Synthetic methods to vary crystallite properties of TON zeolites and their consequences for Brønsted-acid catalyzed propene oligomerization

Abstract

Brønsted acid zeolites catalyze light alkene oligomerization to higher molecular weight alkenes. Rates and selectivities of propene oligomerization in MFI zeolites are regulated by intrazeolite diffusional constraints imposed by heavier alkene products that become occluded within zeolitic micropores during reaction, and by active site properties that influence kinetic rate constants. Propene oligomerization rates and selectivity are also reported to vary with zeolite topology; however, evaluating the relative influences of kinetic and diffusional factors on oligomerization rate and selectivity and their dependence on zeolite framework topology is challenged by a dearth of synthesis methods to crystallize zeolite topologies with independently varied active site and crystallite properties. Herein, we use combinations of different structure directing agents to crystallize TON zeolites, which comprise medium-pore unidirectional straight channels, with varied crystallite sizes (0.9–5.4 μm) at similar H+-site density (∼0.3 H+/unit cell (u.c.)). Propene dimerization rates (per H+, 503 K) decrease systematically with increasing TON crystallite size over a wide range of propene pressures (16–607 kPa C3H6), reflecting the strong influence of intrazeolite diffusion limitations. The effective diffusivity of propene, estimated through the effectiveness factor formalism, systematically decreases (by ∼5×) with increasing propene pressure, indicating that the higher concentrations of heavier alkene products formed at higher propene pressures inhibit intrazeolite diffusion; however, this decrease is less severe compared to that for MFI samples of similar H+-site density. An analysis of product selectivity on zeolite topologies of independently varied pore size and dimensionality (TON, MFI, MOR) reveals that the smaller pore size of TON (∼5 Å diam.) leads to higher rate constants for propene dimerization, but also restricts the growth of larger oligomer products (e.g., ≥ C9), resulting in a lighter composition of occluded products that is also more weakly influenced by changes in propene pressure compared to those present in MFI (∼5–7 Å diam.). Together, these findings reveal the distinct roles of zeolite pore size and interconnectivity in governing kinetic factors and the composition of occluded products that regulate intrazeolite diffusion during propene oligomerization catalysis in medium-pore zeolites, and thereby rate and selectivity.