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Aluminum hydride (AlH3): structure, properties, uses

However, its properties demonstrate the opposite: it is a polymeric solid, the most faithful representation of which would be of the (AlH 3 ) n type , where n is the number of AlH 3 monomeric units that would make up a chain or layer of the crystal. Therefore, AlH 3 is one of those polymers that manages to adopt a crystalline structure.

Crystal structure of aluminum hydride. Source: Benjah-bmm27 / Public domain

Aluminum hydride is not a solid with much commercial diffusion, so there are few available images of it. It is especially intended for organic syntheses, where it serves as a powerful reducing agent. Likewise, it occupies a special place in the technological advance of materials, being a promising alternative for the storage of hydrogen.

Structure

Coordinations

Octahedral coordination of aluminum cations in the AlH3 crystal. Source: Benjah-bmm27 / Public domain.

Regardless of the polymorph or the crystalline phase considered, the coordinates between the aluminum and hydrogen atoms remain constant. In the upper image, for example, as in the first image, the coordination octahedron is shown for aluminum atoms (brown sphere).

Each Al atom is surrounded by six H, establishing six Al-H bonds. The way the octahedrons are oriented in space will make the structural difference between one polymorph and another.

On the other hand, each H atom coordinates with two Al atoms, establishing an Al-H-Al bond, which could be justified by a 3c2e type bond (3 centers-2 electrons). This bond is responsible for joining several AlH 6 octahedra throughout the allan crystal.

Isolated molecule

AlH 3 is considered polymeric because of the AlH 6 networks that make up the crystal. To isolate an individual molecule of allan, it is necessary to apply low pressures in an inert atmosphere of noble gas. In this way, the polymer breaks down and releases AlH 3 molecules of trigonal plane geometry (analogous to BH 3 ).

On the other hand, it is possible to dimerize two AlH 3 to form an Al 2 H 6 , as happens with diborane, B 2 H 6 . However, to achieve this requires the use of solid hydrogen, so it may not have much long-term industrial or commercial value.

Alane or AlH 3 is capable of forming up to seven polymorphs: α, α ‘, β, γ, δ, ε and ζ, of which α is the most stable to changes in temperature. Α-AlH 3 is distinguished by having a cubic morphology and a hexagonal crystal structure. It tends to be the product that other polymorphs transform into when they undergo thermal destabilization.

The morphology of γ-AlH 3 , on the other hand, stands out for being needle-like. That is why solid AlH 3 can contain a mixture of more than two polymorphs, and present varied crystals under the microscope.

Properties

Physical appearance

Aluminum hydride is a colorless to off-white solid with a crystalline appearance with a tendency to show needle shapes.

Molar mass

29.99 g / mol or 30 g / mol

Melting point

150 ° C. But it begins to decompose after 105 ºC.

Water solubility

High, because it reacts with it.

Solubility

Insoluble in diethyl ether and in nonpolar solvents such as benzene and pentane. Reacts with alcohols and other polar solvents.

Decomposition

AlH 3 is susceptible to decompose at different rates depending on external conditions, on the morphology and thermal stabilities of its crystals, or on the use of catalysts. When it does, it releases hydrogen and transforms into metallic aluminum:

2AlH 3 → 2Al + 3H 2

In fact, this decomposition, rather than being a problem, represents one of the reasons why alano is considered interesting in the development of new energy technologies.

Adduct formation

When AlH 3 does not react with the solvent irreversibly, it establishes an adduct with it, that is, a type of complex. For example, it can form a complex with trimethylamine, AlH 3 · 2N (CH 3 ) 3 , with tetrahydrofuran, AlH 3 · THF, or with diethyl ether, AlH 3 · Et 2 O. The latter was the best known when it was introduced the synthesis or production of alane in 1947.

Obtaining

The first appearances of AlH 3 date back to the years 1942 and 1947, being this last year when its synthesis using LiAlH 4 in a diethyl ether medium was presented :

3LiAlH 4 + AlCl 3 + n Et 2 O → 4AlH 3 · n Et 2 O + 3LiCl

The ethereal solution, AlH 3 · n Et 2 O, had to subsequently undergo desolvation, in order to eliminate Et 2 O and obtain pure AlH 3 . In addition to this problem, LiCl had to be removed from the medium of the products.

Thus, from 1950 to 1977, new syntheses were designed to obtain better yields of AlH 3 , as well as purer solids and better thermal and morphological properties. By modifying the quantities, steps and instruments used, it is possible to favor obtaining one polymorph over the other. However, α-AlH 3 is usually the majority product.

Other synthesis methods consist of making use of electrochemistry. For this, an aluminum anode and a platinum cathode are used. The following reaction takes place at the anode:

3AlH  + Al 3+ + n THF → 4AlH 3 · n THF + 3e 

While in the cathode metallic sodium is obtained. Then, the AlH 3 · n THF is also subjected to desolvation to remove the THF and finally obtain the AlH 3 .

Applications

Reducing agent

AlH 3 serves to reduce certain functional groups of organic compounds such as carboxylic acids, ketones, aldehydes and esters. Practically, what it does is add hydrogens. For example, an ester can be reduced to an alcohol in the presence of a nitro group:

Reduction of an ester with aluminum hydride. Source: Gringer / Public domain.

Hydrogen reservoir

Aluminum hydride represents an alternative to serve as a hydrogen reservoir, and thus, to be able to dispense it in a portable way in devices that operate with hydrogen batteries. The volumes obtained from H 2 correspond to a volume greater than double that of AlH 3 .

By taking the AlH 3 , and breaking it down in a controlled manner, a desirable amount of H 2 can be released at any time. Therefore, it could be used as rocket fuel and all those energy applications that seek to take advantage of the combustion of hydrogen.

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