UK-USA study increases theoretical capacity boundaries for MOF-based natural gas storage
USA ARPA-E funded research explores MOF-based Adsorbed Gas Technology for light-weight increased capacity gas storage
In the United Kingdom, the University of Surrey reports what they describe as a major breakthrough by engineers trying to ensure that methane- or hydrogen-powered cars will be able to store enough gas to make them a viable option as the future main source of transport across the world.
Meanwhile, two funded studies, both being carried out by Gas Technology Institute (GTI), headquartered in Des Plaines, Illinois, are focusing on lower-cost, increased-capacity, lighter-weight natural gas fuel storage solutions for vehicles.
Both initiatives are being carried out in collaboration with researchers at Northwestern University, in Evanston, Illinois.
Metal Organic Frameworks (MOF) Model
Their statement is based on a major redefining of theoretical capacity for “metal organic frameworks” (MOFs) to store gases, thereby potentially increasing the operational range for natural gas vehicles (NGVs).
Until now the way the gas has been stored using metal organic frameworks (MOFs) has had a theoretical finite limit so making the idea impractical but now scientists have come up with a way to vastly improve this capacity. Dr Ozgur Yazaydin from Department of Chemical Engineering at University of Surrey and his collaborators at Northwestern University in USA have demonstrated that it is possible to achieve higher surface areas in MOF materials, about 40% higher than previous studies had suggested.
The team has produced two new MOF materials with the highest internal surface area per gram – so therefore gas storage capacity – ever recorded.
“The key is exposing more surface per available space for gas molecules to stick” says Dr Yazaydin who led the theoretical part of the study. “Benzene molecules, which are commonly used in MOFs as organic linkers, are like hexagonal rings, and gas molecules can only stick onto the ring’s outer surface, thus the inner sides of each benzene unit is essentially wasted space. If you break the ring and straighten it then both sides become available for gas adsorption. That is exactly what we did”.
“The breakthrough heralds a whole new dimension to the potential for using gas to power vehicles,” Yazaydin added, but may yet be some years away from practical implementation.
NGV Global technical members point out that while research has proven the potential of new high surface area compounds to store massive amounts of methane or hydrogen, it has yet to be proven if the technology can meet the practical requirements for fuel storage in vehicles.
Lower Cost for Light Duty Vehicles a Possibility
In the USA, Gas Technology Institute (GTI), a research and development organization serving energy and environmental markets, was recently awarded two new cutting-edge research projects by the U.S. Department of Energy (DOE) through its Advanced Research Projects Agency-Energy (ARPA-E), that will also include a focus on MOFs. GTI will serve as lead research organization for two projects focused on the development of new adsorbed natural gas (ANG) technologies for use in light-duty vehicles, for which it received $2.375 million in awards.
GTI received a $1.5 million grant for work they will do to advance gas storage using ANG technology, which enables the installation of more flexible and lighter storage systems using MOF technology. With ANG technology, natural gas is adsorbed by a porous adsorbent material at relatively low pressures, enabling a volumetric efficiency increase of more than 25% compared with traditional CNG storage cylinders.
GTI will partner with Northwestern University, NuMat Technologies, a Northwestern start-up company, and Westport Innovations, Inc. to identify materials with the best characteristics.
The team will characterize methane uptake capacities for these new materials as well as develop new computational models and synthesis routes. This approach will enable researchers to rapidly identify high-potential, low-cost alternatives.
“We’ll produce a small quantity of the material in our laboratory and conduct characterization testing—to determine how well the material adsorbs natural gas, how easily it releases it, what temperatures and pressures it displays when doing so, and how tolerant it is of contaminants,” says Ted Barnes, GTI Senior Project Manager, who will lead the project. “We hope to have an opportunity to follow-up our research with a demonstration of the material on a full-sized vehicle.”
GTI also received a grant of $875,000 for development of a unique low-pressure natural gas storage technology using an adsorbent with thin tailored shell to dramatically reduce the storage pressure while driving down cost. GTI’s innovative shell acts like nano-valves that can be opened and closed on demand to enable vehicle refueling, driving, or storage.
“The thin film coating would act as a ‘liquid valve’ with adsorbents,” says Dr. Shiguang Li, Principal Engineer, who will lead the project. “It would be applied to adsorbents so that, at a given temperature, the coating is impervious to methane when transferred across it—and, at an increased temperature, it can be opened up and allow methane to flow across.”
The research is aimed at the development of engineered adsorbents with high energy densities and storage pressures lower than ARPA-E targets. A lower storage pressure can lower requirements for wall thickness, translating to a lighter storage tank, facilitating the use of natural gas use in transportation.
Other team members include the University of Louisville and University of South Carolina.
Internal Space Explained
If the internal surface area of one gram of one of these record breaking MOFs could be unfolded it would occupy an area equivalent to the Emirates Stadium (see end note), where Arsenal plays its games in the Premier League.
In these materials metal atoms are connected by organic linker molecules. This results in a network of molecular cages with vast internal surface areas ideal for storing gases. The higher the surface area, the larger the amount of hydrogen or methane that can be stored, and the longer the car can go.
The study was published in the September 12, 2012 issue of the Journal of the American Chemical Society.
(This article compiled using information supplied by press releases from The University of Surrey and from Gas Technology Institute.)
End note: Emirates Stadium measures 7140 sq. m. or 8510 sq. yd.