Assessing the influences of kinetics and intrazeolite diffusion on propene oligomerization product selectivity in MFI zeolites

The inherently coupled influences of kinetics and intrazeolite diffusional constraints, together with the concurrent changes in zeolite properties among materials crystallized by conventional synthesis methods, obscure the relationship between propene oligomerization product selectivity and zeolite properties. Herein, we use a suite of MFI zeolites crystallized with independently varied H+-site density (0.3-5.7 H+/u.c.) and crystallite size (0.1-0.9 mu m) together with effectiveness factor formalisms to assess the influences of kinetics and intrazeolite diffusional constraints on propene oligomerization selectivity, and the independent contributions of H+-site density and crystallite size to these influences. Extracting the kinetic influences of MFI properties and reaction conditions on propene oligomerization selectivity requires data measured at conditions where the extent of intrazeolite diffusional constraints imposed by products that occlude within MFI micropores and decrease effective diffusivity is fixed. Applying this framework to analyze product selectivity on MFI samples of varied crystallite size (0.3 H+/u.c.) reveals that selectivity to higher-rank products decreases with crystallite size because larger crystallites cause more severe intrazeolite propene concentration gradients, thereby decreasing rates of trimerization and higher-rank oligomerization reactions disproportionately relative to dimerization. MFI samples of higher H+-site density exhibit lower selectivity to higher-rank products, but a higher selectivity to oligomer products, reflecting differences in both kinetic rate constants and the composition of occluded products among these samples. Together, these findings demonstrate and rationalize the influences of H+-site density and crystallite size on propene oligomerization selectivity on MFI zeolites and provide a framework through which selectivity in transport-limited systems involving chain-growth reactions can be evaluated.