|Neutron-scattering image reveals where hydrogen molecules (red-green circles) connect to a metal organic framework (MOF). The ball-and-stick model of the MOF is superimposed on the neutron image. Image: T. Yildirim/NIST
Two new papers highlight the potential of metal-organic frameworks (MOFs) in two different and critical future applications: hydrogen storage and CO2 capture.
Metal-organic framework (MOF) compounds consist of metal-oxide clusters connected by organic linkers. MOFs are a relatively new class of nano-porous material that show promise for hydrogen storage applications because of their tunable pore size and functionality.
A research team at the National Institute of Standards and Technology (NIST) used neutron scattering techniques to visualize where hydrogen latches onto the lattice-like arrangement of zinc and oxygen clusters in a particular nanoscale material called MOF5.
Taner Yildirim and Michael Hartman found that MOF5 has four types of hydrogen docking sites, including a surprising three-dimensional network of “nano-cages” that appears to form after other sites load up with hydrogen.
They determined that the metal-oxide cluster is responsible for most of the adsorption while the organic linker plays only a secondary role.
These results suggest that by using different organic linkers, which make the hydrogen desorption difficult by narrowing the channels connecting the network of nano-pores in MOF, one may be able to optimize these materials for practical hydrogen storage at ambient conditions.
Equally important, we show that at high-concentration loadings hydrogen molecules form unique 3D networks of hydrogen nano-clusters with intermolecular distances as small as 3.0 Å, which in pure solid hydrogen can only be reached at high pressure. These findings suggest that MOF materials can also be used as templates to create artificial, interlinked hydrogen nano-cages. Such materials could exhibit very unexpected electronic properties due to these small intermolecular distances, such as metallic or superconducting behavior.—T. Yildirim
Yildirim and Hartman found that the two most stable sites in the MOF scaffolding already offer considerable room for storing hydrogen. Earlier studies suggested that, at about –200º Celsius, MOF5 could hold less than 2% of its weight in hydrogen.
The NIST research indicates ample room for improvement. At very low temperatures, hydrogen uptake approached 10% of the material’s weight. (The FreedomCar and Fuel Partnership involving the Department of Energy has set a level of about 6% as a minimum capacity for economically viable hydrogen storage.) The bulk of the hydrogen was held in nanometer-scale cavities inside the box-like arrangements of zinc and oxygen clusters.
These results, reported in Physical Review Letters, suggest that MOF materials might be engineered to optimize both the storage of hydrogen and its release under normal vehicle operating conditions.
Separately, researchers at the University of Michigan have developed a new MOF material (MOF-177) that has the highest carbon dioxide capacity of any porous material.
In a paper published online Dec. 1 in the Journal of the American Chemical Society, Omar Yaghi and co-worker Andrew Millward report that they have been able to increase the material’s storage capacity, making it possible to stuff more gas molecules into a small area without resorting to high pressure or low temperature.
MOF-177 soaks up 140% of its weight in CO2 at room temperature and reasonable pressure (32 bar).
A storage tank filled with MOF-177 could store as much CO2 as would be stored in nine tanks that do not contain MOFS. By comparison, a tank filled with porous carbon—one of the current state-of-the-art materials for capturing CO2 in power plant flues—would hold only four tanks worth of CO2.
MOFs can be made in large quantities from low-cost ingredients, such as zinc oxide and terephthalate, which is used in plastic soda bottles. And finding effective, low-cost ways of reducing CO2 emissions is crucial, according to Yaghi.
Almost every region of the world is using more energy than ever before, and the prediction is that this will continue to increase, not just for petroleum, but also for coal and natural gas. Whenever you’re burning fossil fuels, you’re releasing CO2 into the atmosphere, with devastating environmental effects that include melting the polar ice caps and changing the ocean’s acidity. In the United States alone, each person is responsible for generating more than 15 tons of carbon dioxide a year, largely from automobile and power plant emissions.
I’m not exaggerating when I say that we are digging a big, black hole for ourselves by not addressing the problem of carbon dioxide emissions.—O. Yaghi
Yaghi is also involved in the work at U. Michigan on covalent organic frameworks (COFs) for hydrogen storage. (Earlier post.)
Like 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 covalent bonds with one another.
Direct observation of hydrogen adsorption sites and nano-cage formation in metal-organic frameworks (MOF); T. Yildirim and M.R. Hartman; Phys. Rev. Lett., 95, 215504 (2005); doi:10.1103/PhysRevLett.95.215504
Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature; Andrew R. Millward and Omar M. Yaghi; J. Am. Chem. Soc.; 2005; DOI: 10.1021/ja0570032