Catalyst deactivation during the liquid phase hydrodechlorination of 2,4-dichlorophenol over supported Pd: influence of the support

In the liquid phase hydrodechlorination (HDC) of 2,4-dichlorophenol (DCP) over 1% (w/w) Pd/C and 1% (w/w) Pd/Al2O3, operated in the kinetic controlled regime, 2-chlorophenol is the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone is formed over Pd/Al2O3 prior to the completion of dechlorination. Reuse of the catalysts revealed an appreciable deactivation of Pd/C and a limited loss of activity in the case of Pd/Al2O3. The Pd content, BET surface area, temperature programmed reduction (TPR) and hydrogen chemisorption/temperature programmed desorption (TPD) characteristics of the fresh and used catalyst samples have been recorded. The results indicate that the Pd/C catalyst lost ca. 50% of the total surface area during HDC operation due to occluded organic species but this was readily recovered by TPR. Measurement of the Pd content in the catalysts pre-/post-reaction and in the reaction liquid has demonstrated that Pd leaching is not significant during reaction. The nature of the surface active hydrogen is probed, hydrogen desorption characteristics are discussed and the possible sources of activity loss are considered.