Thermal Decomposition Mechanism of Aqueous Hydroxylammonium Nitrate (HAN): Molecular Simulation and Kinetic Modeling

A detailed
mechanism has been developed for thermal decomposition
of hydroxylammonium nitrate (HAN) solutions, based on quantum mechanical
calculations using the SMD-ωB97X-D method. The mechanism describes
multiple kinetic processes, including nitration and nitrosation of
hydroxylamine, HNO dimerization, and HONO-regeneration pathways involving
H-abstraction reactions. Rate constants of elementary reactions were
estimated using transition state theory with consideration of species’
diffusion effect. Kinetic modeling was performed to predict species’
evolutions in 0.1 m HAN in the temperature range of 463–523
K, and results show reasonable agreement with the experimental data
from flow reactor studies. For more concentrated solutions, strong
autocatalytic behaviors were predicted with the late emergence of
NO2 and HONO, whose regeneration was previously considered
as the major autocatalytic pathway. Sensitivity analysis results suggest
an acid-catalyzed nitration–nitrosation pathway, based on which
the autocatalysis should be caused by the rise of solution acidity.
A linear correlation can be observed in the previously reported apparent
Arrhenius parameters, which may be reconciled via a kinetic compensation
effect.