Insight into induction period of methanol conversion reaction: Reactivity of ethene-precursors over H-ZSM-5 zeolite is independent of Brϕnsted acid site density. (15th January 2023)

• Record Type: Journal Article
• Authors: Liang, Tingyu; Chen, Jialing; Qin, Zhangfeng; Wang, Sen; Wang, Pengfei; Jin, Fang; Dong, Mei; Wang, Jianguo; Fan, Weibin
• Is Part Of: Fuel. Volume 332(2023)Part 1
• Publication Date: 2023-01-15
• Subjects: Methanol to olefins -- H-ZSM-5 -- Acid site density -- Aromatic-based cycle -- Olefin-based cycle; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2022.126062
• Languages: English
• ISSNs: 0016-2361

Abstract

Graphical abstract: 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. Highlights: The methanol pulse micro-reaction confirmed that the MTO reaction was catalyzed by Brϕnsted acid sites (BAS). The reactivity of aromatic Hydrocarbon pool (HCP) remained almost unchanged in spite of different acid density for HZSM-5 zeolites. The higher acid density enhanced the accumulation of confined aromatics in zeolite channels and eventually led to decreased reactivity for olefinic HCPs. 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.