Density functional theory investigation into the epoxidation of propylene over TS-1 catalysts

This density functional theory (DFT) study of the epoxidation of propylene over Au/TS-1 catalysts using H2 and O2 to produce propylene oxide has identified and detailed a viable two step mechanism for the reaction. Initial work on gas-phase gold clusters has established the ability of DFT to account for the unique chemical behavior of Au clusters. Previously unreported geometries for Au9− and Au10 − anions are obtained with several binding sites for O 2 of different energies identified on Au10− . A reaction path is given for formation of hydrogen peroxide over a neutral Au3 cluster with activation barriers less than 10 kcal/mol. The reactions proceed on the edges and one side of the triangular Au 3 cluster which makes this mechanism viable for a cluster in contact with a support surface. Formation of hydrogen peroxide does not involve breaking the O-O bond, but does break the H-H bond in a step that is rate limiting at standard conditions. The highest energy barrier in the cycle is 8.6 kcal/mol for desorption of H2O2 from Au3H2. Adsorption of H2O2 on this site is unactivated. The other reaction step, H2O2 epoxidation of propylene, has been examined on model titanosilicalite (TS-1) Ti centers to explore how microstructural aspects of Ti-sites effect propylene epoxidation reactivity, and shows that Ti sites located adjacent to Si-vacancies in the TS-1 lattice are more reactive than fully coordinated Ti sites, which do not react at all. Propylene epoxidation near a Si-vacancy occurs through a sequential pathway where H2O2 first forms a hydroperoxy intermediate Ti-OOH (15.4 kcal/mole activation energy) and then reacts with propylene by proximal oxygen abstraction (9.3 kcal/mole activation energy). The abstraction step is greatly facilitated through a simultaneous hydride transfer involving neighboring terminal silanol groups arising from the Si vacancy. These results also show that the reactive hydroperoxy intermediates are generally characterized by smaller electron populations on the proximal oxygen atom compared to non-reactive intermediates. Together these mechanisms give a fully plausible, energetically favorable, closed cycle for epoxidation of propylene by H2 and O2 over Au/TS-1 catalysts