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Kinetics of esterification of lactic acid with ethanol
catalyzed by cation-exchange resins
Yang Zhang, Li Ma, Jichu Yang *
Department of Chemical Engineering, Tsinghua University, Beijing 100084, Peoples Republic of China
Received 17 March 2004; accepted 26 April 2004Available online 25 June 2004
Abstract
The esterification of lactic acid with ethanol was carried out in the presence of five different cation-exchange resins.
The effect of catalyst type, catalyst loading, and temperature on reaction kinetics was evaluated. In order to study which
components had the strongest adsorption strength on the resin surface, two simplified mechanisms based on Langmuir
Hinselwood model were compared by correlating the experimental data. FTIR method was used to verify the ratio-
nality of the mechanism. Nonideality of the liquid phase was taken into account by using activities, which were
predicted by UNIFAC method instead of concentrations. The thermal stability and mechanical strength of the resin
catalysts were tested by SEM. 2004 Elsevier B.V. All rights reserved.
Keywords: Lactic acid; Esterification; Ethanol; Kinetics; Cation-exchange resin
1. Introduction
Ethyl lactate is an important organic ester,
which is biodegradable and can be used as food
additive, perfumery, flavor chemicals and solvent,which can dissolve acetic acid cellulose and many
resins [1]. Furthermore, the esterification of lactic
acid with ethanol is a step in the purification of
lactic acid by reactive distillation [2,3].
The conventional way to produce ethyl lactate
is the esterification of lactic acid with ethanol
catalyzed by sulphuric acid. Since this kind of
homogeneous catalyst may cause a lot of prob-
lems, many heterogeneous solid catalysts were
used in the reaction such as ion-exchange resin,
clays and clay supported heteropoly acids [4].Among these catalysts, cation-exchange resin is a
perfect substitute which has many advantages such
as: (a) corrosion problems could be avoided and it
is easier to dispose of the waste liquor from the
reaction mixture; (b) continuous operations in
columns are possible; (c) the catalyst can be easily
removed from the reaction products by decanta-
tion or filtration; (d) the purity of the products is
higher since side reactions can be eliminated or are
less significant [5,6].
* Corresponding author. Tel.: +86-10-62-788568/785514; fax:
+86-10-62-770304.
E-mail address: [email protected] (J. Yang).
1381-5148/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.reactfunctpolym.2004.04.003
Reactive & Functional Polymers 61 (2004) 101114
www.elsevier.com/locate/react
REACTIVE
&
FUNCTIONAL
POLYMERS
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The industrial production of esters by esterifi-
cation of acid and alcohol was first carried out in a
continuous stirred tank reactor (CSTR) and later
in a catalytic distillation column over cation-ex-
change resins. Nowadays, some investigations
have focused on the water-permeable membrane
reactors applied to liquid-phase reversible reac-
tions [1,7,8]. The conversion of the esterification of
lactic acid with ethanol exceeded the equilibrium
limit remarkably with the aid of vapor-permeation
according to their researches.In order to optimize the design of a CSTR, a
reactive distillation column or a membrane reac-
tor, it is necessary to have some information on the
reaction kinetics. Although there have been many
researches about the esterification of lactic acid
with different alcohols over cation-exchange resins
[49], very few reports concern the synthesis of
ethyl lactate with heterogeneous catalysts. In this
paper, the emphasis of our work was to use
LangmuirHinselwood (LH) model [10] in a study
of the esterification kinetics over different cation-
exchange resins. Two simplified LH mechanisms
were compared in order to find which one de-
scribed the reaction kinetics better. Fourier trans-
form infrared spectroscopic (FTIR) analysis was
also used to verify the rationality of the mecha-
nism. Scanning electron microscopy (SEM) was
introduced to evaluate the mechanical strength of
the resins and their thermal stability.
