Sorptive and Catalytic Properties of Partially Sulfonated Resins

2658

Ind. Eng. Chem. Res. 2004, 43, 2658-2668

Sorptive and Catalytic Properties of Partially Sulfonated Resins Florian Lode,†,§ Sergio Freitas,† Marco Mazzotti,‡ and Massimo Morbidelli*,† Institute for Chemical and Bioengineering and Institute for Process Engineering, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland

Sulfonic poly(styrene-co-divinylbenzene) ion-exchange resins are commonly employed as supported acid catalysts or as selective sorbents in a number of different applications. Using the reaction of acetic acid and methanol to form methyl acetate and water as a model system, the competitive sorption behavior of the corresponding nonreactive binary pairs was studied for macroporous, highly cross-linked resins of different acid loading. It was found that a reduction of the acid loading decreases the resins’ selectivity toward the sorption of water and increases its affinity toward less polar species, e.g., methyl acetate. To describe the sorption behavior a phase equilibrium model based on the Flory-Huggins theory of polymer swelling was developed, taking into account the hydration of the acid sites upon the sorption of water. The ability of the model to represent the competitive sorption behavior as a function of the resin’s acid loading has been demonstrated by comparison with various sets of experimental equilibrium data. Finally, initial work to characterize the catalytic performance for different degrees of sulfonation is presented, and conclusions toward the application of these resins as stationary phases for reactive chromatography are drawn. 1. Introduction The sorption of chemical species from liquid mixtures onto polymeric materials increases the volume of the latter. This “swelling” process is utilized in manifold applications including the removal of organic compounds from contaminated water,1,2 the dehydration of organic liquids3, the controlled drug release,4 and the use of polymeric materials as heterogeneous catalysts and selective sorbents in chromatographic reactors.5-8 Each of these applications exhibits different characteristics, and best performances are expected by the use of resins with tailor-made properties. Therefore, in order to optimize the properties of the sorbent phase for a specific application, a detailed understanding of the dependence of the sorptive performance onto the properties of the resin, such as the cross-linking or the derivatization degree, is necessary. An illustrative example arises from the use of sulfonated poly(styrene-co-divinylbenzene) ion-exchange resins as stationary phases for reactive chromatography. Here, the resins combine the properties of a heterogeneous acid catalyst and those of a selective sorbent for the reaction products. Their use offers the possibility to drive conversion beyond the limitations imposed by chemical equilibrium in conventional reactors. Typically, these processes are considered for application to condensation reactions, e.g., esterifications, using alcohols as desorbents. In this case the resin should obviously exhibit a strong selectivity with respect to the two products of the reaction, i.e., water and the ester. On the other hand, with respect to the cost of the process it is clear that the amount of alcohol used as a desorbent plays a crucial role.8 When considering a continuous chromatographic process, as for example SMB units, two elements * To whom correspondence should be addressed. Tel.: +411-6323034. Fax: +41-1-6321082. E-mail: Morbidelli@tech. chem.ethz.ch. † Institute for Chemical and Bioengineering. ‡ Institute for Process Engineering. § Current address: Degussa AG, D-63457 Hanau, Germany.

contribute to the desorbent consumption. The first element is the amount of solvent needed for the complete desorption of water within the resin regeneration step. This amount is determined by the competitive sorption behavior of water and the alcohol, and it is improving if the selectivity for water uptake is reduced. The second element is the amount of alcohol recovered from the solvent regeneration step. This is in turn only determined by the competitive sorption behavior of the least retained product (i.e., the ester) and the alcohol, with higher selectivity toward the ester allowing for better solvent recovery. Regarding their sorptive properties, resins of high sulfonation and cross-linking degree exhibit a strong affinity toward water, while less polar compounds like esters are hardly sorbed at all. Accordingly, for reactive chromatography applications the consumption of alcohol as a desorbent is very high when these resins are employed. In order to optimize resin performance, focus has to be directed toward reducing the selectivity for water and improving the affinity toward the ester, while maintaining sufficient catalytic activity. Besides the resin’s cross-linking degree10 and its ionic form,3,10 the sulfonic acid capacity of the resin is expected to strongly influence its sorptive properties, considering that water is interacting preferentially with the sulfonic acid sites. In this work, the influence of ion-exchange capacity, that is, the number of sulfonic acid groups per unit mass of dry resin, on the sorptive properties and the catalytic activity of highly cross-linked poly(styrene-co-divinylbenzene) resins is investigated. The synthesis of methyl acetate from methanol and acetic acid has been selected as a model system. First, the competitive sorption behavior of water and methanol is studied for each of five different resins with acid loadings ranging from 0.8 to 5.0 mol/kg, that have been obtained by thermal desulfonation of a commercially available ion-exchange resin. To rationalize the sorption behavior observed, a model based on the Flory-Huggins theory of polymer swelling16 is developed and trends in the model parameters are identified and rationalized.

10.1021/ie030664s CCC: $27.50 © 2004 American Chemical Society Published on Web 04/22/2004

Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 2659

Second, the competitive sorption behavior of the remaining nonreactive binary pairs of the species involved in the methyl acetate synthesis is investigated for three resins of different sulfonation degree, i.e., 0.9, 1.9, and 5.0 mol/kg, respectively. The resulting data are modeled on the basis of the developed Flory-Huggins approach, utilizing for the model parameters the same empirical correlations obtained for the water-methanol system. Finally, the catalytic activity of the latter resins is investigated, and conclusions toward their performance in chromatographic reactors are drawn. 2. Experimental Section 2.1. Preparation of the Resins. Conventionally, sulfonated poly(styrene-co-divinylbenzene) ion-exchange resins are prepared from the nonderivatized copolymer backbone through treatment with sulfuric acid.

