Optimal Host–Guest Fit in Metal‐Organic Frameworks to Achieve Record C ₂ H ₂ Packing Density for Trace Acetylene Capture

Abstract 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 C 2 H 2 adsorption performance. The newly synthesized, best‐fit SXU‐10 exhibits the most superior very‐low‐pressure C 2 H 2 capture—reaching near saturation at just 2 kPa. With record C 2 H 2 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 C 2 H 2 at 1 kPa and C 2 H 2 /C 2 H 4 selectivity of 133 for 1/99 C 2 H 2 /C 2 H 4 mixtures (all at 298 K), along with exceptional stability and humidity tolerance, enabling the production of polymer‐grade C 2 H 4 from trace C 2 H 2 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.