Hydrocarbon Products Occluded within Zeolite Micropores Impose Transport Barriers that Regulate Brønsted Acid-Catalyzed Propene Oligomerization.

Abstract

Brønsted acid zeolites catalyze alkene oligomerization to heavier hydrocarbon products of varied size and branching. Propene dimerization rates decrease monotonically with increasing crystallite size for MFI zeolites synthesized with fixed H+-site density, revealing the strong influence of intrazeolite transport limitations on measured rates, which has gone unrecognized in previous studies. Transient changes in dimerization rates upon step-changes in reactant pressure (150-470 kPa C3H6) or temperature (483-523 K) reveal that intrazeolite diffusion limitations become more severe under reaction conditions that favor the formation of heavier products. Together with effectiveness factor formalisms, these data reveal that product and reactant diffusion, and consequently oligomerization rates and selectivity, are governed by the composition of hydrocarbon products that accumulate within zeolitic micropores during alkene oligomerization. This occluded organic phase strongly influences rates and selectivities of alkene oligomerization on medium-pore zeolites (MFI, MEL, TON). Recognizing the coupled influences of kinetic factors and intrazeolite transport limitations imposed by occluded reaction products provides opportunities to competently tailor rates and selectivity in alkene oligomerization and other molecular chain-growth reactions through judicious selection of zeolite topology and reaction conditions.