Very few prospective on-board hydrogen storage materials have garnered as much interest as Mg(BH4)2. The dehydrogenation of the borohydride to MgB2 has a calculated ΔHo of 38.6 kJ/(mol H2) and ΔS of 111.5 J/(K mol H2), predicting a plateau pressure of 1 bar H2 at 73 °C. These thermodynamic properties coupled with high gravimetric density of H2, ca. 14.7 wt% suggest its application as a reversible hydrogen carrier for PEM fuel cell applications. Two critical challenges remain; (i) improvement of the slow rates of hydrogen release at moderate temperatures; and (ii) circumventing the thermodynamic, “dead-end” dehydrogenation product, MgB12H12. The decomposition of Mg(BH4)2 over 5 weeks at 200 °C yielded Mg(B3H8)2 as the near exclusive product. Under 120 bar H2 pressure and 250°C, the Mg(B3H8)2 intermediate was completely converted back to Mg(BH4)2 after 48 hours. The subsequent hydrogenation of independently synthesized Mg(B3H8)2(THF) (THF = tetrahydrofuran), where attempts to remove the solvent were unsuccessful, demonstrated that quantitative re-hydrogenation to Mg(BH4)2 could be achieved under 50 bar H2 and 5 hours at 200°C. We concluded that the faster rate exhibited by the solvated sample resulted from a phase change induced by the coordination of the THF, thus solvent coordination might also enhance dehydrogenation kinetics. To test this hypothesis, authentic sample of Mg(BH4)2(THF) was prepared and dehydrogenation trial was carried out using PCT, indicated rapid release of H2 up to 2.5 wt.% within 5 hours. Subsequent 11B NMR of products after dehydrogenation revealed the selective formation of MgB10H10(THF)x (0 & -30 ppm) with trace amounts of MgB12H12(THF)x and MgB3H8(THF)x (-15 ppm & -31 ppm, respectively). Preliminary results show that MgB10H10 can be cycled back to Mg(BH4)2 in the presence of “fresh MgH2”. Emphasis on 11B and 1H NMR analysis regarding to products distribution and integration values will be discussed in details. Finally, as a basis for relating dehydrogenated of materials containing THF, structures of independently synthesized-MgB12H12 and Mg(B3H8)2-THF adduct were elucidated using PXRD. Unorthodox structure of the latter compound, containing equal quantities of octahedral [Mg(THF)6]2+ and tetrahedral edge-on [Mg(B3H8)4]2- complex ions packed in a FCC lattice, was observed. On the other hand, “traditional” coordination where each Mg2+ binds to one B12H122- and three THF molecules was perceived for MgB12H12(THF)3.

Role of solvent on selective dehydrogenation of magnesium borohydride at moderate conditions

255th National Meeting (2018)

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