Molecular Mechanism of Dioxin Formation from Chlorophenol based on Electron Paramagnetic Resonance Spectroscopy

Abstract

Few studies have investigated the free radical intermediates involved in the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from chlorophenol. This study clarified the reaction pathways during thermochemical formation of PCDDs from 2,3,6-trichlorophenol (TCP) over a Cu(II)O/silica matrix, which was used to simulate fly ash, at 298-523 K. The reaction was studied using electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. In situ EPR indicated the TCP radical (TCPR) formed by hydrogen abstraction of TCP. Five elementary processes including dimerization of TCPR, ortho-chloride abstraction, Smiles rearrangement, ring closure, and intra-annular elimination of Cl were proposed to occur during formation of PCDDs. The proposed mechanism was further confirmed by the detection of PCDD products from thermochemical experiments in a tube furnace. Several dominant congeners, including 1,2,6,9-tetrachlorodibenzo-p-dioxin (TeCDD), 1,2,6,7-TeCDD, 1,2,8,9-TeCDD, and 1,4,6,9-TeCDD were detected by gas chromatography/quadrupole time-of-flight mass spectrometry, and further confirmed by gas chromatography/high resolution mass spectrometry. The detected PCDD products agree with the proposed PCDD formation mechanism. Relatively high temperatures were found to lead to dechlorination of TCPR to form phenoxy radicals in addition to PCDD/Fs. These radicals will be attached to particles, which will increase their lifetimes. These reactions were further verified by molecular orbital theory calculations. The discovery of persistent phenoxy radicals is of environmental significance because of their potential toxicity. The details of this mechanism could be used for controlling PCDD/F formation during industrial thermal processes.

Photoinduced formation of persistent free radicals, hydrogen radicals, and hydroxyl radicals from catechol on atmospheric particulate matter

“…Elimination of ,H means more oxygen-containing functional groups will be produced in the catechol-SiO 2 system, which might contribute to the formation of phenoxy and semiquinone radicals. Density functional theory calculation was used to verify EPFRs formed by hydrogen (H) abstraction of phenolic precursor in silico study (Dellinger et al, 2007;Yang et al, 2017b). This also supported our observations in the present study.…”

Vibrational and Structural Studies of Environmentally Persistent Free Radicals Formed by Phenol-Dosed Metal Oxide Nanoparticles

“…Thus, the high EPFR yield on TiO 2 is a combined effect of the large surface area of the NPs and the superoxide species present, which abstract hydrogen from the organic molecule and thus enable chemisorption of EPFRs on metal oxides surface. 19,26,32 3.2. Vibrational Studies of Phenol Adsorption.…”

Formation mechanism of unintentionally produced persistent organic pollutants in industrial thermal processes

“…有机自由基可能是PCDD/Fs生成反应的重要中间 体. 近期研究开展了以2,3,6-三氯酚为前驱体生成 PCDD/Fs的自由基机理研究 [22] . 发现苯氧自由基作为 反应中间体在PCDD/Fs生成过程中起着重要作用.…”

“…发现苯氧自由基作为 反应中间体在PCDD/Fs生成过程中起着重要作用. 结 合实验室模拟数据和量子化学计算结果, 进一步证实 了在热化学反应过程中, 以氯酚为前驱体形成的有机 自由基中间体可经耦合、脱氯、闭环等一系列基元反 应生成PCDD/Fs [22] . Khachatryan等 [26] 开展了以2-氯酚 为前驱体生成PCDD/Fs的模拟实验, 结果表明, 虽然 PCDDs和PCDFs的生成机理有所不同, 但都涉及自由 基反应, 并且在反应体系中检测到了苯氧自由基.…”