Effects of anions on absorption capacity of carbon dioxide in acid functionalized ionic liquids

A series [Cmmim][X] of imidazolium cation-based ILs, with acid functional group on the alkyl side-chain has been synthesized and characterized by various techniques like 1H, 13C NMR, MS-ESI, FTIR and EA. The absorption capacity of CO2 for various ILs was investigated at 30 and 50 °C at ambient pressure (0–1.6 bar). Specifically, the remarkable effect of different anions on CO2 absorption capacity has been evaluated with same cation. Acid functionalized ILs show significantly high absorption capacity for CO2. The CO2 absorption capacity of ILs increased with a rise in pressure and decreased when temperature was raised. Results showed that absorption capacity reached about 1.0 mol CO2 per mol of IL at 30 °C. The most probable mechanism of interaction of CO2 with ILs was investigated using FTIR and 13C NMR and result shows that the absorption of CO2 in ether functionalized ILs is a chemical process. The CO2 absorption results and detailed study indicate the predominance of 1:1 mechanism, where the CO2 reacts with one IL to form carbamic acid. The CO2 absorption capacity of ILs for different anions follows the trend: BF4 < DCA ~ PF6 ~ TfO < Tf2N. Moreover, the as-synthesized ILs are selective, thermally stable, long life operational and can be recycled at a temperature of 70 °C or under vacuum and can be used repeatedly.