Solid-state MAS NMR studies of Brønsted acid sites in zeolite H-Mordenite

¹⁷O- ¹H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Brønsted acid sites with different O-H distances. The accessibility of the various Brønsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Brønsted acid sites in H-Mordenite (H-MOR) were proposed. The use of ¹⁷O chemical shift correlations to help assign sites is discussed.