Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects

Abstract

The liquid phase hydrodechlorination (HDC) of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) has been studied over 1% (w/w) Pd/C and Pd/Al2O3 under conditions of minimal mass transport constraints. The HDC of 2,4-DCP generated HCl and 2-CP as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al2O3, but not over Pd/C, prior to complete dechlorination. Pd/Al2O3 is characterized (on the basis of TEM analysis) by a narrow distribution of smaller Pd particles to give a surface area weighted mean particle diameter = 2.4nm that is appreciably lower than the value of 13.2nm established for Pd/C, where the latter is characterized by a broader distribution of larger (spherical) particles. The addition of NaOH served to increase fractional dechlorination by suppressing HDC inhibition due to the HCl that is generated. Reuse of the catalysts revealed an appreciable deactivation of Pd/C and a limited loss of activity in the case of Pd/Al2O3. Deactivation of Pd/C can be linked to a decrease (up to ca. 60%) in the initial BET surface area allied to appreciable leaching (up to ca. 40%) of the starting Pd content through the corrosive action of HCl and, while the average Pd diameter is essentially unaffected, there is evidence of a preferred leaching of larger Pd particles. The stronger metal/support interactions prevalent in Pd/Al2O3 results in limited Pd leaching and comparable initial HDC activities during catalyst reuse with/without NaOH addition. Inclusion of HCl in the reaction mixture (pH 5–1.5) resulted in a marked decline in the initial HDC rate associated with Pd/Al2O3 and a lesser drop in HDC activity for Pd/C. The difference 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.