Tuning the ethylene-to-propylene ratio in methanol-to-olefins catalysis on window-cage type zeolites

Decreasing inlet methanol partial pressure and methanol space velocity during methanol-to-olefins (MTO) catalysis on HSAPO-34, HSSZ-13, and HSSZ-39 increases cumulative ethylene-to-propylene ratio by a factor of ~2.3x, ~2.4x, and ~2.2x, respectively. The composition of hydrocarbons occluded in the catalyst when compared at a similar turnover number during MTO suggests that decreasing methanol concentration in the catalyst bed decreases the extent of methylbenzene homologation within the pool of entrained aromatics, resulting in increments in the concentration of aromatic precursors to ethylene relative to those for propylene. Ethylene-to-propylene ratio changed by a factor of ~1.3x within one catalyst turnover and <10 s on-stream after a step-change in the inlet methanol pressure; in this duration the speciation of entrained methylbenzenes is unchanged as less than 1 mol_C mol_H+⁻¹ was converted to hydrocarbons, revealing that instantaneous ethylene and propylene selectivity depends sensitively on local methanol concentration. These mechanistic insights identify local methanol concentration as the salient parameter in modulating product selectivity during MTO on window-cage type materials.