Free-radical triggered formation of dioxins from 2-chlorophenol pyrolysis investigated by synchrotron radiation photoionization mass spectrometry

Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and polychlorinated naphthalenes (PCNs) are a category of highly toxic and environmentally persistent pollutants released particularly via thermal processes of chlorine-containing materials. However, the detailed reaction mechanism, especially the evolution of related radicals remains elusive for decades. Herein we have for the first time characterized the radicals and intermediates during pyrolysis of 2-chlorophenol resulting in PCDD/Fs and PCNs, using a flow tube reactor coupled with in-situ synchrotron radiation photoionization mass spectrometry (SR-PIMS). Transient species including 2-chlorophenoxy (C₆H₄ClO^•), phenoxy (C₆H₅O^•), chloro-cyclopentadienyl (^•C₅H₄Cl), chloro-cyclopentadiene (C₅H₅Cl), fulvenone ketene (C₆H₄O) and o-benzyne (o-C₆H₄), were identified via m/z and photoionization efficiency profile. Potential energy surfaces of the early-stage mechanism and the associated rate constants and branching ratios were elucidated. Successively, the formation mechanisms of PCDD/Fs and PCNs from these transient intermediates at high temperatures were proposed which have experimentally validated and refined the previous mechanism. The results suggested that the combination of 2-chlorophenoxy radicals with another 2-chlorophenoxy, phenoxy, phenyl, or o-benzyne leads to the formation of PCDD/Fs, while PCNs are generated from the self-coupling of chloro-cyclopentadienyl.