Partial Oxidation of Propylene to Propylene Oxide over a Neutral Gold Trimer in the Gas Phase:  A Density Functional Theory Study

We report a B3LYP study of a novel mechanism for propylene epoxidation using H2 and O2 on a neutral Au3 cluster, including full thermodynamics and pre-exponential factors. A side-on O2 adsorption on Au3 is followed by dissociative addition of H2 across one of the AuO bonds (ΔE act = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au3 cluster. The more electrophilic O atom (proximal to the Au) of the AuOOH group attacks the CC of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au3 cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (ΔE act = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (ΔE des = 9.9 kcal/mol), the Au3 is regenerated (closure). In the second subpathway, H2 is added across the AuOH bond to form water and another AuH bond (ΔE act = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au3 dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au3 intermediate with two equivalent AuH bonds, addition of O2 into the AuH bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the ΔG act, which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: ΔE act = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.