Heterogeneous catalysed esterification of propionic acid with n-amyl alcohol over a microporous cation-exchange resin dowex 50wx4

Article abstract of DOI:10.1524/zpch.2010.5498

Kinetics of heterogeneous catalysed esterification of propionic acid with n-amyl alcohol was studied with a microporous cation-exchange resin catalyst, Dowex 50Wx4, in a stirred batch reactor to synthesise amyl propionate. Effects of various parameters such as speed of agitation, catalyst loading, and reaction temperature on reaction rate were investigated. The equilibrium conversion of propionic acid increased with in catalyst loading and reaction temperature. Stirrer speed had virtually no effect on the reaction rate under the experimental conditions. The apparent activation energy was found to be 43.167 kJmolK1 for the formation of amyl propionate and the equilibrium constant, which is independent of temperature ranging from 318 to 348 K, was found to be 4.05. It was also observed that the initial reaction rate decreased with water concentrations and increased with that of acid and increased with that of alcohol linearly. The reaction was found to occur between an adsorbed acid molecule and a molecule of alcohol in the bulk and it was concluded that the reaction mechanism can be represented by Eley-Rideal model. by Oldenbourg Wissenschaftsverlag, Muenchen.

Full text of DOI:10.1524/zpch.2010.5498

Z. Phys. Chem. 224 (2010) 781–793 . DOI 10.1524.zpch.2010.5498
by Oldenbourg Wissenschaftsverlag, München
©
Heterogeneous Catalysed Esterification of
Propionic Acid with n-Amyl Alcohol over a
Microporous Cation-exchange Resin Dowex
5
0Wx4
By Beyhan Erdem1,
and Alime İzci2
*
1
Department of Chemistry, Faculty of Science and Arts, Uludag University, 16059 Bursa,
Turkey
2
Department of Chemical Engineering, Eskişehir Osmangazi University, 26480 Eskişehir,
Turkey
(
Received June 11, 2009; accepted September 24, 2009)
Propionic Acid . Heterogeneous Catalysis . n-Amyl Alcohol, Dowex 50Wx4
Kinetics of heterogeneous catalysed esterification of propionic acid with n-amyl alcohol was
studied with a microporous cation-exchange resin catalyst, Dowex 50Wx4, in a stirred batch
reactor to synthesise amyl propionate. Effects of various parameters such as speed of agitation,
catalyst loading, and reaction temperature on reaction rate were investigated. The equilibrium
conversion of propionic acid increased with in catalyst loading and reaction temperature. Stirrer
speed had virtually no effect on the reaction rate under the experimental conditions. The appar-
K1
ent activation energy was found to be 43.167 kJmol for the formation of amyl propionate
and the equilibrium constant, which is independent of temperature ranging from 318 to 348 K,
was found to be 4.05. It was also observed that the initial reaction rate decreased with water
concentrations and increased with that of acid and increased with that of alcohol linearly. The
reaction was found to occur between an adsorbed acid molecule and a molecule of alcohol in
the bulk and it was concluded that the reaction mechanism can be represented by Eley-Rideal
model.
1. Introduction
Esterification of carboxylic acids with alcohols represents a well-known category
of liquid-phase reactions of considerable industrial interest due to the enormous
practical importance of organic ester products. These ester products include envi-
ronmentally friendly solvents, flavours, pharmaceuticals, plasticizers, polymeri-
*
Corresponding author. E-mail: gbeyhan@uludag.edu.tr
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82
B. Erdem and A. İzci
zation monomers and emulsifiers in the food, cosmetic and chemical industries
1]. Conversions for the esterification reactions have long been known to be
[
limited by a slow reaction rate and the existence of reversible reactions. To
accelerate the reaction rate, catalysts are always employed in a liquid-phase ester-
ification. Despite the strong catalytic effect, the use of homogeneous catalysts,
such as sulphuric acid and p-toluene sulfonic acid suffers from several draw-
backs, such as the existence of side reactions, corrosion of the equipment and
the need to deal with acidic wastes [2–5]. Homogeneous catalysts may also result
in sulphur contamination of final ester product [5–6].
