Ultrathin membranes with intrinsic pores are highly desirable for gas separation applications, because of their controllable pore sizes and homogeneous pore distribution and their intrinsic capacity for high flux. Two-dimensional (2D) covalent organic frameworks (COFs) with layered structures have periodically distributed uniform pores and can be exfoliated into ultrathin nanosheets. As a representative of 2D COFs, a monolayer triazine-based CTF-0 membrane is proposed in this work for effective separation of helium and purification of hydrogen on the basis of first-principles calculations. With the aid of diffusion barrier calculations, it was found that a monolayer CTF-0 membrane can exhibit exceptionally high He and H2 selectivities over Ne, CO2, Ar, N2, CO, and CH4, and the He and H2 permeances are excellent at appropriate temperatures, superior to those of conventional carbon and silica membranes. These observations demonstrate that a monolayer CTF-0 membrane may be potentially useful for helium separation and hydrogen purification.

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Density functional theory (DFT) calculations and molecular dynamic (MD) simulations were performed to investigate the capability of graphene membranes with H-passivated nanopores for the separation of N2/CO2 gas mixtures. We found that the graphene membrane, H-pore-13, with its appropriate pore size of 4.06 Å, can efficiently separate N2 from CO2. Different from the previously reported preferential permeation of CO2 over N2 resulting from size sieving, H-pore-13 can exhibit high N2 selectivity over CO2 with a N2 permeance of 10(5) GPU (gas permeation unit), and no CO2 was found to pass through the pore. It was further revealed that electrostatic sieving plays a cruical role in hindering the passage of CO2 molecules through H-pore-13.