Catalyst For The Reduction Of Nitroarenes

Aromatic amines are industrially important raw materials and intermediates for the manufacture of medicine,pesticides,pigments,spices and functional polymers.The general method for the synthesis of aromatic amines is achieved via the reduction of aromatic nitro compounds.Aromatic amines are traditionally obtained by the reduction of the corresponding nitroarenes using stoichiometric amounts of reducing reagents like Fe/Zn and sulfides.However,these reduction processes produce large amounts of metal sludge and hazardous wastewater containing acids and sulfur,causing serious environmental pollution.Therefore,there is a pressing need to develop highly efficient heterogeneous catalysts with green solvent for the chemoselective reduction of nitroarenes.Noble metal supported catalysts play an important role in the catalytic hydrogenation of aromatic nitro compounds to aromatic amines due to their excellent catalytic activity.However,most of these reported noble-metal catalysts are poorly chemoselective for the reduction of nitro aromatics to the corresponding aromatic amines with other reducible functional groups,especially with halogens?F,Cl,Br,I?.As an effective way to improve the selectivity of aromatic amines,non-noble metal catalysts have also attracted wide attention.The non-noble metals deliver the advantages of abundance and easy availability,yet they often suffer from the oxidation,sintering and leaching problems under relatively harsh conditions,which to some extent limited its application.In this paper,to address these issues,a series of noble metal and non-noble metal catalysts with excellent performance are designed for the reduction of aromatic nitro compounds.Firstly,highly-dispersed Pt nanoparticles supported on nitrogen-modified CMK-3 mesoporous carbon?Pt/N-CMK-3?were first fabricated by a two-step impregnation route.The composite has high specific surface area,regular pore structure and large pore volume.The N species in the carbon matrix promotes the dispersion of metal Pt and prevents the agglomeration of Pt nanoparticles.The Pt/N-CMK-3 quantitatively converted various nitroarenes to the corresponding anilines using hydrazine hydrate or H₂ with unprecedented activities(e.g.,turnover frequency for o-chloronitrobenzene was 30.2 s^–1)and high selectivity.The cooperation effect between Pt and N atoms facilitated heterolytic cleavage of hydrazine to form H^?–/H^?+pairs,and weakened the interaction between the halogen groups and Pt atoms,resulting high catalytic activity and 100%haloaniline selectivity.Moreover,the Pt/N-CMK-3 catalyst was highly stable and could be reused for the selective hydrogenation of nitrobenzene without obvious loss of catalytic performance.Despite the Pt/N-CMK exhibited excellent catalytic activity and selectivity for the selective reduction of aromatic nitro compounds,the high price of these noble metals have limitted the application of the catalyst.In order to solve this problem,we develop silica-supported Co@N-doped carbon?Co@CN/Si O₂?catalysts,which were first prepared by a one-step impregnation with a mixed solution of cobalt nitrate,glucose and urea,followed by in situ carbonization and reduction.The Co@CN/Si O₂-500 carbonized at 500 ^oC exhibited the highest catalytic activity and excellent stability without any decay of activity after 6 cycles for the reduction of ntirobenzene.Turnover frequency?TOF?calculated on the basis of the amount of total Co in Co@CN/Si O₂-500 was 4282 h^-1,which was significantly higher than those over various non-noble metal catalysts ever reported for the selective reduction of nitrobenzene.Both metallic Co atoms and Co–N species formed in the Co@CN/Si O₂ catalysts were active,but the Co–N species were dominant active sites.The high activities of the Co@CN/Si O₂ catalysts were attributed to the synergistic effect between the Co and N atoms,promoting heterolytic cleavage of hydrazine to form H⁺/H^–pairs.The Co@CN/Si O₂-500 could completely transform various halogen-substituted nitro aromatics to the corresponding halogenated anilines with high TOFs and selectivity of>99.5%.Although the monometallic Co@CN/Si O₂ catalyst exhibited high catalytic activity for the selective hydrogenation of nitro aromatics to the corresponding anilines using hydrazine hydrate,the catalytic performance of Co@CN/Si O₂ needed to be further improved when cleaner and cheaper hydrogen was used as the reducing agent.When the Cu species were introduced,Co-Cu bimetallic?Co₁Cu_x@CN/Si O₂?catalysts were first prepared by in situ carbonization and reduction.The material characterization analysis revealed that the presence of Cu species could suppress the gasification of N-containing compounds,so that more nitrogen species are retained in the carbon matrix,which is conducive to the subsequent formation of more active sites?Co-N species?.Compared to the monometallic Co@CN/Si O₂ catalyst,the Co₁Cu_x@CN/Si O₂ exhibited higher catalytic activity for the selective hydrogenation of nitro aromatics to the corresponding anilines using hydrogen.More importantly,benefiting from the protection of copper oxide on the catalyst surface and interaction effect between the interface Co and adjacent Cu species,the Co₁Cu_x@CN/Si O₂catalysts exhibited excellent anti-oxidation ability and reusability for the hydrogenation of nitrobenzene over 12 times without any loss of catalytic performance.This method can be applied to the design of bimetallic catalysts for various catalytic reactions.As is mentioned above,both monometallic and bimetallic Co-Cu catalysts showed excellent catalytic performance in the selective reduction of aromatic nitro compounds,but Co²⁺compounds,used as precursor for catalyst preparation,are carcinogenic and the most expensive of several common non-precious metals.In order to replace Co-based catalysts with other more friendly elements,the Fe₃O₄nanoparticles with controllable size in the range of 2.4–12.6 nm were first synthesized by solvothermal method with glucose as assistant.The preparation method has the advantages of simple process,low toxicity and cost,and easy industrial scale-up.Glucose is the key to the size control of Fe₃O₄.The oxygen-containing functional groups produced by the pyrolysis of glucose adsorbed on the surface of Fe₃O₄ nanoparticles can prevent the aggregation and growth of the particles.The prepared Fe₃O₄ nanoparticles with uniform and controllable size exhibited a good stability and can completely transform various substituted nitro aromatics to the corresponding anilines using hydrazine hydrate with high activity and selectivity.Moreover,the Fe₃O₄ nanoparticles can be easily separated from the reaction mixture for recycling due to its good magnetic properties.It can be recycled for 10 times without obvious loss of catalytic performance.This work makes it possible to develop cheaper and greener non noble metal catalysts for the reduction of nitro aromatics.In summary,a series of highly efficient supported metal catalyst were successfully designed and applied for the chemoselective reduction of nitrarenes.The interaction among the components of catalyst and its influence on the catalytic performance and stability of catalyst were analyzed in detail.Our work partially solves the problem existed in the reported metal heterogeneous catalysts for the catalytic chemoselective reduction of nitroarenes,which could provide some guidance for designing more efficent and environmentally friendly catalytic system.