Newswise — Using
building blocks that make up ordinary plastics,
but putting them together in a whole new way, University
of Michigan researchers have created a class of
lightweight, rigid polymers they predict will be
useful for storing hydrogen fuel.
The work is described in today's (Nov. 17) issue
of the journal Science .
The trick to making the new materials, called covalent
organic frameworks (COFs), was coaxing them to assume
predictable crystal structures---something that never
had been done with rigid plastics.
"Normally, rigid plastics are synthesized by rapid
reactions that randomly cross-link polymers," said
postdoctoral fellow Adrien Côté, who
is first author on the Science paper. "Just as in
anything you might do, if you do it really fast,
it can get disorganized." For that reason, the exact
internal structures of such materials are poorly
understood, making it difficult to predict their
properties. But Côté and colleagues
tweaked reaction conditions to slow down the process,
allowing the materials to crystallize in an organized
fashion instead of assembling helter skelter.
As a result, the researchers can use X-ray crystallography
to determine the structure of each type of COF they
create and, using that information, quickly assess
"Once we know the structure and properties, our
methodology allows us to go back and modify the COF,
making it perform better or tailoring it for different
applications," said Côté.
on the work with Omar Yaghi, who is the Robert
W. Parry Collegiate Professor of Chemistry at U-M.
Over the past 15 years, Yaghi has taken a similar
approach to producing materials called metal-organic
frameworks (MOFs). On the molecular level, MOFs
are scaffolds made up of metal hubs linked together
with struts of organic compounds. By carefully
choosing and modifying the chemical components
used as hubs and struts, Yaghi and his team have
been able to define the angles at which they connect
and design materials with the properties they want.
MOFs, COFs can be made highly porous to increase
their storage capacity. But unlike MOFs, COFs contain
no metals. Instead, they're made up of light elements – hydrogen,
boron, carbon, nitrogen and oxygen – that form strong
links (covalent bonds) with one another.
"Using light elements allows you to generate lightweight
materials," said Côté. "That's very
important for hydrogen fuel storage, because the
lighter the material, the more economical it is to
transport around in a vehicle. The strong covalent
bonds also make COFs very robust materials." Although
the main thrust of the current research is creating
materials for gas storage in fuel cells, Côté,
Yaghi and colleagues also are exploring variations
of COFs that might be suitable for use in electronic
devices or catalytic applications.
"This is the first step to what we think is going
to be a very large and useful class of materials," Côté said.
Yaghi collaborated on the research with assistant
professor of chemistry Adam Matzger and graduate
students Annabelle Benin and Nathan Ockwig, all
of U-M, and Michael O'Keeffe of Arizona State University.
The work was funded by the National Science Foundation,
the U.S. Department of Energy and the Natural Sciences
and Engineering Research Council of Canada.
For more information:
Omar Yaghi--- http://www.umich.edu/%7Emichchem/faculty/yaghi/