Zeolitic imidazolate framework
Zeolitic imidazolate frameworks are a class of metal-organic frameworks that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions 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 and the first ZIF
In 2006, Omar M. Yaghi and his collaborators published a series of Zeolitic imidazolate frameworks, including the iconic ZIF-8, building on his pioneering work in metal-organic frameworks. 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 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 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.
Synthesis
ZIFs are mainly prepared by solvothermal or hydrothermal techniques. Crystals slowly grow from a heated solution of a hydrated metal salt, an ImH, a solvent, and base. Functionalized ImH linkers allow for control of ZIF structure. This process is ideal for generating monocrystalline materials for single-crystal X-ray diffraction. A wide range of solvents, bases, and conditions have been explored, with an eye towards improving crystal functionality, morphology, and dispersity. Prototypically, an amide solvent such as N,N-dimethylformamide is used. The heat applied decomposes the amide solvent to generate amines, which in turn generate the imidazolate from the imidazole species. Methanol, ethanol, isopropanol, and water have also been explored as alternative solvents for ZIF formation but require bases such as pyridine, TEA, sodium formate, and NaOH. Polymers such as poly–poly–poly, polyvinylpyrrolidone, and poly- have been found to act as crystal dispersants, imparting particle-size and morphology control.Due to their promising material properties, significant interest lies in economical large-scale production methods. Sonochemical synthesis, which allows nucleation reactions to proceed rapidly through acoustic generation of localized heat and pressure, has been explored as a way to shorten synthesis times. As with the case of zeolites, microwave-assisted synthesis has also been of interest for the rapid synthesis of ZIFs. Both methods have been shown to reduce reaction times from days to hours, or from hours to minutes. Solvent-free methods, such as ball-milling or chemical vapor deposition, have also been described to produce high-quality ZIF-8. Chemical vapor deposition is of particular promise due to the high degree of uniformity and aspect ratio control it can offer, and its ability to be integrated into traditional lithographic workflows for functional thin films. Environmentally-friendly synthesis based on supercritical carbon dioxide have been also reported as a feasible procedure for the preparation of ZIF-8 at an industrial scale. Working under stoichiometric conditions, ZIF-8 could be obtained in 10 hours and does not require the use of ligand excess, additives, organic solvents or cleaning steps.
Using the traditional melt-quench of metals or sintering of ceramics would cause the collapse of MOF structure as its thermal decomposing temperature is lower than its melting temperature. Moreover, the amorphous form of MOF can be achieved through pressurization or heating, but its network feature would be significantly broken during the amorphization process. Bennett et al found certain members from MOF family can be made into a glassy state. Those carefully selected ZIF crystals are able to form a glassy solid after heating and cooling in an argon atmosphere. Moreover, the melting range can be tuned by their network topologies.
Potential applications
The crystal form of ZIF, or MOF in general, is known for its porosity, but is difficult to mass-produce and incorporate in actual applications due to unavoidable intercrystalline defects. Even more recent study has shown that ZIFs' porosity can be precisely tuned by applying adequate pressures through linker functionalization, for example ZIF-62 shows a continuous phase transition from open pore to close pore phase with increasing the bim− concentration over 0.35 per formular unit. Almost half of the porosity remains after melting. There are several interesting characters about ZIF glasses addressing those challenges to potentially realize promised applications achievable. The first intriguing one is that ZIF glass maintains the porous structure as its crystalline form after melt-quench process, which means it can be applied for applications such as gas separation and storage. The glassy form would also offer unique opportunities for easy processability and mass production. Last but not least, besides pure ZIF glass, composites based on it by tuning the composition and structure has the distinct advantage of a broad design space.Applications to carbon capture
ZIFs exhibit some properties relevant to carbon dioxide capture, while commercial technology still centers around amine solvents.Zeolites are known to have tunable pores – ranging between 3-12 Angstroms – which allows them to separate carbon dioxide. Because a
ZIFs 68, 69, 70, 78, 81, 82, 95, and 100 have been found to have very high uptake capacity, meaning that they can store a lot of carbon dioxide, though their affinity to it is not always strong. Of those, 68, 69, and 70 show high affinities for carbon dioxide, evidenced by their adsorption isotherms, which show steep uptakes at low pressures. One liter of ZIF can hold 83 liters of. This could also be useful for pressure-swing adsorption.