Enhanced CO2 capture by reducing cation–anion interactions in hydroxyl-pyridine anion-based ionic liquids

In this work, an efficient strategy for improving CO2 capture based on anion-functionalized ionic liquids (ILs) by reducing cation–anion interactions in ILs was reported. The influence of the cationic species on CO2 absorption was investigated using 2-hydroxyl pyridium anions ([2-Op]) as a probe. CO2 capture experiments indicated that the CO2 absorption capacity in [2-Op] anion-based ILs varied from 0.94 to 1.69 mol CO2 per mol IL at 30 °C and 1 atm. Spectroscopic analysis and quantum chemical calculations suggested that the increase of the CO2 absorption capacity may be ascribed to the reduction of the strength of cation–anion interactions in ILs, and stronger cation–anion interactions would make one CO2 site in the [2-Op] anion inactive. Furthermore, the effect of the cation unit on the anion was evidenced by FT-IR spectra, implying that strong interactions between ions may lead to the decrease of the IR absorption wavenumber of hydroxy pyridium and work against CO2 capture. Following this strategy, it was finally found that [Ph-C8eim][2-Op] (Ph-C8eim = 1-N-ethyl-3-N-octyl-2-phenylimidazolium) with weaker cation–anion interactions exhibited a significant increase in the CO2 uptake capacity, and extremely high capacities of 1.69 and 1.83 mol CO2 per mol IL could be achieved at 30 and 20 °C, respectively. The study presented here would be helpful for further designing novel and effective ILs for advancing CO2 capturing performance.