Absorption Process of CO₂ into 1,3-Dimethylimidazolium Dimethyl Phosphate Ionic Liquid Revealed by Band Shift and Bandwidth Broadening in In Situ Raman Spectra

Ionic liquids (ILs) have attracted attention as medium capable of highly molecule-selective absorption and separation. In this study, the absorption process of CO2 into ILs was directly observed by in situ Raman scattering measurements using an originally designed high-pressure apparatus. The IL, 1,3-dimethylimidazolium dimethyl phosphate ([C1mim][DMP]), has been developed as an absorption medium for both CO2 and H2O. The band shift, the Raman intensity, and the bandwidth of the CO2 Fermi dyad bands were analyzed in order to evaluate the CO2 absorption mechanism into [C1mim][DMP]. The Raman band of absorbed CO2 was mostly shifted to a lower frequency than that of neat CO2 immediately after the start of absorption and then continuously increased with the absorption time, indicating that the vibrational state of CO2 is stabilized by the interaction with [C1mim][DMP], especially in the early stage of absorption. The result suggests that the CO2 molecules are absorbed into the interaction sites at the early stage of the absorption, and then, as the absorption progresses further, CO2 is preferentially absorbed into the voids in the IL as the next step. The Raman bandwidth gradually increased as the CO2 absorption progressed. The bandwidth broadening supports the absorption mechanism derived from the change in the Raman shift: The increase in bandwidth could be caused by the increase in inhomogeneity of molecular distribution of CO2 due to the change in absorption state from interaction site to free space. For comparison, the CO2 absorption process in an IL with more free volume, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, was also studied.