Sulfonated organic heteropolyacid salts: Recyclable green solid catalysts for esterifications

A family of solid organic heteropolyacid (HPA) salts were prepared by combining Keggin heteropolyanions with ionic liquid (IL)-forming cations functionalized by propane sulfonate (PS). Their catalytic activities were evaluated in esterifications, including the testing of the influence of organic cations, function of heteropolyanions, scope of reactions, optimization of reaction conditions, and catalytic reusability. The obtained catalysts caused a liquid-liquid biphasic esterification system, presenting high yield and selectivity for target esters, and after reaction, they appeared in solid state again, which could be easily recovered and quite steadily reused demonstrated by a seven-run recycling test. The highly efficient and environmentally benign performance of this family of catalysts is due to the pseudoliquid behavior of HPA salts that allows PS acidic species in the bulk of the heteropoly compound acting fully as active centers for esterification, as well as the stabilization function of heteropolyanion on carbonium ion intermediates created at acid sites.