Phenol Oxidation Pathways in Supercritical Water

We oxidized aqueous solutions of phenol in batch and flow reactors at temperatures of 300, 380, and 420 °C, pressures of 218 and 278 atm, and reaction times between 1.2 and 29040 s. Gas chromatographic and mass spectrometric analysis of the oxidation products extracted from the aqueous reactor effluent permitted identification and quantification of the concentrations of multiring compounds such as 2- and 4-phenoxyphenol, 2,2'-biphenol, dibenzofuran, and dibenzo-p-dioxin. The Delplot methodology was used to discriminate between primary and nonprimary products. This analysis revealed that 2- and 4-phenoxyphenol, 2,2'-biphenol, and dibenzofuran formed directly from phenol whereas dibenzo-p-dioxin formed later in the reaction network. The literature suggested that 2-phenoxyphenol was the most likely dibenzo-p-dioxin precursor. A reaction network consistent with these observations was developed and shown to be in quantitative agreement with the experimental results. The reaction model revealed that the formation of the primary, multiring products from oxidation at 380 °C and 278 atm accounted for 43% of the phenol that reacted. This result suggests that compounds such as dibenzofurans and dioxins could be formed during the supercritical water oxidation of phenolic wastes near the entrance of commercial-scale reactors where the design temperature is around 400 °C.