catalytic activity
with an increase in MoO3 loading and the maximum cat-
alytic activity was observed when 20% MoO3/SiO2 was used
as the catalyst. As expected the role of the catalyst in this
reaction is clearly evident as the conversion without cata-
lyst was only 13.6%. Pure silica was also used as the catalyst,
and the conversion was very low (19.6%) comparable with
that of the reaction without the catalyst.
As 20% MoO3/SiO2 showed maximum catalytic activity,
esterification of acetic acid with various alcohols was car-
ried out using this catalyst. The results of esterification of
acetic acid with various alcohols are shown in Table 3. The
conversions varied from 14.7% with tert-butanol to 68%
with 2-butanol.
Fig. 4. NH3-TPD of (a) silica and MoO3/SiO2 with (b) 1% MoO3 (c) 5% MoO3
(d) 10% MoO3, (e) 15% MoO3 (f) 20% MoO3 loading.
As the esterification of mono-carboxylic acids was suc-
cessful, esterification of di-carboxylic acids was also
attempted. Initially oxalic acid was esterified with ethanol
using different loadings of MoO3 on silica as well as pure
silica as the catalyst (Table 4). The conversion without
catalyst was very low (10.6%) which increased to 36.3% in
the presence of pure silica. Acid conversion with 1 and 10%
Mo loadings was 66 and 69%, respectively, which increased
to 82.5% in the presence of 20% MoO3/SiO2. In all the cases
selectivity for diester was maximum. In the case of di-
carboxylic acid also, maximum conversion was obtained
with the maximum acidic catalyst. Hence, esterification of
di-carboxylic acids with various alcohols was carried out
using 20% MoO3/SiO2, and the results are presented in Table
5.
When esterification of maleic acid was carried out with
methanol a very high acid conversion of 91.6% with 96.6%
selectivity for diester was obtained whereas no reaction
was observed in the absence of catalyst. In the case of
esterification of oxalic acid maximum conversion (82.5%)
was obtained with ethanol with 78.3% selectivity for
diester. The oxalic acid conversion decreased in the series
ethanol > 1-butanol > hexanol > isopropyl alcohol with
100% selectivity for diester in the case of hexanol and more
than 90% selectivity in the case of 1-butanol and
Fig. 5. FTIR of adsorbed pyridine on (a) pure silica, (b) 1, (c) 5, (d) 10, (e) 15,
(f) 20 mol% MoO3/SiO2 at 100 ꢀC after subtracting FTIR of neat catalyst at
100 ꢀC.
O
O
R
OH
R' OH
R
OR'
+ H2O
+
O
O
O
OH
OH
OR'
OR'
OR'
R' OH
+
+
+ H2O
OH
O
O
O
Scheme 1. Esterification of mono- and di-carboxylic acids with alcohol.

1252
P.M. More et al. / C. R. Chimie 19 (2016) 1247e1253
Table 2
though with very low conversion compared to the catalytic
reaction. This low conversion may be due to the diffusional
constraints in the presence of a catalyst.
The results clearly showed that the 20% MoO3/SiO2
catalyst, which possesses maximum acidity among the
series, also exhibited the highest catalytic activity. The
catalytic activity observed in the above-mentioned re-
actions appeared to be correlated with the evolved acidity
of the MoO3/SiO2 catalysts.
Esterification of acetic acid with methanol using different loadings of
MoO3/SiO2.
Sr. No.
Catalyst
Acid
conversion, %
Methyl acetate
selectivity, %
1.
2.
3.
4.
5.
6.
7.
Blank
SiO2
13.6
19.6
20.8
24
35.5
48.7
53.7
100
100
100
100
100
100
100
1% MoO3/SiO2
5% MoO3/SiO2
10% MoO3/SiO2
15% MoO3/SiO2
20% MoO3/SiO2
Most of the esterification catalysts used in organic
synthesis are Brønsted acids such as H2SO4, CH3SO3H or
heterogeneous Brønsted solid acids. Hence, in the present
case, the Brønsted acidity present on the MoO3/SiO2 cata-
lyst is responsible for the high esterification activity. The
mechanism of esterification has been studied previously
[28]. In the present case, the catalyst with 20% Mo loading
showed maximum acidity in terms of the number of acid
sites (NH3 desorbed 0.94 mmol/g) as well as acid strength.
