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Carbon nanotubes (CNTs) doped with nitrogen were prepared using two methodologies, chemical vapor deposition (N-CNTs) and post-modification (N-CNTs-P), and characterized by N2 adsorption, TEM, XPS, Raman spectroscopy, and water capacity measurements. Their catalytic performance was assessed for aerobic oxidation of syringyl alcohol, a model of syringyl unit of lignin. With N-CNTs, the reaction readily proceeds under mild conditions (40–80 °C, 1 atm of air, isopropanol, ethanol, or water as solvents), leading to syringaldehyde with 95–97% yield. A correlation between N content in N-CNTs and oxidation rate has been established. Spectroscopic studies coupled with water capacity measurements implicated that a blend of graphitic and pyridinic nitrogen species is crucial for catalytic activity. The catalysts can be easily recovered, regenerated, and reused without significant loss of the catalytic performance. N-CNTs-P catalysts, which endow high content of quinone-type oxygen groups, provide significantly lower syringaldehyde yield due to fast deactivation. |
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