Linker-Doped Zeolitic Imidazolate Frameworks (ZIFs) and Their Ultrathin Membranes for Tunable Gas Separations

Zeolitic imidazolate frameworks (ZIFs) have gained much interest due to their potentials in gas separations. A hybrid approach by mixing metals and/or linkers has been recently investigated to fine-tune the ZIF framework porosity and surface properties and potentially extending their separation applications for many important gas mixtures. In general, the hybrid approach requires mixing of isostructural ZIFs to maintain their topology, thus limiting the options of linkers and metal centers to obtain hybrid ZIFs. Linker-doping, as reported here, can be a strategy to expand the option of imidazolate linkers to obtain mixed-linker hybrid ZIFs. Two linkers are investigated to be doped into the ZIF-8 framework: 2-ethylimidazole (eIm) and 2-phenylimidazole (phIm). The linker-doping strategy is shown to tune the "stiffness" of Zn-N bonding as characterized by FT-IR, thereby the linker flip-flopping motion of ZIF-8, which is analyzed through gas adsorption isotherms. Furthermore, well-intergrown ultrathin eIm-doped ZIF-8 membranes are grown on α-Al₂O₃ substrates, in which the incorporation of eIm affects the morphology and thickness of the polycrystalline membranes, improving the permeance of propylene and propane molecules.