Abstract:

Selective molecular permeation through two-dimensional nanopores is of great importance for nanoporous graphene membranes. In this study, we investigate the selective permeation characteristics of gas molecules through a nitrogen-and hydrogen-modified graphene nanopore using molecular dynamics simulations. We reveal the mechanisms of selective molecular permeation from the aspects of molecular size and structure, pore configuration, and interactions between gas molecules and graphene. The results show that the permeances of different molecules are different, and the following order is observed in our study: H₂O > H₂S > CO₂ > N₂ > CH₄. Molecular permeance is related to the molecular size, mass, and molecular density on the graphene surface. The molecular permeation rate is inversely proportional to the molecular mass based on gas kinetic theory, while the molecular density on the graphene surface exerts a positive effect on molecular permeation. The permeance of H₂O molecules is the highest owing to their smallest diameter, while the permeance of CH₄ molecules is the lowest owing to their biggest diameter; in these cases, the molecular size is a dominating factor. For H₂S and CO₂ molecules, the diameters of H₂S molecules are larger than those of CO₂ molecules, but the interactions between H₂S molecules and graphene are stronger, resulting in a stronger permeation ability of H₂S molecules. Between CO₂ and N₂ molecules, CO₂ molecules show higher permeation rates owing to smaller diameters and stronger interactions with graphene. The graphene surface also shows nonuniform molecular density distribution owing to molecular permeation through graphene nanopores. Because of the doped nitrogen atoms, the CH₄ molecules prefer to permeate from the left and right sides of the graphene nanopore, while the other molecules prefer to permeate from the center of the nanopore owing to their small diameters. For the molecules that show stronger interactions with graphene, the molecular density on the graphene surface is higher; accordingly, the residence time on the graphene surface is longer and the experience time period during permeation is also longer. The mechanisms identified in this study can provide theoretical guidelines for the application of graphene-based membranes. In addition, the permeance of gas molecules in the graphene nanopore adopted in this study is on the order of 10^-3 mol·s^-1·m^-2·Pa^-1, and the selectivity of other molecules relative to CH₄ molecules is also high, showing that the membranes based on this type of nanopore can be employed in natural gas processing and other separation industries.

Key words: Graphene nanopore, Selective permeation, Gas molecules, Molecular dynamics