Oniumsilica-immobilized-Keggin acids: Acidity and catalytic activity for ethyl tert-butyl ether synthesis and acetic acid esterification with ethanol

Keggin heteropolyacids were immobilized on functionalized silica as their onium ( γ-propyl- N-pyridinium, γ-propyl- N-methyl and γ-propyl- N-butyl-imidazolium) salts. Interaction between HPA and the surface-grafted onium cations affords acid salts. In contrast to bare silica, impregnated with HPA, these materials have monoanionic dispersions of HPA on the surface and superior resistance to HPA leaching in polar media. The greatest stability of the Keggin structure and resistance to leaching were found for H 4SiW 12O 40–(SiW)-, and the lowest for H 3PMo 12O 40–(PMo)-based samples. In the two model reactions tested, the liquid-phase synthesis of ETBE and the esterification of AcOH with EtOH, these solids display good catalytic performance (activity per anion, up to 150 and 25 h −1, respectively) and relative high structural stability. Catalysts having a greater coverage of organic functions (revealed by comparing two pyridinium salts) and hydrophobic cations (by comparing two imidazolium salts) have the best performance. Amongst the heteropolyacids studied, H 4SiW 12O 40 is the most active and promising for catalyst design. Oniumsilica-immobilized heteropolyacids display good catalytic performance and relative high structural stability in the liquid-phase synthesis of ETBE and the esterification of AcOH with EtOH.