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M.N. Alaya, M.A. Rabah / Journal of Alloys and Compounds 575 (2013) 285–291
the prepared catalyst. The reaction mixture is kept at 110 °C with stirring speed
of 600 rpm for 4 h, the reaction mixture then immediately filtered and quenched
to stop the reaction. Liquid samples of 0.5 mL volume are withdrawn and the
amount of unreacted acid is determined by the titration with 0.1 N NaOH. The ef-
fects of reaction time, reaction temperature, weight of the catalyst, the initial molar
ratio between the acid and alcohol (PA:B), and calcination temperature are all
studied.
on the amorphous forms of metal oxides. The loading of oxides
with sulfate and tungstate ions causes an increase in the surface
area and surface acidity due to the retardation of oxides crystal
forming [5]. Their textural properties, surface acidity and catalytic
activity are dependent largely on the percentage of loading, prep-
aration methods and thermal treatment [3,7,14,17,37,41–45].
Concerning esterification reaction, it is one of the fundamental
acid-catalyzed reactions. The esters are good solvents and play a
major role in the production of flavors, fragrances, plasticizers,
plastics, medicinal and surface-active agents [20]. n-butyl propio-
nate ester (n-BPE) possesses a high boiling point temperature
(146.7 °C), a high electrical resistance and a good non-VOC solvent.
Many research offers have been given to prepare n-BPE by using
solid acid catalysts utilizing cation exchange resins [18–20], PWP
supported on alumina [2], 12-tungstosilicic acid supported on zir-
conia [21], and fibrous polymer-supported sulphonic acid [22].
Nevertheless, papers concerning the esterification of propionic acid
3. Results and discussion
3.1. Specific surface area
The specific surface areas (SBET) of the catalysts are determined
from the nitrogen adsorption at À196 °C. The SBET values are pre-
sented in Table 1 and reveal the following points: (i) The addition
of sulfate ions to the SnO2 hydrogel causes a pronounced increase
in the surface area. The main reason for the increase in surface area
is the retardation of crystallization by sulfate treatment. (ii) The
SBET of ISS catalyst is decreased with the increase in the calcination
temperature. (iii) The loading of ISS catalyst with WO3 causes a
gradual decrease in SBET for the 200 and 400 °C products, whereas,
the SBET is increased gradually with WO3 loading for 650 °C prod-
ucts. The maximum SBET is observed at 400 °C for WO3-loaded cat-
alysts. (iv) The surface areas at 650 °C are higher than that of pure
SnO2 which means that the addition of both sulfate ions and WO3
hinder the crystallization and sintering of SnO2.
with n-butanol over sulfated SnO2 or WO3/SO2À/SnO2 catalysts are
4
rare [23,24].
We have previously prepared and characterized a series of cat-
alysts based on non-aged 15 wt.% SO24À/SnO2 support loaded with
5–45 wt.% WO3 and exploited to optimize a model catalytic system
for esterification of propionic acid with n-butanol [23]. In the pres-
ent investigation, however, we have prepared aged 15 wt.% SO42À
/
SnO2 hydrogel support, dried, then loaded with 15 and
35 wt.%WO3. The prepared catalysts are calcined at 400 and
650 °C. The effect of WO3 loading and calcination temperature on
the surface area, surface acidity and catalytic activity toward pro-
pionic acid esterification with n-butanol are studied.
3.2. Surface acidity measurements
The surface acidity measurements of the prepared catalysts by
means of potentiometric titration with n-butylamine in acetoni-
trile [48–50] are used to estimate the amount of acid sites and their
relative acid strengths according to the value of the initial elec-
trode potential (Ei). n-butylamine is a strong base and can be ad-
sorbed on acid sites of different strengths and types, thus it
titrates both Lewis and Brønsted sites [26]. On the other hand,
the acid strength of these sites can be classified according to the
following scale [2]: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV
(strong sites), À100 < Ei < 0 mV (weak sites) and Ei < À100 mV
(very weak sites). Fig. 1 shows the titration curves obtained from
the calcined catalysts. The computed amount of the acid sites
(mmol/g) and the number of the acid sites per m2 (N/m2) as well
as the values of Ei are listed in Table 1. For comparison, the surface
acidities of SnO2-400 and SnO2-650 catalysts are presented in the
table.
