N-Alkylpyrrolidine·Alane Compounds for Energy Applications

Abstract

Aluminum hydride (AlH3) has high gravimetric and volumetric hydrogen densities and is thus a promising hydrogen storage material. As part of our effort to develop a cost-effective regeneration pathway for AlH3, we report the synthesis and characterization of N-alkylpyrrolidine·alane complexes (NMPy)2·AlH3, NMPy·AlH3, and NEPy·AlH3 (NMPy = N-methylpyrrolidine, NEPy = N-ethylpyrrolidine); the reversible formation of (NMPy)2·AlH3 from titanium-doped aluminum metal (denoted as Al*), H2 gas, and NMPy; and thermal decomposition studies of these alane adducts. Depending on the stoichiometric ratios of NMPy to AlH3, both the 2:1 complex (NMPy)2·AlH3 and the 1:1 complex NMPy·AlH3 can be selectively synthesized by direct reactions of NMPy with AlH3, whereas NEPy gives only the 1:1 adduct NEPy·AlH3 regardless of the ratio of NEPy to AlH3. In addition, the reversible formation of (NMPy)2·AlH3 from titanium-doped aluminum powder (Al*) and NMPy under a H2 pressure of 1000 psi was observed, while no alane formation was detected with NEPy. Theoretical calculations of the molecular geometries and absolute free energies are in good agreement with experimental observations, and indicate that the dramatic differences between NMPy and NEPy are caused by the steric effects imposed by the alkyl group (methyl or ethyl) on the pyrrolidine ring. Finally, we established the role of LiH in the decomposition of amine·alane adducts, and showed that the hydrogenation of Al* with NMPy and the decomposition of N-alkylpyrrolidine·alane adducts in the presence of LiH could be combined to generate donor-free LiAlH4.