Study Of Pd-N₂H₄·H₂O System For Efficiently Reductive Degradation Of PBDEs

As an additive flame retardant,polybrominated diphenyl ethers（PBDEs）are widely used in polyurethane foams,electronic and electrical equipment,textiles and building materials.Due to the lack of the stable chemical bond to the substrate,it is easy to be released into the environment.PBDEs have the characteristics of high toxicity,persistence,bioaccumulation and semi-volatility,which can cause serious harm to human life and health.Therefore,the efficient and harmless treatment of PBDEs has attracted widespread attention at home and abroad.Reductive debromination is a common strategy to treat PBDEs,but the complete debromination of PBDEs is very difficult because the lower-brominated diphenyl ethers are resistant to the one-electron reduction.The principal reason for this is the LUMO orbital energy of PBDEs become much higher with the decreasing of the number of Br atoms on phenyl ring.By comparing to the electron-transfer process,the hydrogenation process with active hydrogen（H*）is more suitable for the complete debromination of PBDEs.In the thesis,with 2,2’,4,4’-tetrabromodiphenyl ether（BDE47）as a model of PBDEs,a series of system was developed for the rapid and complete debromination of PBDEs by using Pd species as catalysts and N₂H₄·H₂O as reducing agent.The main research contents of this thesis are as follows.（1）A new system for the reduction and degradation of BDE47 was established.In the system,Pd species were used as catalyst,N₂H₄·H₂O was used as reducing agent and CH₃OH was used as solvent.The Pd species was generated from the reduction of Pd salt by N₂H₄·H₂O in situ,which was identified as zero-valent Pd with a small amount of Pd O and Pd（OH）₂ adhered on the surface.Due to the catalytic effect of the in situ generated Pd species and the presence of N₂H₄·H₂O,all the added BDE47 was completely transferred to diphenyl ether（DE）and a little benzofuran（DF）.According to the identification of degradation intermediates and the monitoring of electron spin resonance（ESR）,it was confirmed that the debromination of BDE47 was mainly dominated by the attack of active hydrogen.Based on the identification of N₂H₄·H₂O products,a reasonable catalytic mechanism was proposed:the high utilization rate of N₂H₄·H₂O was attributed to the important role of Pd species on the hydrogen abstraction from N₂H₄·H₂O,which preferentially broken the N-H bond rather than N-N bond and lead to the significantly enhanced efficiency in the utilization of N₂H₄·H₂O and inhibition of the poisoning of catalysts.This system provided an efficient method to remove halogenated organic pollutants.（2）A new catalytic method for the deep debromination of BDE47 at the oil-water interface was proposed.In the new catalytic system,with n-butanol/water mixture as solvent,N₂H₄·H₂O as reducing agent and Pd species as catalyst,the deep debromination reaction of BDE47 took place at the oil-water interface.It was found that all the bromine atoms could be completely removed from BDE47 within 60 min in the mixture of n-butanol/water.By the identification of intermediates on GC-MS,only a small amount of lower-brominated diphenyl ether were detected with the decreasing of BDE47concentration,which suggested that the debromination pathway may be different from the system by using methanol as solvents.The distribution of final products were investigated by detecting the Br ions as inorganic products and DE an DF as organic products.The results demonstrated the inorganic and organic products were existed in the water and n-butanol,respectively.Furthermore,the effect of the volume ratio of two phase and reaction temperature on the debromination of BDE47 were studied.The establishment of oil-water interface in n-butanol/water greatly decreased the accumulation of intermediates and promoted the separation and recovery of inorganic and organic products.