Synthesis, hydrolytic stability, and phase transformation of (Co, Co/Zn, Zn)-ZIFs with cuboid morphology of particles

Abstract

A simple, cost-effective, and environmentally friendly strategy was proposed for synthesis of three types of zeolitic imidazolate frameworks (Co, Co/Zn, and Zn based/containing) with cuboid particle morphology. The structure of (Co, Co/Zn, Zn)-ZIFs was studied by scanning electron microscopy, x-ray diffraction, and low-temperature nitrogen adsorption-desorption. To obtain information about the possibility of potential liquid-phase application of the cuboid (Co, Co/Zn, Zn)-ZIFs, hydrolytic stability testing was carried out at ambient conditions. According to the results of flame atomic absorption spectroscopy of supernatant solutions and x-ray diffraction analysis of solid samples, the negligibly small cleavage of Co–N and especially Zn–N bonds takes place during the first day of experiment with no discernible influence on the structure integrity of the studied ZIF mateials. Long-term water exposure of Co-containing ZIF materials results in transition of metal cations into the solution and noticeable transformation of crystalline structure. The effectiveness of the synthesized (Co, Co/Zn, Zn)-ZIFs in sorption of methyl orange from aqueous solutions was studied in dependence on duration of contact and equilibrium concentration of azo dye. Obtained results were analyzed by kinetic (Lagergren and Ho-McKay) and equilibrium (Langmuir, Freundlich, and Dubinin–Radushkevich) adsorption models. It was found that the kinetics of methyl orange sorption by synthesized ZIF sorbents is best described with the Ho-McKay model. The equilibrium sorption on Co-ZIF and Zn-ZIF with cuboid particle morphology proceeds in accordance with the Langmuir model, whereas interaction with heterometallic Co/Zn-ZIF agrees with the Freundlich one. The mean free adsorption energy for sorption of azo dye by ZIF materials increases in the order Zn-ZIF < Co/Zn-ZIF < Co-ZIF. The x-ray diffraction studies of (Co, Co/Zn, Zn)-ZIF materials after methyl orange sorption proved the complete transformation of mixed ZIF-L/ZIF-67 phase of cuboid Co-ZIF to rhombic dodecahedral ZIF-67.