Selective liquid phase oxidation of benzyl alcohol catalyzed by carbon nanotubes

A HNO3-promoted benzyl alcohol catalytic oxidation system was developed in the presence of carbon nanotubes (CNTs), using molecular oxygen as the terminal oxidant under the mild reaction conditions. The effects of solvent, reaction temperature, amount of HNO3, catalyst loading and surface structure of CNTs on the catalytic performances have been investigated. The CNTs showed excellent catalytic activity exhibiting benzyl alcohol conversion of 96.2% and benzaldehyde selectivity of 88.3% under optimal conditions. In particular, it had remarkable reusability without a significant loss in its activity and selectivity after six consecutive usages. The carbon catalysts were characterized by N-2 adsorption-desorption, FT-IR (Fourier transform infrared spectroscopy), Raman, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) techniques. A possible reaction pathway has been proposed, it is clarified that HNO2 attacks benzyl alcohol to generate benzyl nitrite, which is decomposed to benzaldehyde over the HNO3-promoted CNTs-catalyzed system, and electron transfer in graphene sheets plays an important role in the decomposition of benzyl nitrite. These results not only provide an attractive metal-free alternative to noble-metal-catalyzed systems but also come up with a better understanding of the mechanism of CNTs as a metal-free catalyst for the liquid-phase oxidation of benzyl alcohol. (C) 2012 Elsevier B.V. All rights reserved.