Thermal Decomposition of Lithium Aluminium Hydride

Lithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. Lithium aluminium hydride is often used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.

LAH is metastable at room temperature. During prolonged storage it slowly decomposes to Li3AlH6 and LiH. This process can be accelerated by the presence of catalytic elements, such as titanium, iron or vanadium.
When heated LAH decomposes in a three-step reaction mechanism:
3 LiAlH4 → Li3AlH6 + 2 Al + 3 H2 (R1)
2 Li3AlH6 → 6 LiH + 2 Al + 3 H2 (R2)
2 LiH + 2 Al → 2 LiAl + H2 (R3)
R1 is usually initiated by the melting of LAH in the temperature range 150–170 °C, immediately followed by decomposition into solid Li3AlH6, although R1 is known to proceed below the melting point of LiAlH4 as well. At about 200 °C, Li3AlH6 decomposes into LiH (R2) and Al which subsequently convert into LiAl above 400 °C (R3). Reaction R1 is effectively irreversible. R3 is reversible with an equilibrium pressure of about 0.25 bar at 500 °C. R1 and R2 can occur at room temperature with suitable catalysts.

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