Oxidation of phenol over a transition-metal oxide catalyst in supercritical water

The oxidation kinetics of phenol in supercritical water was examined in the presence of a solid catalyst consisting of supported copper, zinc, and cobalt oxides in an integrally operated fixed-bed reactor. For the conditions studied the rate of phenol disappearance was found to be well described by the Langmuir-Hinshelwood kinetic formulation, which accounts for the equilibrium adsorption of phenol and for dissociative oxygen adsorption processes to the different types of active sites and a bimolecular surface reaction between adsorbed species on adjacent active catalyst sites to be the controlling step. The apparent activation energy and the heat of phenol adsorption in the temperature range 400-440 degrees C were found to be 109 and 24 kJ/mol, respectively. The products identified in the effluent include dimers, single-ring compounds, organic acids, and gaseous end pro ducts. The involvement of a homogeneous-heterogeneous free-radical mechanism is indicated by the intermediates formed; The product distribution suggests that the catalyst is much more selective on the para isomer of phenoxy radical. Comparing the wide spectrum of organic acids formed during the noncatalytic phenol oxidation in supercritical water with only formic and acetic acid found in the effluent of catalytic process, it may be concluded that the intermediates adsorbed on the catalyst surface are probably rapidly oxidized to the low molecular weight acids.