Chemical Characterization and Thermal Stressing Studies of Perfluorohexane Fluids for Space-Based Applications

Abstract

Perfluorohexane (PFH), C 6 F 14 , is a perfluorocarbon fluid. Several PFH fluids with different isomer concentrations were evaluated for use in an upcoming NASA space experiment. Samples tested included two commercially obtained high-purity n-perfluorohexane (n-PFH) fluids and a technical grade mixture of C 6 F 14 branched and linear isomers (FC-72 ). These fluids were evaluated for exact chemical composition, impurity purity and high temperature degradation behavior (pyrolysis). Our investigation involved simulated thermal stressing studies of PFH fluids under conditions likely to occur in the event of an atmospheric breach within the International Space Station (ISS) and subsequent exposure of the vapors to the high temperature and catalyst present in its Trace Contaminant Control Subsystem (TCCS). Exposure to temperatures in the temperature range of 200-450°C in an inert or oxidizing atmosphere, with and without the presence of catalyst was investigated. The most aggressive conditions studied were exposure of PFH vapors to 450°C in air and in the presence of TCCS (palladium) catalyst. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography (GC) analyses were conducted on the perfluorohexane samples before and after pyrolysis. The FC-72 and n-PFH samples showed no significant degradation following pyrolysis even under the most aggressive study conditions. Some trace level impurities associated with the PFH samples such as linear perfluorocarbon monohydrides or monoiodides were destroyed by pyrolysis at the upper limit. Other trace level impurities such as olefinic or cycloolefinic perfluorocarbons were converted into oxidation products by pyrolysis. The purity of PFH following pyrolysis actually increased slightly as a consequence since these trace contaminants were effectively scrubbed from the samples. However, since the initial concentrations of the thermally-impacted impurities were so low, the net effect was trivial. A potential byproduct of exposure of perfluorohexane fluids to high temperatures is the production of perfluoroisobutene (PFiB), which is extremely toxic. An ultra-high sensitivity PFiB-specific analysis based on GC-MS with negative ion chemical ionization (NICI) detection was used to evaluate the samples following thermal stressing. The perfluorohexanes examined here under conditions reflective of the ISS TCCS environment showed no signs of PFiB production with an analytical detection limit of 10 part per billion (ppb v/v). Nomenclature BXF= Boiling eXperiment Facility ECLSS = Environmental Control and Life Support System EI = Electron Ionization FDA = Food and Drug Administration GC = Gas Chromatography HVAC = Heating, Ventilation and Air Conditioning ISRU = In-situ resource utilization

Critical Review of Thermal Decomposition of Per- and Polyfluoroalkyl Substances: Mechanisms and Implications for Thermal Treatment Processes

“…Perfluorohexane (n-C 6 F 14 ) did not decompose at temperatures less than 400 °C67 nor in 450 °C air even in the presence of a palladium catalyst. 68 Pure 2H-heptafluoropropane (HFP, C 3 HF 7 ) did not decompose at temperatures less than 640 °C, 65 and perfluoropentane (n-C 5 F 12 ) was not decomposed below 840 °C. 60 Xiao et al 66 studied thermal stability of PFASs sorbed to GAC and found that the temperatures needed to degrade PFCAs increased with an increasing number of perfluorinated carbons.…”

“…The order of the decomposition rates of PFCs (C x F y ) between 1000 °C and 1250 °C was CF 4 < C 2 F 6 < cyclo-C 5 F 10 < C 3 F 8 < n -C 4 F 10 < n -C 5 F 12 , indicating that the temperature required to decompose PFCs decreases with an increasing number of carbons, although isothermal rates (i.e., rates measured at a fixed temperature) may not perfectly represent overall thermal stability. Perfluorohexane ( n -C 6 F 14 ) did not decompose at temperatures less than 400 °C nor in 450 °C air even in the presence of a palladium catalyst . Pure 2 H -heptafluoropropane (HFP, C 3 HF 7 ) did not decompose at temperatures less than 640 °C, and perfluoropentane ( n -C 5 F 12 ) was not decomposed below 840 °C …”

Esterification Product Protection Strategies for Direct and Selective Methane Conversion

“…Although trifluoroacetic acid is not entirely 'fluorous' in nature, it is miscible in fluorous solvents such as perfluoroalkanes, materials that are inert and extremely stable even under aggressive oxidizing conditions. [18] Wang and co-workers observed 5-fold in product yield for homogeneously-catalyzed direct methane partial oxidation upon adding a relatively small quantity of perfluorooctane (< 20 vol.%) to the trifluoroacetic acid solvent. [19] The improved yields were attributed to the improved mass transfer afforded by the enhanced solubility of gases in the perfluorooctane.…”

Heterogeneously Catalyzed Aerobic Oxidation of Methane to a Methyl Derivative

“…To address the pressing limitations of syngas‐free methane conversion, we propose an approach that combines the heterogeneously catalyzed partial oxidation of methane with subsequent esterification of the product in a reaction medium of trifluoroacetic acid (TFA) diluted in an inert perfluoroalkane co‐solvent. Perfluoroalkanes, such as perfluorohexane, are inert, stable at elevated temperatures, [15] and can exhibit high solubilities for gases [16] . By diluting TFA to manageable concentrations of below 25 wt % in a non‐corrosive and oxidation‐resistant perfluoroalkane, a number of improvements are attained, namely: a strongly reduced corrosivity of the reaction medium; improved stability of a heterogeneous catalyst through operation in a milder environment; enhanced recovery of the methyl ester via a simple liquid‐liquid extraction with a non‐fluorous polar solvent; and improved hydrolysis conditions.…”