(594b) Influence of the Crystallite-Scale Spatial Distributions of Framework Al Sites in MFI Zeolites on Propene Oligomerization Catalysis

Propene oligomerization to heavier molecular weight products are useful reaction steps in converting light hydrocarbons to transportation fuels. During propene oligomerization on Brønsted acidic zeolites, secondary β-scission reaction pathways occur and shorten carbon chains, wherein the selectivity of β-scission relative to oligomerization is affected by intrazeolite residence times. According to reaction-transport formalisms, intrazeolite residence times are proportional to the bulk proton density ([H⁺]) and the square of the diffusion path length (R²), and inversely proportional to the effective molecular diffusivity (D_e). Inhomogeneous crystallite-scale Al distributions, including intracrystalline spatial gradients and acid sites located at extracrystalline surfaces, have been reported to influence rates, selectivities and deactivation during acid-catalyzed reactions. Here, we synthesized core@shell MFI materials to independently control the crystallite-scale Al distribution at fixed diffusion path lengths by varying the Si-MFI shell thickness, and similarly for the inverse gradient consisting of an Si-MFI core and Al-MFI shell. Core@shell samples were synthesized hydrothermally by adapting two-step synthesis from published techniques, including annealing the Al-MFI core and varying the Si/Al ratio in growth solution for the Si-MFI core. Bulk elemental analysis, electron microscopy and transient sorption techniques were used to characterize the effective diffusion path lengths, surface permeability and crystallite-scale Al distributions of core@shell zeolites. In addition, transient changes in observed rates upon step-changes in reactant pressure (0-600 kPa C₃H₈) were measured to investigate the extent of diffusion limitations on dimerization rates and product selectivities, and compared to conventional MFI materials, to assess the influences of inhomogeneous crystallite-scale acid site distributions. This study provides synthetic strategies and characterization techniques to design catalysts with tunable active site distributions at the crystallite length scale and a method to deduce mechanistic insights regarding the roles of kinetics and transport on governing the behavior of complex reaction networks in zeolites.