Mechanistic and kinetic insights into the wet air oxidation of phenol with oxygen (CWAO) by homogeneous and heterogeneous transition-metal catalysts

The activity–selectivity pattern of homogeneous (Cu2+, Fe3+, Mn2+) and ceria-supported (CuCeOx, MnCeOx) transition-metal catalysts in the wet air oxidation of phenol (CWAO) has been probed using a stirred batch reactor with continuous oxygen feeding (T, 150 °C; PO2PO2, 0.9 Mpa). Both non-catalytic and catalytic homogeneous wet air oxidations proceed via an unselective autocatalytic free-radical path leading mostly to refractory C1–C2 acids, while a Langmuir–Hinshelwood (L–H) mechanism accounts for the superior CWAO performance of the MnCeOx system. A thorough kinetic analysis of the studied systems on the basis of homogeneous autocatalytic free-radical and heterogeneous surface L–H reaction paths has been addressed. The kinetic constants of the various reaction steps show that the MnCeOx system prompts a fast adsorption of phenol with the consequent abatement of TOC, though a slow oxidation rate determines the buildup of carbonaceous deposits on the catalyst surface. Lower oxidation strength and extensive leaching definitively argue against Cu-based catalysts for the CWAO process.

Graphical abstractComparison of the performance of homogeneous (Mn2+, Fe3+, Cu2+) and heterogeneous (CuCeOx, MnCeOx) transition-metal catalysts in the wet air oxidation (CWAO) of phenol (T, 150 °C; P, 1.4 MPa).Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ The manuscript addresses a systematic study of the activity–selectivity pattern of transition-metal catalysts in the wet air oxidation of phenol (CWAO), highlighting marked differences in the efficiency of homogeneous (Fe3+, Mn2+, Cu2+) and ceria-supported (CuCeOx, MnCeOx) systems, mostly linked to the occurrence of different reaction mechanisms. ▶ The work represents a significant advance in the knowledge of mechanisms and kinetics of CWAO processes for wastewater treatment, contributing to future advances in catalyst and process design.