Optimal Host-Guest Fit in Metal-Organic Frameworks to Achieve Record C₂H₂ Packing Density for Trace Acetylene Capture

Host-guest fit is critical in enzyme-substrate binding and useful for designing crystalline porous adsorbents to remove trace impurities; however, realization of such remains highly scarce, let alone to elucidate the atomic-level understanding on how to optimize such an effect. Herein, we demonstrate the systematic design of a pillared-layer MOF series (CPL-1) to achieve optimal host-guest fit through increasing the number of bulky groups, progressively enhancing the C₂H₂ adsorption performance. The newly synthesized, best-fit SXU-10 exhibits the most superior very-low-pressure C₂H₂ capture-reaching near saturation at just 2 kPa. With record C₂H₂ packing densities of 483.9 mg cm^-3 at 1 kPa and 733.97 mg cm^-3 at 100 kPa, SXU-10 exhibits an uptake of 1.18 mmol g^-1 C₂H₂ at 1 kPa and C₂H₂/C₂H₄ selectivity of 133 for 1/99 C₂H₂/C₂H₄ mixtures (all at 298 K), along with exceptional stability and humidity tolerance, enabling the production of polymer-grade C₂H₄ from trace C₂H₂ mixtures under a challenging humid condition (80% RH). The unusual steep-rise-uptake phenomenon and significantly improved performance are attributed to the optimized host-guest shape fit at a sub-ångström precision which constrains the gate-opening flexibility, evidenced via in situ single-crystal X-ray diffraction. The present study sheds light on the molecular-level engineering of bioinspired porous adaptive materials.