Synthesis and sorption-induced phase transformation of Co-based zeolitic imidazolate framework polymorphs

Abstract

The synthesis of Co-ZIF polymorphs (C₁₀H₁₆N₅O_3/2Co, C₈H₁₀N₄Co, and their mixture) was realized in aqueous medium under mild conditions. The morphology, crystalline and porous structure of products were investigated in detail by a range of diverse methods including scanning electron microscopy, X-ray diffraction, and low-temperature nitrogen adsorption–desorption. Visualization of the obtained metal–organic particles by SEM proved that variation of reaction mixture composition along with the metal precursor addition strategy opens up new opportunities for preparation of leaf-like, cuboid, pitaya-like, and rhombic dodecahedron Co-ZIFs. Evolution of two- to three-dimensional crystallographic structures, which takes place with the gradual increase of imidazolate ligand to metal precursor molar ratio, was confirmed by X-ray diffraction studies. According to the low-temperature nitrogen adsorption–desorption analysis, structural transformation is accompanied by a substantial increase of surface area and pore volume of the prepared Co-ZIFs. Equilibrium studies of methyl orange sorptive removal by the synthesized materials from aqueous medium proved that despite the higher specific surface area and developed porosity of the Co-ZIF with rhombic dodecahedron morphology, it demonstrates much lower sorption of dye than leaf-like, cuboid, and pitaya-like Co-ZIFs. The SEM analysis indicated that sorptive uptake of methyl orange by the leaf-like and cuboid Co-ZIFs leads to the formation of sponge-like frameworks made up of nano-sized granular structures. Meanwhile, X-ray diffraction and low-temperature nitrogen adsorption–desorption studies confirmed the formation of the rhombic dodecahedron crystalline phase and the evolution of porous structure.