Discovery of Fluidic LiBH₄ on Scaffold Surfaces and Its Application for Fast Co-confinement of LiBH₄–Ca(BH₄)₂ into Mesopores

Generation of fluidic LiBH₄ molecules, <i>f</i>-LiBH₄, was demonstrated by NMR spectroscopy of LiBH₄ bulk powder mixed with silica scaffold surface materials under minor heat treatment. In the presence of the fumed silica or mesoporous MCM-41 and SBA-15, LiBH₄ shows increased translational mobility at relatively low temperature (ca. 95 °C) and becomes liquid-like by evidence from ¹H–¹¹B <i>J</i>-coupling in ¹H and ¹¹B MAS NMR or substantial line narrowing of ⁷Li static NMR. This high diffusional mobility of LiBH₄ at the molecular level has never been seen for bulk LiBH₄, and the property is attributed to the interfacial interaction with the mesoporous scaffold surfaces. While <i>f</i>-LiBH₄ facilitates the confinement of LiBH₄ itself into various scaffold materials, LiBH₄ migrates along the SBA-15 surface to reach other metal borohydride particles, Ca­(BH₄)₂ in this case, and promotes the formation of similarly fluidic LiBH₄–Ca­(BH₄)₂ composite (LC solid solution) for coconfinement into mesopores. <i>In situ</i> variable temperature (VT) NMR spectroscopy detects the co-infiltration process of eutectic LiBH₄–Ca­(BH₄)₂ composite (LC) into mesopores of SBA-15. The infiltration rates measured for LiBH₄ bulk powder or LC composite showed dependence on pore sizes (MCM-41 vs SBA-15) and heat treatment conditions (static vs MAS)