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.
Keywords: NMR spectroscopy; arenes; olefins; reaction mechanisms; zeolites.
© 2020 The Authors. Published by Wiley-VCH GmbH.