Polymer-Coordinated Heterogeneous Zeolitic Imidazolate Framework-8 with Exceptional Propylene/Propane Molecular Sieving Effect.

Zeolitic imidazolate framework-8 (ZIF-8) is a promising material for CH/CH separation; however, its inherent linker flexibility compromises molecular sieving effect. To address this limitation, a novel ZIF-8, namely heterogeneous ZIF-8 (hZIF-8), is developed, consisting of nanocrystalline ZIF-8 phases with a minor amorphous polymer component introduced via in situ coordination. hZIF-8 is synthesized through a Lewis-acid-catalyzed cationic ring-opening polymerization of tetrahydrofuran (THF), generating polytetramethylene ether glycol (PTMEG) chains that competitively coordinate with zinc ions during ZIF-8 crystallization. This coordination-mediated heterogeneous structure exhibits unique characteristics distinct from conventional ZIF-8 and other polymer hybrid MOFs such as PolyMOFs. The polymer phase effectively restricts the linker flexibility of the nanocrystalline phase at the molecular level, leading to significantly enhanced molecular sieving properties. As a result, hZIF-8-containing mixed-matrix membranes (MMMs) exhibit a significant enhancement in CH/CH separation performance, with the separation factor increasing from 13.3 to 75.5 (by 468%) and the CH permeability from 7.4 to 17.4 Barrer (by 135%) at 30 wt.% loading. Furthermore, the intrinsic selectivity of hZIF-8 is predicted to reach ≈3700, representing an order-of-magnitude improvement over that of pristine ZIF-8 (≈180).