Metal cation-promoted hydrogen generation in activated aluminium borohydride ammoniates

Abstract The crystal structure of an aluminum-based borohydride ammoniate – Al(BH4)3·6NH3 – is reported for the first time. The molecular structure of Al(BH4)3·6NH3 is resolved by high-resolution X-ray diffraction. The compound crystallized in the space group Pbcn (No. 60), with lattice parameters of a = 13.2824(5) A, b = 15.2698(7) A and c = 13.1848(6) A. Structure analysis shows that this compound contains complex hexamminealuminum (III) [Al(NH3)6]3+ cations, which are surrounded by BH 4 - anions. The interatomic distances between the Hδ+s from the NH3 units and the Hδ−s from the BH4 units are in the range of 1.91–2.19 A, suggesting the presence of significant Hδ+⋯−δH interactions. Mass spectrometry, thermogravimetry and temperature-programmed desorption studies of metal cation-modified aluminum-based borohydride ammoniates using the reactions of various metal borohydrides M(BH4)n (M = Na, Li, Ca, Mg) and chlorides MCln (M = Sc, Ni, Cu, Zn, Mg, Ca, Li) reveal that their dehydrogenation properties are strongly dependent on the polarizing power of the added metal cations. It is hypothesized that the added metal cations may activate the borohydride ion to such an extent that its Hδ− can easily react with the Hδ+ of the [Al(NH3)6]3+ cation, resulting in an enhanced interaction between the Hδ+ and Hδ−, thus enhancing their dehydrogenation kinetics. Subsequent deuterium isotope and X-ray measurements support the hypothesis that the Hδ+⋯−δH interactions play a role in the dehydrogenation of the metal borohydride ammoniates. Of the systems investigated, 0.5Mg(BH4)2/Li2Al(BH4)5·6NH3 is notable as it releases more than 10 wt.% high-purity H2 within 30 min below 120 °C. This ranks among the highest values currently reported for potential solid-state hydrogen storage materials. These findings provide a feasible and simple route for modifying B–N-based, lightweight materials for highly efficient dehydrogenation.