(509bt) Quantifying the Effect of Surface Density of Aminosilanes on the Fraction of Active Sites in SBA-15 for the Aldol Condensation

Cooperative interactions have been found to accelerate a range of chemical reactions, including the Aldol reaction and condensation. The cooperative interactions can be designed into heterogeneous catalytic materials such as mesoporous silica functionalized with an aminosilane. The key challenges are identifying the nature of highly active catalytic sites and increasing the number of these highly active sites. For heterogeneous catalyst, these challenges are complex since the materials are complex with multiple different types of catalytic sites. In this work, we investigate the catalytic properties of aminosilica functionalized mesoporous silica SBA-15. SBA-15 has been reported to possess both mesopores and micropores, but it is not clear if the catalytic activity is different for aminosilanes grafted in mesopores versus micropores. Our group has synthesized SBA-15 using both conventional methods to produce a material with micropores (REG-SBA-15) and a modified method to produce a material with no micropores (NMP-SBA-15). We have shown that NMP-SBA-15 produces a more active catalyst than REG-SBA-15. The underlying reason is elucidated using site quantification experiments that reveal that a greater fraction of the sites in NMP-SBA-15 are catalytic active than REG-SBA-15. Importantly, this challenges the commonly used assumption that all sites are active in these materials. We have further discovered that the catalytic activity depends on the surface loading of the aminosilane, as shown in Figure. Site quantification experiments have revealed that the observed differences depends on the fraction of active sites. Interestingly, these experiments also reveal a third type of catalytic site that is highly active and has not been previously reported. Overall, these observations provide insights on how to create a more active catalytic material.