Catalytic and non-catalytic formation of 4,4 '-dimethylbiphenyl using p-chlorotoluene

The formation of 4,4'-dimethylbiphenyl is studied from the conversion of p- chlorotoluene over three types of reactions. The first type of reaction is the coupling of two p-chlorotoluene molecules by producing chlorine. The re action is thermodynamically unfavourable and 4,4'-dimethylbiphenyl is not d etected. The second type of reaction is comparable to a classical Ullmann r eaction. It is shown that Cu supported on gamma-alumina is more reactive fo r the coupling of p-chlorotoluene than copper-powder. The third type of rea ction is a catalytic alternative of the Ullmann reaction with hydrogen as r educing agent. The formation of 3,4'-dimethylbiphenyl is feasible. The acti vity and selectivity:are dependent on the type of metal and carrier used an d also on the hydrogen partial pressure. The best catalyst is a Pd:supporte d on carbon. Bimetallic mixtures of Pd-Hg and Pd-Bi show also good performa nce. To our knowledge, it is the first time; that the formation of 4,4'-dim ethylbiphenyl over typical hydrogenation/dechlorination conditions is repor ted. (C) 2000 Elsevier Science B.V. All rights reserved.