Gas phase catalytic dehydrochlorination and hydrodechlorination of aliphatic and aromatic systems

The gas phase dechlorination of cyclohexyl chloride and chlorobenzene, over the temperature range 423 K less than or equal to T less than or equal to 573 K, promoted using a silica supported nickel catalyst in the presence of hydrogen has been studied. Chlorine removal from the chloroalkane is shown to occur by dehydrochlorination via an El type elimination mechanism to yi eld cyclohexene and HCl as the products. The reaction was found to exhibit zero order behaviour with respect to hydrogen partial pressure, a temperatu re dependent reaction order (varying from 0.4 to 0.7) with respect to the c hloroalkane and an apparent activation energy equal to 115 kJ mol(-1). Turn over of the cyclohexyl chloride reactant was subject to a short term loss o f catalytic activity due to a surface poisoning by the HCl that was produce d where the presence of hydrogen served to displace the inorganic halide an d extend the productive lifetime of the catalyst. A steady state conversion of chlorobenzene was however readily achieved where dechlorination in hydr ogen occurs via an electrophilic hydrodechlorination mechanism. Bromine rem oval from cyclohexyl bromide and bromobenzene is also considered for compar ative purposes. The ease of halogen removal and process selectivity are dis cussed in terms of thermodynamic limitations and reactant/catalyst interact ions. (C) 1999 Elsevier Science B.V. All rights reserved.