Studies In The Preparation Of Novel Supported Ni Catalyst And Catalytic Transfer Hydrogenation Coupling Of Nitroaromatics

Aromatic azo/azoxy compounds are of great important raw materials in the production of high-value chemical products such as dyes,pigments,food additives,and medical drugs.But challenges lie in controllable synthesis of desired product from readily available anilines or nitroaromatics.Generally,there are two main methods to produce azo/azoxy compounds:the oxidation coupling of aromatic amines and the reductive coupling of nitroaromatics.The latter is much more effective and environment friendly without the additive of oxidant.Meanwhile it meets the requirements of environmental protection and sustainable development,which has aroused people’s research interest.In large numbers of literature reports,nano-gold-based catalysts have been extensively studied in photocatalysis and thermal catalysis due to their superior activity and selectivity under mild conditions.But its high price limits its large-scale production and application.Moreover,the non-precious metal catalysts exhibit certain performance under harsh conditions,but the activity cannot meet the demand for mass production.In the other hand,the reduction reaction with alcohols as hydrogen donors has attracted people’s research interest due to its mild reaction conditions and wide range of applications.Therefore,looking for non-precious metal catalysts with better performance as substitutes of supported Au catalysts has important academic value and research significance.Lately,Single atom catalysts（SAC）,with atomic metal species on the support surface,have emerged as a new aspect for catalysis owing to the maximum atom utilization efficiency,tunable coordination environment and unique electron structures of active sites.And they have been widely applied in thermo-,electro-,and photocatalysis because of their superior catalytic activity and selectivity compared to their corresponding nanoparticle catalysts.In this work,a quasi-stellate single-atom Ni catalyst was prepared in a convenient microwave pyrolysis method under air atmosphere.And the catalytic properties and structure-activity relationship of reductive coupling of nitrobenzene were further studied.The detailed contents are listed as follows:（1）We developed a single-atom Ni catalyst using stellate mesoporous Si O₂ nanospheres as the template in a convenient microwave pyrolysis method.After leached by Na OH and H₂SO₄,the mesoporous carbon sphere with stellate channel was obtained.The unique structure derived from stellate mesoporous Si O₂template is beneficial to improving mass transfer and increasing the exposure of active sites.The developed catalyst is used for the hydrogenation coupling of nitrobenzene with isopropanol as reagent and hydrogen donor.The result demonstrates that the single-atom Ni SA/NC catalyst achieves total nitrobenzene conversion and produce azoxybenzene as major product with a yield of 94.8%.The value of TOF can reach33.4h^-1.By contrast,supported Ni catalysts can only obtain aniline as the final product under the same reaction conditions with a low yield.The outstanding catalytic efficiency over Ni SA/NC catalyst is also obtained with diverse nitroaromatics for transfer hydrogenation coupling under mild conditions.HAADF-STEM,XAFS,XPS,XRD were used to explore the structure-activity relationship of the catalyst.Control experiments prove that the unique Ni-N₄ sites dispersed on the nitrogen-doped carbon sphere are the active cites for accelerating the hydrogenation of nitrobenzene to nitrosobenzene and the reductive coupling of reaction intermediates.（2）Based on the above work and the reported reaction mechanism about reductive coupling of nitrobenzene.We changed the amount of reagents,base and reaction temperature in the original reaction system to promote the further hydrogenation of azoxybenzene to azobenzene during the reductive coupling process.The reaction results showed that under the condition of complete conversion of nitrobenzene,the selectivity of azobenzene reached 93.8%.Besides,diverse aromatic azo compounds with different substituents were also achieved in good or moderate yield over the developed single atom Ni SA/NC catalyst.The final reaction results proved that the reductive coupling of nitrobenzene followed the reaction route of nitrobenzene-nitrosobenzene（hydroxylamine）-azoxybenzene-azobenzene.In our work,we have successfully realized the transfer hydrogenation coupling of nitroarenes to corresponding azo/azoxy aromatics with total nitroarenes conversion and high corresponding azo/azoxy aromatics selectivity under mild reaction conditions over non-precious single-atom catalyst Ni SA/NC prepared by convenient microwave pyrolysis method.And it also provides a successful case for the deep understanding of the structure-activity relationship of the catalysts from atomic scale.