Chemists aim to produce liquids made out of holes
UK chemists are trying to create the first liquids made from holes. The strange fluids could change the way chemical plants operate, they claim.
Solids filled with microscopic pores are big business for the chemicals industry. Molecular sponges like zeolites, used in washing powders and industrial plants that break down crude oil, have frameworks riddled with holes to soak up molecules.
But the mopping-up process can be relatively slow: molecules must travel through a rigid set of channels to get to pores buried deep in the solid. Stuart James and his team at Queen’s University, Belfast, reasoned that it would be a lot faster if the pores themselves could flow like liquids, rather than stay fixed in regimental position.
As David Rooney, a chemical engineer also at Queen’s University, points out, such holey liquids could be pumped around a chemical plant, so that molecules could be absorbed in one place and released in another. That would also mean every part of the plant could operate continuously; at the moment, packed beds of solid sponges must be regenerated after their mopping up job.
James also thinks that porous liquids might act as fluid sieves - easing the passage of some molecules through a liquid by whisking them into mobile portals, leaving others to take their chances sneaking through transient gaps in the solvent.
In a concept article published in Chemistry - A European Journal, James outlines how the vision might work in practice.1 A liquid would remain microporous as long as its constituent cavernous molecules didn’t collapse when their cavities weren’t filled; and as long as they couldn’t stack into each other. But it’s tough to design molecules with those properties, so James suggests another possibility.
Individual molecules or entire solid porous frameworks could be dissolved in solvents which can’t interfere with the host’s cavities - if they are too large to fit through a restricted access point, for example.
Of course, chemists have been dissolving cave-like molecules in liquids for years. But James is quick to stress that in these cases, the holes are filled up by solvent, which must be pushed out by incoming molecules. With microporous liquids, by contrast, the pores would always be empty, ready to receive guests.
Though the concept is new, some chemists may have already unwittingly created porous liquids, says James. He points to a 1994 paper describing shell-like hemicarcerands,2 and a 2003 description of cube-like cobalt/ruthenium cages bridged with cyanide ligands;3 in both these cases, it seems that even when the molecules are dissolved in liquids, their cavities remain empty.
It’s not yet clear whether the improved mopping-up efficiency of holey liquids would be worth the effort put into their development. But James hopes chemists will want to meet his intriguing challenge.
Richard Van Noorden
1 N O'Reilly, N Giri & S James, Chem. Eur. J., 2007, DOI: 10.1002/chem.200700090
2 T A Robbins et al, J. Am. Chem. Soc., 1994, 116, 111
3 S C N Hsu et al, Angew. Chem., 2003, 115, 2767