Hydrogen–deuterium isotope effects in the reactions of chlorobenzene and benzene on a Pt/γ-Al2O3 catalyst

Abstract

The kinetic isotope effect for combustion of a C₆H₅Cl/C₆D₅Cl mixture on Pt/γ-Al₂O₃ was found to be close to unity between 520 and 580 K. However, in the presence of an excess of heptane, an isotope effect of 1.5 was found between 460 and 490 K. For the combustion of a C₆H₆/C₆D₆ mixture the k_H/k_D value was around 2 between 404 and 439 K. The results show that in the combustion of chlorobenzene per se, C–H bond activation is not a rate-determining step. On Pt sites, C–Cl bond scission probably occurs already at low temperatures. The chlorine and the phenyl group cannot easily react further. Chlorine on the surface is active in chlorination, which is shown by the formation of C₆D₅Cl in an experiment with C₆H₅Cl and C₆D₆. Only at a certain temperature is the chlorine removed, partly as polychlorinated benzenes. The removal of chlorine from the catalyst allows oxygen to take part in the reaction, which determines the rate of the combustion of chlorobenzene. When heptane is present, Cl is removed from the surface and C–H bond scission can become rate determining, as is also the case in the combustion of C₆H₆/C₆D₆. Upon (partial) combustion of C₆H₅Cl/C₆D₅Cl and C₆H₆/C₆D₆ mixtures on a Pt/γ-Al₂O₃ catalyst, hydrogen–deuterium exchange occurs on the γ-Al₂O₃ support.