Liquid Metal-Organic Frameworks

Metal-organic frameworks are porous materials that are proposed for use in a variety of gas storage and separation, catalytic, nuclear waste recycling and drug-delivery applications. Over 60,000 exist, and yet almost all are crystalline solids. Here, we introduce, for the first time, the concept of a MOF-liquid.

This strongly associated porous liquid is obtained by melting a zeolitic imidazolate framework (ZIF) of formula Zn(Im)₂ (Im – C₃H₃N₂^–) with retention of chemical configuration, coordinative bonding modes, and porosity of the parent crystalline framework. The mechanism of melting is investigated, finding the coordination bond switching near melting is analogous to that of hydrogen bond switching in water.

Porous liquids in themselves are extremely rare, and offers practical advantages for liquid gas separation at the industrial level. But liquid MOFs also have value as an intermediary to obtain MOF glasses, allowing for shaping and casting, with resulting macroscopic architectures that have echanical properties superior to crystalline powders, and are a lot more convenient to work with in practical applications and devices.

Figure: Atomic configuration of the melt-quenched glass, gained from modelling synchrotron and neutron total scattering data.