Ab initio density functional theory calculations are presented on some model reactions involved in coke formation during the thermal cracking of hydrocarbons. The reactions under consideration are different cyclization pathways for the butylbenzene radical, which can lead to a further growth of the coke layer. This study enables us to gain more microscopic insight into the mechanistic and kinetic aspects of the reactions. Special attention is paid to the exact treatment of internal rotations and their impact on the kinetic parameters. Pre-exponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. In particular, the relative importance of cyclization toward five and six-membered rings is studied on the basis of the calculated rate constants and concentration profiles of the reactants. The influence of the size of the ring and of the relative stability of the primary and secondary butylbenzene radical on the cyclization reaction is discussed. The activation energy for the formation of six-membered rings is approximately 30 kJ/mol lower than that for five-ring formation. The predicted values for the kinetic parameters enable us to validate some basic assumptions on coke formation. The calculations as presented here are especially important for complex reaction schemes, for which experimental data are not always available.
Abstract