Under this situation the use of solid acid catalysts has received great attention
in the past years. While there exist many solid acid catalysts, ion-exchange resins
are the most commonly used and have been proved effective in liquid-phase
esterification [7]. The use of ion-exchange resins as catalysts holds distinct ad-
vantages: (a) the purity of the products is higher, as the side reactions can be
completely eliminated or are significantly less; (b) the catalyst can be easily
removed from the reaction mixture by filtration; (c) the corrosive environment
caused by the discharge of acid-containing waste is eliminated [4, 8]. Research
in catalysis by ion-exchange resins undoubtedly is interesting, not only from a
purely physicochemical point of view but also in terms of the advantages of
these types of catalyst over the conventional ones [9].
Ion-exchange resins have been used in esterification [10] as well as the hy-
drolysis of methyl acetate in a catalytic distillation column [11]. Furthermore,
they have been also used for the esterification of acetic acid with methanol
[12, 13], liquid-phase esterification of lactic acid with iso-propyl alcohol [14],
esterification of acetic acid with n-amyl alcohol [15], and esterification of pro-
pionic acid with methanol [16].
In the present study, the kinetic behaviour of the heterogeneously catalysed
esterification of propionic acid with n-amyl alcohol was studied in stirred batch
reactor using Dowex 50Wx4 (an acidic microporous-gel type cation-exchange
resin) as a catalyst. In the previous work [17], basic investigation of intrinsic
reaction kinetics in the presence of a heterogeneous catalyst was very limited for
the present system since the reaction was regarded as homogeneous reaction.
Because of their selective adsorption of reactants and swelling nature, these res-
ins not only catalyse the esterification reaction but also affect the equilibrium
conversion [2, 7]. In the literature, it was stated that the selective swelling of the
polymer matrix may lead to values of the reactant to catalyst ratio different
from those typically achieved in homogeneous catalytic systems under similar
conditions. Moreover, the relative compositions of both reactants and products
in the reaction locus (i.e. the polymeric resin) and in the liquid bulk are, in
general, quite different. In particular, if these exhibit different affinities toward
the resin, such that one of the reaction products tends to be selectively removed
from the reaction locus, then the rate of the inverse reaction is minimised, and
it is possible to achieve higher equilibrium conversions than those obtained with
homogeneous catalysts [10]. As a result, this work was undertaken to find the
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Heterogeneous Catalysed Esterification Reaction …
783
kinetic model representing the esterification of propionic acid with n-amyl alco-
hol in the presence of heterogeneous catalyst, Dowex 50Wx4.
2
2
. Experimental
. 1. Chemicals and catalyst
n-Amyl alcohol (purity > 99.0 wt. %), propionic acid (purity > 99.0 wt. %) and
,4-dioxane (purity > 99. wt. %) were obtained from Merck. Amyl propionate
purity > 99.0 wt. %) was obtained from Aldrich. They were used without further
1
(
purification. The microporous sulfonic acid ion-exchange resin Dowex 50Wx4
purchased from Sigma-Aldrich was chosen as the catalyst in this work. It is
cross-linked three-dimensional structure of polymeric material obtained by sulfo-
nation of a copolymer of polystyrene and divinyl benzene [18]. Styrene copoly-
merized with a small amount of divinyl benzene at a low monomers dilution
results in gel-type copolymers, which is characterized by a swollen state porosity.
