Base Effects On The Catalytic Hydrodechlorination Of Typical Organochlorides In Liquid Medium

Chlorinated organic compounds(COCs)including chlorobenzenes,chlorophenols,and chlorinated hydrocarbons are important chemical raw materials,which are widely used as solvents,reagents,pharmaceutical intermediates,and pesticides in industrial and agricultural production.Due to the presence of chlorine atoms in COCs,most of them are high toxicity,high stability and persistence in the environment.In China,hundreds of tons of chlorinated organic industrial waste are produced in the production process of COCs every year.Thus,it must be reduced and controlled from the pollutant source of COCs to avoid and reduce their pollution to the environment.Catalytic hydrodechlorination(HDC)is a nondestructive and environmentally friendly technology,which can effectively remove chlorine atoms of COCs with recovery/reuse of valuable chemicals under mild conditions.In this study,Pd/C and Raney Ni catalyzed liquid-phase HDC of chlorophenols,chlorinated biphenyl,and chlorinated alkanes were investigated with hydrogen used as the hydrogen sources.The effect of base on liquid phase HDC reaction was carried out,and the law and mechanism for base effect on catalyst and reaction substrate in the HDC of COCs containing active hydrogen were clarified with the aid of catalyst characterization and analysis of dechlorinatiton process.Firstly,effect of base on catalytic HDC of COCs containing saturated C-Cl bonds over Pd/C catalyst was studied.Here,the influences of base and Pd/C catalyst on liquid-phase dechlorination of hexachlorocyclohexane(HCH)were carried out to elucidate the dechlorination reaction pathway of HCH and the mechanism for the base effect in the dichlorination of HCH.The major results were summarized as follows: In the dechlorination of HCH,the β-elimination reaction of HCH was promoted by base,and thus dehydrochlorination production pentachlorocyclohexene and trichlorobenzene were generated subsequently.And,the saturated C-Cl bond was converted into the unsaturated C-Cl bond including allylic C-Cl bond,olefin C-Cl bond,and aromatic CCl bond which was easier to be hydrodechlorinated.Therefore,catalytic dechlorination of HCH was further accelerated.Meanwhile,base serves as a proton scavenger,maintaining the catalytic metal in a reduced state and limiting HCl interactions in the liquid-phase catalytic HDC of HCH.Subsequently,effect of base on Raney Ni catalyzed HDC of COCs containing phenolic hydroxyl active hydrogen was further investigated.And,2,4,6-trichlorophenol(2,4,6-TCP)was selected as the representative pollutant.It was found that the base could affect the HDC efficiency of 2,4,6-TCP.Moreover,strong base was more favorable to promote Raney Ni-catalyzed HDC reaction.On the other hand,base affected the HDC pathways of 2,4,6-TCP,the chlorine atom in the para position of hydroxyl group is easier to be removed in the presence of strong base,but was much harder to remove in the presence of weak base.Based on catalyst characterization and analysis of HDC process,it was proposed that the combination of base effect and solvation effect affected the HDC efficiency and pathway of 2,4,6-TCP over Raney Ni catalyst.Further,effect of solutions pH on the HDC of 2,4,6-TCP over Raney Ni was studied.The pH of liquid-phase HDC reaction was controlled by adjusting the added number of stoichiometric bases.When the solution pH was controlled in the range of6.5-8.0,Raney Ni could be exhibited high catalytic activity and stability,and thus the HDC efficiency of 2,4,6-TCP was found to be highest.It was suggested that the negative effect of base effect and solvation effect on the HDC of 2,4,6-TCP could be effectively eliminated when the solution pH was controlled at 6.5-8.0.The base has a significant effect on liquid-phase dechlorination of COCs containing active hydrogen.Base serves as a proton scavenger,maintaining the catalytic metal in a reduced state and limiting HCl interactions,which ensure the smooth progress of the dechlorination reaction.On the other hand,base can also react with COCs containing active hydrogen,which affects the activity and selectivity of the dechlorination of COCs.