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 × 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.