Thermal Degradation of Long-Chain Fluorinated Greenhouse Gases: Stability, Byproducts, and Remediation Approaches

Abstract

Perfluorocarbons (PFCs) are synthetic industrial chemicals, which, once released into the atmosphere, exhibit strong greenhouse effects. They are also potential products of incomplete degradation of per- and polyfluoroalkyl substances in thermal processes. This study aims to fill a significant gap in the literature regarding the thermal stability of PFCs. Among the PFCs examined, perfluorohept-1-ene (C₇F₁₄) and perfluorooct-1-ene (C₈F₁₆) degraded at temperatures as low as 200 °C, achieving near-complete degradation at approximately 300 °C. The mineralization of these two unsaturated PFCs reached up to ∼40 mol % at temperatures between 300 °C and 500 °C. In contrast, their saturated counterparts required significantly higher temperatures (≥600 °C) for similar levels of degradation and yielded less than 10 mol % fluorine. This disparity is likely due to the hemolytic thermal cleavage of the relatively weak C3–C4 bonds in the unsaturated PFCs, initiating radical-chain reactions that release fluorine. The analysis indicates that the thermal degradation pathways of perfluoroalkenes predominantly involve chain scission and cyclization, leading to the formation of various linear and cyclic byproducts, particularly at temperatures below 500 °C. The addition of granular activated carbon enhanced the thermal mineralization of these PFCs, whereas common commercial catalysts were only moderately effective or ineffective.