Natural gas needs to be very tightly compressed and cooling to very cold temperatures will help to get worthwhile volumes suitable for transport use. Those pressures and temperatures pose high levels of engineering and materials quality compared to a simple gasoline tank. But the payoff for a low cost low-pressure ambient natural gas storage solution would offer more millions of vehicles freedom from the foreign oil tether.
A Northwestern University (NU) research team is hot on porous crystals called metal-organic frameworks, with their nanoscopic pores and incredibly high surface areas that are excellent materials for natural gas storage. Metal–organic frameworks (MOFs) are porous materials constructed from modular molecular building blocks, typically metal clusters and organic linkers. These can, in principle, be assembled to form an almost unlimited number of MOFs, yet materials reported to date represent only a tiny fraction of the possible combinations.
Metal Organic Framework Sample Images.
Metal-organic frameworks come in millions of different possible structures, so where does research zero in?
A (NU) research team has developed a computational method that can save scientists and engineers valuable time in the discovery process. Their new computer 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 new 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.
In addition to gas storage and vehicles that could burn natural gas, 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.
Team leader Randall Q. Snurr, professor of chemical and biological engineering in the McCormick School of Engineering and Applied Science explains the import of the research saying, “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? This paper presents a powerful method for answering this question for metal-organic frameworks, a new class of highly versatile materials.”
The team’s study paper is Large-Scale Screening of Hypothetical Metal-Organic Frameworks and was published by the journal Nature Chemistry. It also will appear as the cover story in the February print issue of the journal.
Graduate student in Snurr’s lab and first author of the paper Christopher E. Wilmer 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.
Wilmer takes the explanation of how the research affects the development of metal-organic frameworks, “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 NU team was 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.
The new computer 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 researchers say the algorithm could help remove the bottleneck in the discovery process.
The other people on the team are Michael Leaf, Chang Yeon Lee and Brad G. Hauser, all from NU.
13 million vehicles on the road worldwide today run on natural gas, including many buses in the U.S. The number is expected to increase sharply due to recent discoveries of natural gas reserves with lower prices than gasoline. Converting a vehicle to the fuel isn’t a major matter, albeit complex and includes a drop in available total power, as natural gas is lower than gasoline in energy density. Comparatively speaking, it’s a very cheap fuel.
By. Brian Westenhaus