Ultrafine\nPd Nanoparticles Anchored on Spherical Covalent\nTriazine Frameworks for the Selective Tandem Hydrogenation of Nitrobenzene\nand Benzaldehyde to Secondary Amines

The selective tandem one-pot synthesis of secondary amines\nvia\nnitrobenzene and benzaldehyde coupling–hydrogenation is challenging\ndue to side reactions. Herein, a covalent organic framework (COF)-based\ncatalyst was fabricated by anchoring ultrafine Pd nanoparticles on\na spherical COF material (Pd/DHTA-COF-P123). The surfactant P123 was\nused as a template during solvothermal synthesis to control the morphology\nof DHTA-COF-P123 via micellar encapsulation, which endowed the material\nwith a large surface area. The spherical DHTA-COF-P123 support strengthened\nthe metal–support interactions (SMSIs) and regulated the electronic\nstate of Pd species to an electron-rich state, thereby improving catalytic\nhydrogenation. During Pd/DHTA-COF-P123-catalyzed nitrobenzene and\nbenzaldehyde coupling–hydrogenation, 99.9% conversion of nitrobenzene\nand 99.6% selectivity for <i>N</i>-benzylaniline were obtained\nunder mild reaction conditions. Theoretical calculations confirmed\nthat the stronger SMSIs were responsible for the excellent activity\nof Pd/DHTA-COF-P123 in the tandem reaction, and the hydrogenation\nprocess was exothermic, which strengthened absorption. This work proposes\na green strategy to fabricate an electron-rich Pd-based catalyst by\nenhancing SMSIs to catalyze the synthesis of secondary amines.