Tuning CO2 Absorption in Hydrophobic Protic Ionic Liquids via Temperature and Structure

Conventional amine-based solvents such as monoethanolamine (MEA) and diethanolamine (DEA) are widely used for CO2 removal from natural gas but this technology suffers from drawbacks including high regeneration energy, solvent degradation, and corrosion issues. To overcome these limitations, this study investigates the use of newly synthesized hydrophobic protic ionic liquids (HPILs) composed of ammonium cations coupled with the bis(trifluoromethane)sulfonylimide ([Tf2N]−) anion for CO2 absorption using the pressure-drop method. The results show that CO2 solubility increases with pressure but decreases with temperature. Among the studied ionic liquids (ILs), [BEHA][Tf2N] exhibits the highest CO2 capacity at 298.15 K within the pressure range of 1–20 bar, which is consistent with its free volume (Vf) value. Furthermore, a comparison study indicates that all ILs demonstrate superior CO2 selectivity over methane (CH4) at 298.15 K. The recyclability study shows that [BEHA][Tf2N] maintains its structural integrity over two CO2 absorption cycles at 20 bar across all tested temperatures.