ABSTRACT
Catalytic hydrodehalogenation is established for homogeneous systems (6) and while high turnovers have been achieved, this approach is not suitable for environmental remediation purposes due to the involvement of additional chemicals (solvents/hydrogen donors) and the often difficult product/solvent/ catalyst separation steps. HDC1 in heterogeneous systems has been viewed in terms of both nucleophilic (7) and electrophilic (8,9) attack and, in common with most hydrogenolysis reactions, dechlorination rate is strongly influenced by the electronic structure of the active sites (10). Chlorobenzene (C1B) has been the most widely adopted model reactant to assess catalytic HDC1 activity in both the gas (7-9,11-18) and liquid (19,20) phases using Pd (7,18-20), Pt (19), Rh (17-19) and Ni (8,9,11-17) catalysts. The removal of multiple Cl atoms from an aromatic host has been studied to a lesser extent (9,14-17,21). In the treatment of polychlorinated aromatics, a range of partially dechlorinated isomers has been isolated where product composition depends on the nature of the catalyst and process conditions, i.e. temperature, concentration, residence time etc. HDC1 kinetics has largely been based on pseudo-first order approximations (13,16,17) and there have been few attempts (7,9,18,22) to construct kinetic models from mechanistic considerations. The studies cited above represent a compilation of rate data that needs to be extended in a strategic fashion in order to facilitate a determination of the most active HDC1 catalysts under clearly defined operating conditions. The mechanism of C-C1 bond hydrogenolysis is still open to question and this must be established and combined with a robust "predictive" kinetic model in order to inform reactor design and facilitate process optimization. In a previous study (23), we considered the applicability of a number of mechanistically sound kinetic models to describe the gas phase HDC1 of C1B promoted by Ni/SiO2 over a range of reaction conditions. The kinetic model that best reproduced the experimental data involved reaction between non-competitively and dissociatively adsorbed C1B and spillover hydrogen on a non-uniform surface. We have extended that kinetic study to consider the HDC1 of dichlorobenzene (DCB) isomers and report herein the results of our kinetic modeling.
