Individual crystals of synthetic zhemchuzhnikovite, prepared by Igor Huski?, McGill University. Credit: Igor Huski?, Friš?i? Research Group, McGill University
A surprising discovery reveals that MOFs also exist in nature – albeit in the form of rare minerals found so far only in Siberian coal mines. Metal-organic frameworks are human-made materials introduced in the 1990s, and researchers are working on ways to use them as molecular sponges for applications such as H storage, C sequestration, or photovoltaics.
The finding “completely changes the normal view of these highly popular materials as solely artificial, ‘designer’ solids,” says 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.”
6 years ago 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 structures using X-ray powder diffraction. To Friščić, those parameters hinted that the minerals could be structurally similar to a type of human-made MOF.
His curiosity piqued, Friščić began looking for samples of the rare minerals. 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. With the help of a crystallographer colleague in Venezuela, they connected with two prominent Russian mineralogists. Prof. Krivovichev and Pekov were able to obtain the original samples of the 2 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 from their lab synthesis.
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.
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.” http://www.eurekalert.org/pub_releases/2016-08/mu-hm080116.php
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