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

• Raman spectra of ionic liquid solution with high-pressure CO 2 were measured. • Quantitative relationships between intensity ratio and CO 2 solubility were built. • Solubility of CO 2 in the four types of ionic liquids was expressed on a single curve. To achieve a carbon–neutral society, ionic liquids (ILs) are widely used as solvents for CO 2 separation/recovery and as catalysts for CO 2 fixation reactions. Understanding the dissolution behavior of CO 2 is critical in these applications. Raman spectroscopy enables in-situ analysis of CO 2 dissolution behavior but suffers from limited quantitativeness. In this study, spectral analysis based on Raman spectroscopy was conducted for CO 2 dissolution in four imidazolium-based ILs containing the [Tf 2 N] (bis(trifluoromethylsulfonyl)imide) and the [BF 4 ] (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 CO 2 solubility ranges. The peak intensity caused by symmetric stretching vibrations of CO 2 (about 1380 cm −1 ) increased as the amount of dissolved CO 2 increased although the peak intensity of IL (about 1420 cm −1 ) did not change. The intensity ratios for the peak of CO 2 against that of IL were defined to assess Raman spectra quantitatively. A comparison of the obtained intensity ratios with CO 2 solubility (CO 2 mole ratio to IL: x CO 2 ′ = n CO 2 / n IL , where n i is the mole of component i ) revealed that a consistent linear relationship could represent the data for all four ILs across three temperatures.