(668f) Assessing the Influences of Kinetics and Intrazeolite Diffusion on Propene Oligomerization Selectivity in Brønsted Acid Zeolites

Brønsted acid zeolites catalyze alkene oligomerization to transportation fuels, a critical step in upgrading feedstocks with lower carbon footprints. Rates and selectivity of propene oligomerization on medium-pore zeolites are strongly influenced by kinetics and intrazeolite diffusional constraints imposed by heavy alkene products that accumulate with time within the zeolite micropores. Selectivity is influenced by zeolite material properties, including H⁺-site density (H⁺/u.c.), crystallite size, and topology. However, these influences are obscured by the inherently coupled influences of kinetics and intrazeolite diffusional constraints and by concurrent changes in zeolite properties among materials crystallized by conventional synthesis methods.

Herein, we crystallize H-zeolites (H-MFI, H-TON) with independently varied H⁺-site density (H⁺/u.c. 0.3–5.7) and crystallite size (0.3–5.4 μm) and examine effects of these material properties on propene oligomerization selectivity (7–630 kPa C₃H₆, 503 K). Effectiveness factor formalisms were used to calculate a parameter (k×De)^1/2, where k is the effective rate constant and D_e is the effective reactant diffusivity. (k×De)^1/2 decreased monotonically with time-on-stream for all samples, leading to the selective formation of C₆ alkenes at fixed conversion in MFI. Selectivity compared at fixed conversion and at a fixed extent of intrazeolite diffusional constraints reveals that smaller crystallites and lower H⁺-site density yield higher selectivity to higher-rank products, reflecting differences in intrazeolite propene concentration gradients with crystallite size and kinetic rate constants for chain-growth with H⁺-site density. By contrast, selectivity on TON was weakly influenced by changes in propene pressure or sample properties, reflecting limitations on the growth of heavier oligomers (C₉₊) in the smaller pores of TON (~ 5 Å). Overall, these findings demonstrate and rationalize the distinct influences of H⁺-site density, crystallite size, and pore size on kinetics and intrazeolite diffusional constraints during propene oligomerization and provide an analytical framework for interpreting selectivity in reactions influenced by kinetics and intrapore diffusion.