(635b) Theoretical Study of Gas-Phase Catalyzed Reactions of PFAS

Destroying PFAS compounds through incineration is currently one of the most effective methods available. Kineticists are actively working on developing detailed kinetic mechanisms to better understand the destruction pathways of PFAS.
The EPA has been conducting pilot-scale reactor experiments to measure the incineration products of various PFAS species. Experiments involving the incineration of PFOS and PFOA in a rainbow furnace have revealed the formation of carboxylic acids. Literature suggests that one of the steps in this process is the hydrolysis of perfluoroaldehydes, which produces HF and perfluoroacids. However, our mechanism predicts that this hydrolysis pathway is too slow to yield significant acid formation.

To address this discrepancy -- and to explore additional potential reactions influenced by catalytic effects -- we are investigating reaction pathways in various PFAS molecules that may be catalyzed by H₂O, SO₃H, perfluoroalcohols, perfluorocarboxylic acids, and perfluorosulfonic acids.

Geometry optimization, normal mode analysis, torsional scans, were done using B2PLYPD3/cc-pVTZ level of theory, followed by high-accuracy single-point calculations with DLPNO-CCSD(T)/aug-cc-pVQZ. These calculations were performed for both stable species and transition states.
Reaction rate coefficients were calculated using the Master Equation System Solver (MESS) from Argonne National Laboratory.

For reactions involving HF elimination -- such as the hydrolysis of perfluoroaldehydes, formation of alpha-lactones from perfluorocarboxylic acids, and formation of alpha-sultones from perfluorosulfonic acids -- multiple catalyzed pathways were identified, as mentioned earlier, involving catalysts such as H₂O, SO₃H, perfluoroalcohols, perfluorocarboxylic acids, and perfluorosulfonic acids.
In each case, first-order saddle points corresponding to catalyzed mechanisms were located, which significantly reduced the activation barriers compared to the un-catalyzed reactions.
To evaluate the impact of these catalyzed pathways on PFAS decomposition, a series of simulations were performed using a 1-D plug flow reactor (PFR) in Cantera.
In the first simulation scenario, only uncatalyzed HF elimination reactions were considered.
In the second case, catalyzed reactions were explicitly included by incorporating elementary steps in the mechanism.
In the third case, simulations are performed using the upper limit of termolecular reactions, formulated in an approximate manner by assigning rate coefficients based on the gas-phase bimolecular collision limit and scaled appropriately.
This comparative approach provides insight into the role of gas-phase catalysis in facilitating PFAS destruction.

Ultimately, this work may help reveal viable pathways for the conversion of perfluorosulfonic acids (PFSAs) to perfluorocarboxylic acids (PFCAs), providing new insights into PFAS decomposition under incineration conditions.