| Carbon-based materials are regarded as ideal precursors for metal-free catalysts owing to their excellent physical and chemical properties and abundance.Herein,carbon-based materials were used as supports and modified with suitable nitrogen dopants to prepare nitrogen-doped carbon-based metal-free catalysts for hydrogenation.Methods to improve the catalytic activity were explored and the mechanism of catalytic hydrogenation was studied.First of all,a series of metal-free N-doped carbon nanotubes（NCNTs）are synthesized via a pyrolysis process,using melanin or protoporphyrin IX as the dopant,multi-walled carbon nanotubes（MWCNTs）with various diameters as the supports.The content of doped graphitic N is effectively controlled by varying the heating rate during pyrolysis.The obtained metal-free catalysts are employed to catalyze the hydrogenation of various nitroarenes under mild reaction conditions（2 MPa of H2,120°C）.Efficient and selective hydrogenation of 14 nitroarenes is achieved over NCNTs with outer diameters of≤20 nm synthesized via pyrolysis of melanin at800°C.Graphitic N in the as-prepared NCNTs is found to play a key role in efficient hydrogenation.Kinetic data reveal that a zero-order reaction occurs on the NCNTs,and the apparent activation energy of 4-nitrophenol hydrogenation is as low as 35.69k J mol-1.Recycling experiments indicate that the NCNTs have good stability and recyclability for hydrogenation of 4-nitrophenol.Then we used tripeptides as nitrogen dopants to prepare NCNTs.The effects of the self-assembly and enzymatic oxidation of the tripeptides on the hydrogenation activity of the prepared NCNTs were investigated,and the optimal preparation condition was determined.In addition,NCNTs with high graphitic N content were prepared by using polypeptides with binding site selectivity as nitrogen dopants.Moreover,covalent organic framework or carbon nitride was employed as precursors to prepare nitrogen-doped carbon-based catalysts.The activities of the as-prepared catalysts were initially evaluated. |
