Structural Regulation Of Fe-based Catalyst By Support/Promoter And Their Consequeneces On CO₂ Hydrogenation To Olefins

The China and world energy supply mainly relies on fossil energy such as coal,oil,natural gas,and the use of fossil energy inevitably produces a large amount of carbon dioxide.Facing the increasingly severe pressure to reduce emissions and the huge carbon emissions trading market,how to efficiently utilize carbon dioxide resources is a hot spot of C1 chemistry research.The development of high-efficiency CO₂ conversion and utilization technologies is great significance to China sustainable development strategy and emission reduction tasks.The hydrogenation of CO₂ to olefins by iron-based catalyst is an attractive route.It can not only obtain high-value basic organic chemicals,but also reduce greenhouse gas emissions and dependence on oil imports.However,the iron-based catalysts suffer from low structural strength,high by-product selectivity and low olefins yield during the high-temperature and high-pressure reactions,which become central challenges for the industrial production of CO₂ hydrogenation to olefins.Therefore,this paper separately investigated the effects of support and metal oxide promoter on the structure of iron-based catalysts and their catalytic performance for CO₂ hydrogenation to olefins.Firstly,a series of Na-promoted supported Fe-based catalysts were prepared by a two-step impregnation method to investigate the effect of support materials on the structure of Fe-based catalysts and their consequences on CO₂ hydrogenation to olefins.Various off-line and in-situ characterization（in situ XRD,in situ FTIR,in situ Raman,XPS,M（？）ssbauer spectra,etc.）revealed that the support can modulate the ratio of Fe O_x and Fe C_x in the bulk phase of iron-based catalysts,induce the composition of iron carbides,and at the same time affect the orderliness of carbon species on the catalyst surface during the CO₂ hydrogenation reaction,which in turn mutually modulates the adsorption capacity of CO₂,H₂ and intermediates.The ordered carbon-rich surface is beneficial to the formation and stability of Fe₂C₅,increases the adsorption of CO₂ and decreases the adsorption of H₂,which is beneficial to the formation of CH_x^*intermediate and inhibits the secondary hydrogenation of olefins.The catalyst evaluation results showed that the Fe Na/Zr O₂catalyst exhibited the highest C₂-C₇ olefins selectivity and O/P ratio of 65.2%and 8.1,respectively,at 320°C,2.0 MPa,9000 ml g^-1_cat h^-1 and H₂/CO₂=3.Secondly,Na-promoted Cu and Ga metal promoter modified Fe-based catalysts were prepared by a two-step impregnation method to improve the yield of olefins and inhibit the formation of CO as a by-product.The addition of Cu and Ga promoted the reduction of iron-based catalysts,which is conducive to the formation of iron.During the reaction,the formation of the reversed water gas shift Fe₃O₄ active phase was inhibited and the Fe₅C₂ active phase was stabilized during the reaction.At the same time,the surface basicity of the catalyst was improved,which enhanced the adsorption of CO₂ and CO,and increased the adsorption sites of CO,thus reducing the selectivity of the by-product CO and improving the yield of the target product olefins.The catalyst evaluation results showed that the selectivity of CO decreased from 26.0%（Na/Fe₂O₃）to 16.9%（Ga Na/Fe₂O₃）,and the space time yield of C₂-C₇olefins increased from 133.6 g kg^-1_cat h^-1 to 166.7 g kg^-1_cat h^-1 with a selectivity of C₂-C₇ olefins>70.0%.