Preparation Of Doped Fe-based MOFs Catalysts And Performance Of CO₂ Hydrogenation

Recently,the constant consumption of fossil fuels leads to increasing environmental problems,such as carbon dioxide（CO₂）emissions and energy shortages.The reaction between hydrogen and CO₂ generated by renewable energy such as solar energy and wind energy can not only store hydrogen,but also reduce CO₂ emissions and generate chemicals.Among them,hydrogenation of CO₂ to olefins has aroused wide interest.Currently,supported iron-based catalysts are important catalysts for the generation of olefins from CO₂ hydrogenation reactions,but these catalysts suffer from low loading,poor stability,low conversion and easy sintering with agglomerative deactivation,making them a limiting factor for the industrial development of olefins.Metal-Organic Frameworks（MOFs）have been widely used in the field of catalysis due to their unique properties such as structural versatility,designability,high specific surface area and porosity.Using the physical and chemical properties of MOFs,MOFs can be used to embed metals or metal oxides into the carbon matrix by pyrolysis,reducing catalyst sintering and obtaining highly stable catalytic materials.In this paper,Fe-MIL-88B was prepared by solvothermal method as a precursor template,and Fe-MIL-88B was synthesized by pyrolysis precursor into MOFs-derived Fe-based catalysts doped with different transition metal promotes.The catalysts were characterized by XRD,SEM,TEM,XPS and M（?）ssbauer spectrum.The effect of transition metal promotes on the structure of MOFs-derived iron-based catalysts and the reaction performance of CO₂ hydrogenation to olefins was studied.Based on the structure-effect relationship,the mechanism of CO₂ hydrogenation to olefin in doped Fe-based MOFs catalysts was explored.The research content of this paper is as follows:（1）Using metal salt as metal source and terephthalic acid as organic ligand source,the metal was introduced,and the transition metal promotes doped MFe/C was prepared by solvothermal method.The results showed that Cu,Zn and Mn promoted the carbonization of Fe species,which was conducive to the formation of active phases of Fe₅C₂ promoted C-C coupling and inhibited the secondary hydrogenation of olefins.While Zn promoted the secondary hydrogenation of olefin with enhanced hydrogenation ability.Although the introduction of Ni improves the conversion of CO₂,the strong interaction between Ni and Fe species inhibits the carbonization of Ni Fe/C catalysts,preventing the formation of Fe₅C₂ and further C-C coupling,as well as hydrogenation of CO intermediates.Combined with M（?）ssbauer spectroscopy,the content of specific Fe sites in Fe₅C₂（sites II and III）was positively correlated with the ratio of alkenes to alkanes.Finally,we explored the effect of reaction conditions on the CO₂ hydrogenation performance of Cu Fe/C.The results showed that the conversion rate of CO₂ was consistent with the trend of temperature and pressure,which first increased and then decreased,while the low carbon olefin first decreased and then increased.The reaction temperature had no significant effect on the selectivity of C₅⁺,with the increase of pressure,C₅⁺ rises first,reaches its maximum value at 2 MPa,and then decreases.The optimal reaction conditions were 400 ℃,2 MPa and 12000 m L h^-1g_cat^-1.（2）A series of x Cu Fe Na/C catalysts doped with Na and Cu were prepared by constant volume impregnation method.The effects of Na and Cu content on the performance of CO₂ hydrosynthesis of olefins were studied.The results showed that CO₂ conversion and olefins selectivity increased significantly with the introduction of Na.Meanwhile,the addition of Na promoted the formation of active species Fe₅C₂ and the reduction of Fe during Fe-MIL-88 B pyrolysis.With increasing Cu content,the selectivity of light olefins first increased and then decreased,achieving 19.07% at 0.1 wt.% Cu;it also increased the selectivity of C₅⁺,but had less effect on the conversion of CO₂.In addition,the addition of Cu promoter facilitated the production of the active species Fe₅C₂.At the same time,the interaction between the Cu promoter and the Fe species enhanced the C-C coupling reaction and improved the selectivity of the olefins.