Oxidation Catalyzed by Polyoxometalate-Anion-Intercalated Layred Compounds

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Layred double hydroxides (LDH) consisting of Mg-Al, Zn-Al, and Ni-Al with the atomic ratios varied were prepared, and intercalated with heteropolyoxometalate anions including Mo or W as the addenda atom. It has been found rather difficult to insert SiW_<12>O_<40>^<4-> into the layr space of Zn-Al LDH with the Zn/Al ratio of 2. However, the insertion was promoted with decreasing Al/Zn ratio, To avoid decomposition of polyoxometalate, the pH during the insertion was kept at lower than 4, where the Zn-Al LDH was more stable than the Mg-Al LDH.Zn-Al LDHs intercalated with silicotungstates SiW_<12>O_<40>^<4->, SiW_<11>O_<39>^<8->, and SiW_9O_<34>^<10-> were prepared and the structure of the anions in the gallery space was examined by infrared spectroscopy. The SiW_<11>O_<39>^<8-> intercalate proved to be a good catalyst for olefin epoxidation with H_2O_2 as oxidant. In contrast, the SiW_<12>O_<40>^<4-> intercalate was effective in epoxidizing olefins with O_2-aldehyde, although it showed poor activity in the epoxidation with H_2O_2 Substitution of V for W of the addenda atom resulted in enhancement of activity for epoxidation with O_2-aldehyde. The Ni-Al LDH with no polyoxometalate included exhibited considerable activity in olefin epoxidation with O_2-aldehyde. Thus it has been suggested that the introduction of active metals into both the LDH itself and the gallery space may lead to emergence of bifunctional catalysis, which would be applied to novel catalytic reactions.