The formation of benzene and chlorobenzene during the oxidation of tolueneover rhodium-based catalysts

The oxidation of toluene and benzene in a four hydrocarbon mixture with and without carbon monoxide present has been investigated over Rh/Al2O3 and Rh /CeO2/Al2O3 catalysts containing residual chlorine. Up to 12% of toluene ca n be converted to benzene with a near stoichiometric feed containing carbon monoxide. Much less benzene is formed in the absence of carbon monoxide, e specially with the ceria-alumina based catalyst which is more active for th e oxidation of the aromatics. Formation of benzene probably proceeds within areas where toluene coverages are sufficiently high to provide the hydroge n atoms required for addition to a benzyl intermediate. With fresh catalyst s chlorobenzene can be formed as a byproduct in concentrations up to 10% of that of the aromatic in the feed. Toluene is the largest source of chlorob enzene with the alumina-based catalyst when CO is present. Benzene is a lar ger source with Rh/CeO2/Al2O3 when the feed is dry but with water present t he behaviour resembles that of Rh/Al2O3. The processes involved in chlorobe nzene formation are uncertain but may include migration of chlorine from al umina to rhodium or the metal-support interface. (C) 1999 Elsevier Science B.V. All rights reserved.