Decomposition of chlorinated hydrocarbons on iron-group metals

The decomposition of 1,2-dichloroethane and chlorobenzene on nickel-alumina , cobalt-alumina, and iron-alumina catalysts at 400-600 degreesC was studie d. Thermodynamic calculations demonstrated that the susceptibility of metal s to chlorination under exposure to HCl increases in the order Ni < Co < Fe . The addition of hydrogen to the reaction mixture was found to dramaticall y decrease the rate of carbon deposition in the decomposition of 1,2-dichlo roethane because of the intense hydrogenation of intermediates that are gra phite precursors. Two fundamentally different reaction paths were found in the degradation of 1,2-dichloroethane: decomposition via a carbide-cycle me chanism with the formation of carbon as the main product (under conditions of a deficiency of hydrogen) and 1,2-dichloroethane hydrodechlorination acc ompanied by methanation of the formed carbon (under conditions of an excess of hydrogen). The degradation of chlorobenzene diluted with hydrogen in a molar ratio of 1 : 50 was not accompanied by carbon formation on the cataly st. A comparison between the selectivity for reaction products on nickel-al umina and cobalt-alumina catalysts indicated that the former catalyst is mo re active in the rupture of C-C bonds and in the methanation reaction of de posited carbon, whereas the latter is more favorable for hydrodechlorinatio n. The optimum conditions and thermal regime for efficient and stable opera tion of the catalysts were found.