Raman spectroscopic measurement of oxidation in supercritical water .1. Conversion of methanol to formaldehyde

The oxidation rate of methanol and the subsequent production and destruction of the primary intermediate, formaldehyde, were investigated using Raman spectroscopy as an in situ analytical method. Experiments were conducted in supercritical water over temperatures ranging from 440 to 500 degrees C at 24.1 MPa and at a nominal feed concentration of 0.05 mol/L (1.5 wt %). Effluent samples were also examined using gas chromatography. In these experiments, feed concentrations ranging from 0.011 to 1.2 wt % and temperatures from 430 to 500 degrees C were examined and showed that the effective first-order reaction rate for the oxidation of methanol is dependent on the initial feed concentration. Raman measurements reveal a temperature-dependent induction period of less than 1 s over the range of conditions investigated. In addition, quantitative measurements of the production of formaldehyde indicate it is a key metastable intermediate. An elementary reaction mechanism, which reproduces accurately the quantitative features of methanol oxidation and formaldehyde production, is used to identify key rate controlling reactions during the induction period and the transition to the primary oxidation path.