Synthesis of Amines by Oxidative Coupling of Benzylamine over a Vanadium-Nitrogen Co-doped Porous Carbon Catalyst

Synthesis of imine compounds via benzylamine oxidative coupling has become one of the most ideal methods due to its high atom economy and environmental friendliness. The key is to develop high-performance non-noble metal-based heterogeneous catalysts. In this work, a vanadium-nitrogen co-doped porous carbon (V-N-C) catalyst was prepared via high-temperature pyrolysis (900 degrees C for 2 h in an inert atmosphere) combined with acid-leaching (1 mol.L-1 HCl solution at 120 degrees C for 12 h) approach by using biomass chitosan as the sacrificial template, vanadium acetylacetonate as the source of metal vanadium, and ZnCl2 as the pore-forming agent. Various characterization techniques including a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) investigation were used to analyze the composition, structure, vanadium species size, content, and other physical and chemical properties of the catalyst, and its catalytic performance was evaluated in the oxidative coupling reaction of benzylamine. The characterization results showed that the specific surface area of the V-N-C catalyst was as high as 1 470 m(2).g(-1), the pore volume was 1.06 cm(3).g(-1), and the mass fraction of the vanadium species was 0.19% that were highly dispersed on the support likely in the form of single atoms (VNx). In the oxidative self-coupling reaction of benzylamine to the imine (reaction conditions: toluene as solvent, 110 degrees C, 1.01x10(5) Pa O-2, 12 h), the developed V-N-C exhibited excellent activity (99%), exclusive selectivity (99%), outperforming the homogeneous VO(acac)(2) and heterogeneous V2O5 catalysts. Moreover, V-N-C was repeatedly used 9 times without any decay in reactivity and stability. Furthermore, V-N-C presented excellent universality for a series of substrates containing different functional groups. Mechanism studies indicated that the reaction steps were involved in the initial formation of benzylimine and H2O2 intermediates by activating benzylamine and oxygen molecules, respectively, on the VNx and defect sites of V-N-C, then benzylimine and benzylamine condensed to release an NH3 molecule to generate the target product imine.

Automated summary

A vanadium-nitrogen co-doped porous carbon (V-N-C) catalyst was prepared via high-temperature pyrolysis and acid-leaching, and it exhibited excellent catalytic activity and selectivity in the oxidative coupling reaction of benzylamine, as well as good universality for various substrates.