2. Materials and catalysts
Ethanol (purity >99.7 wt%) was purchased
from Yili fine chemical Co., Beijing. Ethyl
LL-lactate was purchased from SigmaAldrich.LL-
lactic acid (80 wt%) was obtained from PURAC
Biochem, Netherlands. The catalysts used in the
experiments were commercial strong-acid cation-
exchange resins. Their physical properties are
listed in Table 1. Before use, the fresh catalysts
Nomenclature
aiactivity of i (mol/l)
ci concentration of i at the surface of the
catalyst (mol/l)
DA molecular diffusion coefficient
De effective diffusion coefficient
EA apparent activation energy (kJ mol1)
DH enthalpy change (kJ mol1)Keq reaction equilibrium constant
k reaction rate constant (mol g1 min1)k0 pre-exponential factor (mol g1 min1)k0i adsorption coefficient at initial temper-
ature
ki adsorption coefficientM molar mass of the solvent
MRD mean relative deviation
N number of data points
nLA;0 initial molar amount of lactic acid (mol)
R gas constant (J mol1 K1)r reaction rate (mol g1 min1)r0 radius of catalyst particle
S vacant site on catalyst surface
SRS sum of residual squares
Ttemperature (K)
t time (min)
W dry catalyst weight (g)
X conversion
VA molar volume at boiling point
Greek symbols
c activity coefficient
e porosity
s tortuosity
/ thiele modulus
U0 association factorl viscosity (Pa s)
Subscripts
calc calculated values
exp experimental values
LA lactic acid
E ethanol
EL ethyl lactate
W water
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were dried at 90 C over 12 h to remove moisture
completely after being washed with pure water,
ethanol, hydrochloric acid and pure water
sequentially.
3. Kinetics experimental apparatus and procedure
3.1. Apparatus
The esterification reactor consisted of a three-
necked flask of 250 ml capacity fitted with a con-
denser, a sampling device and a thermometer. The
temperature was controlled by a thermostating
bath, which ensured a temperature constancy of
0.2 C in the reactor. A magnetic stirrer was usedto mix the reactants, and the frequency was about
450 rpm.
3.2. Procedure
Lactic acid and cation-exchange resins were
charged into the reactor, and then heated to the
desired temperature. Finally, ethanol was added.
This was taken as zero time for a run. The initial
molar ratio of lactic and ethanol was 1:3, and the
total volume of the reactant was 196.77 ml. About
1 ml of liquid sample was withdrawn from the
reactor at regular intervals for gas chromatogra-
phy (GC) analysis. In a typical run, about 10
samples were taken from the system.
4. GC analysis
GC (Shimadzu GC-9AM) was used to analyze
the sample, which was equipped with a flame
ionization detector (FID). The sample size for GC
was 0.2 ll. The injection port and detector tem-
peratures were set to be 240 and 250 C separately.
A capillary column (25 m 0.5 mm, SE-30) wasused. The column temperature was programmed
to rise from an initial value of 100125 C at
2.5 C/min, and then held constant at 125 Cfor an additional 4 min. High purity helium gas
(99.999%) was used as a carrier gas. The flow rate
of the carrier gas was 40 ml/min.
5. FTIR and SEM
The catalyst samples, which had been used in
the kinetics experiments, were tested in FTIR
analysis. Before test, they were treated under 100
C with different desorption times from 0 to 8 h inan oven, and then sealed in plastic bags.
The FTIR analysis was carried out at room
temperature using NICOLET 560 equipment. The
samples were ground to fine powders using an
agate mortar immediately prior to analysis. KBr
was used as the embedding medium to make every
sample tablets containing around 1.3 mg of ion-
exchange resin powder [11,12].
