Highly selective C₂H₄/C₂H₂ separation using a cyano-functionalized alkadiyne-pyrene conjugated framework membrane

Effective separation of gas molecules is of significant importance and presents a considerable challenge in industrial processes, particularly for species with similar kinetic diameters such as ethylene and acetylene. Here, we demonstrate that incorporating cyano (CN) groups into a poly(1,3,6,8-tetraethynylpyrene) (PTEP) conjugated framework membrane not only narrows the pore size of the membrane, but also introduces C-H⋯N hydrogen bonding interactions and the resulting orientational matching with ethylene (C₂H₄), thereby facilitating its selective and efficient permeation. In contrast, acetylene molecules, which cannot form such interactions, are effectively hindered, enabling highly efficient separation of C₂H₄/C₂H₂ mixtures. Density functional theory (DFT) calculations reveal a diffusion barrier of only 0.87 kcal mol^-1 for C₂H₄ through the CN-functionalized membrane, compared to 6.87 kcal mol^-1 for C₂H₂, resulting in an ideal selectivity of approximately 1 × 10⁴ at 298 K. Molecular dynamics (MD) simulations further demonstrate a dynamic selectivity of 46.7 ± 4.1 at room temperature, which is about 47 times higher than that of the pristine PTEP membrane, surpassing most reported covalent organic framework (COF) membranes. The corresponding permeance was measured as 2419 ± 145 GPU for C₂H₄ and 84.7 ± 18.8 GPU for C₂H₂. This work proposes a versatile strategy for precise regulation of both membrane pore size and chemical functionality to achieve advanced gas separation performance, providing valuable insights for the rational design of next-generation separation membranes.