Direct and reversible synthesis of AIH3-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 H-2 and Al at around 100 degrees 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. Delta H-f and Delta G(f298K) for alpha-AlH3 are -9.9 and 48.5 kJ/mol AlH3, respectively. The latter value yields an equilibrium hydrogen fugacity of similar to 5 x 10(5) atm at 298 K, which is equivalent to a hydrogen pressure of similar to 7 x 10(3) atm. Thus, the direct regeneration of AlH3 from spent Al with gaseous H-2 is deemed impractical. This paper describes an alternate approach to the regeneration of AIH(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 AlH3-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 degrees 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.