Formation of Al2H7− anions — indirect evidence of volatile AlH3 on sodium alanate using solid-state NMR spectroscopy

Abstract

After more than a decade of intense research on NaAlH₄ doped with transition metals as hydrogen storage material, the actual mechanism of the decomposition and rehydrogenation reaction is still unclear. Early on, monomeric AlH₃ was named as a possible transport shuttle for aluminium, but never observed experimentally. Here we report for the first time the trapping of volatile AlH₃ produced during the decomposition of undoped NaAlH₄ by an adduct of sodium alanate and crown ether. The resulting Al₂H₇⁻ anion was identified by solid-state ²⁷Al NMR spectroscopy. Based on this indirect evidence of volatile alane, we present a simple description of the processes occurring during the reversible dehydrogenation of NaAlH₄.