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

5 years ago

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

Tobias Uesbeck, Hubert Koller, Michael Hunger, Michael Ryan Hansen

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, Brønsted acid sites that typically show large quadrupolar coupling constants (C_Q) 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 C_Q were used to fit the experimental data. This approach provides direct information on the neighborhood between Brønsted and Lewis acid sites. Finally, a theoretically expected frequency shift of the ¹H MAS NMR signals as a function of neighboring ²⁷Al nuclei in different spin states with large quadrupolar coupling is discovered experimentally for the first time for a zeolite Brønsted acid site by using offset REAPDOR.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b09544

DOI: 10.1021/acs.jpcc.7b09544