Kinetics of the Mono-esterification Between Terephthalic Acid and 1,4-Butanediol

CATALYSIS, KINETICS AND REACTORS Chinese Journal of Chemical Engineering, 18(3) 391—396 (2010)

Kinetics of the Mono-esterification Between Terephthalic Acid and 1,4-Butanediol TIAN Wenyu (田文玉)1, ZENG Zuoxiang (曾作祥)2,*, XUE Weilan (薛为岚)2, LI Yingbin (李应宾)2 and ZHANG Tianyu (章添钰)2 1 2

Department of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China Institute of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

Abstract The chemical kinetics of the monoesterification between terephthalic acid (TPA) and 1,4-butanediol (BDO) catalyzed by a metallo-organic compound was studied using the initial rate method. The experiments were carried out in the temperature range of 463-483 K, and butylhydroxyoxo-stannane (BuSnOOH) and tetrabutyl titanate [Ti(OBu)4] were used as catalyst respectively. The initial rates of the reaction catalyzed by BuSnOOH or Ti(OBu)4 were measured at a series of initial concentrations of BDO (or TPA) with the concentration of TPA (or BDO) kept constant. The reaction orders of reagents were determined by the initial rate method. The results indicate that the reaction order for TPA is related with the species of catalyst and it is 2 and 0.7 for BuSnOOH and Ti(OBu)4 respectively. However, the order for BDO is the same 0.9 for the two catalysts. Furthermore, the effects of temperature and catalyst concentration are investigated, and the activation energies and the reaction rate constants for the two catalysts were determined. Keywords esterification, 1,4-butanediol, terephthalic acid, kinetics

1

INTRODUCTION

The consumption and production of hot-melt adhesives are quickly growing at present in all branches of industry due to a variety of advantages [1]. For example, hot-melt adhesives ensure a fairly high strength of the bond line and are operative over a wide temperature range [2]. Additionally, they are environmental friendly. Copolyester hot-melt adhesive is one of the most important commercial adhesives. Various processes for the manufacture of copolyester have been proposed in the literature [3-7]. Most of them start from dimethyl terephthalate (DMT) and 1,4-butanediol (BDO) by transesterification. A potential technology for the synthesis of polyester begins with direct esterification of terephthalic acid (TPA) with BDO [8, 9]. In the absence of catalyst, the rate of esterification is very low, and the reaction must be carried out at a high temperature, which will result in some side reactions [10, 11]. This situation can be improved by adding an appropriate catalyst. So it is important to understand the esterification kinetics in the presence of a catalyst. El-Kinawy et al. investigated the kinetics of esterification of various saturated fatty acids with isobutanol [12]. Kulawska et al. studied the esterification of maleic anhydride over an acid ion-exchange resin catalyst in an isothermal semi-batch reactor, and the first-order kinetics with respect to acid has been observed [13]. Tesser et al. [14] investigated the kinetics of esterification between oleic acid and methanol in the presence of triglycerides. The data are well interpreted using a second-order pseudo-homogeneous kinetic model [14]. However, the kinetics of esterifica-

tion between TPA and BDO has not been clarified [4, 15]. In general, the esterification reaction involves a two-step process: (1) formation of monohydroxybutyl terephthalate (MHBT); and (2) formation of bishydroxybutyl terephthalate (BHBT) [16]. Although the reaction system of TPA and BDO is heterogeneous [17], it can be assumed that the esterification occurs only in the liquid phase [18, 19]. In this work, the initial rate method is used to study the kinetics of the first step promoted by a catalyst such as butylhydroxyoxo-stannane (BuSnOOH) and tetrabutyl titanate [Ti(OBu)4]. The effects of various reaction variables on the conversion of TPA are investigated, including amount and species of catalyst, temperature and the concentration of TPA and BDO. Kinetic models are proposed to explain the characteristics of mono-esterification between TPA and BDO. 2 REACTION MECHANISM AND KINETICS RATE MODEL 2.1

Reaction mechanism

According to the mechanism of catalysis of Ti(OBu)4 elucidated by Pilati et al. [20, 21], the mono-esterification of TPA may follow a coordinative mechanism shown in Fig. 1 which involves the formation of an adduct between a carbonyl group and Ti atom. Because of the similarity in coordination number and electronegativity between Ti and Sn, the mechanism shown in Fig. 2 should be reasonable for the mono-esterification catalyzed by BuSnOOH.

