In Situ Self‐Locked Crystalline–Amorphous ZIF‐8 Composite Membranes for Ethylene/Ethane Separation via Sacrificial Coordination Disruption

Metal–organic framework (MOF)‐derived amorphous materials combine the processability of disordered solids with the molecular‐sieving characteristics of crystalline microporous frameworks; however, controlling crystalline domain regeneration within a chemically compatible amorphous matrix remains challenging. Here, a sacrificial coordination‐disruption strategy is introduced in which formic acid (FA) transiently perturbs the Zn–2‐methylimidazolate coordination network of ZIF‐8 through proton transfer and ligand exchange, producing a formic acid‐treated ZIF‐8 precursor (FZIF‐8). Heat‐only treatment converts FZIF‐8 into a ZIF‐8‐derived amorphous intermediate (aZIF‐8), whereas 2‐methylimidazole (mim)‐assisted coupled heat‐and‐pressure treatment promotes removal of FA‐derived species, structural reorganization, and in situ crystallization, yielding self‐locked crystalline–amorphous ZIF‐8 composite membranes (scaZIF‐8). Increasing mim content systematically modulates the extent of in situ crystallization, crystallite size, accessible microporosity, and domain distribution. The resulting crystalline–amorphous interfacial architecture is associated with attenuated gate‐opening behavior and a narrower effective molecular‐sieving window relative to crystalline ZIF‐8. These features enable ethylene/ethane separation with an ethylene permeability of 2871 Barrer and an ethylene/ethane separation factor of 10.1 for equimolar mixtures at 0°C. This work demonstrates sacrificial coordination disruption as a route to crystalline–amorphous MOF composite membranes and highlights crystalline–amorphous structural design for tailoring molecular‐sieving behavior and gas‐transport properties.