Hybrid Zeolitic Imidazolate Frameworks: Controlling Framework Porosity and Functionality by Mixed-Linker Synthesis

Hybrid zeolitic imidazolate frameworks (ZIF), a class of metal-organic frameworks with zeolitic topologies, have the ability to be synthetically fine-tuned to exhibit properties such as microporosity and chemical functionality. This study focused on different methods for synthesizing ZIF-7-8 and ZIF-8-90 by varying linker and solvent ratios in order to further understand the structural and chemical properties. The ZIF-8 linker (2-methylimidazole, 2-MeIM) was incorporated into hybrid frameworks, ZIF-7-8 and ZIF-8-90, creating a combination of different ligands in differing relative quantities. These hybrids were synthesized using a nonsolvent induced crystallization technique (NSIC). This study demonstrated the effects of varying linker ratios, ZIF-7 linker (benzimidazole, Bz-IM) and ZIF-90 (carboxaldehyde-2-imidazole, OHC-IM), against the ZIF-8 linker as well as varying the solvent ratios in the synthesis stage. ¹H NMR showed that the ZIF hybrid frameworks preferentially include OHC-IM or Bz-IM over 2-MeIM due to ZIF-7 and ZIF-90 linkers’ lower solubility in the nonsolvent. XRD demonstrated the ZIF-8-90 unit cell volume increased with OHC-IM loading while maintaining the same cubic crystal structure. XRD patterns of the ZIF-7-8 hybrid shifted from a cubic to a rhombohedral space group above 35% Bz-IM loading. Additionally, TEM images showed only single phase mixtures at all hybrid loadings, an indication of one crystal phase. Nitrogen physisorption also supported hybrids of single phases with different microporosity than their parent frameworks due to the contrasting inflection points seen from the 50-50 physical mixture of ZIF-8 and ZIF-90 linkers versus the ZIF-8-90 hybrid of similar composition (43% OHC-IM). Overall, the ZIF-7-8 hybrid demonstrated tunable pore sizes while the ZIF-8-90 hybrid introduced an opportunity for post-synthetic modification by incorporating a functional group into the hybrid network. Alterations in the chemical and structural properties enable hybrid ZIFs to be viable candidates for a variety of gas separations.