Influence of Brønsted Acid-Site Density on Reaction-Diffusion Phenomena that Govern Propene Oligomerization Rate and Selectivity in MFI Zeolites

Brønsted acid zeolites catalyze propene oligomerization to higher-molecular-weight alkenes. Previous studies that only consider kinetic factors have reached contradictory conclusions regarding the dependence of oligomerization rates on H+-site density in MFI zeolites. Herein, we account for both the reaction and diffusion phenomena that govern the rates and selectivity of propene oligomerization and interrogate suites of MFI samples synthesized with widely varying H+-site density (0.3–5.7 H+/unit cell; Si/Al 13–250) and crystallite size (0.03–2.65 μm). Heavier alkene products become occluded within zeolitic micropores during reaction and impose intrazeolite diffusional restrictions for all MFI samples over wide ranges of reaction conditions (483–523 K, 7–615 kPa C3H6). This occluded organic phase becomes heavier in composition at higher propene pressures and lower reaction temperatures, which favor the growth of higher-molecular-weight oligomers, resulting in effective diffusivities of propene and product alkenes that systematically decrease with increasing propene pressure. Analysis of product selectivity reveals that MFI samples of lower H+-site density possess higher rate constant ratios for trimerization relative to dimerization, leading to the formation of heavier alkenes within their micropores and, in turn, to a greater decrease in the effective diffusivity of propene and product alkenes with increasing propene pressure. As a result, measured dimerization rates on MFI samples, which convolve the influences of kinetic factors and intrazeolite diffusional constraints imposed by the occluded organic phase, exhibit an increasingly negative-order dependence on propene pressure with decreasing H+-site density. The combined influences of kinetic factors and intrazeolite diffusional constraints result in different dependences of oligomerization rate on H+-site density when measured at different propene pressures, reconciling apparent contradictions among previous reports. Overall, these findings reveal the strong influences of H+-site density in MFI zeolites on both the kinetics of propene oligomerization reactions and the composition of the product-derived intrazeolite organic phase that influences effective diffusivities, and consequently measured propene oligomerization rates and product selectivity.