Equimolar CO₂ capture by imidazolium-based ionic liquids and superbase systems

Imidazolium-based ionic liquids continue to attract interest in many areas of chemistry because of their low melting points, relatively low viscosities, ease of synthesis, and good stabilities against oxidative and reductive conditions. However, they are not totally inert under many conditions due to the intrinsic acidity of hydrogen at the C-2 position in the imidazolium cation. In this work, this intrinsic acidity was exploited in combination with an organic superbase for the capture of CO₂ under atmospheric pressure. During the absorption of CO₂, the imidazolium-based ionic liquid containing an equimolar superbase reacted with CO₂ to form a liquid carboxylate salt so that the equimolar capture of CO₂ with respect to the base was achieved. The effects of ionic liquid structures, types of organic superbases, absorption times, and reaction temperatures on the capture of CO₂ were investigated. Our results show that this integrated ionic liquid–superbase system is capable of rapid and reversible capture of about 1 mol CO₂ per mole of ionic liquid. Furthermore, the captured CO₂ can be readily released by either heating or bubbling N₂, and recycled with little loss of its capture capability. This efficient and reversible catch-and-release process using the weak acidity of the C-2 proton in nonvolatile imidazolium-based ionic liquids provides a good alternative to the current CO₂ capture methods that use volatile alkanols, amines, or water.