Impact of Hierarchical ZSM-5 Catalyst Structure and Compositional Gradients on the Methanol-to-Hydrocarbons Reaction

Abstract

The design of hierarchical zeolites through post-synthetic modification offers versatile routes to tune their catalytic performance by engineering properties, such as morphology and aluminum distribution, which impact diffusion and reaction(s) within their confined porous networks. In this study, we use secondary growth to produce a library of finned and core-shell MFI-type zeolites with tailored nanosized features and mesoscopic gradients in acid site density. Structure–performance relationships are developed using methanol to hydrocarbons (MTH) to assess catalysts with distinct active (ZSM-5) and inactive (silicalite-1) regions. We show that catalysts with inactive interiors used as seeds to generate ZSM-5 fins and shells function as pseudo nanoparticles and nanosheets, respectively. These materials outperform their silicon-zoned counterparts; however, the introduction of Si-rich exteriors (fin or shell) achieves longer catalyst lifetime compared to conventional ZSM-5 catalysts due to enhanced mass transfer through siliceous regions and surface passivation, which mutually reduce external coking. Comparisons between finned and core-shell catalysts of similar compositional architectures reveal that finned structures consistently achieve higher cumulative turnovers and longer lifetimes, highlighting the benefits of creating localized high densities of pore openings for rapid exchange of molecules into and out of microporous channels. Combined Al X-ray absorption spectroscopy, solid-state NMR, and acid titrations reveal that secondary growth generates a distribution of tetrahedral Al sites with distorted Al–O bonds. The ensemble average of active sites is distinct for each hierarchical catalyst, countering a common belief that framework Al sites have identical Brønsted acidity. Collectively, our findings demonstrate the advantages of introducing fin or shell configurations by secondary growth; and the significance of controlling spatial distributions of active sites on exterior regions of hierarchical zeolite catalysts to optimize catalytic performance. These results are demonstrated for the MTH reaction, but we envision similar outcomes could be achieved for other hydrocarbon upgrading processes.