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Magnesium hydride
Magnesium-hydride-unit-cell-3D-balls.png
Magnesium-hydride-xtal-3D-ionic-B.png
IUPAC name
Identifiers
CAS number 7693-27-8 Yes check.svgY
PubChem 107663
Properties
Molecular formula MgH2
Molar mass 26.3209 g/mol
Appearance white crystals
Density 1.45 g/cm3
Melting point

327 °C

Solubility in water decomposes
Hazards
EU Index Not listed
Main hazards pyrophoric[1]
Related compounds
Other cations Beryllium hydride
Calcium hydride
Strontium hydride
Barium hydride
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.[2]

Contents

Preparation

The compound was first discovered in 1912,[3] when pyrolysis of ethyl magnesium iodide (a Grignard reagent) produced small amounts 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 MgI2 catalyst:[4]

Mg + H2 → MgH2

Lower temperature production from Mg and H2 using nano crystalline Mg produced in ball mills has been investigated. [5] Other preparations include:

  • the hydrogenation of magnesium anthracene under mild conditions:[6]
Mg(anthracene) + H2 → MgH2
  • 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]

Structure and bonding

The room temperature form β-MgH2 has a rutile structure.[8] There are two high pressure forms, α-MgH2 with α-PbO2 structure[9] and γ-MgH2.[10] Additionally a non stoichiometric MgH(2-δ) has been characterised.[11]
The bonding in the rutile form is sometimes described as being covalent in nature rather than purely ionic;[12] 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.[13] 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.[14] The Mg2H4 molecule has a bridged structure analogous to dimeric aluminium hydride, Al2H6.[14]

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:[15]

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.[15][16] 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.[2]

References

  1. ^ a b Synthesis of magnesium hydride by the reaction of phenylsilane and dibutylmagnesium , Michalczyk M.J., Organometallics; (1992); 11(6); 2307-2309. doi:10.1021/om00042a055
  2. ^ Catalytic Synthesis of Organolithium and Organomagnesium Compounds and of Lithium and Magnesium Hydrides - Applications in Organic Synthesis and Hydrogen Storage, Bogdanovic B., Angewandte Chemie International Edition in English, 24, 4, 262–73, doi:10.1002/anie.198502621
  3. ^ Jolibois, Compte Rendu, 155, 353,(1912)
  4. ^ Wiberg, Goeltzer, Bauer,1951, Z naturforsch 6b, 394, (1951)
  5. ^ Nanocrystalline magnesium for hydrogen storage, A Zaluska, L Zaluski, JO Ström–Olsen, Journal of Alloys and Compounds, 288, 1-2, 1999, 217-225, doi:10.1016/S0925-8388(99)00073-0
  6. ^ Catalytic Synthesis of Magnesium Hydride under Mild Conditions, Bogdanovic B., Liao S-T, Schwickardi M, Sikorsky P., Spliethoff B., Angewandte Chemie International Edition in English, 19,(1980), 10, 818 – 819, doi:10.1002/anie.198008181
  7. ^ The Preparation of the Hydrides of Zinc, Cadmium, Beryllium, Magnesium and Lithium by the Use of Lithium Aluminum Hydride, Barbaras G.D., Dillard C., Finholt A. E., Wartik T, Wilzbach K. E., Schlesinger H. I., J. Am. Chem. Soc.; 1951; 73(10); 4585-4590, doi:10.1021/ja01154a025
  8. ^ Neutron diffraction study of magnesium deuteride, Zachariasen W.H., Holley C.E, Stamper J.F. Jnr, Acta Cryst. (1963) 16, 352-353, doi:10.1107/S0365110X63000967
  9. ^ Michael D. Hampton, Dmitry V. Schur, Svetlana Yu. Zaginaichenko, V. I. Trefilov, Hydrogen Materials Science and Chemistry of Metal Hydrides: Proceedings of the NATO Advanced Research Workshop, Katsiveli, Yalta, Ukraine, 2002, Springer, ISBN 1402007302
  10. ^ Structure of the high pressure phase γ-MgH2 by neutron powder diffraction, Bortz M., Bertheville B., Böttger G., Yvon K., Journal of Alloys and Compounds, 287, 1-2, (1999), L4-L6, doi:10.1016/S0925-8388(99)00028-6
  11. ^ Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium,Schimmel, H. G.; Huot, J.; Chapon, L. C.; Tichelaar, F. D.; Mulder, F. M.,J. Am. Chem. Soc.; (Article); 2005; 127(41); 14348-14354. doi:10.1021/ja051508a
  12. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5  
  13. ^ Charge density measurement in MgH2 by synchrotron X-ray diffraction, T. Noritake, S. Towata, M. Aoki, Y. Seno, Y. Hirose, E. Nishibori, M. Takata and M. Sakata, Journal of Alloys and Compounds, 356-357, (2003), 84-86, doi:10.1016/S0925-8388(03)00104-X
  14. ^ a b Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride, Xuefeng Wang and Lester Andrews, J. Phys. Chem. A, 108 (52), 11511 -11520, 2004. doi:10.1021/jp046410h
  15. ^ a b Hydrogen-storage materials for mobile applications, L. Schlapbach and A. Züttel, Nature 414, 353 (2001).doi:10.1038/35104634[1]
  16. ^ J Huot Hydrogen in Metals (2002) in New Trends in Intercalation Compounds for Energy Storage, Chrisitan Julien, J. P. Pereira-Ramos, A. Momchilov, Springer, ISBN 1402005946
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