Nitrogen-Doped Graphene
Nanosheets as Metal-Free Catalysts
for Aerobic Selective Oxidation of Benzylic Alcohols
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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 latticepyridinic
N, pyrrolic N, and graphitic Nthe graphitic sp<sup>2</sup> 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<sup>–1</sup> for the benzyl alcohol oxidation) close to that (51.4
kJ·mol<sup>–1</sup>) proceeding on the catalyst Ru/Al<sub>2</sub>O<sub>3</sub> 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 sp<sup>2</sup> N–O<sub>2</sub> adduct transition state, which can oxidize alcohols directly
to aldehydes without any byproduct, including H<sub>2</sub>O<sub>2</sub> and carboxylic acids, may be a key element step. Our results advance
graphene chemistry and open a window to study the graphitic sp<sup>2</sup> nitrogen catalysis