Acidity of Amorphous Silica-Alumina (ASA) Catalysts

First-principles calculations reveal the dominant acid sites on the amorphous silica-alumina (ASA) catalyst surface. Based on the strength of interaction with Lewis base probe molecules, the bridging and pseudo-bridging silanol (PBS) species are determined to be the most acidic and therefore catalytically active groups. Evident in the calculations is the interplay between the Lewis acid and the Brønsted acid sites on the ASA catalyst surface, giving rise to the enhanced acidity of the PBS species. This work provides mechanistic insight to inform efforts at rationally engineering enhanced ASA-based solid acids.