Discovery of Fluidic LiBH_4 on Scaffold Surfaces and Its Application for Fast Co-confinement of LiBH_4−Ca(BH_4)_2 into Mesopores
Generation of fluidic LiBH_4 molecules, ƒ-LiBH_4, was demonstrated by NMR spectroscopy of LiBH_4 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, LiBH_4 shows increased translational mobility at relatively low temperature (ca. 95 °C) and becomes liquid-like by evidence from ^1H–^(11)B J-coupling in ^1H and ^(11)B MAS NMR or substantial line narrowing of ^7Li static NMR. This high diffusional mobility of LiBH_4 at the molecular level has never been seen for bulk LiBH_4, and the property is attributed to the interfacial interaction with the mesoporous scaffold surfaces. While ƒ-LiBH_4 facilitates the confinement of LiBH_4 itself into various scaffold materials, LiBH_4 migrates along the SBA-15 surface to reach other metal borohydride particles, Ca(BH_4)_2 in this case, and promotes the formation of similarly fluidic LiBH_4–Ca(BH_4)_2 composite (LC solid solution) for coconfinement into mesopores. In situ variable temperature (VT) NMR spectroscopy detects the co-infiltration process of eutectic LiBH_4–Ca(BH_4)_2 composite (LC) into mesopores of SBA-15. The infiltration rates measured for LiBH_4 bulk powder or LC composite showed dependence on pore sizes (MCM-41 vs SBA-15) and heat treatment conditions (static vs MAS).
© 2015 American Chemical Society. Received: January 5, 2015. Revised: April 3, 2015. Published: April 6, 2015. The authors gratefully acknowledge the support by Seung-Ju Lee, Se-Bi Jung, and Kiho Park for the synthesis and characterization of the SBA-15. This work has been sponsored by the Korea Research Council of Fundamental Science and Technology, and the HyFill-Fast project. The NMR facility at the California Institute of Technology was supported by the National Science Foundation (NSF) under Grant Number 9724240 and partially supported by the MRSEC Program of the NSF under Award Number DMR-520565.
Supplemental Material - jp5b00111_si_001.pdf