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

A new ammine dual-cation borohydride, LiMg(BH₄)₃(NH₃)₂, has been successfully synthesized simply by ball-milling of Mg(BH₄)₂ and LiBH₄.NH₃. Structure analysis of the synthesized LiMg(BH₄)₃(NH₃)₂ revealed that it crystallized in the space group P6₃ (no. 173) with lattice parameters of a=b=8.0002(1) Aa, c=8.4276(1) Aa, α=β=90 , and γ=120 at 50 C. A three-dimensional architecture is built up through corner-connecting BH₄ units. Strong N-H..H-B dihydrogen bonds exist between the NH₃ and BH₄ units, enabling LiMg(BH₄)₃(NH₃)₂ to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH₄)₃(NH₃)₂/LiBH₄ composite is able to release over 8 wt % hydrogen below 200 C, which is comparable to that released by Mg(BH₄)₃(NH₃)₂. More importantly, it was found that release of the byproduct NH₃ in this system can be completely suppressed by adjusting the ratio of Mg(BH₄)₂ and LiBH₄.NH₃. This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)