Synthesis of Metal Complex Hydrides for Hydrogen Storage

Abstract

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.

Preparation of a new Ti catalyst for improved performance of NaAlH4

“…These results were similar to results reported in previous studies by Ritter and co-workers on the sonochemical doping of Ti-doped NaAlH 4 [25], on the dehydrogenation and hydrogenation cycling of LiAlH 4 [8], and on the direct synthesis of Na and Li alanates [9]. In the sonochemical doping study [25], they showed that the Ti catalyst obtained when using THF during the sonochemical pretreatment was more effective at improving the dehydrogenation and hydrogenation performance than that obtained when using Et 2 O during the sonochemical pretreatment.…”

Complex Hydrides

“…The weakly exothermic hydrogenation of LiH and Al to form Li 3 AlH 6 might enable a rehydrogenation under very high pressures. Recently Wang et al confirmed the previous results of Clasen [59] and Ashby et al [60] for the rehydrogenation of Li 3 AlH 6 , LiH, and Al in the presence of THF [88,89]. Dehydrogenation takes place below 100 C and provides about 4 wt.% H 2 .…”

Ti-doped LiAlH4 for hydrogen storage: Rehydrogenation process, reaction conditions and microstructure evolution during cycling

“…Using this approach, Ashby [22,23]. This method involves the necessary separation of catalyst by filtration after each cycle.…”