A Liquid With Holes In It

Supramolecular Chemistry: Dissolved cage compounds created a fluid with permanent porosity

Stu Borman

A new type of material developed this year, a liquid with permanent porosity, combines the merits of porous solids and continuous fluids. The systems developed by Stuart James of Queen’s University Belfast and coworkers could be used to separate mixtures of gases by size and to catalyze chemical processes, if the holey liquids can be made at a reasonable cost (Nature 2015, DOI: 10.1038/nature16072). Porous solids such as zeolites have proved useful for separating molecules and as catalysts. But being solids limits their applications. They can’t readily flow through channels or be smoothed onto surfaces. Liquids flow, but they aren’t usually holey. They do have intermolecular spaces, but those spaces are tiny, and bubbles blown into liquids to instill porosity generally dissipate quickly. The new holey liquids are made from hollow organic cage molecules built by coupling 1,3,5-triformylbenzene with one or two types of diamines and dissolving the cage molecules in a solvent. The cage openings are too small for the solvent molecules to enter and block, ensuring the liquids’ porosity. In their first effort, the researchers made a cage from triformylbenzene and a crown ether-functionalized diamine. The diamine decorated the cage surface with crown ether groups to make it soluble in a crown ether solvent. This initial holey liquid was viscous and hard to synthesize. So collaborators Andrew I. Cooper and Rebecca L. Greenaway at the University of Liverpool made a second version from triformylbenzene and two simple alkyldiamines in hexachloropropene solvent. The resulting liquid was less viscous and easier to make. Sheng Dai of Oak Ridge National Laboratory, who earlier helped design nanoporous liquids made up of surface-functionalized hollow colloidal silica nanoparticles, commented that the James group’s holey liquids “will open up new frontiers in how we think about porosity.”

A porous molecular organic cage with crown ether surface decorations (left) dissolves in a crown ether solvent (center) to generate a permanently porous liquid (right); C = gray, O = red, N = blue, H = white. Credit: Nature

A porous molecular organic cage with crown ether surface decorations (left) dissolves in a crown ether solvent (center) to generate a permanently porous liquid (right); C = gray, O = red, N = blue, H = white.
Credit: Nature

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