Nitrogen-Doped Graphene Nanosheets as Metal-Free Catalysts for Aerobic Selective Oxidation of Benzylic Alcohols

Abstract

This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a high-temperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene lattice—pyridinic N, pyrrolic N, and graphitic N—the graphitic sp2 N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir–Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ·mol–1 for the benzyl alcohol oxidation) close to that (51.4 kJ·mol–1) proceeding on the catalyst Ru/Al2O3 reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp2 N–O2 adduct transition state, which can oxidize alcohols directly to aldehydes without any byproduct, including H2O2 and carboxylic acids, may be a key element step. Our results advance graphene chemistry and open a window to study the graphitic sp2 nitrogen catalysis.