Metal-organic frameworks as hydrogen storage materials: effects of framework reduction and cation dopingPublic Deposited
The safe and efficient storage of hydrogen is possibly the chief obstacle to its use as a fuel on a large scale. Metal-organic frameworks (MOFs) are well poised to provide unique solutions to hydrogen storage, and gas storage in general, a result of their crystalline, porous networks that present the potential for immense structural and chemical tunability. However, the H2-MOF interactions that govern the storage properties are too weak to realistically use MOFs as a storage medium. The focus of this research is to discover and fully understand methods by which to augment the interactions of hydrogen with MOFs in particular and potentially solid porous materials more generally. Several new mixed-ligand MOFs based on Zn(II)-paddlewheel geometry have been designed and synthesized. Chemical reduction of the framework struts through either direct contact with solvated lithium metal or alkali-metal naphthalenide solutions and subsequent framework doping with Li+, Na+, or K+ has been successfully accomplished. At low cation loading levels, H2 uptake is significantly enhanced, but with little change in the H2 binding energy. Parallel nitrogen adsoption studies indicate that the enhanced uptake is not necessarily from H2-cation interactions, but more likely structural changes induced by the dopant cations. Additionally, lithium and magnesium alkoxide-functionalized frameworks were pursued to introduce strong specific H2 binding sites with two new framework materials. These metal-alkoxide frameworks exhibit increasing H2 heat of adsorption with loading, uncharacteristic of normal H2 physisorption. This behavior, coupled with slight increases in uptake, suggests site-specific H2 binding within the alkoxide functionalized frameworks.