Anionic molecular metal-oxygen cluster compounds polyoxometalates show the strong acidity and redox properties. In addition, the compounds are thermally and oxidatively stable. These properties can be controlled to a great extent by changing the constituent elements. In this study, we attempt to construct the catalytically active sites and to arrange them in a ultrasmall space.
Epoxides are an important class of industrial chemicals that have been used as chemical intermediates. Catalytic epoxidation of olefins affords an interesting production technology. We found a widely usable green route to the production of epoxides.
A silicotungstate compound, [γ-SiW10O34(H2O)2]4- (1), is synthesized by protonation of a divacant, lacunary, Keggin-type polyoxometalate of [γ-SiW10O36]8- and exhibits high catalytic performance for the epoxidation of various olefins, including propylene, with a hydrogen peroxide at 305 K. In contrast, the other mono- and trivacant lacunary compounds, [α-SiW11O39]8- and [α-SiW9O34]10-, as well as a complete silicododecatungustate, [α-SiW12O40]4-, were almost inactive.
The effectiveness of this catalyst is evidenced by ?99% selectivity to the corresponding epoxide, ?99% efficiency of hydrogen peroxide utilization, high stereospecificity,and easy recovery of the catalyst from the homogeneous reaction mixture. Moreover, high stereospecific reactivity of 1 suggests the contribution of a structurally rigid non-radical oxidant generated on 1
An ionic crystal [Cr3O(OOCH)6(H2O)3] [α-CoW12O40]?7.5H2O (2) was synthesized by the complexation of a Keggin-type polyoxometalate of [α-CoW12O40]6- with a Cr(III) macro cation of [Cr3O(OOCH)6(H2O)3]+ in the presence of Cs+. The exposure of 2 to warm N2 stream resulted in the desorption of the water of the crystallization in 2 giving the corresponding dihydrated complex (2′). The crystal structures of 2 and 2′ were almost identical except the number of the water of crystallization. This dihydrated compound 2′ could adsorb water molecules but excluded MeOH and EtOH. Therefore, 2′ colud separate only water from the azeotropic mixture of EtOH/H2O.
Hydrogen peroxide, Hydrocarbons, Selective oxidation, Polyoxometalates, Molecular recognition