Molecular design of two-dimensional graphdiyne membrane for selective transport of CO2 and H2 over CH4, N2, and CO

CO2 capture and H2 purification are the main challenges in syngas and flue gas processing. Two-dimensional graphdiyne (GDY) membrane, with intrinsic uniform pores, provides a promising candidate but lacks both a design strategy and an atomic understanding for these separations. In this study, for the first time, flexible GDYs are computationally designed with the engineering of various functional groups, namely GDY-H, GDY-F, GDY-OH and GDY-NH2, to gain molecular-level insights into CO2 capture and H2 purification from binary mixtures of CO2/CH4, CO2/CO, CO2/N2, H2/CH4, H2/CO and H2/N2. Gas separation performance is enhanced by the co-effect of size sieving and membrane-gas interactions. Both the hydroxylated GDY-OH and the aminated GDY-NH2 membranes exhibit unprecedented performance for both CO2 and H2 separation. Despite processing a small-sized aperture, the GDY-NH2 achieves the highest performance for CO2 separations with permeance above 8.9 x 104 GPU and selectivities over CH4, CO and N2 up to 13935, 12356 and 809, respectively, which far surpass the 2008 upper bound. Structural and energetic analyses show that the flexible GDY-NH2 tends to open its nanowindows and evoke concerted motions that enhance gas permeation, thereby promoting prompt diffusion. Additionally, the ultra-high H2 separation performance of the functional GDY-NH2 and GDY-OH is also several orders of magnitude higher than state-of-the-art membranes. This computational study reveals two dominant effects that govern the gas permeation process and suggests the great potential of hydroxylated and aminated GDYs for both CO2 capture and H2 purification.