Deep Debromination Of BDE47 By Using Pd/TiO₂ As A Catalyst

As flame retardants with excellent flame resistance and good stability,polybrominated biphenyl ethers?PBDEs?are widely used in building materials,electronic products and textile products and other products.However,due to their high toxicity,bioaccumulation,persistence and semi-volatility,and the easiness of their releasing into the environment,PBDEs have caused great harms to human beings and have attracted much attention from the academia of environment for many years.The high toxicity of PBDEs is mainly caused by bromine atoms on the benzene ring,and it is needed to make PBDEs debromination to reduce their toxicity.PBDEs have strong electrophilicity and it is difficult to oxidize,so reduction methods are often used to degrade PBDEs.However,with the decrease of bromination degree of PBDEs,the degradation becomes more and more difficult.As noble metals have strong catalytic hydrogenation properties,Pd/TiO₂ composites were prepared in this thesis for the catalytic reduction of 2,2',4,4'-tetrabromodiphenyl ether?BDE47?.The main research contents of this thesis are as follows.?1?Using PdCl₂ as precursor,Photo-Cl-Pd/TiO₂ was prepared by photoreduction method.It was found that the catalyst could efficiently degrade BDE47and achieve its deep debromination.The experimental results showed that the strong adsorption force between PdCl₂ and the TiO₂ support was very strong,which was conducive to improving the actual content of Pd in Photo-Cl-Pd/TiO₂ and reducing the diameter of Pd nanoparticles.The effect of catalyst dosage and Pd load on the degradation of BDE47 was investigated.As Pd was the catalytic active site,more active sites were provided with the increase of catalyst dosage and Pd load,so the degradation rate and debromination rate of BDE47 increased.The effects of reaction temperature,reaction solvents on the degradation of BDE47 were studied.When the reaction temperature was 40^oC and the solvent was isopropanol,the degradation ratio of BDE47 reached 100%within 35 min and the debromination efficiency reached 100%within 60 min.Increasing the reaction temperature would accelerate the dissociation of isopropanol and the hydrogen capture process of Pd/TiO₂,so the degradation and debromination rate of BDE47 would be accelerated.Due to the different hydrogen donate capacity and polarity of the reaction solvents,the degradation of BDE47 was also very different.?2?The mechanism of catalytic reduction of BDE47 on Pd/TiO₂ was studied in depth.According to the identified intermediates,it was proved that the reduction of BDE47 is a step-by-step process in which bromine is removed mainly by para-position.The products consisted of not only biphenyl ether,but also the coupling product benzofuran.There was a competition between the two final products.Through isotope experiments,it was found that the active hydrogen attacking BDE47mainly came from the solvent isopropanol.In addition,the increase of NaOH concentration and reaction temperature in the system would shorten the induction period.This was because the NaOH solution can neutralize HBr generated in the reaction process,which reduced the deactivation of the catalyst.It could also increase the solubility of isopropanol ions,which coordinated with metal?M?and then forms M-H reduction species,and accelerated the hydrogenation reduction process of BDE47.