(337a) Mechanistic Implications of Low-Pressure Feeds for Methanol-to-Olefins Conversion on MFI

The production of methanol and its subsequent conversion on zeolites to produce important petrochemical precursors, such as ethylene and propylene, provides a route for the commercial-scale valorization of unconventional carbon sources such as coal, biomass, and natural gas. The complex network of reactions governing this chemistry can be summarized by a dual catalytic cycle description where olefins-based chemistries of methylation and ß-scission are interconnected with the aromatics-based chemistries of methylation and dealkylation via hydrogen transfer and cyclization. Transient isotopic labeling studies implicate ethylene production predominantly from the aromatics-based cycle, thereby, suggesting that process or material parameters that result in the selective propagation of the olefins-based cycle would in turn result in high propylene-to-ethylene ratios. We implicate inlet methanol partial pressure as the crucial process parameter for preferential propagation of the olefins-based cycle over its aromatics counterpart, regardless of the reaction temperature, on H-ZSM-5. We report

(i) ~10-fold increase in the propylene-to-ethylene ratio (~2.7 – 27) as the inlet methanol pressure was decreased from 52 to 0.6 kPa; carbon selectivities towards propylene increased from ~29 to 41%C when compared at iso-conversion (~30 %C) levels. This effect persisted regardless of the reaction temperature (623 – 773 K). These results clearly suggest that low inlet pressures of methanol can effectively decouple the olefins- and aromatics-based cycles;

(ii) The previously unrecognized role of formaldehyde, obtained from persistent methanol dehydrogenation events, in initiating the aromatics-based cycle. A small co-feed of formaldehyde (~24 Pa) with 0.6 kPa of methanol resulted in ~15-fold increase in ethylene selectivity (from ~1.5 to 22 %C) while the propylene-to-ethylene ratio dropped from ~27 (in the absence of formaldehyde) to ~1 (in the presence of formaldehyde), thereby, advocating suppressed formaldehyde production at low inlet methanol pressures as the critical factor in selective propagation of the olefinic cycle.

These mechanistic studies provide guidance for process conditions to minimize the formation of undesirable products during methanol-to-hydrocarbons conversion on zeolites.