Entropic selectivity in air separation via a bilayer nanoporous graphene membrane

Abstract

Membranes represent 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 by 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 of up to 26 with a permeance of over 10⁵ GPU (gas permeation unit). We find that 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 nanopores of the bilayer membrane. Such motion is absent in the single-layer graphene membrane with pores of similar size and shape which yields an O₂/N₂ selectivity of only 6 via molecular sieving alone. Hence the bilayer nanoporous graphene membrane provides a novel way to enhance the entropic selectivity for gas separation by controlling both the pore size and the 3D pore shape.