A new ammine dual-cation (Li, Mg) borohydride: Synthesis, structure, and dehydrogenation enhancement

摘要

A new ammine dual-cation borohydride, LiMg(BH _4) _3(NH _3) _2, has been successfully synthesized simply by ball-milling of Mg(BH _4) _2 and LiBH _4·NH _3. Structure analysis of the synthesized LiMg(BH _4) _3(NH _3) _2 revealed that it crystallized in the space group P6 _3 (no. 173) with lattice parameters of a=b=8.0002(1) ?, c=8.4276(1) ?, α=β=90°, and γ=120° at 50 °C. A three-dimensional architecture is built up through corner-connecting BH _4 units. Strong N-H?H-B dihydrogen bonds exist between the NH _3 and BH _4 units, enabling LiMg(BH _4) _3(NH _3) _2 to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH _4) _3(NH _3) _2/LiBH _4 composite is able to release over 8 wt % hydrogen below 200 °C, which is comparable to that released by Mg(BH _4) _3(NH 3) _2. More importantly, it was found that release of the byproduct NH _3 in this system can be completely suppressed by adjusting the ratio of Mg(BH _4) _2 and LiBH _4·NH _3. This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems.