Discovery of New Ionic Liquids for Co2 Capture

CO2 concentrations in the atmosphere have been steadily increasing over the years, mainly due to CO2 being emitted from the burning of fossil fuels. The ever increasing concentration of CO2 is starting to have substantial effects on the global climate and the effects will continue to multiply if the problem is not addressed. While the ideal would be to have all energy sources be carbon-neutral, coal will still be a major energy source for the foreseeable future. Because of this, finding new and alternative processes for controlling CO2 emissions becomes a vital necessity.

Post-combustion CO2 capture is the most clear-cut and promising path to limiting carbon emissions but its implementation relies on the discovery of energy-efficient means of separating CO2 from other flue gas components such as N2, O2, H2O and other trace gases. Using the presently available amine absorption technologies, separating the CO2 from the flue gas stream is estimated to consume more than 30% of the power produced by the plant, which is far above the theoretical minimum work needed. An alternative liquid absorbent is needed that overcomes the pitfalls of the amine absorbents, such as ionic liquids.

Ionic liquids (ILs) are low-melting salts of bulky cations and anions and are effectively non-volatile. Common ILs physically absorb CO2 selectively over O2 and N2, but the physical solubility of CO2 in the IL is too low to be practical. Therefore, to achieve the chemical absorption seen in the amine absorbents but also keep the properties of ILs, an amine group is tethered to the ionic liquid.

In this work, we use density functional theory and first principles thermodynamics to describe the ionic liquid systems. We show that by tethering an amine functional group to the anion, such as an amino acid, on an ionic liquid, we are able to achieve a one CO2 to one IL reaction ratio, which is twice as much as an aqueous amine or tethering the amine functional group to the cation. This was corroborated by experimental data. Unfortunately, when amino acid based ionic liquids react with CO2, their viscosity increases significantly, making them undesirable for CO2 capture.

We then developed the N-aprotic heterocyclic anion (AHA) ionic liquids for CO2 capture. They are aromatic and react with CO2 with a 1:1 reaction stoichiometry. When they react with CO2, no acidic proton is produced, which corresponds to the viscosity staying the same. This is supported by experimental data. We have studied the use of attaching electron withdrawing substituent groups to the AHA in order to tune its CO2 reaction enthalpy (ÌãH). Depending on the substituent group and its location relative to the reactive nitrogen, we have been able significantly change the ÌãH (-1 to -100 kJ/mol).

Finally, we developed an isotherm model, based on a Langmuir model, to predict the CO2 isotherm of any AHA based on its ÌãH. We also used this model to predict the optimal ÌãH at given conditions. This will aid in the process of choosing which proposed AHAs to test experimentally. Future work should involve experimentally testing the proposed AHAs and computationally developing new AHAs that can capture CO2 with higher reaction ratios than proposed here.