The pore size only depends on the proportion of divinyl benzene because of the
restricted swelling of polymer structure by cross-linking. Therefore, the gel-type
resins are completely ineffective as ion exchangers or catalysts in non-polar
media. If the gel beads are dry, the polymer matrix collapses and the polymer
chains will be as close as atomic forces allow [19, 20]. The acid catalytic activity
of sulfonated polystyrene ion-exchange resins shows a pronounced dependence
on the degree to which the polymer is sulfonated. This is, in part, due to the
increased concentration of acid sites, but a more significant factor is that the acid
strength also increases with the level of sulfonation. When the resins are used as
anhydrous catalysts (as far as they can be), this increased acid strength is a direct
result of structural features such as the degree of di-substitution, the presence of
sulfone bridges and interactions between neighbouring groups [21]. For this rea-
son, the catalyst washed with distilled water was dried in a vacuum oven (under
53.3 kPa) at 348 K, since drying at higher temperature involves the risk of
loosing sulfonic acid sites in the form of SO because of desulfonization of the
3
polystyrene matrix of the catalyst [22]. The dried catalyst was stored in a desicca-
tor until further use. The properties of the catalyst used for the present work are
listed in Table 1.
2.2. Apparatus and procedure
Esterification of propionic acid with n-amyl alcohol over a fibrous polymer-
supported sulfonic acid catalyst, Dowex 50Wx4, was carried out in an isother-
mally operating glass reactor. The reactor was equipped with a heating jacket
and a reflux condenser, which was placed on top of the reactor to prevent the
escape of volatile components. The temperature was controlled within ± 0.1 K
by circulating water from a thermostat in to a cylindrical water-jacket of the
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84
B. Erdem and A. İzci
Table 1. Some physical and chemical properties of Dowex 50Wx4 catalyst.
Matrix active group
Matrix
sulfonic acid functional group
styrene-divinylbenzene (gel- microporous)
Particle size
20–50 mesh
Mean particle size (μm)
Thermal stability (K)
Cross-linkage
570
423
4%
reactor. In a typical run, dioxane and propionic acid were placed in the reactor.
A known weight amount of the catalyst was added and the reactor content was
well mixed. After a steady value of the desired temperature was attained, n-amyl
alcohol, at the same temperature, was added and this was taken as zero time for
a run. One millilitre of liquid sample was withdrawn from the reactor at regular
intervals and the amount of unreacted acid was analysed by titration with 0.1 M
3
NaOH. The liquid volume in all of the experiments was 0.100 dm . A separate
set of experimental runs under different conditions was performed to validate
the model and its parameters.
The effect of water on reaction is important in esterification because H O is
2
continuously produced as a product. Certainly, the presence of water promotes
the reverse reaction; however, the effect of water on catalyst activity is even
more critical [23]. Hence, kinetic measurements focused particularly on the ini-
tial period of reaction (<10 %) to minimize the impact of reverse hydrolysis.
The values for initial reaction rates were calculated as K(r ) = C (dX .dt),
A0
A,0
A
where CA,0 is the initial concentration of the carboxylic acid and X is the con-
A
version of the carboxylic acid. In addition, autocatalysis was negligible compared
with Dowex 50Wx4 catalysis under the experimental conditions used; thus it did
not have to be accounted for in the rate determinations.
3
3
. Results and discussion
. 1. Effects of speed of agitation and particle size
Sharma et al. observed that the internal mass-transfer resistance could be ne-
glected and the conversion could be enhanced when a smaller particle size of
the resin was used [23]. Yadav and Kulkarni stated that for average particle size
less than 700 μm there was no effect of particle size on the conversion of lactic
acid [14]. The influence of external or pore diffusion can be neglected as reported
in previous studies [8] To ensure the insignificance of the external mass-transfer
resistance in kinetics measurements, experiments with different spinning speeds
were carried out. From these literature assumptions, preliminary experiments
were conducted varying the stirring speed, to quantify the influence of external
resistances to heat and mass transfer. These experiments showed that there was
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785
Fig. 1. Effect of stirring speed on conversion of propionic acid at 333 K (feed mole ratio: 1:1;
catalyst: Dowex 50Wx4).
very little effect of speed of agitation in the range 300–700 rpm on the overall
rate of the reaction (see Fig.1). A high degree of agitation was maintained in
order to eliminate the liquid film resistance. It was found that stirrer speed of
about 500 rpm was enough to keep all the catalyst particles in suspension and
that the stirrer speed did not affect the rate of reaction until 700 rpm. Hence, in
the range of stirrer speeds employed (300–700 rpm) the reaction rate was not
affected by the liquid film resistance.
