Combined Effects of Sb-Doping and of Preparation via Lacunary Precursor for P/Mo-Based, Keggin-Type Polyoxometalates, Catalysts for the Selective Oxidation of Isobutane to Methacrylic Acid

Catalysts for the selective oxidation of isobutane to methacrylic acid, based on ammonium salts of phosphorus/molybdenum Keggin-type polyoxometalates, were prepared by precipitation of a lacunary precursor at pH 4.0, and by calcination of the precipitate at 350 °C in air. The thermal treatment led to the transformation of the lacunary polyoxometalate into (NH4)3PMo12 O40. This procedure yielded a catalyst that was active, but poorly selective, from the beginning of its reaction time, since the active sites were generated during the calcination, and thus during the transformation of the lacunary precursor into the Keggin compound. Nevertheless, for the reaction under isobutane-rich conditions, the catalyst exhibited an initial period during which the progressive reduction of the polyoxometalate led to an improvement of the selectivity to methacrylic acid. An alternative method to develop a partially reduced compound was by preparation of a Sb3+-doped polyoxometalate. A solid-state redox reaction between Sb3+ and Mo6+ occurred in the Keggin framework during the calcination of the precursor. With this procedure, a catalyst was obtained, which did not require the preliminary equilibration, and which therefore was active and selective from the beginning of its reaction time. Furthermore, the catalyst was found to be surprisingly very active under isobutane-lean conditions.