Ind. Eng. Chem. Res. 1997, 36, 3-10
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of Liquid-Phase Esterification Catalyzed by Acidic Resins Marco Mazzotti,† Bernardo Neri, Davino Gelosa,‡ Alexey Kruglov,§ and Massimo Morbidelli* Laboratorium fu¨ r Technische Chemie LTC, ETH Zentrum CAB C40, Universita¨ tstrasse 6, CH-8092 Zu¨ rich, Switzerland
The characteristics of ion-exchange resins provide the basis for many processes of practical interest involving both sorption separations and catalytic reactions. The optimal design and operation of these processes require a proper understanding of the equilibrium behavior of multicomponent liquid mixtures in contact with cross-linked polymeric resins, in terms of both the amount and composition of the sorbed mixture. For this, a model which describes the equilibrium between a polymer phase, described through the extended Flory-Huggins theory, and a liquid phase, which does not contain the polymer, has been developed. This has then been coupled with a kinetic model describing the catalytic reaction inside the resin particles. The model has been validated through an appropriate experimental analysis involving both equilibrium partitioning and reactive experiments, for the case of a highly cross-linked sulfonated resin in the presence of various mixtures of the components involved in the esterification of ethanol with acetic acid. The results indicate the ability of the resin not only to catalyze the esterification reaction but also to shift the corresponding equilibrium conversion, due to its swelling capability. This approach is believed to apply to a wide class of reactions catalyzed by polymeric resins, and it is suitable for the optimal design of the corresponding processes. 1. Introduction When a dry polymeric material is brought into contact with a liquid, it swells; i.e., a portion of the liquid component is sorbed by the resin up to reaching equilibrium with the liquid phase (cf. Flory, 1953). Several applications are based on this phenomenon such as, for example, gel collapse (Tanaka, 1978), environmentally sensitive membranes (Siegel and Firestone, 1988), or controlled drug release (Hoffman, 1987). In the case where the liquid phase is constituted of a multicomponent mixture, in principle all components are sorbed but each of them to a different extent. Thus, we have a selective swelling which results in substantial partitioning of the liquid components between the polymeric and the liquid phases. The partitioning ratio of each component between the two phases, as well as the overall relative increase of the volume of the resin (i.e., the swelling ratio), depends upon the physicochemical characteristics of the system. These include not only the liquid phase composition and temperature but also the composition and structure of the polymeric resin, i.e., the type of polymer, the degree of cross-linking, the presence of functional groups, and so on. Although all these different aspects of the multicomponent partition equilibrium are discussed in * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: +41-1-6323034. Fax: +41-1-16321082. † Permanent address: Dipartimento di Chimica, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy. ‡ Permanent address: Dipartimento di Chimica Fisica Applicata, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy. § Permanent address: Lexan Technology, General Electric Co., Mt. Vernon, IN 47620. S0888-5885(96)00450-2 CCC: $14.00
the classical book by Flory (1953), a consolidated quantitative treatment is still missing. On the other hand, there are a number of processes of practical interest which are based on the resin selective swelling in multicomponent liquid mixtures. Examples include the use of polymeric resins to remove organic compounds from contaminated water (cf. Cornel and Sontheimer, 1986; Garcia and King, 1989; Gusler et al., 1993), to remove small amounts of water from organic liquids (Wymore, 1962; Sinegra and Carta, 1987), and to recover sucrose from beet or cane molasses (Hongisto, 1977; Munir, 1976). In this work we focus on the use of these resins as heterogeneous catalysts, due to the acidic (or basic) nature of the added functional groups. In this application the selective swelling of the polymer matrix leads to values of the reactant to catalyst ratio different from those typically achieved in homogeneous catalytic systems under similar conditions (Pitochelli, 1980). Moreover, the relative compositions of both reactants and products in the reaction locus (i.e., the polymeric resin) and in the liquid bulk are, in general, quite different. In particular, if these exhibit different affinities toward the resin, such that one of the reaction products tends to be selectively removed from the reaction locus, then the rate of the inverse reaction is minimized, and it is possible to achieve higher equilibrium conversions than those obtained with homogeneous catalysts. All of this clearly affects the reaction performance in terms of both yield and selectivity; thus, a clear understanding of selective swelling is of fundamental importance also for the optimal design of these materials when used as catalysts. Particularly relevant in applications are the reactions between olefins and alcohols catalyzed by acidic resins, as, for instance, those involved in the production of © 1997 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997
Table 1. Physicochemical Characteristics of the Resin (Amberlyst 15) skeleton type structure functional groups ionic form cross-linking degree particle size internal porosity concentration of acid sites bulk density polymer density
styrene-divinylbenzene strong acid macroreticular sulfonic (SO3H) hydrogen 20% 0.35-1.2 mm 0.36 4.53 mequiv/g of dry resin 600 kg/m3 1410 kg/m3
