Heterogeneous H2O2-based selective oxidations over zirconium tungstate α-ZrW2O8

Abstract

Catalytic properties of a crystalline zirconium tungstate, ZrW₂O₈, 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 CC, OH, S and other functional groups using aqueous hydrogen peroxide as the green oxidant. Samples of ZrW₂O₈ were prepared by hydrothermal synthesis and characterised by N₂ adsorption, PXRD, SEM, EDX, FTIR and Raman spectroscopic techniques. Studies by IR spectroscopy of adsorbed probe molecules (CO and CDCl₃) revealed the presence of Brønsted acidic and basic sites on the surface of ZrW₂O₈. It was demonstrated that ZrW₂O₈ is able to activate H₂O₂ under mild conditions and accomplish the epoxidation of CC 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 ZrW₂O₈ 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 ZrW₂O₈ 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.