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Ind. Eng. Chem. Res. 2008, 47, 3840–3846
Kinetics of the Hydrodechlorination of 4-Chlorophenol in Water Using Pd, Pt, and Rh/Al2O3 Catalysts Elena Díaz,* José A. Casas, Ángel F. Mohedano, Luisa Calvo, Miguel A. Gilarranz, and Juan J. Rodríguez Sección de Ingeniería Química, Facultad de Ciencias, UniVersidad Autónoma de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain
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 (k1 ) 0.42–0.73 h-1) and Pt (k1 ) 0.20–0.42 h-1) catalysts, while in the case of Rh, the three reactions, hydrodechlorination of 4-chlorophenol to phenol (k1 ) 0.43–0.64 h-1) and hydrogenation of phenol to cyclohexanone (k3 ) 0.42–0.78 h-1) and to cyclohexanol (k4 ) 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. 1. Introduction Chloroaromatics are known as persistent toxic compounds for which the environment has little assimilative capacity. Among them, chlorophenols, characterized by their acute toxicity and poor biodegradability, have been widely used in the manufacture of pesticides, disinfectants, wood preservatives, and personal care formulations, and they are a byproduct of several industrial processes. Several techniques have been investigated for the treatment of waste streams containing highly toxic and refractory chlorinated organic contaminants. In the case of liquid effluents, wet oxidation and advanced oxidation processes, such as Fenton or photochemical processes, have been studied extensively, showing some drawbacks or limitations such as relatively high temperatures and/or pressures, large amounts of reagents, and complex equipment, respectively.1–3 Biological treatment is fairly slow because the microbial activity is affected by the high toxicity of chlorinated compounds.4 Catalytic hydrodechlorination is emerging in the environmental field as a potential technique to treat waste streams containing these chlorinated compounds. This technique is gaining interest as an environmentally friendly treatment.5 Although this process does not usually cause the complete destruction of pollutants, it can result in their convenient transformation to less toxic compounds. Advantages of hydrodechlorination include the ability of reaching high conversion levels at low temperature and pressure.6,7 The most active hydrodechlorination catalysts are supported precious metals such as Pd,6,8–10 Pt,9,11,14 and Rh,9,11,15,16 with metal loadings ranging from 0.5 to 10% (w/w). In the same way, Pd-based catalysts have been identified as the most suitable for liquid-phase hydrodechlorination showing a high activity at low temperatures and atmospheric pressure.17 Hydrodechlorination of chloroaromatic compounds, in both organic and aqueous phases, has been successfully promoted using Pd/AC and Pd/Al2O3 catalysts. These catalysts have shown high activity * To whom correspondence should be addressed. Tel.: +34 914973991. Fax: +34 914973516. E-mail:
[email protected].
and selectivity in the conversion of chlorobenzenes and chlorophenols under relatively mild conditions (
