The solvation and regeneration of ammonia borane : a hydrogen storage material

Ammonia borane (AB), NH₃BH₃, is considered a promising medium for the storage of hydrogen gas in the solid state, capable of releasing ≤ 16 wt% H₂ at temperatures near 150 ºC. The solvation structures of AB in three solvents: tetrahydrofuran (THF), liquid ammonia, and diglyme, have been probed in depth through neutron scattering experiments. The results were analysed through Monte Carlo simulations based on the empirical potential structure refinement (EPSR) method, in which the simulations are refined to the experimental diffraction data. Hydrogen/deuterium isotopic substitution was used to add constraints to the simulations and improve their accuracy. Full 3D pictures of the bonding in each solvent were determined, all of which were found to be rich in hydrogen and dihydrogen bonding interactions. This approach enabled the answering of several questions pertaining to the spatial and orientational structure of hydrogen and dihydrogen bonding in solution. Among others, the reason for the extremely high solubility of AB in liquid ammonia (72.2 wt%, cf. 20.0 wt% in THF) was discovered to be due to the numerous dihydrogen bonds formed between the AB BHs and ammonia hydrogens, enabling bonding to both ends of the AB molecule unlike in the other solvents. A major challenge preventing the widespread adoption of AB as a green fuel is in the regeneration of the dehydrogenated waste. This has typically been attempted through a three-step process of digestion-reduction-ammoniation, though none have succeeded with mild reagents. In this project the addition of a transesterification step after digestion has been attempted. The digestion product B(OEt)₃ was successfully converted into the more easily reducible B(OC₆F₅)₃ through reaction with pentafluorophenol. Reduction was then attempted with hydrazine in liquid ammonia. A discovered incompatability of pentafluorophenol with ammonia prevented this scheme from working, but B(OC₆F₅)₃ remains a promising target for future attempts at mild reduction.