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Comparison of polyvinylbenzene and polyolefin supported sulphonic acid catalysts in the esterification of acetic acid
Applied Catalysis A: General 184 (1999) 25±32
Comparison of polyvinylbenzene and polyole®n supported sulphonic acid catalysts in the esteri®cation of acetic acid PaÈivi MaÈki-Arvelaa, Tapio Salmia,*, Mats Sundellb, Kenneth Ekmanb, Robert Peltonenb, Juha Lehtonenc Laboratory of Industrial Chemistry, Process Chemistry Group, AÊbo Akademi, FIN-20500, AÊbo, Finland b Smoptech Ltd., VirusmaÈentie 65, FIN-20300, AÊbo, Finland c Neste Engineering, Box 310, FIN-06101, BorgaÊ, Finland
a
Received 16 November 1998; accepted 4 March 1999
Abstract New polyole®n supported sulphonic acid catalysts were used in the esteri®cation of acetic acid with methanol. The new catalysts turned out to be stable and active in esteri®cation. The esteri®cation kinetics was modelled with a mechanistic rate equation, the parameters of which were determined by non-linear regression. The esteri®cation rate constant of the most active modi®cation of the new catalyst was 9.610ÿ10 dm9/(mol2 g min) at 558C, which clearly exceeds the corresponding value obtained with a traditional polyvinylbenzene supported catalyst, 1.510ÿ10 dm9/(mol2 g min). # 1999 Elsevier Science B.V. All rights reserved. Keywords: Esteri®cation; Kinetics; Fibre catalyst Abbreviations: Dtex, length-to-mass ratio of ®bres, 1 Dtex10 km/g; M, concentration unit, 1M1 mol/dm3
1. Introduction Esteri®cation of carboxylic acids with alcohols belongs to classical chemical reactions, the kinetics and equilibria of which have been investigated throughout the history of physical chemistry; the pioneering efforts date back to the time of Berthelot and de Saint Gilles [1]. Today, organic esters are valuable intermediates in several branches of chemical industry.
*Corresponding author. Tel.: +358-2-2154427; +358-2-2154479; e-mail: [email protected]
Esteri®cation proceeds in the absence and in the presence of an added catalyst. In the absence of a catalyst, the reaction is, however, extremely slow, since its rate depends on the autoprotolysis of the carboxylic acid. Therefore the esteri®cations are carried out in the presence of an acid catalyst, which acts as a proton donor to the carboxylic acid. Typical homogeneous acid catalysts are inorganic mineral acids, such as H2SO4, HCl, HI, and ClSO3OH. The disadvantage of mineral acids is their miscibility with the reaction medium, which causes separation problems and equipment corrosion at higher catalyst concentrations. Therefore heterogenized acid catalysts provide an attractive alternative to homoge-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 0 8 1 - 2
26
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
neous catalysts. There exist quite a few publications in literature having polystyrene-based ion exchange ®bre as acid catalyst [2]. New heterogeneous catalysts have been developed in order to surmount the separation problem. Ion exchange resin catalysts have been used for some years in esteri®cation reactions [3,4]. Typical resin catalysts are sulphonic acids ®xed to a polymer carrier, such as polystyrene crosslinked with divinylbenzene (DVB). Several catalysts of this kind are commercially available, e.g. the Amberlyst family. In the use of polyvinylbenzene-based catalysts, the process typically becomes diffusion limited, and the polymer is swelled and deactivated in aggressive reaction media. Compared to polystyrene, polypropene is a chemically very resistant polymer. A new type of polymeric ®bre catalyst, which is commercially available from Smoptech, having sulphonic acid as active group has been recently developed for different types of acid catalysed organic reactions [5]. The endeavour of the current work is to demonstrate the ability of a polypropene-supported sulphonic acid catalyst in the esteri®cation process and to compare its performance with a classical polyvinylbenzene-supported catalyst. Esteri®cation of acetic acid with methanol was selected to the model system. 2. Experimental procedure The esteri®cation of acetic acid was carried out in an isothermal batch reactor equipped with a heating jacket, where glycol from a thermostat batch was circulated. A re¯ux condenser was placed on top of the reactor to prevent the escape of volatile liquid components. Methanol and the catalyst were preheated to the reaction temperature in the reactor, and the esteri®cation was commenced by injecting preheated acetic acid into the mixture. Stoichiometric amounts of acetic acid and methanol were used in all experiments performed at 56±618C. The total liquid volume was about 427 cm3. The following catalysts were used: Amberlyst 15 (Rohm and Haas Co., particle sizes 0.35±1.2 mm, crosslinking density 20% DVB), Smopex 101 (Smoptech) and PE-UHS (Smoptech). Samples were withdrawn from the reactor and the amount of unreacted acetic acid was determined by titration with sodium hydroxide.
