Development of a reaction mechanism for liquid-phase decomposition of ammonia borane

Using quantum mechanics (QM) calculations, a detailed reaction mechanism for liquid-phase decomposition of ammonia borane (AB) is developed. The primary objective of this study is to provide comprehensive understanding regarding the chemical kinetics involved in H₂ release process during AB dehydrogenation, to explore the pathways of oligomerization leading to ring formation during the second phase of AB decomposition and to explain the experimental observations using the proposed mechanism. Along with QM studies, we have also performed TGA/DSC studies coupled with Fourier transform infrared (FTIR) spectroscopy. In previous experimental studies, there is no consensus regarding release of NH₃ in gaseous phase during thermal decomposition of AB. Experiments performed in this study using FTIR spectroscopy confirm the presence of NH₃ in gaseous phase during both steps of AB decomposition. Furthermore, we have also studied the effect of different heating rates on the trend of NH₃ evolution during AB decomposition, which has not been done previously. To identify reaction pathways based on QM studies, transition state theory (TST) is used in this study. Using G4(MP2) compound method, thermodynamic properties are calculated for both equilibrium species and transition states using the Gaussian 09 program package. For these condensed-phase calculations, Solvation Model based on Density (SMD) is used with acetonitrile as the solvent. Sensitivity analysis is performed to identify important reaction pathways for the formation of gaseous species (H₂, NH₃, BH₂NH₂ and borazine) observed during experiments. Using a control-volume analysis, a numerical simulation is also performed. A comparison of simulated results with experimental data shows that the proposed mechanism captures all the key features of AB decomposition.