Mechanistic Investigation into Olefin Epoxidation with H₂O₂ Catalyzed by Aqua‐Coordinated Sandwich‐Type Polyoxometalates: Role of the Noble Metal and Active Oxygen Position

Abstract

Aqua‐coordinated sandwich‐type polyoxometalates (POMs), {[WZnTM2(H2O)2](ZnW9O34)2}n− (TM=RhIII, PdII, and PtII), catalyze olefin epoxidation with hydrogen peroxide and have been well established, and they present an advance toward the utilization of olefins. To elucidate the epoxidation mechanism, we systematically performed density functional calculations. The reaction proceeds through a two‐step mechanism: activation of H2O2 and oxygen transfer. The aqua‐coordinated complexes show two distinct H2O2 activation pathways: “two‐step” and “concerted”. The concerted processes are more facile and proceed with similar and rate‐determining energy barriers at the Rh‐, Pd‐, and Pt‐containing transition states, which agrees well with the experimental results. Next, the resulting TM−OH−(μ‐OOH) intermediate transfers an O atom to olefin to form an epoxide. The higher reactivity of the Rh‐containing POM is attributed to more interactions between the Rh and hydroperoxo unit. We also calculated all active oxygen positions to locate the most favorable pathway. The higher reactivity of the two‐metal‐bonded oxygen position is predominantly ascribed to its lower stereoscopic hindrance. Furthermore, the presence of one and two explicit water solvent molecules significantly reduces the energy barriers, making these sandwich POMs very efficient for the olefin epoxidation with H2O2.

Solvothermal synthesis of Co-substituted phosphomolybdate acid encapsulated in the UiO-66 framework for catalytic application in olefin epoxidation

“…Introducing a small amount of transition metals might adjust the electron density of the neighboring Mo species forming TM-O-Mo bonds, thus activating the olefin epoxidation activity [34]. Moreover, recent literature work that combined a variety of characterization results with DFT calculations revealed that substituted transition metals such as Ti or Nb can act as a new active site to directly activate oxidants like t-BuOOH and H2O2 [56,60,61]. In our case, the electron density of Mo atoms in POM decreased slightly after introducing a small amount of Co, as suggested by the XPS results.…”

Pathways towards true catalysts: computational modelling and structural transformations of Zn-polyoxotungstates

“…Interestingly activation, so that they are rather unlikely to participate in alcohol oxidation in the present case. 87 From a dynamic perspective, the complex spatial distribution of inequivalent POM units and the defective half-occupancy of Zn atoms provide energetic and structural flexibility to the de novo created POM architecture while preserving the bonding around the active W/Zn catalytic centres responsible for the activation of hydroperoxo and peroxo-tungstate species. This could also explain the catalytic stability observed in the {ZnW 9 O 34 } Keggin anion under catalytic cyclohexanol oxidation.…”