Tailoring novel deep eutectic solvents for efficient CO2 capture: a molecular dynamics perspective





Development of CCS technologies requires efficient capture of CO2 by solvents, which are environmentally friendly and cost-effective. Currently, aqueous solutions of alkanolamines are industrially used to capture CO2, however, being highly volatile and corrosive, the use of amines is not sustainable. Over the last decade, several Deep Eutectic Solvents (DESs) have been developed to possibly replace amines to absorb CO2 more efficiently. Among them, mixtures of choline chloride and monoethanolamine (MEA), tetrabutylammonium bromide (TBABr) and MEA, and methyltriphenylphosphonium bromide (MTPPBr) and MEA at molar ratio of 1:6 have shown phenomenal capacities to absorb CO2. However, mechanism of CO2 capture remains obscured, thus, impeding their commercial development and future research. Hence, in current work, we use molecular dynamics simulations to systematically analyze the molecular structure of these DESs and interactions between the DESs’ components and CO2 to examine the CO2 absorption mechanism. The results, based on intermolecular energy and radial distribution functions, interestingly reveal strong interactions between MEA/CO2 and anion/CO2, while interactions between CO2/CO2 are weakened suggesting that CO2 is mainly absorbed by anions and MEA. Although cations (Ch+/MTPP+/TBA+) do not have any significant interactions with CO2, they are required to maintain intermolecular structure of DESs. Simulations further validate high absorption capacities of these DESs. Furthermore, using diffusivity analyses high viscosity of DESs was observed which is a technical barrier for their industrial usage. Therefore, we examined the effect of temperature on the diffusion coefficient of the DESs’ component. Analysis of systems at 25°C and 55°C shows that rise in temperature increases diffusion of components, corresponding to the decrease of viscosity, while maintaining similar absorption capacities. Summing up, the work studies molecular structure and properties of DESs, as well as CO2 absorption mechanism to assist in the optimization of suitable process conditions for the commercial development of these novel DESs.