Characterization of a new phase of ammonia borane

Abstract

An uncharacterized phase of ammonia borane (AB) was previously identified while investigating the thermal aging of this hydrogen rich solid state material. This phase was identified as a key intermediate for the release of hydrogen from AB prompting the current characterization study. The new phase (AB*) was investigated extensively using in situ solid state MAS NMR spectroscopy, including ¹¹B, ¹⁵N and ¹H NMR. Single-pulse excitation, cross polarization and T₁ relaxation experiments for all nuclei collectively demonstrated a significant increase in mobility of AB in the new phase compared to the room temperature phase of AB, consistent with a highly mobile solid. This implies a disruption of the extensive dihydrogen bonding network, allowing increased motional freedom. Complementary studies by in situ X-ray diffraction showed AB* to have the same crystal lattice as AB, but suggested an expanded lattice, providing support for increased mobility. NMR measurements of AB embedded into a mesoporous scaffold were also consistent with high mobility in AB*. These data suggest that a breakdown of the extensive hydrogen bonding network occurs before hydrogen can be released and will be an important factor in the practical use of AB as a hydrogen storage material.