Porous
crystals called metal-organic frameworks, with their nanoscopic pores
and incredibly high surface areas, are excellent materials for natural
gas storage. But with millions of different structures possible, where
does one focus?
A
Northwestern University research team has developed a computational
method that can save scientists and engineers valuable time in the
discovery process. The new algorithm automatically generates and tests
hypothetical metal-organic frameworks (MOFs), rapidly zeroing in on the
most promising structures. These MOFs then can be synthesized and tested
in the lab.
Using
their method, the researchers quickly identified more than 300
different MOFs that are predicted to be better than any known material
for methane (natural gas) storage. The researchers then synthesized one
of the promising materials and found it beat the U.S. Department of
Energy (DOE) natural gas storage target by 10 percent.
There
already are 13 million vehicles on the road worldwide today that run on
natural gas—including many buses in the U.S.—and this number is
expected to increase sharply due to recent discoveries of natural gas
reserves.
In
addition to gas storage and vehicles that burn cleaner fuel, MOFs may
lead to better drug-delivery, chemical sensors, carbon capture materials
and catalysts. MOF candidates for these applications could be analyzed
efficiently using the Northwestern method.
“When
our understanding of materials synthesis approaches the point where we
are able to make almost any material, the question arises: Which
materials should we synthesize?” said Randall Q. Snurr, professor of
chemical and biological engineering in the McCormick School of
Engineering and Applied Science. Snurr led the research. “This paper
presents a powerful method for answering this question for metal-organic
frameworks, a new class of highly versatile materials.”
The
study is published by the journal Nature Chemistry. It also will appear
as the cover story in the February print issue of the journal.
Christopher
E. Wilmer, a graduate student in Snurr’s lab and first author of the
paper, developed the new algorithm; Omar K. Farha, research associate
professor of chemistry in the Weinberg College of Arts and Sciences, and
Joseph T. Hupp, professor of chemistry, led the synthesis efforts.
“Currently,
researchers choose to create new materials based on their imagining how
the atomic structures might look,” Wilmer said. “The algorithm greatly
accelerates this process by carrying out such ‘thought experiments’ on
supercomputers.”
The
researchers were able to determine which of the millions of possible
MOFs from a given library of 102 chemical building block components were
the most promising candidates for natural-gas storage. In just 72
hours, the researchers generated more than 137,000 hypothetical MOF
structures. This number is much larger than the total number of MOFs
reported to date by all researchers combined (approximately 10,000
MOFs). The Northwestern team then winnowed that number down to the 300
most promising candidates for high-pressure, room-temperature methane
storage.
In
synthesizing the natural-gas storage MOF that beat the DOE storage
target by 10 percent, the research team showed experimentally that the
material’s actual performance agreed with the predicted properties.
The
new algorithm combines the chemical “intuition” that chemists use to
imagine novel MOFs with sophisticated molecular simulations to evaluate
MOFs for their efficacy in different applications. The algorithm could
help remove the bottleneck in the discovery process, the researchers
said.
The
Defense Threat Reduction Agency and the U.S. Department of Energy,
Office of Science, Basic Energy Sciences, supported the research.
Large-Scale Screening of Hypothetical Metal-Organic Frameworks