R-C6H4-H + H2SO4 h R-C6H4-SO3H + H2O (1) Under conditions of long contact times between polymer and sulfuric acid this method yields a monosulfonation of all styrenic groups present and thus a spatially uniform molar site density within the resin particle. The resulting acid loading of the resin depends on the cross-linking degree of the polymer matrix, and for resins containing 20% of divinylbenzene a value of about 5 mol of acid groups/kg of dry resin is obtained. While for typical ion-exchange applications, e.g., water softening, a high capacity of the resin and accordingly the highest sulfonation degree possible is sought, different degrees of derivatization can be advantageous for operations utilizing the resin as adsorbent or heterogeneous catalyst, as discussed in the above section. On an industrial scale, the most reasonable way to obtain such a partial sulfonation is the proper adjustment of the sulfonation conditions in terms of residence times, reaction temperature, and swelling agent. For the laboratory scale quantities required in this work, however, a partial cleavage of the sulfonic acid moieties from the monosulfonated polymer matrix through thermal treatment is more convenient. The desulfonation technique applied here has been shown to lead to a uniform distribution of sulfonic acid sites over the resin particle,11,12 with the acid loading decreasing slowly for increasing reaction times. The polymer matrix itself remains unaffected. In particular, six resins of different sulfonation degree have been obtained starting from a commercially available, monosulfonated ion-exchange resin through the following procedure. Amberlyst-15, a macroporous ionexchange resin (5 mol/kg), was washed with an excess amount of water, which was of doubly distilled quality for this and all of the experiments described in the following. Suspended in water at a concentration of about 0.1 g of wet resin/g of water,12 the resin was transferred to a PTFE-lined autoclave and heated to 240 °C for a certain period of time, depending on the desired level of desulfonation. After removal from the autoclave the resin was again washed with water and dried at a temperature of 130 °C and an absolute pressure of 15 kPa for 16 h. The ion-exchange capacity of each resin was measured by titration with NaOH, while the density was determined using a Micromeritics AccuPyc1330 gas pycnometer, operated using helium.

Table 1. Acid Loading and Density of the Resins Obtained by Partial Thermal Desulfonation of Amberlyst-15 (R50) resin name:

R50

R34

R24

R19

R12

R09

R08

acid loading [mol/kg] 5.0 3.4 2.4 1.9 1.2 0.9 0.8 density [103kg/m3] 1.40 1.30 1.18 1.18 1.14 1.11 1.11

Following the procedure described above, the resins listed in Table 1 were obtained with R50 denoting the starting material, that is, the unmodified Amberlyst15. In order to verify that the distribution of acid sites over the particle diameter remained uniform throughout the thermal treatment, representative samples of the resins have been examined by energy dispersive analysis (EDX) in a scanning electron microscope. Using this technique, the concentration of sulfur can be measured along a straight line within the cross-section of a resin particle cut in half. For all resins no gradients in the sulfonic acid site distribution were observed. 2.2. Competitive Sorption of Binary Mixtures. In order to determine the competitive sorption behavior of binary solvent mixtures on the ion-exchange resins, aliquots of about 700 mg of dry resin are put into glass vials of 4 mL volume, transferred to an oven, and dried at a temperature of 130 °C and an absolute pressure of 15 kPa for 6 h. After removal from the oven, the vials are immediately sealed with a septum and the dry resin mass in each sample is determined. To each sample is added approximately 3 mL of premixed solvent solution through the septum using a syringe, and the total mass of the sample is again determined. After equilibrating within a thermostated shaker for 3 h at 25 °C, samples are taken from the supernatant liquid phase and their composition is analyzed using a Hewlett-Packard 5890 gas chromatograph equipped with a heat conductivity detector, using helium as carrier gas. To ensure that thermodynamic equilibrium has been established, a few samples are allowed to equilibrate for 96 h, leading to identical results as for the case of 3 h of equilibration. Using this experimental procedure, we can measure the excess sorbed amounts, qEi , that is, the difference between the number of moles of the ith component sorbed at equilibrium and those that would be sorbed if the sorbed phase had the same composition as the liquid phase, through the following relation:13

qEi (xLi )

)

(x0i

-

n0 L i xi )

+ n0j ) qi - xLi (qi + qj) mdry

(2)

where the superscripts “L” and “0” denote the liquid phase in the equilibrated and in the initial state, i.e., the state before the solution is brought into contact with the resin, respectively; mdry represents the mass of the dry polymer; ni, xi, and qi are the molar amount of species i present in the batch equilibration experiment, its molar fraction in the liquid phase, and the moles sorbed per unit mass of dry resin, respectively. The subscript “j” denotes the second component. As an example, the experimentally measured excess sorbed amounts for the binary mixture water-methanol are shown in Figure 1 for Amberlyst-15 and four partially desulfonated resins. For each of the resins the sorptive excess with respect to water increases steeply from its limiting value at low water mole fractions, before reaching a maximum and then decreasing toward even negative values at sufficiently large water mole fractions for the desulfonated resins. This behavior

2660 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004

Figure 1. Excess sorbed amount of water, qEW, as a function of water mole fraction for resins with various acid loading; lines are simple data interpolation. R50: downward pointing triangles, solid curve. R34: diamonds, dashed curve. R24: squares, dash-dotted curve. R12: circles, dotted curve. R08: upward pointing triangles, solid curve.

implies a strong selectivity of the resin with respect to water at low water concentrations, which is where the regeneration of the chromatographic reactors mentioned above has to operate. However, as the acid loading of the resin decreases, the sorptive excess with respect to water decreases too, thus indicating lower selectivities toward water. In order to compute the absolute sorbed amounts, qi, the knowledge of the excess amounts is not sufficient and additional experimental information is needed. For the system under investigation here, the common assumption of constant liquid phase volume throughout the equilibration process is not applicable due to the large swelling of the ion-exchange resins upon sorption. Accordingly, the increase of mass of the resin particles from the dry to the swollen state has to be measured. For this, the septa sealing the equilibration vials were replaced by wire mesh and the equilibrated resin phase was separated from the liquid solution by centrifugation. Immediately after centrifugation, the mass increase of the swollen polymer phase with respect to the dry polymer was determined. Focusing again on the sorption of water and methanol, in Figure 2 it is shown that for resins with low acid site concentration, i.e., R08, R12, and R24, the mass increase upon sorption is characterized by a relative maximum at intermediate water concentrations in the liquid phase, and that it increases monotonically with ion-exchange capacity. For the highly sulfonated resins, that is, R34 and R50, the gain in mass upon swelling increases monotonically from pure methanol toward pure water. Given the resins’ macroporous character, the mass increase of the wet resin particles with respect to their dry state, ∆m/mdry, comprises not only the mass of the sorbed molecules but also that of the liquid inside the macropores:

