Unveiling the acid properties of metallosilicate nanospheres: a combined experimental and computational study

The design of solid acid catalysts with tunable Brønsted and Lewis acidity is essential for biomass valorization reactions. This study explores the synthesis, characterization, and catalytic behavior of extra-small mesoporous aluminosilicate nanospheres for the acetalization of glycerol to solketal. The synthesis was optimized by varying the Si/Al ratio, the nature of the aluminum precursor and the dissolution medium, concluding that ethanol afforded uniform nanospheres with high intra-framework Al incorporation. In-depth characterization via TEM, ICP-OES, N2 physisorption, 27Al ssNMR and FT-IR spectroscopy using NH3 and CO probes revealed that increasing the Al content enhances overall acidity, particularly Brønsted sites. However, materials with overly strong Brønsted acidity exhibited reduced activity due to too strong interaction with reaction intermediates leading to catalyst poisoning. The aluminosilicate catalyst synthesized using Al(NO3)3 with a Si/Al ratio of 74 showed the best performance, with a balanced Brønsted/Lewis ratio and moderate acid strength. Comparative studies with gallosilicates and Al–Ga bi-functional materials demonstrated that both Brønsted and Lewis sites contribute synergistically to the reaction path, with optimal performance at an intermediate B/L ratio of 3.5. DFT-based simulations of IR spectra, structure, and deprotonation energies further rationalized the experimental findings, thus providing a solid background for a straightforward structure/activity correlation. These insights offer a set of valuable design principles for the synthesis of advanced heterogeneous acid catalysts.