Rational plug-bridge design to enhance catalytic efficiency of sulfonic acid-functionalized plugged...

A series of sulfonic acid-functionalized plugged and unplugged bifunctional periodic mesoporous organosilicas incorporating either ethyl-imidazolium or phenyl-imidazolium bridging units (P/U BFPMO-IL-PrSO₃Hs, including Et-U-BFPMO-IL-PrSO₃H (1a), Et-P-BFPMO-IL-PrSO₃H (1b), Ph-U-BFPMO-IL-PrSO₃H (1c) and Ph-P-BFPMO-IL-PrSO₃H (1d)) were synthesized and systematically evaluated as solid acid catalysts in direct esterification and Biginelli reactions. The synthetic protocol involves a modified co-condensation method using organosilica and silica precursors in the presence of Pluronic P123 as a structural directing agent and KCl as an additive under acidic conditions, followed by grafting with mercaptopropyl groups and oxidation to create sulfonic acid-functionalized bi-functional PMO nanostructures. The key difference between the plugged and unplugged variants lies in the order of introducing the silica and organosilica precursors during synthesis. Among these materials, the catalyst 1b exhibited the highest catalytic performance, surpassing its unplugged analogs as well as reference catalysts such as mesoporous silica SBA-15-PrSO₃H (2) and periodic mesoporous organosilica Et-PMO-Me(PrSO₃H) (3). Considering a number of comparative studies, the superior activity of this catalyst may be largely attributed to its combination of organic bridges and channel plugs. These structural features enhance the accessibility and reactivity of sulfonic acid sites while prolonging the residence time of reactants near active centers and expelling out the by-produced water from the interior of the mesochannels. The 1b catalyst demonstrated excellent efficiency in the esterification of various alcohols and carboxylic acids and in the synthesis of dihydropyrimidinones via the Biginelli reaction under mild conditions. Moreover, it exhibited remarkable stability and reusability, retaining high catalytic performance over multiple reaction cycles without significant deactivation.