Transition-metal incorporated heteropolyacid-ionic liquid composite catalysts with tunable Brønsted/Lewis acidity for acetalization of benzaldehyde with ethylene glycol.

A series of transition-metal (M = Mn, Fe, Co, Ni, Cu) ion-exchanged tungstophosphoric acid (H 3 PW 12 O 40 ; TPA)-based ionic liquid (IL) catalysts, synthesized by incorporating metal and methylimidazolium propyl sulfobetaine (MIMPS) zwitterionic IL precursors onto TPA, were exploited for acetalization of benzaldehyde (BzH) with ethylene glycol (EG). These organic-inorganic, water-soluble M-MIMPS-TPA composite salts possess not only strong Brønsted and Lewis acidities but also exhibited unique self-separation property desirable for catalyst separation and recycling. The superior acetalization activities observed for these novel M-MIMPS-TPA catalysts are attributed to their strong acidity, Brønsted/Lewis acid synergy, and low mass transport resistance. Among them, the Ni[MIMPSH]PW 12 O 40 catalyst showed extraordinary catalytic performance and durability with a benzaldehyde glycol acetal selectivity and yield of 99.0 and 94.6%, respectively, in excellent agreement with results from response surface methodology. Further kinetic study gave rise to an activation energy of 34.11 kJ/mol for the acetalization reaction, surpassing most reported catalysts. [ABSTRACT FROM AUTHOR]