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Materials

Liquid MOFs debut

Framework compound retains crystal structure and porosity in a high-temperature liquid state

by Mitch Jacoby
October 16, 2017 | APPEARED IN VOLUME 95, ISSUE 41

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Credit: Nat. Mater.
A framework compound known as ZIF-4 (ball and stick) retains its structure and porosity (yellow indicates void volume) in the liquid state.
Credit: Nat. Mater.
A framework compound known as ZIF-4 (ball and stick) retains its structure and porosity (yellow indicates void volume) in the liquid state.

Metal-organic framework (MOF) compounds are a large family of porous crystalline materials composed of metal ions joined by organic linkers. Because of their extreme porosity, which can be tailored via synthesis, MOFs are widely touted for their usefulness in gas separation, gas storage, and catalysis. Liquid versions of these materials could be especially useful because liquids are often more robust and easier to process than crystalline powders. But porous liquids with molecular orderliness are almost unheard of. So a team led by Thomas D. Bennett of the University of Cambridge and François-Xavier Coudert of the French National Center for Scientific Research (who is a C&EN advisory board member) decided to try to make a liquid version of a MOF. They succeeded (Nat. Mater. 2017, DOI: 10.1038/nmat4998). On the basis of X-ray and neutron-scattering methods and various computational techniques, the researchers conclude that heating ZIF-4, which is composed of zinc imidazolate units, to 856 K causes the solid to melt but the high temperature does not cause bond cleavage and decomposition. The data indicate instead that the hot liquid retains the chemical configuration, coordinative bonding, and porosity of the crystal state. The liquid’s properties and structure differ from those of the glassy state obtained upon cooling the material. The team suggests that this type of processing might be used to shape MOFs at the macroscale by using the liquid as a transient state en route to forming a glass or recrystallized product.

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