3.2 Effect of catalyst loading
In the absence of mass transfer resistance, the rate of reaction is directly propor-
tional to catalyst loading based on the entire liquid phase volume. It was also
stated that the reaction rate is linearly depended on resin concentration [8]. Cata-
lyst loading was varied from 0.7 % to 2.0 % (w.w) (i.e. weight of catalyst .
weight of propionic acid) at a temperature of 333 K, feed mole ratio (alcohol to
acid) of 1:1, and stirrer speed of 700 rpm. It is clear from Fig. 2 that the conver-
sion of propionic acid increases with an increase in catalyst concentration. With
an increase in the number of resin functional groups, the concentration of the
carbonium ion formed increased. With the utilisation of cationic resins a rela-
tively high yield of ester was assumed [24]. The higher the catalyst loading, the
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86
B. Erdem and A. İzci
Fig. 2. Effect of catalyst loading on conversion of propionic acid at 333 K (feed mole ratio:
:1; catalyst: Dowex 50Wx4; stirrer speed: 700 rpm).
1
faster the rate at which equilibrium conversion was reached because of the in-
crease in the total number of available active catalytic sites for the reaction.
3.3 Effects of reactant and product concentrations
The calculation of kinetic parameters for heterogeneous acid catalyst is compli-
cated because of the complex catalyst surface-adsorbate interactions [25]. Initial
reaction rates were examined by varying the concentration of one reactant while
K1
fixing that of the other at 1.0 molL . The results obtained experimentally are
illustrated in Figs. 3–5.
As seen in Fig. 3, the initial reaction rate, measured at different acid concen-
trations, increased linearly with low acid concentrations, but it was nearly inde-
pendent of acid concentrations at high levels. Fig. 4 indicated that the initial
reaction rate increased linearly with alcohol concentration. On the other hand,
we observed from Fig. 5 that, for fixed concentrations of propionic acid and n-
amyl alcohol (1.0 M) the rate decreases with increasing concentration of water.
The type of dependence of this rate suggests that water is adsorbed on the cata-
lyst surface [26]. In the subsequent tests, it was determined that the adsorption
of amyl propionate and solvent (dioxane) could be negligible. From these analy-
ses, it was concluded that the reaction mechanism can be represented by Eley-
Rideal model, that is, the reaction takes place between adsorbed molecules of
acid and the molecules of alcohol free in the bulk solution. This model is derived
based on the assumption that the rate-limiting step is the surface reaction between
an adsorbed molecule and molecule in the bulk. In this mechanism, propionic
acid molecules are adsorbed on the active site of the resin and protonated first
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Heterogeneous Catalysed Esterification Reaction …
787
Fig. 3. Effect of propionic acid concentration on initial reaction rate at different tempera-
tures (catalyst loading: 1.4 % (w.w); catalyst: Dowex 50Wx4; stirrer speed: 700 rpm; CB0
=
K1
1
.0 mol L ; CE0 = CW0 = 0).
Fig. 4. Effect of n-amyl alcohol concentration on initial reaction rate at different tempera-
tures (catalyst loading: 1.4 % (w.w); catalyst: Dowex 50Wx4; stirrer speed: 700 rpm; CA0
=
K1
1
.0 mol L ; CE0 = CW0 = 0).
and then reacted with n-amyl alcohol molecules in the bulk liquid. The products
formed, namely, amyl propionate and water, are then desorbed from the surface.
3.4 Reaction mechanism
The preliminary mathematical modelling of the observed esterification kinetics
was based on second order kinetics, which principally could be applied to both
homogeneous and heterogeneous catalyst [27]. This acid catalysed esterification
reaction is an equilibrium limited chemical reaction:
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88
B. Erdem and A. İzci
Fig. 5. Effect of water concentration on initial reaction rate at different temperatures (cata-
lyst loading: 1.4 % (w.w); catalyst: Dowex 50Wx4; stirrer speed: 700 rpm; CA0 = CB0
=
K1
1.0 mol L ; CE0 = 0).
Scheme 1.
