Partial framework-Al in lamellar H-[Al]-RUB-18: acidity by probe TMPO adsorption and catalytic study in the presence and absence of water†

While framework-Al (Al_Td) sites on [Al]-RUB-18 materials with Brønsted acidity were active in ethanol dehydration, the acidity of partial framework-Al sites (Al_Oh or aluminol sites) remains unknown. ³¹P-MAS-NMR of adsorbed trimethylphosphine oxide (TMPO) was used together with catalytic reactions, in the presence and absence of water, to elucidate the acidity features of aluminol sites on lamellar RUB-18. Herein, spectroscopic analysis indicates that the ³¹P signals arise from TMPO adsorbed on the lamella surface of RUB-18 (δ_³¹P = 40–49 ppm), silanol/aluminol sites (δ_³¹P = 49–56 ppm), and Brønsted acid sites (60–75 ppm). The crystallinity degree of the structure, Si/Al and Al_Td/Al_Oh molar ratios, and TMPO loadings, under wet and dry conditions, all lead to changes in ³¹P resonances. Two resonance signals at 64 ppm and 69 ppm were attributed to partial and complete framework-Al sites, respectively. A clear correlation between the adsorption capacity of the active sites and peak intensities in the BAS region proves the influence of water in the adsorption process. This study brings additional evidence that the (SiO₄)₄Al and (SiO₄)₃Al–(OH)(H₂O)₂ species played a crucial role in biomass-derivative conversion reactions. The simultaneous activity of Brønsted/Lewis acid sites was revealed, with the possibility of the (SiO₄)₃Al–(OH)(H₂O)₂ sites acting as LAS. The present study clarifies the acidity features of aluminol sites on lamellar [Al]-RUB-18 and brings attention to the evaluation of solid catalysts under aqueous-phase conditions.