Thermochemistry of Species in Gas-Phase Thermal Oxidation of C2 to C8 Perfluorinated Carboxylic Acids

Computational investigation of the kinetic and thermodynamic parameters governing polytetrafluoroethylene chain shortening

Mechanisms of fluoropolymer pyrolysis are poorly understood. Oversimplifications in both experimental and computational studies related to high temperature decomposition of PTFE have led to the elucidation of potentially incomplete and inaccurate pathways. While most conflicts within the literature claim that individual decomposition occurs through either CF2 elimination or C2F4 elimination, one mechanism that is largely ignored is the thermodynamically favored 1,2-F atom transfer. Quantum chemical calculations with M06-2X/6-311+G(d,p) level of theory were applied to a PTFE model to investigate the thermodynamic and kinetic barriers of their primary thermal decomposition mechanisms, and to a six carbon perfluoroalkyl radical to investigate the kinetics and thermodynamics of chain shortening. The present computational study aims first to provide activation energies and kinetic rate constants at a range of temperatures from 500 to 1500 K for three primary mechanisms of perfluoroalkyl radical chain shortening. These primary mechanisms include: 1) α-scission forming difluorocarbene, 2) β-scission forming C2F4 monomer fragments, and 3) 1,2-F atom transfer followed by β-scission to form C3F6 monomer fragments. Reaction energetics of these three pathways showed that CF2 elimination required the highest activation energy barrier regardless of chain length and temperature, and C2F4 elimination was kinetically favored at all temperatures.

Roadmap to Catalytic Abatement of Gas Phase Per- and Polyfluoroalkyl Substances (PFAS)

While the outstanding stability of per- and polyfluoroalkyl substances (PFAS) paved the way for their widespread application in a huge variety of applications, it also resulted in their nickname "forever chemicals". The rising awareness for PFAS-related environmental and health concerns drives a discussion on the most effective ways to abate PFAS emissions into the environment, i.e. water, soil, and air, and remediation of contaminated matter. In order to address the knowledge gap regarding air pollution by PFAS, this minireview summarizes the current corpus of work in the field and outlines how catalysis can contribute to PFAS abatement in the gas phase. Beyond a mere collection of state-of-the-art knowledge, overarching challenges in catalytic PFAS removal are identified, spanning from fundamental organic and inorganic chemistry, i.e. C-F-bond activation, to heterogeneous catalysis, i.e. surface reactions at the gas-solid interface, to reaction engineering, i.e. scaling relations and technical hurdles. In addition, the article introduces concepts and workflows that aim at providing guidance during the design of technological solutions for the efficient control of gaseous PFAS emissions.

Direct measurement of fluorocarbon radicals in the thermal destruction of perfluorohexanoic acid using photoionization mass spectrometry

Thermal destruction is a critical cornerstone of addressing the rampant contamination of natural resources with per- and polyfluoroalkyl substances (PFAS). However, grave concerns associated with stack emissions from incineration exist because mechanistic studies have thus far relied on ex situ analyses of end products and theoretical calculations. Here, we used synchrotron-based vacuum ultraviolet photoionization mass spectrometry to study the pyrolysis of a representative PFAS-perfluorohexanoic acid-and provide direct evidence of fluorocarbon radicals and intermediates. A key reaction pathway from perfluorocarboxylic acids to ketenes via acyl fluorides is proposed. We furthermore propose CF2/CF3 radical-centered pyrolysis mechanisms and explain their roles in the formation of other products that may form in full-scale incinerators. These results have not only unveiled the role of radicals and intermediates in thermal PFAS decomposition and recombination mechanisms but also provide unique insight into improving the safety and viability of industrial PFAS incineration.

