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 laboratory.

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%.

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?” says 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 will be published in Nature Chemistry.

Christopher E. Wilmer, a graduate student in Snurr’s laboratory
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
says. “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 hr, 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%, 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 say.

SOURCE – Northwestern University