Chemoselective Hydrogenation of Halonitrobenzenes by Platinum Nanoparticles with Auxiliary Co-N₄ Single Sites in Sandwiched Catalysts

The chemoselective hydrogenation of halonitrobenzenes to haloanilines is of great importance but remains challenging to simultaneously achieve high catalytic activity, excellent selectivity, and good reusability, especially for ortho-substituted substrates. This is due to the occurrence of hydrogenolysis of halogen groups, as well as the easy migration and aggregation of active species on the catalyst surface during the hydrogenation of nitro groups. In this study, we integrate Pt nanoparticles (NPs) with auxiliary Co-N₄ single sites from a porphyrinic metal-organic framework [known as PCN-221(Co)] in a sandwiched nanostructure as a catalyst for the chemoselective hydrogenation of ortho-halonitrobenzenes at 80 °C and 1 MPa H₂ in a 50 mL batch microreactor. This sandwiched catalyst achieves 97.3% selectivity for ortho-chloroaniline at nearly complete conversion of ortho-chloronitrobenzene, with an exceptionally high turnover frequency (TOF) of 11,625 h^-1 and good reusability over ten cycles, outperforming state-of-the-art heterogeneous supported metal catalysts. Theoretical and experimental investigations reveal that the nitro group in ortho-chloronitrobenzene is preferentially hydrogenated by Pt NPs, while the ortho-chloro group is selectively adsorbed by Co-N₄ single sites in PCN-221(Co), preventing its hydrogenolysis and enhancing selectivity for ortho-chloroaniline. Furthermore, the PCN-221(Co) shell in the sandwiched catalyst plays a key role in enriching ortho-chloronitrobenzene and stabilizing the supported Pt NPs, thus leading to high catalytic activity and good reusability. Additionally, at nearly complete conversion of ortho-fluoronitrobenzene and ortho-bromonitrobenzene, this sandwiched Pt catalyst displays 100% selectivity for ortho-fluoroaniline with a TOF of 8680 h^-1 and 99.2% selectivity for ortho-bromoaniline with a TOF of 5859 h^-1, respectively. When meta- and para-halonitrobenzenes are used as substrates, high activity and excellent selectivity for the corresponding haloanilines are also achieved by the sandwiched Pt catalysts.