A SEM equipment (KYKY-2800, KYKY
Technology Development Ltd.) was used to
Table 1
Physical properties of the strong-acid cation-exchange resins
Property Amberlyst-15 (Mp) D001 (Mp) D002 (Mp) NKC (Mp) 002 (G)
Shape Bead Bead Bead Bead BeadSize (mm) 0.5P90% 0.321.25P 95% 0.41.25P 95% 0.41. 25P 95% 0.51.25P95%
Internal surface
area (m2/g)
50 N/A 3540 77 N/A
Weight capacity
(mEq/g)
4.7 P 4.35 P 4.8 P 4.7 P 5.0
Temperature
stability (C)
120 100 120 100 180
Manufacturer Rohm and Haas Co.,
USA
Shandong Dongda
Chemical Industry
Co., PRC
Jiangyin Organic
Chemical Plant,
PRC
Chemical Plant of
Nankai Univ.,
PRC
Jiangyin Organic
Chemical Plant,
PRC
Mp Macroporous; G Gel; N/A Not available.
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evaluate the mechanical strength and the thermal
stability of the catalysts, and the acceleration
voltage is 20 kV.
6. Results and discussion
6.1. External and internal diffusion
In a heterogeneous catalytic reaction, there are
several processes that influence the rate of reac-
tion, which are external and internal diffusion of
reactants, adsorption, surface reaction, desorp-
tion, internal and external diffusion of reaction
products. In order to study the intrinsic kinetics atthe catalyst surface, external and internal diffusion
should not be the rate limiting steps [8]. According
to Sanzs work [9], if the speed of stirring was
between 300 and 700 rpm for the similar cation-
exchange resins, the influence of external diffusion
could be neglected. Therefore, the stirring speed
was set around 450 rpm in all further experiments
to ensure the absence of external mass transfer
resistance.
To evaluate the effect of internal diffusion on
the cation-exchange resins, Eq. (1) was used [13].
/ r20k
9De; 1
where r0 and De denote the radius of catalyst
particle and the effective diffusion coefficient re-
spectively. k is the reaction rate constant, and / is
the thiele modulus. If the calculated value of/ was
smaller than 1, the internal diffusion could be
neglected.
The effective diffusion coefficient was defined as
follows:
De DAes
; 2
where DA is the molecular diffusion coefficient. s is
the particle tortuosity and e is the porosity. For
most resin catalysts, the values of e=s are between0.25 and 0.50 [14]. In the calculation, the value of
e=s was taken 0.50. The molecular diffusion coef-ficient for liquid phase diffusion can be evaluated
from the WilkeChang equation (Eq. (3)) [15].
DA 7:4 108 ffiffiffiffiffiffiffiffiffiffiffiffiU0MTp
VA0:6l: 3
In Eq. (3), U0 and M denote the associationfactor and the molar mass of the solvent. VA is the
molar volume at boiling point and l is the vis-
cosity of the solution. For solvent mixtures, the
association molar mass can be calculated byffiffiffiffiffiffiffiffiffiU0M
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPxiU0iMip . Le Bas method was used in
order to calculate VA [15].
With the increase of temperature, the viscosity
of the reaction solution will decrease while the
diffusion coefficient will increase. As the result, if
the influence of internal diffusion could be ne-
glected at lower temperature, the influence at
higher temperature could also be neglected ac-
cording to Eq. (1). At 25 C, through experiment,
the viscosity of the equilibrium reaction solution
was 2.3963 103 Pa s. Then the calculation re-sults of Eqs. (1) and (2) were / 0:02 andDe 2:2 106 m2 s1, which indicated that theeffect of internal diffusion on the reaction rate
could be ignored reasonably. Based on the dis-
cussion above, the experimental kinetic results
could be considered to reflect the intrinsic kinetics
of the esterification reaction catalyzed by cation-
exchange resins.
