Inverse CO₂ /C₂ H₂ Separation with MFU-4 and Selectivity Reversal via Postsynthetic Ligand Exchange

Although many porous materials, including metal-organic frameworks (MOFs), have been reported to selectively adsorb C₂ H₂ in C₂ H₂ /CO₂ separation processes, CO₂ -selective sorbents are much less common. Here, we report the remarkable performance of MFU-4 (Zn₅ Cl₄ (bbta)₃ , bbta=benzo-1,2,4,5-bistriazolate) toward inverse CO₂ /C₂ H₂ separation. The MOF facilitates kinetic separation of CO₂ from C₂ H₂ , enabling the generation of high purity C₂ H₂ (>98 %) with good productivity in dynamic breakthrough experiments. Adsorption kinetics measurements and computational studies show C₂ H₂ is excluded from MFU-4 by narrow pore windows formed by Zn-Cl groups. Postsynthetic F^- /Cl^- ligand exchange was used to synthesize an analogue (MFU-4-F) with expanded pore apertures, resulting in equilibrium C₂ H₂ /CO₂ separation with reversed selectivity compared to MFU-4. MFU-4-F also exhibits a remarkably high C₂ H₂ adsorption capacity (6.7 mmol g^-1 ), allowing fuel grade C₂ H₂ (98 % purity) to be harvested from C₂ H₂ /CO₂ mixtures by room temperature desorption.