VPoresFL ∆m mwet - mdry ) ) qiM ˜ i + qjM ˜j+ (3) mdry mdry mdry with M ˜ denoting the molar mass, VPores the volume of the resin bead macropores, and FL the liquid phase density. The volume of the macropores is calculated on

Figure 2. Relative mass increase upon sorption of water and methanol as a function of liquid phase water mole fraction for resins with various acid loading; lines are simple data interpolation. Symbols as in Figure 1.

the basis of a pore void fraction of p ) 0.36 given by the manufacturer, and it assumed to remain constant throughout the desulfonation and the swelling processes. Furthermore, by assuming ideal volume additivity for both sorbed and liquid phases, the following relationship is obtained:

(

p VPores 1 ) + qi V ˜ i + qjV ˜j mdry 1 - p Fdry

)

(4)

with V ˜ i and Fdry representing the molar volume of species i and the density of the dry polymer backbone, respectively. Calculating the liquid phase density according to

FL )

xLi M ˜ i + xLj M ˜j xLi V ˜ i + xLj V ˜j

(5)

and combining eqs 2 to 5, the molar amounts of each species sorbed in the polymer phase are finally obtained as a function of only measured (or otherwise known) quantities:

qi )

( ) (

p x0i n0i + n0j p ∆m FL - 1- L M ˜ j+ V ˜ jFL mdry Fdry 1 - p mdry 1 - p x

(

1 - xLi xLi

i

)

p p M ˜j+V ˜ j FL +M ˜i+V ˜ iFL 1 - p 1 - p

)

(6)

2.3. Chemical Reaction Kinetics. In order to characterize the catalytic activity, the reaction kinetics for the synthesis of methyl acetate has been evaluated from batch reaction runs using resins R50, R19, and R09 as heterogeneous catalysts. For all experiments, a certain amount of catalyst is equilibrated in a known amount of the reactant present in excess. After addition of the second reactant, the evolution of the liquid phase composition is monitored by GC analysis of samples taken after specified time intervals. The reaction temperature was maintained at 25 °C by using a thermostated bath. In the case of resin R50 the experimental protocol has already been described in detail earlier,8 and it was found that mass transfer resistances as well


as the influence of the reactant used for preswelling the resin could be neglected. 3. Modeling of Sorption Equilibria Within the framework of the Flory-Huggins theory, the uptake of solvent species is considered as a phase equilibrium between a bulk liquid phase and the swollen polymer or gel phase. While the cross-linked polymer cannot leave the gel phase, the solvent species enter the gel phase, so that the gel volume expands and the pressure within the gel phase increases due to the elastic properties of the three-dimensional polymeric network. Eventually, a state of mechanical as well as thermodynamic equilibrium is established. The general equilibrium condition for each ith component partitioning between the gel and the liquid phase is given by Maurer et al.,14

aLi (T, pL, nL) ) aPi (T, pP, nP) ) aiP(T, pL, nP)

[ ( )]

ViP ∂AP exp RT ∂VP

(7)

T

with aiL and aiP denoting the activity of the ith component in the liquid and the polymer phase, respectively, VPi the partial molar volume of the ith component inside the polymer phase, AP the Helmholtz energy for the elastic deformation of the polymer phase, and VP the volume of the polymer phase, respectively. Thus, the variation of the Gibbs energy in the gel phase upon sorption of the ith species comprises a chemical contribution as well as a mechanical one accounting for the elastic deformation of the three-dimensional polymeric network. In order to compute the equilibrium partitioning between the two phases, suitable expressions for the thermodynamic activities as a function of phase composition are needed. Here, the liquid phase activities were calculated employing the modified UNIFAC group contribution method.15 For the gel phase, on the other hand, the original model by Flory16 was chosen among the numerous approaches presented in the literature: N+1

ln aiP(T, pP, nP) ) 1 + ln vi -

N+1

mijvj + ∑ χijvj ∑ j)1 j)1

N+1 j-1

(

)

1 mikvjvkχkj + ηV ˜ i v1/3 P - vP (8) 2 k)1

∑ ∑ j)1

Here, vi and mij ) V h i/V ˜ j denote the volume fraction of species i in the gel phase and the ratio of the molar volumes of the components i and j (miP ) 0), respectively. While the first three terms on the right-hand side of eq 8 represent the change in entropy upon mixing, the fourth and the fifth terms account for the enthalpic contribution of the mixing process based on a lattice model. In particular, the parameter χij characterizes the molecular interaction between species i and j and is subject to the following constraints:

χij ) χjimij, χii ) 0

(9)

Finally, the last term of eq 8 comprises the elastic contribution, assuming a tetrafunctional polymeric network as well as a Gaussian distribution of the elastic

chain lengths. Here, the molar volume V ˜ i is used as an approximation for the partial molar volume, VPi , while vP denotes the volume fraction of the polymer in the gel phase, and η represents a measure for the elasticity of the cross-linked polymer, that is, the number of moles of elastic chains per unit volume of dry resin. Note that the summations in eq 8 involve all N + 1 species present in the gel phase including the polymer itself. Several applications of Flory-Huggins based models to the description of the swelling behavior of hydrogels and polyelectrolyte gels are reported in the literature and have been briefly surveyed in a recent publication.10 Mazzotti et al.9 used a variation of the model described in eq 8 where the elasticity term had been derived for a non-Gaussian distribution of chain lengths. This approach had also been followed in this work, but did not lead to superior results as compared to eq 8. Prange et al.18 extended the lattice approach by introducing three different types of interaction forces between adjacent lattice sites, that is, they distinguished between a dispersion and two types of hydrogen bond interactions (electron pair accepting and donating). For the case of ionic hydrogels, Ishidao et al.19 accounted for the possibility of dissociation of the ionic groups, thus introducing the dissociated ion as an additional species in the gel phase. In this work, it is intended to develop an equilibrium model that explicitly accounts for the presence of sulfonic groups attached to the polymeric backbone. As it is apparent from the experimental data shown in Figure 1, the sulfonated resins exhibit a very strong selectivity toward water, particularly at low water concentrations. This is compatible with a rather specific interaction between the sulfonic groups and the water molecules, which can effectively be described by the following dissociation equilibrium:

