Nanoconfinement and Catalytic Dehydrogenation of Ammonia Borane by Magnesium-Metal-Organic-Framework-74

Ammonia borane (NH3BH3, AB) has recently received much attention as a promising hydrogen-storage medium among a very large number of candidate materials because of its satisfactory air stability, relatively low molecular mass (30.7 gmol-1), and remarkably high energy-storage densities (gravimetric and volumetric hydrogen capacities are 19.6wt% and 140 gL-1, respectively).However, the direct use of pristine AB as a hydrogen energy carrier in onboard/ fuel-cell applications is prevented by its very slow dehydrogenation kinetics below 100 °C and the concurrent release of detrimental volatile by-products such as ammonia, borazine, and diborane. Many different methods have been adopted to promote efficient H2 generation from AB, including catalytic hydrolysis in aqueous solution, ionic liquids, organic solvents,[and thermodynamic modifications by formation of hybrid structures with transition metals, alkali-, or alkaline-earth metal/hydrides, or nanoconfined phases using porous scaffolds. However, many of these methods rely on the usage of heavy metal catalysts, aqueous or nonaqueous solutions, and ionic liquids, all of which make the hydrogen density of the systems unacceptably low for practical applications. Furthermore, the vigorous reactions, hygroscopic properties, and water solubility of borohydrides have negative impacts on the dehydrogenation performance and make it difficult to control the release of hydrogen. The other approach is made, in particular, nanocomposition of AB within porous scaffoldings. However, systems still suffers one or more of the followings: either the nanocomposite is heavier or cannot prevent the generation of all the volatile by-products. Hence, more work needs to be done to explore the potential role that catalysts can play to further improve the controllable H2-release kinetics under moderate conditions while at the same time preventing the generation of detrimental byproducts © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.