Spontaneous generation of singlet oxygen on microemulsion-derived manganese oxides with rich oxygen vacancies for efficient aerobic oxidation

Developing innovative catalysts for efficiently activating O(2) into singlet oxygen ((1)O(2)) is a cutting-edge field with the potential to revolutionize green chemical synthesis. Despite its potential, practical implementation remains a significant challenge. In this study, we design a series of nitrogen (N)-doped manganese oxides (N(y)-MnO(2), where y represents the molar amount of the N precursor used) nanocatalysts using compartmentalized-microemulsion crystallization followed by post-calcination. These nanocatalysts demonstrate the remarkable ability to directly produce (1)O(2) at room temperature without the external fields. By strategically incorporating defect engineering and interstitial N, the concentration of surface oxygen atoms (O(s)) in the vicinity of oxygen vacancy (O(v)) reaches 51.1% for the N(55)-MnO(2) nanocatalyst. This feature allows the nanocatalyst to expose a substantial number of O(v) and interstitial N sites on the surface of N(55)-MnO(2), facilitating effective chemisorption and activation of O(2). Verified through electron paramagnetic resonance spectroscopy and reactive oxygen species trapping experiments, the spontaneous generation of (1)O(2), even in the absence of light, underscores its crucial role in aerobic oxidation. Density functional theory calculations reveal that an increased O(v) content and N doping significantly reduce the adsorption energy, thereby promoting chemisorption and excitation of O(2). Consequently, the optimized N(55)-MnO(2) nanocatalyst enables room-temperature aerobic oxidation of alcohols with a yield surpassing 99%, representing a 6.7-fold activity enhancement compared to ε-MnO(2) without N-doping. Furthermore, N(55)-MnO(2) demonstrates exceptional recyclability for the aerobic oxidative conversion of benzyl alcohol over ten cycles. This study introduces an approach to spontaneously activate O(2) for the green synthesis of fine chemicals.