A mechanistic basis for the effects of crystallite size on light olefin selectivity in methanol-to-hydrocarbons conversion on MFI

Abstract

Light olefin selectivity in methanol-to-hydrocarbons conversion on MFI increases with an increase in crystallite size because intra-crystalline residence time of methylbenzenes increases as a consequence of increased transport restrictions, which enables these methylbenzenes to undergo multiple methylation/dealkylation reactions before exiting the crystal. Selectivity toward light olefins, for the reaction of dimethyl ether (DME) at 623K, increased monotonically from 22% in 2nm-MFI (∼2nm crystallites) to 47% in 17μm-MFI (∼17μm crystallites) at 46–59% net DME conversion. Transport restrictions were introduced externally in a conventional MFI sample (500nm-MFI) by single-/multi-cycle silylation using tetraethyl orthosilicate. Light olefin selectivity, for the reaction of DME at 623K and at 46–59% net DME conversion, increased from 33% in the conventional MFI sample to 49% in a sample that had undergone three silylation treatments. Adsorption uptake measurements of 2,2-dimethylbutane were used to estimate the “effective” crystallite size of the silylated MFI samples. Total light olefin selectivity and ethene/(2-methyl-2-butene+2-methylbutane) increased monotonically with the effective crystallite size for all zeolite samples used in this study, irrespective of their provenance, thereby suggesting that the mechanistic basis for increase in light olefin selectivity with increasing crystallite size is the enhanced propagation of aromatics-based catalytic cycle.