Synthesis, Characterization And Catalytic Application Of Recyclable Polyoxometalate-based Catalysts

Polyoxometalates(abbreviated as POMs), owing to their inherent acidity and redox properties, and the tunability of their structures, have been widely studied and applied in various selective oxidation reactions as highly efficient catalysts, for example, epoxidation of alkenes, oxidation of alcohols and sulfides, ammoximation of aldehydes and ketones. Although the traditional homogeneous POMs catalysts have much advantage in catalytic activity, they are expensive in price, so the reusability of the POMs catalysts is always attractive and drawn wide attention.In this work, we mainly studied these four POMs-based recyclable catalysts on organic catalytic systems:1. A tri-vanadium substituted Keggin-type POM K6PW9V3O404H2O, was synthesized. It was used as catalyst on the ammoximation of various aldehydes and ketones to their corresponding oximes with H2O2(30%) and ammonia using isopropanol as solvent at room temperature. High conversions and selectivity was obtained. The catalyst was proved to be heterogeneous in the solvent by experiment, so it was recyclable after reaction completed. The reactivity for the catalyst didn’t obviously decrease after recycled for4times. The fresh and used catalysts were characterized by FT-IR,31P NMR and elemental analysis. A possible mechanism was also proposed.2. A temperature-controlled phase transfer catalyst [(C18H37)2(CH3)2N]7PW11O39was synthesized. It was used as catalyst on oxidation of various sulfides including thiophene and thioether to corresponding sulfones with H2O2as green oxidant. Reaction proceeded at60℃for0.5h,1,4-dioxane as solvent, conversions for the substrates tested were all up to96%. The catalyst was insoluble in solvent at room temperature, while with the temperature increased, the catalyst dissolved and behaved as homogeneous during reaction. After reaction, as the temperature decreased, the catalyst precipitated, which realized catalyst recycling. The reactivity for benzothiophene didn’t obviously decrease even after6cycles. The fresh and used catalysts were characterized by FT-IR,31P NMR and elemental analysis. A catalytic mechanism was also proposed. 3. The homogeneous catalyst of Δ-Na8H[PW9O34]·19H2O, was supported on carbon nanotubes(CNTs) using two methods, respectively. The supported CNTs were applied on epoxidation of various alkenes, aiming at improving the reactivity. Reactions were carried out at65℃for3h, acetonitrile was used as solvent, yields for epoxides were all less than20%, indicating no increase in reactivity for the supported catalysts. It may be due to the failure in supporting the homogeneous POMs catalyst, or the amount that was supported was too little.4. Three kinds of POMs, which was substituted by transition metals including Cu, Fe and V, were synthesized. The catalysts were used on the oxidation of C-H for toluene. Reaction conditions were optimized to improve yield and selectivity for benzaldehyde by changing reaction temperature, solvent, oxidant and catalyst amount, and so on. Finally, a-K6[SiW11O39Cu(H2O)] proved to be the best catalyst using TBHP as oxidant, under reaction temperature at80℃, toluene:TBHP=5:1, lml acetonitrile as solvent, catalyst amount of5μmol, benzaldehyde got the highest yield of20%, while the selectivity was56%.