Theoretical and experimental investigation of the effect of proton transfer on the 27Al MAS NMR line shapes of zeolite-adsorbate complexes:: An independent measure of solid acid strength

Assessing the degree of proton transfer from a Bronsted acid site to one or more adsorbed bases is central to arguments regarding the strength of zeolites and other solid acids. In this regard certain solid-state NMR measurements have been fruitful; for example, some C-13, N-15, or P-31 resonances of adsorbed bases are sensitive to protonation, and the H-1 chemical shift of the Bronsted site itself reflects hydrogen bonding. We modeled theoretically the structures of adsorption complexes of several bases on zeolite HZSM-5, calculated the quadrupole coupling constants (Q(cc)) and asymmetry parameters (eta) for aluminum in these complexes and then in turn simulated the central transitions of their Al-27 MAS NMR spectra. The theoretical line width decreased monotonically with the degree of proton transfer, reflecting structural relaxation around aluminum as the proton was transferred to a base. We verified this experimentally for a series of adsorbed bases by way of single-pulse MAS and triple quantum MQMAS Al-27 NMR. The combined theoretical and experimental approach described here provides a strategy by which Al-27 data can be applied to resolve disputed interpretations of proton transfer based on other evidence.