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 generate 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, where 373 K less than or equal to T less than or equal to 573 K: chlorobenzene, chlorotoluene chlorophenol, bromobenzene, dichlorobenzene, dichlorophenol, trichlorophenol, pentachlorophenol. The long term (up to 800 h-on-stream) stability of these catalysts has been assessed where the changes in nickel particle size and morphology as a result of the prolonged catalytic step was probed by TEM; each catalyst irrespective of any loss of initial activity was fully selective in solely promoting dehalogenation. In the case of a polychlorinated feedstock, dechlorination can proceed in a stepwise manner to generate a partially dechlorinated product. Hydrodehalogenation appears to occur via an electrophilic mechanism where the presence of electron-donating substituents on the benzene ring enhances the rate of reaction. The reaction is shown to be structure sensitive over Ni/SiO2 where the hydrodechlorination rates and ultimate yield of the parent aromatic from a polychlorinated reactant is favored by larger nickel particle sizes. A direct contact of the freshly activated catalyst with HCl or HBr gas induced an appreciable growth of the supported metal crystallites. Chlorobenzene hydrodechlorination was suppressed on a HCl or HBr treated Ni/SiO2 which promoted instead the unexpected growth of highly ordered carbon filaments; this carbon growth is characterized by TEM and SEM. The dependence of the experimental hydrodechlorination and hydrodebromination rates on the gas phase aromatic partial pressure (in the range 0.02-0.1 atm) is adequately represented by a kinetic model involving a non-competitive adsorption of hydrogen and halogenated aromatic where the incoming aromatic reactant must displace the hydrogen halide from the catalyst surface. (C) 2000 Elsevier Science B.V. All rights reserved.