Effect of catalyst and substituents on the hydrogenation of chloronitrobenzenes

The liquid phase hydrogenation of chloronitrobenzene isomers (x-CNB, x = 2, 3, 4) to corresponding chloroanilines (x-CAN) at 0.5 MPa and 25degreesC was studied. Palladium catalysts containing 1 wt.% of Pd on charcoal and sulphonated poly(styrene-co-divinylbenzene) Pd/D were used. At stoichiometric consumption of hydrogen (3 mol of H-2/mol of x-CNB) 85 and 90% selectivities were obtained with Pd/C and Pd/D catalysts, respectively. Mathematical treatment of kinetic data revealed the dependence of reaction mechanism on the type of catalyst. A simultaneous hydrogenation of the nitro group and hydrogenolysis of chlorine in x-CNB was observed for the reaction over the Pd/C catalyst. However, over the Pd/D catalyst hydrogenolysis of chlorine is a subsequent reaction. The influence of substituents and sterical hindrance on the substrate reactivity is also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.