Kinetic study of the (trans)esterification catalyzed by gel and macroporous resins

(Trans)esterification reactions with typical production volumes ranging from a few hundred to thousand tonnes per year are commercially important and are used to make esters which serve as precursors or additives for a variety of perfumes and flavours, pharmaceuticals, agrochemicals and polymers. The homogeneous Lewis or BrØnsted acids and bases that are commonly used for (trans)esterification purposes exhibit several well-known disadvantages, such as additional separation and neutralisation steps. A heterogeneously catalysed implementation of the reaction making use of acid ion exchange resins is promising as they are ecofriendly, noncorrosive and reusable. Acid ion exchange resins are promising catalysts for (trans)esterification. Thanks to their pronounced swelling when in contact with polar media, the highest conversions are obtained with a gel type resin, Lewatit K1221. The resins’ swelling significantly affects the catalytic activity and, hence, should also be reflected in the kinetic model. A novel kinetic model, implicitly accounting for resin swelling, was developed based on the following assumptions: (1) all active sites are occupied, (2) the exchange reactions between reagents or products are quasi equilibrated and (3) an Eley-Rideal type surface reaction between protonated (ester)acid with methanol from the bulk occurs as the rate-determining step. This kinetic model describes accurately the (trans)esterification reaction kinetics on different type of resins. Parameter estimates generally adopted physically meaningful values although the exchange coefficients obtained for acetic acid esterification indicated the need for further model refinement, in particular with respect to the description of the thermodynamics inside the resin. The kinetic model can be used as a helpful tool for resin selection. While resin selection and design frequently occurs on a trial and error base, the insight gained by the model, may provide clear guidelines about the desired resin properties. This knowledge may further lead to an enlargement of the use of ion exchange resins as catalysts, and, hence, also be exploited for industrial reactor design and optimization.