Isothermal Dehydrogenation of Ammonia Borane: Insights into BNH Polymers and Challenges in Regeneration

In this study, the BNH polymers produced by ammonia borane (AB) thermolysis under isothermal conditions were investigated. Polyaminoborane (PAB) and diammoniate of diborane (DADB) form upon releasing the first equivalent of H₂ at 85 °C, followed by the formation of cross-linked polyborazylene (PB) at 140 °C. Polyiminoborane (PIB) was not detected under these conditions. The characterization of these BNH polymers, relied on solid-state techniques including IR, Raman, XPS, and ¹¹B MAS NMR. These methods revealed the chemical diversity and structural complexity of PAB and PB, highlighting the presence of different boron environments. The stability of the BNH polymers was also investigated over time and under different atmospheres. Over six months, both PAB and PB exhibited increased polymerization, and PAB showed an interesting ability to adsorb carbon dioxide. Efforts to regenerate AB from PAB and PB through hydrogenation and ammonia-based methods were conducted. The experiments showed that the BNH polymers break into smaller molecules, showing partial rehydrogenation of -NH and -BH groups in some cases, but with limited efficiency. Higher temperatures and hydrogen pressure modified decomposition pathways, though complete regeneration remains challenging. This study offers new insights into the chemical structure of BNH polymers and their potential use as hydrogen storage materials.