Association Kinetics for Perfluorinated n-Alkyl Radicals

Radical-radical reaction channels are important in the pyrolysis and oxidation chemistry of perfluoroalkyl substances (PFAS). In particular, unimolecular dissociation reactions within unbranched n-perfluoroalkyl chains, and their corresponding reverse barrierless association reactions, are expected to be significant contributors to the gas-phase thermal decomposition of families of species such as perfluorinated carboxylic acids and perfluorinated sulfonic acids. Unfortunately, experimental data for these reactions are scarce and uncertain. Furthermore, obtaining reliable theoretical predictions for such reactions is a laborious and computationally intensive task. In this work, the chemical kinetics of the various association/decomposition reactions producing/decomposing the C2-C4 series of unbranched n-perfluoroalkanes (C2F6, C3F8, and C4F10) are examined using state-of-the-art ab initio transition-state-theory-based master-equation calculations. The variable-reaction-coordinate transition-state theory (VRC-TST) formalism is employed in computing the microcanonical and canonical rates for the association reactions. Reaction thermochemistry is obtained via composite quantum chemistry calculations and the laddering of error-canceling reaction schemes via a connectivity-based hierarchy approach employing ANL1/ANL0-style reference energies. Lennard-Jones collision model parameters for the considered systems were estimated by a direct dynamics approach, and collisional energy transfer parameters were obtained from analogies to systems of similar size and heavy-atom connectivity. A one-dimensional master equation approach was used to convert the microcanonical rate coefficients from the VRC-TST analysis into temperature- and pressure-dependent rate constants for the association reactions and the reverse dissociation reactions. The data are reported in standardized formats for usage in comprehensive chemical kinetic models for PFAS thermal destruction.

Thermal transformations of perfluorooctanoic acid (PFOA): Mechanisms, volatile organofluorine emissions, and implications to thermal regeneration of granular activated carbon

Thermal treatment is effective for the removal of perfluorooctanoic acid (PFOA). However, how temperatures, heating methods, and granular activated carbon (GAC) influence pyrolysis of PFOA, and emission risks are not fully understood. We studied thermal behaviors of PFOA at various conditions and analyzed gaseous products using real-time detection technologies and gas chromatography-mass spectrometry (GC-MS). The thermal decomposition of PFOA is surface-mediated. On the surface of quartz, PFOA decomposed into perfluoro-1-heptene and perfluoro-2-heptene, while on GAC, it tended to decompose into 1 H-perfluoroheptane (C7HF15). Neutral PFOA started evaporating around 100 ℃ without decomposition in ramp heating. During pyrolysis, when PFOA was pre-adsorbed onto GAC, it was mineralized into SiF4 and produced more than 45 volatile organic fluorine (VOF) byproducts, including perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs). The VOF products were longer-chain (hydro)fluorocarbons (C4-C7) at low temperatures (< 500 ℃) and became shorter-chain (C1-C4) at higher temperatures (> 600 ℃). PFOA transformations include decarboxylation, VOF desorption, further organofluorine decomposition and mineralization in ramp heating of PFOA-laden GAC. Decarboxylation initiates at 120 ℃, but other processes require higher temperatures (>200 ℃). These results offer valuable information regarding the thermal regeneration of PFAS-laden GAC and further VOF control with the afterburner or thermal oxidizer.

Thermochemistry of Gas-Phase Thermal Oxidation of C2 to C8 Perfluorinated Sulfonic Acids with Extrapolation to C16

New ideal-gas thermochemistry Cp°(T), H°(T), S°(T), and G°(T) are predicted for 53 species involved in the thermal destruction of perfluorinated sulfonic acids (PFSAs) ranging from C2 to C8 in perfluorinated alkyl chain length. Species were selected by considering both the pyrolytic and oxidative pathways of PFSA destruction. After the sulfur-containing moieties are removed, subsequent reactions largely involve species from a prior set of thermochemistry for the thermal destruction of perfluorinated carboxylic acids (Ram et al., J. Phys. Chem. A, 2024, 128, 7, 1313-1326). Enthalpies of formation at 0 K are computed using a new isogyric reaction scheme. Rigid-rotor harmonic-oscillator partition functions were calculated over a 200-2500 K temperature range using rovibrational properties at G4 (≤C3S1 species) and M06-2X-D3(0)/def2-QZVPP (≥C4S1 species), employing the 1D hindered rotor approximation to correct for torsional modes. Seven-coefficient NASA polynomial fits are reported in standardized formats. Bond dissociation energies and important reaction equilibria are examined to provide insights into the reactivity of potentially persistent species. Extrapolated NASA polynomials are also systematically predicted for 126 species larger than C8/C8S1 in size, allowing reasonably accurate estimates of thermochemistry without the need for expensive electronic structure calculations.