Synthesis of Halide- and Solvent free metal borohydrides

Metal borohydrides have been extensively investigated over the last few years as potential hydrogen storage materials for mobile applications, due to their high gravimetric and volumetric hydrogen content, e.g. 18.5 wt% hydrogen in LiBH4.[1] Unfortunately the lightweight alkali metal borohydrides have challenges due to their high desorption kinetics and limited reversibility at moderate conditions.[2],[3],[4]
In this work, we present a new approach to synthesize halide- and solvent free metal borohydrides starting from the respective metal hydride. The synthetic strategy ensures that no metal chloride or LiBH4 is present in the sample. The synthesis pathway has been shown to work for most of the already known metal borohydrides, M = Na, Ca, Sr, Ba, Y, La, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, but also new borohydrides are formed, M = Pr, Nd and Lu. Besides new compounds, new polymorphs of the rare-earth metal borohydrides are found, all crystallizing in the α- and β-Y(BH4)3 structure (except for La(BH4)3).
The synthesis pathway start with hydrogenation of the metal. The formed metal hydride is then activated by high energy ball milling to increase reactivity. The next step involves solvent based Schlenk techniques, where the metal hydride and dimethyl sulfide-borane complex (DMS-BH3) is mixed in appropriate ratios, and left to react at 45 °C, typically for a few days, whereby full conversion to the metal borohydride is generally achieved. For the trivalent M(BH4)3, DMS coordinates to the metal. Hence, the powdered M(BH4)3∙DMS is heated to 140 °C for 4 hours to obtain pure M(BH4)3.
The rare-earth metal borohydrides have been investigated by infrared spectroscopy and thermal analysis (TGA-DSC-MS). Furthermore, the structural trends are investigated by synchrotron radiation powder X-ray diffraction. Rehydrogenation properties are investigated for Tb(BH4)3 and Lu(BH4)3 using in-house PCT equipment. The decomposition pathway of the rare-earth metal borohydrides seems similar, but Lu(BH4)3 stands out by decomposing in two steps, in contrast to the one-step decomposition that are observed for the remaining.
References:
[1] H. I. Schlesinger et al. J. Am. Chem. Soc. 62, 3429–3435 (1940), [2] P. Mauron et al. J. Phys. Chem. B 112, 906–910 (2008). [3] P. Martelli et al. J. Phys. Chem. C 114, 7173–7177 (2010). [4] M. Paskevicius et al. Chem. Soc. Rev. 46, 1565–1634 (2017)