GAS-PHASE OXYCHLORINATION OF BENZENE

Homogeneous gas-phase oxychlorination of benzene has been examined in a flow system between 722 and 1174K. Experiments were performed at atm ospheric pressure with nitrogen as an inert carrier gas, containing ab out equal molar amounts of oxygen, HCl and benzene. Apart from chlorob enzene, substantial amounts of o- and p-chlorophenol are produced. Usi ng benzonitrile rather than benzene, the three isomers of ClC6H4CN are formed in about equal amounts. These findings suggest a homolytic mec hanism with aryl radicals as key intermediates. Thermokinetic analysis points to formation of chlorophenols via reaction of phenoxyl (formed from phenyl and oxygen) with HOCl. Chlorination, C6H6 --> C6H5Cl, is explained by reaction between phenyl radicals and HOCl. With increasin g temperature (and, therefore, oxygen consumption), slow combustion is replaced by pyrolysis as the major reaction channel. As a result, abo ve ca. 1000K, the production of chlorobenzene diminishes, [C6H5Cl] ult imately being governed by the equilibrium: C6H6 + HCl half arrow right over half arrow left C6H5Cl + H-2.