(349j) Synthetic Approaches to Independently Vary Crystallite and Atomic-Scale Acid Site Distributions and Their Catalytic Consequences for Propene Oligomerization

Brønsted acid zeolites catalyze alkene oligomerization, a reaction relevant for upgrading light alkenes to heavier molecular weight products useful as transportation fuels. The distribution of acid sites throughout zeolite crystallites has been reported to affect propene oligomerization rate and selectivity; yet, consensus is lacking among literature reports, in part reflecting concomitant variations in active site density and proximity at atomic and crystallite length scales in commercially available materials. Here, we describe synthetic approaches using organic and inorganic structure-directing agents (SDAs) of different size and charge, and mixtures thereof, to crystallize zeolites of the MFI, MEL, and TON topologies. The use of different SDA mixtures crystallized zeolites in composition ranges (Si/Al) consistent with expectations from charge density mismatch theory, but led to concomitant changes in the local arrangement of framework Al heteroatoms, as quantified by experimental titration of proximal Al sites with Co²⁺. In general, co-occlusion of organic and inorganic SDAs led to greater Al incorporation (lower Si/Al) and higher fractions of Al-Al site ensembles, while competitive occlusion of organic and inorganic SDAs led to crystallization of zeolites with similar composition and Al arrangement. Turnover rates and selectivities for propene reactions on MFI, MEL and TON zeolites in their H-form were measured (7-610 kPa, 503 K) on samples of varying crystallite size, and H⁺ site density and arrangement. Rates and selectivity for propene reactions on TON zeolites were used to study the effects of active site density in zeolites of different pore dimensionality. Overall, this work demonstrates the importance of zeolite synthesis approaches that enable independent variation of active site and crystallite properties for decoupling their effects on propene oligomerization catalysis.