Reaction-dependent heteroatom modification of acid–base catalytic cooperativity in aminosilica materials

Abstract

The effects of heteroatom substitution on the cooperative catalytic activity of a series of bifunctional acid–base aminosilica catalysts are probed in aldol and nitroaldol condensations. Three M3+ (B, Al, and Ga) and three M4+ (Ti, Zr, and Ce) heteroatoms are incorporated into different samples of SBA-15 silica and then grafted with aminosilanes to produce bifunctional acid–base catalysts. The catalytic activity of each material is measured in the aldol condensation of 4-nitrobenzaldehyde with acetone at 50°C and the nitroaldol condensation of 4-nitrobenzaldehyde with nitromethane at 40°C and compared to the catalytic activity of a heteroatom-free aminosilica catalyst. The heteroatom substitutions produce catalysts with larger amounts of strong Lewis acid sites compared to the heteroatom-free aminosilica catalyst. We rationalize these results in the context of the physical (e.g. surface area, pore diameter, particle size) and chemical properties (e.g. total number and strength of acid sites) of each material and the proposed catalytic mechanisms of the two reactions. The increase in the number of strong Lewis acid sites of each heteroatom material decreased its activity in the aldol condensation, though four heteroatom substitutions (B, Al, Ga, and Ti) increased the catalytic activity of the aminosilica catalyst in the nitroaldol condensation. The results suggest that inclusion of a small amount of Lewis acid sites in aminosilica materials can increase the cooperative catalytic activity of the materials in the nitroaldol condensation. The results also suggest that inclusion of Lewis acid sites in aminosilica materials decreases the cooperative catalytic activity of the materials in the aldol condensation.