Solvation of Carbon Dioxide in C4mimBF4 and C4mimPF6 Ionic Liquids Revealed by High-Pressure NMR Spectroscopy

摘要

Imidazolium-based ionic liquids (Im ILs) show relatively high CO, solubility and have been frequently investigated for CO2 separation and absorption processes. In most cases, Im ILs simply absorb the CO, physically, and the solubility is mainly controlled by the nature of the anion with the cation playing a minor role. However, the optimal anion/cation combination or structural modification for tuning CO, solubility and selectivity requires a deeper understanding of the molecular details of CO2 solvation. The rationalization of CO2 solubility in Im ILs relies mainly in the combination of solubilization studies with molecular dynamics (MD) simulations, but no doubt the importance of the theoretical approaches there is still a lack of experimental data aimed to directly probe specific CO2-IL interactions, such as NMR or ATR-IR spectroscopy or X-ray diffraction, to complement the theoretical studies. Herein we combine high-pressure (HP) NMR techniques with MD simulations to study the microscopic behavior of CO2 dissolved in Im ILs in terms of solute-solvent interactions. Using l-butyl-3-methylimidazolium hex-afluorophosphate and l-butyl-3-methylimidazolium tetra-fluoroborate, (C4mimPF6 and C4mimBF4, respectively) we show that CO, solubility is essentially determined by the microscopic structure of the IL which is nanostructured in polar and nonpolar domains, and we present a solvation model that integrates the most relevant previously reported theoretical and experimental data.