Selection of a host material for gas storage and separations, ion transport, and catalysis often requires consideration of trade-offs between the inherent capabilities of porous solids and liquid solvents. Porous solids offer high internal surface areas that enable the accommodation of a high density of guest species, while liquids offer intrinsic fluidity that enables chemical species to be efficiently delivered to where and when they are needed in continuous processes. Developing materials that feature both porosity and fluidity – two properties of matter that rarely coexist in conventional materials – would provide access to new capabilities for both porous solids and liquids and would provide a unique platform to explore fundamental questions related to liquid structure and free volume.
Although significant efforts have been made to fluidize porous solids in systems such as fluidized bed reactors, only limited efforts have been made to impart porosity to inherently fluid materials. Tiny pores form and collapse within conventional molecular liquids as their molecules and atoms move, but the typical size of these transient pores is smaller than most molecules. Here, we will describe efforts to study metal-organic liquids that feature larger and more persistent transient pores than those that exist in conventional liquids. The design of these metal-organic liquids takes guidance from both metal-organic frameworks and ionic liquids to generate materials with low melting temperatures and dynamic – but highly directional – coordination bonds that direct the formation of transient pores. We are interested in establishing relationships between liquid structure – and free volume – and bulk macroscopic properties such as viscosity, surface tension, conductivity, and gas solubility.