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.

Effect of size and oxidation state of size-controlled rhodium nanoparticles on the aqueous-phase hydrodechlorination of 4-chlorophenol

“…They compare closely compare with the obtained in our previous work on 4-CP HDC with Pd nanoparticles [20]. This is an important conclusion since in the case of supported catalysts Pd has been reported in general as more active than Rh in HDC [11,28,44,45] although in some cases equivalent activities have also been found [16]. Fig.…”

“…Cyclohexanone reduction is not completed in 60 h, although progressive conversion of cyclohexanone to cyclohexanol is observed. These results support the reaction mechanism proposed in this work where the hydrogenation of cyclohexanone can lead to obtain cyclohexanol, in contrast to other reaction mechanisms proposed for Rh-supported catalysts [11]. Fig.…”

“…This reaction scheme is in good agreement with that proposed by Fujita et al [47] for the hydrogenation of phenol using supported Rh catalysts. Other useful reaction scheme has been proposed by Diaz et al [11] for the HDC of 4-CP using alumina-supported Pd, Pt and Rh catalysts. This last work proposed the direct hydrogenation of 4-CP to phenol and cyclohexanone and the hydrogenation of phenol as the only path to cyclohexanol.…”

“…A number of authors have prepared colloidal Rh nanoparticles of controlled size using a chemical reduction method to test their behavior as catalysts in different reactions [9,[25][26][27]. However, the dependence of activity on the particle size is so far not clear in HDC, where Rh-based catalysts have shown highly active [11,28].…”

“…Particularly, Rh has shown high catalytic action on carbonylation, hydroformylation, oxidation, reduction and HDC reactions [9][10][11]. Rh has also a higher melting point than other active metals used in catalysis and exhibits high resistance to acid and base attack, so Rh-based catalysts can be singularly stable, even under severe reaction conditions [9].…”