Add time:07/25/2019         Source:sciencedirect.com

The enthalpies of formation of H3PW12O40·25.0H2O and H3PW12O40·20.0H2O (HPW), H4SiW12O40·6.0H2O (HSiW), H3PMo12O40·8.6H2O (HPMo), and H4SiMo12O40·12.0H2O (HSiMo) have been determined as −18.58, −17.20, −13.14, −12.72 and −13.92 × 103 kJ mol−1, respectively. The formation enthalpies of the anhydrous heteropoly acids have been derived and estimated. The estimated data are −11.36, −11.39, −10.23, and −10.44 × 103 kJ mol−1 for H3PW12O40, H4SiW12O40, H3PMo12O40, and H4SiMo12O40, respectively. The data are in good accordance with that calculated from the formation enthalpies of WO3, MoO3, H3PO4, and H4SiO4. 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 the 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 [(1.5H2O or 2H2O)·(0.5P2O5 or SiO2)·12MO3·6H2O]·xH2O, where M=W or Mo, x depends on the species and the condition of preparation of an acid.

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