Theoretical study on the formation mechanism of pre-intermediates for PXDD/Fs from 2-Bromophenol and 2-Chlorophenol precursors via radical/molecule reactions.
Theoretical study on the formation mechanism of pre-intermediates for PXDD/Fs from 2-Bromophenol and 2-Chlorophenol precursors via radical/molecule reactions.
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DOI：
10.1016/j.envpol.2017.03.007
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发表时间：
2017-06
影响因子：
8.9
通讯作者：
Wenxiao Pan;Jianjie Fu;A. Zhang
中科院分区：
文献类型：
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作者：
Wenxiao Pan;Jianjie Fu;A. Zhang
本研究通过自由基/分子机理探讨了前PXDD/F中间体形成的反应途径。详细计算了2-溴苯酚(2-BP)和2-氯苯酚(2-CP)前体与苯氧基、苯基和苯氧基双自由基的结合反应的热力学和动力学参数。苯氧基与2-B(C)P偶联生成卤代邻苯氧基苯酚的前PXDD中间体。苯基和苯氧基二基与2-B(C)P结合生成两种结构，即二羟基联苯和苯氧基苯基，它们专门用作PXDF的前构体。这些缩合反应，特别是那些涉及苯基和苯氧基双自由基中的苯基C原子的缩合反应，被证明是热力学和动力学上都是有利的，并且与自由基/自由基反应中涉及的相应步骤几乎是相似的。最重要的是，苯基和苯氧基双自由基与卤代苯酚的反应只会导致PXDF的生成，这在一定程度上为现实环境中PXDF/PXDD的高比率提供了一个合理的解释。因此，我们的研究揭示了自由基/分子机制在均相气相PXDD/F形成中的关键作用，特别是在PXDF形成中。这些结果填补了迄今存在的关于二恶英生成的知识空白，提高了我们对环境中PXDD/F生成特征的理解。
This study investigates reaction pathways for the formation of pre-PXDD/F intermediatesviaa radical/molecule mechanism. Thermodynamic and kinetic parameters for the combination reactions of 2-bromophenol (2-BP) and 2-chlorophenol (2-CP) precursors with key radical species including the phenoxy radicals, the phenyl radicals and the phenoxyl diradicals were calculated in detail. The couplings of phenoxy radicals with 2-B(C)P tend to produce pre-PXDD intermediates of halogenatedo-phenoxyphenol. The combinations of phenyl and phenoxyl diradicals with 2-B(C)P produce two types of structures, i.e., dihydroxybiphenyl ando-phenoxyphenyl, which exclusively act as prestructures of PXDFs. These condensation reactions, especially those involving the phenyl C atom sites in phenyl and phenoxyl diradicals, are proven to be both thermodynamically and kinetically favorable and are nearly comparable with the corresponding steps involved in the radical/radical reactions. Most importantly, reactions of phenyl and phenoxyl diradicals with halogenated phenols solely lead to the formation of PXDFs, which to some extent provides a plausible explanation for the high PXDF-to-PXDD ratios in the real environment. Therefore, our study reveals the pivotal role of the radical/molecule mechanism in homogeneous gas-phase PXDD/F formation, especially in PXDF formation. The present results fill in a knowledge gap that has hitherto existed regarding dioxin formation and improve our understanding of PXDD/F formation characteristics in the environment.