Characterizing the First and Second 27Al Neighbors of Brønsted and Lewis Acid Protons in Zeolites and the Distribution of 27Al Quadrupolar Couplings by 1H{27Al} Offset REAPDOR

Abstract

The application of 1H{27Al} offset rotational-echo adiabatic-passage double-resonance (REAPDOR) NMR experiments enables the indirect observation of the first-order quadrupolar lineshapes for 27Al nuclei in acidic zeolites H-beta and H,Na-A by observing the neighboring 1H nuclei under the influence of dipolar coupling between them. The technique allows one to resolve the contribution of different Al environments, provided that these are characterized by different quadrupolar broadenings, reflecting different structural distortions. Therefore, Bronsted acid sites that typically show large quadrupolar coupling constants (CQ) can be distinguished from extra-framework Lewis sites, which often have considerably smaller quadrupolar couplings. The 1H{27Al} offset REAPDOR patterns can be simulated employing a Gaussian probability distribution using a library of reference offset REAPDOR curves (depending on quadrupolar coupling constant and asymmetry parameter). Monomodal and bimodal probability distributions of CQ...