Kinetic study of the hydrogenolysis of polychlorobenzenes over a Pd/C catalyst in an alkaline aqueous-n-hexane system Iwona Anusiewicz a , Tadeusz Janiak a, * , Janina Okal b a University of Gdan ´sk, Faculty of Chemistry, J. Sobieskiego 18, 80-952 Gdan ´sk, Poland b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland article info Article history: Received 11 December 2009 Received in revised form 9 February 2010 Accepted 15 February 2010 Available online 19 February 2010 Keywords: Hydrodechlorination Polychlorobenzenes Pd/C catalyst Kinetics abstract The kinetics of the hydrogenolysis of chlorobenzene, dichlorobenzenes and some trichlorobenzenes over a 10% Pd/C catalyst was studied using a multiphase system. The reactions were carried out in a batch reactor with an aqueous NaOH/n-hexane solution of chloroaromatic compound as the liquid phase. Ben- zene was the final product of the hydrogenolysis of all the compounds studied. Hydrogenolysis was more effective in the presence of in situ generated hydrogen than gaseous hydrogen. The initial reaction rates and TOFs of dichlorobenzenes and trichlorobenzenes were slightly lower than those of chlorobenzene. The position of the chlorine atoms in trichlorobenzenes affects the kinetics of the removal of the first chlorine from these molecules. The differences in chlorine reactivity were explained by the inductive and steric effects induced by the benzene-Cl bonds. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Organochlorine compounds, being persistent pollutants, are of great environmental concern; therefore, wastes containing them should be properly treated before final disposal to avoid further unnecessary contamination of the environment [1,2]. One possible, promising solution to this problem is to convert aromatic chlorides to dechlorinated compounds by hydrogenolysis [2] and to use the product as a fuel. Hydrogenolysis of organohalogen compounds has been investigated using different reducing reagents, catalysts and substrates [2,3]. The usual aim of the procedures described in the literature [2–5] is to convert a particular organochlorine compound to the desired reaction product. For example, in the Rosenmund reduction, acid chlorides are converted to aldehydes without further reduction to alcohols [4]. But in the treatment of hazardous wastes, it is effectiveness of halogen removal and simplicity of the reaction system that are in fact required. The hyd- rodechlorination of chloroaromatic compounds is known to pro- ceed over various transition metal catalysts [2,4,5]. In particular, Pd, Rh and Pt supported on carbon, alumina or Raney nickel have been reported to promote hydrogenolysis of the carbon–chlorine bond [2,3,6,7]. Palladium on activated carbon appears to be the cat- alyst of choice [5], and to the best of our knowledge, it is indeed the most effective in hydrodechlorination. Dechlorination in the pres- ence of supported Pd has been studied in the gaseous [8–14] and liquid phases [15–29]. Heterogeneous multiphase liquid systems appear to be useful in the treatment of wastes containing chloroa- romatic constituents. There are reports that the hydrogen chloride produced during the hydrodehalogenation of organochlorine compounds deactivates the catalyst surface [9,26]. However, in li- quid multiphase heterogeneous catalytic hydrogenolysis a base is easily added to restore catalyst activity by reacting with the hydro- gen chloride [23–26]. Moreover, aqueous and nonpolar phases facilitate desorption of both hydrogen chloride and dechlorinated compound from the catalyst surface, each one to a different phase. Although hydrogenolysis of aromatic chlorides with gaseous hydrogen is far from uncommon, the available literature data on the kinetics of the process in the liquid multiphase system are rather sparse; the most comprehensive reports in this respect are those of Marques et al. [21,22] and Janiak et al. [24–26]. Recently, we inves- tigated the kinetics of the hydrogenolysis of o-chlorotoluene [25] and some meta-substituted chlorobenzenes [26]. Generally, we found that in meta-substituted chlorobenzenes the effectiveness of chlorine removal does not depend essentially on the type of sub- stituent [25,26]. In the present work we studied the kinetics of the dechlorina- tion of mono, di and some trichlorobenzenes in a multiphase sys- tem in the presence of a 10% Pd/C catalyst. In particular, we investigated how the number of chlorine atoms in a molecule and their position in polychlorobenzenes influences the suscepti- bility of these heteroatoms to dechlorination. There are few data on the hydrogenolysis of dichlorobenzenes [10,14,26] and trichlo- robenzenes [28,30,31] in the literature, in contrast to the hydrog- enolysis of monochlorobenzene [9–11,14,15,17,23,24,28]. The hydrogenolysis of 1,2,4-trichlorobenzene, as well as 1,2,3,4- and 1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.02.018 * Corresponding author. Tel.: +48 58 523 53 21; fax: +48 58 523 53 57. E-mail address: janiak@chem.univ.gda.pl (T. Janiak). Catalysis Communications 11 (2010) 797–801 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom