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

Abstract

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. Linkerdoping, 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-Al 2 O 3 substrates, in which the incorporation of eIm affects the morphology and thickness of the polycrystalline membranes, improving the permeance of propylene and propane molecules.

Facile Defect Engineering of Zeolitic Imidazolate Frameworks towards Enhanced C3H6/C3H8 Separation Performance

“…Hillman and Jeong reported that the incorporation of bulkier ethyl group of 2-ethylimidazole into ZIF-8 decreased the flexibility of the framework, retarding the second step gate-opening behavior from P/P 0 of 0.02 to 0.05. [23] The presence of either NBA or TBA groups in AZIF8s may prohibit the flipping motion of 2-MIm, resulting in delayed gate-opening behavior. To further support the structural properties of AZIF8, the pore limiting diameter (PLD) of I43m space group, [4a] the most dominant phase in ZIF-8 nanoparticles was calculated under the various pressure conditions.…”

Melamine‐Doped Covalent Organic Framework Membranes for Enhanced Hydrogen Purification

“…[11] Among them, the mixed-linker strategy is one of the most used approaches to improve membrane separation performance via integrate multiple linkers into the micropores. [12] This strategy is accomplished by linker mixing or linker exchange and has been successfully applied to ZIF membranes, [13] silica membranes, [14] and COF membranes. [15] For instance, Jeong et al fabricated eIm/ZIF-8 mixed-linker hybrid membrane by doping 2-ethylimidazole (eIm) into ZIF-8 framework.…”

“…The eIm affected the morphology of ZIF-8 framework and enhanced the permeance of propylene and propane compared to the ZIF-8 membrane. [13] Tsuru et al successfully fabricated composite organosilica membrane via co-condensation of bis(triethoxysilyl)acetylene (BTESA) and bis (triethoxysilyl)benzene (BTESB) precursors. The BTESB tuned the membrane structure due to the steric effect of the phenyl groups and enhanced C 3 H 6 /C 3 H 8 separation performance.…”

Metal–organic framework membranes: Unprecedented opportunities for gas separations

“…In 2019, we showed that some of these “unsuitable” linkers can be doped into the ZIF‐8 framework in a relatively small fraction and be formed into membranes. These doped ZIF‐8 membranes exhibited tunable molecular sieving properties depending on the nature of dopant linkers and their fractions in the framework 46 . This doping strategy is expected to greatly expand the list of linkers that can be used to tune the molecular sieving properties of MOFs/ZIFs for improved gas separations.…”