|Alternative Title||Thermal degradation of polybrominated diphenyl ethers over metal oxide micro/nano-materials and hypothesized mechanism|
|Place of Conferral||北京|
|Keyword||多溴二苯醚,铁基氧化物,锂钛复合氧化物,催化降解,微纳米材料 Polybrominated Diphenyl Ethers, Iron-based Oxide, Lithium-titanium Composite Oxide, Catalytic Degradation, Micro/nano-materials|
1. 本文采用多元醇介导法合成了具有三维结构的Fe3O4微纳米材料，考察了其在300 °C对BDE-209的催化降解活性和相关的加氢脱溴机理，并与Fe3O4微纳米材料催化降解CB-209的相关机理进行比较。结果显示：Fe3O4微纳米材料对BDE-209有很高的降解活性，降解反应符合准一级动力学，反应速率常数为0.15 min?1，高于CB-209降解体系。加氢脱溴路径主要为BDE-209 → BDE-207 → BDE-197 → BDE-183 → BDE-144, BDE-154 → BDE-103 → …… → …… → BDE-7。在BDE-209降解体系中，九溴代和八溴代二苯醚主导产物的生成和CB-209体系类似，即优先脱去间位分别生成BDE-207和-197。然而从七溴二苯醚开始，主导产物BDE-183的邻位溴原子开始脱除。相比而言在CB-209降解体系中，加氢脱氯主导产物中邻位的氯原子一直得到了很好的保存，直至不得不脱除邻位氯原子生成三氯和二氯联苯产物。BDE-209的加氢脱溴路径在反应初期主要由空间位阻效应主导，随着邻位溴原子的脱除，加氢脱溴路径渐渐被热力学主导。这种降解路径的差异可能是因为醚键的存在和不同卤素取代导致的不同结构所致。
2. 通过水热法合成了一系列锂和钛不同初始摩尔掺杂比例的锂钛复合氧化物材料（LiαTiOx，α = 2, 4, 6），并对材料进行了一系列的表征。探究了其在250-350 °C下对BDE-47的催化降解活性，并与市售的锐钛矿TiO2进行了对比。结果表明，在300 °C下LiαTiOx对BDE-47的降解效率远高于锐钛矿TiO2。且它们的降解反应均符合准一级动力学，LiαTiOx催化体系的反应速率常数为0.034-0.055 min-1，远高于锐钛矿TiO2催化体系。LiαTiOx催化降解BDE-47的活化能较低，为39.9-48.1 kJ/mol，尤其是Li4TiOx。研究表明Li具有供电子效应，能促进复合氧化物中氧物种的移动性，从而使LiαTiOx对BDE-47有很好的催化氧化活性。
3. 本文全面检测分析了LiαTiOx对BDE-47降解过程中的中间产物，并推导了相关的降解机制。微量一溴代至三溴代多溴二苯醚的生成，表明加氢脱溴反应极其微弱，而氧化反应占据主导地位。在氧化反应中，活性氧物种的活化遵循Mars-van Krevelen反应机理。同时，我们利用GC-MS结合硅烷衍生化反应，ESI-ICR-MS和离子色谱等一系列仪器手段检测到了包括多溴二苯醚类，单苯环类和开环类产物在内的70余种氧化产物。利用ESI-ICR-MS检测到的产物进行了VK图的分析，发现随着反应时间的增加，开环氧化产物的量也不断增加。这表明在氧化反应中BDE-47首先被亲核的O2-物种攻击生成羟基化的多溴二苯醚类产物，随后进一步生成二溴苯酚，三溴苯酚，苯甲酸和苯二甲酸等的单苯环氧化产物。上述氧化产物均能被亲电子的氧物种（O2-和O-等）进一步攻击，彻底开环生成小分子物种，如甲酸，乙酸，丙酸和丁酸等。
Polybrominated diphenyl ethers (PBDEs), which have been used as brominated flame retardants, are ubiquitous environmental contaminants. It has been established that PBDEs are persistent, stable, and display bioaccumulation and bioactivity, which are harmful to various organisms and human health. Therefore, the reduction of PBDEs contents is a matter of public concern in the context of environmental protection. At present, due to the recalcitrance of PBDEs, various treatment methods are mainly focused on reductive debromination, and the relevant degradation mechanisms remain unclear. Therefore, it is of great significance to develop a cost-effective method for the treatment of PBDEs and to explore associated degradation mechanisms for their reduction and control. Degradation using metal oxides micro/nano-materials might be a new way of disposing of PBDEs.
