Preparation Of Iron-based Catalysts For CO₂ Hydrogenation To Low-carbon Olefins

From the beginning of the 20th century,human society and economy have made rapid progress at the expense of large consumption of fossil energy.The increasing demand for low-carbon olefins produced by petroleum cracking result in the serious oil shortage around the world and the increasing levels of CO₂ in the atmosphere.The low-carbon olefins produced by CO₂ hydrogenation technique can alleviate the shortage of resources and reducing the CO₂ content in the atmosphere,which is beneficial to the improvement of the global ecological environment.This paper focuses on the preparation of iron-based catalysts used for the synthesis of low-carbon olefins via CO₂ hydrogenation technique.Two methods based on the aqueous phase reduction method are used to prepare iron-based catalysts.On the one hand,the aqueous phase reduction method was performed to prepare the catalyst precursor and then the auxiliary agent was loaded via equal volume impregnation method.On the other hand,the"one-pot method"was conducted to prepare catalyst by mixing auxiliary agent,iron salt and sucrose directly.Firstly,the effects of adding additives in different ways on the preparation of iron-based catalysts and their CO₂ hydrogenation performance were studied,and the catalysts were characterized by N₂ physical adsorption,XRD,H₂-TPR,SEM and other instruments.At the same time,the surface additive content of the catalyst prepared by the simultaneous addition of K and Cu additives in the"one pot method"was simulated.The experimental results showed that the performance of the catalyst prepared by the"one-pot method"was better than that of the"two-step method",which was attributed to the catalyst K prepared in the"one-pot method"entered the?-Fe₂O₃ lattice.Since the radius of K⁺is much larger than that of Fe³⁺,the diffraction peak of?-Fe₂O₃ shift to a small angle,and the crystal plane of?-Fe₂O₃ is distorted or even broken to form more defect sites,which is contributed to the reaction.Under the evaluation conditions of reaction conditions of 6000m L/?h·g-cat.?,1.60 MPa,and 235?,the iron-based catalyst with only K promoter had the highest CO₂ conversion rate?28.2%?,which was much higher than the catalysts prepared by simultaneously adding K and Cu promoter,equal volume impregnated K and Cu promoter,and without K and Cu promoter.Secondly,a series of catalysts with different K auxiliary contents were prepared by the"one-pot method".The study found that when the content of the K additive in the"one-pot method"is above K/Fe₂O₃=0.25wt%,the crystal form of the catalyst?-Fe₂O₃ will be moved to a small angle.This crystal form angle shift was caused by the K entered the Fe₂O₃ crystal lattice,which increased the interplanar spacing.The schematic diagram was constructed according to this conclusion.Finally,considering the similar properties of K and Na additives,the catalysts with different contents of Na additives were prepared by"one-pot method"for research.The study found that when Na/Fe₂O₃=0.5wt%,the selectivity of low-carbon olefins is as high as 32.6%,and the O/P is as high as 3.2.Considering that the K additive has a significant improvement in the conversion rate of CO₂,combined with the different improvement effects of the two additives,it is studied to use the"one-pot method"to add K and Na at the same time.The results show that adding K and Na at the same time not only improves the conversion rate of CO₂,but also significantly improves the selectivity of low-carbon olefins;compared with the sample without additives,the yield of low-carbon olefins is improved 4.9%.