Confined Mn2+ enables effective aerobic oxidation catalysis

Effective and mild activation of O-2 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 O-2 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 O-2 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.