3. Catalyst preparation 3.1. Catalyst preparation The ®brous catalysts were prepared by supporting sulphonic acids on either poly(propylene-graft-polystyrene) or poly(ethylene-graft-polystyrene). The graft copolymers were prepared using the following pre-irradiation grafting technique. Polyole®n ®bres were irradiated under an electron beam with a radiation dose of 200 kGy, cut into 4 mm long ®bres and immediately immersed into a nitrogen purged styrene solution. The grafting reaction was allowed to continue for 24 h. The grafted ®bres were then extracted with ethanol and dried at 608C. Two different grafted samples were prepared using either polypropylene ®bres (Polysteen 1 Dtex from Steen and Co, GMBH) or polyethylene ®bres (Borealis HT-®bres 6 Dtex) allowing samples to be prepared from either polyethylene or polypropylene (Smopex 101polypropylene based, PE-UHSpolyethylene based). The PE-UHS ®bres were additionally crosslinked by adding 4% divinylbenzene (Aldrich) to the graft solution. Finally, strong sulphonic acid groups were introduced to the aromatic rings of the graft copolymer by swelling the ®bres in dichloroethane and adding chlorosulphonic acid in a molar ratio 1:1.2 at ambient temperature. The suspension was stirred for 1 h and the ®bres were separated by ®ltration and treated with 1.0 M sodium hydroxide solution. Excess dichloroethane was removed by heating whereafter the ®bres were ®ltered off and treated with 1 M HCl solution. All ion exchangers were pre-treated by three consequent washes with 0.5 M NaOH, distilled water and 0.5 M HCl and converted to H-form with 1 M HCl. Then exchangers were washed with distilled water until the pH of the rinse water exceeded 5.5, air dried overnight and stored in closed bottles. 4. Catalyst characterization 4.1. Ion exchange capacity The ion exchange capacities were determined as follows. Three 0.5 g samples of air dried ion exchanger were weighed into glass bottles and 100 cm3 of 0.1 M NaOH solution containing 50 g/dm3 NaCl, was
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
27
Fig. 1. SEM micrographs of Smopex 101 and PE-UHS catalysts.
added. The bottles were shaken overnight and 20 cm3 samples were titrated. Three parallel 0.5 g samples were dried in oven at 1108C for 24 h to determine the water content of air dried ion exchanger. The ion exchange capacity was determined as average of these results. The acid capacities were following: Smopex 101: 3.2 mmolH/g cat., PE-UHS: 3.7 mmolH/g cat., Amberlyst 15: 4.9 mmolH/g cat. [4]. The acid capacities were included in the calculation of rate constants. 4.2. Microscopic studies The catalysts were investigated with a scanning electron microscope (SEM, LEO S360) and a confocal laser scanning microscope (LEICA TCS 4D). SEM images of the Smopex and PE-UHS catalysts are shown in Fig. 1. The SEM micrographs reveal that the mean particle diameter for Smopex 101 was 20 mm and for PE-UHS 50 mm, respectively. The pictures taken with the laser scanning microscope showed that the catalyst had swelled about 10% under reaction conditions, when it had been exposed to methanol and acetic acid.
increased with increasing temperature and increasing catalyst loading, but when the esteri®cation results obtained with different amounts of the catalyst were plotted against the normalized abscissa (mass of catalyst * reaction time), the curves coincided as demonstrated in Fig. 3. This indicates the absence of external mass transport limitations. The results show that the new acidic polymer catalyst, Smopex 101 (20 mm) is more active than Amberlyst 15. The reaction rate decreased with increasing catalyst crosslinking: with PE-UHS polymer catalyst (50 mm) the reaction proceeded slower than with Smopex 101 (20 mm). Analogous results have been obtained by Rodriguez and SetõÂnek [6]. The quantitative modelling of the kinetic results was based on a plausible reaction mechanism for esteri®cation. The mechanism will be shortly discussed in the sequel, and rate equations will be derived. The sulphonic acid groups on the catalyst surface initiate the esteri®cation by donating a proton to the carboxylic acid molecule: ÿ RCOOH =ÿSO3 H R C O H 2 =ÿSO3 A
5. Kinetic results and mathematical modelling Typical results from kinetic experiments are displayed in Fig. 2. The velocity of esteri®cation
cat
A0
After getting the proton, the carboxylic acid becomes susceptible for a nucleophilic attack by the hydroxyl group (R0OH) after which the reaction continues with water abstraction:
28
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
The proton donating step is usually assumed to be rapid, while the nucleophilic substitution is slow. Provided that the reaction steps succeeding the nucleophilic attack are rapid, the reaction mechanism can be lumped to
The rate-determining step (rds) gives the overall velocity of the process, r k2 cA0 cB ÿ kÿ2 cC cD ccat :
(1)
For the rapid step, the quasi-equilibrium approximation is applied, i.e. c 0 (2) K1 A : cA ccat The concentration of the intermediate (A0 ) is solved from (Eq. (2)) and inserted in the rate equation (1), which becomes r k2 K1 cA cB ÿ kÿ2 cC cD ccat :
(3)
On the other hand, for the overall reaction, the thermodynamic relations KcK1K2 and Kiki/kÿi, i1. . .2 are valid. Using the concentration-based overall equilibrium constant (Kc), the rate equation obtains the form cC cD r k2 K1 ccat cA cB ÿ ; (4) Kc where the lumped constant is k0 k2K1. The concentration-based equilibrium constant (Kc) is related to the thermodynamic equilibrium constant (KT) by KT Kc Kg ; Fig. 2. Esterification kinetics of acetic acid with methanol at 568C (a), 588C (b) and 618C (c) over Amberlyst, Smopex and PE-UHScatalysts. Continuous lines represent the model prediction.
