Detoxification of dichlorophenols by catalytic hydrodechlorination using a nickel/silica catalyst

Heterogeneous catalytic dechlorination is presented as a viable means of treating/detoxifying concentrated chlorinated gas streams. The gas-phase hydrodechlorination of the six individual dichlorophenol (DCP) isomers was studied over the temperature range 473 K less than or equal to T less than or equal 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 catalytic conversion of the three chlorophenol (CP) isomers is also considered for comparative purposes where, in every instance, the catalyst is 100% selective in promoting dechlorination, leaving both the benzene ring and hydroxyl substituent intact. A sequence of increasing chlorine removal rate constants (at 573 K) is established, i.e. 2,3-DCP < 2-CP < 4-CP < 3-CP less than 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 stabilisation effects. Detoxification efficiency is quantified by phenol selectivity and the ultimate partitioning of chlorine in the parent organic or product inorganic host. Hydrodechlorination is shown to be an electrophilic reaction where, in the absence of appreciable steric constraints, chlorine removal is more energetically demanding from DCP than CP. The reaction pathway, with associated pseudo-first-order rate constants, for the conversion of each DCP isomer is presented. (C) 1999 Elsevier Science Ltd. All rights reserved.