Scheme 2.
(1)
K
3
+
The sulfonic acid groups (HSO ) on a heterogeneous catalyst or H in a
homogeneous catalyst (e. g. H SO , HCl) initiate the esterification reaction by
2
4
donating a proton to the carboxylic acid molecule (see Scheme 1).
After the proton transfer, the carboxylic acid is accessible for a nucleophilic
attack by the hydroxyl group from the alcohol (R OH), and the reaction contin-
1
ues with water elimination: (see Scheme 2).
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Heterogeneous Catalysed Esterification Reaction …
789
Scheme 3.
Finally, the catalyst is recovered (see Scheme 3).
The proton-donating step is usually assumed to be rapid, while the nucleo-
philic substitution is slow. The rate-determining step gives the overall rate equa-
tion for the reaction (1)
(2)
where k and kS−1 are the forward and the reverse rate constants, respectively and
S
C denotes concentration for component i.
i
Depending on the experimental results given in section (3.3), Eq. (2) can be
arranged according to Eley-Rideal mechanism as follows:
(3)
where subcripts A, B, E and W refer to acid, alcohol, ester and water, respec-
tively. k is lumped rate constant of the reaction, K is the equilibrium constant
f
e
of the reaction. K and KW are the adsorption equilibrium constants for the acid
A
and water, m is the quantity of dry resin and V is the volume of the reaction
mixture.
For a molar ratios of CB,0.CA,0 = M and CW,0.CA,0 = N, Eq. (3) can be
expressed in terms of propionic acid conversion and it becomes
(4)
By using the ‘‘Method of Averages’’ [28], k , K and K were calculated
f
A
W
for different temperatures and these results are given in Table 2.
3.5 Effect of temperature
The study on the effect of temperature is very important for a heterogeneously
catalysed reaction as this information is useful in calculating the apparent activa-
tion energy. Moreover, the intrinsic rate constants are strong functions of temper-
atures [4]. The effect of temperature on the rate of reaction was studied by
conducting the reactions at 318, 333 and 348 K under the similar conditions (a
feed mole ratio of 1:1, a stirrer speed of 700 rpm, and at 1.4 % (w.w) Dowex
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90
B. Erdem and A. İzci
Fig. 6. Effect of temperature on conversion of propionic acid at a catalyst loading of 1.4 %
w.w) (feed mole ratio: 1:1; catalyst: Dowex 50Wx4; stirrer speed: 700 rpm).
(
Fig. 7. Arrhenius plot of esterification of propionic acid with n-amyl alcohol (T = 318–348 K;
K1
C
A0 = CB0 = 1.0 molL ; stirrer speed: 700 rpm; catalyst loading: 1.4 % (w.w)).
5
0Wx4 catalyst loading). Fig. 6 shows that the higher temperature yields the
greater conversion of propionic acid at a fixed contact time.
The dependence of forward reaction rate constant, k , on temperature was
f
determined from Arrhenius equation, Eq. (5). This is to be shown in Fig. 7.
(5.)
The value of apparent activation energy, E and pre-exponential factor, A
a
were obtained from least square fit of Eq. (5) to the data given in Table 2 and
the computed values are given in Table 3. The reaction equilibrium constant, Ke,
was found to be 4.05, independent of temperature ranging from 318 to 348 K,
in other words, the change in the value of K , was included in the experimental
e
error of it. The dependence of adsorption constants, K and K , on temperature
A
W
is related by [29]:
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Heterogeneous Catalysed Esterification Reaction …
791
Table 2. Parameters for E-R model used to fit the experimental data.
2
−1
−1
−1
(Lmol−1)
K
W
(Lmol−1)
T (K)
k
f
(L (g dry resin) mol min )
K
A
3
3
3
18
33
48
0.000086
0.000201
0.000351
0.2787
0.2148
0.1661
4.5558
3.0744
2.1747
Table 3. Thermodynamic results obtained from for amyl propionate esterification.
.
