Used as storage materials in natural gas tanks, MOFs offer a docking area for gas molecules, which can be stored in higher densities as a result. (Earlier post.) The larger gas quantity in the tank increases the vehicle’s range.

An advantage of the production method developed by BASF is that it uses no organic solvents. The simple method gives a higher material yield from an aqueous medium and is suitable for existing BASF production plants, the company says.

MOFs were discovered toward the end of the 1990s by US chemist Omar M. Yaghi (then at University of Michigan, now UCLA). (Earlier post.) BASF researchers contacted him after reading his 1999 article in the journal Nature and have been collaborating with Professor Yaghi since then on the synthesis of metal-organic frameworks.

The aim is to develop MOFs with the largest possible surface area and storage density. Professor Yaghi recently succeeded in synthesizing MOF-210, a zinc carboxylate with a surface area of more than 10,000 square meters per gram of material. For comparison: the highest surface areas of previous MOFs averaged 5,000 square meters per gram. A paper on the development (Furukawa et al) was published in the 23 July issue of the journal Science.

Given the exceptional properties of such materials, it is expected that MOFs with ultrahigh surface area would exhibit exceptional gas storage capacity. Accordingly, this series of MOFs was subjected to high-pressure hydrogen (77 K) and methane (298 K) adsorption so as to examine their potential utility in the storage of gaseous fuels...The calculated gravimetric hydrogen density in MOF-210 (176 mg g^–1) exceeds that of typical alternative fuels (methanol and ethanol) and hydrocarbons (pentane and hexane). MOF-200 and -205 also show large total hydrogen uptake (163 and 123 mg g–1, respectively); again, these values are higher than MOF-177.

Methane uptake was measured at 298 K and up to 80 bar; under the present experimental conditions, all isotherms were not saturated. Although the excess methane uptake in MOF-200, -205, and -210 (234, 258, and 264 mg g^–1 at 80 bar, respectively) were smaller than that in PCN-14 (253 mg g^–1 at 290 K and 35 bar, respectively), the calculated total uptakes (446, 394, and 476 mg g^–1 for MOF-200, -205, and -210, respectively) were more than 50% greater than those of PCN-14. Moreover, the corresponding volumetric methane densities in the present MOFs are respectively 2, 3, and 2.5 times greater than volumetric bulk density (grams per liter) of methane at the same temperature and pressure. Because the isotherms are nearly linear up to 80 bar, these materials can deliver most of the sorbed methane in the pressure range between 10 to 200 bar.

...The ultrahigh surface areas exhibited by MOF-200 and -210 are near the ultimate limit for solid materials. To appreciate this, it is useful to note that all these compounds have a volume-specific surface area in the range of 1000 to 2000 m² cm^–3 = 1 x 10⁹ to 2 x 10⁹ m^–1, and for a cube of edge d the external surface area/volume is 6d²/d³ = 6/d. Thus, for a monodisperse powder of cubic nanoparticles to have external surface that is equal to that of these MOFs the cube edge would have to be only 3 to 6 nm, which is a size far too small to practically realize in stable dry powders and therefore impossible to access the full surface area of such particles. This analysis emphasizes that MOFs are truly “nanomaterials” in the sense that they can be designed to give volume-specific surface areas that are equal to the external surface areas of nanometer-sized particles.

—Furukawa et al.

Resources

• Hiroyasu Furukawa, Nakeun Ko, Yong Bok Go, Naoki Aratani, Sang Beom Choi, Eunwoo Choi, A. Özgür Yazaydin, Randall Q. Snurr, Michael O’Keeffe, Jaheon Kim, Omar M. Yaghi (2010) Ultrahigh Porosity in Metal-Organic Frameworks. Science Vol. 329. no. 5990, pp. 424 - 428 doi: 10.1126/science.1192160