Effect of Siloxane Ring Strain and Cation Charge Density on the Formation of Coordinately Unsaturated Metal Sites on Silica: Insights from Density Functional Theory (DFT) Studies

Amorphous silica (SiO2) is commonly used as a support in heterogeneous catalysis. However, because of the structural disorder and temperature-induced change of surface morphology, the structures of silica-supported metal catalysts are difficult to determine. Most studies are primarily focused on understanding the interactions of different types of surface hydroxyl groups with metal ions. In comparison, the effect of siloxane ring size on the structure of silica-supported metal catalysts and how it affects catalytic activity is poorly understood. Here, we have used density functional theory (DFT) calculations to understand the effect of siloxane ring strain on structure and activity of different monomeric Lewis acid metal sites on silica. In particular, we have found that large siloxane rings favor strong dative bonding interaction between metal ion and surface hydroxyls, leading to the formation of high-coordinate metal sites. In comparison, metal–silanol interaction is weak in small siloxane rings, resul...