Magnesium hydride
Encyclopedia
Magnesium hydride is the chemical compound MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.
catalyst:
Lower temperature production from Mg and H2 using nano crystalline Mg produced in ball mill
s has been investigated. Other preparations include:
structure. There are two high pressure forms, α-MgH2 with α-PbO2 structure and γ-MgH2. Additionally a non stoichiometric MgH(2-δ) has been characterised, but this appears to exist only for very small particles
(bulk MgH2 is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies).
The bonding in the rutile form is sometimes described as being covalent in nature rather than purely ionic; charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated.
Molecular forms of magnesium hydride, MgH, MgH2, Mg2H, Mg2H2, Mg2H3, and Mg2H4 molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10K, formed following laser ablation of magnesium in the presence of hydrogen. The Mg2H4 molecule has a bridged structure analogous to dimeric aluminium hydride
, Al2H6.
MgH2 + 2H2O → 2H2 + Mg(OH)2
At 300°C decomposes to produce H2 at 1 bar pressure, the high temperature required is seen as a limitation in the use of MgH2 as a reversible hydrogen storage medium:
An alternative approach under investigation is the production of a p umpable slurry of MgH2 which is safe to handle and releases H2 by reaction with water, with reprocessing of the Mg(OH)2 into MgH2.http://www.greencarcongress.com/2006/01/safe_hydrogen_r.html An application (yet to be examined) for a US Patent (US 2010/0163434 A1) http://www.google.com/patents/about?id=6-nRAAAAEBAJ has been made in respect of a hydrogen energy storage system using laser excitation to assist adsorption of hydrogen gas from magnesium hydride.
Preparation
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 MgI2Magnesium iodide
Magnesium iodide is the name for the chemical compounds with the formulas MgI2 and its various hydrates MgI2x. These salts are typical ionic halides, being highly soluble in water...
catalyst:
- Mg + H2 → MgH2
Lower temperature production from Mg and H2 using nano crystalline Mg produced in ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
s has been investigated. Other preparations include:
- the hydrogenation of magnesium anthraceneAnthraceneAnthracene is a solid polycyclic aromatic hydrocarbon consisting of three fused benzene rings. It is a component of coal-tar. Anthracene is used in the production of the red dye alizarin and other dyes...
under mild conditions:
- Mg(anthracene) + H2 → MgH2
- the reaction of diethylmagnesium with lithium aluminium hydrideLithium aluminium hydrideLithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
- the reaction of diethylmagnesium with lithium aluminium hydride
- product of complexed MgH2 e.g. MgH2.THF by the reaction of phenylsilanePhenylsilanePhenylsilane, also known as silylbenzene, a colorless liquid, is one of the simplest organosilanes with the formula C6H5SiH3. It is structurally related to toluene, with a silyl group replacing the methyl group. Both of these compounds have similar densities and boiling points due to these...
and dibutyl magnesium in ether or hydrocarbon solvents in the presence of THFTetrahydrofuranTetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
or TMEDATetramethylethylenediamineTetramethylethylenediamine is a chemical compound with the formula 2NCH2CH2N2. This species is derived from ethylenediamine by replacement of the four N-H groups with four N-methyl groups. Its odor is remarkably similar to that of fish.-As a reagent in organic and inorganic synthesis:TMEDA is...
as ligand.
Structure and bonding
The room temperature form β-MgH2 has a rutileTitanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
structure. There are two high pressure forms, α-MgH2 with α-PbO2 structure and γ-MgH2. Additionally a non stoichiometric MgH(2-δ) has been characterised, but this appears to exist only for very small particles
(bulk MgH2 is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies).
The bonding in the rutile form is sometimes described as being covalent in nature rather than purely ionic; charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated.
Molecular forms of magnesium hydride, MgH, MgH2, Mg2H, Mg2H2, Mg2H3, and Mg2H4 molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10K, formed following laser ablation of magnesium in the presence of hydrogen. The Mg2H4 molecule has a bridged structure analogous to dimeric aluminium hydride
Aluminium hydride
Aluminium hydride is an inorganic compound with the formula AlH3. It is a colourless solid that is pyrophoric. Although rarely encountered outside of research laboratory, alane and its derivatives are used as reducing agents in organic synthesis.-Structure:...
, Al2H6.
Reactions
MgH2 readily reacts with water to form hydrogen gas:MgH2 + 2H2O → 2H2 + Mg(OH)2
At 300°C decomposes to produce H2 at 1 bar pressure, the high temperature required is seen as a limitation in the use of MgH2 as a reversible hydrogen storage medium:
- MgH2→ Mg+ H2
Potential use for hydrogen storage
Its potential as a reversible "storage" medium for hydrogen has led to interest in improving the hydrogenation and dehydrogenation reaction kinetics.An alternative approach under investigation is the production of a p umpable slurry of MgH2 which is safe to handle and releases H2 by reaction with water, with reprocessing of the Mg(OH)2 into MgH2.http://www.greencarcongress.com/2006/01/safe_hydrogen_r.html An application (yet to be examined) for a US Patent (US 2010/0163434 A1) http://www.google.com/patents/about?id=6-nRAAAAEBAJ has been made in respect of a hydrogen energy storage system using laser excitation to assist adsorption of hydrogen gas from magnesium hydride.