Computational Study of the Gas-Phase Thermal Degradation of Perfluoroalkyl Carboxylic Acids

Perfluoroalkyl carboxylic acids (PFCAs) are persistent and ubiquitous pollutants. Environmental remediation is often achieved by absorption on matrices followed by high-temperature thermal treatment to desorb and decompose the PFCAs. Detailed product studies of the thermal degradation of PFCAs have been hampered by the complex nature of product mixtures and associated analytical challenges. On the basis of high-level computational studies, we propose reaction pathways and mechanisms for the high-temperature mineralization of a series of linear PFCAs with a backbone length from C-4 to C-8. The favored initial reaction pathways are nonselective C-C bond homolytic cleavages (with bond dissociation energies of ∼75-90 kcal/mol), resulting in carbon-centered radicals which can undergo β-scissions (E_a ≈ 30-40 kcal/mol) which can be preceded by F atom shifts (E_a ≈ 30-45 kcal/mol). In competing barrierless processes, the carbon-centered radicals can lose ^•F, resulting in the formation of volatile perfluoroalkenes (ΔH ≈ 50-80 kcal/mol). A variety of competing fragmentation processes yield shorter chain perfluorinated PFCAs, isomeric alkenes, alkenoic acids, alkyl, and alkyloic acid radicals. The results provide the energetics for primary, secondary, and tertiary reaction products and insight into the fundamental understanding of the pyrolytic pathways of PFCAs leading to their mineralization.