Study of ammonium, mixed ammonium–cesium and cesium salts derived from (NH4)5[PMo11VIVO40] as isobutyric acid oxidation catalysts: Part II. Synthesis, characterization and catalytic activity in the oxidative dehydrogenation of isobutyric acid of mixed amm

Thermal exchange of cesium for ammonium cations in (NH4)5[PMo11VIVO40] impregnated at incipient wetness by CsNO3 was performed under nitrogen. The structural modification of this salt, previously reported in the Part I of this work, limits the solid state exchange to three cesium per heteropolyanion. Mixed NH4+, Cs+ and VO2+ salts, isotype of the cubic ammonium and cesium 12-molybdophosphates, are obtained. The characterizations of these low surface area compounds show that their formula is Cs2x(NH4)6−2x(VO)[PMo11VIVO40][PMo12O40], with 0≤x≤3. These compounds are active for oxidative dehydrogenation of the isobutyric acid and selective to methacrylic acid. Characterizations of the catalysts corresponding to x<3 after reaction reveal a loss of ammonia and a quantitative release of vanadium from the heteropolyanion. Above x=3, the samples which are inactive, are composed of the oxidized mixed cesium and vanadium salt Cs6(VO2)[PMo11VVO40][PMo12O40] and an excess of cesium nitrate. Like for the mixed ammonium–cesium 12-molybdophosphates, the best catalyst is the nearly stoichiometric cesium salt (x=3) which is stable in the conditions of the reaction. The presence of vanadium improves significantly both the catalytic activity and the selectivity to methacrylic acid.