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 (CuCeo(x), 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 degrees C; P-O2, 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. (C) 2010 Elsevier B.V. All rights reserved.