Reactive molecular dynamics simulation on thermal decomposition of n-heptane and methylcyclohexane initiated by nitroethane

Abstract

Normal alkanes and cycloalkanes are important components of endothermic hydrocarbon fuels. The present work conducts the reactive molecular dynamics (RMD) simulations based on reactive force field to compare the thermal cracking of n-heptane and methylcyclohexane (MCH) with the same carbon atom number initiated by nitroethane, focusing on initial decomposition temperature, intermediates and product distribution, and the related kinetics. From the RMD simulations, we find nitroethane can accelerate both n-heptane and MCH decompositions, since the C–NO2 bond rupture of nitroethane can proceed at relatively lower temperatures to produce the reactive radicals compared to the C–C bond cleavage of n-heptane and MCH to start the free radical chain reactions, but the promoting effect becomes weaker after it reaches a certain level with temperature. Reasonable Arrhenius parameters estimated from RMD simulations based on first-order kinetic analysis compare well with experimental results. The observed difference of pyrolysis behaviors and kinetic parameters between n-heptane and MCH can be ascribed to their molecular structure, coinciding with experimental observations. In the presence of nitroethane, reductions of initial temperature and activation energy for n-heptane decomposition are larger than those for MCH decomposition. More alkenes are observed in n-heptane decomposition compared to MCH decomposition. Due to the endothermic capacities of fuel being sensitive to the yields of alkenes, more heat could be absorbed by the n-heptane decomposition. These results indicate that RMD simulations can provide important insights at the atomistic level on the difference of decomposition behaviors between normal alkanes and cycloalkanes initiated by nitroethane under supercritical conditions.