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 MgI₂ 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:

the hydrogenation of magnesium anthracene under mild conditions:

Mg(anthracene) + H₂ → MgH₂

the reaction of diethylmagnesium with lithium aluminium hydride

product of complexed MgH₂ e.g. MgH₂.THF by the reaction of phenylsilane and dibutyl magnesium in ether or hydrocarbon solvents in the presence of THF or TMEDA as ligand.

Structure and bonding

The room temperature form α-MgH₂ has a rutile structure, which is similar to TiO₂ There are at least four high pressure forms: γ-MgH₂ with α-PbO₂ structure, cubic β-MgH₂ with Pa-3 space group, orthorhombic HP1 with Pbc2₁ space group and orthorhombic HP2 with Pnma space group. Additionally a non stoichiometricetric 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 partially 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 10 K, formed following laser ablation of magnesium in the presence of hydrogen. The Mg₂H₄ molecule has a bridged structure analogous to dimeric aluminium hydride, Al₂H₆.

Reactions

MgH₂ readily reacts with water to form hydrogen gas:

MgH₂ + 2 H₂O → 2 H₂ + Mg(OH)₂

At 287 °C it 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. This can be partially achieved by doping or by reducing the particle size using ball milling. An alternative approach under investigation is the production of a pumpable slurry of MgH₂ which is safe to handle and releases H₂ by reaction with water, with reprocessing of the Mg(OH)₂ into MgH₂.[1] An application (yet to be examined) for a US Patent (US 2010/0163434 A1) [2] has been made in respect of a hydrogen energy storage system using laser excitation to assist desorption of hydrogen gas from magnesium hydride.