(110e) Computational Investigation of the Reaction Mechanism for the Thermal Treatment of Hexafluoropropylene Oxide Dimer Acid [Genx]

Perfluorooctanoic acid (PFOA) and hexafluoropropylene oxide dimer acid (GenX) are environmental contaminants known as poly- and perfluoroalkyl substances (PFAS) with applications in aqueous film forming foam (AFFF) and waterproof/ oil-proof coatings for textiles. Due to its environmental persistence and negative effect on human health, PFOA has been phased out and replaced with GenX. Similar to PFOA, GenX has been detected in drinking water and serum samples of residents living near fluorochemical plants and may also pose a risk to human health. PFAS are commonly destroyed using thermal treatments such as incineration with a goal of complete mineralization to HF and Fâ‚‚.

The kinetic mechanism for GenX transformation in a lab-bench furnace operating at low temperatures of 400-800°C was investigated using computational methods. The rate coefficients for potential reactions were evaluated using master equation simulations with harmonic transition theory and 1-D hindered internal rotors for first order saddle points and phase space theory for barrierless reactions. Geometry optimizations/ normal mode analysis calculations and torsional energy scans were performed in Gaussian09 using B2PLYP-D3/cc-pVTZ and M06-2X/cc-pVTZ, respectively. Single point energies were calculated in ORCA using DLPNO-CCSD(T)/aug-cc-pVQZ. Reaction Mechanism Generator (RMG) simulations of GenX were performed using a GenX reaction library comprised of possible GenX decomposition reactions. The rate coefficients for RMG-generated reactions were calculated explicitly and added to the GenX reaction library. Multiple iterations of RMG simulations with the updated GenX reaction library was used to refine the mechanism. GenX destruction in a furnace was modeled in Cantera using the resulting mechanism. Based on the model, the primary decomposition pathway for GenX was HF elimination to form an alpha-lactone ether which undergoes CO elimination, resulting in an ester. The ester decomposes into the final products of GenX degradation; CF₃CFO and C₂F₅CFO, showing that the degradation of perfluoro-aldehydes is important for achieving complete mineralization.