In situ thermodiffraction to monitor synthesis and thermolysis of hydrazine borane-based materials

• In situ thermodiffraction was used to study N2H4BH3, NaN2H3BH3 and LiN2H3BH3.
• Thermolytic dehydrogenation of N2H4BH3 starts with the liquid phase.
• Dehydrocoupling of N2H4BH3 involves a crystalline intermediate at 90–197 °C.
• NaN2H3BH3 forms from −38 °C, melts at ∼60 °C, and decomposes at >60 °C.
• The 2 polymorphs of LiN2H3BH3 dehydrogenate according to a complex mechanism.

Hydrazine borane N2H4BH3 (HB) and its alkali metal derivatives, i.e. sodium hydrazinidoborane NaN2H3BH3 (NaHB) and lithium hydrazinidoborane β-LiN2H3BH3 (β-LiHB), are novel materials under investigation for solid-state chemical hydrogen storage. Herein, we report in situ thermodiffraction experiments developed to monitor synthesis of NaHB at low temperatures (from −173 °C) and a thermolysis of each of the boranes, manifesting the following main observations. (i) Dehydrogenation of HB starts with the liquid phase and dehydrocoupling involves the formation of a crystalline intermediate (at 90–197 °C). (ii) NaHB forms from −38 °C, melts at ca. 60 °C, and decomposes upon melting at higher temperatures. (iii) β-LiHB dehydrogenates to a small extent starting from 40 °C, but mostly transforms to the high-temperature phase α-LiHB at 91 °C; up to around 150 °C, the two crystalline phases dehydrogenate according to a complex stepwise mechanism. Our results help in gaining insight in synthesis and thermolysis of the aforementioned novel hydrazine borane-based materials.

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