Heteropolyacids 12-tungstophosphoric acid

Cesium salts of 12-tungstophosphoric acid have been compared to the pure acid and to a sulfated zirconia sample for isobutane/1-butene alkylation at room temperature. The salt was found to be much more active than either the acid or sulfated zirconia (201). Heteropolyacids have also been supported on sulfated zirconia catalysts. The combination was found to be superior to heteropolyacid supported on pure zirconia and on zirconia and other supports that had been treated with a variety of mineral acids (202). Solutions of heteropolyacids (containing phosphorus or silicon) in acetic acid were tested as alkylation catalysts at 323 K by Zhao et al. (203). The system was sensitive to the heteropoly acid/acetic acid ratio and the amount of crystalline water. As observed in the alkylation with conventional liquid acids, a polymer was formed, which enhanced the catalytic activity. 

The reaction between a,(3-unsaturated aldehydes and thiophenols is catalyzed by tungstophosphoric acid and in the absence of solvent at room temperature thiochromans are formed rapidly and in good yield (Equation 161). The heteropolyacid is recoverable and reusable <2005TL7567>. 

Relatively high thermal structural stability the Keggin anion in heteropolyacids begins to decompose at temperatures close to 250-300°C. Salification leads to a remarkable improvement in the stability, allowing operations up to 400-450°C to be carried out. Salts can be prepared by ion-exchange in aqueous solutions of proton form of the heteropolyacid, or by direct precipitation of the insoluble salt. In some cases, such as the cesium salts of 12-molybdophosphoric or tungstophosphoric acids, the structure is stable up to 550°C. 

Mukai and co-workers [105] used Keggin-type heteropolyacids immobilized in the network structure of resorcinol-formaldehyde carbon gels as catalysts for the synthesis of methyl tert-butyl ether from methyl alcohol and tert-butyl alcohol. Large amounts of 12-tungstophosphoric and 12-molybdophosphoric acids were immobilized into the support by two methods, pore shrinkage and the ship-in-the-bottle method, which are essentially impregnation methods. The authors reported that these catalysts showed activity in the reaction studied and could be of practical utility as solid acid catalysts in various reactions. 

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