Entropic selectivity in air separation via a bilayer nanoporous graphene membrane

Membranes represent an energy-efficient technology for air separation, but it is difficult to control the pore size to separate N-2 and O-2 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 O-2/N-2 selectivity of up to 26 with a permeance of over 10(5) 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 O-2 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 O-2/N-2 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.