Gas-phase hydrodechlorination of pentachlorophenol over supported nickel catalysts

The gas-phase hydrodechlorination of pentachlorophenol (PCP) over nickel/si lica and nickel/Y zeolite catalysts at 573 K has been studied. Each catalys t was 100% selective in cleaving the C-Cl bonds, leaving the hydroxyl subst ituent and benzene ring intact. The variation of catalytic activity and sel ectivity (in terms of partial and full dechlorination) with time-on-stream is illustrated and catalyst deactivation is addressed. Dechlorination effic iency is quantified in terms of dechlorination rate constants, phenol selec tivity/yield and the ultimate partitioning of chlorine in the parent organi c and product inorganic host. Increasing the nickel loading on silica was f ound to raise the overall level of dechlorination while the use of a zeolit e support introduced spatial constraints that severely limited the extent o f dechlorination. Product composition was largely determined by steric effe cts where resonance stabilisation had little effect. The reaction pathways, with associated pseudo-first-order rate constants, are also presented.