Ab Inito Chemical Kinetics Modeling of Liquid-phase Reactions of Monomethylhydrazine and Nitrogen Tetroxide

Monomethylhydrazine
(MMH) and nitrogen tetroxide (NTO)
have been
utilized as hypergolic bipropellants in spacecraft engines for many
years, but the liquid-phase MMH/NTO reaction mechanism remains underexplored.
This theoretical study investigates the liquid-phase reactions between
MMH and NTO. By employing an implicit solvent model and conducting
high-accuracy quantum chemical calculations at the CCSD(T)/CBS//M06–2X/6–311++G(d,p)/SMD theoretical level, we obtained
the reaction paths for the isomerization reactions of NTO, for the
reactions between MMH and t-ONONO2, and
for the reactions between MMH and NO2. The corresponding
reaction pathways in the gas phase were also computed for comparison
at the CCSD(T)/CBS//M06–2X/6–311++G(d,p)
theoretical level. The results indicate that the energy barriers for
the liquid-phase reactions are lower than their gas-phase counterparts
due to the solvent effects. The rate constants were calculated using
the transition state theory. We established a kinetic model and applied
it to simulate the early stage of the liquid-phase MMH/NTO reactions
under premixed and adiabatic conditions. The results indicate that
the required time for temperature rising from the initial temperature
(−13 °C) to the boiling point temperature of MMH (87.5
°C) is approximately 0.05 μs, consistent with previous
experimental observations in drop tests. In addition, the kinetic
analysis revealed that the reaction steps N2O4 → t-ONONO2 and CH3NHNH2 + t-ONONO2 →
CH3NH2 + N2O + HNO3 play
critical roles in triggering the subsequent reactions, thereby dictating
the rate of temperature rise. This work not only provides a new kinetic
model for liquid-phase MMH/NTO reactions but also clarifies the role
of liquid-phase reactions in the hypergolic ignition of MMH/NTO.