M. Waroquier

The reaction kinetics of two radical reactions that are important for coke formation during the thermal cracking of hydrocarbons is studied by transition-state theory. It is investigated how coupled internal rotations influence the partition functions of molecules with several torsional motions and the reaction kinetics involving such molecules. This is done by applying a general scheme, which is able to treat various rotating tops without restrictions on the symmetry of the rotating parts.

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.

Thermal elimination reactions on polycarbonates are investigated from both theoretical and experimental points of view, to obtain insight into the microscopic aspects that influence the reaction mechanism and rates. In particular, attention is focused on the influence of the type of substituents in the polymer chain on the reaction rates. Ab initio density functional theory calculations are performed on a series of model compound systems for the polycarbonates under study, in particular carbonates differing by the groups attached at the α and β carbon atoms. Reactants, products, and transition states are optimized at the B3LYP/6-311g** level of theory. The structures of the activated complex give insight into the mechanistic details of this type of Ei elimination reactions. The Cα−O bond dissociates before the Cβ−H bond, developing some carbocation character in the transition state on the Cα atom. The kinematics of the thermal decomposition reactions have been studied by means of transition state theory by construction of the microscopic partition functions. It turns out that the rates of the Ei elimination reactions are increased by the presence of those substituents on the Cα and Cβ carbon atoms which are stabilizing the carbocation character in the transition state. In a second part, degradation temperatures have been experimentally measured for some polycarbonates through thermogravimetric analysis. It is investigated whether the relative rates of the model compound carbonate systems are representative of the behavior of the thermal degradation temperatures in polycarbonates. The study as presented here proves that ab initio calculations on small model systems, which are representative for the active area of the degradation process in polycarbonates, can provide insight into the principal ingredients that govern the reaction rates.