NMR-Solved Proton Fine Locations Reveal the Dynamic Flexibility of Brønsted Acid Sites in the SAPO-34 Molecular Sieve.

As Brønsted acid sites (BASs) serve as the active centers in zeolite-catalyzed reactions, comprehending their precise location, population, and dynamic behavior is crucial for catalyst optimization─yet their characterization remains a significant challenge. Here, we demonstrate that 29Si NMR exhibits high environmental sensitivity in silicoaluminophosphate molecular sieves, allowing us to refine proton locations via NMR crystallography and to elucidate their inherent dynamics. Advanced solid-state NMR experiments, supported by 29Si/17O isotopic enrichment, correct the long-standing misassignment in 29Si NMR spectroscopy. Combined with theoretical calculations and neutron diffraction, these experiments precisely identify protons on all four crystallographic oxygen sites within SAPO-34, preferentially located at the intersection of two four-membered rings (O1 + O4 = 76%). Broad-range variable-temperature 1H/29Si NMR spectra (22-650 °C) reveal that protons begin hopping among the four tetrahedral oxygen sites of the T-site above 200 °C (tens to hundreds of Hz), and eventually attain dynamic uniformity above 500 °C (hundreds to over 1000 Hz). Basic molecules such as NH3 and amines ionize the proton even upon single-molecule adsorption, whereas water requires a larger proton-water complex for significant deprotonation (3 molecules) and high fluxionality (>5 molecules). These mechanistic insights elucidate the subtle nature of BASs in SAPO-34, offering a foundation for rational catalyst design.