Study of ammonium, mixed ammonium-cesium and cesium salts derived from (NH4)(5)[(PMo11VO40)-O-IV] as isobutyric acid oxidation catalysts Part I: Syntheses, structural characterizations and catalytic activity of the ammonium salts

The ammonium salt of the 1-vanado-11-molybdophosphate in which vanadium is at the oxidation state IV was synthesized in order to obtain the mixed ammo nium-cesium and pure cesium salts by thermal Cs+/NH4+ solid State exchange. Study of its thermal behavior showed that a phase, named phase A, isotype with the cubic phase of (NH4)(3)[PMo12O40] is Obtained and is stable up to 410 degrees C. This phase A contains two different Keggin heteropolyanions, [(PMo11VO40)-O-IV](5-) and [PMo12O40](3-) in equal quantities and, in addi tion to the ammonium cations, vanadyl cations balance the supplementary neg ative charges of the vanadium containing heteropolyanion. Its molecular com position is (NH4)(6)(VO)[(PMo11VO40)-O-IV][PMo12O40] This phase has a low s urface area (11 m(2)/g), on the contrary of the ammonium cubic phase of (NH 4)(3)[PMo12O40] Obtained by precipitation from aqueous solutions of the het eropolyacid. But it is about twice more active in the oxidative dehydrogena tion of isobutyric acid and more selective to methacrylic acid, as it is ex pected from previous studies on vanadyl salts. Thermal Cs+/NH4+ exchanges p erformed on the phase A are not complete and cannot be carried out without structural modifications. (C) 2000 Elsevier Science B.V. All rights reserve d.