(5)
where Kg is the ratio between the activity coef®cients, Kg
C D :
A B
(6)
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
29
Fig. 3. Esterification of acetic acid with methanol at 568C over different amounts of Amberlyst 15 catalyst: Symbols: (&) 36 g, (*) 20 g and 12 g catalyst (!).
The kinetic model presented above was based on the so-called pseudo-homogeneous approach. It is however, well-known that sulphonic acid-based ion exchange resins are strong adsorbents, particularly for alcohols. The effect of adsorption can be accounted for by including an adsorption term in the kinetic model. Thus the improved kinetic model becomes r
k0 ccat cA cB ÿ cC cD K =KT 1 Ki ci
;
(7)
where Ki denotes the adsorption parameter of compound i. The value of the exponent ( ) depends on the detailed mechanism; 2 for the case that the reaction takes place between adsorbed alcohol and carboxylic acid molecules, whereas 1 for the case that just one of the reagents is adsorbed and reacts with the other reagent from the liquid phase. Since the present work was carried out within a narrow concentration domain, it is not possible to penetrate deeper the reaction mechanism.
Diffusional limitations inside the catalyst particles and ®bres might have an in¯uence on the overall esteri®cation kinetics. Here our aim is, however, to derive a simpli®ed model which enables a comparison of different catalysts; therefore, the diffusional effects inside the particles are lumped with the kinetic effects, and a pseudo-homogeneous model is used. For an arbitrary reacting component in the liquid phase, the mass balance can be written as follows (pseudo-homogeneous model): dni i rmcat : dt
(8)
The symbols are explained in Nomenclature. The amount of substance (ni) is related to the concentration and the liquid volume: niciVL. The change of the liquid volume is included in the modelling, since the liquid volume ± but not the amount of the catalyst ± diminished due to sampling for analysis. Thus we have VL V0L ÿ sVs ;
(9)
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30
Table 1 Build-up of the molecules in the estimation of activity coefficients Molecule
Groupsa
CH3OH CH3COOH CH3COOCH3 H2 O
CH3OH CH3, COOH CH3COO, CH3 H2 O
a
k0 A eÿEA = RT ; KT A0 e
The group members according to Reid et al. [8].
where s and Vs denote the number of samples and the sample volumes, respectively. In practice, the concentration of the carboxylic acid (a) was analysed during the progress of the reaction. The reaction stoichiometry gives the relationship cBcAÿ(c0Aÿc0B) and cCcDc0AÿcA. After introducing these relations and the concentration of A in the rate equation (7), the ®nal model is obtained ( Aÿ1): dcA dt ÿk0 ccat
! c0A ÿ cA 2 Kg cA cA ÿ a ÿ B KT
1KA cA KB cA ÿ a KC KD c0A ÿ cA
;
(10) where ac0Aÿc0B and Bmcat/VL. At t0, cAc0A. The liquid volume is updated according to Eq. (9). In the determination of the kinetic and equilibrium parameters, the following procedure was applied. The activity coef®cients of the reacting compounds were estimated with the UNIFAC-method [7,8]. The buildup blocks of the molecules are listed in Table 1. The density of the liquid phase was taken to constant during the reaction. The validity of this assumption was con®rmed in a previous paper of us [9]. The densities are given in Table 2. The reaction enthalpy at the actual temperature domain was calculated theoretically, and a value of ÿHr0 33±34 kJ/mol was Table 2 The density of the reaction mixture at different temperatures Temperature (8C)
(kg/dm3)
30 60
0.93 0.90
used within the experimental temperature domain [9]. The laws of Arrhenius and van't Hoff were applied on the rate and equilibrium parameters, respectively: ÿHr0 = RT
(11) :
(12)
The adsorption parameters were assumed to be independent of temperature within the actual temperature domain. The parameters determined from the experimental data were: the pre-exponential factor (A), the activation energy (Ea) and the entropy factor (A0 ). Because the model contains very many parameters compared to the experimental information, the following simpli®cation of the model was applied: the adsorption parameters were neglected (a), all of the parameters were set equal (b), and ®nally, the adsorption parameter of methanol (KB) was assumed to be dominant. The values of were: 0 (a), 2 (b), 1 (Eley-Rideal mechanism) or 2 (Langmuir± Hinshelwood mechanism) (c). The differential equation (10) was solved numerically with the backward difference method using the software Odessa [10]. The activity coef®cient calculation and the differential equation solver operated under a parameter estimation routine, which minimized the residual sum of squares, X cA t ÿ ^cA t2 ; (13) Q t
