Deconvoluting the Competing Effects of Zeolite Framework Topology and Diffusion Path Length on Methanol to Hydrocarbons Reaction

Abstract

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 inability to synthesize zeolites with predetermined physicochemical properties. In this study, a series of ZSM-11 (MEL) and ZSM-5 (MFI) catalysts of equivalent acidity, but differing pore architecture, are 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. Operando UV-vis microspectroscopy is used to investigate the evolution of 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 report 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 and HCP speciation for both zeolite framework types; however, differences in pore topology and catalyst size exact different effects on the evolution of intracrystalline hydrocarbon species. Collectively, these findings provide evidence that ZSM-11 is an effective medium-sized pore zeolite catalyst for reactions encumbered by rapid coking that often elicits premature deactivation.