Genesis of V⁴⁺ in heteropoly compounds Cs(x)H(4-x)PVMo₁₁O₄₀ during thermal treatment, rehydration and oxidation of methanol studied by EPR spectroscopy

The development and structural changes of V⁴⁺ and Mo⁵⁺ centers in caesium salts and the acid form of molybdovanadophosphorus heteropoly compounds Cs(x)H(4-x)PVMo₁₁O₄₀·nH₂O (x = 0, 3, 4) of Keggin structure were investigated by electron paramagnetic resonance (EPR) after preparation, thermal treatment in oxygen, reduction by methanol and catalytic oxidation of methanol. The various types of paramagnetic centers are found to characterize the different states and structural transitions of the heteropoly anion. In addition, the EPR spectra reflect well the different reducibility, thermal stability and mobility of vanadium and Keggin ('lattice') oxygen of the Cs₄, the Cs₃H and the H₄ compounds. The V⁴⁺ centers in the Cs₄ salt are different in structure, lower in concentration and thermally more stable than those in the acid form. The reduction of vanadium during thermal treatment due to the loss and oxidation of coordinated ('lattice') oxygen to O₂ is observed in the acid and the Cs₃H salt only, but not in the Cs₄ compound. As deduced from quantitative EPR measurements and manganometric redox titrations, between 0.5 and 8% of the vanadium is present as V⁴⁺ after preparation and thermal treatment in oxygen, whereas the majority of vanadium is reduced by reaction with methanol. V⁴⁺ substitutes Mo atoms in the Keggin unit in the fully exchanged Cs salt (Cs₄). The presence of the bulky caesium counter-cations prevents the removal of vanadium from the Keggin unit to the secondary structure and thus the formation of vanadyl bridged polymers.