RSO3H + H2O h RSO3- + H3O+

(10)

The governing equilibrium constant, Kdiss, can be rewritten as follows:

Kdiss )

qRSO3 × qH3O+ qRSO3HqH2O

)

qH3O+2 (φ - qH3O+)qH2O

(11)

with φ indicating the total concentration of sulfonic groups in the resin, i.e., the ion-exchange capacity of the resin. The resulting physical picture includes a binary liquid phase constituted of water and methanol, and a gel phase constituted by the same components in addition to the polymer. Water can be present in the system also in a third form, i.e., bound to a sulfonic acid group as described by the dissociation equilibrium, eq 10. Therefore, when computing the amount of water sorbed by the resin one has to account both for the water molecules present in the gel and for those bound to the sulfonic groups, that is, qW ) qH3O+ + qH2O. When applying the activity expression given by eq 8 to the ternary gel phase, it should be recalled that the polymer contains sulfonated groups, either in dissociated or in undissociated form, that due to their highly polar nature strongly affect the interaction of the polymer with water and methanol. Accordingly, two different classes of model parameters can be distinguished with respect to their dependence on the sul-


fonation degree. The effective dissociation constant, Kdiss, as well as the interaction parameters between the solvent species can be assumed to be independent of the degree of sulfonation of the polymer matrix, and accordingly the same value of these parameters has been used to regress the experimental equilibrium data for all resins under consideration. The elasticity parameter, η, represents the number of moles of elastic chains, nel, per unit volume of dry resin, Vdry:

η)

nel Fdry ) Vdry M ˜ el

(12)

As the polymer backbone for each of the investigated resins is identical and the increase in dry density due to the higher sulfonation degree is counterbalanced by the increase in the molar mass of the elastic chain between two cross-links, M ˜ el, the elasticity parameter η has also been assumed to be independent of the acid loading. On the other hand, the interaction parameters between the solvent species and the polymer are expected to depend on ion-exchange capacity, as these comprise a superposition of interactions with the styrenic polymer backbone and the highly polar sulfonic acid moieties. In particular, a linear correlation between the acid loading and the solvent-polymer interaction parameters is postulated, 0 s + χiP φ χiP ) χiP

(13)

0 representing the interaction with the parameter χiP s the with the underivatized styrenic backbone and χiP respective interaction with the sulfonic acid sites. Note that the model does not distinguish between dissociated and undissociated sulfonic acid sites, since both of them are much more polar than the polymeric backbone. The interaction parameters between each solvent species and the sulfonated polymer are therefore constant for the different resins and do not depend on the concentration of water in the gel phase. Finally, it has to be mentioned that, due to the lack of information about the volumetric mixing behavior of the gel phase, ideal volume additivity is assumed when the amount of solvent sorbed per unit mass of dry resin, qi, is converted into the volume fraction in the gel phase, vi:

vi )

˜i qi V NG-1

NG-1

1

qj V ˜j + ∑ F j)1

, vP ) 1 -

vj ∑ j)1

(14)

dry

where the liquid phase molar volumes of each species are used as an approximation for the corresponding quantities in the gel phase. Note that this assumption appears to be reasonable because a significant decrease in volume of the equilibrating mixture upon sorption has not been evidenced experimentally. Ishidao et al.19 suggest to account for the strong interaction between the sulfonic groups and the water molecules by introducing a dissociation reaction forming ions that move freely in the gel phase and are therefore regarded as an additional component. Although this view can be disputed on a physical basis, this approach was tested here treating the hydrated proton as a freely

Figure 3. Sorbed amounts of water as a function of liquid phase water mole fraction for resins R50, R24, and R08; lines indicate model calculations. R50: downward pointing triangles, solid curve. R24: squares, dash-dotted curve. R08: upward pointing triangles, dotted curve.

moving species. The model thus comprises four species being present in the gel phase, that is, water, methanol, polymer, and the dissociated proton, and involves six binary interaction parameters in addition to the elasticity parameter, η, and the dissociation constant, Kdiss. By fitting this model to the water/methanol sorption data, a very satisfactory representation of the experimental data was obtained. However, due to the large number of parameters and the tight cross-correlation among them, the values of some of the fitted parameters, and particularly the binary interaction parameters involving the proton, were found to be physically unrealistic. In addition, the sorbed amounts of methanol were predicted to decrease strongly when approaching the limit of vanishing water concentrations, which could not be confirmed experimentally by the sorption data of methanol from mixtures with water and with methyl acetate. Accordingly, this approach was not followed further. 4. The Water-Methanol Binary System The experimentally measured total amounts of water and methanol sorbed at equilibrium are shown for the resins R50, R24, and R08 in Figures 3 and 4, respectively. The equilibrium data for all five resins considered here are summarized in the classical phase equilibrium diagram in Figure 5. Focusing on the total amounts sorbed, the equilibrium data for water in Figure 3 exhibit initially a concave curvature that tends to increase and become convex at larger water mole fractions in the liquid phase. While this behavior is very pronounced for the highly sulfonated resins R50 and R24, a convex shape of the isotherm for high water concentrations can be evidenced only to a smaller degree for resin R08. The equilibrium uptake when in contact with pure water decreases monotonically with reduced sulfonic acid site density from about 26 mol/kg for R50 to 6 mol/kg for R08. At the same time, the initial slope of the water isotherms for the limit of pure methanol in the liquid phase also decreases with decreasing ion-exchange capacity. The equilibrium data of methanol in Figure 4, on the other hand, are characterized by a saturation behavior for all resins investigated. In particular, the amounts

Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 2663 Table 2. Values of the Model Parameters Obtained by Fitting the Water/Methanol Equilibrium Sorption Data on Resins R50, R34, R24, R12, and R08 χ0WP χ0MP

Figure 4. Sorbed amounts of methanol as a function of liquid phase water mole fraction for resins R50, R24, and R08; lines indicate model calculations. Symbols as in Figure 3.

