Engineering Pyrazine Units in Vinylene-Linked Covalent Organic Frameworks for Efficient C₂H₂/CO₂ Separation

Effective separation of C₂H₂/CO₂ is a daunting challenge due to their close molecular sizes and physical properties. Covalent organic frameworks (COFs) have recently attracted widespread attention in the fields of gas adsorption and separation owing to their high surface area and pore designability. In this work, three nitrogen-rich vinylene-linked isostructural COFs were constructed via Aldol condensation between tetratopic monomers containing pyrazine (PZ) groups and terephthalaldehyde. All three PZ-COFs exhibited high specific surface area and decent chemical/thermal stability. Intriguingly, both their adsorption capacity of C₂H₂ (from 57.7 to 107.2 cm³ g^-1) and the separation efficiency of C₂H₂/CO₂ increased along with the growing amount of PZ units within the three frameworks. Dynamic experimental breakthrough results and theoretical calculations further demonstrated that the high-density nitrogen sites along the one-dimensional channels of these COFs played the pivotal role in the C─H···N bond interactions toward C₂H₂ molecules, which subsequently regulated their separation performance of C₂H₂/CO₂ mixtures.