Membranes are an energy-efficient technology for air separation, but it is difficult to control the pore size to separate N2 and O2 due to their similar kinetic diameters. Here we demonstrate from molecular dynamics simulations that a bilayer nanoporous graphene membrane with continuously tunable pore sizes by the offset between the two graphene layers can achieve O2/N2 selectivity up to 26 with a permeance over 105 GPU. By analyzing the mechanism of gas passing through the pore, we find that the entropic selectivity is the main reason behind the high selectivity via the tumbling movement of the skinnier and shorter O2 molecules entering and passing through the elliptic-cylinder-shaped nanopore. In contrast, the longer and fatter N2 molecules wiggle through the nanopore with the rotational degrees of freedom confined. Hence, tuning the pore size and the pore shape via the offset of the bilayer nanoporous graphene membrane provides a novel way to enhance selectivity by confining molecular rotational movement.

Enhanced selectivity in air separation by tumbling movement through a bilayer nanoporous graphene membrane

2019 Fall National Meeting

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