where cA and ^cA denote the experimental and the predicted (Eq. (10)) concentrations, respectively. A combination of the Simplex and Levenberg-Marquardt algorithms was used in the minimization. Typically the parameter estimation was initiated with the Simplex method, which was then switched to the Levenberg-Marquardt method in the proximity of the minimum. The numerical methods were included in the program package Modest [10]. Some estimated values of the activity coef®cients and Kg are listed in Table 3. The table shows that the ratio of the activity coef®cients is de facto changed during the reaction, even though the change is not very prominent. Screening of the different modi®cations of the kinetic model, i.e. the effects of the exponent as well as the adsorption parameters indicated that those parameters can be neglected in modelling: no essential
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
31
Table 3 Estimated activity coefficients and Kg values at different stages of the esterification (T408C) AcOH x
MeOH x
MeOAc x
H2 O x
AcOH
MeOH
MeOAc
H2 O
Kg
0.5 0.275 0.2
0.5 0.275 0.2
0 0.225 0.3
0 0.225 0.3
0.98 0.84 0.785
0.94 1.04 1.06
1.37 1.57 1.61
1.83 1.92 1.96
2.70 3.45 3.79
Table 4 Estimated rate and equilibrium constants for the esterification of acetic acid over three catalysts Error (%) 2
Smopex 101 (R 92.2) PE-UHS (R298.2) Amberlyst (R298.7)
ÿ10
9.610 5.610ÿ10 1.510ÿ10
9.7 4.8 4.7
Ea (kJ/mol) 133 86.3 106
Error (%)
KTmean(558C)
Error (%)
18.7 13.3 10.6
4.0 3.8 5.0
9.6 7.5 6.3
R2degree of explanation; in this case: P P R2 1 ÿ cA;exp ÿ cA model 2 = cA;exp ÿ cA mean 2 ; where `exp'the experimental value, `model'the value predicted by the model, `mean'the mean value of concentrations. 0 k kmean exp ÿEa =R 1=T ÿ 1=T0 . 0 exp ÿHr =R 1=T ÿ 1=T0 . K Kmean T0328 K (550 C). A and A0 (Eqs. (11) and (12)) can be calculated from the above equations.
improvement of the ®t of the model could be achieved by including these parameters. We thus remained with the simplest possible modi®cation (a) of the complete model (Eq. (10)). We determined the kinetic parameters both by using the activity coef®cients and without the activity coef®cients. The ®t of the model was equally good for both cases. The kinetic parameters for the simpli®ed model without activity coef®cients are listed in Table 4. The apparent activation energies are for Amberlyst 15 106 kJ/mol, for Smopex 101 133 kJ/mol, and for PE-UHS 86kJ/mol. The value of the equilibrium constant was estimated to be between 4.0±5.0 at the reference temperature (558C), which is in accordance with previous knowledge. As can be seen from Table 4, the kinetic parameters are well identi®ed, the relative errors of the parameters being always less than 20%. Numerical simulations with the kinetic parameters are displayed in Fig. 2(a)±(c). The proposed kinetic model follows very truly the experimental trends, which suggests that the approach is suf®ciently detailed for the description of the esteri®cation kinetics. The values of the kinetic parameters also con®rm that Smopex 101 is an ef®cient esteri®cation catalyst; it has a rate constant which exceeds that of
Amberlyst 15 by a factor of about 6.4 at the reference (i.e. average) temperature (558C) of the experiments. 6. Nomenclature A A0 a c Ea Hr0 k k0 K m n Q R r s T t
frequency factor (Table 4) pre-exponential factor of the adsorption parameter (dm3/mol) difference between concentrations (Eq. (10)) (mol/dm3) concentration (mol/dm3) activation energy (J/mol) reaction enthalpy (J/mol) rate constant (Table 4) lumped rate constant (Table 4) equilibrium constant (dimensionless) or adsorption parameter (dm3/mol) mass (g) amount of substance (mol) objective function in parameter estimation (mol/dm3)2 gas constant (8.3143J/K mol) reaction rate (mol/g/min) number of samples temperature (K) reaction time (min)
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
32
V x
B
volume (dm3) mole fraction adsorption exponent activity coefficient stoichiometric coefficient catalyst bulk density (g/dm3)
Subscripts and superscripts cat catalyst c concentration based quantity i component index L liquid phase s sample T thermodynamic quantity
a quantity related to the activity coefficients Acknowledgements The authors are indebted Mr. Clifford Ekholm Ê bo Akademi) (Laboratory of Inorganic Chemistry, A for the SEM images and Mr. Thomas Bymark (DepartÊ bo Akademi) for the confocal laser ment of Biology, A scanning microscope pictures. We are grateful to our undergraduate students who performed some of the kinetic experiments.
