Hydrodechlorination of carbon tetrachloride in the liquid phase on a Pd/carbon catalyst: kinetic and mechanistic studies

Activity and selectivity of a 1 wt.% Pd/carbon catalyst in the hydrodechlorination of carbon tetrachloride into chloroform was studied in the liquid phase as a function of temperature (353–413 K) and reaction time. The catalyst was prepared by incipient wetness impregnation and reduced at 523 K, and the reaction performed in a three-phase bubble column reactor at 3 MPa. Reaction is highly selective towards chloroform, with dichloromethane, monochloromethane, hexachloroethane, tetrachloroethylene and methane as by-products. Increasing the temperature mainly favors the production of hexachloroethane and tetrachloroethylene, though the chloroform selectivity remains always above 90% even at conversion levels even higher than 90%. The reaction fits a zero order related to carbon tetrachloride up to 60% conversion at 413 K and even to higher conversions at lower temperatures. From the analysis of both product distribution versus time and reactivity of the reaction products, a reaction pathway is proposed. The results indicate that chloroform, hexachloroethane and methane are primary products, and that the carbon tetrachloride transformation proceeds via three parallel and independent paths. Chloroform, dichloro- and monochloromethanes are formed from tetrachloromethane by successive hydrodechlorination reactions (Path I), while hexachloroethane and tetrachloroethylene follow other parallel consecutive path (Path II). Methane is directly produced from tetrachloromethane (Path III). Paths I and II are conventional consecutive reactions in which the intermediates are desorbed to the gas/liquid phase as stable molecules and further readsorbed in order to give the following chemical species. In contrast, for methane the overall consecutive process occurs in the adsorbed phase, on the same active site, without the appearance of any intermediate molecular species.