Q. Li et al. / Applied Catalysis A: General 484 (2014) 148–153
149
such as solution pH, initial reactant concentration, temperature,
and salt concentration, were all investigated. The catalytic reusabil-
ity of the resin was also evaluated through several continuous batch
experiments.
2
. Experimental
2.1. Materials
All chemicals used in this study were guaranteed grade unless
otherwise specified. The cross-linked PVP resin, which was 25%
cross-linked with divinylbenzene, was purchased from Sigma-
Aldrich U.S. Its particle size distribution ranged mainly from 30
to 60 mesh, and its water content was 60%. Prior to use, the resin
was rinsed with methanol in a glass column at 323 K and dried
under vacuum at 333 K for 8 h. Maleic acid, HCl, NaCl, Na SO ,
2
4
and other chemicals were supplied in analytical grade by Nanjing
Chemical Reagent Company (Jiangsu Province, China). All reagents
were used without further purification. Aqueous solutions were
prepared using distilled deionized water.
Fig. 1. Effect of pH on catalytic activity for isomerization reaction catalyzed by PVP
resin.
2.2. Reaction systems
Catalyzed isomerization of maleic acid to fumaric acid was car-
3. Results and discussion
ried out at atmospheric pressure using PVP resin as catalyst. The
50 mL flask was shaken at 150 rpm in a water-bathing constant
2
3.1. Effect of pH
temperature vibrator (THZ-82, Shanghai Jiangxing Instrument Co.,
Ltd., Shanghai, China). In a typical experiment, 100 mL of 200 mg/L
maleic acid solution and 0.1 g PVP resin were loaded to the reactor
at 333 K. Samples for HPLC analysis were taken at selected reaction
times. The reaction system lasted for 160 h, and was used to study
the effect of pH. The pH value was adjusted by con. HCl (12 M) and
NaOH (1 M) solution. The initial maleic acid concentration was var-
ied in order to obtain data for its effect on the reaction. The effect
of reaction temperature (323, 333, 343, and 353 K) and co-existing
salts were also investigated. Finally, the used catalyst was sepa-
rated through filtration, and regenerated by 0.1 M NaOH solution
after six consecutive uses.
To investigate the effect of pH on the reaction system, the exper-
iments were carried out at pH 1 to 11.3 (Fig. 1). At pH 1.5, fumaric
acid (%) reached the highest conversion (72%), indicating that 1.5
was the optimum pH for the overall reaction. Fumaric acid yield (%)
decreased when pH was less than 1.5 and more than 1.5. Fumaric
acid yield (%) decreased to 2% at pH 5 and 30% at pH 1. When
pH was higher than 7.0, almost no fumaric acid formed. The phe-
nomenon was related to the ionization equilibrium constant of
maleic acid. Given that pKa 1 and pKa 2 of maleic acid were 1.92
and 6.23, respectively [39], pH decrease from 7.0 to 1.92 was propi-
tious to the ionization of maleic acid. When pH was lower than 1.92,
the ionization equilibrium was enhanced, forming more maleate
and contributing to the increase in fumaric acid yield (%). When
pH decreased further from 1.5, increased formation of maleate
resulted in electron pair repulsion, causing decrease of fumaric acid
yield (%). The fumaric acid could not form when the pH was under
alkaline solution, similar to Hayon and Simic’s report. They sug-
gested that maleate anions were formed by an electron transfer
process at acidic pH, which was not observed at higher pH [40].
These findings indicated that fumaric acid yield (%) was closely
related to pH, and was probably decided by the reaction mechanism
The conversion of the reaction was calculated according to the
following equations:
Cm0 − Cmt
maleic acid conversion(%) =
× 100
(1)
(2)
C
m0
Cft
fumaric acid yield(%) =
Cm0
× 100
where Cm0 is the initial concentration of maleic acid and Cmt and
Cft (mg/L) are the concentrations of maleic acid and fumaric acid at
time t in the corresponding solutions, respectively.
(
Scheme 1).
All experiments were duplicated and the average standard devi-
ation of the duplicated experiments was less than ± 10.0% for
the individual composition of the reaction system. The average
standard deviation for conversion was lower than 6%.
According to the study of Karaman and Chatterjee et al. [36,37],
the possible mechanism of isomerization of maleic acid to fumaric
acid catalyzed by PVP resin involved mainly four steps: (1) a proton
transferred from the maleic acid into the nitrogen of PVP resin to
form ion pair S1; in this process, PVP will first break the intermolec-
ular hydrogen bonding of maleic acid. (2) PVP cation approached
the C C double bond of the maleic acid moiety to yield S2; (3) rota-
tion about the central C C single bond of S2 followed by proton
abstraction by PVP molecule to yield unstable S3; and (4) proton
transferred from the PVP moiety and formed into the carboxylate
moiety of the fumarate, leading to the formation of fumaric acid. In
step 1, different pH directly affected the ionization degree of maleic
acid, resulting in different formation numbers of ion pair S1. In the
overall reaction, PVP molecules exhibited an intensely nucleophilic
attack on the C2 of maleate. The C2 C3 double bond was converted
to C2 C3 single bond, which allowed free rotation.
2.3. Sample analysis
All samples were analyzed through high-performance liq-
uid chromatography (HPLC, Agilent 1200, Germany) with
DAD detector and reverse-phase Krornasil-C18 column
a
a
(
150 mm × 4.6 mm × 5 m). The mobile phase consisted of 3%
methanol and 97% phosphoric acid aqueous solution (pH = 2.53)
with flow rate of 1 mL/min and sample size of 20 L. The DAD
detection wavelength was 210 nm and the column temperature
was set at 303 K. All samples were filtered through 0.45 m
membrane filters before detection.