Insight into induction period of methanol conversion reaction: Reactivity of ethene-precursors over H-ZSM-5 zeolite is independent of Brϕnsted acid site density

Abstract

A series of H-ZSM-5 (TZ-m, m represents different Si/Al ratios) has been prepared and their Methanol to Olefin (MTO) performance are investigated. By adjusting the catalyst weight of H-ZSM-5 loaded in the pulse micro-reactor, the total Brϕnsted acid sites (BAS) amount of TZ-m catalysts is unified. The resultant similar methanol conversion (∼24 %) over different TZ-m zeolites confirms that the MTO reaction is dominantly catalyzed by BAS. Nevertheless, the BAS density considerably influences the reaction pathways; more condensed BAS enhances the relative propagation of aromatic-based cycle to alkene-based cycle, and vice versa. However, isotopic co-feeding and switching experiment reveals that the reactivity of aromatic hydrocarbon pool (HCP) species, which has been considered as main ethene precursors, is almost independent of acid site density. Although large amounts of aromatics are generated via hydrogen transfer reactions on the sample with high acid site density, most of the confined aromatics function as coke precursors instead of active HCPs. The accumulation of such confined aromatics in zeolite channels further limits olefins mobility and their access to BAS, leading to decreased reactivity of alkene HCPs on high acid site-density samples.