Evaluating the per site activity of common mesoporous materials as supports for aminosilica catalysts for the aldol reaction and condensation

Mesoporous silicas are regarded as ideal systems; however, they are complex. The materials can be functionalized with aminosilanes to produce highly active catalysts for aldol chemistry. Despite heterogeneous catalysts often having multiple sites, functionalized materials are tested assuming each site contributes equally to catalytic activity. Here, we functionalize common mesoporous silicas (MCM-41, MCM-48, and SBA-15) and determine the per site catalytic activity. The activity of functionalized MCM-41, MCM-48, and SBA-15 are similar if SBA-15 has negligible micropore volume (NMP-SBA-15). Site quantification data reveal that each material has four different types of sites. MCM-41 has the highest fraction of active sites; however, most sites are low activity. Interestingly, NMP-SBA-15 is as active as MCM-41. NMP SBA-15 has a higher fraction of high activity and medium activity sites than both MCM materials. Overall, each material has similar types of sites even though the materials have different pore structures and different pore sizes.