Confined Mn2+ enables effective aerobic oxidation catalysis

Effective and mild activation of O2 is essential but challenging for aerobic oxidation. In heterogeneous catalysis, high-valence manganese oxide (e.g., +4) is known to be active for the oxidation, whereas divalent MnO is ineffective due to its limited capacity to supply surface oxygen and its thermodynamically unstable structure when binding O2 in reaction conditions. Inspired by natural enzymes that rely on divalent Mn2+, we discovered that confining Mn2+ onto the Mn2O3 surface through a dedicated calcination process creates highly active catalysts for the aerobic oxidation of 5-hydroxymethylfurfural, benzyl alcohol, and CO. The Mn2O3-confined Mn2+ is undercoordinated and efficiently mediates O2 activation, resulting in 2–3 orders of magnitude higher activity than Mn2O3 alone. Through low-temperature infrared spectroscopy, we distinguished low-content Mn2+ sites at Mn2O3 surface, which are difficult to be differentiated by X-ray photoelectron spectroscopy. The combination of in-situ energy-dispersive X-ray absorption spectroscopy and X-ray diffraction further provides insights into the formation of the newly identified active Mn2+ sites. By optimizing the calcination step, we were able to increase the catalytic activity threefold further. The finding offers promising frontiers for exploring active oxidation catalysts by utilizing the confinement of Mn2+ and often-ignored calcination skills.