Exploring the potential energy surface of nCO2 (n = 1–5) capture by imidazole-and fluorine-based ionic liquids: A DFT study

Abstract

One of the reasons for global warming is the rising of greenhouse gasses causing the planet's temperature to increase; among these gases, CO2 accounts for 80%. For this reason, it is essential to contemplate environmentally sustainable alternatives for CO2 capture and storage, such as the use of ionic liquids (ILs), which represent an excellent option due to their high affinity for CO2. This theoretical study considered ILs based on imidazole ([bmim]+ and [emim]+) and fluorinated ([TFSI]- and [PF6]-) dimers, thorough exploration of the potential energy surfaces of nCO2[bmim]+[PF6]−, nCO2[emim]+[TFSI]−, nCO2[bmim]+[PF6]− and nCO2[emim]+[TFSI]− molecular clusters, n = 1–5, by using a stochastic SnippetKick searching algorithm. Followed by the optimisation of the molecular cluster geometries using Density Functional Theory (DFT) calculations to describe the effect of physisorption of up to five CO2 in ILs. It was revealed an aggregation of CO2 molecules between the imidazole ring and the anion, mainly surrounding [TFSI]- or [PF6]-, in the form of an orbital in putative global minimum-energy structures, which allows a higher CO2 physisorption. Finally, a detailed analysis of the thermodynamic properties and intermolecular interactions between ILs and CO2 molecules was performed with different approximations of the electron density scalars. Overall, this confirms the use of the ILs studied to capture CO2 efficiently.