Multi-molar CO2 capture beyond the direct Lewis acid–base interaction mechanism

Abstract

Some singly charged ionic liquids (ILs) have been reported to absorb multi-molar CO₂. However, the conventional acid(CO₂)–base(anion) interaction picture leads to too weak CO₂ binding to support the high uptake. Later, a so-called “cation-channel” mechanism assuming the cation-to-anion proton transfer successfully explains the over equimolar CO₂ uptake of some phosphonium-based ILs. Here, by employing the density functional theory (DFT) calculations, we extend the proton transfer mechanism to incorporate imidazole- and ammonium-based ILs as well. For imidazole-based ILs, carbene molecules formed after the proton transfer can react strongly with CO₂. More importantly, for ammonium-based ILs, the proton transfer process is feasible only with the help of CO₂ molecules. Furthermore, compared to the one IL ion pair model, the model consisting of two IL ion pairs can result in stronger CO₂ absorption because it can describe the intermolecular hydrogen bonds more appropriately, especially after incorporating CO₂ molecules. The relative acidity and basicity of cations and anions in ILs may be crucial for understanding their functionalization as ILs.