Mechanistic Investigations of Thermal Decomposition of Perfluoroalkyl Ether Carboxylic Acids and Short-Chain Perfluoroalkyl Carboxylic Acids
Ali Alinezhad, Heng Shao, Kateřina Litvanová, Runze Sun, Alena Kubátová, Wen Zhang, Yang Li, Feng Xiao
Abstract
In this study, we investigated the thermal decomposition mechanisms of perfluoroalkyl ether carboxylic acids (PFECAs) and short-chain perfluoroalkyl carboxylic acids (PFCAs) that have been manufactured as replacements for phased-out per- and polyfluoroalkyl substances (PFAS). C–C, C–F, C–O, O–H, and C═C bond dissociation energies were calculated at the M06-2X/Def2-TZVP level of theory. The α-C and carboxyl-C bond dissociation energy of PFECAs declines with increasing chain length and the attachment of an electron-withdrawing trifluoromethyl (−CF 3 ) group to the α-C. Experimental and computational results show that the thermal transformation of hexafluoropropylene oxide dimer acid to trifluoroacetic acid (TFA) occurs due to the preferential cleavage 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 (CF 3 CF 2 CF 2 OCFCF 3 COOH → CF 3 CF 2 CF 2 · + ·OCFCF 3 COOH) through which perfluorobutanoic acid (PFBA) is formed. The weakest C–C bond in PFPeA and PFBA is the one connecting the α-C and the β-C. The results support (1) the C–C scission in the perfluorinated backbone as an effective PFCA thermal decomposition mechanism and (2) the thermal recombination of radicals through which intermediates are formed. Additionally, we detected a few novel thermal decomposition products of studied PFAS.