Review of Approaches to Synthesizing MOFs for Viable Hydrogen Storage

17 August 2008

A recent paper by UC Berkeley researchers Jeffrey Long and Mircea Dincă (earlier post) reviews the various synthetic approaches being used to produce metal–organic frameworks (MOFs) for viable hydrogen storage.

MOFs—tailorable microporous solids comprising metal building units and organic bridging ligands—offer high uptake capacity and good reversibility kinetics, making them promising candidates for hydrogen storage. (Earlier post.) However, because of their typically weak interaction with H₂, the materials function best at very low temperature, and would require cryogenic cooling for use on-board vehicles. To avoid this, researchers are looking for ways to increase the hydrogen affinity of the materials. Long and Dincă survey a range of different studies and approaches in their paper, published in the journal Angewandte Chemie International Edition. From their conclusion:

Microporous metal–organic frameworks are promising hydrogen storage materials, and the isolation of unsaturated metal ions can be used as a systematic way to increase the H₂ binding affinity. Although many known frameworks may display metal–H₂ interactions, very few experiments have been performed to test this assumption. Critical areas that are likely to produce better results or improve on the ones reported so far are the elucidation of milder methods to desolvate metals within the pores and the synthesis of more robust frameworks that can maintain crystallinity during the thermal evacuation of metal-bound solvent molecules. Possible desolvation strategies for thermally sensitive frameworks may include microwave or photolytic evacuation methods. In turn, the development of new ligands with metal-binding groups that form stronger metal–ligand bonds should lead to materials with increased thermal stability.

Another possible strategy that could yield metal–organic frameworks with increased H₂ affinity involves the incorporation of a larger concentration of charged sites within the pores. This could be achieved either by the use of multi-anionic bridging ligands, which should increase the number of metal atoms per formula unit, or by the incorporation of negative charges, which can also interact electrostatically with the H₂ quadrupole.

Overall, very encouraging results have been reported in a relatively short time, and a few new strategies to obtain unsaturated metal centers were developed only within the last year. Moreover, some of the results reported thus far show that metal–organic frameworks can meet most of the 2010 DoE targets on a materials basis when operating at 77 K [-196°C]. These allow researchers in the area to be optimistic when faced with the challenge of increasing the H₂ binding energy to produce a hydrogen storage system that will ultimately function near ambient temperature.

—Dincă and Long (2008)

Their work was funded by the US Department of Energy and General Motors Corporation.

Resources

• Dincă M. and Long, J. R. (2008) Hydrogen Storage in Microporous Metal-Organic Frameworks with Exposed Metal Sites Angew. Chem. Int. Ed. 47, ASAP doi: 10.1002/anie.200801163.