References [1] P. Berthelot, L. de Saint Gilles, Ann. Chim. Phys. 63 (1863) 385. [2] T. Yoshioka, M. Shimamura, Bull. Chem. Soc. Jpn. 57 (1994) 334±337. [3] M. Streat, Ion Exchange for Industry, Ellis Horwood, Chichester, 1988. [4] Z.P. Xu, K.T. Chuang, Kinetics of acetic acid esterification over ion exchange catalysts, The Canadian J. Chem. Eng. 74 (1996) 493±500. [5] A. KaÈrki, E. Paatero, M. Sundell, Adsorption kinetics of L-phenylalanine on a graft copolymer fibre ion exchanger, in: J. Greig (Ed.), Ion Exchange Developments and Applications, SCI, 1996, pp. 486±493. [6] O. Rodriquez, K. SetõÂnek, Dependence of esterification rates on crosslinking of ion exchange resins used as solid catalysts, J. Catal. 39 (1975) 449±455. [7] Aa. Fredenslund, J. Gmehling, P. Rasmussen, Vapor-Liquid Equilibria using UNIFAC, Elsevier, Amsterdam, 1977. [8] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, McGraw Hill, New York, 1988. [9] R. RoÈnnback, T. Salmi, A. Vuori, H. Haario, J. Lehtonen, A. Sundquist, E. Tirronen, Development of a kinetic model for the esterification of acetic acid with methanol in the presence of a homogeneous acid catalyst, Chem. Eng. Sci. 52 (1997) 3369±3381. [10] H. Haario (1994) Modest ± User's guide, Profmath Oy, Helsinki.
Comparison of polyvinylbenzene and polyole®n supported sulphonic acid catalysts in the esteri®cation of acetic acid PaÈivi MaÈki-Arvelaa, Tapio Salmia,*, Mats Sundellb, Kenneth Ekmanb, Robert Peltonenb, Juha Lehtonenc Laboratory of Industrial Chemistry, Process Chemistry Group, AÊbo Akademi, FIN-20500, AÊbo, Finland b Smoptech Ltd., VirusmaÈentie 65, FIN-20300, AÊbo, Finland c Neste Engineering, Box 310, FIN-06101, BorgaÊ, Finland
a
Received 16 November 1998; accepted 4 March 1999
Abstract New polyole®n supported sulphonic acid catalysts were used in the esteri®cation of acetic acid with methanol. The new catalysts turned out to be stable and active in esteri®cation. The esteri®cation kinetics was modelled with a mechanistic rate equation, the parameters of which were determined by non-linear regression. The esteri®cation rate constant of the most active modi®cation of the new catalyst was 9.610ÿ10 dm9/(mol2 g min) at 558C, which clearly exceeds the corresponding value obtained with a traditional polyvinylbenzene supported catalyst, 1.510ÿ10 dm9/(mol2 g min). # 1999 Elsevier Science B.V. All rights reserved. Keywords: Esteri®cation; Kinetics; Fibre catalyst Abbreviations: Dtex, length-to-mass ratio of ®bres, 1 Dtex10 km/g; M, concentration unit, 1M1 mol/dm3
1. Introduction Esteri®cation of carboxylic acids with alcohols belongs to classical chemical reactions, the kinetics and equilibria of which have been investigated throughout the history of physical chemistry; the pioneering efforts date back to the time of Berthelot and de Saint Gilles [1]. Today, organic esters are valuable intermediates in several branches of chemical industry.
*Corresponding author. Tel.: +358-2-2154427; +358-2-2154479; e-mail: [email protected]
Esteri®cation proceeds in the absence and in the presence of an added catalyst. In the absence of a catalyst, the reaction is, however, extremely slow, since its rate depends on the autoprotolysis of the carboxylic acid. Therefore the esteri®cations are carried out in the presence of an acid catalyst, which acts as a proton donor to the carboxylic acid. Typical homogeneous acid catalysts are inorganic mineral acids, such as H2SO4, HCl, HI, and ClSO3OH. The disadvantage of mineral acids is their miscibility with the reaction medium, which causes separation problems and equipment corrosion at higher catalyst concentrations. Therefore heterogenized acid catalysts provide an attractive alternative to homoge-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 0 8 1 - 2
26
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
neous catalysts. There exist quite a few publications in literature having polystyrene-based ion exchange ®bre as acid catalyst [2]. New heterogeneous catalysts have been developed in order to surmount the separation problem. Ion exchange resin catalysts have been used for some years in esteri®cation reactions [3,4]. Typical resin catalysts are sulphonic acids ®xed to a polymer carrier, such as polystyrene crosslinked with divinylbenzene (DVB). Several catalysts of this kind are commercially available, e.g. the Amberlyst family. In the use of polyvinylbenzene-based catalysts, the process typically becomes diffusion limited, and the polymer is swelled and deactivated in aggressive reaction media. Compared to polystyrene, polypropene is a chemically very resistant polymer. A new type of polymeric ®bre catalyst, which is commercially available from Smoptech, having sulphonic acid as active group has been recently developed for different types of acid catalysed organic reactions [5]. The endeavour of the current work is to demonstrate the ability of a polypropene-supported sulphonic acid catalyst in the esteri®cation process and to compare its performance with a classical polyvinylbenzene-supported catalyst. Esteri®cation of acetic acid with methanol was selected to the model system. 2. Experimental procedure The esteri®cation of acetic acid was carried out in an isothermal batch reactor equipped with a heating jacket, where glycol from a thermostat batch was circulated. A re¯ux condenser was placed on top of the reactor to prevent the escape of volatile liquid components. Methanol and the catalyst were preheated to the reaction temperature in the reactor, and the esteri®cation was commenced by injecting preheated acetic acid into the mixture. Stoichiometric amounts of acetic acid and methanol were used in all experiments performed at 56±618C. The total liquid volume was about 427 cm3. The following catalysts were used: Amberlyst 15 (Rohm and Haas Co., particle sizes 0.35±1.2 mm, crosslinking density 20% DVB), Smopex 101 (Smoptech) and PE-UHS (Smoptech). Samples were withdrawn from the reactor and the amount of unreacted acetic acid was determined by titration with sodium hydroxide.
