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

Abstract

Developing innovative catalysts for efficiently activating O₂ into singlet oxygen (¹O₂) 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-MnO₂, 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 ¹O₂ 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 N₅₅-MnO₂ nanocatalyst. This feature allows the nanocatalyst to expose a substantial number of O_v and interstitial N sites on the surface of N₅₅-MnO₂, facilitating effective chemisorption and activation of O₂. Verified through electron paramagnetic resonance spectroscopy and reactive oxygen species trapping experiments, the spontaneous generation of ¹O₂, 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 O₂. Consequently, the optimized N₅₅-MnO₂ nanocatalyst enables room-temperature aerobic oxidation of alcohols with a yield surpassing 99%, representing a 6.7-fold activity enhancement compared to ε-MnO₂ without N-doping. Furthermore, N₅₅-MnO₂ demonstrates exceptional recyclability for the aerobic oxidative conversion of benzyl alcohol over ten cycles. This study introduces an approach to spontaneously activate O₂ for the green synthesis of fine chemicals.