Hydrogen release from ammonia borane.

摘要

Development of a safe and efficient storage medium for hydrogen is integral to its use as an alternative energy source. The overall goal of the studies described in this dissertation was to investigate the use of a chemical hydride, ammonia borane (AB (19.6 wt% H2)), as a potentially efficient material for hydrogen storage. The specific goals of this study were both to develop new efficient methods for increasing the rate and extent of H2-release from AB and to elucidate the important mechanistic pathways and intermediates in these reactions. Significant achievements that resulted from this work are that AB H2-release is activated in the presence of either ionic liquids or bases. For example, an AB H2-release reaction carried out at 110°C in 50 wt% ionic liquid liberated over 2 equivalents H2 in 15 minutes. Reducing ionic liquid loading to 20 wt% at 110°C yielded a higher materials weight percent (11.4 mat-wt%), while still having fast release rates: 2 equivalents in ∼2.5 hours. The addition of the strong nitrogen base 1,8-bis(dimethylamino)naphthalene, Proton Sponge(TM), to ionic liquid solutions of AB increased the AB H2-release rate at 85°C, with over 2 equivalents of H2 achieved within 3 h. Additional Proton Sponge increased the rate of release; however, the mat-wt% of H2 decreased since the Proton Sponge added significant weight to the system. Solid state and solution 11B NMR and DSC studies of reactions in progress allowed the identification of initial and final products in the H2-release reactions and helped elucidate the overall reaction pathway. The initial formation of diammoniate of diborane, the key intermediate in dehydropolymerization of ammonia borane, was promoted by the addition of ionic liquids. Subsequent H2-release resulted in the formation of polyaminoborane then polyborazylene. Proton Sponge increased the release rate of the second equivalent of H2 by a newly proposed anionic polymerization mechanism. The final product was identified by solid-state 11B NMR and proved to be a sp2-framework of polyborazylene which formed regardless of base additive or amount/type of ionic liquid.