3. Catalyst preparation 3.1. Catalyst preparation The ®brous catalysts were prepared by supporting sulphonic acids on either poly(propylene-graft-polystyrene) or poly(ethylene-graft-polystyrene). The graft copolymers were prepared using the following pre-irradiation grafting technique. Polyole®n ®bres were irradiated under an electron beam with a radiation dose of 200 kGy, cut into 4 mm long ®bres and immediately immersed into a nitrogen purged styrene solution. The grafting reaction was allowed to continue for 24 h. The grafted ®bres were then extracted with ethanol and dried at 608C. Two different grafted samples were prepared using either polypropylene ®bres (Polysteen 1 Dtex from Steen and Co, GMBH) or polyethylene ®bres (Borealis HT-®bres 6 Dtex) allowing samples to be prepared from either polyethylene or polypropylene (Smopex 101polypropylene based, PE-UHSpolyethylene based). The PE-UHS ®bres were additionally crosslinked by adding 4% divinylbenzene (Aldrich) to the graft solution. Finally, strong sulphonic acid groups were introduced to the aromatic rings of the graft copolymer by swelling the ®bres in dichloroethane and adding chlorosulphonic acid in a molar ratio 1:1.2 at ambient temperature. The suspension was stirred for 1 h and the ®bres were separated by ®ltration and treated with 1.0 M sodium hydroxide solution. Excess dichloroethane was removed by heating whereafter the ®bres were ®ltered off and treated with 1 M HCl solution. All ion exchangers were pre-treated by three consequent washes with 0.5 M NaOH, distilled water and 0.5 M HCl and converted to H-form with 1 M HCl. Then exchangers were washed with distilled water until the pH of the rinse water exceeded 5.5, air dried overnight and stored in closed bottles. 4. Catalyst characterization 4.1. Ion exchange capacity The ion exchange capacities were determined as follows. Three 0.5 g samples of air dried ion exchanger were weighed into glass bottles and 100 cm3 of 0.1 M NaOH solution containing 50 g/dm3 NaCl, was
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
27
Fig. 1. SEM micrographs of Smopex 101 and PE-UHS catalysts.
added. The bottles were shaken overnight and 20 cm3 samples were titrated. Three parallel 0.5 g samples were dried in oven at 1108C for 24 h to determine the water content of air dried ion exchanger. The ion exchange capacity was determined as average of these results. The acid capacities were following: Smopex 101: 3.2 mmolH/g cat., PE-UHS: 3.7 mmolH/g cat., Amberlyst 15: 4.9 mmolH/g cat. [4]. The acid capacities were included in the calculation of rate constants. 4.2. Microscopic studies The catalysts were investigated with a scanning electron microscope (SEM, LEO S360) and a confocal laser scanning microscope (LEICA TCS 4D). SEM images of the Smopex and PE-UHS catalysts are shown in Fig. 1. The SEM micrographs reveal that the mean particle diameter for Smopex 101 was 20 mm and for PE-UHS 50 mm, respectively. The pictures taken with the laser scanning microscope showed that the catalyst had swelled about 10% under reaction conditions, when it had been exposed to methanol and acetic acid.
increased with increasing temperature and increasing catalyst loading, but when the esteri®cation results obtained with different amounts of the catalyst were plotted against the normalized abscissa (mass of catalyst * reaction time), the curves coincided as demonstrated in Fig. 3. This indicates the absence of external mass transport limitations. The results show that the new acidic polymer catalyst, Smopex 101 (20 mm) is more active than Amberlyst 15. The reaction rate decreased with increasing catalyst crosslinking: with PE-UHS polymer catalyst (50 mm) the reaction proceeded slower than with Smopex 101 (20 mm). Analogous results have been obtained by Rodriguez and SetõÂnek [6]. The quantitative modelling of the kinetic results was based on a plausible reaction mechanism for esteri®cation. The mechanism will be shortly discussed in the sequel, and rate equations will be derived. The sulphonic acid groups on the catalyst surface initiate the esteri®cation by donating a proton to the carboxylic acid molecule: ÿ RCOOH =ÿSO3 H R C O H 2 =ÿSO3 A
5. Kinetic results and mathematical modelling Typical results from kinetic experiments are displayed in Fig. 2. The velocity of esteri®cation
cat
A0
After getting the proton, the carboxylic acid becomes susceptible for a nucleophilic attack by the hydroxyl group (R0OH) after which the reaction continues with water abstraction:
28
P. MaÈki-Arvela et al. / Applied Catalysis A: General 184 (1999) 25±32
The proton donating step is usually assumed to be rapid, while the nucleophilic substitution is slow. Provided that the reaction steps succeeding the nucleophilic attack are rapid, the reaction mechanism can be lumped to
The rate-determining step (rds) gives the overall velocity of the process, r k2 cA0 cB ÿ kÿ2 cC cD ccat :
(1)
For the rapid step, the quasi-equilibrium approximation is applied, i.e. c 0 (2) K1 A : cA ccat The concentration of the intermediate (A0 ) is solved from (Eq. (2)) and inserted in the rate equation (1), which becomes r k2 K1 cA cB ÿ kÿ2 cC cD ccat :
(3)
On the other hand, for the overall reaction, the thermodynamic relations KcK1K2 and Kiki/kÿi, i1. . .2 are valid. Using the concentration-based overall equilibrium constant (Kc), the rate equation obtains the form cC cD r k2 K1 ccat cA cB ÿ ; (4) Kc where the lumped constant is k0 k2K1. The concentration-based equilibrium constant (Kc) is related to the thermodynamic equilibrium constant (KT) by KT Kc Kg ; Fig. 2. Esterification kinetics of acetic acid with methanol at 568C (a), 588C (b) and 618C (c) over Amberlyst, Smopex and PE-UHScatalysts. Continuous lines represent the model prediction.
