Reactivity of Ionic Liquids: Studies on Thermal Decomposition Behavior of 1-Butyl-3-methylimidazolium Tetrafluoroborate
2020
Knorr, Monika | Icker, Maik | Efimova, Anastasia | Schmidt, Peer
The Ionic Liquid 1-butyl-3-methylimidazolium tetrafluoroborate [C₄C₁im]BF₄ serves as a commonly solvent in inorganic material synthesis and analytics. Nevertheless, its application is frequently associated with trial and error approaches. Thereupon, detailed knowledge on the thermal behavior is the key information for understanding the reactivity of [C₄C₁im]BF₄. 1-butyl-3-methylimidazolium tetrafluoroborate behaves as a glass in the cold, its glass transition temperature being ϑg = −83 °C. During heating with 10 K·min⁻¹ [C₄C₁im]BF₄ appears to be stable above 350 °C with onset temperatures ϑₒₙₛₑₜ, DSC = 375 °C, ϑₒₙₛₑₜ, DTG = 422 °C, and ϑₒₙₛₑₜ, TG = 437 °C. Thereby, thermal decomposition occurs in a single step reaction forming 1-methyl-1H-imidazole (CH₃C₃H₃N₂ or C₄H₆N₂), but-1-ene (C₄H₈), fluoromethane (CH₃F) and boron trifluoride (BF₃) as main species, as determined by thermogravimetry coupled with mass spectrometry and FTIR spectroscopy. To be more specific in thermal behavior, the temperature and time dependent stability is evaluated here on the basis of the kinetic model of maximum operation temperature − MOT. Clearly, thermal stability rises with application time, thus being 193 °C for one hour, while reaching only 141 °C for one day, and 114 °C for one week. The incipient decomposition (≤ 1 %) at the calculated time dependent maximum operation temperature finally is verified by optical analysis, infrared (IR), and nuclear magnetic resonance (NMR) spectroscopy.
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