Figure 5. Water mole fraction in the gel phase as a function of the water mole fraction in the liquid phase at equilibrium. R50: downward pointing triangles, solid curve. R34: diamonds, dashed curve. R24: squares, dash-dotted curve. R12: circles, dashed curve. R08: upward pointing triangles, dotted curve.

sorbed when in contact with the pure alcohol show a much smaller sensitivity toward the polarity of the resin as compared to that of water, ranging only from 10 mol/ kg for R24 to 8 mol/kg for R08. From the representation of the equilibrium data in terms of mole fractions of water in the liquid and in the gel phase shown in Figure 5, a strong selectivity toward water is evidenced for all resins at low water contents. It is only for the Amberlyst-15, though, that water is the preferentially sorbed species over the entire range of water concentrations. For the other resins an inversion of selectivity toward preferential methanol uptake is evidenced at water concentrations decreasing with decreasing acid loading. This behavior can be explained by considering that, when immersed in pure methanol, there exists a strong hydrogen bonding between the sulfonic acid groups of the resin, especially if the spatial distance between two neighboring acid sites is small, that is, for high ionexchange capacity.20 A breakage of these bonds by water as a potent hydrogen bond acceptor/donator is much more favored than the incorporation of methanol into the hydrogen bond network, thus leading to the pref-

-0.486 -6.198

χsWP χsMP

-0.631 kg/mol -0.723 kg/mol

χWM η Kdiss

0.423 190 mol/L 3.326

erential scavenging of water molecules from the liquid phase at very low water mole fractions. As the tight interaction of the acidic sites is more and more relaxed through the incorporation of water molecules between the sulfonic acid moieties, methanol can be incorporated into the hydrogen bond arrangement more easily and accordingly the selectivity of the sorption process diminishes. For resins with lower acid site density, and therefore larger spacing between neighboring acidic groups, the preferential affinity toward water at low concentrations is reduced. The phenomenon of the reversal of selectivity toward favorable methanol uptake for high water contents is probably due to the influence of the apolar copolymer backbone in favor of methanol as compared to water, although both solvents exhibit high polarity. In order to describe the experimentally determined sorption data by the extended Flory model, the values of the model parameters were obtained from a nonlinear regression. The regression included all five data sets, i.e., 120 data points in total, and was aimed at minimizing the sum of squared differences between the sorbed amounts found experimentally and predicted by the model. In detail, the following parameters were regressed: the dissociation constant, Kdiss; the elasticity parameter, η; the water/polymer interaction parameters, χ0WP and χsWP (eq 13); the methanol/polymer interaction parameters, χ0MP and χsMP (eq 13); and the water/methanol interaction parameter, χWM. The obtained parameter values are reported in Table 2. When these values are analyzed, several trends can be identified which are well consistent with the physical picture on which the developed model is based. However, due to the strong cross-correlation and the model assumptions, these parameter values have to be considered to be of a somehow empirical nature. From the data in Table 2 it is seen that for water and methanol the binary interaction parameters with the polymer, χiP, decrease significantly with the increase of the sulfonic acid site concentration, that is, both χsWP and χsMP are negative. This behavior can be explained in the framework of Flory’s theory,16 by considering that smaller values of χiP indicate smaller enthalpies of mixing, that is, higher compatibility of the two species. Here, as the resin turns more and more hydrophilic with increasing acid loading, a higher affinity toward the polar species is expected. It has to be noted, though, that the absolute values of the binary interaction parameters between one species and, say, the polymer are dependent on the number of lattice sites that are occupied by one single molecule of this species.16 Accordingly, the absolute values of binary interaction parameters involving different species have to be scaled by the species molar volume prior to comparison (V ˜ i ) 18.0 and 40.5 × 10-6 m3/mol for water and methanol, respectively). For both water and methanol an attractive interaction with the nonderivatized polymer matrix is found, with the scaled value for methanol, χ0MP/V ˜ M, exceeding the one for water by a factor of approximately 5 (see Table 2). While this difference between the two species seems


physically reasonable, the attractive interaction between both highly polar species and the apolar polymer seems less convincing. About the interaction with the sulfonic sites, both species show an attractive interaction with the scaled parameter for water exceeding the one for methanol by a factor of about 2. Accordingly, the interaction between the unbound water within the gel and the sulfonic sites accounts for about 90% of the water-polymer interaction energy for the most strongly sulfonated resin, R50, and still more than 50% for the least sulfonated one, R08. For methanol, on the contrary, only 37% of the interaction energy with the polymer arises from the sulfonic sites in the case of R50, which seems small given the polarity of this species. For the resin independent parameters, the order of magnitude found for the elasticity parameter, η, is substantially larger than expected from theoretical considerations. Considering an average elastic chain comprising five monomer units based on the nominal DVB content of 20%, an elasticity value of approximately η ) 2 mol/L is expected. The value of η estimated from the sorption equilibrium data of 190 mol/L thus exceeds the expected value by about 2 orders of magnitude. The same disagreement was also encountered in the work of Mazzotti et al.9 using an alternative expression for the elasticity term capable of dealing with a non-Gaussian distribution of chain lengths, where again the fitted values exceeded the expected ones by a factor of more than 100. In order to overcome this problem, alternative models to describe the pressure changes within the resin at different degrees of swelling have been proposed recently,17 taking into account the limited extensibility of strongly cross-linked resins. Finally, a binary interaction parameter for water and methanol of χWM ) 0.423 was found, which is similar to the one found when fitting the Flory-Huggins activity model to binary VLE data of water and methanol in the absence of a polymeric sorbent (χWM ) 0.56). An equilibrium constant for the dissociation reaction of 3.36 seems physically realistic although higher values were expected due to the high tendency of the sulfonic sites toward hydration. A comparison between the sorption data for the resins R50, R24, and R08 calculated from the model and those measured experimentally is shown in Figures 3 and 4 for water and methanol, respectively. It is seen that for water the agreement of the model with the experimental data is very satisfactory. Only for resin R08 the amount of water present in the gel phase is slightly overestimated at high water concentrations. For each resin, the initial curvature of the water isotherm is reproduced well. As compared to water, the model qualitatively predicts strictly concave isotherms in the case of methanol, which is in agreement with the experimental data except for the case of the most strongly sulfonated resin R50 at very low water mole fractions. Quantitatively, the amount of methanol sorbed by the resin is overestimated for resin R08 over almost the entire range of data and for resin R50 at low water concentrations. For the resin of intermediate sulfonation degree, that is, R24, the methanol uptake is underestimated slightly for intermediate water concentrations. Concerning the phase equilibrium diagram in Figure 5, the experimental data are well reproduced for all five resins investigated. In particular, both the initial slope

