Membrane technology has been used for H2 purification.In this paper,the systematic density functional simulations are conducted to study the separation of H2 from the impurity gases (H2,N2,H2O,CO,C12,and CH4) by the bilayer porous graphitic carbon nitride(g-C3 N4) membrane.Theoretically,the bilayer g-C3 N4 membrane with a diameter of about 3.25（A） should be a perfect candidate for H2 purification from these mixed gases,which is verified by the high selectivity (S) for H2 over other kinds of gases (3.43 × 1028 for H2/N2;1.40 × 1028 for H2/H2O;1.60 × 1026 for H2/CO;4.30 × 1014 for H2/Cl2;2.50 × 1055 for H2/CH4),and the permeance (P) of H2 (13 mol/m2.s.Pa) across the bilayer g-C3N4 membrane at 300 K,which should be of great potential in energy and environmental research.Our studies highlight a new approach towards the final goal of high P and high S molecular-sieving membranes used in simple structural engineering.