Gas phase hydrogenolysis mediated by activated carbon: substituted chlorobenzenes

The hydrogenolysis of vapors of chlorobenzene derivatives (ZPhCl, where Ph denotes a benzene ring) when passed over a bed of activated carbon (AC) in a H-2/N-2 atmosphere was investigated. Dehalogenation occurred readily betw een 200 and 550 degreesC, depending on the derivative. The less volatile co mpounds were more strongly adsorbed, and their conversion occurred at lower temperatures. Common dehalogenation pathways seem to hold for most derivat ives, involving both formation of the corresponding desubstituted benzene d erivatives (ZPhH), and attachment to the AC surface. The other product was HCl. Chlorine atoms at ortho and/or para positions to Z = OH in mono- and d ichlorophenols were removed at much lower temperatures and with a high sele ctivity to phenol or less chlorinated phenols. In these cases, at the lowes t temperatures applied (< 300 degreesC) chlorine became partially attached to the carbon surface, and it could be removed only at much higher temperat ures. However, upon hydrogenolysis at temperatures above 300 degreesC chlor ination of the carbon did not occur. Mechanisms are discussed. (C) 2001 Els evier Science Ltd. All rights reserved.