Computational Study of Molecular Complexes Based on Ammonia Alane for Chemical Hydrogen Storage

Electronic structure calculations at the CCSD(T) level with the aug-cc-pVnZ and aug-cc-pV(n+d)Z basis sets (n = D, T, and Q) were employed to construct the potential energy surfaces for H-2 release from a series of derivatives of ammonia alane. H-2 production from AlH3NH3 is facilitated by the addition of alane, which plays the role of a Lewis acid bifunctional catalyst. H-2 release is not competitive from the combination of two AlH3NH3 monomers. in the form of it stable dimer. An alternative route involves the reactions of one AlH3NH3 monomer with two separated molecules of AlH3 and NH3, in two successive steps: (i) an initial condensation of AlH3NH3 with NH3 leads to a stable linear NH3AlH3NH3 species, and (ii) a subsequent combination of the latter with AlH3 gives rise to a bicycle framed by dihydrogen bonds. From the new bicyclic form, H-2 production becomes a facile process. Condensation of dialane (Al2H6) with two separated ammonia molecules (2NH(3)) is also a potential route for H-2 release. The reaction enthalpies and free energies at 298 K were also evaluated for the pathways considered. Although the effects of temperature and entropy are important, the Delta G(298) preferred pathways remain similar to the energy (Delta H(0 K)) profiles.