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 Unit cell of lithium borohydride at room temperature | |
| Names | |
|---|---|
| IUPAC name Lithium tetrahydridoborate(1–) | |
| Other names Lithium hydroborate, Lithium tetrahydroborate Borate(1-), tetrahydro-, lithium, lithium boranate | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.037.277 |
| RTECS number |
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| UNII | |
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| Properties | |
| LiBH4 | |
| Molar mass | 21.784 g/mol |
| Appearance | White solid |
| Density | 0.666 g/cm3[1] |
| Melting point | 268 °C (514 °F; 541 K) |
| Boiling point | 380 °C (716 °F; 653 K) decomposes |
| reacts | |
| Solubility inether | 2.5 g/100 mL |
| Structure[2] | |
| orthorhombic | |
| Pnma | |
a = 7.17858(4),b = 4.43686(2),c = 6.80321(4) | |
Lattice volume (V) | 216.685(3) A3 |
Formula units (Z) | 4 |
| [4]B | |
| Thermochemistry | |
| 82.6 J/(mol⋅K) | |
Std molar entropy(S⦵298) | 75.7 J/(mol⋅K) |
Std enthalpy of formation(ΔfH⦵298) | −198.83 kJ/mol |
| Hazards | |
| > 180 °C (356 °F; 453 K) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Lithium borohydride (LiBH4) is aborohydride and known inorganic synthesis as areducing agent foresters. Although less common than the relatedsodium borohydride, the lithium salt offers some advantages, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle thanlithium aluminium hydride.[3]
Lithium borohydride may be prepared by themetathesis reaction, which occurs upon ball-milling the more commonly availablesodium borohydride andlithium bromide:[4]
Alternatively, it may be synthesized by treatingboron trifluoride withlithium hydride indiethyl ether:[5]
Lithium borohydride is useful as a source ofhydride (H–). It can react with a range ofcarbonyl substrates and other polarized carbon structures to form a hydrogen–carbon bond. It can also react withBrønsted–Lowry-acidic substances (sources of H+) to formhydrogen gas.
As ahydride reducing agent, lithium borohydride is stronger than sodium borohydride[6][7] but weaker than lithium aluminium hydride.[7] Unlike the sodium analog, it can reduce esters to alcohols,nitriles andprimaryamides toamines, and can openepoxides. The enhanced reactivity in many of these cases is attributed to the polarization of the carbonyl substrate by complexation to the lithium cation.[3] Unlike the aluminium analog, it does not react withnitro groups,carbamic acids,alkyl halides, orsecondary andtertiary amides.
Lithium borohydride reacts with water to produce hydrogen. This reaction can be used for hydrogen generation.[8]
Although this reaction is usually spontaneous and violent, somewhat-stableaqueous solutions of lithium borohydride can be prepared at low temperature ifdegassed,distilled water is used and exposure tooxygen is avoided.[9]
Lithium borohydride is renowned as one of the highest-energy-density chemicalenergy carriers. Although of no practicality, the solid liberates 65 MJ/kg heat upon treatment with atmospheric oxygen. Since it has a density of 0.67 g/cm3, oxidation of liquid lithium borohydride gives 43 MJ/L. In comparison, gasoline gives 44 MJ/kg (or 35 MJ/L), while liquid hydrogen gives 120 MJ/kg (or 8.0 MJ/L).[nb 1] The high specific energy density of lithium borohydride has made it an attractive candidate to propose for automobile and rocket fuel, but despite the research and advocacy, it has not been used widely. As with all chemical-hydride-based energy carriers, lithium borohydride is very complex to recycle (i.e. recharge) and therefore suffers from a lowenergy conversion efficiency. While batteries such aslithium-ion carry an energy density of up to 0.72 MJ/kg and 2.0 MJ/L, theirDC-to-DC conversion efficiency can be as high as 90%.[10] In view of the complexity of recycling mechanisms for metal hydrides,[11] such high energy-conversion efficiencies are not practical with present technology.
| Substance | Specific energy, MJ/kg | Density, g/cm3 | Energy density, MJ/L |
|---|---|---|---|
| LiBH4 | 65.2 | 0.666 | 43.4 |
| Regulargasoline | 44 | 0.72 | 34.8 |
| Liquid hydrogen | 120 | 0.0708 | 8 |
| Lithium-ion battery | 0.72 | 2.8 | 2 |
Fourpolymorphs have been described. The stable forms feature tetrahedral BH4- anions.[12]
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