Kinetics of the hydrodechlorination of 4-chlorophenol in water using Pd, Pt, and Rh/Al2O3 catalysts

The hydrodechlorination of 4-chlorophenol in an aqueous phase was studied in a semicontinuous basket stirred tank reactor using Pd, Pt, and Rh on gamma-alumina commercial catalysts (0.5% w/w) under mild reaction conditions. The catalytic activity was studied in the temperature range of 20-40 degrees 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.