Ammonia Borane Hydrogen Release in Ionic Liquids

The rate and extent of H-2-release from ammonia borane (AB), a promising, high-capacity hydrogen storage material, was found to be enhanced in ionic-liquid solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) (50:50-wt %) exhibited no induction period and released 1.0 H-2-equiv in 67 min and 2.2 H-2-equiv in 330 min at 85 degrees C, whereas comparable solid-state AB reactions at 85 degrees C had a 180 min induction period and required 360 min to release similar to 0.8 H-2-equiv, with the release of only another similar to 0.1 H-2-equiv at longer times. Significant rate enhancements for the ionic-liquid mixtures were obtained with only moderate increases in temperature, with, for example, a 50:50-wt % AB/bmimCl mixture releasing 1.0 H-2-equiv in 5 min and 2.2 H-2-equiv in only 20 min at 110 degrees C. Increasing the AB/bmimCl ratio to 80:20 still gave enhanced H-2-release rates compared to the solid-state, and produced a system that achieved 11.4 materials-weight percent H-2-release. Solid-state and solution B-11 NMR studies of AB H-2-release reactions in progress support a mechanistic pathway involving: (1) ionic-liquid promoted conversion of AB into its more reactive ionic diammoniate of diborane (DADB) form, (2) further intermolecular dehydrocoupling reactions between hydridic B-H hydrogens and protonic N-H hydrogens on DADB and/or AB to form neutral polyaminoborane polymers, and (3) polyaminoborane dehydrogenation to unsaturated cross-linked polyborazylene materials.