Ensemble of Active Acid Sites on Amorphous Silica–Alumina Surfaces for Catalytic Cracking

Amorphous silica–alumina are critical materials in catalysis, particularly for fluid catalytic cracking (FCC). However, the atomic scale understanding of the active sites has been challenging, because of the nonuniform atomic distribution and the material's amorphous nature. Here, we use density functional theory (DFT), machine learning potentials and sampling methods to investigate the relationship between structure and acidity in silica-modified alumina. Under FCC conditions, we predict an ensemble of acid sites with diverse local structures and a spectrum of acid strengths, including zeolite-like bridging Brønsted acid sites (BAS) and pseudo-bridging silanol BAS. This distribution is influenced by surface structure, Si coverage, and extent of hydroxylation, shaped by synthesis methods and reaction conditions. Experiments using a model Si-stabilized alumina catalyst confirm that Brønsted acidity increases with Si content, peaking at an optimal value before declining. These insights provide a foundation for designing efficient solid acid catalysts for industrial applications.