Polyoxometalates (POMs) have been extensively studied for catalysis of aerobic oxidation reactions. Commonly, the reaction proceeds in two steps: 1) the oxidation of a substrate by oxidized form of POM; 2) the reoxidation of reduced POM (POMred) by dioxygen, which is usually the rate-limiting step. Here we report a catalytic system to significantly accelerate the re-oxidation step. We used the one-electron reduced POMs, α-PW12O404−, α-SiW12O405− and α-AlW12O406− as model compounds in our previous publication. Cu salts (nitrate or sulfate) were found to be extremely efficient catalysts for POM re-oxidation at low pH, particularly in the case of α-SiW12O405−. The reaction mechanism includes the reversible reduction of Cu(II) by POMred followed by a fast re-oxidation of Cu(I) by O2 to regenerate Cu(II). Dioxygen is reduced to H2O2 in this reaction. We now report that a combination of Cu and Fe salts catalyzes the re-oxidation of α-PW12O404− and α-SiVIVW11O406−, POMs that have higher potentials. 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) to form Cu(I) and Fe(III), followed by a fast oxidation of POMred by Fe(III) and of Cu(I) by O2 to regenerate Fe(II) and Cu(II). In addition, a combination of Cu and Fe complexes efficiently catalyzes the oxidation of POMred by H2O2, resulting in an additional increase in rates. The proposed reaction mechanism quantitatively describes the experimental data over a wide range of experimental conditions.

Synergetic catalysis by copper and iron in oxidation of reduced Keggin heteropolytungstates by dioxygen

254th National Meeting (2017)

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