Toward the Direct Hydrogenation of Aluminum

Aluminum Hydride, or alane (AlH3), is an attractive medium for hydrogen storage, owing to the large amount of hydrogen that can be contained in a dense, lightweight material. It decomposes cleanly in a single step (Equation 1). AlH3→Al+3/2H2 1 This reaction is highly thermodynamically favourable, and not easily reversed. Al metal can only be hydrogenated at high temperatures and pressures (300°C; 2.5 GPa).1 We have investigated supercritical fluids (CO2 and Me2O) as reaction media to lower the temperatures and pressures required for this synthesis.;Use of a donor or co-solvent L can also help stabilize nascent AlH3 (Equation 2), which may be subsequently removed to leave the desired product (Equation 3). Al+3/2H2+L→ AlH3˙L 2 AlH3˙L→D- LAlH3x 3 .;Accordingly, we have explored the stability of a range of AlH3·L complexes using DFT calculations with amines, ethers, and phosphines as stabilizing ligands. Al(Ti)2 has been reacted with hydrogen gas (ca. 1000 psi) below 80°C in the presence of solutions of selected ligands. Hence, five AlH3·L complexes have been formed by direct synthesis; three of these for the first time using this route.