Search

The tetrabutylammonium (TBA) salt of a Keggin-type polyoxometalate (POM), with the chemical formula TBA4H2[BW11Mn(H2O)O39]·H2O, TBABW11Mn, was evaluated as a catalyst in the oxidation by hydrogen peroxide of several organosulfur compounds, namely benzothiophene (BT), 2-methylbenzothiophene (2-MBT), 3-methylbenzothiophene (3-MBT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-diethyldibenzothiophene (4,6-DEDBT), in acetonitrile at room temperature. All compounds were oxidized to their corresponding sulfones with high conversion and selectivity. Following the excellent results achieved, the BW11Mn/H2O2in CH3CN system was tested in the oxidation of model fuels (MFs) consisting of a mixture of BTs and DBTs in hexane (MF1containing mainly BTs and MF2containing predominantly DBTs). In both cases, the organosulfur compounds from the model fuels were fully converted into their corresponding sulfones. Envisaging the development of a promising desulfurization procedure, the extraction of sulfur compounds from MF2was attempted after the catalytic oxidation process. More than 98 mol% was removed using an ethanol/H2O mixture.

The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates with general formula [XW11M(H2O)O39](n−m)−, where X = P, B or Si and M = Mn, Fe or Co, were evaluated as catalysts in the oxidation of styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene, p-chlorostyrene, p-nitrostyrene, and p-methoxystyrene under mild conditions, using aqueous H2O2as an eco-sustainable oxidant. In this study, the influence of the catalysts and of the different styrene substituents on the oxidation reaction profile was evaluated in terms of conversion and selectivity. For all the performed catalytic studies, the main product results from the oxidative cleavage of the vinyl double bond, except in the case of the oxidation of p-methoxystyrene catalysed by BW11Mn, for which p-methoxyphenol is the main product. The catalysts BW11Mn and SiW11Co give rise to 100% conversion for almost all of the substrates, excluding p-methoxystyrene and p-nitrostyrene for both catalysts and α,p-dimethylstyrene only in the case of BW11Mn. The selectivity for CC cleavage products resulting from the oxidative cleavage of the vinyl double bond can be as high as 98%, reaching 98% conversion for p-nitrostyrene when SiW11Co was used as a catalyst. Possible pathways are discussed and the oxidation of a few presumed intermediates was carried out. The systematic study of several substituted styrene derivatives suggests a possible reactivity order for these compounds in the catalytic system considered.

Mono-substituted and sandwich-type copper substituted polyoxoanions ([PW11CuO39]5− and [Cu4(H2O)2(PW9O34)2]10−) were studied as catalysts for the epoxidation of cyclooctene and geraniol, using the green oxidant H2O2, with the results suggesting good efficiency at room temperature. High selectivity and reasonable conversion data were found after the first minutes of reaction. The copper centre may be responsible for this performance and our on-going research focuses in the preparation of new materials incorporating copper into polyoxometalates (POMs). In this context, we have also isolated a new crystalline dimeric sandwich-type polyoxocoppertungstate {H7.05[Cu3WO(H2O)(CuW9O34)(Cu0.6W9O34)]}7.75−. This compound was isolated as potassium salt and structurally characterized by FTIR and single-crystal X-ray diffraction (XRD). The polyoxoanion is composed by the [CuW9O34]12− and [Cu0.6W9O34]12.8− anionic fragments, joined together by a rhomb-like arrangement of three Cu2+ and one W6+ metallic centres, all exhibiting octahedral coordination geometries.

Extended X-ray absorption fine structure spectroscopy(EXAFS) was used to characterize several Keggin-type polyoxotungstates (tetrabutylammonium salts) in the solid state and in acetonitrile solution. The polyoxometalates studied were [PW12O40]3– (the parent Keggin anion), [PW11O39]7– (a lacunary anion), [PW11M(H2O)O39]4–, M = Fe, Ru, and [XW11MnIII(H2O)O39]n–, X = P, Si, B (transition-metal-substituted anions). The role of multiple-scattering contributions was evaluated for [PW12O40]3– for which an accurate structure determination was available. The results from the multiple-scattering analyses were compared with the single-scattering method. The average W-shell inter-atomic distances were determined in relation to a selected tungsten atom (Wc) by W LIII edge transmission analysis. Several shells of up to 3.8–4.0 Å were identified. For the anion [PW12O40]3–, the obtained inter-atomic distances in the solid sample and in acetonitrile were identical but for the lacunary anion the EXAFS spectrum of the solid was different from that in solution (the number of atoms coordinated to Wc in [PW11O39]7– was the same in both cases, but the calculated distances for the shells were slightly different). Metal-substituted anions were studied by W LIII edge transmission and by Fe, Mn, or Ru K edge fluorescence analysis. The average coordination sphere of tungsten was affected by the replacement of a W=O by a M–OH2 group. The EXAFS results indicate that on average the MnO6 octahedron is larger than FeO6 or RuO6 and also larger than the average of all WO6 octahedra in [PW11M(H2O)O39]4–. This was attributed to the occurrence of the Jahn–Teller effect in the coordination sphere of MnIII. A comparison of the W LIII edge analysis EXAFS data for three MnIII-substituted anions (X = P, Si, B) suggested that the average size of the W3O13 groups did not depend on X. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

New lanthanotungstocobaltates with the Keggin structure and general formula KxHy[Ln(CoW11O39)(H2O)3]⋅nH2O or KxHyLn[Ln(CoW11O39)(H2O)3]⋅nH2O, LnIII = Ce, La, Eu, Sm, Tb, were prepared and characterized for the first time. Hybrid silica materials, with the polyoxoanions anchored to spherical silica particles, were also prepared and characterised. Their study by IR spectroscopy and EDS suggested that the structure of the Keggin-type polyanions is maintained when they are immobilized in the silica material.

A layer-by-layer assembly method was employed to fabricate multilayered films containing the terbium heteropolyanions K6[Tb(BW11O39)(H2O)3]·32H2O and Ky[Tb(XW11O39)2]·nH2O (X = Si and P) and the polyelectrolytes poly(4-styrenesulfonate) and poly(allylamine hydrochloride). A comparative study on the optical properties of the films containing distinct polyoxometalates and different types of multilayers was carried out. The photoluminescence of the solid polyoxometalates and of the resulting films was investigated.

Novel luminescent polyoxometalate anion-pillared layered double hydroxides (LDHs) were prepared by aqueous ion exchange of a Zn–Al LDH precursor in nitrate form with the europium-containing polyoxotungstate anions [EuW10O36]9–, [Eu(BW11O39)(H2O)3]6– and [Eu(PW11O39)2]11–. The host–guest interaction has a strong influence on the nature of the final intercalated species, as evidenced by elemental analysis, powder X-ray diffraction (XRD), infra-red (IR) and Raman spectroscopy, solid state magic-angle spinning (MAS) 11B and 31P NMR spectroscopy, and photoluminescence spectroscopy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)