FORMATION OF POLYCHLORINATED BENZENES DURING THE CATALYTIC COMBUSTIONOF CHLOROBENZENE USING A PT GAMMA-AL2O3 CATALYST/

Pt/gamma-Al2O3 has been used for the complete catalytic oxidation of c hlorobenzene, a model for chlorinated aromatic compounds present in fl ue gases. Complete conversion of chlorobenzene is reached at ca. 440 d egrees C, but at this temperature substantial amounts of polychlorinat ed benzenes are formed. Only at 600 degrees C complete selectivity to carbon dioxide is achieved. The addition of 1.75% of water to the carr ier gas reduces formation of polychlorinated benzenes and improves con version. The oxygen partial pressure has a remarkable effect on byprod uct formation: the amounts of polychlorinated benzenes rise sharply wi th 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 app lied, complete conversion of chlorobenzene is reached only at ca. 550 degrees C, without production of polychlorinated benzenes. The selecti vity to carbon oxides however is poor. So platinum seems to be respons ible for the formation of polychlorinated benzenes, which we propose i s brought about by chlorination of adsorbed (chloro)benzene-species th rough platinum (oxy) chlorides. (C) 1998 Elsevier Science B.V.