Molecular Dynamics Study on CO2 Absorption Using Deep Eutectic Solvents

One of the world’s main challenges is to decrease carbon dioxide (CO₂) emissions that greatly contribute to global warming. Currently, aqueous solutions of alkanolamines are industrially used to capture CO₂, however, they have unsuitable drawbacks like high cost and amine losses. Several Deep Eutectic Solvents (DESs) were suggested to be effective solvents to absorb CO₂ that may replace current unsustainable technologies. Among them, mixtures of choline chloride and monoethanolamine (MEA), tetrabutylammonium bromide (TBABr) and MEA, and methyltriphenylphosphonium bromide (MTPPBr) and MEA at molar ratios of 1:6 have shown outstanding capacities to absorb CO₂. Hence, in current work, we use molecular dynamics simulations to systematically analyse the molecular structure of the above mentioned DESs and interactions between DESs’ components and CO₂ to understand the CO₂ absorption mechanism. The results, based on intermolecular energy and radial distribution functions, reveal that interaction energy within ChCl/TBABr/MTPPBr is significantly decreased in the presence of MEA explaining high depression in the melting point. Moreover, Br^- ions form strong hydrogen bonds with MEA molecules. Considering the CO₂ mechanism, strong interactions between MEA/Br- and CO₂ can be observed, while interactions between CO₂/CO₂ weaken suggesting that CO₂ is mainly absorbed by Br^- ions and MEA. Comparing DESs with traditional amine solutions, our solvents, specifically a mixture of MTPPBr and MEA, have higher absorption capacity. Furthermore, using diffusivity analysis high viscosity of DESs was observed that may be a barrier for their industrial implementation. Therefore, we examined the temperature effect on the diffusion coefficient of the DESs’ component. Analysis of systems at 298K, 328K, and 348K shows that rise in temperature increases diffusion of components corresponding to the decrease of viscosity maintaining similar absorption capacity. Summing up, work studies molecular structure and properties of DESs as well as CO₂ absorption mechanism to assist in the optimization of suitable process conditions for their further commercial development.