Boosting H2O2 Activation for the Efficient Alkene Epoxidation over Polyoxometalate-based Tetrakaidecahedron-like Nanodice Assemblies

Abstract

The activation of hydrogen peroxide (H₂O₂) as a terminal oxidant holds great promise for catalytic oxidation processes. However, developing an efficient catalyst for the epoxidation of bulky organic molecules with H₂O₂, including cyclic alkenes, remains challenging. In this research, a lacunary polyoxometalate (POM)-based tetrakaidecahedron nanodices (NDs) are synthesized by [PW₁₁O₃₉]⁷⁻ (PW₁₁) clusters with tetrabutylammonium bromide (TBAB). The NDs exhibit significantly enhanced catalytic activity in the epoxidation of cyclooctene, with a yield 55.4 times greater than that of the raw PW₁₁. Conversely, nanospheres (NSs) assembled from non-lacunary POM clusters of [PW₁₂O₄₀]³⁻ (PW₁₂) under the same conditions achieve only a 2.9 % yield, highlighting the essential role of the lacunary PW₁₁ structure in NDs. Raman spectroscopy and density functional theory (DFT) calculations confirm that the active hydroperoxo species from H₂O₂ activation serves as the effective epoxidizing agent. The reduced energy barrier for H₂O₂ activation on PW₁₁, relative to PW₁₂, corroborates the superior activity of the NDs. Furthermore, the intermediate generated post-H₂O₂ activation shows stronger adsorption of cyclooctene, increasing oxygen transfer to cyclooctene during the epoxidation process. This study demonstrates the exceptional performance of an assembly structure utilizing lacunary POMs for the activation of H₂O₂ and subsequent oxidation reactions.