Chlorobenzene and Dichlorobenzene Reactions in Hydrogen and in Hydrogen/Oxygen Mixtures

Abstract

Thermal Reactions of Chlorobenzene in hydrogen and in hydrogen - oxygen mixtures were studied in tubular flow reactors at 1 atmosphere pressure and temperatures between 833 to 1275 K. 99% conversion of chlorobenzene and dichlorobenzene in hydrogen, 2 seconds residence time, reguired temperatures of 1198 K and 1174 K respectively, while in mixtures containing small guantities, less than 4%, O2 eguivalent conversion was observed to occur at 935 K. Major products in both reaction systems were benzene, carbon solids and HCl, with CH4 and C2H6 also present in low concentrations. Minor products included toluene, cyclopentadiene, biphenyls, C2H4, C2H2 with CO and CO2 also observed when oxygen was present. Decomposition in the presence of hydrogen was observed to occur much faster than pyrolysis in an inert gas. In addition, the presence of hydrogen accelerates the destruction of chlorinated aromatics via a catalytic gas phase process. The specific reaction responsible for this catalytic conversion is a displacement of the aromatic chlorine by atomic hydrogen. Chlorobenzene dissociation to Cl + phenyl radical is the initiation step in He and in H2, with: Cl + H2 --> HCl + H rapidly continuing the chain. The slightly more rapid conversion of dichlorobenzene is attributed to the higher chlorine content and a somewhat weaker carbon - chlorine bond. O2, if present, can initiate the chain mechanism by reaction with hydrogen to form HO2 + H. This explains the lower temperature reguired for conversion when O2 is present. The dissociation of chlorobenzene to phenyl and chlorine atom is not as thermodynamically favorable so that the oxygen/hydrogen system shows much faster reaction.