Catalytic hydrodehalogenation as a detoxification methodology

Catalytic hydrodehalogenation is presented as a viable approach in the non- destructive treatment of concentrated halogenated aromatic gas streams to g enerate reusable raw material. Nickel loaded (from 1.5 to 20.3% w/w) silica catalysts have been used to hydrotreat a range of halogenated feedstock, w here 373 K less than or equal to T less than or equal to 573 K: chlorobenze ne, chlorotoluene chlorophenol, bromobenzene, dichlorobenzene, dichlorophen ol, trichlorophenol, pentachlorophenol. The long term (up to 800 h-on-strea m) stability of these catalysts has been assessed where the changes in nick el particle size and morphology as a result of the prolonged catalytic step was probed by TEM; each catalyst irrespective of any loss of initial activ ity was fully selective in solely promoting dehalogenation. In the case of a polychlorinated feedstock, dechlorination can proceed in a stepwise manne r to generate a partially dechlorinated product. Hydrodehalogenation appear s to occur via an electrophilic mechanism where the presence of electron-do nating substituents on the benzene ring enhances the rate of reaction. The reaction is shown to be structure sensitive over Ni/SiO2 where the hydrodec hlorination rates and ultimate yield of the parent aromatic from a polychlo rinated reactant is favored by larger nickel particle sizes. A direct conta ct of the freshly activated catalyst with HCl or HBr gas induced an appreci able growth of the supported metal crystallites. Chlorobenzene hydrodechlor ination was suppressed on a HCl or HBr treated Ni/SiO2 which promoted inste ad the unexpected growth of highly ordered carbon filaments; this carbon gr owth is characterized by TEM and SEM. The dependence of the experimental hy drodechlorination and hydrodebromination rates on the gas phase aromatic pa rtial pressure (in the range 0.02-0.1 atm) is adequately represented by a k inetic model involving a non-competitive adsorption of hydrogen and halogen ated aromatic where the incoming aromatic reactant must displace the hydrog en halide from the catalyst surface. (C) 2000 Elsevier Science B.V. All rig hts reserved.