The ability to selectively control crystallization to achieve desired material properties requires detailed understandings of the thermodynamic and kinetic factors regulating crystal nucleation and growth. Combining this fundamental knowledge with innovative approaches to tailor crystal size/morphology, structure, and chemical composition can lead to the production of materials with superior properties beyond what is achievable by conventional routes. In catalytic processes, micropore topology and crystal size are two independently adjustable properties that govern the internal mass transport limitations. Deciphering the relative impact of each factor on catalyst performance is often nontrivial owing to the inability to synthesize zeolites with predetermined physicochemical properties. In this talk, comparisons will be made among zeolites MFI (ZSM-5), MEL (ZSM-11), and CHA (SSZ-13 and SAPO-34) over a range of physicochemical properties. Zeolites were prepared with well-defined crystal sizes to elucidate the effects of diffusion path length versus topology on catalyst lifetime and selectivity. For these studies, we selected the methanol to hydrocarbons (MTH) reaction to assess the impact of design variables on the hydrocarbon pool (HCP) mechanism. Our findings reveal that variations in framework topology lead to marked differences in the evolutionary behavior of HCP species within the zeolite pores. We will discuss how diffusion limitations imposed by tortuous channels and/or long diffusion path lengths can have significant effects on MTH selectivity and HCP speciation. Moreover, we will discuss synthesis strategies to mitigate intracrystalline diffusion limitations using commercially viable pathways to tailor zeolite properties for improved catalytic performance.

Mitigating the effects of diffusion limitations in zeolite catalysis

2019 Fall National Meeting

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