Mechanistic Insight Into Catalytic Oxidation Of Cyclohexane Over Carbon Nanotubes

Nano-carbons, which have unique electronic structure and abundant surface functional groups, may replace the traditional metal catalyst in dehydrogenation, oxidation and electrochemical reactions. Catalysis by nano-carbons has become one of the latest frontiers in modem catalysis study. The selective oxidation of cyclohexane (C6H12) is vitally important in chemical industry, but the catalytic oxidation process is extremely complicated. It is of general interest for academic and industrial communities to develop environment-benign methods that can selectively catalyze the oxidation of C6H12with molecular oxygen under mild conditions. In this work, carbon nanotubes (CNTs) were studied in the selective catalytic oxidation of C6H12, which produces cyclohexanol, cyclohexanone and adipic acid. We proposed a strategy to control the interaction bewteen CNTs surface and C6H12through the filled magnetic metal nanowires. The concern was focused on the effect of electronic characteristic of CNTs on catalytic performance, and the structure-activity relationship was studied in detail. In situ spectroscopic technology was employed to monitor the intermediate molecular/radical stabilized on CNT surfaces and the electron transfer between them. Through kinetics study and density functional theory (DFT), we unraveled the mechanism behind CNT-catalyzed C6H12oxidation process at the molecular level, and the structure and evolution of transition-state intermediate confined on carbon surface were clarified.Fe-filled CNTs (Fe@CNTs) were in situ synthesized by chemical vapor deposition (CVD). Defects and metal filling rate of the Fe@CNTs can be controlled by varying the dichlorobenzene (DCB) content in carbon sources. An unprecedented activity of C6H12oxidation was obtained by filling CNTs with about20wt%metallic Fe, maintaining the low-defect nature of the CNTs. Because defects and iron filling have the opposite effect on the electron transfer in graphene sheets, work function was used to measure the ability of materials to donate electrons. The activity of C6H12oxidation has exponential relation with the work function of Fe@CNTs. It proveds that the electron transfer in CNTs plays an important role in the catalytic oxidation of C6H12. The encapsulated Fe endows the Fe@CNTs with high saturation magnetization, allowing for easy separation of catalyst after C6H12oxidation, which greatly reduce the complexity of filtering technique. Beyond the defects and metal filling rate of Fe@CNTs, it was found that wall thickness and the species of filled metal are linked to the electron transfer in CNTs. More excellent catalytic performances were achieved by using Fe@CNTs with thinner walls. Meanwhile, since the work function of Fe is lower than that of Ni, Fe@CNTs are superior on the catalytic performances to Ni@CNTs and FeNi@CNTs prepared from the same carbon sources.We conducted a comparative kinetic study of the C6H12oxidation with or without catalysts. It showed that CNTs, especially N-CNTs, significantly enhanced the reaction fluxes in solvent cages confined on the surface of carbon, thereby the larger radical chain-length improved the reaction rate. The weak interaction between CNTs and radicals in solvent cage was explored with DFT calculation. It may considerably enhance the electron transfer between them, and the doping of N makes a further reduction in the adsorption barrier. The formation of peroxide intermediates confined at CNT surfaces was demonstrated by an in situ FTIR investigation in the catalytic oxidation of C6H12. The weak adsorption of peroxides on CNT surfaces suppresses the diffusive separation of the reactants and products in chain-propagation steps, thus enhance the cage-channel reaction confined on CNT surfaces, which was according with the in situ electron paramagnetic resonance study. Single-walled CNTs (SWCNTs) were adopted for the in situ Raman study. It was found that G band, D band and radial breathing mode of SWCNTs changed in intensity and frequency under the reaction conditions, due to the injection of electrons and the attacking from charged radicals to the SWCNTs. By combining kinetics analysis, in situ spectroscopy and DFT, it was revealed that the catalysis on CNTs origins from a weak interaction between radicals and graphene skeletons, which confines the radicals around the surfaces of CNTs and thus enhances the electro-transfer catalysis of intermediate molecular to yield alcohol and ketone.To summarize, the electron transfer in graphene sheets of CNTs plays an important role in C6H12catalytic oxidation. Based on the mechanism insights, it will help the more rational design for the emerging CNTs catalyst, which is expected to become the fundamentals of environmental-friendly oxidation technology of C6H12in liquid phase.