Gilbert Herve

Structure and catalytic properties of silica-supported polyoxomolybdates: II. Thermal Behavior of Unsupported and Silica-Supported 12-Molybdosilicic Acid catalysts from IR and Catalytic Reactivity Studies

Abstract Thermal behavior of unsupported and silica-supported 12-molybdosilicic acid (with Mo loadings of 17.6 and 9 wt%) are studied by IR spectrometry and catalytic reactivity in the methanol oxidation reaction. In the case of the unsupported acid (SiMo 12 H), IR and catalytic reactivity measurements show that the destruction of the Keggin unit is rather abrupt in the temperature range 300–320°C (collapse of the acidic character in catalysis, formation of Mo0 3 evidenced by IR). The thermal stability of SiMo 12 H is reduced by ∼20–30°C when supported on silica, and the destruction of the Keggin unit extends over a larger temperature range. The nature of the Mo oxo species responsible for the redox activity after thermal treatments at temperatures higher than 330°C is discussed.

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 the acidity of carbon supported and unsupported heteropolyacid catalysts by ammonia sorption microcalorimetry

Abstract Acid property of a series of bulk and carbon-supported heteropolyacids of tungsten were studied by means of microcalorimetry applied to the ammonia sorption. The four bulk samples H 3 PW 12 O 40 , H 4 SiW 12 O 40 , H 5 BW 12 O 40 and H 21 B 3 W 39 O 132 all display a strong acidity, which can be classified in the order of H 3 PW 12 O 40 >H 4 SiW 12 O 40 >H 5 BW 12 O 40 ≫H 21 B 3 W 39 O 132 . A dependence on acidity is shown for the catalytic activity of these bulk heteropolyacids in the methanol dehydration. For the Keggin-type samples, nearly all the protons detected display the same acidity for a given heteropolyacid. Deviation from stoichiometric ammonia sorption is observed with H 4 SiW 12 O 40 and becomes more important with H 5 BW 12 O 40 . The acidity of the three supported Keggin-type heteropolyacids, H 3 PW 12 O 40 /CPX, H 4 SiW 12 O 40 /CPX and H 5 BW 12 O 40 /CPX, is slightly reduced as compared with that of corresponding bulk samples. On the contrary, acidity of H 21 B 3 W 39 O 132 is enhanced with the CPX carbon supported sample; especially the quantity of the strong acidic sites is considerably increased. Heterogeneity in acidity is observed when the heteropolyacids are grafted on the carbon support and could be related to the heterogeneity of adsorption sites on the carbon surface.

Oxidation of isobutane catalyzed by vanadyl, copper and cesium substituted H3PMo12O40

Effects of Cs+, H+ and Cu2+ counterions in the vanadium containing heteropoly compounds CsxH1-xVO[PMo12O40] and CsyH0.5-yCu0.25VO[PMo12O40] on the catalytic oxidation of isobutane and characterization by TGA, IR and ESR spectroscopies are reported. A high selectivity of 76% for methacrylic acid and methacrolein together has been obtained with Cs0.75H0.25VO[PMo12O40] catalysts at a reactivity of 5.3x10-1 mmol/h cm3.

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.