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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: yjc-dce@mail.tsinghua.edu.cn (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

http://mail%20to:%20yjc-dce@mail.tsinghua.edu.cn/http://mail%20to:%20yjc-dce@mail.tsinghua.edu.cn/

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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.

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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.

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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