Metal-organic frameworks are porous materials with applications in, e.g., catalysis or gas separation and storage. They generally are crystalline solids. Usually, the porous properties are not retained when the material is melted.
Thomas D. Bennett, University of Cambridge, UK, François-Xavier Coudert, Chimie ParisTech, PSL Research University, CNRS, Paris, France, and colleagues have obtained a strongly associated MOF liquid by melting a zeolitic MOF (ZIF-4) at high temperature. The team has studied the melting phenomenon and the nature of the liquid with a combination of variable temperature X-ray, neutron diffraction experiments, and molecular dynamics (MD) simulations. Structural, dynamical, and thermodynamical information show that the chemical configuration, coordinative bonding, and porosity of the parent crystalline framework are retained in the MOF liquid.
The MOF is made up of pyramidal molecular frameworks, each consisting of a zinc atom and four imidazolates. The porosity results from the presence of gaps between the pyramidal structures that exist even in the liquid state. After cooling and solidification, the MOF turns into a glass (a disordered, non-crystalline structure) that also retains the same properties in terms of porosity.