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 Brønsted 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 13C, 15N, or 31P resonances of adsorbed bases are sensitive to protonation, and the 1H chemical shift of the Brønsted 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 (Qcc) and asymmetry parameters (η) for aluminum in these complexes and then in turn simulated the central transitions of their 27Al 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 27Al NMR. The combined theoretical and experimental approach described here provides a strategy by which 27Al data can be applied to resolve disputed interpretations of proton transfer based on other evidence.