Synthesis of polyoxovanadates from aqueous solutions

Polyoxovanadates have been synthesized from aqueous solutions of V-V in the presence of inorganic Na+ and organic [N(CH3)(4)](+) cations. Decavanadate crystals are precipitated at room temperature around pH 7, whereas layered compounds Na[V3O8].H2O and N(CH3)(4)[V4O10] are formed at the same pH under hydrothermal conditions. In situ V-51 NMR experiments show that upon heating solute decavanadate species are progressively transformed into cyclic metavanadates [V4O12](4-). A chemical mechanism is suggested for the formation of these layered vanadium oxides. It involves the ring-opening polymerization of cyclic metavanadates arising from co-ordination expansion favored by protonation or vanadium reduction. (C) 2001 Elsevier Science.