Selective Oxidation of Acetoin to Diacetyl by Superoxo Species on Fe-MIL-88B_NH2 and NHPI under Oxic Conditions

Diacetyl (2,3-butanedione) is an important ingredient used in the food and perfume industries, whose synthesis is largely constrained by environmental pollution and low selectivity. In this study, Fe-MIL-88B_NH2 has been successfully synthesized via the microwave-assisted solvothermal method, which exhibits high reactivity in acetoin oxidation, affording 81.3% conversion and 95.8% selectivity toward diacetyl under mild conditions in the presence of NHPI (N-hydroxyphthalimide). Raman and a series of characterizations reveal that O2 is activated by surface Fe(II) on Fe-MIL-88B_NH2 with the generation of ≡Fe-OO• superoxo, which then abstracts H from acetoin with the formation of diacetyl and H2O2. Quenching experiments indicate that ≡Fe-OO• superoxo on Fe-MIL-88B_NH2 is the primary oxidizing species in the oxidation of acetoin. EPR and DPD analyses further reveal that the NHPI/PINO cycle effectively shuttles H from the acetoin to ≡Fe-OO• superoxo, thus enhancing the oxidation efficiency of acetoin. This study enriches our understanding on the importance of surface superoxo species and the H-shuttle in the selective oxidation of alcohols toward ketones.