Kinetics and Mechanism of the Heterogeneous Catalytic Hydrogenolysis of Chlorobenzenes and Chlorocyclohexanes

The catalytic hydrogenolysis of hexachlorobenzene and hexachlorocyclohexanes (isomer mixture) on a nickel–chromia catalyst and hexachlorobenzene hydrogenolysis intermediates (1,2,4,5-tetrachlorobenzene, 1,2,4-trichlorobenzene, 1,2-dichlorobenzene, and chlorobenzene) are studied. The hydrogenation of an aromatic ring does not occur in the presence of chemisorbed chlorine atoms on the catalyst surface. A reaction mechanism for chlorobenzene hydrogenation was proposed taking into account experimental evidence that, in the presence of chemisorbed chlorine on the catalyst surface, hydrogen in a dissociated state is firmly bound to the surface. It is found that the desorption of the resulting hydrogen chloride is the slowest step in chlorobenzene hydrogenolysis. The hydrogenolysis of hexachlorocyclohexanes occurs via a dehydrochlorination stage with the formation of trichlorobenzenes, which are subsequently converted into chlorobenzene and benzene.