Insight into the deactivation mode of methanol-to-olefins conversion over SAPO-34: Coke, diffusion, and acidic site accessibility

Abstract

The whole reaction course of methanol-to-olefins (MTO) conversion over SAPO-34 catalyst with crystal size 7–10 µm has been investigated by multiple spectroscopic techniques. The reaction and deactivation of methanol conversion are closely correlated to the amount, size, and location of coke formed in the CHA nanocages. A nonuniform spatial distribution of these coke species in the individual SAPO-34 crystals is revealed by a combination of hyperpolarized (HP) 129Xe NMR and confocal fluorescence microscopy (CFM) techniques. Combined with acidic properties and diffusion performance, the bulky coke species formed at the shell layer of the crystals led to an increase of intracrystalline diffusion resistance and a decrease in accessible acidic sites in the deactivated catalyst. The overall catalyst deactivation mode of the MTO reaction is presented in terms of coke formation and location, mass transport variation, and accessibility of Brønsted acid sites in the SAPO-34 catalyst.