Tunable CO₂ Adsorbents by Mixed-Linker Synthesis and Postsynthetic Modification of Zeolitic Imidazolate Frameworks

The incorporation of accessible amine functionality in zeolitic imidazolate frameworks (ZIFs) is used to improve the adsorption selectivity for CO₂/CH₄ gas separation applications. Two synthetic approaches are described in this work to introduce functionality into the ZIF: (i) mixed-linker ZIF synthesis with 2-aminobenzimidazole as a substitution linker and (ii) postsynthetic modification of a mixed-linker ZIF with ethylenediamine. Using 2-aminobenzimidazole, a linker with a primary amine functional group, substitution of the ZIF-8 linker during synthesis allows for control over the adsorption properties while maintaining the ZIF-8 structure with up to nearly 50% substitution in the mixed-linker ZIF framework, producing a material with tunable pore size and amine functionality. Alternatively, postsynthetic modification of a mixed-linker ZIF containing an aldehyde functional group produces a ZIF material with a primary amine without detrimental loss of micropore volume by controlling the amount of functional group sites for modification. Both approaches using mixed-linker ZIFs yield new materials that show improvement in adsorption selectivity for the CO₂/CH₄ gas pair over ZIF-8 and commercially available adsorbents as well as an increase in the heat of adsorption for CO₂ without significant changes to the crystal structure. These results indicate that tuning the surface properties of ZIFs by either mixed-linker synthesis and/or postsynthetic modification may generate new materials with improved gas separation properties, thereby providing a new method for tailoring metal–organic frameworks