Hierarchical Carbon Nanocages as Efficient Catalysts for Oxidative Coupling of Benzylamine to N-Benzylidene Benzylamine

N-Benzylidene benzylamine is an important pharmaceutical intermediate and industrial chemical. Traditionally, it is synthesized via the reaction between benzylamine and benzaldehyde by using Lewis acid catalysts, which usually suffers from the difficulty of product separation and environmental unfriendliness. An alternative approach is the oxidative coupling of benzylamine catalyzed by metal-based catalysts, which is also beset by the contamination of metallic impurities. Thus, the development of green, metal-free and reusable heterogeneous catalysts is highly attractive. Herein, we report an efficient metal-free catalyst, hierarchical carbon nanocages (hCNCs) synthesized by the in-situ magnesium oxide template method with benzene precursor at different temperatures (700, 800 and 900 ), for the oxidative coupling of benzylamine to N-benzylidene benzylamine. The hCNCs feature multi-scale hierarchical pore structure, high specific surface area and abundant surface defects. The hCNC700 exhibits excellent catalytic performance for the solvent-free oxidative coupling of benzylamine to N-benzylidene benzylamine under mild conditions (100 , atmospheric O2). Specifically, after reacting for 8 h, both benzylamine conversion and N-benzylidene benzylamine selectivity are larger than 98%, far better than the counterparts of carbon nanotubes, reduced graphene oxide and activated carbon, as well as the reported mesoporous carbon and graphene oxide. The hCNC700 also presents high mass activity, significantly better than the reported carbon-based catalysts. Its catalytic performance is almost unattenuated after 6 times of recycling, exhibiting good stability. By comparison experiments, the catalytic activity of hCNCs results from the intrinsic defects of carbon, and the excellent performance of hCNC700 is mainly attributed to the following aspects: (i) its ultra-high specific surface area can provide abundant surface active sites (defects), (ii) the unique hierarchical pore structure is very conducive to mass transfer in the reaction process, enabling the full utilization of these active sites. In addition, hCNC700 shows good catalytic performance for different substrates, with high catalytic activity (90% conversion rate for 12 h reaction) and high selectivity (95%) for oxidative coupling of aromatic methylene amines. This study provides a new avenue for the development of cheap and efficient carbon-based metal-free catalysts.