Study on hydrodechlorination of polychlorinated biphenyls during reductive pyrolysis in the presence of a catalyst

Abstract

For the first time an experimental set-up, called atmospheric pressure temperature programmed reduction (AP-TPR) applied in the speciation study of sulphur compounds in coal, has been used in the hydrodechlorination study of polychlorinated biphenyls (PCBs) with several specifically designed catalysts. Quantitative results concerning the degree of dechlorination were obtained using a potentiometric detection system equipped with a Cl − selective electrode. Using a MS detection system, qualitative interpretation of the hydrodechlorination process (HDCl) of PCB as a function of the systematical increasing temperature could be formulated. Highly chlorinated organic compounds, i.e. decachlorobiphenyl (CB 209) and a technical mixture of PCBs (Aroclor 1254), were subjected to reductive pyrolysis in a flow of hydrogen in the presence of a catalyst. A number of nickel catalysts on carbon and silica were prepared using an optimized spray drying technique. A commercial carbon supported palladium catalyst was also tested. Pd/C leads to a complete removal of chlorine from CB 209 and to a smaller extent from Aroclor 1254 and at a much lower temperature than for the NiC catalysts. The HDCl of Aroclor 1254 and CB 209 catalysed by Ni/C starts at 300 and 350 °C, respectively. The highest removal of chlorine is achieved using Ni/C catalyst with the lowest amount of NiO being 3 wt%. While nickel on silica support is found to be inactive for the HDCl reaction under the conditions of AP-TPR analysis. AP-TPR-MS study shows that biphenyl structure is decomposed to benzene, toluene, styrene and propenyl benzene with a maximum evolution temperature higher than the HDCl process but depending on the catalyst and PCBs. It can be concluded that AP-TPR method can be considered as a simple method for preliminary testing the activity of catalysts for HDCl process.