Low-temperature Hydrodechlorination of Chlorobenzenes on Palladium-supported Alumina Catalysts

Abstract

Hydrodechlorination of monochlorobenzene (MCB), 1,4-dichlorobenzene (DCB), and 1,3,5-trichlorobenzene (TCB) was carried out on palladium-supported γ-alumina and alumina Lewis superacid catalysts at 298 K and ambient pressure in a stream of hydrogen, using a fixed bed flow reactor for the gas-phase reaction and a fixed bed semi-batch reactor for the liquid-phase reaction. Both catalysts showed higher activity for hydrodechlorination in the gas-phase reactions than the platinum catalysts supported on the same carriers. The completely hydrodechlorinated products consisted only of benzene and cyclohexane. Hydrodechlorination of MCB and DCB at 3900 g . h . mol -1 resulted in conversions and cyclohexane yields of 100%, and for TCB the cyclohexane yield was also over 95% with a conversion of 90%. Slight activity decrease of the Pd catalysts due to chlorine accumulation on the catalyst was observed, but the chlorine amount was about one-fourth at 353 K compared to 298 K in spite of the remarkable increase in the number of reacted (dechlorinated) molecules, so catalyst deactivation was effectively reduced at higher reaction temperatures. Furthermore, even in the liquid-phase reactions at ambient temperature, which was lower than the reaction temperatures previously used, the palladium-supported catalysts had moderate activity for hydrodechlorination and formed benzene and cyclohexane. The hydrodechlorination of chlorobenzenes probably proceeded through reactions between the hydrogen atoms spilling over from the Pd surface to the alumina carrier surface and the chlorobenzenes adsorbed and activated on the Lewis acid sites of the alumina surface.