(5)
where Kg is the ratio between the activity coef®cients, Kg
C D :
A B
(6)
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Fig. 3. Esterification of acetic acid with methanol at 568C over different amounts of Amberlyst 15 catalyst: Symbols: (&) 36 g, (*) 20 g and 12 g catalyst (!).
The kinetic model presented above was based on the so-called pseudo-homogeneous approach. It is however, well-known that sulphonic acid-based ion exchange resins are strong adsorbents, particularly for alcohols. The effect of adsorption can be accounted for by including an adsorption term in the kinetic model. Thus the improved kinetic model becomes r
k0 ccat cA cB ÿ cC cD K =KT 1 Ki ci
;
(7)
where Ki denotes the adsorption parameter of compound i. The value of the exponent ( ) depends on the detailed mechanism; 2 for the case that the reaction takes place between adsorbed alcohol and carboxylic acid molecules, whereas 1 for the case that just one of the reagents is adsorbed and reacts with the other reagent from the liquid phase. Since the present work was carried out within a narrow concentration domain, it is not possible to penetrate deeper the reaction mechanism.
Diffusional limitations inside the catalyst particles and ®bres might have an in¯uence on the overall esteri®cation kinetics. Here our aim is, however, to derive a simpli®ed model which enables a comparison of different catalysts; therefore, the diffusional effects inside the particles are lumped with the kinetic effects, and a pseudo-homogeneous model is used. For an arbitrary reacting component in the liquid phase, the mass balance can be written as follows (pseudo-homogeneous model): dni i rmcat : dt
(8)
The symbols are explained in Nomenclature. The amount of substance (ni) is related to the concentration and the liquid volume: niciVL. The change of the liquid volume is included in the modelling, since the liquid volume ± but not the amount of the catalyst ± diminished due to sampling for analysis. Thus we have VL V0L ÿ sVs ;
(9)
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Table 1 Build-up of the molecules in the estimation of activity coefficients Molecule
Groupsa
CH3OH CH3COOH CH3COOCH3 H2 O
CH3OH CH3, COOH CH3COO, CH3 H2 O
a
k0 A eÿEA = RT ; KT A0 e
The group members according to Reid et al. [8].
where s and Vs denote the number of samples and the sample volumes, respectively. In practice, the concentration of the carboxylic acid (a) was analysed during the progress of the reaction. The reaction stoichiometry gives the relationship cBcAÿ(c0Aÿc0B) and cCcDc0AÿcA. After introducing these relations and the concentration of A in the rate equation (7), the ®nal model is obtained ( Aÿ1): dcA dt ÿk0 ccat
! c0A ÿ cA 2 Kg cA cA ÿ a ÿ B KT
1KA cA KB cA ÿ a KC KD c0A ÿ cA
;
(10) where ac0Aÿc0B and Bmcat/VL. At t0, cAc0A. The liquid volume is updated according to Eq. (9). In the determination of the kinetic and equilibrium parameters, the following procedure was applied. The activity coef®cients of the reacting compounds were estimated with the UNIFAC-method [7,8]. The buildup blocks of the molecules are listed in Table 1. The density of the liquid phase was taken to constant during the reaction. The validity of this assumption was con®rmed in a previous paper of us [9]. The densities are given in Table 2. The reaction enthalpy at the actual temperature domain was calculated theoretically, and a value of ÿHr0 33±34 kJ/mol was Table 2 The density of the reaction mixture at different temperatures Temperature (8C)
(kg/dm3)
30 60
0.93 0.90
used within the experimental temperature domain [9]. The laws of Arrhenius and van't Hoff were applied on the rate and equilibrium parameters, respectively: ÿHr0 = RT
(11) :
(12)
The adsorption parameters were assumed to be independent of temperature within the actual temperature domain. The parameters determined from the experimental data were: the pre-exponential factor (A), the activation energy (Ea) and the entropy factor (A0 ). Because the model contains very many parameters compared to the experimental information, the following simpli®cation of the model was applied: the adsorption parameters were neglected (a), all of the parameters were set equal (b), and ®nally, the adsorption parameter of methanol (KB) was assumed to be dominant. The values of were: 0 (a), 2 (b), 1 (Eley-Rideal mechanism) or 2 (Langmuir± Hinshelwood mechanism) (c). The differential equation (10) was solved numerically with the backward difference method using the software Odessa [10]. The activity coef®cient calculation and the differential equation solver operated under a parameter estimation routine, which minimized the residual sum of squares, X cA t ÿ ^cA t2 ; (13) Q t
