New acid medium sol-gel synthesis of metal phosphates

New sol-gel routes based on peroxo complexes of early transition metals in a highly acidic medium were developed, to prepare metal oxide phosphates that feature structural protons. A sol-gel synthetic route was chosen because it allows atomic level mixing of precursors and lower heating temperatures, which are preferable in exploring metastable phases. Titanium and molybdenum sol-gel chemistries were the focus of the initial studies and the synthesis of Ti1-xMoxP2O7 (x = 0 – 0.5) and Mo1-yTiyP2O8-y (y = 0 – 0.4) type metal oxide phosphates were explored. For the synthesis of the metal oxide phosphates, hydrogen peroxide was employed to prepare the respective precursor solutions. The peroxide ligand suppressed the immediate precipitation of metal cations in aqueous medium, by coordinating to Ti4+ and Mo6+ ions, and produced a soft wet-gel following polycondensation. Phosphoric acid was used to acidify the reaction medium and to provide protons and phosphate ions as structural components. From this synthetic route, a series of Ti1-xMoxP2O7 (x = 0 – 0.5) and Mo1-yTiyP2O8-y (y = 0 – 0.4) crystalline compounds, with various degrees of purity, were synthesized. For x = 0 and y = 0, the crystalline compounds TiP2O7 and MoP2O8 were produced, respectively, after calcining at 600 °C.

In pursuit of new metastable molybdenum oxide phosphate compounds, peroxo-molybdenum precursor mixtures with different molar ratios were treated gently by low-temperature heating. After controlled drying in a lab oven, MoO2(H2O)(HPO4) crystals were obtained as a highly crystalline pure product instead of a gel. The dissolution of MoO2(H2O)(HPO4) in water and precipitation with a CsCl solution produced a new crystalline compound with a cubic unit cell (a = 11.8(2) Å). Further studies will lead to crystal structure determination and elucidation of the aqueous chemistry of MoO2(H2O)(HPO4).