Quantifying the fraction and activity of catalytic sites at different surface densities of aminosilanes in SBA-15 for the aldol reaction and condensation

Abstract

The complexity of aminosilica materials manifests as catalysts with multiple types of active sites. Whereas previous work has assumed that each site contributes equally to the observed catalytic behavior, this work rigorously investigates the catalytic activity on a per site basis using site quantification experiments. These experiments reveal that aminosilica materials have four types of catalytic sites, including high, medium, and low activity sites as well as inactive sites. The fraction of each of these sites depends on the surface density. The low activity sites are associated with amines clustered on the surface. Additional experiments reveal that the high activity sites are likely not associated with amines grafted in pairs or amines located on the external surface, leading us to speculate that the difference is amines cooperatively interacting with different types of silanols. Overall, the results demonstrate the potential to create highly active catalytic materials using cooperative interactions.