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

Abstract

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: $x_{{\text{CO}}_2}^{\prime }=n_{{\text{CO}}_2}/n_{\text{IL}}$, 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.