Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Fe, Co, Zn) connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies. As of 2010, 105 ZIF topologies have been reported in the literature. Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIFs are being investigated for applications such as carbon dioxide capture. ZIF glasses can be synthesized by the melt-quench method, and the first melt-quenched ZIF glass was firstly made and reported by Bennett et al. back in 2015. ZIFs remain porous even after forming glasses, recent studies have revealed that the linker modification can really modulate the melting behaviour of ZIFs. ZIF glasses are a newly discovered type of material that has been garnering increasing interest in recent years, with around 13 different ZIFs, including ZIF-4, ZIF-62, and ZIF-76, being successfully prepared in their glassy state. In traditional materials science, glasses can be divided into three major families: inorganic, organic, and metallic. The chemical bonds that make up the structure of members of each family are mixed ionic/covalent bonds, covalent bonds, and metallic bonds, respectively. ZIF glasses, on the other hand, are an organic-inorganic coordinated glass discovered only recently, and have a completely different structure than the three traditional glass families. They thus represent a fourth type of glass.
History In 2006, Omar M. Yaghi and his collaborators published a series of Zeolitic imidazolate frameworks (ZIFs), including the iconic ZIF-8, building on his pioneering work in metal-organic frameworks (MOFs). Yaghi introduced ZIFs as a novel class of materials that combined the structural characteristics of zeolites — such as their tetrahedral coordination and robust chemical stability— with the tunability and porosity of MOFs. By linking metal ions (typically zinc or cobalt) with imidazolate linkers, ZIFs achieved the zeolite-like topology while maintaining the modularity and versatility of MOF chemistry. This innovation significantly broadened the potential applications of porous materials in areas such as gas storage, separation, and catalysis. The breakthrough material ZIF-8, composed of zinc ions and 2-methylimidazolate linkers, exemplifies the unique properties of ZIFs. ZIF-8 demonstrated exceptional chemical and thermal stability, coupled with a highly selective pore system, making it suitable for demanding applications like carbon dioxide capture and hydrocarbon separation. The introduction of ZIFs not only expanded the capabilities of MOFs but also bridged the gap between traditional zeolites and modern framework materials, solidifying Yaghi’s reputation in the development of advanced porous materials.
Glassy structure The structure of melt-quenched ZIF glasses maintains a certain amount of short-range order, although the chemical configuration and coordination environments, after melting, lose long-range order completely. From a microscopic view, the linkages between metal nodes and organic ligands (e.g., Zn-N linkages) partially break at high temperature and the resulting undercoordinated metal ions have the potential to link with other neighboring organic ligands for exchange. One notable discovery regarding the structure of ZIF glass was made by Rasmus et al. Before this research was published, the short-range structural order at the scale of the cation-ligand units remained unknown given the limitations of the analytical techniques available. The short-range structural disorder of the tetrahedral ligand environment around metal nodes in the ZIF glass was detected for the first time by performing zinc-67 nuclear magnetic resonance. This finding clearly showed that ZIF glasses are structurally very different from the other known glass types, overturning the traditional view that a glass structure has short-range order and long-range disorder, providing a broader view of what qualifies as a glass.
… excerpt ends here. Continue reading the full article.
