Relay Catalysis for Epoxidation of Cyclohexene by Mg–Mo Bimetallic Oxides and O₂ under Solvent-Free Conditions

Selective oxidation of cyclohexene (Cy) to cyclohexene oxide (Cy-ep) by O2 under solvent- and initiator-free conditions is a challenge in chemical processes, which is still constrained by its low selectivity (theoretical selectivity of 50.0%). Given the low epoxide selectivity (<20.0%) of single metal catalysts, we propose a relay catalysis strategy on the oxidation of Cy to Cy-ep with high selectivity by the spherical Mg–Mo bimetallic oxide (MMO). Such a relay catalyst exhibits a desirable Cy conversion of 31.0% and a high selectivity of 49.3% toward Cy-ep. Meanwhile, the high stability and reusability of MMO also indicate its potential to be used for industrial scale-ups. A series of characterizations and DFT calculations reveal that the negatively charged lattice oxygen in [MgII–O–II] adsorbs allylic H of Cy to weaken the bond energy of the allylic C–H bond and form Cy•, and then Cy• rapidly traps O2 to give CyOO•, which subsequently abstracts H from allylic C–H of another Cy to form 2-cyclohexene-1-hydroperoxide (Cy-OOH) and Cy•, achieving the propagation cycle. On the other hand, the Mo(VI) site in MMO catalyzes the produced Cy-OOH peroxo, which then oxidizes Cy to Cy-ep and 2-cyclohexene-1-ol. This work provides insight into the relay catalyst for the epoxidation of olefin with O2 with high selectivity under solvent- and initiator-free conditions.