ENTHALPIES OF FORMATION AND DSC AND TG RESULTS OF HETEROPOLY ACIDS CONTAINING TUNGSTEN AND MOLYBDENUM

The enthalpies of formation of H(3)PW(12)O40 . 25.0H(2)O and H3PW12O40 . 20.0H(2)O (HPW), H4SiW12O40.6.0H(2)O (HSiW), H3PMo12O40. 8.6H(2)O (H PMo), and H4SiMo12O40. 120H(2)O (HSiMo) have been determined as -18.58 , -17.20, -13.14, -12.72 and -13.92 x 10(3) kJ mol(-1), respectively. The formation enthalpies of the anhydrous heteropoly acids have been d erived and estimated. The estimated data are -11.36, -11.39, -10.23, a nd -10.44 x 10(3) kJ mol(-1) for H3PW12O40, H4SiW12O40, H3PMo12O40, an d H4SiMo12O40, respectively. The data are in good accordance with that calculated from the formation enthalpies of WO3, MoO3, H3PO4, and H4S iO4. The differential scanning calorimeter (DSC) curve of the hydrated heteropoly acids covers two endothermic and one exothermic peaks. In the light of the temperature at which the Keggin unit collapses and th e temperature of the second endothermic peak, the order of the thermal stability should be HPW > HSiW > HPMo > HSiMo. Thermogravimeter (TG) results verify the general formula of hydrated heteropoly acids as [(1 .5H(2)O or 2H(2)O).(0.5P(2)O(5) or SiO2). 12MO(3) . 6H(2)O]. xH(2)O, w here M = W or Mo, x depends on the species and the condition of prepar ation of an acid. (C) 1997 Elsevier Science B.V.