6.2. Assumption of the model and parameters
estimation
The heterogeneous reaction has been described
with many models such as LangmuirHinselwood,
EleyRideal (ER) and pseudo-homogeneous
model [6,10]. Among them, LH model was found
to be more appropriate for this kind of esterifica-
tion reaction [9,14,16]. The reaction mechanism
can be described as follows:
LA S () LA S
E S () E S
E S LA S () EL S W S
EL S () EL S
W S () W S
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Therefore, the LH model could be written as:
r nLA;0W
dX
dt k
aLAaE aELaWKeq1 kWaW kEaE kLAaLA kELaEL2
;
ki ciSaicS
; Keq aEL aWaLA aE
eq
;
i Water, Ethanol, Lactic acid, Ethyl lactate4
In the equations, nLA;0 is the initial molar con-
centration of lactic acid and W is the catalyst load-
ing, k represents the reaction rate constant and ki
represents the adsorption coefficient. cS and ciSdenote the concentration of vacant site on catalyst
surface and the concentration of component i at the
catalyst surface respectively. ai is the activity for
each component andKeq is the reaction equilibrium
constant. The activities were calculated by the
UNIFAC method [15]. The splittingof the groups is
shown in Table 2. The volume and area parameters
of the groups and their interaction parameters were
taken from the book of Fredenslund et al. [16].
Since the denominator of Eq. (4) is a multi-
component complex, the parameters cannot beregressed correctly from the experimental results.
It was assumed that the adsorption of the mole-
cules was competitive on the same active site, and
only those that had the strongest adsorption were
taken into account in the simplified mechanisms.
There were some different opinions about which
components had the strongest adsorption on the
catalyst surface [810,14,17]. Two mechanisms
were tested in our work to find which one was
more reliable. Mechanism A assumes that ethanol
and water adsorbed much stronger than other
components in the esterification solution, so the
adsorption of ethyl lactate and lactic acid wasneglected. We get
r kaLAaE aELaWKeq
1 kWaW kEaE2: 5
In contrast to mechanism A, it was assumed
that lactic acid and water had the strongest ad-
sorption in mechanism B (Eq. (6)).
r kaLAaE aELaWKeq
1 kWaW kLAaLA2: 6
In mechanisms A or B, there are three param-eters instead of five (Eq. (4)) to be estimated at a
constant reaction temperature. In mechanism A,
they are k, kW and kE. While for mechanism B, the
corresponding parameters are k, kW and kLA. A
two-stage optimization procedure was adopted for
parameter evaluation. Firstly, the regression of
these parameters was carried out by minimizing
the sum of residual squares (SRS) between the
experimental and calculated reaction rates. Then,
numerical integration method was used for inte-
grating the calculated reaction rates with previ-ously determined parameters to get the
conversions. The calculated conversion values
were then compared with experimental values
through the mean relative deviation (MRD).
SRS XN
rexp rcalc2; 7
MRD 1N
XN
Xcalc XexpXexp
! 100%: 8
Table 2UNIFAC group identification of the components
Molecule Group identification vij Volume parameter Rj Area parameter Qj
Group name Main Secondary
Ethyl Lactate CH3 1 1 1 0.9011 0.8480
CH3OHCH 5 15 1 1.8780 1.660
CH2COO 11 26 1 1.6764 1.420
Water H2O 7 20 1 0.92 1.40
Ethanol CH3CH2OH 5 17 1 2.1055 1.972
Lactic acid COOH 17 40 1 1.3013 1.224
CH3OHCH 5 15 1 1.8780 1.660
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Regression results are summarized in Table 3,
which included the values of the regression pa-
rameters, SRS and MRD at 353 K. The differences
of the two mechanisms were compared through
the values of SRS and MRD. From the regression
results, the rate equation for mechanism A de-
scribed more accurately the experimental data
than mechanism B. Therefore, the former was
more reliable for this kind of esterification reac-
tion. Similar mechanisms were applied in Sanz and
Wei Songs work [9,17]. In order to verify this
conclusion, FTIR analysis was used to test the
component absorbing strength on the catalyst
surfaces as discussed below.
