Gas phase hydrogenolysis mediated by activated carbon: Monosubstituted benzenes

Easy hydrogenolysis of benzene derivatives in flow reactors packed with activated carbon (AC) and hydrogen as a bath gas is reported. Fast desubstitution (below 600 degrees C) - PhX --> PhH - is encountered for chloro-, bromo-, and iodobenzene, modest for X = OCH3, NO2 and CH3, and slow for X = NH2, F, OH, CN and Ph. In general, compared with homogeneous hydrogenolysis, AC lowers the reaction temperature for complete conversion with 200 degrees C. As to the mechanism, hydrogen transfers from hydroaryl and/or dihydroarene type surface groups appear to play a role in the displacement of X from aryl-X. The relatively fast conversion of chloro-, bromo- and iodobenzene is explained by a process in which an adsorbed substrate adds to a hydroaryl-type stabilized radical present on the AC surface, with elimination of the substituent as X. Copyright (C) 1996 Elsevier Science Ltd