Michel Fournier

Polyoxometallates as models for oxide catalysts: Part I. An UV-visible reflectance study of polyoxomolybdates: Influence of polyhedra arrangement on the electronic transitions and comparison with supported molybdenum catalysts

Abstract Diffuse reflectance spectroscopy is a technique commonly used to determine the symmetry and the environment of transition metal ions in supported oxide catalysts by comparison with reference samples either in solid or liquid state. However, the examination of model polyoxomolybdates with well-established structures such as Na2MoO4 · 2H2O, TBA2Mo2O7 (TBA = tetrabutylammonium), α-TBA4Mo8O26, α- and β-TBA4SiMo12O40, TBA2Mo6O19, H8As4Mo12O50, α-TBA6P2Mo18O62, etc., indicates that earlier attributions of the UV-visible bands of supported Mo catalysts may be reconsidered. It is shown that Mo dispersion (Mo cluster size and distance between clusters) or Mo-support interactions have more influence than local Mo symmetry. Indeed, analysis of experimental spectra of polyoxomolybdates indicates that: • —in contrast to an earlier belief, the effect of the local symmetry on the lowest energy transition band of ligand-metal charge transfer is negligible compared to the effect of the more distant environment of the metal. • —a broadening and a red shift of this band are observed when cluster size increases, or when the polarizing effect increases and/or the size of the counter-cation decreases. On the contrary, a narrowing and a blue shift are observed when a polyoxomolybdate is supported, i.e., when its dispersion increases. • —the increasing effects of the different parameters may be classified as follows: local symmetry overall symmetry condensation degree (polyanion size) polarizing effect of the counter-cation size of the counter-cation dispersion on a support • —UV-visible spectra do not distinguish the effect of strong interaction between Mo (via oxobonding Moue5f8Oue5f8Mo) exhibited by the influence of polyanion size from weaker effects such as polyanion interaction (for example, via hydrogen bonding), occurring when the counter-cation is small enough, or from electrostatic effects due to polarization by the counter-cation.

Structure and catalytic properties of silica-supported polyoxomolybdates I. MO/SiO2 catalysts prepared from hexamolybdate

Abstract Silica-supported hexamolybdate catalysts prepared by impregnation and two series obtained by calcination of the former are studied by different physicochemical techniques (IR and Raman spectroscopies, X-ray diffraction, and electronic microscopy) and tested in the catalytic oxidation of methanol. The nature of the surface species and the effect of water on the calcined catalysts are discussed as a function of the Mo loading (3 to 25 wt% Mo) from solid-state characterization and catalytic behavior. The catalysts unexposed to water (series 1) essentially exhibit a redox catalytic behavior, while the water-exposed catalysts (series 2) exhibit an acidic behavior. At high coverages (≥ 10 wt% MO), MoO3 is the main species present on the surface. At lower coverages, a molybdenum oxo species is formed which is mainly responsible for the catalytic activity. This species is constituted of trimolybdic groups, fragments of the Keggin unit SiMo12, in interaction with silica. Upon prolonged exposure to water, 12-molybdosilicic acid is also detected, but plays a minor role in the catalytic reaction.

Silica-supported 12-molybdophosphoric acid catalysts: Influence of the thermal treatments and of the Mo contents on their behavior, from IR, Raman, X-ray diffraction studies, and catalytic reactivity in the methanol oxidation

Abstract The thermal behavior of silica-supported 12-molybdophosphoric acid catalysts (PMoH- x series, x = Mo wt%, varying from 16.2 to 0.9) was investigated by using different techniques (IR and Raman spectroscopies, X-ray diffraction, and catalytic reactivity in methanol oxidation). The silica support used in the study (surface area 376 m 2 g −1 ) induces destabilization of 12-molybdophosphoric acid. The transformation into β-MoO 3 begins from 250°C, and is achieved at 350°C. β-MoO 3 is the main species on silica support in a wide range of temperatures, and seems to be stabilized by the support. The role of this molybdenum oxide is discussed in terms of active species in the catalytic reaction, and the capability to rebuild the Keggin unit under exposure to water vapor.

Polyoxometallates as models for oxide catalysts: Part II. Theoretical semi-empirical approach to the influence of the inner and outer Mo coordination spheres on the electronic levels of polyoxomolybdates

Abstract In complement to Part I, theoretical MO calculations (EHMO level) show that the main factor which influences the electronic energy levels of polyoxomolybdates is not always the local Mo symmetry (tetrahedral or octahedral). The effect of protonation and/or grafting on a surface oxide is weak but slightly larger for tetrahedral than for octahedral species and could lead to wrong attributions. The condensation of several sites in polyoxomolybdates has a greater effect since it involves a MO delocalization over the whole compound, and therefore a loss of the individuality of each octahedral or tetrahedral unit. It leads to a decrease in the HOMO-LUMO gap yielding to an absorption red shift. The effect of distortion of local symmetry by bond elongation tends to decrease the HOMO-LUMO gap in an isolated species. In a condensed system, this distortion lowers the Moue5f8Mo interactions and moderates the red shift due to the effect of condensation. This latter effect, however, remains predominant.

