Novel Tetramethylammonium Cation-Based Ionic Liquid Solutions for Highly Efficient CO2 Absorption

In this work, a series of novel CO2 absorption solvents were prepared using solid tetramethylammonium cation-based ionic liquids and water. Herein, for the first time, tetramethylammonium cations-based ionic liquids ([N1111]­[AHA], namely [N1111]­[Py], [N1111]­[Im], and [N1111]­[1,2,4-Tz]) with aprotic heterocyclic anion were synthesized. The viscosity of these absorbents ([N1111]­[AHA]/H2O), composed of different contents of [N1111]­[AHA] and H2O, was measured, indicating that their viscosity was below 50 mPa·s at 313.15 K. Moreover, the anion structure significantly affected the CO2 absorption performance. The CO2 absorption capacity increased in the order of [N1111]­[Py]/H2O > [N1111]­[Im] /H2O > [N1111]­[1,2,4-Tz]/H2O, with only [N1111]­[1,2,4-Tz]/H2O presenting excellent reversibility (5 cycles). The optimal solvent composition was 80 wt % [N1111]­[1,2,4-Tz] and 20 wt % H2O, yielding a CO2 absorption capacity of 0.268 g CO2/g absorbent and the ideal selectivity for CO2/N2 and CO2/O2 was 1217.8 and 911.1, respectively, at 313.15 K and 100 kPa CO2. The study of the CO2 absorption mechanism was investigated using 13C NMR, in situ FTIR, and DFT calculations, showing that water can cooperate with ILs to absorb CO2. Carbamate and carbonate were formed during CO2 absorption, which is beneficial for CO2 capture. These novel [N1111]­[AHA] solutions are potential absorbents for CO2 separation in flue gas.