(664f) Kinetics and Mechanism of Selective C-Cl Hydrogenolysis By Pd/C Catalysts

C-Cl hydrogenolysis is a well-documented reaction for many chlorinated molecules. However, there are fundamental discrepancies in the reaction kinetics analyses for C-Cl bond cleavage in aromatic systems, which arises from a paucity of molecular-level studies. We have studied the reaction kinetics of catalytic hydrodechlorination (HDC) of chlorobenzene (PhCl) on a Pd/C catalyst in a continuous stirred tank reactor (CSTR), and we use this data to propose a reaction mechanism for C-Cl hydrogenolysis. The kinetics analysis suggests that the general mechanism for C-X hydrogenolysis proposed by Sinfelt¹ is operative for this system. Assuming C-Cl bond cleavage is the rate controlling step and further assuming that adsorbed chlorine and PhCl are abundant on the catalyst surface, we derived a rate expression which is in agreement with the kinetic behavior of this system. The reaction is approximately first-order with respect to PhCl, half-order with respect to H2, and inverse first-order with respect to HCl. The lack of a primary kinetic isotope effect (KIE) when using D₂ confirmed that hydrogen does not participate in the rate controlling step. Abundant surface intermediates will be identified by in situ FT-IR of adsorbed PhCl as well as other probe molecules, including CO and ethylene.

References:

1. Sinfelt, J. H. Catalytic hydrogenolysis on metals. Catal. Letters 9, 159–171 (1991).