Individual and competitive liquid-phase hydrodechlorination of chlorinated pyridines over alkali-modified Pd/ZrO 2 Jose ´ M. Moreno, Maria A. Aramendı ´a, Alberto Marinas, Jose ´ M. Marinas, Francisco J. Urbano * Department of Organic Chemistry, University of Cordoba, Marie-Curie Building, Campus de Rabanales E-14014, Cordoba, Spain Received 31 January 2007; received in revised form 27 April 2007; accepted 1 May 2007 Available online 6 May 2007 Abstract Individual and competitive liquid-phase hydrodechlorination of chlorinated pyridines has been studied over alkali-modified zirconia-supported palladium catalysts. All experiments proceed without catalyst deactivation as a result of the interaction of the reaction products (HCl and pyridine) forming pyridinium chloride, thus avoiding the detrimental effect of HCl on palladium particles. Individual experiments indicate a different behaviour for 2-chloropyridine and 3-chloropyridine. Catalyst modification with alkali metals improves the catalytic activity for 3-chloropyridine but not for 2-chloropyridine hydrodechlorination. 2,3-Dichloropyridine hydrodechlorination yields lower reaction rates than 2- or 3-chloropyr- idine. Moreover, no partially chlorinated compounds were detected. Competitive hydrodechlorination of equimolecular mixtures of 2-and 3- chloropyridine allow us to obtain both the reactivity and adsorption coefficient of 3-chloropyridine with respect to 2-chloropyridine. Again, in competitive hydrodehalogenation, 3-chloropyridine reacts faster than 2-chloropyridine for all catalysts. However, results indicate that the alkali- modified catalysts adsorb preferably 2-chloropyridine instead of 3-chloropyridine. Moreover, competitive hydrodehalogenation carried out for the couples 3-chloropyridine/chlorobenzene, 2-chloropiridine/chlorobenzene and 2,3-dichloropyridine/chlorobenzene revealed that chlorinated pyridines are reduced to the detriment of chlorobenzene due to the preferential adsorption of chlorinated pyridines against chlorobenzene. All data support a reaction mechanism based on an electrophilic attack to the carbon bearing the chlorine atoms. # 2007 Elsevier B.V. All rights reserved. Keywords: Alkali-modification; Zirconia; Palladium catalyst; Hydrodechlorination; Hydrodehalogenation; Chloropyridine; Competitive reaction; Adsorption coefficient 1. Introduction Polychlorinated organic substances are important pollutants due to their environmental impact and noxious effects. Their transformation by oxidative processes leads to the formation of chlorinated dioxins and furans, which are also polluting agents. Many pyridine derivatives have found widespread use as pesticides and the chloropyridine structure is present in many of them. The challenge, in this field of research, is to develop disposal methods, which are ecologically safe. Catalytic hydrodechlorination (HDC) is a promising technology whereby the chlorinated waste can be converted into products of commercial value [1]. It is simple, safe, effective, and it ensures, in many instances, the regeneration of the initial raw material [2]. Moreover, the selective cleavage of C–S, C–O, C–Cl and C– N bonds are reactions, which are used both in the laboratory and on industrial scale for the production of fine chemicals. Milder conditions can be achieved with noble metals as catalysts. However, the mechanisms and kinetics of these reactions, especially hydrodechlorination, remain uncertain and con- troversial, and more efficient catalytic formulations have to be found. The most important problem in the hydrodehalogenation of organic halides is the strong deactivation of the catalysts due to the hydrogen halide formed as a by-product of the reaction. Although deactivation is also observed in the gas-phase hydrodechlorination [3], it is especially severe in processes carried out in a batch reactor (liquid-phase HDC) [4–7]. The interaction of Cl with transition metals is known to lower the metal electron density and impact on hydrogen uptake www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 76 (2007) 34–41 * Corresponding author. Tel.: +34 957 218638; fax: +34 957 212066. E-mail address: FJ.Urbano@uco.es (F.J. Urbano). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.05.008