(445c) Deconvoluting the Competing Effects of Zeolite Framework Topology Versus Diffusion Path Length on Methanol-to-Hydrocarbon Reactions
Micropore topology and crystal size are two independently adjustable properties that govern the internal mass transport limitations of zeolite catalysts. Deciphering the relative impact of each factor on catalyst performance is often nontrivial, owing to the difficulty in synthesizing zeolites with predetermined physicochemical properties. In this talk, we describe the preparation of a series of ZSM-11 (MEL) and ZSM-5 (MFI) catalysts of equivalent acidity, but differing pore architecture, which are prepared with well-defined crystal sizes to elucidate the effects of diffusion path length versus topology on catalyst lifetime and selectivity. To this end, we selected the methanol to hydrocarbons (MTH) reaction to assess the impact of design variables on the hydrocarbon pool (HCP) mechanism. In situ UV-Vis spectroscopy is used to investigate the evolution of both active HCP species and heavier aromatic coking species during the transient start up period over both catalysts. Our findings reveal that slight variations in framework topology between MEL and MFI zeolites lead to marked differences in their catalytic performance as well as the evolutionary behavior of HCP species within the zeolite pores. We postulate that the diffusion limitations imposed by the tortuous channels in ZSM-5 catalysts are analogous to increasing the channel length in ZSM-11 catalysts via larger crystal sizes. Notably, we observe similar (albeit slightly offset) trends in MTH selectivity, HCP speciation, and catalyst lifetime for both zeolite framework types; however, differences in pore topology and catalyst size exact different effects on the evolution of intra-crystalline hydrocarbon species. Collectively, the combination of catalyst synthesis, MTH reactions, and in situ characterization confirm the advantage of reduced catalyst size and provide compelling evidence that ZSM-11 is an optimal medium-sized pore zeolite catalyst for reactions where rapid coking can lead to premature deactivation.