Thermal decomposition mechanism of some hydrocarbons by ReaxFF-based molecular dynamics and density functional theory study

In order to investigate the decomposition mechanism of hydrocarbons, pyrolysis processes of 11 typical hydrocarbons (isobutane, isopentane, isohexane, n-butane, n-pentane, n-hexane, cyclobutane, cyclopentane, cyclohexane, benzene and toluene) are performed by using ReaxFF MD and DFT method. The results show that the initial pyrolysis reactions of these hydrocarbons can be divided into two types: homolytic cleavage of C-H bond and C-C bond. The bond dissociation energies of C-H bonds are higher than that of C-C bonds in these hydrocarbons except for toluene. The thermal decomposition rates of branched-chain hydrocarbons are faster than that of straight-chain hydrocarbons. The thermal decomposition rates of chain hydrocarbons gradually increase with the increases of C atom number. The main product molecules of hydrocarbon pyrolysis are H-2, CH4, C2H2 and C2H4. The apparent activation energies of 4 hydrocarbons (n-pentane, isohexane, neopentane and cyclopentane) pyrolysis are calculated by the kinetic analysis. In further reactions, CH3, C2H5 and H radicals are collided with hydrocarbons to undergo H-abstraction reactions. The energy barriers of H-abstraction reactions are calculated by DFT.