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 H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) 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 Au(3) 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 (DeltaE(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 (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) 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 Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(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: DeltaE(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.