Epoxidation of Propylene by Hydrogen Peroxide Catalyzed by the
Silanol-Functionalized Polyoxometalates-Supported Ferrate: Electronic
Structure, Bonding Feature, and Reaction Mechanism
Abstract
Hydrogen peroxide (H2O2), as clean oxidant, has long suffered from low
efficiency and selectivity for the oxidation of olefins. In the present
paper, the redox important ferrate anion (FeO42-) has been anchored into
a silanol-decorated polyoxometalates (POM) to form single–site
supported Fe-POM catalyst. And possible reaction mechanism for the
epoxidation of propylene with hydrogen peroxide (H2O2) catalyzed by the
Fe-POM catalyst have been investigated based on density functional
theory with 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 the Fe
center, resulting in the replenishing of 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
acts as the role of electron donor in the rest reaction steps to
eliminate the radical feature, reduce the reactivity, and stop the
reaction at the stage of the desired epoxypropane product.