12646
J. Phys. Chem. 1994, 98, 12646-12652
Reaction Mechanism for Phenol Oxidation in Supercritical Water Sudhama Gopalan and Phillip E. Savage* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 Received: August 17, I994@
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, COZconsistently formed in higher yields than CO, and carboxylic acids and single-ring oxygenates formed as stable reaction intermediates. We examine the literature for freeradical 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 HO;! 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.
Introduction Supercritical water oxidation' (SCWO) is an advanced oxidation process for the treatment of hazardous wastes. It involves the oxidation of organic pollutants in aqueous streams at reaction conditions beyond the critical point of water (T, = 374 "C, Pc = 218 atm) to C02. The advantage of the supercritical water reaction environment is the high solubility it affords to both organics and oxygen, thereby obviating interphase transfer and leading to fast reaction rates. Fundamental research on supercritical water oxidation of real pollutants has been restricted to determining global kinetics and reaction paths. The disappearance kinetics of phen01,~
