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Porous ionic liquid captures alcohols and CFCs

Cage-like structures could enable selective separations in continuous-flow systems

by Mark Peplow, special to C&EN
February 23, 2020 | A version of this story appeared in Volume 98, Issue 8

A porous ionic liquid that can trap guest molecules, including simple alcohols and halocarbons, could pave the way for new separation technologies (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0419-2). Chemists often use porous solids to separate molecules. But while solids typically require batch separations, porous liquids containing cage-like structures could perform separations in continuous-flow systems. Until now, though, porous liquids haven’t trapped guest molecules larger than carbon dioxide or methane. Jonathan R. Nitschke at the University of Cambridge and colleagues created a porous liquid containing a tetrahedron of zinc ions surrounded by coordinating ligands, which forms a 6.2 Å wide cavity between the ions. The ligands have long poly(ethylene glycol) tails that render the compound liquid at room temperature, along with charged imidazolium groups that repel one another, ensuring the tails cannot enter the cavity. The cage can capture propanol and butanol, with branched isomers binding more strongly than linear isomers. It also traps several types of gaseous chlorofluorocarbons (CFCs). All guest molecules can be removed under vacuum, enabling the porous liquid to be recycled. The compound’s performance is not yet good enough for efficient separations, though. “It’s got too little cavity and too much stuff on the outside,” Nitschke says. So the researchers want to make similar liquids with larger cavities and improved guest binding.

Line structure of a cage-like porous ionic liquid compound and a crystal structure of a related cage compound.
An organic ligand coordinates zinc ions to form a tetrahedral cage (left), creating a cavity that can host guest molecules. (R = poly(ethylene glycol)–imidazolium.) The crystal structure (right) of a related compound (R = CH3) shows that the cage bears a ligand on each of its four faces. Yellow = Zn, blue = N, black = C, and white = H.


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