Bronsted and lewis acidity of the BF3/γ-Al2O3 alkylation catalyst as revealed by solid-state NMR spectroscopy and DFT quantum chemical calculations

Multinuclear solid-state NMR techniques and DFT quantum chemical calculations were employed to investigate the detailed structure of acid sites on the BF3/gamma-Al2O3 alkylation catalyst. The NMR experiment results indicate that gaseous BF3 is able to react with the hydroxyl groups present on the surface of gamma-Al2O3, leading to the formation of new Bronsted and Lewis acid sites. The H-1/B-11 and H-1/Al-27 TRAPDOR (TRAnsfer of Population in DOuble Resonance) experiments suggest that the 3.7 ppm signal in H-1 NMR spectra of the BF3/gamma-Al2O3 catalyst is due to a bridging B-OH-Al group that acts as a Bronsted acid site of the catalyst. On the other hand, a Lewis acid site on. the surface of the catalysts, as revealed by (31)p MAS and (31)p/Al-27 TRAPDOR NMR of adsorbed trimethylphosphine, is associated with three-coordinate -OBF2 species. C-13 NMR of adsorbed 2-C-13-acetone indicates that the Bronsted acid strength of the catalyst is slightly stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which is in good agreement with theoretical prediction. In addition, DFT calculations also reveal the detailed structure of various acid sites formed on the BF3/gamma-Al2O3 catalyst and the interaction of probe molecules with these sites.