Environmentally Benign Bifunctional Solid Acid and Base Catalysts

Abstract

Solid bifunctional acid–base catalysts were prepared in two ways on an amorphous silica support: (1) by grafting mercaptopropyl units (followed by oxidation to propylsulfonic acid) and aminopropyl groups to the silica surface (NH2–SiO2–SO3H), and (2) by grafting only aminopropyl groups and then partially neutralizing with phosphotungstic acid, relying on the H2PW12O40– ion for surface acidity (NH2–SiO2–NH3+[H2PW12O40–], denoted as NH2–SiO2–PTA). Surface acidity and basicity were characterized by adsorption calorimetry, using SO2 as a probe for surface basicity and NH3 for surface acidity. Catalytic activities were compared in a two-stage cascade: an acid-catalyzed deacetalization followed directly by a base-catalyzed Henry reaction. Overall, the NH2–SiO2–SO3H catalysts showed higher concentrations and strengths of both acid and base sites, and higher activities than NH2–SiO2–PTA. Both catalysts showed evidence of cooperative acid–base catalysis. Importantly, the bifunctional catalysts exhibited catalytic advantage over physical mixtures of singly functionalized catalysts.