Direct Observation of H2O2 during Alcohol Oxidation by O2 in Supercritical Water

This paper presents the direct quantitative measurement of the presence of hydrogen peroxide as an intermediate produced during the oxidation of methanol, ethanol, and 1-propanol by oxygen in supercritical water. The concentration of H2O2 is measured as a function of reaction time in an optically accessible, high-pressure flow reactor by monitoring the intensity of a strong resonance in the Raman spectrum at 874 cm-1 that is characteristic of the O−O vibrational stretch. Transient concentrations of H2O2 as high as 0.0065 mol/L (0.1% mole fraction) are measured during the oxidation of ethanol at feed mole fractions of 0.4% at 430 °C and 24.5 MPa. Significantly lower transient concentrations of H2O2 are observed during methanol oxidation. The experimentally observed transient H2O2 concentration during methanol oxidation is compared to that predicted by two similar elementary reaction mechanisms and found to be distinctly lower than the models predict.