Supported transition metal catalysts for hydrodechlorination reactions

As halogenated waste is xenobiotic with no analogous compounds in nature, there is no natural means of ameliorating the negative environmental impact caused by halo-organic emissions. The presence of organo-halogens in effluent discharges is of increasing concern, owing to the mounting evidence of adverse stratospheric ozone, ecological effects, and the impact on public health. There is a pressing need, driven by legislative and financial imperatives, to develop robust and efficient treatment technologies. This Review considers the application of catalytic hydrodechlorination (HDC) as a progressive approach to the conversion and recycling of toxic chloro-compounds. An overview of the existing treatment technologies is provided with an assessment of the benefits of catalytic HDC over separation/oxidation methodologies. Taking the transformation of chloro-aromatics, the catalytic HDC of liquid and gas phase reactants is considered in turn with a critical analysis of the pertinent literature. Two "case studies" are considered in detail: i) the liquid phase HDC of chlorophenols over alumina supported palladium, focusing on the critical process and catalyst structural characteristics that influence HDC performance; ii) the gas phase conversion of a range of halogenated (Cl and Br) aromatics over silica supported nickel with an examination of the dehalogenation activity/selectivity response as a function of the nature of the aromatic reactant.