LAUSR

Molecular‐sieving membranes from metal–organic frameworks (MOFs) are promising candidates for separating olefin/paraffin mixtures, a critical demand in sustainable chemical processes and a grand challenge in molecular separation. Currently, the inherent lattice flexibility of MOFs severely compromises their precise sieving ability. Here, a proof‐of‐concept of “alloy” membranes (AMs), which are fabricated by incorporating quaternary ammonium (QA)‐functionalized covalent organic frameworks (COFs) into a zeolitic imidazolate framework‐8 (ZIF‐8) matrix is demonstrated. The Coulomb force between the COFs and the ZIF‐8 restricts the linker rotation of the ZIF‐8, generating a distinct alloying effect, by which the lattice rigidity of ZIF‐8 can be conveniently tuned through varying the content of the COFs, similar to the flexible‐to‐rigid transition in aluminum alloy manufacturing. Such an alloying effect confers the AM's superior propylene/propane separation performance, with a propylene/propane separation factor surpassing 200 and a propylene permeance of 168 GPU. Hopefully, the AMs concept and the concomitant alloying effect can update the connotation of mixed matrix membranes and stimulate the re‐envisioning about the design paradigm and development of advanced membranes for energy‐efficient separations.