−1
2
−1
−1
−1
0
−1
0
−1 −1
E
a
(kJmol ) A (L (g dry resin) mol min ) ΔH (kJmol )
ΔS (kJmol K )
K0.060(foracid)
K0.059(forwater)
4
3.167
1093.676
K15.872(foracid)
K22.694(forwater)
Fig. 8. Comparison of experimental data with values predicted by Eq. (4) for esterification of
propionic acid with n-amyl alcohol on Dowex 50Wx4 loading of which is 1.4 % (w.w); symbol
represents the experimental data; dashed lines represent the model prediction (T = 333 K;
e
feed mole ratio: 1:1; stirrer speed: 700 rpm; M = 1; N = 0; K = 4.05).
(
6)
0
0
The values of ΔS and ΔH were also obtained by least square fit of Eq. (6)
and the values are given in Table 3. The heats of adsorption for propionic acid
and water were found to be negative, as they are exotermic process, and therefore
K and K decrease with the increase of temperature.
A
W
Using numerical integration (Runga-Kutta), propionic acid conversion at a
given time can be predicted from Eq. (4) under the condition of 333 K; M =
C .C = 1; N = CW0.CA0 = 0; K = 4.05; m.V = 1 g dry resin.0.1L; C
=
A0
B0
A0
e
K1
1
.0 mol L . The dotted lines in Fig. 8 give the prediction of reaction conversion
based on Eq. (4). According to this figure, Eq. (4) satisfactorily represents the
esterification of propionic acid and n-amyl alcohol on Dowex 50Wx4.
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B. Erdem and A. İzci
4. Conclusion
In this work, the catalytic performance of the Dowex 50Wx4 microporous cation
exchange resin was evaluated for the esterification of propionic acid with n-
amyl alcohol. Kinetic and thermodynamic parameters such as activation energy,
adsorption energies for propionic acid and water were obtained for the model
reaction. As for the reaction equilibrium constant (K ), which is independent of
e
temperature ranging from 318 to 348 K, was determined to be 4.05.
The conversion of propionic acid increased with an increase in the amount
of catalyst. But the equilibrium conversion was not affected. Therefore, the time
required to reach the reaction equilibrium was shortened due to the increase in
the total number of acid sites available [30].
The esterification reaction on Dowex 50Wx4 appears to involve single-site
catalysis with a probable rate-controlling Eley-Rideal surface reaction and this
kinetic model successfully predicts the esterification rate of Dowex 50Wx4 as
reaction progresses.
In the esterification of propionic acid and n-amyl alcohol, a good agreement
between the calculated curve and the experimental points is observed. It is con-
cluded that Dowex 50Wx4 is an influent catalyst to accelerate the esterification
reactions.
Appendix
Nomenclature
A, B, E, Wpropionic acid, n-amyl alcohol, amyl propionate and water, respec-
tively
rA0initial reaction rate of propionic acid
k forward reaction rate constant
s
−
1
k reverse reaction rate constant
S
mquantity of dry resin
Vreaction mixture volume
CA,0initial concentration of propionic acid (mol. L
CB,0initial concentration of n-amyl alcohol (mol. L
K1
)
)
K1
K1
C initial concentration of amyl propionate (mol. L
)
E,0
K1
CW,0initial concentration of water (mol. L
)
k lumped rate constant (L . (g dry resin) . molK1. minK1
2
K1
)
f
K equilibrium constant of the reaction
e
K adsorption equilibrium constant for propionic acid (L. molK1)
A
K1
K adsorption equilibrium constant for water (L. mol
)
W
˙
X conversion of propionic acid
A
A Arrhenius preexponential factor (L . molK1. minK1
2
)
K1
Eaapparent activation energy (kJ. mol
Ttemperature (K)
)
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Heterogeneous Catalysed Esterification Reaction …
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Rgas constant (kJ. molK1. KK1
)
0
K1
ΔG adsorption free energies for propionic acid and water (kJ. mol
)
0
K1
K1
ΔS adsorption entropies for propionic acid and water (kJ. mol . K
ΔH heats of adsorption for propionic acid and water (kJ. molK1)
ttime (min)
)
0
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