Quantitative analysis of physical absorption behavior of CO2 in imidazolium-based ionic liquids containing bis(trifluoromethylsulfonyl) imide and tetrafluoroborate anion by Raman spectroscopy

To achieve a carbon-neutral society, ionic liquids (ILs) are widely used as solvents for CO2 separation/recovery and as catalysts for CO2 fixation reactions. Understanding the dissolution behavior of CO2 is critical in these applications. Raman spectroscopy enables in-situ analysis of CO2 dissolution behavior but suffers from limited quantitativeness. In this study, spectral analysis based on Raman spectroscopy was conducted for CO2 dissolution in four imidazolium-based ILs containing the [Tf2N] (bis(trifluoromethylsulfonyl)imide) and the [BF4] (tetrafluoroborate) anion temperature at 298-333 K and pressure up to 6.3 MPa. These four types were selected as model ionic liquids with different cation chain lengths and CO2 solubility ranges. The peak intensity caused by symmetric stretching vibrations of CO2 (about 1380 cm(-1)) increased as the amount of dissolved CO2 increased although the peak intensity of IL (about 1420 cm(-1)) did not change. The intensity ratios for the peak of CO2 against that of IL were defined to assess Raman spectra quantitatively. A comparison of the obtained intensity ratios with CO2 solubility (CO2 mole ratio to IL: x '(CO2) = nCO(2) /nIL, where ni is the mole of component i) revealed that a consistent linear relationship could represent the data for all four ILs across three temperatures.