Brønsted acid zeolites catalyze alkene oligomerization, a route relevant for upgrading light hydrocarbons to heavier products useful as liquid transportation fuels. Zeolite crystallite size and acid site density and proximity have been reported to affect oligomerization rate, selectivity, and deactivation; 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 on measured rate data. Here, we synthesize MFI and other zeolites with independently varied crystallite size and acid site density, location and proximity, and determine the effects of these material properties on oligomerization catalysis. 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 molecular transport rates. The coupled kinetic and diffusional influences imposed by occluded intrapore hydrocarbons lead to complex dependences of rates and selectivity on alkene pressure and material properties for 10-MR zeolite catalysts. These insights provide guidance for catalyst design strategies to alleviate transport limitations allowing zeolite materials to show rates and product selectivity that remain stable with time-on-stream.
