Mechanistic aspects of propene epoxidation by hydrogen peroxide. Catalytic role of water molecules, external electric field, and zeolite framework of TS-1

We have assessed various aspects of the epoxidation of propene by hydrogen peroxide, a reaction of considerable industrial importance, and elucidated some of the important factors that govern its mechanism. Quantum chemical calculations on the reactants, products, and transition states were performed both in the gas phase and using models to represent the TS-1 (titanosilicalite-1) catalyst. The reaction energy for the uncatalyzed process is computed as -52.6 kcal/mol with a barrier of 35.2 kcal/mol in the gas phase using the B3LYP hybrid density functional and a 6-31+G(d, p) basis set. The reaction appears to occur via a concerted mechanism. The competing reaction of ionic addition of hydrogen peroxide to the double bond to form a hydroperoxopropane is computed to have a reaction energy of only -17.1 kcal/mol with a barrier of 34.8 kcal/mol and is therefore expected not to be thermodynamically preferable. Introduction of water molecules to the model is calculated to reduce the reaction barrier to 25.7 kcal/mol in the case of a single molecule but did not significantly affect the reaction energy. The competing addition reaction barrier appears to be significantly less sensitive to the presence of water molecules, suggesting that the concerted epoxidation reaction is also kinetically favored in the polar environment. Introduction of additional water molecules does not result in a noticeable enhancement. The water molecules appear to mediate proton transfer between the peroxide oxygens in the rate determining step of the concerted epoxidation reaction. The introduction of a background solvent field was also found to reduce the activation energy. For example, a model with a single explicit water molecule and the solvent field gives an activation barrier of 16.9 kcal/mol. A similar effect is observed if an external electric field is applied to the model with the dipole component directed along the 0-0 bond direction. Calculations were also performed on the same reaction occurring in the vicinity of a model for the active site of the TS-1 catalyst using a cluster model. The activation barrier for the cluster model is calculated to be 25.8 kcal/mol with a reaction energy of -55.5 kcal/mol, which is comparable to the gas phase model with a single water molecule added. No significant changes are observed with the addition of water molecules in this model.