Vieira-Linhares and Seaton studied the transport mechanism of H 2/CH 4 mixture in the 2D graphite channels by a combination of grand canonical molecular dynamics (GCMC) and dual control volume GCMC (DCV-GCMD) methods. For 2D graphene channels, MD simulation showed that the water molecules cannot fill the 2D graphene capillaries with interplanar distance below 0.6В nm, whereas the capillaries with interplanar distance ranging from 0.6 to 1.0В nm could be filled by one and two layers of water molecules. To explore the mechanism of molecular transport, molecular dynamic (MD) simulation, as a powerful tool, has been employed. In addition, the permeability and selectivity of graphene-based membranes to ions and molecules have also been investigated in aqueous media. Recently, Li prepared an ultrathin 1.8-nm-thick graphene oxide membrane by a facile filtration method, which exhibits the ultra-high selectivity for H 2/CO 2 and H 2/N 2 mixtures. They suggested that this seemingly incompatible phenomenon is attributed to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. found that the submicrometer-thick graphene oxide membranes could completely reject liquids, vapors, and gases (He, Ar, H 2, and N 2), but water is allowed to permeate facilely. reported that the nanochannels within chemically converted graphene sheets can be controlled by hydrothermal treatment, leading to the selective passing of water and small metal nanoparticles.

The 2D channels between two stacked graphene nanosheets may allow the special molecules to pass through but reject the unwanted molecules. Now, the interlayer channel size could be tuned by oxidation and intercalating different-sized cross-linking molecules, nanoparticles, and nanowires. Alternatively, we could prepare a separation membrane with stacked 2D graphene sheets, and the gas molecules could selectively permeate through the 2D channels by controlling the interlayer spacing. In the real-word, however, it is extremely difficult to fabricate a large-area monolayer graphene material with controllable and uniform high-density nanopores. For molecular separation, therefore, a graphene-based membrane needs to be functionalized with nanopores or nanochannels.ДЇor the porous membranes, the selective molecular permeation could be enabled by opening and controlling the holes on the 2D graphene sheet. Because of the high impermeability, the perfect graphene is a good barrier layer for gases and liquids. Among various membrane materials, graphene-based materials have the two-dimensional (2D) carbon sheets with large surface area, chemical stability, mechanical robustness, and high impermeability, and thus, they are considered as one of the most potential classes of separation membranes. In the past few decades, membrane separation technologies exhibit many fascinating properties including low energy consumption, facile operation, and high cost effectiveness and thus have attracted much research attention. For the largest pore width of 1.336В nm, the influence of edge functionalization becomes small, and a comparable CH 4/H 2 selectivity is observed for all the considered membranes. Meanwhile, small H 2 molecules start to enter the channel of GMвЂ“COOH at the pore width up to 0.728В nm. For GMвЂ“NH 2, it exhibits an excellent hydrogen molecular sieve at 0.64В nm and then turns into a significant H 2/CH 4 selectivity at 0.728В nm. At pore widths of 0.64 and 0.728В nm, both вЂ“H and вЂ“F edge-functionalized GMs show a preferential selectivity of methane over hydrogen, while the favorable transport for GMвЂ“OH is changed from H 2 molecules at 0.64В nm to CH 4 molecules at 0.728В nm.

At the pore width of 0.6В nm, the H 2 permeability of molecular sieve is enhanced by вЂ“H, вЂ“F, and вЂ“OH groups but restrained by вЂ“NH 2, especially вЂ“COOH with a passing rate of zero. The edge functionalization by вЂ“H, вЂ“F, вЂ“OH, вЂ“NH 2, and вЂ“COOH groups could significantly alter gas permeability by modifying the active surface area of the pore and tuning attractive and/or repulsive interaction with molecules at the entrance of channel. At pore widths of 0.64~1.366В nm, both H 2 and CH 4 molecules could fill into the GM channel, where the permeability of methane is more preferential than that of hydrogen with the largest CH 4/H 2 selectivity (1.89) at 0.728В nm. Pristine graphene membrane (GM) with pore width of 0.515~0.6В nm is found to only allow H 2 molecules to enter rather than CH 4, forming a molecular sieve. A molecular simulation technique is employed to investigate the transport of H 2/CH 4 mixture through the two-dimensional (2D) channel between adjacent graphene layers.

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