EMPA: Solvent-free synthesis of borohydrides

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Scientists at EMPA in Switzerland have developed a new method for the solvent-free synthesis of borohydrides at room temperature.

Borohydrides are reagents that are widely used in organic and inorganic synthesis. In addition, they have high gravimetric hydrogen density, which means they are very promising compounds for hydrogen storage in mobile applications.

The new synthesis method is performed by milling the corresponding metal hydrides in a borane/hydrogen atmosphere.

The synthesis of borohydrides is currently performed in wet chemical reactions with organic solvents such as ether or isopropyl amine, making use of metathesis reactions with different halides or chlorides.

In order to obtain a pure product, the solvents and the salts forming as by-products have to be separated from the final product. This makes the synthesis process complex, time consuming and costly. As a result, it is difficult to obtain pure borohydrides as end products. As a consequence many borohydrides are not yet commercially available, or are very expensive.

The new synthesis method presents a general way to prepare pure borohydrides that are solvent-free at room temperature. After-treatment of the final product is not necessary, making this process more effective and economic than other preparation methods.

Additionally, the new synthesis method allows the combination of different metals for borohydride formation. This might enable the synthesis of new, mixed borohydride systems with tunable sorption properties.

Borohydrides are applied in a wide range of different fields. In organic and inorganic chemistry they are used as reducing agents, starting compounds for the synthesis of organometallic derivatives, precursors for the production of borides and hydrides, as well as other inorganic materials and catalysts for hydrogenation, isomerisation, oligomerisation, and polymerisation.

Furthermore, borohydrides have potential as future energy carriers due to their high volumetric and gravimetric hydrogen density. The gravimetric energy density of LiBH4, for example, exceeds that of gasoline by a factor of three.