Magnesium hydride
White solid
Fixed
1.45 g · cm -3
280-300 ° C ( decomposition)
- Reacts with water
- Insoluble in diethyl ether
Risk
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Magnesium hydride is a hydride of the light metal magnesium with the formula unit MgH2.
Synthesis
For the synthesis of magnesium hydride of the elements magnesium and hydrogen methods have been described, however, require either a very high pressure and temperatures or difficult to handle and partly toxic catalysts. These methods are therefore economically ( and ecologically ) unfavorable.
The reaction can be recovered as a catalyst, for example with a mixture of alkyl iodide, propargyl bromide iodine.
Another possibility is the high-pressure reaction of a magnesium halide such as magnesium iodide is dissolved in ether with sodium hydride.
In the hydrogenation of magnesium energy (heat) is free, so it should actually continue spontaneously. However, the reaction is in its execution speed significantly inhibited (kinetic inhibition). Catalysts can reduce these inhibitions. Magnesium hydride itself catalyzes its own formation ( autocatalysis ), as Wilfried Knott found out. In the presence of magnesium little further formation is possible under medium pressure and temperature conditions.
The product thus produced is a gray powder, with little magnesium impurities.
Also possible is the production by heating magnesium dialkyls (eg magnesium diethyl, Magnesiumdibutyl, Magnesiumdiphenyl ) or corresponding Grignard compounds in a high vacuum.
Properties
Activated finely divided magnesium hydride is pyrophoric, macrocrystalline magnesium but does not ignite on contact with air, but needs to more than 300 degrees Celsius. Magnesium reacts as most metal hydrides, violently with water liberating hydrogen.
Use
One kilogram hydride can be up to 800 liters of hydrogen gas ( about 7 % of the combined mass is then hydrogen ) are stored as hydride. While having ordinary fuel such as gasoline, a higher energy density, but compared to batteries or liquefied Save the energy density is higher. Moreover, because the hydrogen can be delivered reversible at elevated temperatures, use as hydrogen storage is possible. A current research focus is in the reduction of the required higher temperature for hydrogen release to allow practical use.
By hydrogen, which is released from magnesium, a metal foam with advantageous properties (for example, lighter than water ) can be generated.