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

CO2 hydrogenation to light olefins is one of an effective and valuable progress for CO2 utilization.Generally speaking,reverse water gas shift reaction(RWGS)and Fischer-Tropsch synthesis are the main progress in CO2 hydrogenation to light olefins.Therefore,it is necessary to design a bifunctional catalyst to activate CO2.Fe-based catalysts are frequently reported in literatures.The structure,morphology,and reduction condition of catalyst sample have significant influence on the catalytic performance.Meanwhile,the structure is highly influenced by the preparation methods.In this work,preparation methods,catalyst structure and reduction condition are studied on C02 hydrogenation to olefins.K/Fe-Zn catalyst samples are prepared by coprecipitation method,hydrothermal method,solvothermal method respectively.The characterization results indicated that the phase structure and morphology of the samples are significantly affected by the synthesis methods.Catalytic activity is carried out in a fixed bed reactor,which confirmed that catalytic performance is of difference.Furthermore,surface basicity is the highest in C-2Fe-1Zn/K with highest CO2 conversion at 54.8%and light olefins selectivityof 57.4%.BET surface area is the over S-2Fe-1Zn/K with O/P of 6.28.Lower CH4 is obtained over H-2Fe-1Zn/K at 22.3%.The readsorption of the generated olefins leads to second adsorption and causes undesired secondary olefin reaction such as polymerization,isomerization,and other side reactions.Product distribution is influenced by secondary olefin reaction.In order to restrain the side reaction,the component,ratio,surface basicity and morphology are tuned.Layered structure materials have a relative shorter transmission distance and duct benefiting to the adsorption and diffusion of reactant and product compared with granule structure catalyst.It is benefited to CO2,H2 adsorption and product desorption for products.Diffusion restriction probably decreased to a certain extent.A series of K-Fe-Ti layered structure catalyst is prepared by solid-state reaction process.K-Fe-Ti molar ratio is investigated in CO2 hydrogenation to light olefin reaction.When the K/Fe/Ti ratio is 0.8/2.4/1.3,the catalyst exhibited the highest olefin selectivity to 59.3%with O/P of 7.3.What's more,the layered structure is retained during the reaction.High olefin selectivity and stability are obtained over the K-Fe-Ti layered structure catalyst.Unexpected high CO selectivity of about 70%could be observed due to higher content of potassium,which will accelerate the RWGS to generate a large amount of CO during the reaction.As is known that Fe3O4 and FesC2 are responsible for the CO2 hydrogenation reaction,the ratio of Fe3O4 and FexCy will affect the CO formation and re-consumption.Thus,we intend to alter the reduction atmosphere by employing CO and syngas(H2/CO = 2).The experiment data has demonstrated that the reduction of CO and syngas(H2/CO = 2)decrease the CO selectivity with high olefin selectivity.Acid treatment also changes the distribution of product by decreasing the K content in the catalyst sample,which is contributed to the inhibition of RWGS and then enhances the hydrocarbon selectivity.