Hydrodechlorination of 4-chlorophenol in aqueous phase using Pd/AC catalysts prepared with modified active carbon supports

Abstract

The hydrodechlorination of 4-chlorophenol with hydrogen in aqueous phase has been studied using different home-made Pd/active carbon catalysts. The active carbons employed were subjected to oxidation with nitric acid, hydrogen peroxide and ammonium persulfate and to thermal treatment under nitrogen atmosphere at temperatures ranging from 200 to 900°C in order to modify their surface chemistry. High conversion values for 4-chlorophenol, well above 95%, were obtained working under mild conditions of temperature (50–75°C) and pressure (2.4bar). Modifications on the surface composition of the active carbon support upon oxidation and thermal treatment proved to affect to both conversion of 4-chlorophenol and selectivity towards cyclohexanol, the less toxic product in the reaction pathway. Increasing thermal treatment temperature showed to be detrimental in both respects whereas oxidation with nitric acid favored a higher selectivity to cyclohexanol. Those effects can be attributed more specifically to the relative presence of some oxygen groups on the surface of the support. Thus, the highest 4-chlorophenol conversions were obtained with the catalysts whose supports yielded higher amounts of CO2 upon temperature programmed desorption analysis. In particular, carboxylic acid and lactone groups where found to be relevant for a high conversion of 4-chlorophenol and their presence was also important to improve the selectivity to cyclohexanol. The presence of such groups can be related with a more homogeneous distribution of palladium on the catalysts surface.