JELLIES that harden into solids riddled with holes could soon be put to work
as molecular sieves, say chemists in the US and the Netherlands. The holey
materials could separate different chemicals, or speed up chemical reactions for
industry.
Chemists have used porous minerals called zeolites for many years as
molecular sieves for separating molecules鈥攐nly small molecules with
certain shapes can fit through the holes. Zeolites can also be used to trap
molecules so that they can be analysed. Alternatively, they can act as catalysts
and speed up chemical reactions. This is because when a molecule is trapped in a
small space, its structure may change slightly in a way that makes it more
reactive.
Zeolites, however, come with a limited range of pore shapes and sizes which
restricts their versatility. Chemists would like to create 鈥渄esigner zeolites鈥�,
solids with tailor-made pores for a wide range of uses.
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Now Richard Weiss and his colleagues at Georgetown University in Washington
DC have found a way to make gels that harden into porous materials. They have
found several gelling agents, such as tetraoctyldecylammonium bromide (TOAB),
which can turn organic compounds into jelly.
The researchers mixed TOAB with a brew of either styrene鈥攖he building
blocks of polystyrene 鈥攐r methyl methacrylate, which is used to make
Perspex. They added a polymerising agent, then heated the mixture until the
solids dissolved. As the mixture cooled, the TOAB molecules formed a mesh,
creating a gel. Next, they blasted it with ultraviolet light to trigger a
polymerisation reaction that made the gel solidify.
By heating the gel in an alcohol solvent, they were able to boil off the
stringy TOAB molecules, leaving a solid material with micrometre-sized channels
and pores where the TOAB had been. Weiss says that in future, this kind of
material could be used in the same way as zeolites.
鈥淲e have demonstrated the principle,鈥� says Weiss, 鈥渂ut to put it into
industrial practice will require a lot of engineering.鈥� Unlike zeolites, the new
materials break down at high temperatures. But with more work, Weiss hopes to
tailor the size and shape of the channels so that they can be put to use as
low-temperature 鈥渟ieves鈥� and catalysts in a wide range of reactions.
Martinus Feiters and his colleagues at the University of Nijmegen have
developed a similar technique using compounds called gluconamides as gelling
agents. Both teams describe their results in the latest issue of Chemical
Communications (issue 6, p 543 and 545).
