Peroxotungstate catalyst

Figure 12. Schematic representation of the active peroxotungstate catalyst covalently anchored to silica via phosphoramide group [136].

G. Gelbard, F. Breton, M. Quenard, D. C. Sherrington, Epoxidation of cyclohexene with poly-methacrylate-based peroxotungstic catalysts, /. Mol. Catal. A 153 (2000) 7. 

The supported peroxotungstate catalyst proved to be very efficient for the epoxi-dation of various olefins using H2O2 (Fig. 5.6-5), with a catalytic activity comparable with a corresponding homogeneous tetraalkylammonium analog under the same conditions. The catalyst could be easily recovered by filtration and reused at least three times for the epoxidation of cyclooctene without loss of catalytic performance (99% yield, >99% selectivity). In accordance with this result no tungsten species... 

Gelbard, G., Breton, R, Quenard, M., et al. (2000). Epoxidation of Cyclohexene with Polymethacrylate-Based Peroxotungstic Catalysts, J. Mol. Catal. A Chem., 153, pp. 7-18 Gelbard, G., Gauducheau, T., Vidal, E., et al. (2002). Epoxidation with Peroxotungstic Acid Immobilised onto Silica-Grafted Phosphoramides, J. Mol. Catal. A Chem., 182-183, pp. 257-266. 

One problem associated with the peroxotungstate-catalyzed epoxidation system described above is the separation of the catalyst after the completed reaction. To overcome this obstacle, efforts to prepare heterogeneous tungstate catalysts have been conducted. De Vos and coworkers employed W catalysts derived from sodium tungstate and layered double hydroxides (LDH - coprecipitated MgCU, AICI3, and NaOH) for the epoxidation of simple olefins and allyl alcohols with... 

Over the years it has been shown that complexes prepared from organophosphorous ligands in combination with peroxotungstic acid or its quaternary ammonium salts exhibit efficient catalytic properties. In order to make an efficient recycling of the catalyst possible and get tungsten-free products and effluents, some of these catalysts were immobilized onto polystyrene, poly benzimidazole and polymethacrylate copolymers modified by the introduction of the phosphorous(V)-containing ligands. 

Gelbard has used peroxotungstates supported on polypyridine polymers in the epoxidation of cyclohexene with hydrogen peroxide.66 Polypyridine polymers were also used to support heteropolyperoxometallates for use in the oxidation of alcohols with hydrogen peroxide.67 The tetranuclear complex [cetylpyridinium chloride][P04 WO(02)2 4] supported on polypyridine was found to be an effective catalyst for the oxidation of cyclohexanol to cyclohexanone, also with hydrogen peroxide. 

K. Yamaguchi, C. Yoshida, S. Uchida, N. Mizuno, Peroxotungstate immobilized on ionic liquid-modified silica as a heterogeneous epoxidation catalyst with hydrogen peroxide, J. Am. Chem. Soc. 127 (2005) 530. 

In conclusion, this report introduces a truly heterogeneous catalyst based on the in situ generation of active peroxotungstate complexes. By taking profit of the variability of the anion population, a new class of tailor-made catalysts is available. The hydrophobicity of the surface can be tuned, and heteronuclear anion oligomers can be synthesized at the sohd-liquid interphase. 

Finally, it should be mentioned that Yamaguchi et al. used the SILP concept to immobilize an oxidation catalyst [156]. They found that peroxotungstate [ W(=0)(02)2(H20) 2(/U-0)] can be immobilized on a dihydroimidazolium-based ionic liquid-modified Si02 surface to get an eSicient heterogeneous... 

In another approach, heterogeneous olefin epoxidation catalysts were prepared by electrostatic attachment of binuclear peroxotungstate complexes onto an imidazolium-grafted silica support by ion-exchange [78]. The catalyst was made by initial grafting of the imidazolium-cations onto a pre-dried silica support by treatment with l-octyl-3-(3-triethoxysilypropyl)-imidazolium hexafluorophosphate. 

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