Heterogeneous HO-based selective oxidations over zirconium tungstate α-ZrWO

Catalytic properties of a crystalline zirconium tungstate, ZrW 2 O 8 , the material known mainly for its isotropic negative coefficient of thermal expansion, have been assessed for the liquid-phase selective oxidation of a range of organic substrates comprising C&z.dbd;C, OH, S and other functional groups using aqueous hydrogen peroxide as the green oxidant. Samples of ZrW 2 O 8 were prepared by hydrothermal synthesis and characterised by N 2 adsorption, PXRD, SEM, EDX, FTIR and Raman spectroscopic techniques. Studies by IR spectroscopy of adsorbed probe molecules (CO and CDCl 3 ) revealed the presence of Brønsted acidic and basic sites on the surface of ZrW 2 O 8 . It was demonstrated that ZrW 2 O 8 is able to activate H 2 O 2 under mild conditions and accomplish the epoxidation of C&z.dbd;C bonds in alkenes and unsaturated ketones, oxidation of thioethers to sulfoxides and sulfones, along with the oxidation of alcoholic functions to produce ketones and aldehydes. The oxidation of tetramethylethylene and α-terpinene over ZrW 2 O 8 revealed the formation of peroxidation products, 2,3-dimethyl-3-butene-2-hydroperoxide and endoperoxide ascaridole, respectively, indicating the involvement of singlet oxygen in the oxidation process. The ZrW 2 O 8 catalyst preserves its structure and morphology under the turnover conditions and does not suffer from metal leaching. It can be easily recovered, regenerated by calcination, and reused without the loss of activity and selectivity. A stable and reusable crystalline α-ZrW 2 O 8 catalyst is capable of activating H 2 O 2 under mild conditions and performing various selective liquid-phase oxidations.