Hydrodechlorination of polychlorinated biphenyl

Abstract

The catalytic hydrodechlorination of a complex polychlorinated biphenyl (PCB) mixture was studied in a batch reactor using a 61% Ni on kieselguhr catalyst from 25 to 100 °C at 50 atm of H 2. Ethanol was used as a solvent and NaOH as an acid acceptor. Aromatic hydrodechlorination of chlorinated biphenyls is a consecutive single-step process; ortho-substituted chlorine is the least labile due to steric effects. In the reaction sequence below, φφ i indicates a biphenyl molecule substituted with i chlorine atoms distributed over the two rings. First-order pseudo-homogeneous relative rate constants for the liquid phase study are noted: ▪ Similar behavior was observed in a gas-phase pulse microreactor over a series of 0.00, 0.05, 0.10, and 0.35 wt% Pd on α-Al 2O 3 catalysts at 220 ± 5 °C, 2.3 atm of H 2, and space time of ~10 −2 sec (based on H 2 carrier gas flow rate). The corresponding values of the relative rate constants were 1.0, 0.85, 0.50, 0.70, and 0.95. These values of relative rate constants pass through a minimum as a result of the combined influence of statistical and steric factors. Highly ortho-substituted materials cannot adsorb in a planar configuration. The relative first-order rate constant for ortho to ( meta + para) hydrodechlorination k o k mp = 0.52 for the nickel catalyst. Reaction behavior is consistent with the mechanism requiring adsorption of the benzene ring and charge delocalization at the aromatic chlorine: ▪ For hydrodechlorination, the ▪ carbon-chlorine double bond is regarded as the reactive species.