Epoxidation of propylene by hydrogen peroxide catalyzed by the silanol‐functionalized polyoxometalates‐supported ferrate: Electronic structure, bonding feature, and reaction mechanism

Abstract

AbstractHydrogen peroxide (H2O2), as clean oxidant, has been disregarded due to its low efficiency and selectivity for the oxidation of olefins. In the present paper, the redox‐important ferrate anion (FeO42−) has been anchored to a silanol‐decorated polyoxometalates (POM) to form a single site‐supported Fe‐POM catalyst. Possible reaction mechanisms for the epoxidation of propylene with H2O2 catalyzed by the Fe‐POM catalyst have been investigated based on density functional theory with the M06L functional. The study of molecular geometry, electronic structure, and bonding feature shows that the Fe‐POM complex can be viewed as a high‐valent Fe‐oxo (FeO) species. The propylene molecule was activated by the Fe‐POM catalyst via an effective electron transfer from propylene to the Fe‐POM catalyst to form a cation propylene radical. Due to the high reactivity of radical species, the calculated activation energy barrier is only 4.50 kcal mol−1 for epoxidation of propylene to epoxypropane catalyzed by the Fe‐POM catalyst. Subsequently, the calculated free energy profiles show that H2O2 was decomposed into a H2O molecule and a surface O species over the Fe‐POM catalyst, and the remaining O atom attaches to the exposed Fe center, resulting in the replenishing of the Fe‐POM catalyst via a two‐state reaction pathway. The calculated activation energy barrier for this process is 23.42 kcal mol−1, and thus, decomposition of H2O2 is the rate‐determining step for the whole reaction. The Fe center serves as an electron acceptor, accepting electrons from the binding propylene molecule to form radical species in the first half of the reaction and, in the role of electron donor in the remaining reaction steps to eliminate the radical feature, reduces the reactivity and stops the reaction at the stage of the desired epoxypropane product.