Mechanistic insights into the conversion of dimethyl ether over ZSM-5 catalysts : a combined temperature-programmed surface reaction and microkinetic modelling study

The rates of adsorption, desorption, and surface reaction of dimethyl ether (DME) to olefins over fresh and working ZSM-5 catalysts with different Si/Al ratios (36 and 135) were decoupled using a combination of temperature-programmed surface reaction experiments and microkinetic modelling. Transient reactor performance was simulated by solving coupled 1D nonlinear partial differential equations accounting for elementary steps during the induction period based on the methoxymethyl mechanism on the zeolite catalyst and axial dispersion and convection in the reactor. Propylene is the major olefin formed, and site-specific scaling relations between the activation energies of DME desorption and barriers to the formation of methoxymethyl and methyl propenyl ether are observed. Six ensembles of sites are observed with a maximum of three adsorption/desorption sites and three adsorption/desorption/reaction sites. Barriers are generally higher for working catalysts than fresh catalysts. Activation energies of propylene formation of ca. 200 kJ mol−1 are obtained, corroborating direct mechanistic proposals