A benign synthesis of alane by the composition-controlled mechanochemical reaction of sodium hydride and aluminum chloride

Solid-state mechanochemical synthesis of alane (AlH₃) starting from sodium hydride (NaH) and aluminum chloride (AlCl₃) has been achieved at room temperature. The transformation pathway of this solid-state reaction was controlled by a stepwise addition of AlCl₃ to the initial reaction mixture that contained sodium hydride in excess of stoichiometric amount. As in the case of previously investigated LiH–AlCl₃ system, complete selectivity was achieved whereby formation of unwanted elemental aluminum was fully suppressed, and AlH₃ was obtained in quantitative yield. Reaction progress during each step was investigated by means of solid-state NMR and powder X-ray diffraction, which revealed that the overall reaction proceeds through a series of intermediate alanates that may be partially chlorinated. The NaH–AlCl₃ system presents some subtle differences compared to LiH–AlCl₃ system particularly with respect to optimal concentrations needed during one of the reaction stages. Based on the results, we postulate that high local concentrations of NaH may stabilize chlorine-containing derivatives and prevent decomposition into elemental aluminum with hydrogen evolution. Complete conversion with quantitative yield of alane was confirmed by both SSNMR and hydrogen desorption analysis.