Liquids with permanent porosity

Porous materials find use in applications such as gas separation, drug delivery and energy storage, but have hitherto been solid rather than liquid; now a combination of cage molecules and a crown-ether solvent that cannot enter the cage molecules is used to create a porous liquid that can solubilize methane gas better than non-porous liquids. Materials that are intrinsically porous, consisting of molecules organized in cage-like structures, are generally solid at usable temperatures. A few liquefiable, rigid cavity-containing molecules have been described, but they do not contain permanent pores as liquids inevitably flow to fill any available spaces. Now Nicola Giri et al. have developed a new type of 'porous liquid' consisting of a high concentration of modified cage molecules in a crown ether solvent that combine to generate permanent, unoccupied cavities in the liquid phase. The authors show that this free-flowing liquid can solubilize methane gas more efficiently than non-porous liquids. Porous liquids have potential in applications such as catalysis, extraction, and gas capture or separation. Porous solids such as zeolites1 and metal–organic frameworks2,3 are useful in molecular separation and in catalysis, but their solid nature can impose limitations. For example, liquid solvents, rather than porous solids, are the most mature technology for post-combustion capture of carbon dioxide because liquid circulation systems are more easily retrofitted to existing plants. Solid porous adsorbents offer major benefits, such as lower energy penalties in adsorption–desorption cycles4, but they are difficult to implement in conventional flow processes. Materials that combine the properties of fluidity and permanent porosity could therefore offer technological advantages, but permanent porosity is not associated with conventional liquids5. Here we report free-flowing liquids whose bulk properties are determined by their permanent porosity. To achieve this, we designed cage molecules6,7 that provide a well-defined pore space and that are highly soluble in solvents whose molecules are too large to enter the pores. The concentration of unoccupied cages can thus be around 500 times greater than in other molecular solutions that contain cavities8,9,10, resulting in a marked change in bulk properties, such as an eightfold increase in the solubility of methane gas. Our results provide the basis for development of a new class of functional porous materials for chemical processes, and we present a one-step, multigram scale-up route for highly soluble ‘scrambled’ porous cages prepared from a mixture of commercially available reagents. The unifying design principle for these materials is the avoidance of functional groups that can penetrate into the molecular cage cavities.