6.3. Desorption experiment and FTIR analysis
In the model assumption section, through the
comparison of two different mechanisms, it was
concluded that ethanol and water absorbed
stronger than other components in the reaction
solution. In order to examine the previous results,
FTIR analysis was introduced. The used resins
(002) were treated at high temperature in order to
determine the desorption information which was
inverse to the reaction of adsorption. The resinsamples were withdrawn from the esterification
reactor, and then treated at 100 C in an oven for
different period of time (0, 0.5 and 8 h). In Fig. 1,
the overall spectra of three samples are presented
(wave numbers from 400 to 4000 cm1), in whichany component changes on the surfaces of the
catalysts could be observed. There is a broad band
in the range from 3200 to 3600 cm1, which is dueto the symmetric and asymmetric stretching of the
OH groups [18]. Fig. 1 shows that the intensity of
Table 3
Values of parameters for the kinetic equations based on the mechanisms A and B
Mechanism Parameters 002 NKC
Value SRS MRD (%) Value SRS MRD (%)
k (molg1 min1) 1.704 1.701A kW 4.973 1.83 109 4.27 5.073 2.10 109 4.16
kE 2.093 2.245
k (molg1 min1) 1.650 1.449B kW 5.016 5.88 109 8.61 5.510 9.06 _109 7.30
kLA 1.554 2.010
Fig. 1. Absorbance FTIR spectrum of 002 at different de-sorption times: 0 h (), 0.5 h (- - - - -), 8 h ( ), under 100 C,wave number region: 4004000 cm1.
Fig. 2. Absorbance FTIR spectrum of 002 at different de-
sorption times: 0 h (), 0.5 h (- - - - -), 8 h ( ), under 100 C,wave number region: 15001800 cm1.
106 Y. Zhang et al. / Reactive & Functional Polymers 61 (2004) 101114
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the band decreases with desorption time, which
means that the extent of the desorption reaction
took place on the catalyst surface can be measured
by FTIR.
Fig. 2 shows two important bands, which are
located at 15001800 cm1. One is at around 1640cm1, and the other is located at 1740 cm1. Theformer band is attributed to the bending vibration
of water and the latter to the stretching vibration of
the C@O [18]. It can be seen that the absorbance
of the band at 1640 cm1 decreases as a function ofdesorption time, which indicates that water held a
large proportion of active sites on the catalyst
surfaces. The intensity of the band at 1740 cm1 isrelatively much smaller in comparison with the
band at 1640 cm1. However, the intensity of thestretching vibration of the C@O group is always
very strong and sharp, which can be testified by the
Fig. 3. Absorbance FTIR spectrum of lactic acid, wave number
region: 4004000 cm1.
Fig. 4. Absorbance FTIR spectrum of ethyl lactate, wave
number region: 4004000 cm1.
Fig. 6. Conversion versus time for the esterification with NKC
at different temperatures, 333 K ., 343 K d, 353 K N,361 K j. The continuous lines represent the results of the LHmodel. All the reactions were carried out with an initial molar
ratio 3:1 (ethanol:lactic acid). The catalyst loading was 4%
(w/w) in all experiments.
Fig. 5. Conversion versus time for the esterification with 002 atdifferent temperatures, 333 K ., 343 K d, 353 K N, 361K j. The continuous lines represent the results of the LHmodel. All the reactions were carried out with an initial molar
ratio 3:1 (ethanol:lactic acid). The catalyst loading was 4%
(w/w) in all experiments.
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Table 5
Regression results of NKC at different reaction temperatures
Temperature (K) k (molg1 min1) kW kE SRS MRD (%)
333 0.578 5.921 3.922 3.99 1010 3.94343 1.101 5.507 2.945 2.58 109 4.05361 2.505 5.501 2.902 3.60 10
10
2.33The results at 353 K are shown in Table 3.
Table 4
Regression results of 002 at different reaction temperatures
Temperature (K) k (molg1 min1) kW kE SRS MRD (%)
333 0.762 5.999 3.254 8.01 1010 4.58343 1.015 5.475 2.601 2.84 1010 4.79361 2.555 5.500 2.900 7.23 1010 2.68
The results at 353 K are shown in Table 3.
