Kinetics of Carbon Dioxide Formation From the Oxidation of Phenols in Supercritical Water

Global power-law rate expressions were determined for the formation of CO₂ from the oxidation of aqueous solutions of phenol and 2-chlorophenol. The oxidation experiments were accomplished in a plug-flow reactor at temperatures between 300 and 420 °C and pressures from 185 to 278 atm. These conditions included oxidations in both near-critical and supercritical water. Reactor residence times ranged from 1.2 to 109 s. The initial reactant concentrations were between 8.0 × 10^-5> and 2.1 × 10^-3 M, and the initial oxygen concentrations ranged from 5.5 × 1CT3 to 4.8 × 10^-2 M. The phenolic compound was always the limiting reactant, and the excess oxygen was between 180% and 1360%. Nonlinear regression analysis of the kinetics data for the formation of CO₂ from phenol oxidation revealed that the reaction was 0.82 ± 0.20 order in total organic carbon and 0.71 ± 0.26 order in oxygen. The activation energy was 6.2 ± 2.6 kcal/mol. The rate law for CO₂ formation from 2-chlorophenol was 0.51 ± 0.08 order in total organic carbon and 0.80 ± 0.13 order in oxygen. The activation energy for this rate law was 9.0 ± 5.8 kcal/mol. The uncertainties given here represent 95% confidence intervals. There was no statistically significant dependence of the rate of CO₂ formation on the water concentration for either compound. The low activation energies suggest that the major pathways leading to CO₂ do not include carbon monoxide or acetic acid as refractory intermediates. For both phenolic compounds, the conversion of organic carbon to CO₂ was always less than the conversion of the phenol, indicating that products of incomplete oxidation were present.