“One-step” synthesis of nonsolvated aluminum hydride

Abstract

The direct reaction of lithium tetrahydroaluminate with aluminum chloride in an ethertoluene mixture at temperatures ≥90 °С was studied. Under these conditions, nonsolvated aluminum hydride is formed in a yield ≥80%. The product formed represents agglomerates with the highly developed surface and has satisfactory hydrolytic and thermal stability but an increased lithium chloride content (0.25-2.2 wt.%). The decrease in the percentage of this impurity to 0.2 wt.% is achieved by using chloroalanes AlH 3-n Cl n •xOEt 2 (n ≤ 1) instead of aluminum chloride along with vigorous stirring and an increase in the synthesis-crystalliza tion temperature to ≥100 °С. The use of compounds based on magnesium tetrahydroaluminate, LiAlH 4 •Mg(AlH 4 ) 2 or LiCl•Mg(AlH 4 ) 2 , as modifying agents increases the thermal stability of the hydride and decreases the agglomeration of the crystals.The problems associated with the search for efficient hydrogen sources and materials for its storage are among the most important in the context of hydrogen power engineering. Diverse types and classes of substances that can be used as sources and/or accumulators of this gas have been considered and studied over the recent years. First of all, these are simple and complex hydrides of light metals containing the largest amount of hydrogen per mass unit. Among them, aluminum hydride can be recog nized as one of the most efficient in chemical and physi cochemical properties, because its thermolysis requires low energy expenses but produces 10 wt.% hydrogen, while twice as large amount of hydrogen is evolved upon hydrolysis.Unfortunately, aluminum hydride in the nonsolvated state is one of the most inaccessible and expensive binary hydrides. Presently, the most popular method for the syn thesis of this hydride is the thermal cleavage of etherate {AlH 3 •xEt 2 O}, which was obtained by the Schlesinger reaction 1 (Eq. (1)) in the presence of small amounts of lithium tetrahydridoaluminate and tetrahydridoborate (10-15 wt.% of aluminum hydride) followed by crystal lization of nonsolvated aluminum hydride (hereinafter, NAH) from solutions in an ether-ArH mixed solvent. 2 3 LiAlH 4 solv + AlCl 3 •xEt 2 O solv 4 AlH 3 •xEt 2 O solv + 3 LiCl solid (1) It has been shown 3 that the concentration of alumi num hydride in a 1 : 1 (v/v) Et 2 O-ArH solution should not exceed 4 g L -1 for the process to occur successfully. The concentration can be increased to 5-6 g L -1 with addition of a large amount of LiAlH 4 and LiBH 4 (up to 25-45 wt.% each of aluminum hydride). 3 All further attempts to increase the concentration resulted in the spontaneous precipitation of the "dead" solvate {AlH 3 •0.33 Et 2 O} n (see Ref. 4) and a simultaneous decrease in the yield of the nonsolvated material and deterioration of its quality.Some increase in the hydride concentration (more exactly, effective concentration (С eff AlH 3 ) determined from its amount calculated from the reaction stoichio metry and related to the total weight of solvents used in crystallization) was achieved 5 b...

Aluminum hydride as a hydrogen and energy storage material: Past, present and future

“…The "direct" synthesis of non-solvated alane (any polymorph) occurs by a reaction between an alanate and aluminum bromide in an aromatic hydrocarbon in the absence of ether [16]. Another example is a high-temperature synthesis of ␣-AlH 3 similar to reaction (3), but performed by mixing the solutions of alanate and aluminum chloride in an ether-toluene solution at high temperature (>90 • C) [17]. It should be noted, however, that reactions (3) and (4) are the preferred synthesis methods for the preparation of ␣-AlH 3 and in most cases the use of other reactants and synthesis techniques results in a less dense material with a non-ideal morphology and a lower thermal stability.…”

“…Therefore, a variety of stabilization methods were developed over the years, one example includes the addition of magnesium compounds, LiAlH 4 ·Mg(AlH 4 ) 2 and LiCl·Mg(AlH 4 ) 2 [17], which was shown to significantly increase the activation energy of decomposition. Probably the most successful and widely used method of alane stabilization is the application of a thick surface oxide and/or hydroxide layer [48][49][50][51].…”

Wet Synthesis of Energetic Aluminum Hydride

“…On the other hand, alanes prepared from process II began to thermally decompose sharply at 168 °C, slightly above referenced temperatures (160–165 °C) and lost about 9.9 % their mass up to 182 °C (see Figure (b)). In fact, after the first mass decrease, no further mass variation was found up to 300 °C.…”

Luminescent properties of aluminum hydride

“…Description of synthesis procedure can be found in [10]. They have form of a powder consisting of cubic particles with an edge length of 10-15 μm.…”