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A. Kara, B. Erdem / Journal of Molecular Catalysis A: Chemical 349 (2011) 42–47
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Dowex which are the best known catalysts for the esterification
reactions.
percentages of H2SO4 (Samples II and III) beads were subjected to
elemental analysis with LECO CHNS-932 model elemental analyzer.
The presence of magnetite nano-powders in the beads was
investigated with an electron spin resonance (ESR) spectropho-
tometer (EL 9, Varian, USA).
2. Experimental
FTIR measurements were performed on a Thermo Nicolet 6700
series FTIR spectrometer in normal transmission mode with a KBr
detector over the range 4000–400 cm−1 at a resolution 8 cm−1 aver-
aged over 32 scans.
Thermal stabilities of the m-poly(DVB-4VP) and its sulfonic
acid functionalized forms were examined by TG analyses with SII-
EXSTAR TG/DTA 6200. The samples (∼5–10 mg) were heated from
room temperature to 800 ◦C under dried-air atmosphere at a scan-
ning rate of 10 ◦C/min.
The acid exchange capacities of the m-poly(DVB-4VP-SO3H)
prepared with 10% and 20% H2SO4 were measured by means of
titration, using sodium chloride as exchange agent. In a typical
experiment, 0.05 g of solid was added to 10 g of aqueous solution
of sodium chloride (2 M). The resulting suspension was allowed to
equilibrate and thereafter titrated potentiometrically by drop-wise
addition of 0.01 M NaOH (aq) [34].
2.1. Materials
Divinilbenzene (DVB) was obtained from Merck (Darmstadt,
Germany), and inhibitor was rendered by washing with NaOH solu-
tion (3%, w/w) prior to use. 4-Vinylpyridine (4VP) was obtained
from Fluka (Steinheim, Germany). 2,2ꢀ-Azobisisobutyronitrile
(AIBN) was obtained from Merck (Darmstadt, Germany). Poly(vinyl
alcohol) (PVAL; Mw: 72.000, 98% hydrolyzed) was supplied from
Merck (Darmstadt, Germany). Magnetite nanopowder (Fe3O4;
diameter 20–30 nm) was obtained from Aldrich (Steinheim,
Germany). All other reagents were of analytical grade and were
used without further purification.
2.2. Preparation of m-poly(DVB-4VP-SO3H) microbeads
DVB and 4VP were copolymerized in suspension by using AIBN
and poly(vinyl alcohol) as the initiator and the stabilizer, respec-
tively. Toluene was included in the polymerization recipe as the
diluent (as a pore former). A typical preparation procedure was
exemplified below: Continuous medium was prepared by dissolv-
ing poly(vinyl alcohol) (200 mg) in the purified water (50 mL). For
the preparation of dispersion phase, DVB (2.9 mL; 20 mmol) mag-
netite Fe3O4 nanopowder (0.5 g) and toluene (10 mL) were stirred
for 10 min at room temperature. Then, 4VP (8.6 mL; 80 mmol) and
AIBN (100 mg) were dissolved in the homogeneous organic phase.
The organic phase was dispersed in the aqueous medium by stir-
ring the mixture magnetically (500 rpm), in a sealed-cylindrical
pyrex polymerization reactor. The reactor content was heated to
polymerization temperature (i.e., 65 ◦C) within 4 h and the poly-
merization was conducted for 2 h with a 600 rpm stirring rate at
80 ◦C. The final microbeads were extensively washed with ethanol
and water to remove any unreacted monomer or diluent and
then dried at 50 ◦C in a vacuum oven. The microbeads then were
sieved to different sizes. An inspection with a microscope showed
that almost all the microbeads were perfectly spherical. Table 1
shows recipe and polymerization conditions for preparation of the
mesoporous m-poly(DVB-4VP) microbeads. The m-poly(DVB-4VP-
SO3H) catalysts were prepared by mixing of different percentages
of H2SO4 solution (10% and 20%, respectively) with m-poly(DVB-
4VP) polymer at 298 K in a sealed cylindrical pyrex reactor for 2 h.
The solid was filtered and vacuum dried at 343 K overnight and
stored in the glove box for characterizations.
2.4. Catalytic conditions
The esterification of propionic acid with methanol was carried
out in a glass flask placed in the shaking water bath the tempera-
ture of which was controlled within 0.1 ◦C. Stoichiometric ratio of
propionic acid to methanol was (1:1) in the experiments performed
at 333 K. 1,4-Dioxane was used as solvent in the all experiments.
In a typical run, catalyst (about 0.5 g), methanol and dioxane of
known amount were charged in to the reactor and preheated to
the reaction temperature and the esterification was commenced
by injecting preheated propionic acid in to the mixture. This was
considered as the zero time for a run. The total liquid volume was
100 cm3. Samples were withdrawn and the amount of unreacted
acid was analysed by titration with 0.1 M sodium hydroxide.
2.5. Effect of temperature
The effect of temperature is very important for a hetero-
geneously catalysed reaction as this information is useful in
calculating the apparent activation energy. Moreover, the intrinsic
rate constants are strong functions of temperatures [35]. To calcu-
late the apparent activation energy the reaction temperature was
changed from 318 K to 348 K by keeping the same experimental
conditions such as (1:1) mole ratio, 0.5 g catalyst loading, and by
using 1,4-dioxane as solvent.
2.6. Reuse of catalyst
2.3. Characterization of m-poly(DVB-4VP) and its sulfate
microbeads
At the end of the reaction, catalyst was separated from the reac-
tion mixture in the presence of external magnetic fields and was
thoroughly washed with deionised water several times. After the
reaction the catalyst was activated in the potentially containing 20%
H2SO4 solution for 2 h and dried under vacuum at 333 K and then
used as catalyst for recycling experiments. This whole process has
been continued four times.
The porosity of the microbeads was measured by N2 gas adsorp-
tion/desorption isotherm technique (Quantachrome Corporation,
Poremaster 60, USA). The specific surface area of the beads in a
dry state was determined by a multipoint Brunauer–Emmett–Teller
(BET) and pore volumes and average pore diameter for the beads
were determined by the BJH (Barrett, Joyner, Halenda) model. In
addition, the average size and size distribution of the beads were
determined by screen analysis performed using standard sieves
(Model AS200, Retsch Gmb & Co., KG, Haan, Germany).
3. Results and discussion
The surface structures of the beads were visualized and exam-
ined by scanning electron microscopy (SEM, CARL ZEISS EVO 40,
UK).
In order to evaluate the degrees of (4VP) incorporation and to
test whether sulfur enters to the polymeric structure, m-poly(DVB-
4VP) (Sample I), and m-poly(DVB-4VP-SO3H) having 10 and 20
The suspension polymerization procedure provided cross-
linked mesoporous m-poly(DVB-4VP) microbeads in the spherical
form of 106–212 m in diameter. The N2 adsorption/desorption
isotherm for the m-poly(DVB-4VP) and the calculated pore size
distributions are plotted in Fig. 1. The BET surface area (SBET),
pore volume (VP) and pore size are given in Table 2. The sample