Kinetics of the Hydrodechlorination of 4-Chlorophenol in Water Using Pd, Pt, and Rh/Al₂O₃ Catalysts

Abstract

The hydrodechlorination of 4-chlorophenol in an aqueous phase was studied in a semicontinuous basket stirred tank reactor using Pd, Pt, and Rh on γ-alumina commercial catalysts (0.5% w/w) under mild reaction conditions. The catalytic activity was studied in the temperature range of 20−40 °C. Pd and Rh showed a higher catalytic activity than Pt. From consumption of 4-chlorophenol and evolution of the reaction products, phenol, cyclohexanone, and cyclohexanol, a reaction scheme based on a parallel-series pathway and a kinetic model based on pseudo-first-order dependence on 4-chlorophenol have been proposed. Hydrodechlorination of 4-chlorophenol to phenol exhibits the largest apparent kinetic constant for Pd ( k 1 = 0.42–0.73 h −1 ) and Pt ( k 1 = 0.20–0.42 h −1 ) catalysts, while in the case of Rh, the three reactions, hydrodechlorination of 4-chlorophenol to phenol ( k 1 = 0.43–0.64 h −1 ) and hydrogenation of phenol to cyclohexanone ( k 3 = 0.42–0.78 h −1 ) and to cyclohexanol ( k 4 = 0.38–0.65 h −1 ), have comparable values of the kinetic constant. The apparent activation energy for 4-chlorophenol disappearance was determined, and values of 21.0, 26.2, and 15.3 kJ/mol were obtained for Pd, Pt, and Rh, respectively.