Properties of Alkylimidazoles as Solvents for CO₂ Capture and Comparisons to Imidazolium-Based Ionic Liquids

Density, viscosity, and CO 2 solubility of a series of 10 1- n -alkylimidazoles with chain lengths ranging from methyl (C 1 ) to tetradecyl (C 14 ) were characterized. Density and viscosity values were comparable to many common organic solvents over a temperature range of 20–80 °C. The measured data were utilized to develop empirical models for these physical properties with respect to temperature and the contribution of the n -alkyl chain. Solubility of CO 2 in 1- n -alkylimidazoles at ambient temperature (25 °C) and low pressures (3–7 atm) was found to be less than most common organic solvents, even though the 1- n -alkylimidazole motif offers opportunities to tune the molecule’s solubility parameter. This effect was attributed to lower fractional free volume (FFV) that is available in molecules with systematically variable side chains. 1- n -Alkylimidazoles were less dense than most 1- n -alkyl-3-methylimidazolium-based ionic liquids (ILs), with differences ranging from 10 to 50%, based on contributions to increased density from the anion. However, much larger differences in viscosities were observed between the two classes of compounds, with ILs at least an order of magnitude more viscous than their neutral 1- n -alkylimidazole counterparts. Solubility levels of CO 2 were similar in both types of solvents, indicating that no advantage (or disadvantage) in CO 2 uptake is achieved by transforming a neutral 1- n -alkylimidazole to a charged IL solvent. While CO 2 solubility levels in each are insufficient to provide a viable solvent for postcombustion CO 2 capture applications, 1- n -alkylimidazoles can be used in combination with amines as a low-volatility, high-capacity solvent to capture CO 2, using only inexpensive and readily available components. The viscosity of a highly CO 2 -rich liquid phase formed between monoethanolamine (MEA) and 1-butylimidazole was found to be 85–100 cP at 298 K, which is less viscous than many neat ILs. Initial data indicate 1-butylimidazole has a synergistic effect on CO 2 capture as the solution can easily exceed the stoichiometric limitations of 2 mol of MEA/(1 mol of CO 2 ). Thus, while certainly more volatile than IL-based analogues, 1- n -alkylimidazole solvents may offer some unique capabilities and advantages in CO 2 capture processes.