Figure 6. Sorbed amounts of water (solid) and methanol (open symbols) as a function of liquid phase water mole fraction for resins R50, R19, and R09; lines indicate model calculations. R50: upward pointing triangles, solid curve. R19: squares, dash-dotted curve. R09: circles, dotted curve.

of the data at low water concentrations and the point of selectivity inversion are mapped with satisfactory accuracy. 5. Extension to Other Binary Sorption Systems In order to complete the study of the multicomponent sorption equilibria involved in the esterification of acetic acid with methanol, the sorptive behavior of the following nonreactive binary systems were investigated: water/ acetic acid, methyl acetate/methanol, and methyl acetate/ acetic acid. From the knowledge of these systems, in addition to the water/methanol system analyzed above, a description of the multicomponent sorption equilibria involving the quarternary reaction mixture is possible based on a suitable equilibrium model such as the Flory model described above. For this investigation, two partially desulfonated resins of an ion-exchange capacity of 1.9 mol/kg (denoted R19) and 0.9 mol/kg (R09) were prepared (see Table 1). For each of these resins, as well as for the Amberlyst15 (R50), the binary sorption equilibria of each of the four nonreactive pairs were investigated experimentally following the protocol described above. The experimentally determined sorption data are shown in Figures 6-9. With respect to the case of single components it has been found that, contrary to water and methanol, for acetic acid and methyl acetate the equilibrium amounts sorbed by the resin increase monotonically as the sulfonation degree decreases. In particular, the saturation capacity of the acid and the ester increase from 4.5 and 3.5 mol/kg for resin R50, respectively, to 5.2 and 4.7 mol/kg for resin R09, respectively. The binary water/methanol uptake data are consistent with the data presented above for other resins and therefore do not need to be discussed in detail. The equilibrium isotherms for mixtures of water and acetic acid shown in Figure 7 are similar to those for the water/ methanol system. In particular, they are characterized by the typical S-shaped behavior of the water isotherms, and the saturation type behavior of the acetic acid isotherms. The resins exhibit a strong selectivity toward water at low water mole fractions in the liquid phase, which decreases as the concentration of water in the liquid phase increases. While for the highest sulfonation


Figure 7. Sorbed amounts of water (solid) and acetic acid (open symbols) as a function of liquid phase water mole fraction for resins R50, R19, and R09; lines indicate model calculations; symbols as in Figure 6.

Figure 9. Sorbed amounts of methyl acetate (solid) and acetic acid (open symbols) as a function of liquid phase methyl acetate mole fraction for resins R50, R19, and R09; lines indicate model calculations; symbols as in Figure 6. Table 3. Values of the Model Parameters Obtained by Fitting the Equilibrium Sorption Data of the Binary Pairs Water/Acetic Acid, Methanol/Methyl Acetate, and Acetic Acid/Methyl Acetate on Resins R50, R19, and R09 resin name: χAP χEP χWA χME χAE

Figure 8. Sorbed amounts of methyl acetate (solid) and methanol (open symbols) as a function of liquid phase methyl acetate mole fraction for resins R50, R19, and R09; lines indicate model calculations; symbols as in Figure 6.

degree (R50) a selectivity in favor of water is found for the whole range of concentrations, an inversal in selectivity toward favorable acid uptake is evidenced for the desulfonated resins. For the system methyl acetate/methanol shown in Figure 8, a situation strongly in favor of methanol uptake is found for resin R50. Upon reduction of the sulfonation degree this selectivity is substantially reduced, with the methanol isotherm turning toward an S-shaped behavior similar to the one found for the water isotherms, and the ester isotherms evolving into a typical saturation type behavior. In fact, for R09 an uptake selectivity in favor of the ester is found over a significant range of liquid phase compositions. Finally, the sorption equilibrium data of methyl acetate from mixtures with acetic acid shown in Figure 9 follow a concave isotherm for all three resins, with the corresponding ones for acetic acid being of approximately linear (R50) to convex type. In particular, both species are taken up nearly nonselectively by Amberlyst 15, while both partially sulfonated resins exhibit a sorptive trend in favor of methyl acetate.

R50

R19

R09

-10.26 -15.12

-9.54 -14.99

-9.70 -15.03

-0.439 -0.027 -1.11

In order to model the sorptive behavior of these resins the extended Flory-Huggins model described above has been applied. The competitive sorption of water and acetic acid is treated in the same way as that of water and methanol. Accordingly, the sorptive behavior is governed by the binary interaction parameters between water and acetic acid, χWA, between acetic acid or water and the polymer, i.e., χAP and χWP, in addition to the dissociation constant, Kdiss, and the polymer elasticity, η. A similar set of parameters govern the equilibrium model for the two remaining binary pairs except that in these cases the dissociation mechanism is not applicable since no water is involved, and the model thus reduces to the original Flory model described by equation 8. Given the detailed analysis of the water/methanol system developed above, the same values found for the dissociation constant, the polymer elasticity, and the water-methanol interaction parameter, as well as the same functional dependence of the interaction parameters χWP and χMP onto the ion-exchange capacity of the resin (Table 2), have been used. Only the remaining parameters have been determined by fitting of the experimental data. The obtained parameter values are reported in Table 3, while the corresponding calculated model results are compared with the experimental data in Figures 6-9. First of all, since the parameters describing the water/ methanol system have already been determined above, the comparison of the model predictions and the experimental data in Figure 6 offers the possibility to check the reliability of the developed model for the two new resins R19 and R09. It is seen that the sorbed amounts of water are represented satisfactorily well by the model, with only the saturation capacity in the case of resin


