Synergetic Catalysis of Copper and Iron in Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O2-based oxidations is reoxidation of the reduced POM by O-2. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POMred (alpha-PW12O404- and alpha-SiVW11O406-) reacting with O-2, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu(II) and Fe(II), followed by a fast oxidation of POMred by Fe(III) and Cu(I) by O-2 to regenerate Fe(II) and Cu(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, alpha-PW12O403- and alpha-SiVW11O405-, are far better oxidants of organic substrates than the previously studied POMs, alpha-SiW12O404- and alpha-AlW12O405-, this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O-2/air-based processes.