Monoethanolamine based DESs for CO2 absorption: Insights from molecular dynamics simulations

In order to mitigate issues related to greenhouse gases, several ionic liquids have been designed to efficiently absorb CO2. Among them, novel deep eutectic solvents (DESs), with their commercially attractive properties, have recently been studied for CO2 capture process. Earlier, we have analyzed molecular structure of methyltriphenylphosphonium bromide (MTPPBr) and mono ethanol amine (MEA) based DESs at molar ratio 1:6, 1:7 and 1:8, which are known to have high CO2 absorption capacity. Herein, we further explore the interactions between the above mentioned DESs and CO2 using molecular dynamics simulations to systematically explore the mechanism(s) of CO2 absorption. Results based on interaction energy and radial distribution functions (rdfs) reveal strong interactions between MEA/CO2 and Br/CO2, suggesting the role of these key components of the DESs in CO2 capture. The observed absorption capacity is significantly higher than the traditionally used MEA solvent. Furthermore, the DESs have high viscosity, or low diffusion of the components, which is an impediment to its commercial development. Hence, we examined the effect of temperature and addition of water onto diffusion of the DESs' components and CO2 absorption capacity. Results show that structure of DESs and its CO2 absorption capacity remain unaffected up to 70 mol% water in the DESs; however, the increase in diffusion is also insufficient. On the other hand, a 30 K rise in the temperature led to a significant increase in the diffusion while maintaining a high absorption capacity. Taken together, work assists in understanding interactions between CO2 and DESs' components and the absorption mechanism of these DESs and additionally suggests methods to decrease DESs' viscosity, along with optimized process conditions, for further industrial implementation.