Magnesium hydride

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:[3]

Mg + H₂ → MgH₂

Lower temperature production from Mg and H₂ using nano crystalline Mg produced in ball mills has been investigated.[4] Other preparations include:

• the hydrogenation of magnesium anthracene under mild conditions:[5]

Mg(anthracene) + H₂ → MgH₂

• the reaction of diethylmagnesium with lithium aluminium hydride[6]
• 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.[1]

The room temperature form α-MgH₂ has a rutile structure.[7] There are at least four high pressure forms: γ-MgH₂ with α-PbO₂ structure,[8] cubic β-MgH₂ with Pa-3 space group,[9] orthorhombic HP1 with Pbc2₁ space group and orthorhombic HP2 with Pnma space group.[10] Additionally a non stoichiometric MgH_(2-δ) has been characterised, but this appears to exist only for very small particles[11]
(bulk MgH₂ is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies[12]).

The bonding in the rutile form is sometimes described as being partially covalent in nature rather than purely ionic;[13] 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.[14] 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.[15] The Mg₂H₄ molecule has a bridged structure analogous to dimeric aluminium hydride, Al₂H₆.[15]

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,[16] the high temperature required is seen as a limitation in the use of MgH₂ as a reversible hydrogen storage medium:[17]

MgH₂ → Mg + H₂

Its potential as a reversible "storage" medium for hydrogen has led to interest in improving the hydrogenation and dehydrogenation reaction kinetics.[17][18] This can be partially achieved by doping or by reducing the particle size using ball milling.[19][20][21] 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. A series of U.S. patents (US9123925B2, US8651268B2, US8651270B2, US9732906B2) and international patents have been granted for a rechargeable low pressure, low temperature hydrogen storage system employing nano-structured magnesium hydride for high density hydrogen absorption and laser-activated desorption. A practical system employs treated polymer substrates in the form a disc or tape for use in long term storage, transportation and grid stabilization. The system has a unique advantage in the ability to passively capture hydrogen from industrial flues and waste treatment plants before it can bond to form greenhouse gases.