Structure and catalytic properties of silica-supported polyoxomolybdates III. 12-molybdosilicic acid catalysts: vibrational study of the dispersion effect and nature of the mo species in interaction with the silica support

Silica-supported 12-molybdosilicic acid catalysts prepared by impregnation are studied by IR and Raman spectrometries as a function of the Mo content (Mo wt% varying from 24.6 to 1.2). A dispersion effect is evidenced, especially by Raman spectrometry. The 12-molybdosilicate anion in interaction with the silica support is preserved down to 2% Mo. The results are discussed in relation with 29Si magic-angle-spinning NMR and reactivity studies in the methanol oxidation.

Synthesis of polypyrrole and polythiophene in aqueous solution of Keggin-type structure heteropolyanions

Abstract Immobilization of Keggin-type heteropolyanions (HPA) such as: PMo 12 O 40 3− , SiW 12 O 40 4− , and PW 12 O 40 3− in polypyrrole, poly(3-methyl-thiophene), and polyaniline is performed by electropolymerization of pyrrole (Py), 3-methyl-thiophene (3-MeTh), and aniline (An) in aqueous and nonaqueous solutions in the presence of corresponding heteropolyacids and salts. It was found possible to obtain electroactive films of polypyrrole and polythiophene from aqueous solutions of HPA due to formation of respective HPA-Py and HPA-3-MeTh complexes. Application of acidic environment to the Py polymerization leads to its rapid trimerization and deterioration of electroactive properties of the film obtained. These electrodes were characterized by cyclic voltammetry which provides two one-electron reversible HPA redox systems. They are stable in acidic solutions for poly(3-methylthiophene) and in acidic and neutral solutions for polyaniline.

Infrared spectroscopic evidence of bipolaron delocalization in reduced heterododecamolybdates

Abstract Upon reduction of heteropolymolybdates of Keggin structure by two electrons, substantial changes take place in the IR spectra which are not observed in the substituted anions or in the one-electron reduced form. These peculiar changes are interpreted by a strong coupling of a bipolaron with the vibrations of the bridging bonds.

Synthesis and characterization of Keggin derivatives containing an [Mo(NO)]3+ unit: (n-Bu4N)4[PM11O39{Mo(NO)}] (M Mo, W)

Abstract The diamagnetic complexes (n-Bu4N)4[PM11O39{Mo(NO)}] (M = Mo, W) have been synthesized in acetonitrile by reaction of the lacunary [PM11O39]7− species, generated in situ through degradation of the patent complete Keggin anions by n-Bu4NOH, with either (n-Bu4N)2[Mo5O13(OMe)4(NO){Na(MeOH)}]·3MeOH or [Mo(NO)(OMe)(Me2CNO)2]2 acting as a source of the [Mo(NO)]3+ unit. These new nitrosyl-oxometalates have been characterized by elemental analysis, infrared spectroscopy, electrochemistry, and 31P and 183W spectroscopy. Comparison of electrochemical and spectral data of the nitrosyl derivatives to those of the parent Keggin anions demonstrate that the electron-withdrawing effect of the nitrosyl ligand is opposed to the overall increase in charge of the heteropolymetalate framework arising from the replacement of an MO4+ unit by an [Mo(NO)]3+ unit. In the case of [PW11O39{Mo(NO)}]4−, 183W NMR data also support some delocalization of the Mo(II) d-electrons onto the polytungstate ligand.

Vibrational investigations of polyxometalates. 4. Valence force fields of anions related to the lindqvist structure

Abstract Normal-coordinate treatments of W 6 O 19 2− and of the one-electron reduced hexamolybdate Mo 6 O 19 3− were performed by assuming the transferability of the internal valence force field of the closely related hexamolybdate Mo 6 O 19 2− . The principal force constants are discussed in terms of electronic distribution. For the hexatungstate, the results are consistent with a greater rigidity of the tungstic framework with respect to the corresponding molybdic one. For the reduced hexamolybdate, the results obtained for the force constants in both V and VI metal oxidation states, and for the potential energy distribution, are used to discuss the approximations of the theoretical treatments of mixed-valence systems.

Synthesis of conducting polymers doped with heteropolyanions of Dawson-type structure

Abstract Immobilization of Dawson-type heteropolyanions (HPA) such as: P 2 W 18 O 62 6− in poly(3-methyl-thiophene), and polyaniline is performed by electropolymerization of 3-methyl-thiophene (3-MeTh) and aniline (An) in nonaqueous solutions in the presence of corresponding heteropolyacids and salts. It was found possible to obtain electroactive films of polyaniline and polythiophene from acetonitrile solutions of HPA due to formation of respective HPA-An and HPA-3-MeTh complexes. These electrodes were characterized by cyclic voltammetry which provides three one-electron reversible HPA redox systems. They are stable in acidic solutions for poly(3-methylthiophene) and in acidic and neutral solutions for polyaniline.