Synthesis of Metal Complex Hydrides for Hydrogen Storage (original) (raw)

2007, Journal of Physical Chemistry C

A new procedure for the direct synthesis of metal complex hydrides, NaAlH 4 and LiAlH 4 , has been developed through hydrogenation of their respective metal hydrides, NaH and LiH, with Al, both in the presence of a catalyst and in the presence of a liquid complexing agent. This procedure involves three steps: blending the TiCl 3 catalyst precursor with reactants using dry high-pressure ball milling (HPBM) in hydrogen, hydrogenation using HPBM in hydrogen and tetrahydrofuran (THF), and vacuum filtration and drying. Essentially, pure NaAlH 4 and LiAlH 4 products were produced in high yield at ambient temperature and near ambient pressure. For the Na and Li systems, the Ti nanoparticles, stabilized in THF, together with the formation of a MAlH 4 ‚ xTHF adduct (M) Na or Li), both facilitated the hydrogenation of MH at such mild conditions. For the Na system, x was equal to 4 or 8, but the interaction was very weak, and for the Li system, x was equal to 4, but the interaction was very strong. In general, THF, the Ti catalyst, and wet HPBM all played essential roles in promoting the hydrogenation reactions, which proceeded through either a direct pathway (2MH + 2Al + 3H 2 f 2MAlH 4), or a sequential pathway (6MH + 2Al + 3H 2 f 2M 3 AlH 6 followed by M 3 AlH 6 + 2Al + 3H 2 f 3MAlH 4), with the formation of sodium hexahydridoaluminate limiting in the latter case. The same procedure was tried unsuccessfully for the direct synthesis of Mg(AlH 4) 2 from MgH 2 and for AlH 3 from Al.