Controlled synthesis of phase-pure zeolitic imidazolate framework Co-ZIF-9

The synthesis of phase-pure Co-ZIF-9, an important cobalt-based zeolitic imidazolate framework, could be achieved by modification of the reported synthesis procedure through pH adjustment of the starting synthesis mixture. The phase-pure Co-ZIF-9 material obtained has been characterized by a combination of UV/Vis, FTIR, and Raman spectroscopy as well as by thermogravimetric analysis (TGA) and XRD and possesses a lower overall crystallinity. This can be explained by the addition of the base for the pH adjustment method. On the basis of these findings, a synthesis pathway for the formation of the secondary phase, cobalt formate, is proposed along with its relationship to the flexibility of the coordination environment of cobalt ions. The crystal structures of both phases have been determined by single-crystal X-ray crystallography, and the resolved structures also reflect the coordination flexibility of the framework cobalt ions.