Role of base addition in the liquid-phase hydrodechlorination of 2,4-dichlorophenol over Pd/Al2O3 and Pd/C

he aqueous-phase batch hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP), over 1 wt/wt% Pd/Al2O3 and Pd/C has been investigated with/without the addition of NaOH, NH4OH, LiOH, KOH, RbOH, and CsOH; bulk solution pH spanned the range 1.5-13. The reaction was operated in the kinetic-controlled regime with 2-chlorophenol (2-CP) as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al2O3, but not Pd/C, prior to the completion of dechlorination. An increase in fractional dechlorination with the addition of base was observed and can be attributed to a suppression of HDC inhibition due to the HCl that is generated. The initial HDC activity and selectivity delivered by both catalysts were pH dependent and differences in response to bulk solution pH variations are discussed in terms of the nature of the reactive species in solution and the amphoteric behavior of the Pd supports. The Pd/Al2O3 catalyst is characterized by a high surface charge density while Pd/C bore a low basic functionality density on a high surface area carrier. A maximum initial HDC rate over Pd/Al2O3 was attained at pH 7-9 while a higher pH (greater than or equal to 9) results in more effective HDC for Pd/C, effects that are linked to chloroarene dissociation and surface charge effects. In the case of Pd/C, the initial HDC rate with the addition of alkali metal hydroxide (0.074 mol dm(-3)) increased in the order Li+ approximate to Na+ < K+ approximate to Rb+ < Cs+, a sequence that matches the adsorption affinity on a negatively charged carbon surface; HDC over Pd/Al2O3 was insensitive to the nature of the alkali metal cation. The action of NH4OH, as a weak base, is also considered. (C) 2004 Elsevier Inc. All rights reserved.