Abstract
The chemical industry has exploited zeolite shape selectivity for more than 50 years, yet our fundamental understanding remains incomplete. Herein, the zeolite channel geometry–reactive intermediate relationships are studied in detail using anisotropic zeolite ZSM‐5 crystals for the methanol‐to‐hydrocarbon (MTH) process, and advanced magic‐angle spinning solid‐state NMR (ssNMR) spectroscopy. The utilization of anisotropic ZSM‐5 crystals enabled the preferential formation of reaction intermediates in single‐orientation zeolite channels, as revealed by molecular dynamics simulations and in situ UV/Vis diffuse‐reflectance spectroscopy. The ssNMR results show that the slightly more constrained sinusoidal zeolite channels favor the olefin cycle by promoting the homologation of alkanes, whereas the more extended straight zeolite channels facilitate the aromatic cycle with a higher degree of alkylation of aromatics. Dynamic nuclear polarization experiments further indicate the preferential formation of heavy aromatics at the zeolite surface dominated by the sinusoidal channels, providing further insight into catalyst deactivation.
Highlights
The shape or topology of internal pore structures of zeolites strongly affects the product selectivity by permitting the configuration of different reactants, intermediates or products.[1,2,3] This contributes to highly cost-effective process-[+] These authors contributed to this work
The negligible enhancement for methyl species is due to their fast rotational dynamics even at cryogenic temperatures, which provides an efficient relaxation sink under typical dynamic nuclear polarization (DNP) MAS experimental conditions.[65]
For the aromatics at % 150 ppm, DNP enhancements of 22 and 18 were observed for the a-oriented and b-oriented zeolite crystals, respectively. This indicates the formation of a greater fraction of the large aromatics at the sinusoidal channel dominated surface of the a-oriented zeolite
Summary
The shape or topology of internal pore structures of zeolites strongly affects the product selectivity by permitting the configuration of different reactants, intermediates or products.[1,2,3] This contributes to highly cost-effective process-. The research from our group, as well as from the Gascon and van Speybroeck groups, highlighted the complexity and significance of the host-guest chemistry during the MTH process.[32,33,34,35,36,37] Recently, we have shown distinct product distributions and coking behaviors for zeolite channels with different geometries.[38] These findings altogether motivated us to comprehensively scrutinize the distinctive host-guest chemistry exclusively between zeolite channel geometry and reaction intermediates (Scheme 1) This information is crucial to the design of catalyst materials with maximized product yields and minimized costs.[39,40]. We have applied dynamic nuclear polarization (DNP)[36,42,43,44,45,46] to improve the sensitivity and examine the spatial distribution of the hydrocarbons within the MTH-reacted materials
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