Tailoring Pore Environments in Metal–Organic Frameworks for Efficient C₂H₂/CO₂ and C₂H₂/C₂H₄ Separations

The efficient separation of acetylene (C₂H₂) from carbon dioxide (CO₂) and ethylene (C₂H₄) is of significant industrial importance but remains highly challenging due to their similar physicochemical properties. In this study, three isostructural metal-organic frameworks, [Co₄(µ₄-SO₄)₂(tpim)₂(L)₂] (tpim = 2,4,5-tri(4-pyridyl)-imidazole, L = dicarboxylate; termed as Co₄-L), are constructed from sulfate-capped Co₄ clusters extended by N-rich tpim linkers and different-sized dicarboxylate ligands. Among them, the Co₄-1,4-ndc framework, incorporating 1,4-naphthalenedicarboxylic acid (1,4-ndc), offers optimal pore confinement that promotes host-guest interactions between C₂H₂ molecules and framework. This material exhibits high C₂H₂ capture capacities of 4.30 and 3.40 mmol g^-¹ from equimolar C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures, respectively. Theoretical simulations reveal that the enhanced selectivity arises from multiple cooperative hydrogen-bonding interactions between C₂H₂ and the framework Co₄-1,4-ndc, generating strong and preferential binding sites. This study highlights the role of pore-environment engineering of metal-organic frameworks (MOFs) for selective C₂H₂ separation, providing a foundation for the development of advanced adsorbents in industrial gas purification.