Microcalorimetric study of the acidity of tungstic heteropolyanions

Abstract

The number and strength of the acid centres of tungstic heteropolyacids have been determined by absorption calorimetry of ammonia. The initial heats are in the order H₃PW₁₂O₄₀>H₄SiW₁₂O₄₀>H₆P₂W₂₁O₇₁(H₂O)₃>H₆P₂W₁₈O₆₂, varying from 200 to 155kJ mol^–1. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. Moreover, the influence of the activation temperature on the acidity has been studied and confirms that there is a drastic modification of the solid at ca. 350 °C. The heat capacities of the heteropolyanions and of the corresponding ammonium salts, the thermokinetic parameters of ammonia absorption and the heat capacities of the acids and ammonium salts have been related to the porosity of the various samples.