Esterification has been already reported using cation-
exchanged montmorillonite [29]. The catalytic activity is
correlated with the electronegativity of exchanged cations,
which in turn shows variable polarizability of water mol-
ecules coordinated to the cations thus exhibiting protonic
(Brønsted) acidity. Similarly, in the present case polariza-
tion of water molecules coordinated to Mo center can give
rise to protonic acidity essential for the catalysis of the
esterification reaction. The nature of catalytically active
species in the case of MoO3/SiO2 studied in detail shown in
situ formation of silicomolybdic acid associated with silica
fine particles to be responsible for the very high acidity of
the catalyst [25]. The characterization also revealed the
reduction of formed SMA/SiO2 in the presence of water/
alcohol rendering blue color to the catalyst. The in situ
formation of reduced SMA during the esterification reac-
tion as given in Scheme 2 was responsible for very high
acidity and in turn very high activity for esterification.
Reaction conditions: acetic acid, 5 g; methanol, 1:5 M equiv; catalyst;
0.5 g; reaction time, 8 h; reaction temperature, 60 ꢀC.
Table 3
Esterification of acetic acid with different alcohols.
Sr. No.
Acid
Alcohol
Temp., ꢀ
C
Conversion, %
1.
2.
3.
4.
5.
6.
Acetic acid
Acetic acid
Acetic acid
Acetic acid
Acetic acid
Acetic acid
Methanol
Ethanol
60
70
80
80
80
53.7
59.0
62.2
68
14.7
45.3
1-Butanol
2-Butanol
tert-Butanol
Benzyl alcohol
100
Reaction conditions: acetic acid, 5 g; alcohol, 1:5 M equiv; catalyst 20%
MoO3/SiO2, 0.5 g; time, 8 h.
Table 4
Study of different MoO3 loadings on SiO2 for esterification of oxalic acid
and ethanol.
Sr. No. Catalyst
Acid conversion Mono-ester, % Di-ester, %
1.
2.
3.
4.
5.
Blank
SiO2
1% MoO3/SiO2 66.3
10% MoO3/SiO2 69.2
20% MoO3/SiO2 82.5
10.6
36.3
0.0
0.0
0.0
0.0
21.7
100
100
100
100
78.3
Reaction conditions: oxalic acid, 5 g; ethanol, 1:5 M equiv; temperature,
75 ꢀC; time, 8 h.
4. Conclusion
isopropanol. The monitoring of the reaction of oxalic acid
with ethanol with time shows that the formation of diester
proceeds through mono-ester. Initially selectivity for
mono-ester was high and decreased with time whereas the
selectivity for diester was increased. Interestingly selec-
tivity for diester in the absence of catalyst was more (100%)
Mesoporous high surface area silica has been prepared
using ethyl silicate-40 as a silica source using a very simple
hydrolysis/solegel synthesis process without any use of the
organic template and hydrothermal treatment. Ethyl
silicate-40, being an industrial bulk chemical, provides a
simple and cheaper process for making thermally stable
Table 5
Esterification of di-carboxylic acids with different alcohols.
Sr. No.
Acid
Alcohol
Time, h
Acid conversion, %
Selectivity, %
Mono-ester
Di-ester
1
2
3.
Maleic acid
Oxalic acid
Oxalic acid
Methanol
Methanol
Ethanol
8
8
1
2
3
5
8
8
8
8
91.58
69.7
22
3.4
26.6
65.2
56.4
44
39
21.7
8.3
96.6
73.4
34.7
43.6
56
39
47.9
64.5
82.5
77.2
62.58
63.89
61
78.3
91.7
100
91.7
4.
5.
6
Oxalic acid
Oxalic acid
Oxalic acid
1-Butanol
Hexanol
Isopropyl alcohol
0.0
8.3
Reaction condition: acid, 5 g; alcohol, 1:5 M equiv; temperature, 75 ꢀC; catalyst, 20% MoO3/SiO2, 10 wt%.

P.M. More et al. / C. R. Chimie 19 (2016) 1247e1253
1253
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Acknowledgments
PM acknowledges UGC for fellowship. Financial support
from Council of Scientific and Industrial Research, New
Delhi for the XII FYP project CSC0125 is acknowledged.
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