Hybrid Zeolitic Imidazolate Frameworks: Controlling Framework Porosity and Functionality By Mixed-Linker Synthesis of ZIF-8-90

Hybrid zeolitic imidazolate frameworks (ZIF), a class of metal-organic frameworks with zeolitic topologies, exhibit synthetically tunable properties such as microporosity and chemical functionality. The ZIF-8 linker (2-methylimidazole, 2-MeIM) and the ZIF-90 linker (carboxaldehyde-2-imidazole,OHC-IM) are incorporated into the hybrid framework of  ZIF-8-90, creating a combination of different ligands in differing relative quantities. This study focused on improving ZIF-8-90 synthesis methods to increase ZIF-8 inclusion and tune the crystal size by changing linker ratio and using different Zn precursors. These hybrids were synthesized using a nonsolvent induced crystallization technique (NSIC). This study showed the effects of various ZIF-90 and ZIF-8 linkers’ ratio on the crystal size and composition of the ZIF-8-90 hybrid framework. ¹H NMR showed that the ZIF hybrid frameworks preferentially include OHC-IM over 2-MeIM due to ZIF-90 linker’s lower solubility in the nonsolvent. Using Zn precursors with lower disassociation constants might increase ZIF-90 solubility in the non-solvent phase and then enhancing ZIF-8’s ability to be included in the hybrid structure.  XRD demonstrated the ZIF-8-90 unit cell volume increased with OHC-IM loading while maintaining the same cubic crystal structure. Therefore, higher ZIF-8 incorporation results in smaller unit cell volumes. The ZIF-8-90 hybrid introduces an opportunity for post-synthetic modification by incorporating a functional group into the hybrid network. By increasing ZIF-8 incorporation, the hybrid may have new applications for hydrocarbon separations. Alterations in the chemical and structural properties enable hybrid ZIFs to be viable candidates for a variety of gas separations.