(126i) Thermal Stability and CO2 Uptake of Dicationic Ionic Liquids Containing 2-Cyanopyrrolide Anions

Aprotic-heterocyclic anion (AHA) ionic liquids (ILs) have the potential to be separation and reaction media for carbon capture and utilization (CCU) processes. While CO₂ capture may occur at moderate temperatures (e.g., between ambient and 50 ⁰C), thermochemical conversion of the CO₂ requires good thermal stability of the ILs, so that they can withstand reaction at elevated temperatures. ILs with non-coordinating anions such as bis(trifluoromethylsulfonyl)imide ([Tf₂N]^-) have shown improved thermal stability when the cation is modified to have 2+ charge; i.e., so called dicationic ILs. To our knowledge, no previous studies have paired dications with AHAs. A series of dicationic ILs composed of di(tetraalkylphosphonium) cations with tributyl chains and linkers ranging from propyl to undecyl, paired with 2-cyanopyrrolide anions, do show improved thermal stability compared to their monocationic AHA IL equivalents. Interestingly, the dicationic ILs all have similar CO₂ uptake per anion, although exhibiting CO₂ capacity intermediate between monocationic ILs with the 2-cyanopyrrolide anion and tetraalkylphosphonium cations with longer (trihexyltetradecyl) and shorter (triethyloctyl) chains. This is consistent with differences in the standard entropy of reaction between the anion and CO₂. Like their monocationic AHA counterparts, dicationic AHA ILs are sensitive to presence of oxygen, which results in the formation of phosphine oxides, as determined by long-term isothermal stress tests with air purging.