where cA and ^cA denote the experimental and the predicted (Eq. (10)) concentrations, respectively. A combination of the Simplex and Levenberg-Marquardt algorithms was used in the minimization. Typically the parameter estimation was initiated with the Simplex method, which was then switched to the Levenberg-Marquardt method in the proximity of the minimum. The numerical methods were included in the program package Modest [10]. Some estimated values of the activity coef®cients and Kg are listed in Table 3. The table shows that the ratio of the activity coef®cients is de facto changed during the reaction, even though the change is not very prominent. Screening of the different modi®cations of the kinetic model, i.e. the effects of the exponent as well as the adsorption parameters indicated that those parameters can be neglected in modelling: no essential
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Table 3 Estimated activity coefficients and Kg values at different stages of the esterification (T408C) AcOH x
MeOH x
MeOAc x
H2 O x
AcOH
MeOH
MeOAc
H2 O
Kg
0.5 0.275 0.2
0.5 0.275 0.2
0 0.225 0.3
0 0.225 0.3
0.98 0.84 0.785
0.94 1.04 1.06
1.37 1.57 1.61
1.83 1.92 1.96
2.70 3.45 3.79
Table 4 Estimated rate and equilibrium constants for the esterification of acetic acid over three catalysts Error (%) 2
Smopex 101 (R 92.2) PE-UHS (R298.2) Amberlyst (R298.7)
ÿ10
9.610 5.610ÿ10 1.510ÿ10
9.7 4.8 4.7
Ea (kJ/mol) 133 86.3 106
Error (%)
KTmean(558C)
Error (%)
18.7 13.3 10.6
4.0 3.8 5.0
9.6 7.5 6.3
R2degree of explanation; in this case: P P R2 1 ÿ cA;exp ÿ cA model 2 = cA;exp ÿ cA mean 2 ; where `exp'the experimental value, `model'the value predicted by the model, `mean'the mean value of concentrations. 0 k kmean exp ÿEa =R 1=T ÿ 1=T0 . 0 exp ÿHr =R 1=T ÿ 1=T0 . K Kmean T0328 K (550 C). A and A0 (Eqs. (11) and (12)) can be calculated from the above equations.
improvement of the ®t of the model could be achieved by including these parameters. We thus remained with the simplest possible modi®cation (a) of the complete model (Eq. (10)). We determined the kinetic parameters both by using the activity coef®cients and without the activity coef®cients. The ®t of the model was equally good for both cases. The kinetic parameters for the simpli®ed model without activity coef®cients are listed in Table 4. The apparent activation energies are for Amberlyst 15 106 kJ/mol, for Smopex 101 133 kJ/mol, and for PE-UHS 86kJ/mol. The value of the equilibrium constant was estimated to be between 4.0±5.0 at the reference temperature (558C), which is in accordance with previous knowledge. As can be seen from Table 4, the kinetic parameters are well identi®ed, the relative errors of the parameters being always less than 20%. Numerical simulations with the kinetic parameters are displayed in Fig. 2(a)±(c). The proposed kinetic model follows very truly the experimental trends, which suggests that the approach is suf®ciently detailed for the description of the esteri®cation kinetics. The values of the kinetic parameters also con®rm that Smopex 101 is an ef®cient esteri®cation catalyst; it has a rate constant which exceeds that of
Amberlyst 15 by a factor of about 6.4 at the reference (i.e. average) temperature (558C) of the experiments. 6. Nomenclature A A0 a c Ea Hr0 k k0 K m n Q R r s T t
frequency factor (Table 4) pre-exponential factor of the adsorption parameter (dm3/mol) difference between concentrations (Eq. (10)) (mol/dm3) concentration (mol/dm3) activation energy (J/mol) reaction enthalpy (J/mol) rate constant (Table 4) lumped rate constant (Table 4) equilibrium constant (dimensionless) or adsorption parameter (dm3/mol) mass (g) amount of substance (mol) objective function in parameter estimation (mol/dm3)2 gas constant (8.3143J/K mol) reaction rate (mol/g/min) number of samples temperature (K) reaction time (min)
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V x
B
volume (dm3) mole fraction adsorption exponent activity coefficient stoichiometric coefficient catalyst bulk density (g/dm3)
Subscripts and superscripts cat catalyst c concentration based quantity i component index L liquid phase s sample T thermodynamic quantity
a quantity related to the activity coefficients Acknowledgements The authors are indebted Mr. Clifford Ekholm Ê bo Akademi) (Laboratory of Inorganic Chemistry, A for the SEM images and Mr. Thomas Bymark (DepartÊ bo Akademi) for the confocal laser ment of Biology, A scanning microscope pictures. We are grateful to our undergraduate students who performed some of the kinetic experiments.
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