Crosslinked polymer resins are known to exhibit a remarkable selective sorption and swelling behaviour when brought in contact with a multicomponent mixture. Additionally, they constitute an interesting class of heterogeneous catalysts for several organic synthesis reactions. However, when developing kinetic models for these reactions, the partitioning of the components between the liquid and polymer phases and its effect on their thermodynamic activities is rarely accounted for. In this work, the performance of a kinetic model if this equilibrium is explicitly accounted for is compared to the its performance if equality of the liquid and polymer phase activities is implicitly assumed. As case study, the esterification of acetic acid with methanol catalysed by the gel-type polystyrene-based resin Lewatit K1221 is used. It was found that the thermodynamic activities within the polymer phase are overestimated in case they are assumed to be equal to those in the liquid phase. As a result, the pre-exponential factors of the kinetic rate and adsorption coefficients have to compensate and, hence, are estimated at slightly lower values then in case the phase equilibrium is explicitly accounted for. Moreover, remarkably, the confidence intervals of the parameter estimates appear to be more narrow, if the polymer phase activities are explicitly determined. Overall, it can be concluded that for the considered case study the performance of both models is very similar. It should, however, be noted that the difference between the liquid and polymer phase activities will strongly vary between datasets acquired using multiple resin catalysts or in various solvents. Hence, for studies aiming at reconciling such datasets, explicitly accounting for the selective sorption and swelling behaviour is strongly recommended.
The authors acknowledge the Eurokin consortium (www.eurokin.org) for the constructive discussions, particularly on the thermodynamics methodology, and the permission to publish.