Reaction Thermochemistry for Carbon Dioxide Absorption by Aprotic <i>N</i>-Heterocyclic Anion Ionic Liquids

Ionic liquids (ILs) containing aprotic N-heterocyclic anions (AHAs) are attractive CO2 capture solvents. However, CO2 uptake has only been measured in a limited subset of AHAs (27 tetraalkylphosphonium AHA ILs with 14 unique AHAs). To evaluate the broader range of possible AHA ILs with tetraalkylphosphonium cations, we develop a CO2 uptake model which is a function of temperature, pressure, and anion–CO2 reaction enthalpy calculated using electronic structure methods (DFT). The resulting model accurately describes CO2 uptake within ±0.046 mol CO2 per mole of AHA ILs on average. We use this model to predict the CO2 uptake for ILs of an additional 267 AHAs, whose uptakes have not been measured, for post-combustion carbon capture. Several identified candidates demonstrate similar or greater CO2 cyclic capacity than the best performing aqueous amines. Additionally, we show that the anion–CO2 reaction enthalpy, and hence the anion’s basicity, can be correlated with a novel COSMO-RS descriptor─the σ-potential slope of the uncomplexed anion. This requires only a single DFT calculation, roughly halving the computational requirement to construct the uptake model. Finally, our findings highlight the potential applicability of the σ-potential slope as an electronic descriptor in screening chemical reactivity across various molecular motifs.