Cavity Controls the Selectivity: Insights of Confinement Effects on MTO Reaction

Abstract

Organic reaction intermediates confined in zeolite cavities are very important for product formation during methanol-to-olefins (MTO) conversion; however, direct evidence is still required to understand the particular function of these intermediates. Herein, for the first time, by careful selection of SAPO molecular sieves with different cavity size but identical 8MR pore openings, the reactivity and role of these intermediates played in olefin generation are verified by isotopic tracing method and theoretical calculations based on the observation of two types of carbenium intermediates, polyMB+ and polyMCP+, under the real MTO reaction conditions. It demonstrates that cavity size controls the molecular size and reactivity of these confined species, which results in different MTO activity and product selectivity.

Mechanism of Methanol‐to‐hydrocarbon Reaction over Zeolites: A solid‐state NMR Perspective

“…Besides SSZ‐13, heptamethylbenzenium ion has been also observed over other topologies with 12‐ring pores or large cages, such as Beta and DNL‐6 . Moreover, the cavity size of zeolite controls the molecular size and reactivity of carbenium intermediates, which results in different MTO activity and product selectivity …”

Further Studies on How the Nature of Zeolite Cavities That Are Bounded by Small Pores Influences the Conversion of Methanol to Light Olefins

“…For both reactions, the small pores are important.F or the MTO reaction, there appears to be ar equirement of ac age with sufficient size to accommodate and confine aromatic intermediates in order for the reaction to proceed with high selectivity to light olefins, [1,11] and the product distributions can dependo nt he cage geometries. [7,12,13] Additionally,t he Si/Al ratios of the zeolite catalysts can be critical to their performance, as Bronsteda cid sites that are in close proximity to one another (at lower Si/Al ratios and quantified by divalentc ation exchange capacity) have been shown to be detrimental to the lifetimes andolefin selectivities in the MTO reaction( with SAPOs, the Bronsted acid sites are typicallyi solated). [14] The synthesiso fs mall pore zeolitesw ith cages and higher Si/Al typicallyi nvolve the use of organic quaternary ammonium cations (Structure-DirectingA gents,h ere denoted SDAs).…”

Direct production of olefins via syngas conversion over Co2C-based catalyst in slurry bed reactor

“…The indirect process mainly refers to the methanol to olens (MTO or DMTO) technology, where an intermediate such as methanol or dimethyl ether is synthesized from syngas at rst, then dehydrated to form lower olens using zeolite catalysts. 11,12 Due to the simplied operation and low energy consumption, direct production of olens from syngas has attracted increasing attention. The typical routes of direct production of olens from syngas include bifunctional catalytic reactions and the Fischer-Tropsch to olen (FTO) process.…”