Reaction mechanism for phenol oxidation in supercritical water

We postulate the elementary steps in the reaction mechanism for phenol oxidation in supercritical water that account for key findings from earlier experiments. These findings are competing primary paths (dimerization and ring-opening) for phenol disappearance, CO₂ consistently formed in higher yields than CO, and carboxylic acids and single-ring oxygenates formed as stable reaction intermediates. We examine the literature for free-radical chemistry of phenol oxidation in both gas and solution phases to find elementary processes that could describe our findings for oxidation in supercritical water. We discriminate between potential elementary processes using thermodynamic and kinetic arguments and select the most appropriate mechanisms for the primary pathways. Our results indicate that phenol dimerization is a radical-radical process involving phenoxy radical recombination. The likely ring-opening processes also involve radical-radical addition. The two potential ring-opening paths involve the addition of HO₂ and OH to the phenoxy radical as the initial step to form a hydroperoxide and a benzenediol, respectively. Subsequent ring-opening reactions that form carboxylates like acids and acyl radicals are consistent with the intermediates detected. Ring-opening schemes involving oxygen addition reactions are not likely to be important. The mechanism reported here can be used as a foundation upon which to build a quantitative detailed chemical kinetics model for the primary paths for phenol oxidation in supercritical water.