Fig. 7. (a) Arrhenius plot of esterification catalyzed with 002. The continuous line represents the result of linear regression. (b) vant
Hoff plot of adsorption coefficient of water catalyzed with 002. The continuous line represents the result of linear regression. (c) vant
Hoff plot of adsorption coefficient of ethanol catalyzed with 002. The continuous line represents the result of linear regression.
108 Y. Zhang et al. / Reactive & Functional Polymers 61 (2004) 101114
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FTIR spectra of lactic acid and ethyl lactate in
Figs. 3 and 4. Thus it was concluded that lactic acid
and ethyl lactate were not absorbed strongly in
comparison with water on the resin surfaces.
Through the FTIR experiments, it is proved
that water absorbs much stronger on the resin
surfaces than lactic acid and ethyl lactate do. Al-
though the adsorption intensity of ethanol couldnot be confirmed in this way, mechanism B is
Fig. 8. (a) Arrhenius plot of esterification catalyzed with NKC. The continuous line represents the result of linear regression. (b) vant
Hoff plot of adsorption coefficient of water catalyzed with NKC. The continuous line represents the result of linear regression. (c) vant
Hoff plot of adsorption coefficient of ethanol catalyzed with NKC. The continuous line represents the result of linear regression.
Table 6
Regression results ofDH for 002 and NKC
002 NKC
Water Ethanol Water Ethanol
DH (kJ/mol) )6.42 )21.33 )7.47 )26.97
Fig. 9. Conversion versus time for the esterification with 002.
The catalyst loading was 2% (w/w) j, 4% N and 6% drespectively. All the reactions were carried out with an initial
molar ratio 3:1 (ethanol:lactic acid), at temperature 343 K.
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surely unreasonable since the adsorption of lactic
acid is thought as one of the strongest.
6.4. Esterification of lactic acid with ethanol
6.4.1. Effect of reaction temperature
The effect of temperature on the esterification
reaction was studied over a temperature range
from 333 to 361 K, under atmospheric pressure,
with 002 and NKC as catalysts. The boiling point
of this mixture is about 361 K. As mentioned
above, the stirring speed was set to around 450
rpm. So the influence of external and internal
diffusion could be neglected under the given con-
ditions. The experimental and regression resultsare displayed in Figs. 5 and 6. From the figures,
the reaction rate increases with increasing tem-
perature. The same trend was described in the re-
action of lactic acid with methanol by Seo and
Fig. 10. Conversion versus time for the esterification with fivedifferent cation-exchange resins. The symbols: 002 j, Am-berlyst-15 N, D001 d, D002 ., NKC r. All the re-actions were carried out with an initial molar ratio 3:1
(ethanol:lactic acid), at temperature 353 K. The catalyst loading
was 4% (w/w) in all experiments.
Fig. 11. SEM photomicrographs of the surface of the resins: (a) fresh catalyst, (b) after esterification experiment and (c) reused in the
catalytic distillation column. Catalyst: 002. Magnification: 150.
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Sanz et al. [6,9], and in the reaction of isopropanol
with lactic acid [4]. The reaction constant, ad-
sorption coefficients, SRS and MRD are provided
in Tables 4 and 5. In both tables, the values of SRSwere smaller than 3.0 109 and MRD weresmaller than 5%. The adsorption coefficient of
water is greater than that of ethanol, which sug-
gests that water adsorbs more strongly than etha-
nol on the active sites of the resins.
6.4.2. Activation energy and adsorption coefficient
Figs. 7 and 8 show the Arrhenius plots for the
esterification reaction with 002 and NKC as cata-
lysts at different temperatures. Arrhenius relation
was introduced to describe the change of the re-
action rate constant with temperature. The linear
correlation coefficients were higher than 0.995 for
both catalysts. From the slopes of the straight lines
in Figs. 7(a) and 8(a), the activation energies can be
calculated by Eq. (9), which are 51.58 kJ/mol for
002 and 52.26 kJ/mol for NKC. The high values of
the activation energy indicate that the reaction is
kinetically controlled. From Figs. 7(b) and (c), it
can be deduced that the variation of adsorptioncoefficient can be described by vant Hoff law (Eq.
