Molecular simulation of confined ethaline-based deep eutectic solvents for separations of carbon dioxide from methane

JM Xu and FR Hung, AICHE JOURNAL, 69, e18093 (2023).

DOI: 10.1002/aic.18093

Classical molecular dynamics (MD) simulations were used to study the separation of carbon dioxide from methane by three formulations of the deep eutectic solvent (DES) ethaline (choline chloride: ethylene glycol at 1:2, 1:4, and 1:8 molar ratios), confined inside graphite and titania (rutile) slit pores of two different pore widths, 2 and 5 nm. In addition, equivalent DES systems in the bulk were studied, which can also be viewed as a model supported DES membrane with mu m-sized pores. Our results indicate that variations in the ratio of ethylene glycol, which in turn affect the interactions of all DES species with the gas molecules and the different pore walls, plus confinement effects resulting from varying the pore sizes, can affect the gas separation performance of these systems in complex ways. The highest permselectivities (similar to 20), computed as the product of the diffusivity and solubility selectivities, are observed for 1:2 ethaline in a 5 nm graphite pore, followed by the 1:4 DES in a 5 nm graphite pore, 1:2 ethaline in a 2 nm graphite pore, and the 1:8 bulk DES. In bulk systems, all three selectivities reach their highest values for 1:8 ethaline. When the DESs are confined in the nanopores, the solubility selectivity for most systems improves compared to the equivalent bulk systems, with the graphite pores having the largest solubility selectivities for any given ethylene glycol ratio. In contrast, the diffusivity selectivities in confined systems tend to be similar to the values observed in the bulk DESs. Interaction energies and local density profiles were used to rationalize absorption and diffusivity of gases in our systems. Confining ethaline in graphite and rutile nanopores tends to weaken the CO2-ethylene glycol and CO2-cation interactions compared to the values observed in equivalent bulk ethaline systems, which also affect the local density profiles. Our results confirm that variations in ethylene glycol ratio, pore size, and pore wall material can lead to significant changes in gas separation performance. Other porous matrices, for example, nanoporous polymer formulations and graphene oxides, should be considered in follow-up studies as they may lead to significant improvements in gas separation performance as compared to the bulk DES.

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