Aqueous phase hydrodechlorination of 2,4-dichlorophenol over supported palladium catalysts

The feasibility of aqueous phase chlorophenol hydrodechlorination (HDC) reaction at 273-303 K has been demonstrated for commercial available Pd/C, Pd/Al2O3 and Rh/C catalysts. The relative importance of the transport/kinetic processes were considered in the case of 2,4-dichlorophenol (DCP) HDC over Pd/C and Pd/Al2O3 catalysts. In 2,4-DCP HDC over a 1% w/w Pd/C and 1% w/w Pd/Al2O 3 at 303 K, 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 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 that is generated during HDC. Inclusion of HCl in the reaction mixture (pH = 5-1.5) resulted in a marked decline in the initial HDC rate associated with Pd/Al2O 3 and a lesser drop in HDC activity for Pd/C. 2,4-DCP HDC with/without the addition of NaOH, NH4OH, LiOH, KOH, RbOH and CsOH (solution pH = 13-1.5) revealed an increase in fractional dechlorination with the addition of base, which can be attributed to a suppression of HCl inhibition. 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. Starting in the range 7-11.6, solution pH during 2,4-DCP HDC over Pd/Al2O3 can be controlled to within +/-0.13 with a continuous supply of NaOH solution. HDC operating under controlled pH exhibited a maximum activity in the pH range 7-8. The initial conversion of 2,4-DCP to 2-CP follows an Langmuir-Hinshelwood kinetic mechanism with rate inhibition at high 2,4-DCP concentrations (>20 mmol dm-3 ). The pH effect on initial HDC activity was analyzed by a generic correlation.