Study On ZIF-4 Crystal Synthesis,Glass Formation And Glass Stability

Metal-organic frameworks(MOFs)are a class of crystalline porous materials,having a periodic network structure formed by self-assembly of inorganic metal centers(metal ions or metal clusters)and bridged organic ligands.MOFs is an organic-inorganic hybrid material that combines the rigidity of inorganic materials with the flexibility of organic materials.It shows great development potential and attractive prospects in modern material application research.In recent years,some MOFs have been found to be melt and vitrification.The discovery of molten and glassy MOFs broadens their processing methods,making MOFs equivalent to other liquid/glass materials(such as metals,oxides,chalcogenides,and organic polymers)that are important in industrial production.More importantly,the mechanical properties of MOFs glass obtained by melt-quenching were greatly improved,which provided a good research direction for solving the problems such as the difficulty of powder forming and poor mechanical properties during the practical application of MOFs materials.As an important subset of MOFs,zeolitic-imidazolate frameworks(ZIFs)with high thermal stability and chemical stability are one of the best candidates for vitrification in MOFs by melt-quenching.In addition,the discovery of ZIFs glass provides a new way to study the fundamental problems of glass transition and glass formation.(1)Synthesis of ZIF-4 crystal.Controlling the morphology and particle size of the solvothermal synthesis products is very important for the application of ZIFs materials.It is important to optimize the process parameters that affect the crystal morphology and size.In order to better understand the solvothermal synthesis mechanism of ZIFs materials,and to link the solvothermal synthesis conditions with the properties of the prepared ZIFs materials,we used the typical ZIF-4(Zn/Co[C₃N₂H₃]₂)crystals as an example,the synthesis conditions and process parameters of ZIF-4(Zn)and ZIF-4(Co)were systematically studied.The conclusions are as follows:During the synthesis of ZIF-4(Zn)crystals,high temperature(above 383 K)or long duration(above 96 h)will lead to the production of ZIF-zni(Zn)crystals.Pure ZIF-4 crystals can only be obtained at low solvothermal temperatures(below 383 K).The solvothermal process of ZIF-4(Co)crystals only produces pure ZIF-4(Co).The formation of ZIF-zni(Zn)crystal phase undergoes the dissolution and recrystallization of the mesophase ZIF-4(Zn),indicating that ZIF-4(Zn)is not the most thermodynamically stable phase and requires to be further converted into a thermodynamically stable product(ZIF-zni(Zn)).The dissolution of the intermediate ZIF-4(Zn)formed in the first stage may release an active substance that directs the growth of the final product ZIF-zni(Zn).An amorphous phase was detected during the crystallization of ZIF-4(Co).Although this intermediate amorphous phase was also observed during the synthesis of inorganic zeolites,it is not common in the study of ZIFs synthesis.The transition from the intermediate amorphous phase to the thermodynamic metastable or steady state crystalline phase is a rearrangement of the topology.Although only the types of metal ions have been changed,the formation of crystals in the solvothermal synthesis of the two ZIF-4 crystals is completely different.(2)Preparation of ZIF-4 glass.To explore the relationship between the synthesis conditions of ZIFs crystals and the properties of glassy ZIFs,it is helpful to deepen the understanding of the new ZIFs glass family,which is of great significance for obtaining ideal ZIFs glass.Taking ZIF-4(Zn)crystal as an example,we studied the effects of solvothermal synthesis conditions(solvothermal temperature and reaction time)on the melting and vitrification of ZIFs crystals.The conclusions are as follows:The change of solvothermal synthesis conditions affects the melting temperature of ZIF-4 crystal by affecting the morphology and particle size of the synthesized crystal,and the melting enthalpy is affected by the crystal quality.The melting process of ZIF-4 crystal is as follows:the coordination bond between the imidazole ring and the zinc ion is partially broken as the temperature increases,the imidazole ring leaves,and at the same time,the imidazole ring fractured from the adjacent zinc ion will be reconnected to the zinc ion.This dynamic process occurs in a very short period and produces tri-or even di-coordinated zinc ions,and suspended nitrogen orphaned electrons in the imidazole ring.That is,the melting of ZIF-4 crystal depends on partial dissociation and recombination of the Zn-N coordination bond.Therefore,the strength of the coordination bond between the metal cation and the organic anion ligand plays a key role in the melting process,and the lower interaction facilitates the melting.It is possible to reduce the melting temperature by changing the synthesis conditions of the crystal ZIFs,which makes it possible to promote the melting of other ZIFs(even MOFs),thereby making it possible to prepare more ZIFs(or MOFs)glass by melt quenching.(3)Stability of ZIF-4 glass.Nuclei or crystals that may form in the glass will have an effect on the thermal stability of the system and the properties of the material.Considering the high temperature environment that may be involved in the application of glass materials and the rich structural evolution of ZIFs during heating,it is important to understand the structure of ZIFs glass and the stability of their structure(anti-crystallization).The structural evolution and stability of melt-quenched ZIF-4 glass were studied by constant temperature heat-treatment and dynamic heat-treatment.The conclusions are as follows:No crystals were detected in ZIF-4 glass after a long time and high temperature heat-treatment or multiple-cycle heat-treatment,indicating that ZIF-4 glass has good thermal stability(resistance to crystallization),which is very beneficial to the practical application of ZIF-4 glass.The structural changes of ZIF-4 glass during heat-treatment are as follows:constant temperature heat-treatment or dynamic multiple-cycle heat-treatment strengthens the medium-range structure ordering of ZIF-4 glass.The short-range structure of ZIF-4 crystal remains unchanged during melt-quenching and heat-treatment,while the medium-range structure is destroyed during melting and frozen by quenching(the medium-range structure of ZIF-4 glass is disordered).The medium-range structure of ZIF-4 glass is ordered during heat-treatment.The study of ZIFs glass openes up a new direction in the field of MOFs research.The study of the midium-range structural changes during heat-treatment of ZIFs glass may help to understand the nature of glass formation and transformation,which is considered to be one of the most interesting and complex problems in condensed matter science.