(29c) Ionic Liquids for Post-Combustion Carbon Dioxide Capture

Post-combustion CO₂ capture is a necessary strategy for reducing CO₂ emissions to the atmosphere since over 50% of electricity in the U. S. is currently produced by conventional coal-fired power plants. The currently available capture technology involves absorbing the gas into an aqueous amine solution. This process suffers from a number of problems, including a large energy penalty for regeneration of the absorbent that translates into an unacceptably large increase in the cost of electricity. Thus, new breakthrough technologies are needed. Ionic liquids present one such possibility. Ionic liquids (ILs) are organic salts with low melting points, many below room temperature. Even though they are liquids, they have negligible vapor pressure. Thus, they have an advantage over conventional solvents for absorption of CO₂ from flue gas because they would not contaminate the purified gas stream. Typical ILs are composed of a imidazolium, pyridinium, ammonium or phosphonium cation with any of a wide variety of anions. Their properties can be varied tremendously by the choice of anion, cation and substituents.

Using accurate gravimetric microbalances, combined with atomistic simulations, we have found that even common ILs like 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) have excellent selectivity for CO₂ relative to N₂ and other components in flue gas based on pure physical absorption. Here we report on the solubility of CO₂ in a wide variety of ILs that include various functionality designed to further increase CO₂ solubility. These include fluoroalkyl chains, sulfates and ethers. In addition we show results for ILs designed to chemically complex with the CO₂, including acetates, lactates, malonates and other functional groups. The goal with these compounds is to form weak chemical complexes such that the carrying capacity is high but the energy requirements to regenerate the sorbent are not prohibitive. These efforts have resulted in a thirty five fold increase in the uptake of CO₂ by ILs, while maintaining good selectivity. We have achieved carrying capacities comparable to aqueous amine solutions, but with dramatically reduced regeneration energy requirements. We also show that SO₂ can be removed with the CO₂ in a single step, which presents a significant advantage of IL sorption over amine scrubbing. These results show how careful design of the ionic liquid can lead to an optimal combination of both physical absorption and weak chemical complexation, resulting in high carbon dioxide solubilities and selectivities, plus reasonable desorption conditions.