Theoretical study of helium separation performance of crown ether-graphane membranes

Abstract

Helium (He) is widely used in many scientific and industrial fields, and the shortage of He resources and the growing demand make He separation extremely important. In this work, the He separation performances of a series of graphanes containing crown ether nanopores (crown ether graphane, CG-<i>n</i>, <i>n</i> = 3, 4, 5, 6) are studied by first-principles calculations. At first, the minimum energy paths of He and other 10 gas molecules (Ne, Ar, H₂, CO, NO, NO₂, N₂, CO₂, SO₂ and CH₄) passing through CG-<i>n</i> membranes are calculated, and the factors affecting the energy barriers are also investigated. The calculated results show that He is the easiest to pass through all the four CG-<i>n</i> membranes with energy barriers of 4.55, 1.05, 0.53 and 0.01 eV, respectively. He can be separated by CG-5 and CG-6 with very low energy barriers, and the energy barrier of He passing through CG-6 is the lowest, so far as we know. Moreover, all gas molecules can pass through CG-6 with low energy barriers, including many molecules with large kinetic diameters, such as CO (0.13 eV) and N₂ (0.16 eV). Therefore, CG-6 is also expected to be used in the screening field of other gas molecules. In addition, it is found that the energy barriers of gas molecules passing through CG-<i>n</i> are synergistically affected by the size of the crown ether nanopore, the kinetic diameter and the type of the gas molecules. Secondly, the diffusion rates of gas molecules passing through CG-5 and CG-6 and the He selectivity towards other 10 gases of CG-5 and CG-6 at different temperatures are calculated. It is found that CG-5 exhibits extremely high He selectivity in a wide temperature range (0–600 K). In summary, the crown ether graphanes CG-5 and CG-6 can serve as excellent He separation membranes with high He selectivity. This work is expected to inspire one to develop other graphene-based two-dimensional separation membranes for separating He and other gas molecules.