Inverse CO2/C2H2 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 C2H2 in C2H2/CO2 separation processes, CO2‐selective sorbents are much less common. Here, we report the remarkable performance of MFU‐4 (Zn5Cl4(bbta)3, bbta=benzo‐1,2,4,5‐bistriazolate) toward inverse CO2/C2H2 separation. The MOF facilitates kinetic separation of CO2 from C2H2, enabling the generation of high purity C2H2 (>98 %) with good productivity in dynamic breakthrough experiments. Adsorption kinetics measurements and computational studies show C2H2 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 C2H2/CO2 separation with reversed selectivity compared to MFU‐4. MFU‐4‐F also exhibits a remarkably high C2H2 adsorption capacity (6.7 mmol g−1), allowing fuel grade C2H2 (98 % purity) to be harvested from C2H2/CO2 mixtures by room temperature desorption. MFU‐4 shows high selectivity for adsorption of CO2 over C2H2 owing to a large activation energy for C2H2 diffusion through narrow pore apertures. Postsynthetic F−/Cl− ligand exchange increases the pore aperture size, allowing MFU‐4‐F to adsorb a large quantity of C2H2 and exhibit reversed selectivity for C2H2/CO2 separation.