R19 being slightly overestimated. For methanol, the agreement between model predictions and experimental data is excellent for resin R09, while the calculated values for resin R19 are slightly smaller than the experimental ones for small water mole fractions. For the competitive uptake of water and acetic acid, the agreement between model and experiments in Figure 7 is reasonable for all three resins examined, with the sorbed amounts of acetic acid being overestimated only for resin R50. For the resin with the lowest sulfonation degree, R09, reliable data for the case of high water concentrations could not be obtained due to problems of the equilibrating mixture not being able to wet the resin particles sufficiently, and the quality of the model predictions therefore cannot be evaluated in this region. The resins’ sorption behavior with respect to methyl acetate and methanol can also be modeled well by the Flory model. In Figure 8 it is seen that the model reproduces the change in the shape of the sorption isotherms for reducing sulfonation degree well both in qualitative as well as in quantitative terms. The same applies to the model performance in the case of the methyl acetate and acetic acid system shown in Figure 9. When analyzing the values of the model parameters reported in Table 3, physically reasonable values for the three interaction parameters among the sorbing species are found. In particular, an attractive interaction is found both between water and acetic acid and between methyl acetate and acetic acid, while hardly any enthalpic interaction between methanol and methyl acetate is predicted. The interactions between both acetic acid and methyl acetate with the polymer matrix are also of an attractive nature for all three resins. In particular, values on the order of -10 and -15 are found for the parameters describing the interaction acetic acid/polymer and methyl acetate/polymer, respectively, with the former exhibiting a slight decrease for lower acid loadings of the polymer and the latter one remaining substantially constant. While these values appear quite large at first sight, they compare well with the parameters found for methanol after scaling with the species molar volumes. ˜E In particular, when considering that V ˜ A ) 57.1 and V ) 79.6 × 10-6 m3/mol, it is seen that the values of χiP/V ˜i found for the ester, the acid, and methanol on resins R19 and R09 are all within the range of 0.17-0.19 × 106 mol/m3. For the most hydrophobic resin R09 the parameters for acetic acid and methyl acetate exceed the respective one for water by about a factor of 3, while on the most hydrophilic resin, R50, χWP is about 5% and 10% larger than the respective quantity for the ester and the acid, respectively. To summarize, the extended Flory-Huggins model, in combination with the parameters determined above from the investigation of the competitive sorption behavior of water and methanol, has been shown to be a well-suited tool to describe also the competitive sorption of other binary nonreactive mixtures of the species involved in the synthesis of methyl acetate. 6. Catalytic Activity for Methyl Acetate Synthesis Reducing the acid loading of the resin changes not only its sorptive properties but also its catalytic activity. In order to investigate the effect of sulfonation degree

Figure 10. Initial reaction rate as a function of the initial composition of the reacting mixture for three different resins. Experimental data: R09, circles; R19, downward pointing triangles; R50, squares. Model prediction based on extended Flory model and χMA ) 0: R09, solid line; R19, dashed line; R50, dashdotted line.

on the catalyst performance, the initial rates of methyl acetate formation were measured for resins R50, R19, and R09. The results are shown in Figure 10 as a function of the initial composition of the reaction mixture, i.e., for mole fractions of methanol in acetic acid xM ) 0.10, 0.50, and 0.95, respectively. As expected, faster reaction rates are observed for resins with higher acid loading for all compositions investigated, while for each resin the reaction rate is fastest when a stoichiometric mixture of the reactants is employed. In general, it can be observed that the decrease in catalytic activity becomes significant only when the acid loading falls below about 1-2 mol/kg. With respect to reactive chromatography applications this means that we can play with the resin’s acid loading in order to improve its separation performance without significantly losing in terms of catalytic activity as long as we do not fall below the acid loading values mentioned above. However, when attempts have been made to model the kinetic results shown in Figure 10, some significant difficulties have been encountered. In the first place, since the reaction occurs only in the gel phase, a pseudohomogeneous model would not be appropriate, and in fact it has been shown to provide a rather poor representation of the experimental data. Accordingly, the simple bimolecular reaction rate expression, r ) kRqAqM, has been used, which requires that the gel phase composition be computed by coupling the kinetic model with the phase partitioning model developed above. The problem is that the latter is not complete for this purpose since the interaction parameters for the reacting pair methanol/acetic acid, χMA, could not be evaluated due to the presence of the reaction. A comparison with the other binary interaction parameters, χij, suggests that a range of -1 < χMA < 1 would be plausible, but on the other hand χMA can be adjusted together with kR to best fit the reaction rate data. As a first trial, it was set that χMA ) 0 and, by fitting the experimental data for each resin separately, values of kR ) 8.7, 14.7, and 18.7 × 10-4 (kg/mol)/s were found for resins R09, R19, and R50, respectively. The resulting model predictions are compared with the experiments in Figure 10. It is seen that the model predictions for


the resin with the lowest acid loading, R09, are in satisfactory agreement with the experimental data. On the other hand, for the remaining two resins, R19 and R50, the reaction rates at low methanol content are predicted to be faster than found experimentally, while exactly the opposite is found for reaction rates at high methanol content. This behavior can be explained by the phase partitioning model predicting a preferential sorption of methanol as compared to acetic acid, which is the more pronounced the higher the acid loading of the resin. According to the model, the liquid phase composition leading to the maximum rate of reaction is thus shifted from approximately xM ) 0.50 for R09 to xM ) 0.35 and 0.12 for resins R19 and R50, respectively. However, this behavior is clearly not confirmed experimentally. Finally, it can be observed that the value of the reaction rate referred to the single acid site does not really remain constant for the three resins, that is, kR/φ ) 9.7, 7.7, and 3.7 (kg/mol)2/s for resins R09, R19, and R50, respectively. In order to improve the agreement with the experimental data, the interaction parameter for methanol and acetic acid can be adjusted to change the competitive sorption behavior toward a much more favorable uptake of acetic acid. Minor variations of χMA within the range of values described above, though, do not affect the quality of the model predictions significantly. Accordingly, when a combined regression of χMA and kR is applied, the system converges toward a shallow optimum at χMA ) -13, where model predictions are in fact significantly improved. However, the isotherms for methanol and acetic acid under these conditions are of parabolical shape with strongly developed maxima in sorbed amounts and seem to be of poor physical relevance. Thus summarizing, it appears that the kinetic model considered is probably too simple to describe this reaction system. A more detailed study would be required in order to better quantify the effect of acid loading on reaction kinetics. On the other hand, and more relevant to this work, it could be shown experimentally that the sulfonation degree can be reduced from 5.0 to 1.9 mol/kg without substantial losses in catalytic activity. For even smaller sulfonation degrees catalytic performance decreases rapidly, but resins could still be used in applications where proper adjustment of sorptive properties is more important than catalyst optimization. 7. Conclusions It was shown that by decreasing the ion-exchange capacity of sulfonated poly(styrene-divinylbenzene) resins the competitive sorption behavior of water and methanol can be varied toward smaller selectivities for water as compared to methanol. This effect is particularly pronounced for the limiting case of small water contents in the liquid phase. Investigating the remaining binary, nonreactive pairs involved in the synthesis of methyl acetate, a general trend of decreasing selectivity of the resin toward the uptake of water and methanol, as well as an increasing affinity toward acetic acid and methyl acetate, was evidenced for decreasing acid loadings. In addition, a modest change in catalytic activity as a function of acid loading was observed for the resins of acid loading higher than 1.0-2.0 mol/kg investigated in this work. Using a Flory-Huggins model to describe the swollen polymer phase, properly extended to account for the

