Catalytic Hydrodechlorination Of High Concentration Organochlorides In Alcohol-water Biphasic Systems

Chlorinated organic compounds（COCs）including organochlorine pesticides,chlorinated phenols and polychlorinated biphenyls were important chemical raw materials for the production of pesticides,industrial solvents,pharmaceutical intermediates,transformer oil and capacitor oil,which were widely used in agriculture,industry and medicine.Due to the presence of chlorine atoms,COCs have the characteristics of high toxicity,high chemical stability and bioaccumulation,which seriously endangered human health and ecological environment safety.Catalytic hydrodechlorination（HDC）was a kind of harmless treatment and resource utilization technology of COCs,which could effectively remove chlorine atoms of COCs and reduce its toxicity and stability under mild conditions.In this study,effect of reaction system and catalyst system on the catalytic HDC of4-chlorobiphenyl were investigated with Pd/C and Pd/C-TiO₂used as catalysts.On the one hand,the effect of solvent system,base and phase transfer agent on Pd/C-catalyzed HDC of 4-chlorobiphenyl in organic-water biphasic system and its mechanism were examined in detail.On the other hand,Pd/C-TiO₂ amphiphilic catalysts were prepared by sol-gel method and hydrothermal method,and the process and kinetics of Pd/C-TiO₂catalytic HDC of 4-chlorobiphenyl were explored.Moreover,the mechanism of Pd/C-TiO₂ catalytic HDC of 4-chlorobiphenyl was further elucidated with the aid of catalyst characterization techniques.It was applied to the degradation of high concentration transformer oil-contained PCBs in stock.Firstly,effect of reaction system on the catalytic HDC of 4-chlorobiphenyl was studied.It was found that the inorganic salts Na Cl generated in the organic solvent could deposit on the surface of Pd/C catalyst,thus reducing the activity of the catalyst.Meanwhile,the aqueous phase of organic-water biphasic system could avoid the inorganic salts Na Cl from depositing on surface of the catalyst and thus enabling the catalyst to maintain high activity and stability.The results demonstrated that the HCl formed during the HDC reaction could be effectively assimilated when Et₃N was used as base.On the other hand,Et₃N·HCl was generated from the neutralization of Et₃N and HCl in turn.Furthermore,Et₃N·HCl as phase transfer agent could increase the contact between the n-butanol phase and aqueous phase and thus promoting the HDC reaction.Subsequently,the preparation of Pd/C-TiO₂ by sol-gel method and its effect on the catalytic HDC of 4-chlorobiphenyl were investigated.It can be seen that the pretreatment of activated carbon by different solvents did not change its surface hydrophilicity.Although C-TiO₂ prepared by sol-gel method had a better affinity for water than activated carbon,and loaded smaller Pd particles.Therefore,Pd/C-TiO₂ could fully contact with 4-chlorobiphenyl of n-butanol phase and Na OH of aqueous phase,which was conducive to the adsorption and activation of 4-chlorobiphenyl at the active site of catalyst and thus facilitating the HDC reaction.Increasing the calcination temperature was beneficial for promoting the growth of TiO₂grain,thereby improving the catalytic activity of Pd/C-TiO₂.With the increased of TiO₂loading,the grain size of TiO₂ increased and the agglomeration was aggravated,resulting in the decreased of specific surface area of C-TiO₂ and the increased of Pd particle size and the aggregation phenomenon,thus reducing the catalytic activity of Pd/C-TiO₂.Further,the preparation of Pd/C-TiO₂ by hydrothermal method and its effect on the catalytic HDC of 4-chlorobiphenyl were investigated.The results showed that the TiO₂crystals with well-crystallized and high purity could be directly prepared by hydrothermal method.The catalytic activity of Pd/C-TiO₂ was affected by hydrothermal temperature,hydrothermal time,p H and TiO₂ loading capacity.Increasing the hydrothermal temperature,extending the hydrothermal time were beneficial for enhancing the growth of TiO₂ grain,thus improving the catalytic activity of Pd/C-TiO₂.With the increased of TiO₂loading,the grain size of TiO₂ increased and the agglomeration was aggravated,leading to the decreased of specific surface area of C-TiO₂ and the increased of Pd particle size and the aggregation phenomenon,and the catalytic activity of Pd/C-TiO₂ were decreased.