Hydrodechlorination of organic and inorganic substances by metal catalysts

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Hydrodechlorination of chorobenzenes Experimental results of hydrodechlorination and disproportionation of industrial mixtures of isomer trichlorobenzenes (TCB, 85% 1,2,4-TCB) to mono and di-chlorobenzenes by palladium, nickel and copper catalysts and also aluminosilicate base catalysts have been obtained. The performances of different catalysts were compared at different temperatures: at low temperature (220·C) the Pd catalyst is the most active for the hydrodechlorination reaction. When increasing temperature (380·C) the difference between the activities of Pd and Ni and Cu catalysts diminishes and favours the Ni or Cu catalysts for this reaction. The reaction was studied mainly on a Ni catalyst within 350-450·C temperature range, with a ratio H2 to TCB of between 2.6 and 4 and with a contact time varying from 2 to 8 sec. These kinetic studies have confirmed that the following consecutive scheme operates: trichlorobenzene => dichlorobenzene => monochlorobenzene => benzene, coke The zeolite (NaY, NaX) catalyst has definitely poor activity in hydrodechlorination reaction. However this catalyst becomes favourable because it works for the disproportionation reaction which takes place between TCB and benzene leading to mono and di-chlorobenzene. This reaction occurs on zeolite without hydrogen in the gas phase, which is not the case for metal catalysts. The practical interest of this reaction is to find possible ways to obtain useful mono and dichlorobenzenes from chloroaromatic wastes and create ecologically pure technology using a solid aluminosilicated catalyst without the use of hydrogen which is explosive and flammable. Therefore, the research on intermolecular disproportionation of chlorine between benzene and trichlorobenzene on catalysts is of great interest. This reaction appears not have been reported in the literature. Therefore explorative work is being done in this field. Different types of zeolites, natural and synthetic have been tested as catalysts for this reaction. We have found that HNaY type is the most active catalyst. The reaction has been carried out between 450-500·C temperature range, with contact time between 4 and 10 sec and a 1: 1 ratio between trichlorobenzene and benzene. The products obtained are mono and di-chlorobenzene, selectivity tends to 76%, while the benzene and trichlorobenzene conversion are respectively 60 and 45% respectively. These preliminary results need to be complimented by chemical kinetic and mechanism studies. Hydrodechlorination of CCl4 This reaction has been studied for its use of the CCl4 largely produced in the non-selective hydrocarbon chlorination process which itself acts as a model for hydrodechlorination of organic compounds in general. A.K. Avetisov and coworkers have built the catalytic apparatus, circulation and plug flow reactors, combined with the analytical system in order to study the kinetics of this reaction and the performances of different catalysts. Subsequently the research were focussed on the influence of platinum concentration upon platinum/gamma-alumina catalysts in the hydrodechlorination reaction of of CCl4 It was found that the Pt concentration influences mainly the selectivity in this reaction. While, at 80·C, on catalyst with 3%Pt content, about 65% of chloroform (CHCl3), 15% of methane (CH4) and 20% of coupling product (C2H3Cl3) are produced, on 0.4% Pt content catalyst, only 40% chloroform, 10% methane but 50% C2H3Cl3 are formed. The coupling product was assigned to CHCl2-CH2Cl. The formation of this product is not a trivial process and demands further investigation to understand the reaction mechanism. Such a selective reaction was not reported in the literature. The influence of water vapour was also studied, it was found that water vapour influences greatly the selectivity and the activity of the two catalysts under study and in different ways depending on the content of metal. This means that, by the introduction of water pressure into this reaction, it is possible to regulate the way of transformation of CCl4 to CHCl3 or to C2H3Cl3. This could be of technical importance and also the subject of patent; C2H3Cl3 could be considered as a raw material for the synthesis of chloroethylene compounds. From a fundamental point of view it is vital to know the reasons why these two catalysts under study have different catalytic properties, knowing that their preparation differs only by the content of metal. It could be concluded that CCl4 hydrochlorination reaction is a structure sensitive reaction, however this sensitivity cannot be attributed to a difference in particle size as shown by the TEM analysis. Tn both catalysts the particle size was in the 1-2.5 nm range having no significant difference between them. XPS analyses have shown a large chlorine deposit on the catalysts after the reaction in proportions which depend upon the catalyst. More investigations are needed to understand the structure sensitivity of this reaction.

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