Elucidating trichloroethylene pyrolysis pathways using synchrotron radiation photoionization mass spectrometry

Given its extensive industrial use and adverse effects on human health, trichloroethylene (TCE) is a priority environmental pollutant and a Group 1 carcinogen (IARC). Understanding its transformation under thermal conditions is essential for evaluating environmental risks associated with thermal remediation and incineration. Herein, the thermal pyrolysis of TCE was investigated in a flow-tube reactor using in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) to directly detect reactive radicals and short-lived intermediates. Key transient species, including methyl radical (•CH 3 ), propargyl radical (•C 3 H 3 ), chloromethyl radical (•CH 2 Cl), chloro-cyclopentadienyl radical (•C 5 H 4 Cl), chlorocyclopentadiene (C 5 H 5 Cl), and trichloromethyl radical (•CCl 3 ), and small molecular intermediates, were identified by their mass-to-charge ratios and photoionization efficiency (PIE) spectra. A comprehensive reaction network was constructed, encompassing initial TCE decomposition and subsequent radical-driven and molecular combination pathways leading to chlorinated and non-chlorinated aromatic species. Initial decomposition proceeds primarily through homolytic C–Cl cleavage. Chlorinated species, including C 2 Cl 2 , C 2 Cl 4 , C 4 Cl 4 , C 3 Cl 6 , C 4 Cl 6 , C 5 Cl 6 , C 6 Cl 6 , and C 8 Cl 6 , were major products at 923–1173 K and 760 Torr, while others were present in trace amounts. Reduced pressure (30 Torr) suppressed aromatic formation and favored smaller molecular species. This work provides a mechanistic framework for chlorinated hydrocarbon transformation and highlights SR-PIMS capability to probe transient intermediates in complex pyrolytic systems. Furthermore, these findings may help predict toxic byproduct formation during thermal treatment of chlorinated solvents and provide mechanistic insights relevant to emission control and remediation design.