Molecular Dynamics Insights of CO2 Capture through Phosphonium-Based Deep Eutectic Solvents for Direct Air Capture

Abstract

Negative emission technologies (NETs) have emerged as a mandatory step to achieve the respective sustainable goals of this generation, and direct air capture (DAC) is the newest addition to this class. Conventional amine- and alkaline-based solvents commonly used for DAC pose various operating disadvantages, and for that reason, deep eutectic solvents (DESs) are seen as a promising sustainable alternative solvent for this purpose. In this study, three DESs based on different compositions of tetrabutylphosphonium bromide (TBPBr) and triethylene glycol (TEG) are computationally studied with the help of molecular dynamics (MD) and density functional theory (DFT). The systems were analyzed based on intermolecular interactions such as radial distribution function, combined distribution function, hydrogen bonds, interaction energy, and diffusion behaviors. The results indicated the ability of the long-chained phosphonium cations to facilitate CO₂ molecules in a cage-like structure, whereas TEG molecules proved to be the deciding factor due to their higher polarity. DES2, with a 1:2 molar ratio of TBPBr:TEG showed better structural distribution and affinity toward CO₂ overall, which was confirmed with DFT studies. Finally, DES2 was studied under atmospheric conditions (400 ppm of CO₂) to confirm the possibility of this solvent being used as a DAC solvent. This showed very promising selectivity for CO₂ molecules over nitrogen and oxygen molecules.