Acid–base cooperativity in condensation reactions with functionalized mesoporous silica catalysts

The Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was used to test the acid–base cooperativity of SBA-15 mesoporous silica co-condensed with aminopropyl groups as well as silica co-condensed with dihydroimidazole groups. Surface silanols were capped with trimethylsilyl groups using hexamethyldisilazane to determine the effects of the silanol groups on catalytic activity. The activities of the mesoporous silicas with or without capped silanols were compared to the activity of propylamine free in solution. Silylation resulted in a significant loss of activity, where turnover frequencies dropped nearly to those of the homogeneous base. The same behavior was also observed with dihydroimidazole-functionalized SBA-15, which catalyzes the Knoevenagel condensation by a different mechanism. Not only did these results indicate cooperative effects in heterogeneous systems, but cooperativity between a homogeneous base and silanols on unfunctionalized SBA-15 was also observed to a lesser extent. These results demonstrated that acid–base cooperativity seen in well-defined single sites can also be demonstrated on an extended catalytic domain on the surface of mesoporous silica where the active sites are not rigorously isolated.

Acid–base cooperativity in the Knoevenagel condensation reaction was found for amine-functionalized mesoporous silica catalysts operating through either an imine intermediate or ion-pair mechanism. The role of silanols as weak acids rather than through hydrogen bonding was demonstrated using different solvents.Figure optionsDownload high-quality image (47 K)Download as PowerPoint slide