Methanol conversion to light olefins over SAPO-34. Sorption, diffusion, and catalytic reactions

The effects of adsorption and diffusion of the reactants on methanol to olefins (MTO) and propene conversion over SAPO-34 have been studied in an oscillating microbalance reactor. The adsorption parameters of methanol and propene at reaction conditions (698 K) were determined by a pulse method, and the results were identical to the values obtained by extrapolation from low temperatures (323-398 K). Inverse uptake diffusion times were calculated from adsorption data at low temperatures, and these results were dependent on the temperature and the adsorbed amount. The inverse steady-state diffusion times calculated from the inverse uptake diffusion times were independent of the temperature and the adsorbed amount. The influence of diffusion on the reaction rates was estimated on the basis of the inverse steady-state diffusion time, using the Weisz-Prater criterion. The methanol conversion over SAPO-34 was influenced by diffusion of the reactant, while the propene conversion was not. A kinetic study revealed that both the rate constant and the site coverage of propene were much lower than that of methanol at 698 K. The deactivation behavior during the MTO reaction over SAPO-34 was studied by measuring both the adsorbed amount of methanol and the conversion at different coke contents. Catalyst deactivation was proposed to be due to a decreasing number of sites available for adsorption at high coke contents and a lower diffusivity, hence a lower effectiveness factor due to coke deposition.