Detoxifying chlorine rich gas streams using solid supported nickel catalysts

Catalytic hydrogen treatment is presented as a viable low energy means of t reating/detoxifying concentrated chlorinated gas streams to generate recycl able 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) an d pentachlorophenol (PCP) over the temperature interval 473 K less than or equal to T less than or equal to 573 K. In every instance the nickel cataly sts were 100% selective in cleaving the chlorine component from the ring, l eaving the aromatic nucleus and hydroxyl substituent intact. The effects of varying process time and temperature are considered in terms of phenol yie ld and the ultimate partitioning of chlorine in the parent organic and prod uct inorganic hosts. Chlorine removal rates, hydrodechlorination selectivit y and apparent activation energies are also provided. Prolonged exposure of the catalysts to the concentrated chlorine gas streams resulted in an irre versible loss of activity which is related to the total concentration of ch lorine that had been hydroprocessed, Hydrodechlorination proceeds via irrev ersible stepwise and/or concerted routes as is illustrated for the treatmen t of 2,3,5-TCP. Increasing the nickel content was found to raise the overal l detoxification efficiency while the use of a zeolite support introduced s patial constraints which had a strong bearing on process selectivity. (C) 1 999 Elsevier Science B.V. All rights reserved.