Direct and Reversible Synthesis of AlH₃−Triethylenediamine from Al and H₂

Aluminum hydride, AlH 3, is the most well-known alane. Though thermodynamically unstable under ambient conditions, it is easily prepared in a metastable state that will undergo controlled thermal decomposition to produce H 2 and Al at around 100 °C. AlH 3 contains 10.1 wt % hydrogen and has a density of 1.48 g/mL and is therefore of interest for on-board automotive hydrogen storage. Δ H f and Δ G f298K for α-AlH 3 are −9.9 and 48.5 kJ/mol AlH 3, respectively. The latter value yields an equilibrium hydrogen fugacity of ∼5 × 10 5 atm at 298 K, which is equivalent to a hydrogen pressure of ∼7 × 10 3 atm. Thus, the direct regeneration of AlH 3 from spent Al with gaseous H 2 is deemed impractical. This paper describes an alternate approach to the regeneration of AlH 3 via a low-temperature, low-pressure, reversible reaction using Ti-doped Al powder and triethylenediamine (TEDA). The adduct is formed in a slurry of the Al powder and a solution of TEDA in THF in contact with H 2 . The AlH 3 −TEDA product is insoluble and precipitates from solution. The reaction, forward or reverse, depends on the departure of the actual pressure of H 2 gas above or below the equilibrium pressure. Pressure−composition isotherms in the range of 70−90 °C are presented from which thermodynamic data were calculated. A possible reaction mechanism is described. The relevance of this system to hydrogen storage applications is also noted.