MOLTEN MOFS

The study of scaffolding-like crystals known as metal–organic frameworks (MOFs) has dominated much of the field of solid state chemistry over the last decade. Favoured for their ability to store gases such as hydrogen and carbon dioxide, much is known of the chemistry of MOFs. But relatively little is known of their physical properties, their underlying thermodynamics, and their relationship to other, more familiar, forms of matter.

We have recently found that MOFs can exist in a liquid-like state, forming amorphous structures similar to that of window glass. This discovery opens a number of new exciting possibilities for the design of MOFs that couple the glassy state with favourable properties such as chirality, fluorescence, charge transfer and catalytic activity.

MOFs are assembled by connecting together metal atoms or inorganic clusters of atoms using organic molecules. Because there exists a huge range available of different metal types, clusters, and organic molecules from which to make MOFs, the family is essentially limitless. Moreover, the one set of components can connect in different ways: even for the one chemical system there are often many different framework topologies accessible, each with their own particular properties.

This is no truer than for the family of zeolitic imidazolate frameworks (ZIFs), a sub-family of MOFs. ZIFs contain a single type of metal atom (usually zinc, and sometimes cobalt) and a single type of pentagonal organic molecule known as imidazole. What is particularly interesting about this family is that the arrangement of zinc atoms and imidazole molecules is completely analogous to the arrangement of silicon and oxygen atoms in silicates. Just as silica can form many different “polymorphs” — zeolites, window glass and crystalline quartz are essentially the same but for a change in connectivity — so too can ZIFs be found in many different crystalline forms.

What we have found is that on heating a number of different ZIFs (only one is studied in the paper below), it is possible to “melt” the structure into an amorphous state. This amorphous form can be cooled down to room temperature without crystallising and appears in many ways to resemble a traditional melt-quenched glass. Even probing its atomic-scale structure using neutron and x-ray total scattering reveals a structure similar in its connectivity to that of silica glass. We now have the fun of trying to work out how best to exploit this new state of MOF matter….

Structure and properties of an amorphous metal–organic framework. Phys Rev Lett 104, 115503 (2010)

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