(10)) [8] in the temperature range from 333 to 353
K, but there was a big deviation at 361 K (the
boiling point of the reaction mixture). Since a lot of
bubbles appeared in the liquid reaction mixture at
the boiling point, the concentrations ai of com-ponents in the liquid phase, mainly ethanol and
water, became lower because of their higher vola-
tility. Meanwhile, as the surface reaction was the
rate-determining step, the concentration of ab-
sorbed components on catalyst surface was con-
sidered unchanged. In Eq. (4), ciS and cS did notchange, while ai got smaller. So the regression value
ofki was higher than that estimated by vant Hoff
law. Figs. 7 and 8 show that the adsorption of
water and ethanol on cation-exchange resin surface
Fig. 12. SEM photomicrographs of the surface of the resins: (a) fresh catalyst, (b) after esterification experiment and (c) reused in the
catalytic distillation column. Catalyst: 002. Magnification: 5000.
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is an exothermic process, while the whole esterifi-
cation reaction is an endothermic reaction. The
regression results of DH for both catalysts were
listed in Table 6.
lnk lnk0 EART
; 9
lnki lnk0i DHi
RT: 10
6.4.3. Effect of catalyst loading
Fig. 9 shows the effect of catalyst loading on re-
action rates. It was observed that the equilibrium
constant nearly did not change with the increase of
catalyst loading. The time required to reach theequilibrium was reduced as the catalyst loading in-
creased. The reason was that the more catalysts loa-
ded, the more active sites were available for reaction.
6.4.4. Effect of catalyst type
Different types of cation-exchange resins were
used to assess their efficiency in the esterification
reaction. Fig. 10 shows the plots of conversion oflactic acid against time for the various catalysts. It
can be concluded that the catalytic activity
increased in the order D002 < D001
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more than 40 h at around 363 K in the lactic acid
ester catalytic distillation process [19].
Figs. 11 and 12 show the SEM photographs of
002 under magnification of 150 and 5000. In these
figures, the surface of 002 was very smooth like
most gel resins. Figs. 12(b) and (c) show that after
being used in the esterification and catalytic dis-
tillation experiments, there were few changes on
the gel surfaces. Some small defects were observed
on their surfaces, which were probably caused byimpurities in the column or by a small amount of
poly-lactic acid formed in the experiments, just like
what happened on other catalysts after being used
for a long time.
In the same way, SEM micrographs of NKC
are shown in Figs. 13 and 14. The surface had
obvious differences in comparison with 002. The
pore diameters on the surfaces of fresh NKC
ranged from 50 to 1000 nm. The surfaces of NKC
had much more cracks and mass loss than 002
after the same intensity of stirring according to
Figs. 13(b) and 14(b). In Fig. 14(c), the pore sizes
of NKC became larger than the fresh catalysts. It
was due to the swelling effect of reaction liquid
under high temperature in the catalytic distillation
column. All these indicated that 002 had better
mechanical strength than NKC.
For industrial applications, mechanical strength
is an important factor in evaluation of catalyst.
Taking lactic acid ester production for example,cation-exchange resins were used in a CSTR or a
catalytic distillation column. Under the stirring
force, the resins may be destroyed if they do not
have enough mechanical strength. And that would
cause a lot of problems in further separation and
purification processes. In a reactive distillation
column, the destroyed resin fragments would plug
the packing section and lead to much pressure
drop in it. Taking this factor into account, 002 is a
better choice for industrial applications.
Fig. 14. SEM photomicrographs of the surface of the resins: (a) fresh catalyst, (b) after esterification experiment and (c) reused in the
catalytic distillation column. Catalyst: NKC. Magnification: 5000.
Y. Zhang et al. / Reactive & Functional Polymers 61 (2004) 101114 113
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7. Conclusions
Esterification of lactic acid with ethanol over
different kinds of cation-exchange resins was stud-ied in this paper. The order of catalytic activity was
found to be: D002