A. Tézé

ESR characterization of V4+ as a counter-ion of the 12-molybdophosphate. Influence of thermal treatments

Abstract Vanadyl cations were introduced as counter-ions of the 12-molybdophosphate Keggin anion and crystals of [VO(H 2 O) 5 ]H[PMo 12 O 40 ] · 23H 2 O were obtained. Dehydration under nitrogen of this hydrate occurs step by step which were studied by thermogravimetry and ESR characterization of vanadium after thermal treatment. Vanadyl exists as an aquo complex [VO(H 2 O) 5 ] 2+ up to 75°C. Between 75 and 210°C, loss of the water molecules leads to VO 2+ becoming bound to the outer oxygen atoms of the molybdophosphate. All these transformations are readily reversible in the presence of water. At 320°C, a progressive decomposition of the Keggin structure occurs, characterized by a decrease of the intensity of the ESR signal of V 4+ and the emergence of a signal of Mo 5+ . This transformation is only partly reversible in the presence of water vapor.

Oxidative dehydrogenation of isobutyric acid: Characterization and modeling of vanadium containing polyoxometalate catalysts

The catalyzed oxidative dehydrogenation of isobutyric acid by H4[PMo11VO40] and VOH[PMo12O40] was studied at 593 K. Thermal evolution of H4[PMo11VO40] at 593 K leads to a complex mixture of different species with or without vanadium inside the Keggin structure. After reaction, the one and only heteropolyanion is partly reduced [PMo12O40]3− and vanadium is present as vanadyl cations. Vanadyl cations are bound to the oxygen atoms of the heteropolyanion at 593 K and enhance the selectivity to methacrylic acid with respect to acetone. On the other hand, copper cations improve the reoxidation of the catalyst and increase the IBA conversion. Synergism between copper and vanadyl cations was observed.

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 ammonium–cesium and cesium salts

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.

Polyoxotungstates with Lone-Pair Assembling Atoms: Part 2 [1]: 183W NMR Structural Study of the Inorganic Cryptates [MAs4W40O140](28-n)- and [MSb9W21O86](19-n)- (Mn+ = Alkaline or Alkaline Earth Cation)

The cryptates [MAs4W40O140](28-n)-, (Mn+ = Na+, K+, Rb+, Cs+, NH4 + or Ba2+), have been investigated by 183W solution NMR. The six 183W lines have been assigned with the help of homonuclear 2JW-W coupling constants and by ID-COSY NMR. Depending on M, shielding of the resonance line of the W’s near the cryptant site is correlated with increasing stability of the M-cryptate. Slow M exchange (on the NMR time scale) for [MAs4W40O140](28_n)- contrasts with the rapid exchange observed for the parent cryptates [MAs4W42O142]20-n)-: this difference is discussed with respect to the geometry of the cavity.