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

Aluminum hydride, AlH3, 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 H2 and Al at around 100 °C. AlH3 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. ΔH f and ΔG f298K for α-AlH3 are −9.9 and 48.5 kJ/mol AlH3, respectively. The latter value yields an equilibrium hydrogen fugacity of ∼5 × 105 atm at 298 K, which is equivalent to a hydrogen pressure of ∼7 × 103 atm. Thus, the direct regeneration of AlH3 from spent Al with gaseous H2 is deemed impractical. This paper describes an alternate approach to the regeneration of AlH3 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 H2. The AlH3−TEDA product is insoluble and precipitates from solution. The reaction, forward or reverse, depends on the departure of the actual pressure of H2 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.