Crystalline, Amorphous, and Liquid Metal-Organic Frameworks at Non-Ambient Conditions
Metal-organic frameworks (MOFs), in the traditional sense, are crystalline and microporous materials. MOFs are of incessant scientific interest due to their enormous potential as host structures for a range of chemical process applications, including molecular separation, catalytic reaction, and gas storage. In the course of investigating their physical and chemical properties, it was soon discovered how diverse the structural responses of these low-density materials are when, for example, heated or compressed. Such structural changes range from the simple loss of pore-occupying solvent molecules to intricate sequences of structural phase transitions. Some MOFs are also capable of forming glasses, i.e. dense amorphous structures, with chemical composition and short-range order that is reminiscent of their crystalline precursor. This generated a new area of research within the MOF discipline at the interface of glass science, supramolecular chemistry, and condensed matter physics. Here, two isotopological zeolitic imidazolate frameworks, ZIF-4 and ZIF-62, have been studied under the influence of a range of physical variables. Pressure- and temperatureinduced crystalline–amorphous and crystalline–crystalline phase transitions have been investigated using in-situ high-pressure/high-temperature powder X-ray diffraction and Raman spectroscopy. Furthermore, these transitions were also studied as a function of varying chemical composition of the ZIFs, as well as of dynamic variables such as heating- and compression-rate. The two ZIFs were shown to be related by a continuous solid solution and this chemical substitution was found to systematically control the high-temperature behaviour. Moreover, simultaneous heating and compression of the endmember compositions ZIF-4 and ZIF-62 up to 600 ◦C and 8 GPa resulted in strikingly different pressure-temperature phase diagrams. For ZIF-4, four, previously unknown, high-pressure-temperature polymorphs were found. The crystal structures of two new phases were solved by powder diffraction methods. The other two new phases could be assigned with a unit cell and space group. In contrast, the crystalline starting phase of ZIF-62 undergoes pressure- and temperature-induced amorphisation without subsequent recrystallisation. Importantly, it was found that the melting temperature of ZIF-62 decreases with increasing pressure. Furthermore, based on the topology of the two phase diagrams, it could be concluded that the respective pressure- and temperature-amorphous phases of ZIF-4 and ZIF-62 must be different from each other – density contrasts and observations on the reversibility of the crystalline–amorphous transitions clearly indicate the polyamorphic character of these phases. The use of a pressure-assisted sintering techniques allowed for the production of macroporous crystalline, dense amorphous, and dense recrystallised monoliths of ZIF-4, in accordance with the previously established phase diagrams. The results from mechanical testing and microstructural analysis of the monoliths correlate well, which establishes useful materials characteristics for potential industrial applications. The interconnected porosity of the macroporous crystalline material provides an immense interface for gas–solid interaction, while the dense amorphous and recrystallised monoliths have distinct mechanical robustness. The structural collapse of of the ZIFs was also studied as a result of the interaction of these materials with X-rays. The underlying mechanisms were investigated by kinetic analysis of the crystalline-amorphous transformation at ambient and elevated temperature. It was found that the ZIFs display a rare example of transient effects which lead to increasing local Avrami exponents in the course of amorphisation. This was attributed to the structural complexity and the density contrast between the crystalline and the amorphous forms of the frameworks. These findings have essential practical implications - any synchrotron-based experiments on MOFs should ideally be preceded by an assessment of their stability in the beam.
Metal-organic frameworks, Diamond anvil cell, Non-ambient XRD
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