Properties Of Metal/nitrogen Doped Mesoporous Carbon For Catalytic Reduction Of Nitrarenes

Aromatic amines are industrially important organic chemical raw material and chemical product,widely used in dyes,pharmaceuticals,pesticides,rubber and imaging chemicals.In industry,aromatic amines are mainly prepared by selective reduction of nitrarenes.The recent works on green synthetic of aromatic amines were mainly focused on the greenization of the chemical reactions,raw materials,catalysts,solvents,as well as the products.Aromatic amines are mainly prepared by the reduction of nitrarenes.Heterogeneous catalytic reaction is considered as an environmental friendly procedure,as the easy separation of the products and the catalysts can be recycled conveniently.However,for the selective reduction of nitrarenes,expensive noble metal catalysts and auxiliaries are usually required,and the reaction should be carried out under harsh conditions.In addition,the reduction of some nitrarenes containing reductive groups or halogenated groups under heterogeneous catalytic reaction system usually achieved low selectivity.To address these issues,a series of catalysts with excellent performance under different catalytic reduction systems are designed.Firstly,for the catalytic hydrogenation of nitrarenes,in order to make the reaction process greener,which can be achieved by avoiding the use of toxic organic solvents,we designed a supported metal catalyst supported on nitrogen-doped mesoporous carbon material(Pd/N-CMK-3)by a post-enrichment of nitrogen method.The as-prepared catalyst possesses well distributed small Pd particles(1-2 nm),while the particle size of Pd nanoparticles in Pd/CMK-3 were around 5 nm.When the catalysts were used for hydrogenation of nitrarenes,it was found that Pd/N-CMK-3 showed a much higher activity under solvent-free condition,which was about three times as high as that of Pd/CMK-3 catalyst.The combination of characterizations revealed that the high activity of the catalyst could be attributed to the nitrogen doped in CMK-3 that promoted the interaction between the support and metal Pd nanoparticles,leading to a higher content of the active sites Pd0 on the surface of Pd/N-CMK-3 catalyst;on the other hand,much smaller Pd particles size together with strengthened anchoring were obtained by the incorporation of nitrogen.This method could also be extended to the preparation of other carbon-supported catalysts,such as activated carbon,carbon nanotubes and so on.Considering the high price of noble metals and in order to make the preparation of catalyst more economic,for the first time,a transition metal(Ni,Co,Fe)supported on nitrogen-doped ordered mesoporous carbon composites were successfully prepared by a one-step chelates and nitrogen-containing complexes assisted method.The prepared composites possess a high surface area of 520 m2g-1 and an average pore size of up to 4.1 nm.The metal nanoparticles were evenly distributed on the nitrogen-doped ordered mesoporous carbon with an average particle size about 20 nm.The catalyst preparation process was simple,economical,easy to operate and to scale up.At the same time,the hydrophilicity of the carbon material was inhanced after nitrogen doping,so that the prepared catalyst exhibits high catalytic activity in the selective hydrogenation of nitrarenes(more than 20 samples)in aqueous solution with high selectivity.The high catalytic activity and stability of the catalysts were resulted from the synergistic effect between the components,the special nitrogen-doped carbon coating and activation of the metal.Considering that the hydrogenation of nitroarenes using non-noble metal catalysts should proceed in high temperature and pressure,and dehalogenation were inevitable for selective hydrogenation of nitrarenes containing halogenated groups,development of more efficient catalytic system is necessary.To solve this problem,the hydrazine hydrazine was used as the hydrogen donor for selective hydrogen transfer reduction of nitrarenes.Mesoporous y-alumina supported Co oxides(Co3O4/γ-MA)with high specific surface areas,large pore volumes and narrow pore size distributions were prepared by one-pot hydrolysis inorganic metal nitrates through a sol-gel method,followed by a low temperature roasting.The well-defined cobalt metal supported on the N-doped mesoporous carbon/γ-Al2O3 composites(Co-NC/γ-Al2O3)were successfully synthesized by combining the Co3O4/y-MA with 1,10-phenanthroline,followed by roasting in an inert atmosphere.The mesoporous structure of γ-Al2O3 was remained after the combination of nitrogen doped carbon,accompanied with most of the metal particles were coated with a defective nitrogen-doped carbon layer.Subsequently,the selective hydrogen transfer reduction of 4-chloronitrobenzene was used as a model reaction to determine the performance of the catalysts.The prepared Co-NC/γ-Al2O3 could enable the efficient transformation of 4-chloronitrobenzene to 4-chloroaniline under mild condition,with no significant dechlorination occurring.The prepared Co-NC/γ-Al2O3 catalysts also obtanied excellent catalytic activity and perfect chemoselectivity(>99%)for a wide range of other substituted nitroarenes.Although the Co-NC/γ-Al2O3 catalysts exhibited excellent activity,selectivity and stability in the selective hydrogen transfer reduction of nitrarenes,the activity of the catalyst remained to be further improved.To solve this problem,we combined transition metal,molybdenum oxide,nitrogen-doped mesoporous carbon and SBA-15 together to successfully prepare Co-Mo03/CN@SBA-15 catalysts for the first time.The results showed that the Co-MoO3/CN@SBA-15 catalysts could efficiently catalyze the reduction of nitrarenes under mild conditions(30℃,atmospheric pressure)with high selectivity and the catalytic activity was significantly improved compared to the Co-NC/β-Al2O3 catalysts with single metal components.At the same time,the catalysts could be recycled up to 10 times with unconspicuous loss of activity.The synergic effects between each of the component of the catalyst were fully utilized to ensure the efficient conversion of the substrates.In summary,a series of highly efficient metal/nitrogen-doped mesoporous carbon composites have been successfully designed and were applied for the chemoselective reduction of nitrarenes.The synergic effects among each component,which greatly affected the performance of catalysts in terms of activity,selectivity and stability,were investigated 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 powerful and environmentally friendly catalytic system.