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

Abstract

Solid-state mechanochemical synthesis of alane (AlH 3 ) starting from sodium hydride (NaH) and aluminum chloride (AlCl 3 ) has been achieved at room temperature. The transformation pathway of this solid-state reaction was controlled by a step-wise addition of AlCl 3 to the initial reaction mixture that contained sodium hydride in excess of stoichiometric amount. As in the case of previously investigated LiH-AlCl 3 system, complete selectivity was achieved whereby formation of unwanted elemental aluminum was fully suppressed, and AlH 3 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-AlCl 3 system present some subtle differences compared to LiH-AlCl 3 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 both by SSNMR and hydrogen desorption analysis. AbstractSolid-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 step-wise 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 present 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 both by SSNMR and hydrogen desorption analysis.

Preface to the special section on mechanochemical synthesis

“…There are three articles [6][7][8] dealing with different materials for hydrogen production and storage. Garroni et al [6] and Hlova et al [7] focused on the use of mechanical milling to induce a metathesis or displacement reaction and produce metal hydrides. Garroni et al [6] showed a fine example of the differences in the reaction pathways of a specific system, when it is either thermally or mechanically activated.…”

“…Garroni et al [6] showed a fine example of the differences in the reaction pathways of a specific system, when it is either thermally or mechanically activated. Hlova et al described [7] a novel, quantitative, and solvent-free mechanochemical method to produce alane at room temperature, from a mixture of NaH and AlCl 3 . Huot and Tousignant [8] analyzed the effect of ball milling and cold rolling on the hydrogenation sorption properties of a CaNi 5 alloy.…”

Alanates, a Comprehensive Review

“…This absence of data can be related to the instability of some alanates and the consequent difficulty of their synthesis. Among the examples is the work of Hlova et al [36,37,38]. They produced the reaction of LiH with AlCl 3 or NaH with AlCl 3 in several molar proportions, with the objective of forming AlH 3 .…”

“…However, it must be considered that the mechanochemical version of Equation (5), and Equation (7), in general, or any similar reaction that involves hydrides or alanates plus aluminum salts, would compete with the formation of Al, for example [36]:3MH + AlCl 3 → Al + 3MCl + 1.5H 2 , (M = Li, Na).…”