Pore architecture of mixed ligand zeolitic imidazolate frameworks investigated using complementary techniques

Zeolitic Imidazolate framework-8 (ZIF-8) consisting of Zn metal nodes connected to 2-methylimidazole is highly flexible. As a result, its selectivity towards CO₂/CH₄ separation is very limited. In order to enhance the separation selectivity, rigidity of the framework is enhanced incorporating a bulkier ligand i.e. benzimidazole, bIm. Mixing of ligands also bring a complete modification in the crystal morphology and pore architecture. In order to investigate the modifications in pore architecture and morphology, mixed ligand frameworks with varying bIm (0-100%) have been synthesized using fast current driven synthesis method. FTIR and Raman measurements have confirmed the presence of both the ligands in all the frameworks. XRD patterns show that crystal structure varies from Cm to R3 space groups with the increase in bIm fraction. FE-SEM micrographs shows different morphologies such as μm size single crystal (bIm~ 0%), spherical particles made of interpenetrating nanocrystals (bIm ~ 40-70%) and nanocrystals (bIm ~ 90-100%). Positronium lifetime measurements confirmed opening of aperture and closing of central cavity on increase of bIm fraction. Small angle X-ray scattering measurement shows the evolution of mesopores in inter-crystal regions in mixed ligand frameworks due to the evolved morphology which is distinct from pure ZIF-8_Cm (0% bIm) and ZIF-7 (100% bIm). (author)