Formation of polychlorinated benzenes during the catalytic combustion of chlorobenzene using a Pt/γ-Al2O3 catalyst

Pt/gamma-Al2O3 has been used for the complete catalytic oxidation of chlorobenzene, a model for chlorinated aromatic compounds present in flue gases. Complete conversion of chlorobenzene is reached at ca. 440 degrees C, but at this temperature substantial amounts of polychlorinated benzenes are formed. Only at 600 degrees C complete selectivity to carbon dioxide is achieved. The addition of 1.75% of water to the carrier gas reduces formation of polychlorinated benzenes and improves conversion. The oxygen partial pressure has a remarkable effect on byproduct formation: the amounts of polychlorinated benzenes rise sharply with increasing oxygen concentration. Total conversion of chlorobenzene remains stable, hence the selectivity to carbon dioxide increases with decreasing oxygen pressure. When the gamma-Al2O3 support alone is applied, complete conversion of chlorobenzene is reached only at ca. 550 degrees C, without production of polychlorinated benzenes. The selectivity to carbon oxides however is poor. So platinum seems to be responsible for the formation of polychlorinated benzenes, which we propose is brought about by chlorination of adsorbed (chloro)benzene-species through platinum (oxy) chlorides. (C) 1998 Elsevier Science B.V.