Preparation and characterization of catalysts based on oniumsilica-immobilized Keggin acids

A new method has been developed for preparing acid onium salts of Keggin heteropolyacids via ion exchange on amorphous silica functionalized with pyridinium and alkylimidazolium cations (SiO2–Q). The interaction between HPA and the surface-grafted cations affords acid salts of HPA with intact Keggin structure. This method offers excellent dispersion of HPA on the surface and higher resistance to HPA leaching in polar media, compared to the silica-based materials. The solids are thermally stable up to 250–300 °C, depending on the onium cation and anion, the most stable being the samples with immobilized H4SiW12O4 and H3PW12O40.