The gas phase hydrodechlorination of chlorobenzene over nickel/silica

Abstract

The gas phase hydrodechlorination of chlorobenzene over the temperature range 473 K⩽T⩽573 K has been studied using a 1.5% (w/w) Ni/SiO2 catalyst. Reproducible turnover frequencies are quoted and the effects of varying such process variables as reaction time and temperature, contact time, chlorobenzene and hydrogen partial pressures are presented. The catalyst was 100% selective in promoting hydrodechlorination and the aromatic ring remained intact in every instance. Under reaction conditions far removed from equilibrium conversions, the catalyst exhibited no appreciable short term deactivation while the maintenance of long term activity was also established. Chlorine coverage of the catalyst surface under reaction conditions was probed indirectly by monitoring, via pH changes in an aqueous NaOH trap, HCl desorption after completion of the catalytic step. The hydrogenolysis of bromobenzene, 2-chlorophenol and 3-chlorotoluene under the same reaction conditions were considered for comparative purposes where the turnover frequencies decrease (at 573 K) in the order 2-chlorophenol>3-chlorotoluene>chlorobenzene>bromobenzene; reactivity is discussed in terms of thermodynamic limitations and reactant/catalyst interactions. Reaction orders with respect to hydrogen and chlorobenzene partial pressures were obtained at different reaction temperatures and the experimental rate data are adequately represented by an extended power rate expression that approximates the Langmuir–Hinshelwood model for non-competitive adsorption. © 1999 Society of Chemical Industry