Site-Specific Activity Maps Observed during Methanol-to-Olefin Conversion over ZSM-5 Catalysts

Abstract

The Sabatier principle has been applied to nonuniform site ensembles of ZSM-5 catalysts of different compositions (Si/Al = 25, 36, and 135) during the induction period of the conversion of methanol and dimethyl ether to propylene. For the first time, site-specific volcano-plots and site-specific activity-maps have been observed following microkinetic simulations of the temperature-programmed surface reaction of methanol and dimethyl ether over ZSM-5 catalysts in a temporal analysis of products reactor. The microkinetic simulations are constructed using coupled 1D nonlinear partial differential equations that connect reactions at the active site to convection and dispersion through the reactor. The methoxymethyl cation pathway predicts the formation of propylene from methanol and dimethyl ether via dimethoxyethane. Site-specific scaling relations are observed between the acid site density and the barriers of the dissociative desorption of dimethyl ether with the barrier of methoxymethyl cation formation during the induction period of propylene formation from methanol and dimethyl ether, respectively. During the induction period of methanol conversion to propylene, depending on the barrier to methoxymethyl cation formation, the active site density should not be too low such that site cooperation is inhibited and not too high such that site competition is promoted. The optimum site density is ca. 5 × 10–4 mmol g–1 of active sites over the high-temperature active sites of ZSM-5 (Si/Al = 36) catalysts. Over the high-temperature active sites on ZSM-5 (25) catalysts, we observe a maximum rate of propylene formation at an optimum site density of 7 × 10–4 mmol g–1 active sites. During the induction period of dimethyl ether conversion, the binding energies should not be too weak to activate dimethyl ether and not too strong to inhibit its release into the gas phase. The binding energy of methoxymethyl cation should be between −80 kJ mol–1 and −100 kJ mol–1 for maximum propylene formation over the low-temperature active sites of ZSM-5 (Si/Al = 36) catalysts. Over the medium-temperature active sites of ZSM-5 (Si/Al = 36) catalysts, the maximum rate constant of propylene formation is observed between a binding energy of dimethyl ether of −135 kJ mol–1 and −145 kJ mol–1. The site-specific activity maps show that the catalytic activity of methanol-to-olefin conversion varies across the different site ensembles. In each site-specific activity map, the direction of the maximum rate constant of propylene formation changes with zeolite composition and site-ensemble. The precision given by this site-specific control shows that the catalytic activity can be tailored to the site characteristics.