Effects of NH4+, Cs+, and H+ counterions of the molybdophosphate anion in the oxidative dehydrogenation of isobutyric acid

Mixed ammonium-cesiun salts of the molybdophosphoric acid (Cs-x(NH4)(3-x)[P Mo12O40]) have been prepared by solid-state cationic exchange using the amm onium salt impregnated with the suitable quantity of cesium nitrate at inci pient wetness. Structural characterization of the solids showed that the ex change is quantitative and that ammonium and cesium cations are randomly di stributed in the lattice. Catalytic activity of these salts in the oxidativ e dehydrogenation of isobutyric acid increases as the amount of cesium incr eases up to 3 Cs per heteropolyanion. Between 3 and 3.1 Cs, the catalytic a ctivity abruptly falls and, simultaneously, the major product switches from methacrylic acid to acetone. This abrupt change in the activities for x ap proximate to 3 corresponds to the strong decrease of the rates of formation of methacrylic acid and propene, whereas the rate of formation of acetone remains almost unmodified. Under the conditions of reaction, departure of a mmonia was total for x greater than or equal to 2 and partial for x<2. Ther efore, catalysts formed at steady state have to be considered as acid ammon ium-cesium salts when x<2 and acid cesium salts when 2<x<3. So, very few pr otons are needed in order to obtain an active and selective catalyst in MAA . (C) 2000 Academic Press.