Low-viscous diamino protic ionic liquids with fluorine-substituted phenolic anions for improving CO2 reversible capture

Ionic liquids (ILs) have been widely investigated as the promising green absorbents for CO2 capture. In this work, four low-viscous diamino protic ionic liquids (PILs), including N,N-dimethyl-1,3-propane diamine 2-fluorophenolate ([DMAPAH][2F-PhO]), N,N-dimethyl-1,3-propane diamine 3-fluorophenolate ([DMAPAH][3F-PhO]), N,N-dimethyl-1,3-propane diamine 3-fluorophenolate ([DMAPAH][4F-PhO]), and N,N-dimethyl-1,3-propane diamine 3,5-difluorophenolate ([DMAPAH][3,5F-PhO]), were prepared and evaluated for CO2 capture. The solubilities of CO2 in these PILs were measured using an isochoric saturation technique at temperatures from 303.2 to 333.2 K and pressure up to 1.5 bar. It was found that the PILs are excellent absorbent for CO2 capture with the best absorptive capacity of 3.99 mol CO2/kg IL (0.86 mol CO2/mol IL), which is superior or competitive to most reported ILs. Furthermore, the captured CO2 was easy to release by heating and decompression, leading regeneration of the PILs. In addition, a plausible mechanism of CO2 absorption in these PILs was proposed via a combination of chemical and physical absorption by NMR, FTIR, and mass spectrum (MS).