摘要/Abstract

苄胺氧化偶联制N-苄烯丁胺通常需使用贵金属催化剂, 开发廉价催化剂具有重要研究价值. 本工作以具有大比表面积和丰富表面缺陷的分级结构碳纳米笼(hCNCs)作为无金属催化剂, 在无溶剂、100 ℃和常压O₂条件下即可实现苄胺到N-苄烯丁胺的高效转化, 反应8 h的苄胺转化率和N-苄烯丁胺选择性均可达98%, 远优于碳纳米管、还原氧化石墨烯、活性炭等典型碳材料. hCNC700样品循环使用6次后催化性能基本无衰减, 且具有优秀的底物拓展性. hCNC700的优异催化性能源于其超高的比表面积可提供大量的缺陷活性位点, 而独特的分级孔结构十分有利于反应过程中的传质, 使丰富的表面活性位点(缺陷)得以充分利用.
关键词: 分级结构碳纳米笼, 无金属催化剂, 缺陷催化, 苄胺, N-苄烯丁胺
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.
Key words: hierarchical carbon nanocage, metal-free catalyst, defect catalysis, benzylamine, N-benzylidene benzylamine


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