Matthew S. Shannon

Reactive and Reversible Ionic Liquids for CO2 Capture and Acid Gas Removal

The use of ionic liquids (ILs) for CO2 capture and the removal of acid gases from natural gas and other industrial processes has been one of the foremost research applications for this unique class of non-volatile solvents. However, most of the most broadly studied ILs lack sufficient capacities for CO2 and other acid gases such as H2S, SO2, etc. to be viewed as viable replacements for aqueous amine technologies which have been used industrially for acid gas removal for nearly a century. Furthermore, many of the most well-known ILs are too viscous to be used within conventional process equipment and are likely too costly for use at large scales. As the negligible vapor pressure of ILs is an attractive property for gas separations, it is desirable to find new ILs with improved properties that can be synthesized from lower cost starting materials and/or natural products. Recently, new reactive and reversible IL solvents have emerged in efforts to improve upon the CO2 capacity, physical properties and costs of IL-based gas separation technologies. In this review, we detail the differences between these novel approaches and the standard crop of ILs that have been reported in the literature. The various strategies that have been employed to develop these materials for energy-related separation applications will be examined, with an emphasis on how chemistry and physical properties relate to the demands of efficient chemical process engineering. Where applicable, comparisons to conventional (i.e., aqueous amine) solvents will be made so as provide baselines to commercial technologies. Finally, we introduce the concept of imidazoles and imidazole-amine hybrid solvents as another tunable platform for the removal of CO2, SO2, and H2S.

Properties of alkylbenzimidazoles for CO 2 and SO 2 capture and comparisons to ionic liquids

To date, few reports have been concerned with the physical properties of the liquid phases of imidazoles and benzimidazoles-potential starting materials for a great number of ionic liquids. Prior research has indicated that alkylimidazole solvents exhibit different, and potentially advantageous physical properties, when compared to corresponding imidazolium-based ionic liquids. Given that even the most fundamental physical properties of alkylimidazole solvents have only recently been reported, there is still a lack of data for other relevant imidazole derivatives, including benzimidazoles. In this work, we have synthesized a series of eight 1-n-alkylbenzimidazoles, with chain lengths ranging from ethyl to dodecyl, all of which exist as neat liquids at ambient temperature. Their densities and viscosities have been determined as functions of both temperature and molecular weight. Alkylbenzimidazoles have been found to exhibit viscosities that are more similar to imidazolium-based ILs than alkylimidazoles, owed to a large contribution to viscosity from the presence of a fused ring system. Solubilities of CO2 and SO2, two species of concern in the emission of coal-fired power generation, were determined for selected alkylbenzimidazoles to understand what effects a fused ring system might have on gas solubility. For both gases, alkylbenzimidazoles were determined to experience physical, non-chemically reactive, interactions. The solubility of CO2 in alkylbenzimidazoles is 10%–30% less than observed for corresponding ILs and alkylimidazoles. 1-butylbenzimidazole was found to readily absorb at least 0.333 gram SO2 per gram at low pressure and ambient temperature, which could be readily desorbed under an N2 flush, a behavior more similar to imidazolium-based ILs than alkylimidazoles. Thus, we find that as solvents for gas separations, benzimidazoles share characteristics with both ILs and alkylimidazoles.

Poly(ionic liquid) superabsorbent for polar organic solvents.

A simple, polymerized ionic liquid (poly(IL)) based on methylimidazolium cations tethered to a polystyrene backbone exhibits superabsorbent behavior toward polar organic solvents, most notably propylene carbonate (PC) and dimethyl sulfoxide (DMSO), wherein the poly(IL) was observed to swell more than 390 and 200 times (w/w) its original mass, yet absorbs negligible quantities of water, hexanes, and other solvents, many of which were miscible with the IL monomer. Although solubility parameters and dielectric constants are typically used to rationalize such behaviors, we find that poly(IL)-solvent compatibility is most clearly correlated to solvent dipole moment. Poly(IL) superabsorbency is not reliant upon the addition of a cross-linking agent.