Undifferentiated Permeation of Long-Chain Hydrocarbon Molecules through Graphene Nanopores and Its Implications for Alkane Separations

Graphene nanopore-based two-dimensional membranes are very promising for gas separation. However, the potential of porous graphene membranes for alkane separations has not been fully tapped, owing to the complex effects of molecular chain length on their permeation. Here, we systematically study the permeation of hydrocarbon molecules through graphene nanopores via the molecular dynamics simulation method. The results show that the permeation of hydrocarbon molecules with different chain lengths nearly has no difference for pores with various sizes. This undifferentiated permeation is reflected not only in the penetration rate but also in the permeation time, molecular incident angle, and density in the pore. The mutual offset between the contributions of molecular adsorption and molecular mass to the permeation rate can be responsible for the undifferentiated permeation. Finally, we propose that the selectively separation of alkanes through graphene nanopores can be realized by the molecular-orientation sieving effect, that is, the diverse difficulty level for the hydrocarbon molecules of adjusting to the upright orientation in the nanopore with a certain depth. The proposed scheme is numerically verified by a selectivity of 13.6 for ethane relative to n-hexane through a three-layer pore with the same size.