The 3D structures of ZIFs crystals consist of transition metal cations (M) such as Zn 2+ , and Co 2+ , which are coordinated to an imidazole-based (Im) linker (Figure ). The coordination bonds are formed between the tetrahedral metal centers and nitrogen atoms in the 1,3-positions of the imidazolate linker . The nature between the N atom and the metal node is not well known. However, the crystal combines both types of covalent and coordination bonds. The bond angle of M-Im-M is 145 o, similar to the Si-O-Si bond angle ) in zeolites. Thus, the name was 'Zeolitic Imidazaolte Frameworks', meaning 'zeolite-like materials based on imidazolate. ZIFs exhibit zeolite topology such as SOD, RHO, or LTA. ZIFs crystals show the framework's flexibility concerning gas adsorption. More than 105 ZIF topologies are in the literature (Figure ) .