Metallated derivatives of ammonia borane with a view to their potential as hydrogen storage materials

摘要

Ammonia borane has attracted significant interest in the field of hydrogen storage due to its high gravimetric hydrogen content. In this study the reaction of NH(_3)BH(_3) with various metal hydrides was investigated. The reactions with hydrides of lithium and sodium required a molar ratio of 1:2 in favour of NH(_3)BH(_3) and the reaction products were characterised as [Li(NH(_3))](^+)[BH(_3)NH(_2)BH(_3)](^−) and [Na](^+)[BH(_3)NH(_2)BH(_3)](^−), respectively, through solid state (^1)(^1)B and (^2)(^3)Na MAS NMR and Raman spectroscopy. The reaction of CaH(_2) with NH(_3)BH(_3) required a reaction stoichiometry of 1:4 and this reaction proceeded through a different reaction mechanism, forming Ca(BH(_4))(_2)•2NH(_3). The crystal structures of Ca(BH(_4))(_2)•2NH(_3) and Ca(BH(_4))(_2)•NH(_3) were determined by powder diffraction methods and the reaction pathway investigated through solid state (^1)(^1)B MAS NMR spectroscopy. The thermal desorption properties of these hydrogen rich materials were investigated and hydrogen was released from all the materials. However, under certain conditions ammonia was the major gaseous desorption product from Ca(BH(_4))(_2)•2NH(_3) and was observed as a minor product from the decomposition of [Li(NH(_3))](^+)[BH(_3)NH(_2)BH(_3)](^−). Ammonia was also released during the synthesis of [Na](^+)[BH(_3)NH(_2)BH(_3)](^−), but the decomposition of this material was free from ammonia release. Ramped thermal desorption studies of [Na](^+)[BH(_3)NH(_2)BH(_3)](^−) and [Li(NH(_3))](^+)[BH(_3)NH(_2)BH(_3)](^−) to 350°C resulted in weight losses due to hydrogen desorption of 7.5 wt% and 12.5 wt%, respectively. Heating Ca(BH(_4))(_2)•2NH(_3) to 350°C resulted in a total weight loss of 27.5 wt%, which was predominantly due to NH(_3) desorption. Powder XRD and solid state (^1)(^1)B MAS NMR spectroscopy were employed to identify the solid decomposition products and decomposition pathways were proposed. Metal borohydrides were identified in all cases as well as polymeric products possessing B–N chains.