As some of the most interesting metal-free catalysts, carbon nanotubes (CNTs) and other carbon-based nanomaterials show great promise for some important chemical reactions, such as the selective oxidation of cyclohexane (C6H12). Due to the lack of fundamental understanding of carbon catalysis in liquid-phase reactions, we have sought to unravel the role of CNTs in the catalytic oxidation of C6H12 through a combination of kinetic analysis, in situ spectroscopy, and density functional theory. The catalytic effect of CNTs originates from a weak interaction between radicals and their graphene skeletons, which confines the radicals around their surfaces. This, in turn, enhances the electron-transfer catalysis of peroxides to yield the corresponding alcohol and ketone.
Inside job: New applications of carbon materials pave the way towards greener chemical syntheses. The encapsulation of metallic Fe within CNTs improves electron transfer between the metal and the CNTs. The resulting material offers a high catalytic activity and easy magnetic separation of catalyst in the heterogeneous selective oxidation of cyclohexane.
Highlights▪ A simple in-situ oxidation approach was developed to prepare MnO2/NCNT. ▪ Prepared samples showed boosted activity for HCHO degradation at low temperature.▪ MnO2/NCNT exhibited better activity and selectivity than MnO2/CNT. ▪ Mechanism of improved HCHO oxidation by MnO2/NCNT was revealed.
Fe-, Ni-, and alloyed FeNi-filled carbon nanotubes (Fe@CNT, Ni@CNT, and FeNi@CNT) were prepared by a general strategy using a mixture of xylene and dichlorobenzene as carbon source, and ferrocene, nickelocene, and their mixture as catalysts. By tailoring the composition of the carbon precursor, the filling ratio and the wall thickness of metal@CNT could be controlled. For the catalytic oxidation of cyclohexane in liquid phase with molecular oxygen as oxidant, the highest activity was obtained over Fe@CNT synthesized from pure dichlorobenzene. However, Ni filling did not improve the activity of CNTs. The effects of metal filling, wall thickness, and defects on catalytic activity were investigated to determine the structure–activity relationship of the filled CNTs. The enhanced catalytic performance can be attributed to a combined contribution of thin walls of CNTs and confined electron-donating metals, which are favourable to electron transfer on the surfaces of CNTs. The modification of the electronic structure of CNTs upon Fe and Ni fillers insertion was elucidated through density functional theory calculations.
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