Received 2009-11-23, accepted 2010-05-06. * To whom correspondence should be addressed. E-mail: [email protected]

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Figure 1 The mechanism of the mono-esterification between BDO and TPA catalyzed by Ti(OBu)4

Figure 2

2.2

The mechanism of the mono-esterification between BDO and TPA catalyzed by BuSnOOH

Kinetics rate model

The common approach is to lump the detailed mechanism into a set of global reactions which involve only molecular species, whose concentration can be, in principle, easily monitored as a function of time. However, the complexity of this reacting system prevents the ability of measuring the concentration of adducts (shown in Figs. 1 and 2) in the system. So, in this work, the mono-esiterification kinetics is studied using the initial rate method as follows. The reaction between TPA and BDO accords to the following reaction:

ries of initial reactant concentrations. The advantage of this method is that it avoids complication due to products. In practice, it is convenient to determine the initial rates for mixtures in which the concentration of only one of the substances is changed at a time. For reaction (2), when a fixed high concentration of BDO is used and the concentration of TPA changes, the kinetic equation can be simplified to

v = kCAn CBm (2) where CA and CB represent the concentrations of TPA and BDO, respectively, k is the reaction rate constant which is related to temperature and catalyst , n and m represent the reaction order with respect to A and B. The reaction order can be obtained from experiments in which the initial rates are measured at a se-

(3)

k ′ = kCBm

(4)

According to Eqs. (3) and (4), measuring the initial velocities v0 at a series of initial concentration (CA0) of TPA, n and k are easy to be obtained by plotting lgv0 versus lg CA0 using the method of least squares. 3

(1) where A, B, C and D represent TPA, BDO, MHBT and water, respectively. Generally, when the etherification reaction is not limited by mass-transfer, the reaction rate v can be described as

v = k ′CAn

3.1

EXPERIMENTAL Materials

Terephthalic acid (TPA) (Sinopec Yangzi Petrochemical Co.) and 1,4-butanediol (BDO) (Shanxi Sanwei Group Co.) used here are fiber forming grade. Moisture is removed from BDO by boiling and condensing it at 427 K under vacuum. Before use, TPA is dried at 353 K under vacuum. BuSnOOH and Ti(OBu)4 are of analytical grade and purchased from Shanghai Chemical Reagent Co., China. Dimethyl terephthalate (DMT) with a mass fraction purity better than 99% is provided by Asahi Kasei Chemicals Corporation and used as received. Bishydroxybutyl

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Table 1

Experiment data of the initial rate (v0) at different concentrations CA0 of TPA CA0/mol·L−1

0.1% (by mass) BuSnOOH

0.1% (by mass) Ti(OBu)4

v0/mmol·L−1·min−1 T = 463 K

T = 473 K

T = 483 K

0.0601

0.350

0.446

0.660

0.1202

1.600

2.001

3.450

0.1804

3.290

4.480

5.790

0.2402

6.400

8.200

12.00

0.0601

8.402

11.85

15.59

0.1202

14.81

21.86

24.23

0.1804

17.99

27.80

34.76

0.2402

34.66

33.40

42.72

terephthalate (BHBT) is prepared by transesterification between BDO and DMT. 3.2

Experimental method

The experiments are carried out in a 250 ml four-neck flask equipped with a two-blade stirrer, a mercurial thermometer, sampling port and a fractionating column allowing the withdrawal of water and tetrahydrofuran (THF) during the reaction. The reactor is heated with an oil jacket, and the temperature in the reactor is controlled automatically by adjusting the oil temperature. The temperature in the chamber is maintained within ±0.5 K. 3.2.1 Mono-esterification reaction with different initial concentrations of TPA A known amount of BDO (about 100 g) and catalyst is placed in a four-neck flask prior to the experiment and heated to the desired temperature. The TPA reactant (1-4 g) is warmed to the reaction temperature in another vessel, and the mono-esterification reaction is started by feeding the acid into the reactor. The sample withdrawn from the reaction mixture is taken after 5 min and immediately cooled with solid carbon dioxide to avoid further reaction. The concentration of TPA in the sample is determined by high performance liquid chromatography (Waters Breeze 1515 HPLC) with refractometer 2414 as a detector. The column used is SunFire C18 stainless steel (4.6 mm×250 mm). The optimum operation conditions of HPLC are: column temperature, 313 K; flow phase, methol(A)-potassium dihydrogen phosphate buffer solution (B, 0.2 mol·L−1) (VA ︰VB = 10︰90); flow rate, 1.0 ml·min−1; sample volume, 10 μl. 3.2.2 Mono-esterification with different initial concentrations of BDO Known amounts of solvent (80-95 g), TPA and catalyst are added in the four-neck flask and heated to the desired temperature. The amount of TPA is slightly more than its solubility in the solvent at the temperature in order to ensure the concentration of TPA to be constant during the reaction. BDO (5-20 g) is warmed

separately to the reaction temperature, and the reaction is started by feeding BDO into the reactor. The solvent is different for different catalysts. DMT and BHBT are used as solvent for BuSnOOH and Ti(OBu)4, respectively. The methods of sampling and analysis are the same as the above. 4 4.1

