(712g) Tuning Ethylene-to-Propylene Ratio in Methanol-to-Olefins Conversion on Window-Cage Type Zeolites

Methanol-to-olefins (MTO) conversion on window-cage type zeolite or zeotype catalysts produces streams rich in ethylene and propylene (>80% carbon selectivity) as the topological characteristics of these materials prevent egress of heavy hydrocarbons. While cumulatively ethylene and propylene selectivity is high, tuning relative selectivity of the effluent hydrocarbons towards ethylene without deleterious effect on catalyst lifetime remains a major challenge. Here, we demonstrate and rationalize a strategy that modulates ethylene-to-propylene (E/P) ratio during MTO on materials where methylbenzenes entrained within the cavities engage with methanol to give ethylene and propylene.

We demonstrate that varying inlet methanol pressure from 30 kPa to 0.5 kPa at 673K engenders a ~2.3x increment in E/P ratio when compared at similar turnover numbers during MTO on HSAPO-34 at sub-complete methanol conversion while maintaining high olefins selectivity (>85 %C) and increments in catalyst lifetime (>4x). This trend in selectivity can be rationalized based on the effect of methanol pressure on the relative dealkylation rate of tetra-methylbenzene—aromatic precursor to ethylene—to that of penta-methylbenzene and hexa-methylbenzene—aromatic precursors to propylene—as the extent of homologation within the pool of entrained methylbenzenes on HSAPO-34 changes with methanol concentration. This is corroborated by dissolution experiments where reactions were quenched rapidly at similar turnover numbers and catalysts were dissolved in aqueous hydrochloric acid to liberate entrained methylbenzenes. The molar ratio of tetra-methylbenzene to the sum of penta-methylbenzene and hexa-methylbenzene increases from 0.9 to 9.5 with methanol pressure decreasing from 30 kPa to 0.5 kPa, suggesting that decreasing methanol pressure shifts the distribution of entrained methylbenzenes towards tetra-methylbenzene, which, consequently, increases the ethylene selectivity during MTO. This strategy and its mechanistic understanding can be extended to modulate product selectivity on HSSZ-13 where E/P ratio increases by a factor of ~2.4x upon decreasing methanol pressure from 30 kPa to 0.025 kPa.