methyl tert-butyl ether (MTBE) (Miracca et al., 1996) and ethyl tert-butyl ether (ETBE) (cf. Jensen and Datta, 1995) fuel oxygenates, which exhibit reaction orders strongly dependent upon the overall composition of the reacting mixture. These findings are justified by the selective swelling of the resin, which, for instance, in the case of MTBE, prefers methanol with respect to isobutene (Ancillotti et al., 1978). Other examples of ether production using sulfonic ion-exchange resins include tert-amyl methyl ether (TAME), tert-amyl ethyl ether (TAEE) (cf. Kitchaiya and Datta, 1995), and 3-methyl-3-ethoxypentane (THEE3) (Zhang and Datta, 1996). Esterifications and transesterifications constitute another class of important reactions catalyzed by acidic resins. Examples include, among others, the esterification of acetic acid with ethanol to produce ethyl acetate and that of lactic acid with butanol (cf. Dassy et al., 1994). The ability of the resin to simultaneously catalyze the esterification reaction and separate the reaction products (water and ester) has been exploited to develop both batch (Sardin and Villermaux, 1979) and continuous (Kawase et al., 1996; Mazzotti et al., 1996) chromatographic reactors, where the equilibrium conversion is driven to completion. In all the cases above, the reaction performance is strongly affected by the selective swelling of the resin. However, in most kinetic models reported in literature for these reactions, selective swelling was not considered and the classical Langmuir/Hinshelwood reaction rate expression used in heterogeneous catalysis was adopted (cf. Rehfinger and Hoffman, 1990; Fite´ et al., 1994). In this work we consider, as a model reaction, the synthesis of ethyl acetate catalyzed by a highly cross-linked polystyrene-divinylbenzene resin functionalized with sulfonic groups (i.e., Amberlyst 15 by Rohm and Haas). In order to develop the kinetic model of this system, we first describe the partition of the various components between the liquid bulk and the resin. Then, we model the rate of the esterification reaction using the concentration values in the reaction locus (i.e., the resin). The reliability of this approach, which we believe to be rather general and valid for all the classes of more complex reactions mentioned above, is tested by comparison with appropriate experimental data. 2. Experimental Procedure Reagent-grade ethanol, ethyl acetate, acetic acid, and distilled water have been used in all the experimental runs. The relevant characteristics of the used resin (Amberlyst 15) are summarized in Table 1. The phase equilibrium partitioning data have been obtained by contacting a given amount of the dry resin with either single chemical species or binary mixtures at 25 °C. Binary mixtures were constituted of waterethanol, water-acetic acid, ethyl acetate-ethanol, and
ethyl acetate-acetic acid, thus avoiding the two pairs which make esterification or hydrolysis occur. After physical equilibrium conditions were attained, the volume of the swollen resin and the liquid phase composition were measured. In particular, ethyl acetate and ethanol were analyzed through gas chromatography, and water was analyzed through by Karl-Fischer titration and acetic acid through sodium hydroxide titration. The rate of the chemical reaction has been studied through experimental runs conducted in an isothermal well-stirred batch reactor. The conversion behavior as a function of time has been monitored through periodic sampling of the liquid phase and measurements of its composition. The effect of various variables on the rate of reaction has been investigated. These include temperature, the initial reactants to resin ratio, and the initial reactants’ relative composition. 3. Development of the Kinetic Model As we have pointed out in the introduction, the selective sorption, i.e., selective swelling, of the resin has a strong influence on the catalytic behavior of the system. Thus, it is necessary to develop a thermodynamic model which describes the phase equilibrium partition of the various involved components. This will then be used to model the reaction rate kinetics in the polymer phase. 3.1. Multicomponent Phase Equilibria. The equilibrium model to be developed should describe both the amount of liquid phase sorbed at equilibrium conditions by the resin through the selective swelling process, and its composition. For this, selective swelling is described in terms of the equilibrium between a multicomponent polymeric phase containing all compounds and a liquid phase which does not contain the polymer. The activity of each component in the liquid phase has been evaluated using the UNIFAC group contribution method (Fredenslund et al., 1977), which does not involve any adjustable parameter. The reliability of the results of this model for the specific system under examination has been checked by comparison with experimental vapor-liquid equilibrium data (Gmehling and Onken, 1977). The behavior of the polymeric phase has been described in the framework of the extended FloryHuggins model. In particular, the activity aPi of the ith component of a multicomponent polymeric solution is given by N+1
ln aPi ) 1 + ln vi -
N+1
mijvj + ∑ χijvj ∑ j)1 j)1
N+1 j-1
(
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7
mikvjvkχkj + ηVi vp1/3 - vp ∑ ∑ 3 6 j)1 k)1
)
(1)
where vi and vp are the volume fractions of the ith component and of the polymer in the polymer phase, respectively; χij represents the molecular interaction between components i and j; N is the number of components in the system (excluding polymer which is the (N + 1)th species); Vi is the molar volume of the ith component and mij ) Vi/Vj (mip ) 0). The parameter η represents the number of moles of active elastic chains per unit volume (Flory, 1953) and is defined as η ) Fp/ Mc, where Fp is the polymer density. The parameter Mc represents the molar mass of the active elastic chain; accounting for the cross-link degree and the concentra-
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 5
tion of sulfonic acid sites in the resin under examination (see Table 1), its theoretical value is estimated to be approximately 900 g/mol, thus yielding η ) 0.0015 mol/ cm3. In eq 1, the first three terms in the right-hand side represent the entropic contribution to the free energy of mixing (Flory, 1942; Huggins, 1942), computed by regarding the system as a lattice with interchangeable polymer units and solvent molecules. The fourth and fifth terms account for the enthalpy of mixing (Flory, 1953), whereas the last term represents the elastic deformation contribution, which restricts swelling of the cross-linked resin. Among several forms of the elastic contribution proposed in the literature, in eq 1 we have adopted the one developed by Gusler and Cohen (1994) for the case of isotropic swelling of polymer networks consisting of short chains (