RESULTS AND DISCUSSION Reaction order of TPA

The esterification experiments were finished with 0.1% (by mass) BuSnOOH or 0.1% Ti(OBu)4 in the mixture as a catalyst. BDO and TPA molecular ratios were 46, 92, 138, 184, respectively. The reaction temperature was in the range of 463-483 K. The experimental data were listed in Table 1. 4.1.1 BuSnOOH catalyst Figure 3 is the relationship of lgv0 versus lgCA0 under the different reaction temperatures (463, 473, 483 K) using 0.1% (by mass) BuSnOOH as catalyst. As seen in Fig. 3, when the volume of BDO is in far excess of the stoichiometric amount and its concentration is constant (CB = 11.1 mol·L−1), the initial reaction

Figure 3 Plots of lgv0 vs. lgCA0 for mono-esterification of TPA and BDO catalyzed by BuSnOOH at different temperatures ▼ T = 483 K; ○ T = 473 K; ■ T = 463 K; —— regression line

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rates increase with the concentration of TPA, and a reasonable liner dependence of lgv0 vs. lgCA0 is observed at constant temperature. The reaction order (n) of TPA obtained from the slope of the lines shown in Fig. 3 is 2.07, 2.09 and 2.03 at the temperature of (463, 473 and 483) K, respectively. Hence, it is considered that the reaction is a second-order reaction when the concentration of BDO is constant. So, the rate equation of the mono-esterification catalyzed by BuSnOOH is described by ′ CA2 = kSn CBm1 CA2 v = kSn

(0.06%, by mass) are used as catalyst separately. The experimental data were listed in Table 2. Table 2

−1

CA0/mol⋅L

(5)

where m1 and kSn are the order of BDO and the rate constant, respectively. 4.1.2 Ti(OBu)4 catalyst Figure 4 is the relationship of lgv0 vs. lgCA0 under different reaction temperatures (463, 473, 478 K) using 0.1% (by mass) Ti(OBu)4 as catalyst. It can be seen from Fig. 4 that a reasonable liner dependence of lgv0 vs. lgCA0 is observed and the lines obtained at different temperatures are almost parallel. The slope of these lines is 0.74, 0.74 and 0.73 at (463, 473 and 478) K, respectively, which shows that the reaction order for TPA is nearly a constant of 0.7 in the range of 463-478 K. So, the rate equation of the esterification catalyzed by Ti(OBu)4 is as follows: v=

′ CA0.7 kTi

=

kTi CBm2 CA0.7

Experiment data of the initial rate (v0) at different concentrations CA0 of BDO (T = 473 K) v0/mmol⋅L−1⋅min−1 0.06% (by mass) Ti(OBu)4

0.1% (by mass) BuSnOOH

0.55

1.099

0.736

1.11

2.028

1.335

1.66

2.859

1.982

2.22

3.895

2.564

Figure 5 shows the relationships of initial rate and initial concentration of BDO. It can be seen from Fig. 5 that the plots of lgv0 versus lgCB0 are linear relationship and the lines obtained for different catalysts are almost parallel. The apparent reaction orders for BDO are m1 = 0.91 and m2 = 0.9, which shows that the reaction order for BDO is nearly 0.9 with different catalyst. Eqs. (5) and (6) can be rewritten as follows:

′ CA2 = kSn CA2 CB0.9 v = kSn

(7)

′ CA0.7 = kTi CA0.7 CB0.9 v = kTi

(8)

(6)

where m2 and kTi are the order of BDO and the rate constant, respectively.

Figure 5 Plots of lgv0 vs. lgCA0 for mono-esterification of TPA and BDO using different catalysts at 473 K ○ Ti(OBu)4; ■ BuSnOOH; —— regression line Figure 4 Plots of lgv0 vs. lgCA0 for mono-esterification of TPA and BDO catalyzed by Ti(OBu)4 at different temperatures ■ T = 478 K; ○ T = 473 K; ▼ T = 463 K ; —— regression line

4.2

Reaction order of BDO

The reaction order of BDO can be estimated from the data of initial rate obtained at different concentrations of BDO. The experiment follows the procedure in Section 3.2. The concentrations of BDO are 0.55, 1.11, 1.66, 2.22 mol·L−1, respectively. The reaction temperature is 473 K and the concentration of TPA is constant (equal to its solubility at 473 K, 0.241 mol·L−1). BuSnOOH (0.1%, by mass) and Ti(OBu)4

