Reaction Characteristics Analysis Of Ethylene Hydroformylation And Tailor-Design Of Rh-based Catalysts

Propionaldehyde is an important chemical intermediate,mainly used in the production of propanol,propionic acid and so on.The main approach for the industrial production of propionaldehyde is ethylene hydroformylation,where the most widely used catalyst is Rh-based homogeneous catalyst.However,the real application of this catalyst still suffers some drawbacks,such as the complex procedure for the catalyst preparation,the usage of excess solvent and the low yield of Rh.Moreover,the poor stability of this homogeneous catalyst causes significant metal loss during the reaction.Based on this,the development of highly efficient and heterogeneous Rh catalysts with simple preparation process and low metal loading is desirable to further improve the operational stability of the process and reduce the production cost,which has gradually become a hot and difficult research point in this field.This paper takes the supported rhodium catalyst system as the main research object.The reaction characteristics were analyzed based on thermodynamic calculations to optimize the operation conditions,and which the effects of Rh catalyst support,particle size and promoter are further studied by changing the catalyst support,metal loading and promoter,respectively.As a result,the regulation mechanism of the geometric and electronic properties of Rh catalyst on catalytic performance is disclosed,which provides guidance for the design and preparation of highly efficient and supported Rh catalysts for ethylene hydroformylation.The main findings are listed as follows:(1)Thermodynamic analysis was performed to investigate the reaction characteristics of ethylene hydroformylation,and the thermodynamic trends of the main and side reactions were comparatively studied.Based on which,the appropriate reaction conditions were determined and the production process parameters were optimized.It was found that the side reaction of ethylene hydrogenation is more thermodynamically favorable.Therefore,more efforts should be devoted to manipulate the active site structure of catalyst to promote the activation of ethylene for the generation of more reaction intermediate species of propionaldehyde,and inhibit the direct hydrogenation of ethylene to further increase the selectivity of propionaldehyde.The reaction process conditions were further optimized through Aspen simulation,and it was found that 90℃ and 1.2 MPa were the optimal reaction conditions,in which the ethylene conversion could reach 85.93%.(2)The support effect and particle size effect of Rh-based catalyst for the ethylene hydroformylation were explored.It was found that the SiO2 support has a larger specific surface area,which is beneficial for the dispersion of active metals.Moreover,the abundant weak acid sites on its surface were found to promote the insertion of CO into ethylene,thus delivering high catalytic performance.On this basis,the catalysts with different Rh particle sizes were prepared by adjusting the Rh loadings to explore the particle size effect.It was found that the catalytic performance showed a volcanic curve as a function of Rh particle size,in which the 1.6 nm-sized rhodium catalyst has more edge active sites to deliver the highest propionaldehyde yield of 1.7 mmol.(3)The Rh-based bimetallic catalyst was prepared by the co-impregnation method to introduce metal promoters(Co,Fe,Ni),based on which the regulation mechanism of the metal promoter on the structure and catalytic performance of Rh catalyst was clarified.It was found that the Co promoter can reduce the electron density of Rh surface by attracting Rh electrons without changing the geometric structure of the rhodium particles,thereby promoting the adsorption of the reactants and increasing the yield of propionaldehyde.The introduction of Fe and Ni promoters increased the metal particle size and the surface electron density of Rh,which inhibited the adsorption of reactants and catalyst activity.The above works provide guidance for the preparation of highly efficient Rh-based catalysts for ethylene hydroformylation.