CO2 capture by carboxylate ionic liquids: fine-tuning the performance by altering hydrogen bonding motifs†

The use of ionic liquids (ILs) for CO₂ capture has drawn significant attention due to their tuneable structural design and non-volatility. Among these, carboxylate ionic liquids, particularly in the presence of water as a hydrogen bond donor, show great promise due to their effective chemical sorption mechanism, leading to bicarbonate, and low regeneration energy requirements. The additional presence of hydroxyl groups in their structures is expected to affect both hydrogen bonding network and CO₂ capture capacity. This study systematically investigates the role of hydroxyl moieties in tetraalkylammonium cations of carboxylate ionic liquid hydrates on their physicochemical properties and CO₂ solubility. The ILs studied are based on the trimethylpropylammonium cation or choline as a hydroxyl-containing analogue, paired with either acetate or propionate. The solubilities of CO₂ in each IL at different H₂O/IL hydration ratios were determined by a headspace gas chromatography method. The effects of water, in addition to those of cationic hydroxyl, on CO₂ capture performance were evaluated. To this end, nuclear magnetic resonance and infrared spectroscopy results are presented and analyzed to propose distinct chemical sorption mechanisms in either scenario.