Mixed ammonium-cesiun salts of the molybdophosphoric acid (Cs-x(NH4)(3-x)[PMo12O40]) have been prepared by solid-state cationic exchange using the ammonium salt impregnated with the suitable quantity of cesium nitrate at incipient wetness. Structural characterization of the solids showed that the exchange is quantitative and that ammonium and cesium cations are randomly distributed in the lattice. Catalytic activity of these salts in the oxidative dehydrogenation of isobutyric acid increases as the amount of cesium increases up to 3 Cs per heteropolyanion. Between 3 and 3.1 Cs, the catalytic activity abruptly falls and, simultaneously, the major product switches from methacrylic acid to acetone. This abrupt change in the activities for x approximate 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 ammonia was total for x greater than or equal to 2 and partial for x<2. Therefore, catalysts formed at steady state have to be considered as acid ammonium-cesium salts when x<2 and acid cesium salts when 2

Keywords:12-MOLYBDOPHOSPHORIC ACID;HETEROPOLY COMPOUNDS;12-TUNGSTOPHOSPHORIC ACID;SALTS;CATALYSTS;MECHANISM;TUNGSTEN;ALKALI