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

Abstract

Density, viscosity, and CO2 solubility of a series of 10 1-n-alkylimidazoles with chain lengths ranging from methyl (C1) to tetradecyl (C14) 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 CO2 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 CO2 were similar in both types of solvents, indicating that no advantage (or disadvantage) in CO2 uptake is achieved by transforming a neutral 1-n-alkylimidazole to a charged IL solvent. While CO2 solubility levels in each are insufficient to provide a viable solvent for postcombustion CO2 capture applications, 1-n-alkylimidazoles can be used in combination with amines as a low-volatility, high-capacity solvent to capture CO2, using only inexpensive and readily available components. The viscosity of a highly CO2-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 CO2 capture as the solution can easily exceed the stoichiometric limitations of 2 mol of MEA/(1 mol of CO2). Thus, while certainly more volatile than IL-based analogues, 1-n-alkylimidazole solvents may offer some unique capabilities and advantages in CO2 capture processes.

Molecular Simulation of the Thermophysical Properties of N-Functionalized Alkylimidazoles

“…3 These NFI compounds are essentially neutral analogues to their IL counterparts, but they still retain the low volatility and favorable solvation properties associated with ILs. Although there have been many molecular simulation studies performed to characterize the thermophysical properties of imidazolium-based ILs, we instead focus on the molecular-level behavior of a series of alkylimidazoles, and we compare this behavior to recent experimental benchmarks available in the literature.…”

Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity

“…[1][2] Other non-aqueous alternative solvent classes under development are also limited by high viscosity behavior upon loading with CO 2 . Task-specific ionic liquids 7 are two-component systems that turn into carbamate-ammonium or carbamate-imidazolium salts after carbon binding 8 .…”

Imidazole-Based Solvents and Membranes for CO2 Capture Applications

“…[17] While 1methylimidazole was obtained commercially, the rest of these imidazoles were synthesized in our laboratory at scales approaching 100 g with yields typically > 85%. [17] While 1methylimidazole was obtained commercially, the rest of these imidazoles were synthesized in our laboratory at scales approaching 100 g with yields typically > 85%.…”