Highly Selective H2O2-Based Oxidation of Alkylphenols to p-Benzoquinones Over MIL-125 Metal-Organic Frameworks

The titanium-based metal-organic framework MIL-125 and its amine-functionalized analog, MIL-125_NH2, have been synthesized and characterized by elemental analysis, XRD, SEM, TEM, N-2 adsorption measurements, and spectroscopic techniques, including FTIR, Raman, and DR UV/Vis spectroscopy. Catalytic properties of MIL-125 and MIL-125_NH2 were evaluated in the selective oxidation of two representative alkyl-substituted phenols, 2,3,6-trimethylphenol and 2,6-di-tert-butylphenol, with the clean oxidant H2O2. With both MIL-125 and MIL-125_NH2, the selectivity toward the corresponding p-benzoquinones was 100%. Samples of MIL-125 with different sizes of crystallites (0.5, 1.5, and 5 m) demonstrated similar reaction rates, thus indicating the absence of diffusion limitations. The efficiency of the oxidant utilization and stability of the MIL-125 structure increased upon decreasing the amount of water in the reaction mixture. Even if the structural integrity of MIL-125 was destroyed by the reaction medium, the metal-organic framework acted as a precursor for the highly active, selective, and recyclable catalyst. The MIL-125-derived materials were stable toward titanium leaching, behaved as true heterogeneous catalysts, and could easily be recovered by filtration and reused several times without the loss of the catalytic properties.