In this study, iron-based oxides micro/nano-material and lithium?titanium composite oxides were prepared using polyol-mediated and hydrothermal methods, and the PBDEs degradation activities of the materials produced and the mechanisms involved were studies. The methods used and the main results are described below.
1. The three-dimensional Fe3O4 micro/nano-material was synthesized by polyol-mediated method. The catalytic degradation of BDE-209 at 300 °C and the relevant degradation mechanism were investigated. The results were compared with the results found when CB-209 was degraded using Fe3O4 micro/nano-material. Fe3O4 micro/nano-material had a high reactive activity for the degradation of BDE-209, and degradation followed pseudo-first-order kinetics with kobs=0.15 min?1 higher than that for CB-209. The major reaction pathway was BDE-209 → BDE-207 → BDE-197 → BDE-183 → BDE-144, BDE-154 → BDE-103 → ?????? → ?????? → BDE-7. Similar to the case for CB-209, two initial hydrodebromination steps are favored at the BDE-209 meta-positions, giving the major products BDE-207 and BDE-197. However, the variance about the preferred products began to emerge from the start of heptabromodiphenyl ethers (hepta-BDEs). The majorly produced hepta-BDE isomer with BDE-183 is unbrominated at one ortho-position. However, this is different from the reported degradation of CB-209, which always produced the products chlorinated at all four ortho-positions until the ortho-position had to be removed for the formation of trichlorobiphenyls and dichlorobiphenyl still majorly chlorinated at three or two ortho-positions. The early BDE-209 hydrodebromination steps appear to be strongly influenced by steric effects, whereas subsequent hydrodebromination steps, as more bromine atoms are removed, will be gradually governed more by thermodynamics. The difference might be attributed to the flexibility of the linkage bond between two benzene rings and halogen substituents.
2. A series of lithium-titanium composite oxides prepared at various initial Li/Ti mole ratios of 2:1, 4:1, and 6:1 (LiαTiOx, α = 2, 4, 6) were synthesized by hydrothermal method and characterized using a number of techniques. The catalytic activity of the LiαTiOx toward the degradation of BDE-47 was evaluated at 250–350 °C and compared with that of commercially available anatase TiO2. The degradation efficiencies and pseudo-first-order rate constant values (0.034–0.055 min-1) of BDE-47 over LiαTiOx at 300 °C were higher, when compared with those obtained over anatase TiO2. And lower activation energy with 39.9–48.1 kJ/mol is required for the degradation of BDE-47 to proceed over LiαTiOx, especially Li4TiOx. The Li dopant, which exerts an electron-donating effect, could enhance the mobility of the reactive oxygen species, which is expected contribute to the excellent performance of LiαTiOx.
3. The degradation products during the catalytic degradation of BDE-47 over LiαTiOx were comprehensively analyzed, and the associated degradation pathways were proposed. The oxidative reaction was believed to be the dominant degradation pathway following Mars–van Krevelen mechanism, being accompanied by the weak hydrodebromination occurrence generating the trace mo- to tri-BDEs. More than 70 types of oxidation products containing diphenyl ether backbone, single-benzene rings, and ring-opened products were detected by GC-MS with derivatization, ESI-FT-ICR-MS, and ion chromatography. An increase in the number of ring-cracked oxidative products under prolonged reaction was observed by ESI-FT-ICR-MS analysis according to van Krevelen diagram. In the oxidative reaction, a series of oxidative products, such as OH-tri-BDEs and OH-tetra-BDEs, first formed via the nucleophilic O2? attack and subsequently transformed into dibromophenol, tribromophenol, and benzenedicarboxylic and benzoic acids, etc. They could be further attacked by electrophilic O2? and O? and completely cracked to small molecules such as formic, acetic, propionic, and butyric acids.
|李倩倩. 金属氧化物微纳米材料对多溴二苯醚的热催化降解及机理研究[D]. 北京. 中国科学院生态环境研究中心,2018.|
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