Highly Selective H 2 O 2 ‐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_NH 2 , 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_NH 2 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 H 2 O 2 . With both MIL‐125 and MIL‐125_NH 2 , the selectivity toward the corresponding p ‐benzoquinones was 100 %. Samples of MIL‐125 with different sizes of crystallites (0.5, 1.5, and 5 μm) 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.