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Magnesium hydride: structure, properties and uses

It belongs to the family of saline (or ionic) hydrides, those defined by a negatively charged H-ion. These hydrides are considered those that are formed from alkali metals and alkaline earth metals, but in the case of magnesium (and beryllium) they have covalent bonds, in addition to those ionic that characterize this family of hydrides.

Unit cell model of magnesium hydride, MgH2.

Preparation and formula

Magnesium hydride is formed through the direct hydrogenation of metallic magnesium (Mg) under high pressure and temperature conditions (200 atmospheres, 500 ° C) with a MgI 2 catalyst . Your reaction equates to:

The production of MgH 2 at lower temperatures with the use of nanocrystalline magnesium produced in ball mills has also been investigated .

There are also other preparation methods, but they represent more complex chemical reactions (hydrogenation of magnesium-anthracene; the reaction between diethylmagnesium with lithium-aluminum hydride; and as a product of a MgH 2 complex ).

Chemical structure

Crystal structure of magnesium hydride

This atom has a rutile structure at room temperature, with a tetragonal crystalline structure. It has at least four different forms under high pressure conditions, and a non-stoichiometric structure with hydrogen deficiencies has also been observed; the latter only occurs in very small amounts of particles when it is formed.

As mentioned above, the bonds that exist in the rutile structure have partially covalent properties rather than being purely ionic like other salt hydrides.

This makes the magnesium atom have a spherical shape, fully ionized, but its hydride ion has an elongated structure.

Physical properties

  • Appearance: White crystals.
  • Molar Mass: 26.3209 g / mol
  • Density: 1.45 g / cm 3
  • Melting Point: 285 ° C decomposes
  • Solubility: In water it decomposes.

This chemical compound has a molecular weight of 26.321 g / mol, a density of 1.45 g / cm³ and has a melting point of 327 ºC.

Chemical properties

  • Precursor for the manufacture of other chemical substances.
  • Hydrogen storage, as a possible source of energy.
  • Reducing agent in organic synthesis.

It is important to point out that this compound cannot be brought to a liquid state , and when it is brought to its melting point or introduced into water, it decomposes. This hydride is insoluble in ether.

It is a highly reactive and highly flammable substance, and it is also pyrophoric, that is, it can ignite spontaneously in air. These three conditions represent security risks that will be mentioned in the last section of this article. 

Uses / applications

Hydrogen storage

Magnesium hydride easily reacts with water to form hydrogen gas, through the following chemical reaction:

MgH 2 + 2H 2 O → 2H 2 + Mg (OH) 2

Furthermore, this substance decomposes at a temperature of 287 ºC and a pressure of 1 bar, as follows:

MgH 2 → Mg + H 2

Therefore, the use of magnesium hydride has been proposed as a hydrogen storage medium for its use and transport.

The hydrogenation and dehydrogenation of a quantity of metallic magnesium is proposed as a way of transporting quantities of gaseous hydrogen, thus ensuring that it does not leak during its transport and representing a safer and more practical way than with the use of high pressure vessels. .

Hydrogenation and dehydrogenation reactions

Although the decomposition temperature of magnesium hydride represents a limitation for its use, methods have been proposed to improve the kinetics of hydrogenation and dehydrogenation reactions. One of these is with the reduction of the size of the magnesium particle with the use of ball mills. 

Mud

In addition, a system has been proposed that produces a magnesium hydride in the form of sludge (more manageable and safer than that in powder or other solid particles), which would be reacted with water to obtain the desired hydrogen.

It is estimated that the aforementioned sludge would be formed by a finely ground hydride, protected with a protective layer of oils and suspended in dispersing agents to ensure that it maintains its consistency without loss of material, and that it does not absorb humidity from the environment.

This sludge has the advantage that it can be pumped through any common diesel, gasoline or water pump, making this proposal economical as well as efficient.

Fuel cells

Magnesium hydride can be implemented in the production of advanced fuel cells, and also in the creation of batteries and energy storage. 

Transport and energy

During the last decades, the use of hydrogen as a source of energy has been considered. The introduction of hydrogen as a fuel requires finding safe and reversible storage systems with high volumetric capacities (quantity of hydrogen per unit of volume ) and gravimetric (quantity of hydrogen per unit of mass).

Alkylation

Alkylation (adding CH 3 R alkyl groups ) of organic compounds in a basic medium, where there is the presence of -OH groups at low concentrations and at temperatures above the melting point of the hydride.

In this case the hydrogens present in the magnesium hydride (MgH 2 ), bind to the -OH groups, forming water. Free magnesium can receive the halogen that often accompanies the alkyl molecule that is intended to join the hydrocarbon chain.

Risks

Reaction with water

As already mentioned, magnesium hydride is a substance that reacts very easily and violently with water, presenting the ability to explode in higher concentrations.

This occurs because its exothermic reaction generates enough heat to ignite the hydrogen gas released in the decomposition reaction, leading to a rather dangerous chain reaction.

It is  pyrophoric

Magnesium hydride is also pyrophoric, which means that it can spontaneously ignite in the presence of moist air, forming magnesium oxide and water.

Inhalation in solid state or contact with its vapors is not recommended : the substance in its natural state and its decomposition products can cause serious injuries or even death.

It can generate corrosive solutions in contact with water and its contamination. Contact with the skin and eyes is not recommended, and it also causes irritation to the mucous membranes.

Magnesium hydride has not been shown to cause chronic health effects, such as cancer, reproductive defects or other physical or mental consequences, but it is recommended to use protective equipment when handling it (especially respirators or masks, due to its fine powder character).

When working with this substance, keep the humidity of the air at low levels, extinguish all sources of ignition and transport it in drums or other container containers.

Working with large concentrations of this substance should always be avoided when it can be avoided, as the possibility of an explosion is significantly reduced.

Should a magnesium hydride spill occur, the work area should be isolated and dust collected with a vacuum tool. You should never use the dry sweep method; increases the chances of some reaction with the hydride.

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