Selective Reversible Hydrogenation of Mg(B3H8)2/MgH2 to Mg(BH4)2: Pathway to Reversible Borane-Based Hydrogen Storage?

Mg(B3H8)(2)center dot 2THF (THF = tetrahydrofuran) was prepared by the addition of BH center dot THF to Mg/Hg amalgam. Heating a 1:2 molar mixture of Mg(B3H8)(2)center dot 2THF and MgH2 to 200 degrees C under 5 MPa H-2 for 2 h leads to nearly quantitative conversion to Mg(BH4)(2). The differential scanning calorimetry profile of the reaction measured under 5 MPa H-2 shows an initial endothermic feature at similar to 65 degrees C for a phase change of the compound followed by a broad exothermic feature that reaches a maximum at 130 degrees C corresponding to the hydrogenation of Mg(B3H8)(2) to Mg(BH4)(2). Heating Mg(B3H8)(2)center dot 2THF to 200 degrees C under 5 MPa H-2 pressure in the absence of MgH2 gives predominantly MgB12H12 as well as significant amounts of MgB10H10 and Mg(BH4)(2). Hydrogenation of a mixture of Mg(B3H8)(2)center dot 2THF and LiH in a 1:4 molar ratio at 130 degrees C under 5 MPa H-2 yields [B12H12](2) in addition to [BH4](-), while a 1:4 molar ratio of Mg(B3H8)(2)center dot 2THF and NaH yields [BH4](-) and a new borane, likely [B2H7](-). Hydrogenation of the NaH-containing mixture at 130 degrees C gives primarily the alternative borane, indicating it is an intermediate in the two-step conversion of the triborane to [BH4](-). The solvent-free triborane Mg(B3H8)(2), derived from the low-temperature dehydrogenation of Mg(BH4)(2), also produces Mg(BH4)(2), but higher temperature and pressure is required to effect the complete transformation of the Mg(B3H8)(2). These results show that the reversible transformation of the triborane depends on the stability of the metal hydride. The more stable the metal hydride, that is, LiH > NaH > MgH2, the lower is the regeneration efficiency.