One of the hottest new materials is a
class of porous solids known as metal-organic frameworks, or MOFs. These
man-made materials were introduced in the 1990s, and researchers around the
world are working on ways to use them as molecular sponges for applications
such as hydrogen storage, carbon sequestration, or photovoltaics.
Now, a surprising discovery by
scientists in Canada and Russia reveals that MOFs also exist in nature—albeit
in the form of rare minerals found so far only in Siberian coal
mines.
The finding, published in the journal Science
Advances, "completely changes the normal view of these highly popular
materials as solely artificial, 'designer' solids," says senior author
Tomislav Friščić, an associate professor of chemistry at McGill University in
Montreal. "This raises the possibility that there might be other, more
abundant, MOF minerals out there."
The twisting path to the discovery
began six years ago, when Friščić came across a mention of the minerals
stepanovite and zhemchuzhnikovite in a Canadian mineralogy journal. The crystal
structure of the minerals, found in Russia between the 1940s and 1960s, hadn't
been fully determined. But the Russian mineralogists who discovered them had
analyzed their chemical composition and the basic parameters of their
structures, using a technique known as X-ray powder diffraction. To Friščić,
those parameters hinted that the minerals could be structurally similar to a
type of man-made MOF.
His curiosity piqued, Friščić began
looking for samples of the rare minerals, reaching out to experts, museums and
vendors in Russia and elsewhere. After a promising lead with a mining museum in
Saint Petersburg failed to pan out, Igor Huskić, a graduate student in the
Friščić research group at McGill turned his attention to synthesizing analogues
of the minerals in the lab - and succeeded. But a major journal last year
declined to publish the team's work, in part because the original description
of the minerals had been reported in a somewhat obscure Russian mineralogical
journal.
Then, the McGill chemists caught a
break: with the help of a crystallographer colleague in Venezuela, they
connected with two prominent Russian mineralogists: Sergey Krivovichev, a
professor at Saint Petersburg State University, and Prof. Igor Pekov of
Lomonosov Moscow State University.
Krivovichev and Pekov were able to
obtain the original samples of the two rare minerals, which had been found
decades earlier in a coal mine deep beneath the Siberian permafrost. The
Russian experts were also able to determine the crystal structures of the
minerals. These findings confirmed the McGill researchers' initial results
Stepanovite and zhemchuzhnikovite have
the elaborate, honeycomb-like structure of MOFs, characterized at the molecular
level by large voids. The two minerals aren't, however, representative of the
hottest varieties of MOFs—those that are being developed for use in
hydrogen-fueled cars or to capture waste carbon dioxide.
As a result, Friščić and his collaborators are now broadening their research to determine if other, more abundant minerals have porous structures that could make them suitable for uses such as hydrogen storage or even drug delivery.
As a result, Friščić and his collaborators are now broadening their research to determine if other, more abundant minerals have porous structures that could make them suitable for uses such as hydrogen storage or even drug delivery.
In any event, the discovery of MOF
structures in the two rare minerals already is "paradigm-changing"
Friščić says. If scientists had been able to determine those structures in the
1960s, he notes, the development of MOF materials "might have been
accelerated by 30 years."
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