Thermal Hydrogenolysis of Dibenzo‐ p ‐dioxin and Dibenzofuran

The behaviour of dibenzo‐ p ‐dioxin (DD) and dibenzofuran (DF) was studied in flow reactors in an excess of hydrogen, at atmospheric or elevated pressure (up to 35 bar), in the temperature range 890−1262 K. The experiments at atmospheric pressure were performed with DF or DD as such, while at increased pressures the compounds were introduced as an admixture with benzene. DD gave CO, but also DF as an important product. The rates clearly depended on the hydrogen concentration, and were about an order of magnitude higher than that of the hydrodechlorination of chlorobenzene. The reaction apparently started with the fission of a C−O bond, induced by H atom attack. DF reacted much more slowly, to give CO and hydrocarbons, especially naphthalene and benzene. Its rate was insensitive to the concentration of H 2 , and the degradation has been interpreted as thermolysis, through C−O bond homolysis, isomerisation and fragmentation, primarily to naphthalene, and C 2 O as the intermediate to CO. The apparent resistance of DF to hydrogenolysis can be understood from its relatively favourable thermodynamic stability. The consequences for the behaviour of polychlorinated DDs and DFs under similar conditions — relevant for the possible application of thermal hydrogenolysis as a waste management technology — are also discussed briefly. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)