Kinetics of 10% Pd/C-catalysed hydrogenolysis of chlorobenzene dissolved in n-heptane by gaseous hydrogen in contact with aqueous NaOH or water

The kinetics of the hydrogenolysis of chlorobenzene by gaseous hydrogen in the presence of 10% Pd/C catalyst was investigated in two-phase systems: n-heptane containing the substrate and the chromatographic reference substance (n-dodecane) (upper phase), and aq. NaOH or water (lower phase). The main product was benzene although traces of diphenyl were always found in post-reaction mixtures. The kinetics of hydrogenolysis followed a model of zero-order reaction when the catalyst was pre-conditioned in the upper phase. If pre-conditioning of the catalyst took place in aq. NaOH or water, the kinetics followed a model of parallel zero-order and first-order reactions. Zero-order rate constants for a catalyst pre-conditioned in the upper phase increase with the rpm (revolutions per minute) and the size of the stirring bar, whereas they first rise but then fall with increasing NaOH concentration in the lower phase. A knowledge of rate constants enables reaction completion times to be determined. The results of the kinetic analysis suggest that the main contribution to the zero-order pathway is due to the features of the catalyst surface and the transport of catalyst grains from water to the organic phase. The first-order contribution arises from the adsorption and desorption of reactants on the catalyst surface, and diffusion. The proposed approach forms a convenient framework within which to consider and design the catalytic hydrodechlorination of chlorobenzene under various conditions. (C) 2004 Elsevier B.V. All rights reserved.