Enhanced oxygen transfer over bifunctional Mo-based oxametallacycle catalyst for epoxidation of propylene

Activation of inert propylene to produce propylene oxide (PO) is critical, but still faces some challenges in realizing higher PO selectivity and productivity. Herein, a temperature-controlled phase transfer catalyst (MoOO·DMF) is prepared for the liquid-phase epoxidation of propylene with tert-butyl hydroperoxide (TBHP) as oxidant, which exhibit the selectivity of 90.6% and the productivity of 1286.42·h^-1 for PO (catalyst/propylene = 0.77 mol‰). Some experimental factors (solvent types, reaction temperature, contact time, the dosage of catalyst, TBHP and substrate) were investigated, and the reaction kinetics and thermodynamics are discussed. MoOO·DMF has the characteristic of both homogeneous and heterogeneous catalysts, which can be dissolved in the solvent at higher temperatures and separated from the solvent after reaction by lowering the temperature. Importantly, MoOO·DMF has a wonderful epoxidation performance for many olefins (e.g., light olefins, linear α-olefins, cyclic olefins and others). The mechanisms are proved by in-situ FT-IR, ESR and HRMS spectrum to be the selective oxygen transfer from tert-butyl peroxide radical and the MoOO bridge in MoOO·DMF to propylene. Density functional theory (DFT) calculations show that the MoOO bridge in catalyst is the key role for the activation of both the OH bond in TBHP and the CC bond in propylene, thus enhanced the epoxidation of propylene.