Novel methods for synthesizing halide-free alane without the formation of adducts

Abstract

Many of the current synthesis methods for aluminum hydride (alane-AlH 3 ) involve reacting AlCl 3 and LiAlH 4 in solvents. The reaction requires the formation of an alane adduct such as AlH 3 ·[(C 2 H 5 ) 2 O] prior to obtaining crystallized stable α-AlH 3 . This process requires several hours of pumping in a vacuum system to remove the ether and convert the alane etherate into stable α-alane. This crystallization process is both costly and hazardous because a large amount of highly flammable material (e.g. ether) is removed by vacuum pumps over several hours. Conversely, the work presented herein describes novel methods to synthesize adduct-free alane. It is demonstrated here that AlH 3 can form by mixing AlCl 3 and LiAlH 4 in the solid state and heating to 75 • C; only α-AlH 3 was obtained. The α-AlH 3 product can be washed with minimal solvents leading to zero formation of alane adducts. In addition, the unwanted LiCl by-product is also removed during the solvent wash, resulting in halide-free α-alane. Although simply mixing and heating the reactants led to a 40% yield of alane, having the reactants compacted and mechanically pressed while heating increases the yield to 60% crystalline α-AlH 3 .

Aluminum hydride for solid-state hydrogen storage: Structure, synthesis, thermodynamics, kinetics, and regeneration

“…The synthesis method described above requires the use of a large amount of solvents such as ether and the removal of such solvents is both costly and hazardous. Dinh et al [74] presented a novel method to synthesize adduct-free AlH 3 . They directly heated the mixtures of LiAlH 4 and AlCl 3 at 75 °C in the solid state.…”

Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art

“…LiAlH4 and AlCl3 although alternative electrochemistry, [202] organic adducts, [203] and solvent free methods [204] have been developed. In the current state, AlH3 may be suitable for hydrogen generation at low temperatures, although its cost effective regeneration could be problematic.…”

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

“…Processing and chemical related challenges associated with traditional methods, such as precise temperature control, handling of volatile organic liquids and significant loss of yields have prompted development of other syntheses routes that (1) may afford adduct-free alane [18][19][20][21][22][23][24] and (2) utilize small amounts solvents or are entirely solvent-free [13][14][15][16][17]. We and others have recently demonstrated that these shortcomings can be partly circumvented by solvent-free mechanochemical syntheses of alane at room temperature using LiH or LiAlH4 and AlCl3 [20][21][22][23][24].…”