The results of the acidity measurements reveal the following
points: (i) The investigated catalysts possess very strong acid sites,
with Ei values that are in the range of 135–396 mV. (ii) Mixing of
SnO2 hydrogel with 15 wt.% sulfate ions enhances the acid strength
as well as the amount of surface acidity of the catalyst due to the
formation of new strong acid sites. (iii) Calcination of the catalysts
at 400 °C causes a pronounced increase in the surface acidity,
whereas, the raising of the calcination temperature to 650 °C is
accompanied with a sharp decrease in the strength and amount
of acid sites which may be due to the loss of surface OH groups
as well as the decomposition of sulfate ions. (iv) Loading of ISS cat-
alyst with WO3 causes a slight decrease in the surface acidity for
400 °C products, whereas, no change in the surface acidity is ob-
served for 650 °C products. The number of acid sites per m2
(DSA) is decreased slightly due to the gradual increases of the sur-
face area with an increase in the WO3 loading. (v) The increase of
WO3 loading causes a slight increase in the acid strength, total
amount and the number of acid sites for 400 °C products.
2. Experimental
2.1. Catalysts
Pure tin oxide hydrogel is prepared by a dropwise addition of ammonia solution
(10 wt.%) to 0.5 M solution of SnCl4Á5H2O (Riedel–deHaen) up to the final pH of 8
with continuous stirring for further 4 h. The gel is left overnight then washed by
decantation in sequence with solution of 1% ammonium acetate (Merck) [46,47],
followed by bidistilled water. Appropriate amount of 1 M H2SO4 solution is added
with vigorous stirring for 4 h to the hydrogel making the final sulfate ion percentage
up to 15 wt.%. The obtained gel is aged for 24 h at room temperature, then filtered
and finally dried at 120 °C. The resulting product is designated hereafter as ISS.
Appropriate amounts of ammonium paratungstate (Prolabo) solution are added
to known amounts of ISS catalyst, with stirring for 4 h, to make the percentage load-
ing up to 15 and 35 wt.% WO3, and then dried at 120 °C. The obtained materials are
calcined in air at 400 and 650 °C for 4 h. The catalysts are designated hereafter as 15
IWSS and 35 IWSS, respectively, followed by a number indicating the calcination
temperature.
2.2. Techniques
The specific surface area (SBET) is determined by the analysis of data of nitrogen
adsorption at À196 °C, using Gemini III 2375 Surface Area Analyzer. Prior to any
adsorption measurement, the sample is degassed at 200 °C for 6 h under a reduced
pressure of 10À4 Torr.
The surface acidity (strength and amount) of the prepared catalysts is measured
by means of potentiometric titration method [48,49]. That is performed by sus-
pended 0.2 g of the dry solid in 20 mL acetonitrile (Lab-Scan), and agitated for
3 h. Then, the suspension is titrated with 0.02 N or 0.1 N n-butylamine (Merck) in
acetonitrile at 0.05 mL/min. The electrode potential variation is measured with Ino-
lab Digital pH-mV model using a double junction electrode. The initial electrode po-
tential (Ei, mV) measures the strongest acid strength. The nature of acid sites
presented on the surface of the catalyst is determined from FT-IR transmission
spectra of adsorbed pyridine (Scharlau) within the range of 1200–1700 cmÀ1 (at a
resolution of 4 cmÀ1) using Jasco FTIR-460 spectrophotometer. Prior to the pyridine
adsorption [37,49], the samples are degassed at 200 °C for 3 h under high vacuum
followed by suspending in a dried pyridine. The excess pyridine is removed by
evaporation at 70 °C. A mixture of 0.005 g sample with 0.1 g of KBr is pressed into
13 mm disks.
The Brønsted and Lewis acidic sites of the catalyst can be char-
acterized using pyridine as a probe molecule. The distribution of
both types of acidic sites on the catalysts is confirmed by FT-IR
techniques. The FT-IR spectra of the chemisorbed pyridine on the
The catalytic activity of the prepared catalysts is tested for the esterification of
propionic acid (Merck) with n-butanol (SRL). The esterification reaction is carried
out in 100 mL flat-bottomed flask, equipped with a reflux condenser, containing a
stirring mixture of propionic acid (0.05 mol), n-butanol (0.10 mol) and 0.2 g of