Brønsted acid zeolites catalyze alkene oligomerization, a reaction relevant for upgrading light alkenes to heavier products useful as liquid transportation fuels. For widely studied zeolites, the crystallite size and acid site density and proximity are reported to affect alkene oligomerization rate and selectivity; yet, consensus among prior reports is lacking, reflecting the dissimilar reaction conditions used, the concurrent variations in active site and crystallite properties in zeolites crystallized via typical hydrothermal routes, and the influence of intrazeolitic diffusional constraints that are unaccounted for in kinetic-based interpretations of rate data. Here, we synthesize MFI and other zeolite topologies with independently varied crystallite size and acid site density and proximity, and determine their effects on oligomerization rates and selectivity. We provide evidence that bulky alkenes gradually accumulate within micropores of ten-membered ring (10-MR) during oligomerization catalysis, and that this pool of occluded hydrocarbons changes in composition with reaction conditions and material properties, imposing intracrystalline diffusion barriers that influence rates of molecular transport. Measured rates and selectivities of alkene oligomerization reflect the inextricably linked influences of the kinetic properties of active sites and diffusional constraints imposed by occluded intrapore hydrocarbons, leading to complex dependences of rates and selectivity on alkene pressure and material properties among 10-MR zeolite catalysts. This work illustrates how synthetic methods to independently vary atomic and bulk properties of zeolite catalysts, combined with transient and steady-state kinetic measurements, can elucidate how the confluence of kinetic and transport effects determines rates and product selectivities in complex reaction networks.

Hydrocarbon products occluded within zeolite micropores impose transport barriers that regulate Brønsted acid-catalyzed alkene oligomerization

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.