Kinetics and mechanism of methanol oxidation in supercritical water

We oxidized methanol in supercritical water at 246 atm and temperatures between 500 and 589°C. Pseudo-first-order rate constants calculated from the data led to Arrhenius parameters of A = 10^21.3±5.3 s^-1 and E_a = 78 ± 20 kcal/mol. The induction time for methanol oxidation decreased from 0.54 s at 525 °C to 0.093 s at 585 °C and the reaction products were formaldehyde, CO, and CO₂. Formaldehyde was a primary product, while CO and CO₂ were secondary products. Formaldehyde was more reactive than methanol and its yield was always less than 24%. The temporal variation of the CO yield exhibited a maximum, whereas the CO₂ yield increased monotonically. The experimental data were consistent with a set of consecutive reactions (CH₃OH → CH₂O → CO → CO₂) with pseudo-first-order global kinetics. The experimental data were also used to validate a detailed chemical kinetics model for methanol oxidation in supercritical water. With no adjustments, this elementary reaction model quantitatively predicts the product distribution as a function of the methanol conversion, and it accurately predicts that this distribution is nearly independent of temperature. A sensitivity analysis revealed that only eight elementary reaction steps most strongly influenced the calculated species' concentrations. A reaction path analysis showed that the fastest reactions that consumed methanol involved OH attack and the resulting radicals produced formaldehyde, which was attacked by OH to form, eventually, CO. The CO was then oxidized to CO₂ via reaction with OH. This work shows that the chemistry for methanol oxidation in supercritical water at temperatures around 500-600 °C is quantitatively analogous to combustion chemistry within the same temperature range.