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Porous metal-organic frameworks offer useful properties for gas storage
A new family of highly crystalline, porous materials in which the size and chemical functionality of the pores can be tailored systematically shows promise for gas-storage applications, according to a research team led by chemistry professor Omar M. Yaghi of the University of Michigan, Ann Arbor [Science, 295, 469 (2002)].
Several members of this family have pore sizes in the mesoporous range (greater than 20 Å) and densities that are the lowest for any crystalline materials reported to date, according to the researchers. And one of the materials "has the highest methane storage capacity measured thus far," they report.
Yaghi and his coworkers call the materials IRMOFs--which stands for isoreticular metal-organic frameworks. They consist of cubical 3-D networks of zinc-oxygen clusters connected by molecular struts such as 1,4-benzenedicarboxylate. By choosing connectors based on longer molecules such as terphenyl, the chemists have shown that they can expand the pore size in increments from 3.8 to 28.8 Å. And there's no limit "as far as I can see," Yaghi tells C&EN.
Unlike zeolites, IRMOFs don't have walls to impede diffusion of guest molecules, he points out. They're basically wiry scaffolds, with all the pores in any given IRMOF being identical in size. Furthermore, the frameworks are rigid and robust, retaining their dimensions and shape even when the pores are empty.
The 16 IRMOFs described by the Michigan group have more free volume than some of the most open zeolites, with the "airiest" framework boasting a pore volume of 91%. Six of these materials have densities between 0.45 and 0.21 g per cm3--lower than the density of any previously reported crystalline material, the researchers note. By comparison, lithium metal has a density of 0.53 g per cm3.
The synthesis of IRMOFs is simple and the starting materials are inexpensive, Yaghi says. And because the frameworks have organic struts, the pores can be decorated with bromo, amino, alkoxy, alkyl, aryl, or other groups. These groups influence how the pores interact with guest molecules, Yaghi says. For example, functionalized pore openings can behave like valves, regulating the passage of guests.
This effect was seen in the case of one member of the series, IRMOF-6, which was found to be especially adept at storing methane. At room temperature and 36 atm of methane, each cubic centimeter of this sorbent takes up 155 cm3 of methane at standard temperature and pressure. This exceeds the methane uptake of other crystalline materials, including zeolites, Yaghi notes.
On a volume basis, the amount of methane sorbed by IRMOF-6 at 36 atm--regarded as a safe and cost-effective pressure--"represents 70% of the amount stored in compressed methane cylinders in laboratories, where much higher, unsafe levels of pressure (205 atm) are used," the researchers point out in their paper. "Reducing the pressure represents an advance that we believe will affect the future use of these materials in automobile fueling."
Methane uptake by IRMOF-6 is comparable with that of the best carbon adsorbents, says K. Mark Thomas, professor of carbon science at the University of Newcastle upon Tyne, in England. "In principle, if you can get a crystalline adsorbent--for example, a metal-organic framework, which has a specific optimum pore size for adsorption of a particular gas--you should be able to improve on the performance compared with amorphous adsorbents like active carbons, which, by their very nature, have a distribution of pore sizes."
Thomas also points out that "to get a big advantage over the current technology, you would need to get significantly above the 150 cm3 per cm3 standard achieved for carbon."
In any case, he says Yaghi's work is "first-rate. I think there is a great future" in it.
||BIG EMPTINESS In one of the highly porous metal-organic frameworks synthesized by Yaghi's group, 91% of the crystal volume is calculated to be open space. The large orange spheres are included to emphasize the size of the cavities.
IMAGE BY NATHANIEL ROSI
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