High CO2 absorption capacity by chemisorption at cations and anions in choline-based ionic liquids

Abstract

The effect of CO₂ absorption on the aromaticity and hydrogen bonding in ionic liquids is investigated. Five different ionic liquids with choline based cations and aprotic N-heterocyclic anions were synthesized. Purity and structures of the synthesized ionic liquids were characterized by ¹H and ¹³C NMR spectroscopy. CO₂ capture performance was studied at 20 °C and 40 °C under three different pressures (1, 3, 6 bar). The IL [N_1,1,6,2OH][4-Triz] showed the highest CO₂ capture capacity (28.6 wt%, 1.57 mol of CO₂ per mol of the IL, 6.48 mol of CO₂ per kg of the ionic liquid) at 20 °C and 1 bar. The high CO₂ capture capacity of the [N_1,1,6,2OH][4-Triz] IL is due to the formation of carbonic acid (-OCO₂H) together with carbamate by participation of the -OH group of the [N_1,1,6,2OH]⁺ cation in the CO₂ capture process. The structure of the adduct formed by CO₂ reaction with the IL [N_1,1,6,2OH][4-Triz] was probed by using IR, ¹³C NMR and ¹H-¹³C HMBC NMR experiments utilizing ¹³C labeled CO₂ gas. ¹H and ¹³C PFG NMR studies were performed before and after CO₂ absorption to explore the effect of cation-anion structures on the microscopic ion dynamics in ILs. The ionic mobility was significantly increased after CO₂ reaction due to lowering of aromaticity in the case of ILs with aromatic N-heterocyclic anions.