Design And Preparation Of Fe-based/Ni-AlMCM-41 Bifunctional Catalyst And Its Catalytic Performance For CO₂ Hydrogenation To Heavy Hydrocarbons

The hydrogenation of CO2 to produce higher-carbon hydrocarbons(C5+)can both relieve the pressure of CO2 emissions and is expected to realize its high-value utilization.In recent years,a bifunctional catalyst composed of "metal oxide" and "molecular sieve" has been used to achieve selective preparation of high-value hydrocarbons by CO2 hydrogenation.First,CO2 undergoes RWGS reaction at the active site of the metal oxide to form CO,and then CO pass FTS reaction to form light olefins,followed by the light olefins migrate and diffuse to the acid sites of the molecular sieve for oligomerization,hydrogenation,and isomerization,so as to achieve high selectivity for CO2 hydrogenation to produce heavy hydrocarbons.In this paper,Fe-based/Ni-AlMCM-41 bifunctional catalyst was designed and prepared,and its performance in CO2 hydrogenation to heavy hydrocarbon was tested in a fixed-bed reactor.Combined with XRD,H2-TPR,NH3-TPD,Olefins-TPD-Mass,XPS,SEM,TEM and other characterization methods explored the effect of catalyst structure,surface properties,etc.on the performance of CO2 hydrogenation reaction.The main research contents and results are as follows.(1)The Fe3O4 catalyst with uniform morphology was prepared by solvothermal method.The CO2 hydrogenation reaction performance was investigated through the modification of Na and K additives and the reduction of CO+H2,CO2+H2,and high purity H2.Experiments show that when the Fe3O4 catalyst modified with K additive and reduced with high purity H2 is used in the CO2 hydrogenation reaction,it has a strong ability to regulate and control the reaction product and shows a high CO2 conversion rate and light olefins selectivity.Whereafter,the amount of K additives modified on the Fe3O4 catalyst was investigated.The Fe-based catalyst with a mass content of 0.5%K after the reduction of high purity H2 was applied to the CO2 hydrogenation reaction under the conditions of 320?,2 MPa,1000 h-1,H2/CO2=3,the conversion rate of CO2 was 30.9%,the selectivity of CO was 16.6%,the selectivity of CH4 was 20.5%,the selectivity of light olefins was 51.5%,and the yield of heavy hydrocarbon was 3.1%.(2)Ni-AlMCM-41 mesoporous silica-aluminum material with regular pore structure and weak acid,capable of providing active sites for oligomerization,hydrogenation and isomerization of low carbon alkenes,was prepared by the commercial MCM-41 mesoporous material modified with Ni and Al element.Fe/Ni-ALMCM-41 bifunctional catalysts were combined in a section of stratified combination and two sections in series,respectively,to compare the performance of CO2 catalytic hydrogenation.The experiment shows that when Fe and Ni-AlMCM-41 adopt a layered combination for CO2 hydrogenation,the combination distance between the two catalysts is too close.As a result,the acid-base molecules of different active components migrate between each other,thereby causing catalyst deactivation.The influence of acid content on the hydrogenation performance of CO2 was explored by changing the ratio of Si/Al in Ni-AlMCM-41 to adjust acid content.The results showed that when 0.5wt%K-Fe3O4 was connected in series with the Ni-ALMCM-41 mesoporous material(Si/Al=50),the conversion rate of CO2 was 32.9%,the selectivity of CO was 7.1%,the selectivity of CH4 was 10.9%,and the yield of heavy hydrocarbon can reach 15.2%.Compared with 0.5wt%K-Fe3O4,the yield of heavy hydrocarbons increased from 3.1%to 15.2%,indicating that the bifunctional catalyst system realized CO2 hydrogenation to produce heavy hydrocarbons,and showed a higher yield of heavy hydrocarbons.