(52g) Elucidating the Acid Site Strength and Distribution on Amorphous Aluminosilicates.

In heterogeneous catalysis, amorphous aluminosilicates (AS) primarily function as supports for active materials in catalytic reactions due to its superior thermal/ mechanical stability and high surface area. The acid sites on amorphous AS significantly differ from those on crystalline AS like zeolites. The complex and often non-intuitive interactions between alumina and silica can yield unique Lewis and Brønsted acid sites. The acid site strength and distribution are influenced by various factors, including synthesis, pretreatment, and reaction conditions, presenting challenges in developing an accurate atomistic model. The origins of the Brønsted and Lewis acidity of AS remains an ongoing research question.

Computational investigation of these systems can help elucidate the atomistic details of the acid sites. However, current thermodynamic approaches have limitations in accurately describing the system due to their inability to capture the wide range of structurally and chemically diverse site environments. To address these limitations, we developed new sampling techniques that integrate global optimization techniques such as basin hopping with graph-based enumeration schemes. Additionally, the simulations operate within a grand canonical ensemble to precisely capture surface hydroxylation under specific pressure and temperature conditions, thereby accurately representing the dynamic behavior of the acid sites. We validate the acid site strengths against the model Si-doped γ-alumina surfaces, using the extensive literature data available for these systems. Furthermore, we predict acid site distributions as a function of hydrogen and oxygen chemical potential. This comprehensive study on amorphous AS contributes to establishing correlations between catalytic activity and specific experimental synthesis and reaction conditions.