dissociation of the acidic groups upon contact with water, correlations for the dependence of the model parameters on the acid loading of the polymer resin were developed. The model was shown to be applicable to the sorption of water and methanol, as well as to the remaining nonreactive binary mixtures involved in the model system, over the entire range of liquid phase compositions as well as over a wide range of acid loadings. Accordingly, this provides a useful tool in describing the sorptive properties of sulfonated ionexchange resins and can potentially be used in order to optimally design the sulfonation degree with respect to a specific application. With respect to the application of such resins as stationary phases for reactive chromatography, the data reported in this work show that their sorptive properties can be varied toward lower selectivity for water and higher affinity toward apolar substances. Accordingly, the use of partially sulfonated resins promises to allow for substantial reductions in the consumption of alcohol as the desorbent during the resin regeneration step. At the same time, the catalytic activity of the resins remained comparably high so as not to significantly affect the unit productivity. On the other hand, a simple kinetic model that could explicitly relate the involved kinetic parameters to the acid loading of the resin could not be identified. Acknowledgment Financial support of Roche AG, Basel, Switzerland, is gratefully acknowledged. Notation ai ) activity of species i Kdiss ) dissociation constant kR ) solid phase reaction rate constant m ) mass of polymer resin ∆m ) mass increase upon swelling mij ) ratio of molar volumes M ˜ el ) molar mass of an elastic chain M ˜ i ) molar mass of species i n ) number of moles nel ) number of elastic chains p ) pressure qi ) molar amount of species i sorbed per unit mass of dry resin qEi ) excess sorbed amount of species i r ) solid phase reaction rate per unit mass of dry resin T ) temperature vi ) volume fraction of species i in the gel phase V ˜ i ) molar volume of species i VP ) total volume of polymer phase VPores ) total pore volume VPi ) partial molar volume in the polymer phase xi ) mole fraction of species i Greek Symbols p ) particle porosity η ) elasticity parameter νi ) stoichiometric coefficient of species i χi,j ) binary interaction parameter F ) density φ ) ion-exchange capacity

2668 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 Subscripts A ) acetic acid dry ) polymer gel in the dry, nonswollen state E ) methyl acetate i ) species i j ) species j M ) methanol P ) polymer W ) water wet ) swollen state in equilibrium with fluid phase Superscripts L ) liquid phase P ) polymer (gel) phase s ) per sulfonic acid group 0 ) in absence of sulfonic groups 0 ) at the beginning of an experiment

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(8) Lode, F.; Houmard, M.; Mazzotti, M.; Morbidelli, M. Continuous Reactive Chromatography. Chem. Eng. Sci. 2001, 56, 269. (9) Mazzotti, M.; Neri, B.; Gelosa, D.; Morbidelli, M. Kinetics of liquid-phase esterification catalyzed by acidic resins. Ind. Eng. Chem. Res. 1997, 36, 3. (10) Tiihonen, J.; Laatikainen, M.; Markkanen, I.; Paatero, E. Sorption of neutral components in ion-exchange resins. 1. Effect of cross-link density and counter-ion on selective sorption of water-ethanol mixtures in sulfonated PS-DVB resins. Ind. Eng. Chem. Res. 1999, 38, 4832. (11) Bothe, N.; Doscher, F.; Klein, J.; Widdecke, H. Thermal stability of sulphonated styrene-divinylbenzene resins. Polymer 1979, 20, 850. (12) Bothe, N. Struktur, Stabilita¨t und katalytische Aktivita¨t sulfonsaurer Styrol- Divinylbenzolharze. Ph.D. Dissertation, Technical University of Braunschweig, Braunschweig, Germany, 1980. (13) Kipling, J. J. Adsorption from solutions of non-electrolytes; Academic Press: New York, 1965. (14) Maurer, G.; Prausnitz, J. Thermodynamics of phase equilibrium for systems containing gels. Fluid Phase Equilib. 1996, 115, 113. (15) Gmehling, J.; Li, J.; Schiller, M. A Modified UNIFAC Model. 2. Present Parameter Matrix and Results for Different Thermodynamic Properties. Ind. Eng. Chem. Res. 1993, 32, 178. (16) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, 1953. (17) Tiihonen, J.; Markkanen, I.; P. Aanismaa; Laatikainen, M.; Paatero, E. Modelling the sorption of water-ethanol mixtures in cross-linked ionic and neutral polymers. Chem. Eng. Sci. 2002, 57, 1885. (18) Prange. M.; Hooper, H.; Prausnitz, J. Thermodynamics of Aqueous Systems Containing Hydrophilic Polymer Gels. AIChE J. 1989, 35, 803. (19) Ishidao, T.; Iwai, Y.; Hashimoto, Y.; Arai, Y. Correlation for volume phase transition of polymeric gel (vinyl alcohol-sodium acrylate copolymer) in water-alcohol mixtures. Polym. Eng. Sci. 1994, 34, 507. (20) Zundel, G. Hydration and Intermolecular Interaction; Academic Press: New York, 1969.

Received for review August 12, 2003 Revised manuscript received February 9, 2004 Accepted February 18, 2004 IE030664S

Sorptive and Catalytic Properties of Partially Sulfonated Resins

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