Study On The Catalytic Performance And Mechanism Of Iron-based Catalysts For CO₂ Hydrogenation To Light Olefins

In recent years,with the consumption of fossil resources and the massive emission of carbon dioxide（CO₂）,more and more attention has been paid to greenhouse gas and increasing energy demand.Direct synthesis of light olefins by hydrogenation of CO₂ has a broad prospect,it can not only achieve CO₂ emission reduction and CO₂ conversion,but also alleviate energy crisis and greenhouse effect.Iron-based catalysts has been widely used for CO₂ hydrogenation to synthesis light olefins.The synthesis of light olefins could be effectively promoted by adding transition metals,alkali metals and supports on iron oxide,but its promotion mechanism and reaction mechanism need to be studied for further.At the same time,since the selectivity of C₂-C₄ hydrocarbons in the catalytic product follow by the Anderson-Schultz-Flory（ASF）distribution,it is difficult to further increase the yield of light olefins of the iron-based catalysts through traditional modification methods.For these problems,the aim of this paper is improving the efficiency of iron-based catalysts for the producing of light olefins,and focusing on the preparation of the carriers of zirconia（ZrO₂）,potassium（K）promoter and the introduction of SAPO-34 molecular sieves.The main contents list as follows:First,ZrO₂ was prepared by hydrothermal,precipitation,sol-gel and direct calcination methods,and FeCoK/ZrO₂ catalyst was obtained by impregnation and using it as a support.The effect of CO₂ hydrogenation performance for support with different preparation method was studied.The results show that the mainly crystal form of ZrO₂ prepared by direct calcination,hydrothermal and precipitation methods is monoclinic,while ZrO₂ prepared by sol-gel method contains both monoclinic and tetragonal crystals.The activity test results show that the mixed crystal ZrO₂ prepared by the sol-gel method is not beneficial to the CO₂ adsorption,and the reaction activity is low.At the same time,FeCo/ZrO₂ and FeCoK/ZrO₂ catalysts were prepared with ZrO₂ obtained by direct calcination method as the support.The influence of K content on the catalytic activity of CO₂ hydrogenation reaction was investigated.The results showed that the addition of K increased the electron cloud density of iron species.Enhances the adsorption of CO₂ on the surface of Fe species,promotes the formation of iron carbide（χ-Fe₅C₂）,and optimizes the path of CO₂ adsorption and activation,thereby increasing CO₂ conversion and the selectivity of light olefins.And the space-time yield（STY）increased from 0.013 mmol·g_cat^-1·h^-1 to 4.11 mmol·g_cat^-1·h^-1.Secondly,the FeZnK catalyst was prepared by the precipitation and the impregnation method,and the FeZnK/SAPO-34 catalyst was prepared by the three integrated methods of dual-bed,granule stacking and physical mixing.The performance of CO₂ hydrogenation reaction for the above three integrated methods was investigated.The results show that the close contact between FeZnK and SAPO-34 in the physically mixed catalyst,leading to K ion exchanges with SAPO-34 at high temperature,which reduces the alkalinity of FeZnK and the acidity of SAPO-34,thereby suppressing the formation ofχ-Fe₅C₂,reduces the yield of low-carbon olefins.A trace of ion exchange was detected in granule-stacking catalyst,and the acidity and alkalinity have almost no change.At the same time,due to the cracking ability of SAPO-34,the ASF distribution of hydrocarbons in the products of FeZnK was broken,and the STY of light olefins increased from 4.29 mmol·g_cat^-1·h^-1 to 5.05 mmol·g_cat^-1·h^-1,and the catalyst shows good reaction stability.Finally,FeCoK,FeMnK and FeZnK catalysts were prepared by precipitation and impreganation method,and FeMK/SAPO-34（M=Co,Mn and Zn）catalysts were prepared by granule-stacking method.The catalytic reaction performance test results show that due to the cracking effect of hydrocarbons by SAPO-34,the selectivity of ethylene is significantly improved,and the order of the promotion effect of the reaction performance for three transition metals on FeMK/SAPO-34（M=Co,Mn and Zn）catalysts is Zn>Co>Mn.At the same time,the effect of SAPO-34 with different acidity on the reaction performance of FeZnK/SAPO-34was investigated.The results showed that the increase of SAPO-34 acidity is beneficial to the cracking of long-chain hydrocarbons into short-chain hydrocarbons.However,excessive acidity will aggravate the ion exchange between K in FeZnK and SAPO-34,reduce the alkalinity of FeZnK,and increase the secondary hydrogenation of olefins.Therefore,moderately acidic SAPO-34 is more conducive to the production of light olefins.