Cyclohexene oxidation with h ₂ o ₂ over metal-organic framework mil-125(Ti): The effect of protons on reactivity

The catalytic performance of the titanium-based metal-organic framework MIL-125 was evaluated in the selective oxidation of cyclohexene (CyH) with environmentally friendly oxidants, H ₂ O ₂ and ^t BuOOH. The catalytic activity of MIL-125 as well as the oxidant utilization efficiency and selectivity toward epoxide and epoxide-derived products can be greatly improved by acid additives (HClO ₄ or CF ₃ SO ₃ H). In the presence of 1 molar equivalent (relative to Ti) of a proton source, the total selectivity toward CyH epoxide and trans-cyclohexane-1,2-diol reached 75-80% at 38-43% alkene conversion after 45 min of reaction with 1 equivalent of 30% H ₂ O ₂ at 50 °C. With 50% H ₂ O ₂ as the oxidant, the total selectivity toward heterolytic oxidation products increased up to 92% at the same level of alkene conversion. N ₂ adsorption, powder X-ray diffraction (PXRD), and infrared (IR) spectroscopy studies before and after the catalytic oxidations confirmed the absence of structural changes in the Metal-organic framework (MOF) structure. MIL-125 was stable toward titanium leaching, behaved as a truly heterogeneous catalyst, and could easily be recovered and reused several times without any loss of the catalytic properties.