|Schematic representation for proposed material Mg-C60@MOF showing a MOF cavity impregnated with magnesium-decorated C60. Credit: ACS. Click to enlarge.|
Researchers in Australia have proposed a new concept for hydrogen and methane storage materials involving the incorporation of magnesium-decorated fullerenes within metal-organic frameworks (MOFs). According to their modeling, the magnesium-decorated Mg10C60 fullerenes show a volumetric methane uptake of 265 v/v, the highest reported value for any material, which significantly exceeds the US Department of Energy target of 180 v/v.
They also predict a very high hydrogen adsorption enthalpy of 11 kJ mol-1 with relatively little decrease as a function of H2 filling. This value is close to the calculated optimum value of 15.1 kJ mol-1 and is achieved concurrently with saturation hydrogen uptake in large amounts at pressures under 10 atm.
A paper describing their work was published online 7 July in the Journal of the American Chemical Society.
There are a number of candidates for advanced gaseous on-board storage, including MOFs, light-metal hydrides, and fullerenes.
Given that MOFs, exposed metal sites (for example, light-metal hydrides), and fullerenes each hold potential for gas storage, a material that has the combined attributes of each of these systems is particularly desirable. Here we propose and model a material combining all of these components. On the basis of the impregnation concept first demonstrated by Chae et al., who show that C60 fullerenes readily impregnate the porous network of MOF-177 (denoted by C60@MOF), we predict that the magnesium-decorated Mg10C60 within similar structures (denoted as Mg-C60@MOF) should deliver a very high gas storage enhancement.—Thornton et al. (2009)
To simulate adsorption, the team used a novel method (topologically integrated mathematical thermodynamic adsorption model, TIMTAM). The results obtained using Mg-C60@MOF for the TIMTAM approach show large hydrogen uptake capacities at low pressures, which are achieved with one of the highest heats of adsorption for a physisorption-based storage material.
Perhaps surprisingly, capacity was found to increase despite an apparent loss of free volume related to pore filling by fullerenes. The increase in capacity was related to the tunability of pore sizes in conjunction with a drastic increase in adsorption enthalpy. The TIMTAM approach was designed to be deliberately conservative to ensure that our results display a high degree of verisimilitude, so that the actual physical materials may possibly display an even higher performance. Moreover, TIMTAM is verified using published experimental results.—Thornton et. al. (2009)
Australia’s CSIRO laboratories are working toward the experimental realization of these materials.
Aaron W. Thornton, Kate M. Nairn, James M. Hill, Anita J. Hill and Matthew R. Hill (2009) Metal-Organic Frameworks Impregnated with Magnesium-Decorated Fullerenes for Methane and Hydrogen Storage. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja9036302