Ternary amide-hydride system: A study on LiAl(NH2)4-LiAlH4 interaction

Abstract

LiAl(NH2)4 is a ternary amide that readily decomposes to release ammonia at temperatures as low as ∼ 90 °C. Owing to such instability as compared to binary amides, we hypothesize that the dehydrogenation mechanism involving ternary amide-hydride interaction would be significantly different from those of binary metal amide-hydride interaction. Therefore, in this study, interaction of LiAl(NH2)4 and LiAlH4 has been investigated by means of mechanical milling and thermal method. It was found that dehydrogenation occurred spontaneously during the milling process and the rate of dehydrogenation increased with increasing amount of LiAlH4, suggesting an ion migration mediated dehydrogenation. As reaction progressed, the formation of Li3AlH6 as an intermediate was detected and a total of 8 equiv. of H2 (7.5 wt%) can be released, forming LiH and AlN as the final product. In contrast, heating the homogenously ground LiAl(NH2)4 and LiAlH4 sample resulted in the release of NH3 at low temperatures, indicating that NH3 mediation would take place in case of dehydrogenation. Further increase in temperature resulted in a rapid release of hydrogen from the interaction of the LiAl(NH)2 and LiAlH4. It was also found that hydride with higher basicity is required to trigger amide-hydride interaction for dehydrogenation at low temperatures.