A rational kinetic model of dimethyl ether conversion over ZSM-5 catalysts: Induction period and steady-state

The formation of the first C–C bond and primary olefins from methanol or dimethyl ether has been debated for 50 years. Numerous mechanisms have been proposed. Step response studies of dimethyl ether (DME) conversion were conducted over ZSM-5 catalysts in a temporal analysis of products reactor. Rational kinetic models have been developed for describing the kinetic dependencies of DME conversion over ZSM-5 catalysts using common intermediates (surface methoxy groups and adsorbed ethylene). Seven rational kinetic models have been compared. These include the consecutive adsorption mechanism (CAM), CAM with buffer steps, parallel adsorption mechanism (PAM), extended PAM, combined CAM and PAM, an enhanced PAM, and an enhanced two-route PAM. Of these, the enhanced (two-route) PAM, which agrees with the methoxymethyl mechanism when considering the global stoichiometry, gives the best agreement for describing both steady-state and transient kinetic behaviour over ZSM-5 catalysts, while reproducing ethylene and propylene profiles. The enhanced two-route PAM is more amenable to reactor design via computational fluid dynamic simulations as it has four steps and two routes (competitive adsorption on Brønsted acid sites, nonlinear kinetics for propylene formation, and an additional route for ethylene formation). It contains basic intermediates (surface methoxy groups, and adsorbed ethylene) and describes almost all steady-state and transient features of DME conversion over ZSM-5 catalysts.