Environmentally Benign Bifunctional Solid Acid and Base Catalysts

Solid bifunctional acid-​base catalysts were prepd. in two ways on an amorphous silica support: (1) by grafting mercaptopropyl units (followed by oxidn. 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 concns. 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 phys. mixts. of singly functionalized catalysts.