Detoxification of dichlorophenols by catalytic hydrodechlorination using anickel/silica catalyst

Heterogeneous catalytic dechlorination is presented as a viable means of tr eating/detoxifying concentrated chlorinated gas streams. The gas-phase hydr odechlorination of the six individual dichlorophenol (DCP) isomers was stud ied over the temperature range 473 K less than or equal to T less than or e qual to 573 K using a 1.5% w/w Ni/SiO2 catalyst. The variation of catalyst activity and selectivity with time on stream and temperature is illustrated while the possible role of thermodynamic limitations is addressed. The cat alytic conversion of the three chlorophenol (CP) isomers is also considered for comparative purposes where, in every instance, the catalyst is 100% se lective in promoting dechlorination, leaving both the benzene ring and hydr oxyl substituent intact. A sequence of increasing chlorine removal rate con stants (at 573 K) is established, i.e. 2,3-DCP < 2-CP < 4-CP < 3-CP less th an or equal to 2,5-DCP < 2,4-DCP less than or equal to 2,6-DCP < 3,4-DCP < 3,5-DCP, and discussed in terms of steric, inductive and resonance stabilis ation effects. Detoxification efficiency is quantified by phenol selectivit y and the ultimate partitioning of chlorine in the parent organic or produc t inorganic host. Hydrodechlorination is shown to be an electrophilic react ion where, in the absence of appreciable steric constraints, chlorine remov al is more energetically demanding from DCP than CP. The reaction pathway, with associated pseudo-first-order rate constants, for the conversion of ea ch DCP isomer is presented. (C) 1999 Elsevier Science Ltd. All rights reser ved.