Enhanced activity for aerobic oxidative of alcohols over manganese oxides stimulated with interstitial nitrogen doping

Abstract

Heteroatom-doped transition-metal oxides are potential non-noble-metal-based catalysts for the aerobic oxidation of alcohols because of the synergistic effects of their multiple active sites. However, the design of efficient heteroatom-doped transition-metal oxide catalysts is hindered by the complexity of the synergistic effects of multiple active sites. We have developed a practical strategy for fabricating nitrogen-doped MnO2 (N–MnO2), in which nitrogen is selectively doped at interstitial sites in MnO2. Use of N–MnO2 as a catalyst enabled additive-free aerobic oxidation of alcohols in > 99% yield at 30 ​°C. The catalytic activity of commercial MnO2 (< 1%) was negligible. Systematic studies of a series of Mn-based oxide catalysts, namely commercial MnO2, MnNxO2−x (prepared by traditional N doping by treatment with NH3), and N–MnO2 showed that N–MnO2 with abundant interstitial nitrogen dopant (1.8 ​mol%) had the best catalytic activity. Mechanistic studies suggested that the introduction of interstitial nitrogen as a dopant in MnO2 enhanced the adsorption and dissociation of oxygen on the N–MnO2 catalyst and promoted oxidative dehydrogenation of alcohols at interstitial nitrogen and oxygen vacancy sites. In situ infrared spectroscopy showed that C–N bonds (1161 ​cm−1) were formed and broken during benzyl alcohol oxidation. This confirms the importance of interstitial nitrogen in the catalytic cycle. This work provides an alternative strategy based on interstitial nitrogen doping for designing nitrogen-doped transition-metal oxides for aerobic oxidative reactions.

Keywords

• Interstitial nitrogen
• N–MnO2
• Alcohol oxidation