Solvothermal synthesis of mixed-ligand metal–organic framework ZIF-78 with controllable size and morphology

ZIF-78 is a prototypical mixed-ligand compound among metal–organic frameworks (MOFs), with the composition of divalent zinc cations bridging by 2-nitroimidazolate (nIm) and 5-nitrobenzimidazolate (nbIm) anions. ZIF-78 belongs to hexagonal space group (P63/mmc) with a GME topology. Owing to the anisotropic pore structure and good affinity to CO2, ZIF-78 has potential applications in adsorption and separation fields. Herein, morphological control of ZIF-78 crystals was achieved via solvothermal synthesis. XRD, SEM, NMR, and adsorption characterizations were performed to study the crystalline phase, morphology, composition and adsorption behavior of obtained crystals, respectively. ZIF-78 micro-crystals with shape of the hexagonal rods were prepared with the assistance of triethylamine. The size and the aspect ratios of ZIF-78 microrods were manipulated from the “slender” to the “squat” via varying the nutrient concentration, the ligand concentration or relative molar ratio of nIm to nbIm.

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► Morphological control of mixed-ligand metal–organic frameworks was achieved via solvothermal synthesis.
► ZIF-78 hexagonal microrods with well defined crystal faces were prepared with the assistance of triethylamine.
► The size and the aspect ratios of the ZIF-78 microrods were modulated by varying the synthesis parameters.