Ultrafine Pd Nanoparticles Anchored on Spherical Covalent Triazine Frameworks for the Selective Tandem Hydrogenation of Nitrobenzene and Benzaldehyde to Secondary Amines

The selective tandem one-pot synthesis of secondary amines via nitrobenzene and benzaldehyde coupling–hydrogenation is challenging due to side reactions. Herein, a covalent organic framework (COF)-based catalyst was fabricated by anchoring ultrafine Pd nanoparticles on a spherical COF material (Pd/DHTA-COF-P123). The surfactant P123 was used as a template during solvothermal synthesis to control the morphology of DHTA-COF-P123 via micellar encapsulation, which endowed the material with a large surface area. The spherical DHTA-COF-P123 support strengthened the metal–support interactions (SMSIs) and regulated the electronic state of Pd species to an electron-rich state, thereby improving catalytic hydrogenation. During Pd/DHTA-COF-P123-catalyzed nitrobenzene and benzaldehyde coupling–hydrogenation, 99.9% conversion of nitrobenzene and 99.6% selectivity for N-benzylaniline were obtained under mild reaction conditions. Theoretical calculations confirmed that the stronger SMSIs were responsible for the excellent activity of Pd/DHTA-COF-P123 in the tandem reaction, and the hydrogenation process was exothermic, which strengthened absorption. This work proposes a green strategy to fabricate an electron-rich Pd-based catalyst by enhancing SMSIs to catalyze the synthesis of secondary amines.