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 °C), thermochemical conversion of the CO2 requires good thermal stability of the ILs so that they can withstand reaction at elevated temperatures. ILs with noncoordinating 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 they exhibit CO2 capacity intermediate between those of monocationic ILs with the 2-cyanopyrrolide anion and tetraalkylphosphonium cations with longer (trihexyl(tetradecyl)) and shorter (triethyloctyl) chains. This is consistent with differences in the standard entropy of the reaction between the anion and CO2. Like their monocationic AHA counterparts, dicationic AHA ILs are sensitive to the presence of oxygen, which results in the formation of phosphine oxides, as determined by long-term isothermal stress tests with air purging.