¹⁷O magic angle spinning NMR studies of Brønsted acid sites in zeolites HY and HZSM-5

High-resolution ¹⁷O/¹H double resonance NMR spectra were obtained for two zeolites, one with a low Si/Al ratio (zeolite HY) and one with a high Si/Al ratio (HZSM-5), to investigate their local structure and Brønsted acidity. Two different oxygen signals, corresponding to Brønsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D) ¹H-¹⁷O heteronuclear correlation (HETCOR) NMR spectra allowing the ¹⁷O isotropic chemical shift (δ_cs) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, η) for the two oxygen atoms to be extracted. Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY. ¹⁷O-¹H rotational echo double resonance (REDOR) NMR was applied to probe the O-H distances in zeolites HY and HZSM-5. Weaker ¹⁷O-¹H dephasing was observed for zeolite HZSM-5 in comparison to that of HY, consistent with longer O-H bonds and/or increased proton mobility.