Insights into the H2/CH4 Separation Through Two-Dimensional Graphene Channels: Influence of Edge Functionalization

A molecular simulation technique is employed to investigate the transport of H-2/CH4 mixture through the two-dimensional (2D) channel between adjacent graphene layers. Pristine graphene membrane (GM) with pore width of 0.515 similar to 0.6 nm is found to only allow H-2 molecules to enter rather than CH4, forming a molecular sieve. At pore widths of 0.64 similar to 1.366 nm, both H-2 and CH4 molecules could fill into the GM channel, where the permeability of methane is more preferential than that of hydrogen with the largest CH4/H-2 selectivity (1.89) at 0.728 nm. The edge functionalization by -H, -F, -OH, -NH2, and -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 the pore width of 0.6 nm, the H-2 permeability of molecular sieve is enhanced by -H, -F, and -OH groups but restrained by -NH2, especially -COOH with a passing rate of zero. At pore widths of 0.64 and 0.728 nm, both -H and -F edge-functionalized GMs show a preferential selectivity of methane over hydrogen, while the favorable transport for GM-OH is changed from H-2 molecules at 0.64 nm to CH4 molecules at 0.728 nm. For GM-NH2, it exhibits an excellent hydrogen molecular sieve at 0.64 nm and then turns into a significant H-2/CH4 selectivity at 0.728 nm. Meanwhile, small H-2 molecules start to enter the channel of GM-COOH at the pore width up to 0.728 nm. For the largest pore width of 1.336 nm, the influence of edge functionalization becomes small, and a comparable CH4/H-2 selectivity is observed for all the considered membranes.