Stabilized aluminum hydride polymorphs

Details Stabilized aluminum hydride polymorphs Stabilized aluminum hydride polymorphs

2001-03-29

2003-09-09

Langel, Wayne A. (Department: 1754)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C149S037000, C149S043000, C149S046000, C149S061000, C149S109400, C423S265000, C423S274000, C423S645000, C429S206000, C526S159000, C568S881000, C568S885000, C585S250000

Reexamination Certificate

active

06617064

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to aluminum hydride, or “alane,” and more particularly relates to a novel method for preparing aluminum hydride polymorphs such as &agr;-AlH
3
and &agr;′-AlH
3
. The invention additionally relates to a stabilized form of &agr;-AlH
3
, to energetic compositions, particularly propellant compositions, containing, as a fuel, stabilized &agr;-AlH
3
and/or &agr;′-AlH
3
prepared using the method of the invention, and to methods of using the alane polymorphs prepared herein as chemical reducing agents, as polymerization catalysts, and as a source of hydrogen gas such as in batteries and fuel cells.
BACKGROUND
Aluminum hydride, also referred to as “alane,” is usually prepared as a solution by the reaction of lithium aluminum hydride with aluminum trichloride. A. E. Finholt et al. (1947)
J. Am. Chem. Soc.
69:1199. The alane-containing solution, however, is not stable, as an alane.ether complex precipitates from solution shortly after preparation. In addition, attempts to isolate the nonsolvated form of alane from the ether solution result in the decomposition of the complex to aluminum and hydrogen. M. J. Rice Jr. et al. (1956) Contract ONR-494(04) ASTIA No. 106967, U.S. Office of Naval Research.
In a method for preparing non-solvated alane, alane.etherate may be desolvated in the presence of a small amount of lithium aluminum hydride. See, for example, A. N. Tskhai et al. (1992)
Rus. J. Inorg. Chem.
37:877, and U.S. Pat. No. 3,801,657 to Scruggs. Non-solvated alane exhibits six crystalline phases, with each having different physical properties. The phase designated as &agr;′-alane is essentially non-solvated and appears under a polarizing microscope as small multiple needles growing from single points to form fuzzy balls. The &ggr; phase appears as bundles of fused needles. The &ggr; phase is produced in conjunction with the &ggr; phase, and both &ggr;- and &bgr;-alane are metastable nonsolvated phases that convert to the more stable &agr;-alane upon heating. The &agr;-alane is the most stable, and is characterized by hexagonal or cubic shaped crystals that are typically 50-100 &mgr;m in size. The other two forms, designated &dgr; and &egr;-alane, are apparently formed when a trace of water is present during crystallization, and the &zgr;-alane is prepared by crystallizing from di-n-propyl ether. The &agr;′, &dgr;, &egr; and &zgr; polymorphs do not convert to the &agr;-alane and are less thermally stable than the &agr;-form. For a discussion of the various polymorphs, reference may be had to F. M. Brower at al. (1976)
J. Am. Chem. Soc.
98:2450.
Alane consists of about 10% hydrogen by weight, thereby providing a higher density of hydrogen than liquid hydrogen. Because of the high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, alane can be used as a fuel for solid propellants or as an explosive.
Solvated alane can be synthesized by the reaction of LiAlH
4
with aluminum chloride, resulting in the alane.etherate complex (equation 1).
In an alternative synthesis, LiAlH
4
is reacted with sulfuric acid to give the alane.etherate complex (equation 2).
The AlH
3
-ether complex is then treated with a mixture of LiAlH
4
and LiBH
4
, and heated (equation 3).
The combination of LiBH
4
/LiAlH
4
enables use of a lower processing temperature, and &agr;-alane is the final product after heating at 65° C. under vacuum. In an alternative synthesis, Bulychev reports that &agr;-alane can be prepared at pressures greater than 2.6 GPa and at temperatures in the range of 220-250° C. B. M. Bulychev et al. (1998)
Russ. J. Inorg. Chem.
43:829. Under those conditions, apparently only the &agr;-alane form is observed.
In addition, alane can be directly synthesized by metathesis of aluminum alkyls followed by removal of the alkylaluminum byproduct in vacuum (equation 4).
Still another method of preparing nonsolvated alane is by bombarding an ultrapure aluminum target with hydrogen ions. However, alane thus produced has poor crystallinity.
One of the obstacles to large scale production of &agr;-alane is the handling of the diethyl ether solution of the alane.ether complex. At concentrations of about 0.5 M or higher and temperatures above 0° C., the alane.ether phase prematurely precipitates out of solution. In addition, &agr;-alane can be contaminated with other phases of alane, and is not stable over time as the complex decomposes to hydrogen and aluminum.
Thus, although alane is potentially promising as a high energy density fuel, because of its high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, the lack of a suitable method for synthesizing alane in a stabilized form has severely limited its applicability.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the invention to address the above-mentioned need in the art and provide a method for synthesizing &agr;-alane in a stabilized form.
It is another object of the invention to provide stabilized &agr;-alane as a novel composition of matter, prepared using the aforementioned method.
It is an additional object of the invention to provide a method for synthesizing &agr;′-alane.
It is a further object of the invention to provide energetic compositions containing stabilized &agr;-alane or &agr;′-alane, prepared using the methods described herein.
It is still a further object of the invention to provide such energetic compositions in the form of a propellant composition.
It is also an object of the invention to provide methods for using stabilized &agr;-alane or &agr;′-alane, prepared using the methods described herein, as an energy dense fuel, as a chemical reducing agent, as a polymerization catalyst, and as a source of hydrogen gas such as in batteries and fuel cells.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one embodiment, then, the invention relates to a method for preparing stabilized &agr;-AlH
3
wherein: (a) an alkali metal hydride is initially reacted with AlCl
3
in diethyl ether to form an initial AlH
3
product and an alkali metal chloride; (b) the reaction mixture is filtered to remove the alkali metal chloride; (c) an excess of toluene is added to the filtrate of step (b), resulting in a diethyl ether-toluene solution; (d) the diethyl ether-toluene solution is heated and distilled to reduce the amount of diethyl ether in solution, until a precipitate is formed; (e) the precipitate is isolated; and (f) the isolated precipitate is added to an acidic solution effective to dissolve and thus remove materials other than &agr;-AlH
3
. In a preferred embodiment, the acidic solution contains an &agr;-AlH
3
stabilizing agent, e.g., a compound that coordinates to the Al
3+
ion, an electron donor, or an electron acceptor.
In another embodiment, the invention provides a method for synthesizing &agr;′-AlH
3
wherein (a) an alkali metal hydride is initially reacted with AlCl
3
in diethyl ether to form an initial AlH
3
product and an alkali metal chloride; (b) the reaction mixture is filtered to remove the alkali metal chloride; (c) an additional alkali metal hydride and an excess of toluene are added to the filtrate of step (b), providing a diethyl ether solution containing &agr;′-AlH
3
and optionally other AlH
3
polymorphs; and (d) removing the &agr;′-AlH
3
is from the solution.
In a further embodiment of the invention, a propellant composition is provided containing fuel, a binder material, and an oxidizer, wherein the fuel is a stabilized &agr;-AlH
3
product or an &agr;′-AlH
3
product prepared using the aforementioned techniques. The alane polymorphs of the invention are compatible with a wide range of binder materials, oxidizers, secondary fuels, and other propellant components, and provide for a propellant

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