Interactions of phosphorous molecules with the acid sites of H-Beta zeolite: Insights from solid-state NMR techniques and theoretical calculations

The local structures of various Brønsted and Lewis acid sites in H-Beta zeolite were resolved with the combined 31P MAS NMR, 31P–27Al TRAPDOR NMR experiments and theoretical calculations at different levels. In addition, the interacting mechanisms of these acid sites with probe molecules such as trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO) were clarified, which greatly aids the understanding of acid catalysis. Owing to the narrow chemical shift range and close Brønsted acid strengths, only an average resonance at −4.5 ppm was observed in TMP adsorbed H-Beta zeolite, consistent with the calculated data of acidities (substitution energies and proton affinities), geometries, adsorption energies as well as 31P chemical shifts. However, two types of Brønsted acids were distinguished by TMPO, and the HF/DZVP2 (MP2/DZVP2) chemical shifts were calculated at 68.1 (69.5) and 69.7–72.1 (71.7–74.9) ppm, respectively. Two types of Lewis acids were identified at −32.0 and −47.0 ppm with the latter exhibiting strong 31P–27Al TRAPDOR effects. With theoretical calculations, these two peaks were attributed to the extra-lattice oxo-AlOH2+ species and the three-fold coordinated lattice-Al, extra-framework Al(OH)3, oxo-AlO+ species, respectively. The peak at −60.0 ppm was conventionally assigned to the TMP physisorption, but the calculations indicated that the EFAL monovalent Al(OH)2+ species coordinating with two lattice-O atoms near the framework Al atom can contribute to it as well.

Acids play an important role in zeolite catalysis whereas their natures are currently under hot debates. With solid-state NMR techniques and theoretical calculations at various levels, two types of Brønsted acid sites and three types of Lewis acid sites in H-Beta zeolite were resolved. In addition, the interaction modes of TMP with all these acid sites were provided.Figure optionsDownload as PowerPoint slide