Conversion of Methanol to Alkenes over Medium-and Large-Pore Acidic Zeolites : Steric Manipulation of the Reaction Intermediates Governs the Ethene/Propene Product Selectivity

Methanol conversion over H-beta and H-ZSM-5 zeolite catalysts is compared at identical reaction conditions (temperature = 350 °C, WHSV = 7.0 gg-1 h-1), and it is found that H-ZSM-5 yields seven times more ethene relative to propene than H-beta yields. By switching from a 12C methanol feed to 13C methanol, the transient incorporation of 13C atoms from methanol into the gas-phase products and reaction intermediates located within the zeolite voids is followed. For H-beta, penta- and hexamethylbenzene are involved in a hydrocarbon-pool-type mechanism, leading predominantly to propene and the higher alkenes, whereas for H-ZSM-5, the lower methylbenzenes are the intermediates and represent the only route for ethene formation. This work outlines how the zeolite topology determines the type of intermediate participating in the catalytic cycle, thereby controlling the ethene/propene product selectivity in the methanol to olefins reaction.

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