Cooperative Catalysis with Acid–Base Bifunctional Mesoporous Silica: Impact of Grafting and Co-condensation Synthesis Methods on Material Structure and Catalytic Properties

The structural and cooperative catalytic characteristics of acid and base co-functionalized mesoporous silica synthesized through grafting and co-condensation methods are investigated. It is shown that incorporation of the mutually reactive amine and carboxylic acid functional groups is aided by a protecting group in the grafting method. Using a thermally cleavable protecting group on the carboxylic acid organosilane, the differential effect of silanol removal and acid group functionalization on catalytic activity is studied. For samples prepared here by both the co-condensation and grafting procedures, the removal of silanols and the introduction of the carboxylic acid has a negative impact on activity of the catalyst in aldol condensations under the conditions used here. These results demonstrate that a weaker Brønsted acid silanol is more effective in cooperatively catalyzing the aldol condensation in combination with an amine base than the stronger carboxylic acid for all the materials prepared in this study.