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

Abstract

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.