Computational Study of the Gas-Phase Thermal Degradation and the Reaction Rate Coefficients of Perfluoroalkyl Ether Carboxylic Acids.

Abstract

Perfluoroalkyl ether carboxylic acids (PFECA), which are replacements for legacy per- and polyfluorinated alkyl substances (PFAS), exhibit undesirable properties and often require thermal remediation. Detailed kinetic evaluation of the pyrolysis of PFECA was achieved computationally using density functional ωB97xD/6-311+G (d,p) to establish homolytic bond dissociation energies for the carboxylic acid and carboxylate forms of ∼90-100 kcal/mol and as low as 65 ± 3 kcal/mol, respectively. The negatively charged oxygenated radical products collapse with activation energies (E_a) of E_a(β-scission) ∼ 12-42 kcal/mol, E_a(1,2-F-shift) ∼ 24-47 kcal/mol, and E_a(oxygen atom-shift) ∼ 33-35 kcal/mol and enthalpies (ΔH) of ΔH(F-loss) ∼ 56-71 kcal/mol. The perfluoroalkoxyl radical intermediates transform via E_a(β scission) ∼ 2-9 kcal/mol and E_a(F-loss) ∼ 25-43 kcal/mol. The radical intermediates have lifetimes in the microsecond-to-nanosecond range at 1000 K and 1 atm, with some radicals stable for hours or even days with respect to the unimolecular processes. The results provide new fundamental thermodynamic and kinetic parameters for the partitioning of the degradation pathways of PFECA and establish specific structure-activity relationships of intermediates, leading to the final degradation products. These results are critical for modeling the thermal treatment of PFECA and related PFAS.