Multinuclear solid-state NMR characterization of the Brønsted/Lewis acid properties in the BP HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve using adsorbed TMPO and TBPO probe molecules

•New approach to unambiguously assign internal/external and Lewis/Brønsted acidity.•Using probe molecules to identify the acid species using 1D/2D 11B, 31P and 1H SSNMR.•Proposed model based on NMR explaining boron species generated from in situ steaming.•Proton affinities estimated based on 31P NMR for all identified acid sites.•NMR quantification of boron species associated with Brønsted/Lewis acid sites.

The acid properties of a dehydrated borosilicate, HAMS-1B (H-[B]-ZSM-5), including the acid types, strengths, location, and quantities are investigated by means of trialkylphosphine oxides through multinuclear 1D/2D MAS NMR experiments. 11B DQF-STMAS combined with 1H MAS NMR studies revealed B–OH and distinct Si–OH protons associated with trigonal boron. 31P NMR spectra of TMPO-treated HAMS-1B reveal three Brønsted and three Lewis acid sites. We have found a number of limitations applying the TMPO/TBPO method to identify internal/external acidity. Therefore, we propose a new approach to unambiguously discriminate external/internal acid sites by treating a pore-free and pore-blocked HAMS-1B zeolite. This method provided unique structural insight regarding the identification of boron species/coordinations associated with Brønsted/Lewis acid sites. Additionally, ICP analysis in tandem with solid-state NMR enabled full assignment of the detected internal/external acid species and the study of their acid strength. Moreover, we identify the nature of TMPO complexes arising from Brønsted/Lewis interactions.

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