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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