Magnesium hydride

Description

MgH2 contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen-storage medium. It was discovered in 1912, during the pyrolysis of ethyl magnesium iodide (a Grignard reagent), which produced small amount of MgH2. In 1951, preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500°C)with magnesium iodide as a catalyst:
Mg＋H2→MgH2
Lower temperature production from Mg and H2 using nano-crystalline Mg produced in ball mills has been investigated. Other preparations include:
1. The hydrogenation of magnesium anthracene under mild conditions:
Mg(anthracene)＋H2→MgH2＋C14H10 2. The reaction of diethyl magnesium with LiAlH4
3. An adduct of complexed MgH2, e.g. MgH2·THF by the reaction of phenylsilane (C6H8Si) and dibutyl magnesium in ether or hydrocarbon solvents in the presence of THF (C4H8O) using TMEDA (Tetramethylethylenediamine＝(CH3)2NCH2CH2N (CH3)2) as a ligand.

Chemical properties

Pure magnesium hydride is a white, nonvolatile powder. Although its properties are predominantly salt-like, it shows signs of a transition towards the covalent polymeric hydride structure of compounds such as beryllium and aluminum hydride. Magnesium hydride decomposes without melting at 280 ℃. Its reactivity depends on the method of preparation. The product obtained by direct synthesis from the elements is stable in air. Nevertheless, if it is prepared by pyrolysis of dialkyl magnesium or by reaction of lithium aluminum hydride with dimethyl magnesium, the product is very pure and finely divided and ignites spontaneously on contact with air.

The Uses of Magnesium hydride

Magnesium hydride contains a high proportion of hydrogen (7.65 wt %), which can be liberated by heat. The catalytically produced compound displays rapid rates of hydrogenation and dehydrogenation and it is therefore of interest as a hydrogen reservoir. The high activity of catalytically produced magnesium hydride also enables it to take part in an addition reaction with 1-alkenes to form organomagnesium compounds:
MgH2+2 CH2 = CHR→Mg（CH2CH2R）2
These dialkylmagnesium compounds have become industrially important in the production of very active Ziegler catalysts. The Mg – H2 system can also be used for the simultaneous removal and purification of pure hydrogen from gas mixtures. Magnesium hydride is also used as a drying agent for organic solvents and gases and in chemical syntheses.

Production Methods

Magnesium hydride is synthesized industrially from the elements at 300 – 400 ℃ and 10 – 15 MPa hydrogen pressure in a rotary autoclave provided with steel grinding balls:
Mg+H2→MgH2
The commercial product is gray due to the presence of impurities, being about 97 % pure (Ventron). Less drastic reaction conditions are required when a homogeneous catalyst is used. The hydrogenation can be carried out at 60 – 70 ℃and 8 MPa in THF in the presence of halides of chromium, titanium, or iron and magnesium – anthracene complex. In contrast to the product from an uncatalyzed synthesis, this has a high specific surface area (90 m2 /g compared with 0.5 m2 /g), and is very reactive and pyrophoric.

General Description

Magnesium hydride is a white crystalline solid. Magnesium hydride reacts violently with water and may ignite upon contact with air. Magnesium hydride is used to make other chemicals.

Air & Water Reactions

Highly flammable. Pyrophoric, ignites spontaneously in air and forms magnesium oxide and water [Merck 11th ed. 1989]. Reacts with water to form caustic solution of magnesium hydroxide and hydrogen gas (H2) and heat sufficient to ignite the evolved hydrogen [Merck 11th ed. 1989].

Reactivity Profile

Hydrides, such as Magnesium hydride, are reducing agents and react rapidly and dangerously with oxygen and with other oxidizing agents, even weak ones. Thus, they are likely to ignite on contact with alcohols. Hydrides are incompatible with acids, alcohols, amines, and aldehydes.

Health Hazard

Inhalation or contact with vapors, substance or decomposition products may cause severe injury or death. May produce corrosive solutions on contact with water. Fire will produce irritating, corrosive and/or toxic gases. Runoff from fire control may cause pollution.

Fire Hazard

Produce flammable gases on contact with water. May ignite on contact with water or moist air. Some react vigorously or explosively on contact with water. May be ignited by heat, sparks or flames. May re-ignite after fire is extinguished. Some are transported in highly flammable liquids. Runoff may create fire or explosion hazard.

Potential Exposure

Used in hydrogen production and in production of magnesium alcoholates.

First aid

If this chemical gets into the eyes, remove anycontact lenses at once and irrigate immediately for at least15 min, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts theskin, remove contaminated clothing and wash immediatelywith soap and water. Seek medical attention immediately. Ifthis chemical has been inhaled, remove from exposure,begin rescue breathing (using universal precautions, including resuscitation mask) if breathing has stopped and CPR ifheart action has stopped. Transfer promptly to a medicalfacility. When this chemical has been swallowed, get medical attention. Give large quantities of water and inducevomiting. Do not make an unconscious person vomit.

storage

Color Code—Red Stripe: Flammability Hazard:Store separately from all other flammable materials. Storeunder nitrogen in tightly closed containers in cool (decomposes .250℃), well-ventilated area. Keep containers dry atall times. Magnesium hydride reacts violently with water,releasing caustic material, heat, and flammable gas. Sourcesof ignition, such as smoking and open flames, are prohibited where magnesium hydride is handled, used, or stored.

Shipping

UN2010 Magnesium hydride, Hazard Class: 4.3; Labels: 4.3-Dangerous when wet material.

Incompatibilities

A strong reducing agent. Pyrophoric: the powder or dust may ignite spontaneously in air, or in the presence of moisture. Contact with water or steam forms magnesium hydroxide, flammable hydrogen gas, and enough heat to ignite the hydrogen. Violent reaction with oxidizers, alcohols, halogens, chlorinated solvents. Incompatible with strong acids, acid chlorides, alcohols, amines and aldehydes. Store under nitrogen. Decomposes @ 280C in high vacuum