Development of an empirical force field CRACK for n-alkanes that allows for classical molecular dynamics simulations investigating the pyrolysis reactions

A novel empirical force field CRACK was developed for n-alkane that allowed for covalent bond breaking and forming, and can be used to investigate the pyrolysis of alkanes by classical molecular dynamics simulation (MD). The shape of CRACK potential-energy curve was for the pyrolysis reactions, with a deep harmonic well, an activation energy barrier and another shallow Lennard-Jones well. To validate CRACK force field, classical MD was performed to simulate the pyrolysis reactions of n-octane and n-decane. The defined pyrolytic temperature Tp could be adjusted by the displacement constant de in accord with the pyrolysis energy barrier presented by CRACK. Tp of n-octane was found higher than that of n-decane at the same de is consistent with the pyrolysis fundamentals. The results of time histories of pyrolysis reactions and of distribution of pyrolysis product revealed that CRACK force field certainly reflected the recombination between different molecules. The microscopic pyrolysis kinetics was discussed and the pyrolysis reaction was found to reach its equilibrium within the simulation time 100 ps. The evaluated rate constant in apparent first order increased with the elevated temperature or the decreased value of de. The distribution of the pyrolysis products was analyzed in details and was compared with the experimental data. The results demonstrate that CRACK force field can be applied to predict the product distribution at least qualitatively by MD.