Fine tuning of pore architecture and morphology of stiffened Zeolitic Imidazolate Frameworks synthesized using fast current driven method and mixed ligand strategy

Zeolitic Imidazolate Frameworks (ZIFs) show great promise in molecular separation as well as recognition owing to their crystalline morphology consisting of a well designed pore architecture. However, the sieving performance of ZIFs remains limited due to their fixed pore sizes and framework flexibility. In the present study, fast current driven synthesis (FCDS) has been used to prepare the stiffened phase of mixed ligands (2-methylimidazole and benzimidazole) based ZIFs having tunable pore sizes and morphologies. X-ray diffraction has been used to determine the crystal structure of the frameworks, which were observed to be crystalline through a wide range of ligand ratio (benzimidazole fraction: -0.206-0.972). The random distribution of both the ligands in the frameworks has been established through complementary methods viz. Raman spectroscopy, Fourier transform infrared and scanning electron microscopy. Morphology of the frameworks was observed to vary from micrometer size individual crystals to spherical aggregates of inter-grown nanocrystals to individual nanocrystals. Pore sizes determined using positronium lifetime measurements were observed to be consistent with the literature for pure phase of ZIF-7 and stiffened ZIF-8. The pore sizes were observed to vary with the benzimidazole ligand fraction confirming that mixed ligand strategy can be used to efficiently tune the pore sizes of the frameworks. Positronium diffusion investigation confirms that pore interconnectivity is inferior as compared to stiffened ZIF-8 but superior than ZIF-7 in these mixed ligand frameworks.

Automated summary

This study utilized fast current driven synthesis to prepare stiffened phase ZIFs based on mixed ligands, allowing for tunable pore sizes and morphologies. X-ray diffraction was used to determine the crystal structure, while positronium lifetime measurements and scanning electron microscopy confirmed the relationship between pore sizes and benzimidazole ligand fraction in the frameworks.