Study of ammonium, mixed ammonium-cesium and cesium salts derived from (NH4)(5)[(PMo11VO40)-O-IV] as isobutyric acid oxidation catalysts Part II. Synthesis, characterization and catalytic activity in the oxidative dehydrogenation of isobutyric acid of mixed ammonium-cesium and cesium salts

Thermal exchange of cesium for ammonium cations in (NH4)(5)[(PMo11VO40)-O-I V] 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 heteropo lyanion. Mixed NH4+, Cs+ and VO2+ salts, isotype of the cubic ammonium and cesium 12-molybdophosphates, are obtained, The characterizations of these l ow surface area compounds show that their formula is Cs-2x(NH4)(6-2x)(VO)[( PMo11VO40)-O-IV][PMo12O40], with 0 less than or equal to x less than or equ al to 3. These compounds are active for oxidative dehydrogenation of the is obutyric acid and selective to methacrylic acid. Characterizations of the c atalysts 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 v anadium salt Cs-6(VO2)[(PMo11VO40)-O-V][PMo12O40] and an excess of cesium n itrate. Like for the mixed ammonium-cesium 12-molybdophosphates, the best c atalyst is the nearly stoichiometric cesium salt (x=3) which is stable in t he conditions of the reaction. The presence of vanadium improves significan tly both the catalytic activity and the selectivity to methacrylic acid. (C ) 2000 Elsevier Science B.V. All rights reserved.