Oxidation of substituted phenols in supercritical water. Final technical report, September 1992--August 1996

Wastewaters from coal-conversion processes contain phenolic compounds in appreciable concentrations. These compounds need to be removed so that the water can be discharged or reused. Oxidation in supercritical water is one potential means of treating coal-conversion wastewaters, and this project examined the reactions of model pollutants in supercritical water. The decomposition of cresols, hydroxybenzaidehydes, nitrophenols, and benzenediols was studied in dilute aqueous solutions in both the presence and absence of oxygen at 460{degrees}C and 250 atm. Experimental data from the oxidation of these compounds were fit to global, power-law rate expressions. The resulting rate laws showed that the reactivity of the different isomers at 460{degrees}C was in the order of ortho > para > meta for cresols and hydroxybenzaldehydes. Moreover, the CHO-substituted phenol was more reactive than the analogous CH{sub 3}-substituted phenol, and all of these substituted phenols were more reactive than phenol itself. Identifying and quantifying the reaction products of incomplete oxidation allowed us to assemble a general reaction network for the oxidation of cresols in supercritical water. This network comprises parallel primary paths to phenol, to a hydroxybenzaldehyde, and to ring-opening products. The hydroxybenzaldehyde reacts through parallel paths to phenol and to ring-opening products. Phenol also reacts via two parallel paths, but these lead to phenol dimers; and ring-opening products. The dimers are eventually converted to ring-opening products, and the ring-opening products are ultimately converted to CO{sub 2} The relative rates of the different paths in the reaction network are strong functions of the location of the substituent on the phenolic ring