MOFs are usually highly crystalline and researchers normally go out of their way to avoid introducing defects and structural disorder in these materials. However, it is now coming to light that these defects may in fact be advantageous in some cases.

Examples of non-crystalline or amorphous MOFs include the zeolite imidazolate framework (ZIF) family, which have similar structures to inorganic zeolites. They are formed from Mn+ (where Mn+ can be Li+, B+ or Zn+) inorganic nodes connected by Im-based ligands (C3H3N2-). One member of this family, ZIF-4, consists of Zn atoms surrounded by four organic molecules (the imidazolates). Recently, researchers observed a molten ZIF state forming from a crystalline phase and found that they could quench this liquid phase into a glass.

“Nothing was known about the nature and properties of this MOF liquid, however, and our work aimed to provide information on its structure and dynamics using both experimental and computational techniques,” explains co-team leader François-Xavier Coudert at CNRS/Chimie Paris Tech. “We found that the coordination nodes in the liquid (and thus its ‘chemical nature’) are very similar to that of the crystal form, except that the atoms move around much more and there is exchange between the metals in the compound and the linkers. Secondly, we saw that the liquid retains most of the porosity of its parent crystalline phase, which is a very surprising – and rare – feature.”

The porous structure remains thanks to the fact that the liquid MOF retains the same pyramidal structure as in the solid, he says.

"Strongly associated" nature of the MOF liquid

“The ‘strongly associated’ nature of the MOF liquid means that it shows preferential tetrahedral order, with on average close to four organic linkers coordinated to one Zn cation,” he adds. “This structure is very similar to that of liquid water, which also shows strong association and tetrahedral ordering.

“This strongly associated nature poses very fundamental questions such as: does this MOF liquid behave like a normal liquid or will it share some of water’s anomalies (for example, non-monotonic density and non-classical transport mechanisms)?”

Porous liquids in themselves are extremely rare and could be used in practical applications in industry, such as liquid-gas separation. They are also valuable as intermediaries to obtain MOF glasses, allowing for shaping and casting, with resulting macroscopic architectures that have better mechanical properties than crystalline powders, and are much more convenient to work with in practical applications and devices than their solid counterparts.

First report of a liquid MOF state

“Our work is the first report of the liquid MOF state and will hopefully help diversify the MOF field and allow it to move forwards into the amorphous domain,” adds co-team leader Thomas Bennett of the University of Cambridge.

The team, which includes scientists from Air Liquide in Jouy-en-Josas, the Advanced Photon Source at the Argonne National Laboratory in Illinois and the Rutherford Appleton Laboratory in Didcot, says that it will now be studying different MOF crystals (with different chemical compositions and different porosities). “We will be trying to better understand how the melting of different MOF crystals can lead to different properties of the liquid, and ultimately of the melt-quenched glasses, obtained,” Coudert tells nanotechweb.org.

The research is detailed in Nature Materials doi:10.1038/nmat4998.