Glass formation and meltability of metal-organic frameworks

Melt-quenched (MQ) glasses derived from metal-organic frameworks (MOFs) have emerged very recently as tunable organic-inorganic hybrid glasses, showing potential applications in gas separation. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs which have shown high meltability. ZIF-62, a mixed-linker ZIF, has been investigated extensively because of its low melting temperature and high thermal stability. Application of glasses formed from MOFs on an industrial scale requires large-scale production of parent crystals. However, in large-scale production of mixed-linker MOFs, both kinetics and thermodynamics of synthesis play significant roles. The importance of both factors originates from a heterogeneous linker distribution in mixed-linker MOFs where each type of linker can form different crystalline phases during synthesis, altering thermal properties and more importantly, the meltability. The parameters which affect the formation of different crystalline polymorphs in ZIF-62, and the methods required to detect such structural heterogeneity in the final material are investigated, while possible phase transformations are also discussed. Among a huge number of crystalline MOFs (over 70,000), only small number of them have shown the ability to melt. Decomposition of the framework prior to its melting transition is an obstacle hindering the transition to the liquid state while heating. Porosity and metal-ligand interactions have been identified as parameters determining meltability of such frameworks. To overcome these constraints, an ionic liquid (IL) containing its sodium salt was incorporated into the cages of ZIF-8, a highly porous, normally non-meltable ZIF. After mechanical amorphization, the structure of the resultant composite and stability of IL molecules in the collapsed pores were examined, and the effect of structural collapse on ionic conduction, as a macroscopic property, was investigated.



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