Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anions for CO₂ Separation

Development of innovative carbon dioxide (CO₂) capture technologies is critical for maintaining fossil fuel as an affordable and environmentally benign energy resource. Post-combustion CO₂ capture offers one of the best near-term potentials for reducing greenhouse gas emissions, and commercially available technologies use aqueous amine solutions to mitigate CO₂ emissions from coal-fired power plants. However, due to the large heat of reaction and the presence of water, this process requires large amounts of energy to regenerate the trapping solvent draining up to 30 % of the energy created by burning the coal in the first place.

Non-volatility, good thermal stability and high CO₂ solubility make Ionic Liquids (ILs) an attractive replacement for current volatile solvents. ILs are low-melting (T_m<100 °C) salts of bulky cations and anions and their properties can be carefully tailored by choosing or modifying the anion, the cation, and their substituents.

In this study, ILs with Aprotic Heterocyclic Anions (AHAs) are examined. Taking advantage of 'tunability', the enthalpy of reaction with CO₂ can be controlled over a wide range suitable for gas separations. Lowering the enthalpy of reaction can help to reduce the heat required to regenerate the solvent, or in this case, the IL. A new type of ILs termed Phase Change Ionic Liquids (PCILs) are also introduced. They undergo a phage change from solid to liquid when they react with CO₂. Utilizing the heat of fusion (ΔHfus), the ideal PCIL process has a potential to reduce the parasitic energy from 30% down to about 14%. Additionally, the effect of different types of cations on the physical properties and CO₂ solubility are examined by paring AHAs with imidazolium, ammonium, or phosphonium cations with different alkyl substituents.

To summarize, the new families of tunable ILs are explored as a viable alternative for current CO₂ capture technologies. While the field of ILs is relatively new and ample opportunity should exist to optimize the properties of ILs for CO₂ capture, this study offers a promising step forward in the search for energy-optimal carbon capture materials.