The Study On The Kinetics Of Hydrogenation Of Nitrobenzene Catalyzed By MOF Derived Co-Zn@NC And The Active Sites Of Co-Nx Hydrogenation

Aromatic amines are important industrial organic chemical products and intermediates.At present,the preparation of aromatic amines in industry mainly adopts catalytic hydrogenation method,but the applied precious metal catalysts lead to high production costs of the process and limit its promotion and application.Therefore,efficient non-precious metal additions have been developed hydrogen catalyst has important research and application value.Based on the previous research of the group,this thesis continue to study the stability of the highly active nitrogen-doped carbon-coated cobalt-based bimetallic catalyst（Co-Zn@NC）derived from Co-Zn-BMZIFs and the kinetics of the nitrobenzene hydrogenation reaction.At the same time,through the design of N-doped Co-Zn-MOF-71-derived Co-Zn@NC material,the important role of Zn metal and Co-Nx hydrogenation active sites in the catalyst for improving the catalyst activity was further discussed,and combined with DFT theoretical calculations,Co-Nx hydrogenation active sites were studied.Second,through the design and preparation of nitrogen-doped Co-Zn-MOF-71material and its derived Co-Zn@NC catalyzed hydrogenation of nitrobenzene.The catalyst characterization analysis shows that the introduction of N and Zn will not change the crystal structure of the MOF-71 material,and the introduction of an appropriate amount of N can improve the thermal stability of the MOF-71 material and form the Co-Nx active site;while the introduction of Zn leads to the thermal stability deteriorate.The experimental results show that Co@NC has better catalytic hydrogenation activity than Co@C,which confirms that Co-Nx sites have better hydrogen activation characteristics than Co elemental sites.The catalyst activity becomes worse after the introduction of Zn,which also verifies the research conclusion that"Zn improves the thermal stability of the material and the dispersion of the active metal,thereby increasing the catalytic activity of the catalyst."Combined with the catalytic performance and characterization results,it is confirmed that the existence of Co-Nx active sites is beneficial to improve the catalytic activity.To eliminate the interference caused by the structure of the material and other factors on the performance of Co@NC catalyst,based on MOF-71 material was prepared to obtain N-doped Co-Zn@NC catalyst to start research.The analysis of TG and experimental results shows that the introduction of nitrogen can increase the catalytic activity,indicating that Co-Nx has better hydrogenation activity than single Co sites.The introduction of Zn on the basis of Co-MOF-71 to synthesize bimetallic materials,TG characterization results and experimental data show that the introduction of Zn does not improve the thermal stability and catalytic activity of Co-Zn@NC,which further confirms the conclusion that Zn improves the catalytic activity by improving the thermal stability of the material.By calculating and comparing the dissociation energy of H₂ on the Co@NC and Co@C models,it is found that H₂ is dissociated into H⁺and H^-through hybridization,and H⁺on the Co@NC model has more electron scarcity than that on Co@C,indicating that the introduction of nitrogen will affect the entire electronic environment,and there is electron transport between elements,which is conducive to H₂ hybridization and dissociation.The calculation shows that in the Co@NC model,the dissociation energy and reaction barrier of H₂ are 76.04 KJ/mol and 1.197 e V,which are significantly lower than Co@C（140.77 KJ/mol and 2.062 e V）,indicating that Co-Nx species can effectively reduce the reaction barrier,thereby increasing the catalytic activity.Comparing the conclusions of the kinetic study,it is determined that the hydrogen dissociation step in the first step of the reaction is the rate determining step for the Co-Zn@NC catalyzed nitrobenzene hydrogenation reaction.