Mechanistic Investigations of Thermal Decomposition of Perfluoroalkyl Ether Carboxylic Acids and Short-Chain Perfluoroalkyl Carboxylic Acids

In this study, weinvestigated the thermal decomposition mechanismsof perfluoroalkyl ether carboxylic acids (PFECAs) and short-chainperfluoroalkyl carboxylic acids (PFCAs) that have been manufacturedas replacements for phased-out per- and polyfluoroalkyl substances(PFAS). C-C, C-F, C-O, O-H, and C Cbond dissociation energies were calculated at the M06-2X/Def2-TZVPlevel of theory. The & alpha;-C and carboxyl-C bond dissociation energyof PFECAs declines with increasing chain length and the attachmentof an electron-withdrawing trifluoromethyl (-CF3) group to the & alpha;-C. Experimental and computational resultsshow that the thermal transformation of hexafluoropropylene oxidedimer acid to trifluoroacetic acid (TFA) occurs due to the preferentialcleavage of the C-O ether bond close to the carboxyl group.This pathway produces precursors of perfluoropropionic acid (PFPeA)and TFA and is supplemented by a minor pathway (CF3CF2CF2OCFCF3COOH & RARR; CF3CF2CF2 & BULL; + & BULL;OCFCF3COOH)through which perfluorobutanoic acid (PFBA) is formed. The weakestC-C bond in PFPeA and PFBA is the one connecting the & alpha;-Cand the & beta;-C. The results support (1) the C-C scissionin the perfluorinated backbone as an effective PFCA thermal decompositionmechanism and (2) the thermal recombination of radicals through whichintermediates are formed. Additionally, we detected a few novel thermaldecomposition products of studied PFAS. Thisstudy provides insights into the thermal decompositionmechanisms of PFECAs and short-chain PFCAs.