Mechanistic insight into thermal decomposition of PFOA using computational nanoreactors

PFAS or per- and polyfluoroalkyl substances are a large, complex, and ever-expanding group of manufactured chemicals that are widely used to make several types of everyday products and are now inescapable throughout the environment. This has earned these compounds the nickname “forever chemicals” due to the strength of their carbon-fluorine atoms keeps them from easily degrading in the environment. Due to their extreme stability PFAS molecules only decompose under extreme conditions typically using elevated temperatures and high pressures. However, little is still known about the mechanistic pathways for this thermal decomposition as well as the products of their decomposition. In this study, a nanoreactor using the GFN2-xTB method by Grimme and coworkers was set up in which a single perfluorooctanoic acid (PFOA) molecule was thermally degraded, along with subsequent products, until decomposition was no longer possible. This process produced a wide range of radical species which were systematically combined. These were also added to the nanoreactor to find multiple pathways for decomposition. The fragments were then optimized using Gaussian16 to refine the geometry and the electronic energies. Thermodynamically PFOA was found to favorably decompose completely above 910 K (~640 °C), consistent with known experimental pyrolysis temperatures of 700 °C. However, to explore the kinetics of this process a microkinetic model was built with DFT-predicted activation energies and preexponential factors modeled using either a plug-flow or constant-pressure-and-temperature reactor. Through manipulation of the parameters of the microkinetic model, the kinetically varied products can be tuned to produce materials that would be practical for processing into starting materials for other industrial uses.