4.3

Reaction rate constant and effect of temperature

′ and kTi ′ ) The apparent reaction rate constants ( kSn at different temperatures are obtained from the intercepts of the lines in Figs. 3 and 4. According to Eqs. (7) and (8), the reaction rate constants, kSn and kTi, are calculated, respectively. The kSn values (10−2×L1.9⋅mol−1.9⋅min−1) using 0.1% (by mass) BuSnOOH as catalyst are estimated to be 1.38, 1.88, 2.47 at (463, 473, 483) K, respectively. From Fig. 3 we can find the reaction rate increases with the temperature. Based on the Arrhenius equation, the apparent activation energy (Ea) of the esterification with different catalysts can be estimated from reaction rate constants. Fig. 6 shows the relationship between

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Chin. J. Chem. Eng., Vol. 18, No. 3, June 2010 Table 3

the reaction rate constant and temperature for esterification promoted by 0.1% (by mass) BuSnOOH and indicates that ln kSn against 1/T plot is linear. The fitting equation and its correlation coefficient (R2) is obtained by the least square method as follows:

Reaction rate constant at different catalyst mass concentrations wTi

wTi/%

103kTi/L0.6·mol−0.6·min−1

0.06

5.01

0.10

12.50

( R 2 = 0.999 ) (9)

0.15

15.31

and the Ea value of 53.38 kJ⋅mol−1 is determined for the esterification using BuSnOOH as catalyst.

0.20

16.52

0.30

17.13

0.40

17.41

3

ln kSn = 9.58 − 6.42 × 10 / T

Figure 6 Arrhenius plots for mono-esterification of TPA with BDO catalyzed by BuSnOOH (■) and Ti(OBu)4 (▼) in the temperature rang of 463-483 K

Similarly, the kTi values (10−2×L0.6·mol−0.6·min−1) using 0.1% (by mass) Ti(OBu)4 as catalyst are also estimated to be 0.795, 1.14, 1.37 at (463, 473, 478) K, respectively. Not surprisingly, as the reaction temperature increases, the rate of ester formation also increases. The relationship between the reaction rate constant and temperature for esterification promoted by 0.1% (by mass) Ti(OBu)4 is also shown in Fig. 6, and the linear regression is obtained as follows:

ln kTi = 11.73 − 7.67 × 103 / T

( R 2 = 0.997 ) (10)

Based on the equation, the active energy of mono-esterification catalyzed by Ti(OBu)4 is 63.77 kJ⋅mol−1. The high active energy values of esterification catalyzed by the two catalysts further support the fact that under the reaction conditions used, esterifiction is kinetically controlled and no mass-transfer limitations are present in the present experiments. 4.4

Effect of amount of catalyst

When Ti(OBu)4 was used as catalyst at six levels of mass concentrations wTi (%): 0.06, 0.1, 0.15, 0.2, 0.3, 0.4 at 473 K, the effect of concentration of catalyst was investigated. The corresponding reaction rate constants were also determined by the initial reaction rate method. The results are listed in Table 3. Plots of the reaction rate constant versus concentration of Ti(OBu)4 are shown in Fig. 7, suggesting that the rate of reaction is greatly increased with the

Figure 7 Effect of amount of Ti(OBu)4 on the apparent rate constant at 473 K

mass concentration of Ti(OBu)4 up to 0.15%, and then tend to level off. 5

CONCLUSIONS

The kinetics of esterification reaction from BDO and TPA to MHBT was investigated by using BuSnOOH and Ti(OBu)4 as catalyst, respectively. The reaction orders and rate constants were obtained by the initial rate method. The following conclusions can be drawn from the results: (1) The order of mono-esterification for TPA shifts from 0.7 to 2 as the catalyst is changed from Ti(OBu)4 to BuSnOOH, and that for BDO is 0.9 for the two catalysts; (2) The reaction rate increases with the catalyst concentration, especially at a low catalyst concentration; (3) The activation energy of the esterification promoted by Ti(OBu)4 (63.77 kJ⋅mol−1), is larger than that promoted by BuSnOOH (53.38 kJ·mol−1), which indicates that kTi depends more strongly on temperature. NOMENCLATURE CA CA0 CB Ea k kSn kTi m

concentration of terephthalic acid in solution, mol·L−1 initial concentration of terephthalic acid, mol·L−1 concentration of 1,4-butanediol, mol·L−1 activation energy, kJ⋅mol−1 reaction rate constant reaction rate constant with BuSnOOH as catalyst reaction rate constant with Ti(OBu)4 as catalyst reaction order of BDO

396

m1 m2 n R2 T t v v0 wTi

Chin. J. Chem. Eng., Vol. 18, No. 3, June 2010 reaction order of BDO using BuSnOOH as catalyst reaction order of BDO using Ti(OBu)4 as catalyst reaction order of TPA correlation coefficient absolute temperature, K reaction time, min esterification rate, mmol⋅L−1⋅min−1 initial reaction rate, mmol⋅L−1⋅min−1 catalyst mass concentration, %

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