Detoxifying chlorine rich gas streams using solid supported nickel catalysts.

Catalytic hydrogen treatment is presented as a viable low energy means of treating/detoxifying concentrated chlorinated gas streams to generate recyclable raw materials. Nickel (1.5% w/w and 15.2%) loaded silica and nickel (2.2% w/w) exchanged Y zeolite catalysts have been used to hydrotreat a range of chlorophenols (CPs), dichlorophenols (DCPs), trichlorophenols (TCPs) and pentachlorophenol (PCP) over the temperature interval 473 K</=T</=573 K. In every instance the nickel catalysts were 100% selective in cleaving the chlorine component from the ring, leaving the aromatic nucleus and hydroxyl substituent intact. The effects of varying process time and temperature are considered in terms of phenol yield and the ultimate partitioning of chlorine in the parent organic and product inorganic hosts. Chlorine removal rates, hydrodechlorination selectivity and apparent activation energies are also provided. Prolonged exposure of the catalysts to the concentrated chlorine gas streams resulted in an irreversible loss of activity which is related to the total concentration of chlorine that had been hydroprocessed. Hydrodechlorination proceeds via irreversible stepwise and/or concerted routes as is illustrated for the treatment of 2,3,5-TCP. Increasing the nickel content was found to raise the overall detoxification efficiency while the use of a zeolite support introduced spatial constraints which had a strong bearing on process selectivity. Copyright 1999 Elsevier Science B.V.