Blending liquid MOFs produces new glass materials
New research on liquid MOFs shows they can blend together to form a new glass MOF
Metal-organic frameworks (MOFs) are a class of crystalline materials with a structure of inorganic nodes connected by organic ligands. There are currently more than 60,000 known MOFs, and they are being investigated as promising materials for gas storage, including CO2 sequestration and hydrogen storage, and can even be used to harvest water in the desert. Research so far has concentrated on MOFs in the solid state, but these soft, microcrystalline powders are hard to process industrially, as they can’t be sintered and are hard to form into pellets.
Curiously, several examples of the melting of crystalline MOF structures have been observed, along with the formation of a liquid of identical composition to the parent framework. Cooling of these liquids then results in a new family of gasses. The fundamental novelty of the liquid and glass MOF states, compared with their more widely known crystalline states, provoked early research1 into the reactivity of the liquid state, and particularly how a liquid MOF may interact with another MOF component.
Using new techniques to investigate amorphous MOFs
The idea of using small and wide angle X-ray scattering (SAXS and WAXS) on these materials was first raised by I22 beamline scientist Dr Andy Smith.
Unlike with many of Diamond’s techniques we cannot see individual atoms with SAXS since the technique works at longer length scales, those of large molecules or molecular assemblies. In this work we were able use SAXS to watch the changes occurring as the microcrystals of MOF melted into the glassy state and correlate this with simultaneous WAXS data showing the gradual loss of crystallinity. Combination of SAXS/WAXS on I22 with PDF measurements on I15-1 allows a fuller understanding of these complex materials under processing conditions. It is an area we hope to do a lot more work in.”
Another innovation was to use ex situ energy-dispersive X-ray spectroscopy (EDS) to give element-specific results, and EDS tomography. The resulting 3D images show that the two MOF phases bind across the interface between them, in a ligand swapping process. The two MOFs used in these experiments were quite viscous; further experiments showed that starting with smaller particles of the crystalline MOFs resulted in a greater degree of mixing, or blending.
What’s next for liquid MOFs?
The next steps for this research are to investigate which MOFs can be blended together to make useful new materials.
Gaillac R et al. Liquid metal–organic frameworks. Nature materials, 16 (1149–1154) (2017). DOI: 10.1038/nmat4998.
Longley L et al. Liquid phase blending of metal-organic frameworks. Nature Communications 9, 2135 (2018). DOI: 10.1038/s41467-018-04553-6.