Lithium tetrahydridoborate：Properties，Production and Uses
Lithium tetrahydridoborate (known industrially as lithium borohydride or lithium boranate), LiBH4, is a white, microcrystalline powder that decomposes at the melting point (280℃) with considerable evolution of hydrogen. It is stable in vacuum to about 200℃. It is extremely hygroscopic, being rapidly decomposed by moist air, sometimes with spontaneous ignition.
Properties
Lithium tetrahydridoborate is therefore stored and handled in an atmosphere of nitrogen or argon. It is hydrolyzed only slowly by water at 0 ℃:
LiBH4+2 H2O→LiBO2+4 H2
By acidification of the solution or by addition of certain metal salts as catalysts, the hydrolysis is rapid and large volumes of hydrogen are liberated (4.11 L/g at STP). Lithium borohydride is a more powerful reducing agent than sodium borohydride although much less reactive than lithium aluminum hydride.
Lithium borohydride is used for the selective reduction of esters, carboxylic acids, amides, and epoxides in preference to other functional groups such as nitriles, nitro compounds, and halides [5]. The effect of solvents has been investigated [40]; diethyl ether and tetrahydrofuran are preferred. The addition of a stoichiometric amount of methanol leads to a very large increase in reducing power without significantly affecting the selectivity. Among the reactions of lithium borohydride with inorganic compounds, special mention should be made of reactions yielding borohydrides or hydrides of other metals, e.g.,
4 LiBH4+ZrCl4→Zr(BH4)4+4 LiCl
Production
Industrial production of lithium borohydride makes use of the heterogeneous metathetical reaction between LiCl or LiBr and sodium borohydride in ethers (diethyl ether, tetrahydrofuran) or amines:
NaBH4+LiX→LiBH4+NaX X = Cl, Br
The mixture is ball-milled while the reaction takes place in order to ensure a satisfactory rate of reaction and a good yield (up to 90 %) [42]. Synthesis from the elements is not used industrially. The commercial product (Chemetall, Ventron) has a bulk density of 0.6 – 0.7 g/cm3 and a purity of 94 %.
Uses
Lithium borohydride is used to a limited extent as a selective reducing agent in organic chemistry. It has been investigated as a means of storing hydrogen, but has not been used on an industrial scale for this purpose.
