Solubility of carbon dioxide in amine-functionalized ionic liquids: Role of the anions

Abstract

A series of amine-functionalized imidazolium cation-based ionic liquids (ILs) with different anions were synthesized and characterized by 1H NMR, 13C NMR, MS-ESI, FTIR, and EA. The ILs were investigated as potential absorbents for CO2 capture. The effects of replacing the anion with a corresponding cation were studied. The absorption capacities of the ILs for CO2 were investigated at 30°C and 50°C at ambient pressures (0–1.6bar). Amine-functionalized ILs displayed high absorption capacities toward CO2. The CO2 absorption capacities of the ILs increased with increasing pressure and decreased with increasing temperature. The results showed that the absorption capacities reached 0.45mol CO2 per mol of IL at 30°C. The most probable mechanism underlying the interactions between CO2 and the ILs was investigated using FTIR and 13C NMR, and the results showed that the absorption of CO2 in the amine-functionalized ILs could be described as a chemical process. The CO2 absorption results and detailed study indicated the predominance of a 1:2 mechanism, whereby the CO2 reacted with two IL molecules to form a carbamate group. The CO2 absorption capacity of the ILs toward each anion followed the trend: BF4-<DCA-<PF6-<TfO-<Tf2N-. The as-synthesized ILs were selective, thermally stable, provided long-lived operational stability, could be recycled at 70°C or under vacuum conditions, and could be used repeatedly.