CATALYTIC LIQUID-PHASE OXIDATION OF PHENOL AQUEOUS-SOLUTIONS - A KINETIC INVESTIGATION

Catalytic oxidation of aqueous phenol solutions was studied in a differential,liquid-full operated fixed-bed reactor. A proprietary catalyst comprised of supported copper, zinc, and cobalt oxides was found to be effective for converting phenol to benzenedioles and benzoquinones, the C-4 intermediates in total oxidation route, and carbon dioxide. The proposed intrinsic rate expression for the phenol disappearance is based on the Langmuir-Hinshelwood kinetic approach, considering both equilibrium phenol and dissociative oxygen adsorption processes on different types of active sites and assuming a bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. Apparent activation energies for catalytic phenol oxidation and heat of phenol adsorption, in the temperature range 150-180 degrees C, were found to be 139 and -62 kJ/mol, respectively. It is believed that the liquid-phase oxidation of an aqueous phenol solution undergoes a combined redox and heterogeneous free-radical mechanism. The involvement of a free-radical mechanism is indicated by the intermediates formed and by pH as well as radical initiator effects on observed phenol disappearance rates.