Efficient solid-base catalysts for aldol reaction by optimizing the density and type of organoamine groups on nanoporous silica

A highly efficient solid-base catalyst for aldol condensation reaction was synthesized by grafting site-isolated secondary amines onto the channel walls of mesoporous silica, MCM-41, in a polar-protic solvent. These site-isolated organoamines pair up with the neighboring residual surface silanol (weak acid) groups to form optimized acid and base groups, which cooperatively catalyze the aldol condensation reaction with high TON (turn over number) and selectivity (alcohol products over alkene products). The organoamine samples grafted in a polar-protic solvent, isopropanol, showed higher catalytic efficiency toward aldol reaction than those grafted in a non-polar solvent, toluene, because the former gave a sample with less dense loading of grafted organoamines (or more silanols present in it). To elucidate the role of surface silanols as acidic sites and their ability to activate the substrates in aldol condensation, control experiment with diethylamine as a homogeneous catalyst in the presence of MCM-41, silica microspheres, methyl-capped MCM-41 or methyl-capped catalyst was carried out. MCM-41 resulted in significant enhancement of catalytic activity compared to the corresponding reactions conducted in the absence of MCM-41, or in the presence of methyl-capped catalysts or silica spheres. By testing materials with different grafted organoamine groups as catalysts, we also found that secondary amine functionalized sample produced the best acid and base pairs and most efficient catalytic activity in aldol reaction. This was followed by primary amines, while the tertiary amine functionalized samples showed negligible catalytic property.