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

For H-ZSM-5 zeolites with different Si/Al ratios (TZ-m), their catalyst weights are adjusted to maintain a similar amount of Brϕnsted acid sites (BAS); meanwhile, their acid density per volume of catalyst bed remained different. After well mixing quartz sands with all catalysts to guarantee similar volume of catalyst bed for different samples, the catalytic performance of H-ZSM-5 zeolites in methanol to olefins is then inspected by means of multiple pulses tests combined with isotopic experiments. For TZ-m zeolite with higher acid density, the more steric-hindered aromatic hydrocarbon pool (HCP) species are confined in zeolite channels (most probably the intersections); with less mobility of contacting other BAS in different channels, the reactivity of their generating ethene via aromatic methylation/cracking pathway remained similar in spite that the acid site density is different for TZ-m zeolites. However, the higher acid site density enhanced hydrogen transfer reactions on low-silica TZ-m zeolites, leading to massive accumulation of aromatics (most of which functioned as coke precursors instead of aromatic HCPs) and increased propagation of aromatic-based cycle to olefin-based cycle. In comparison, on TZ-m zeolite with lower acid site density, the dispersed acid sites are less occupied by aromatics; and the less steric-hindered olefinic HCP species managed to gain access to BAS in different channel multiply, which reinforced the generation of C3+ olefins through olefin methylation/cracking routes. Therefore, it resulted in selectively more generation of propene for high-silica TZ-m zeolites.