Highly Efficient Liquid-phase Hydrodehalogenation Of Halogenated Aromatic Compounds Over Pd/C And Raney Ni Catalysts

Due to the existence of halogen atom, persistent organic pollutants(POPs) exhibit high toxicity, bioaccumulate in the food web, persist in the environment, and pose a risk of causing adverse effects to human health and the environment. Thus, it is urgent to develop an efficient and cost-effective method to detoxify and destroy them with the ever increasing concern about the environment protection. Catalytic hudrodehalogenation(HDH) represents a viable alternative and non-destructive treatment for transforming toxic hazardous material into less toxic products that can be more easily degraded or even possess commercial importance, and was received more and more attention.In this study, liquid-phase HDH of chlorophenols, halobenzenes, 4-halophenols, PCBs, and other HACs using Pd/C or Raney Ni catalyst under mild conditions were investigated. In liquid-phase system, the influence of solvent system on the activity of catalyst and HDH reactivity were studied in detail, and the reason for the deactivation of catalyst in organic solvent and the catalyst keep high activity and stability was elucidated by means of catalyst characterization. On the basis of these studies, alcohol-water homogeneous solvent system was developed for catalytic HDH of high concentrations PCBs and other POPs. And catalytic HDH of high concentration chlorophenols over Raney Ni was also achieved. Moreover, catalysts could keep high activity and stability in water containing solvent systems. This provided apractical strategy to establish solvent system which was suitablefor application in catalytic HDH of high concentration POPs. Meanwhile,we developed a combination technique of solvent extraction and catalytic HDH, and designed an experimental apparatus for the remediation of DDTr contaminated soil. DDTr contaminated soil could be efficiently detoxified by combination of solvent extraction and catalytic HDH. The combination method described in this manuscript is a good way for lab detection and removing of the contamination.Furthermore, effect of solvent and alkali on HDH reactivity and stability were also described. Meanwhile, comparative study on catalytic hydrodehalogenation of halogenated aromatic compounds over Pd/C and Raney Ni catalysts were carried out. To clarify the reason why hydrogenolytic scission reactivity of C-X bonds over Pd/C and Raney Ni catalysts exhibits different trends, liquid-phase HDH of mixed HACs over Pd/C and Raney Ni catalysts were studied, and catalysts were characterized by SEM, EDSX, and XRD techniques. The major results were summarized as follows:Solvent systems are considerably important factors to influence the liquid-phase HDC reaction. Compared with an aprotic solvent, protic solvents are much better solvents for liquid-phase HDH. Moreover, HDH rate increased with solvent polarity for the HDC carried out in protic solvents.Inorganic salts, such as Na Cl, produced insitu, could deposit on the surface of Raney Ni catalyst, andthus caused deactivation of the catalyst in organic solvents. Moreover, the deactivation of the catalyst used in organicsolvents, which was caused by the accumulation of inorganic salts,such as Na Cl, was a universal phenomenon in liquid-phase HDH. Meanwhile, alcohol-water mixed solvent as the reaction medium can prevent the inorganic salts from accumulating on the surface ofcatalyst and thereby enabled the catalyst to keep high activity andstability. On the basis of the mechanism above, alcohol-water solvent system was developed for catalytic HDH of high concentration POPs under mild conditions and the catalyst could be recovered and reused. This report provided apractical strategy to establish solvent system which was suitable for application in catalytic HDC of high concentration POPs.Based on the studies mentioned above, alcohol-water mixed solvent system applied to treatment of transformer oil-contained PCBs. Meanwhlie, the influences of temperature, pressure, solvents and water addition on HDH of transformer oil-contained PCBs were studied. This provides a practical and efficient solvent system for the HDH of high concentration transformer oil-contained PCBs.For the remediation of DDTr contaminated soil, we developed a combination technique of solvent extraction and catalytic HDC, and designed an experimental apparatus. DDTr could be completely extracted from the soil sample at ambient temperature and pressure. Then the extract from DDTr contaminated soil was effectively hydrodechlorinated over Pd/C with GC-MS analysis monitoring the HDC process of DDTr. The combination method described in this manuscript is a good way for lab detection and removing of the contamination. It provides a practical strategy and direction for rapid abatement of POPs contaminated soil under mild condition. However, for big scale application in industry or for a polluted soil field, this method might still need further investigation.Liquid-phase HDH of single and mixed halobenzenes/4-halophenols with H2 over 5% Pd/C and Raney Ni catalysts were investigated and compared. For liquid-phase HDH of single HACs, hydrogenolytic scission reactivity of C-X bonds decreases in order of C-Br > C-Cl > C-I > C-F over Pd/C catalyst, and in order of C-I > C-Br > C-Cl > C-F over Raney Ni catalyst. To clarify the reason why hydrogenolytic scission reactivity of C-X bonds over Pd/C and Raney Ni catalysts exhibits different trends, liquid-phase HDH of mixed HACs over Pd/C and Raney Ni catalysts were studied, and catalysts were characterized by SEM, EDX, and XRD techniques. It was found that the high adsorption of iodoarenes on Pd/C catalyst caused the HDH reactivity of iodoarenes to be lower than that of chloroarenes and bromoarenes in the HDH of single HACs. Moreover, the adsorption of in situ produced iodine ion(I-) to catalyst surface would result in the decline of catalytic activity, which might be the main reason why the HDH reactivity of HACs in the presence of Na I is rather low.For liquid-phase HDH of single HACs, Pd/C catalyst exhibits high catalytic activity for the HDH of bromoarenes and chloroarenes, but shows low catalytic activity for the HDH of iodoarenes and fluoroarenes. According to catalyst characterization(SEM, EDX, and XRD), I-, produced in situ, will adsorb on the catalyst surface. The high adsorption of in situ produced I- can lead to the decrease of the catalytic activity in liquid-phase HDH of HACs, and thereby the HDH reactivity of mixed HACs is lower than that of the HDH reactivity of single HACs. This may be the main reason why the HDH reactivity of HACs in the presence of Na I is rather low.According to the analysis of SEM under high magnification, itwas found, for the first time, that the surface micro-topographyof Raney Ni catalyst could be affected by solvent in liquid-phase HDC. This presumably accounted for the higher activity and sta-bility of Raney Ni catalyst in water than that in water–alcoholmixed solvents.In summary, water was the most effective solvent for dispos-ing high-concentration chlorophenols over Raney Ni catalyst undermild conditions(30-40 ℃, 1 atm). In water, a variety of chlorophe-nols could be efficiently hydrodechlorinated, and high catalyticactivity and stability could be preserved.