In collaboration with Umicore, SIM² KU Leuven researchers investigated the thermal stability of the ionic liquid Cyphos IL 101 under various experimental conditions for possible high-temperature applications. The work has been published in Phys. Chem. Chem. Phys. (Leuven, 21-5-2018)

Rationale

Ionic liquids (ILs) are solvents that consist entirely of ions, mostly bulky organic cations and large weakly coordinating anions, resulting in solvents being liquid below 100 °C. Due to their wide liquidus range, low volatility and flammability, and high thermal and electrochemical stability, multiple high-temperature applications for ionic liquids have been proposed already. These applications require ionic liquids with a high long-term thermal stability. Thermogravimetric analysis (TGA) is the most commonly used technique to determine the thermal stability of a substance.

Dynamic TGA studies of phosphonium ionic liquids have led to reported thermal stabilities of 300 °C or higher for many species. Phosphonium ionic liquids are considered for high-temperature industrial processes due to their availability and relatively low cost. Dynamic TGA studies give often an overestimation of the real thermal stability. The chosen technique as well as the experimental parameters can influence the thermal stability.

In this paper, the thermal stability of commercially available Cyphos IL 101 is studied. The effect of the nature of the atmosphere (air or inert gas), the purity of the sample, the heating rate and presence of a metal on the short-term and long-term stability of commercial Cyphos IL 101 is investigated. The thermal decomposition products are characterized using thermogravimetric analysis coupled to mass spectrometry (TGA-MS). The presence of impurities in commercial Cyphos leads to an underestimation of the thermal stability due to their decomposition at lower temperatures. Also, the environment and the presence of metal salts has an influence on the thermal stability. A lower activation energy for the oxidative decomposition than for the nonoxidative degradation was reported. The opposite is true for the addition of metal chlorides to the ionic liquid. The chloride anions are coordinated to the metal ion, so that the Lewis basicity of the anions is reduced and the thermal stability of the ionic liquid is increased. Dynamic TGA results are an overestimation of the real thermal stability even at low heating rates. From an industrial application point of view, ionic liquids must be able to endure higher temperature for longer periods of time. In this case, static TGA offers a better alternative to dynamic TGA.

Full reference paper

Clio Deferm, Arne Van den Bossche, Jan Luyten, Harald Oosterhof, Jan Fransaer, Koen Binnemans,  Thermal stability of trihexyl(tetradecyl)phosphonium chloride, Phys. Chem. Chem. Phys., 2018, 20, 2444–2456, DOI: 10.1039/c7cp08556g

Acknowledgements

This research was supported by the Flemish Institute for the Promotion of Innovation by Science and Technology (IWT Vlaanderen) via a Baekeland PhD fellowship to Clio Deferm (IWT 130305) and a PhD fellowship to AVDB and by the Umicore Group Research & Development. TGA-MS measurements were performed in the analytical laboratory of Umicore Group Research & Development. Mass spectrometry was made possible by the support of the Hercules Foundation of the Flemish Government (grant 20100225–7). We thank Prof. Ken Seddon (Queen’s University Belfast, UK) for providing a copy of the PhD thesis of G. Adamová.

Bio main author

clio defermClio Deferm is a doctoral researcher working with Prof. Koen Binnemans in the group of LIC at SIM² KU Leuven. She graduated in Chemistry (BChem) in 2011 at UHasselt and (MChem) in 2013 at KU Leuven. Her PhD is in collaboration with Umicore.