| China’s carbon emission task is arduous,and the utilization of CO2 resources has important practical significance and broad application prospects.Among them,hydrogenation of CO2 to produce high valueadded products is one of the effective ways to utilize CO2.A series of progress has been made in the research on short-chain products such as methane,methanol,formic acid and light olefins by hydrogenation of CO2.However,due to its high thermodynamic stability and C-C coupling energy barrier,the selective conversion of CO2 to heavy hydrocarbons is still a huge challenge.Coupling "metal catalyst"and "zeolite" can effectively increase the content of heavy hydrocarbon in the product.However,it is still difficult to obtain the desired selectivity of high-carbon hydrocarbons while reducing the CO selectivity.In addition,key mechanisms,such as C-O bond fracture and C-C bond formation,as well as the exact structure and synergistic mechanism of multiple active centers,need to be further studied.In response to the above problems,a series of K-Fe3O4/Ni-zeolite(ZSM-5,β,MCM-22)catalysts which couple the two processes of CO2 hydrogenation to synthesize light olefins on metal oxides and light olefins oligomerization,isomerization,and aromatization on zeolite were designed to directly produce heavy hydrocarbons such as gasoline and aromatics by catalyzing the hydrogenation of CO2.Combined with XRD,SEM-mapping,ICP-OES,BET,H2-TPR,CO2-TPD,NH3-TPD,Py-IR,XPS,TPSR-MS,in situ DRIFTS,GC-MS and other characterization methods,the reaction performance of tandem catalyst for CO2 hydrogenation and the reaction mechanism of F-T reaction primary products on Ni modified zeolite were studied,and the influence law and action mechanism of the product distribution of Ni-zeolite catalyst were mainly explored.The main research contents and results are as follows:1.Fe3O4 catalysts with uniform morphology were prepared by solvothermal method.The results show that using alkaline promoter K to modify Fe3O4 to catalyze the hydrogenation of CO2 can improve the CO2 conversion and selectivity of low olefin.In the hydrogenation of CO2,A 34.3%CO2 conversion rate and 27.9%C2-3 olefins selectivity were obtained,and the catalytic performance was stable.No deactivation occurred after 100h of reaction.2.Fe3O4 catalyst and Ni modified zeolite were connected in tandem.The effects of reaction temperature,topological structure of zeolite,Ni loading capacity,SiO2/Al2O3 ratio of zeolite,Ni loading method,and the combination of the two materials on the catalytic performance of CO2 hydrogenation were investigated.The results showed that:(1)The acidity of zeolite has a significant effect on its catalytic performance.Ni modification weakened the cracking ability of the Bronsted active acid site and improved the dehydrogenation and aromatization properties of the Lewis acid site.Meanwhile,the addition of Ni provides more mesoporous structure,showing the shape selection effect on heavy hydrocarbons.On K-Fe3O4/1%Ni-ZSM-5(100)catalyst,the C4+selectivity increased from 51.9%to 74.7%,the CH4 conversion rate decreased from 15.2%to 10%,and the CO2 conversion rate was 38.2%.(2)The proximity of the two active centers of the tandem catalyst is not conducive to the conversion of CO2 and the generation of heavy hydrocarbons.Filling the two active centers of the catalyst into a tandem reactor can prevent the interaction between the two components and better control the optimum process conditions for each step of the tandem reaction.3.The reaction mechanism and regularities of CO2-FTS primary products(light olefins rich in H2)on zeolites were investigated by TPSR-Mass,in situ DRIFTS and GC-MS.The results showed that:(1)TPSR-MS experiment showed that the increase of total acid and Lewis acid after Ni loading enhanced the oligomerization,isomerization,aromatization and hydrogen transfer rate of light olefin.Increasing the pressure will accelerate the mass transfer efficiency and increase the raw material conversion rate,which is conducive to the oligomerization reaction of light olefin to produce heavy hydrocarbons,but not conducive to the hydrocracking reaction of heavy hydrocarbons.(2)In situ DRIFTS experiment investigated the transformation process of light olefin to aromatic,that is,light olefin as the reaction intermediate,through oligomerization,cyclamation,dehydrogenation and aromatization of light olefin to generate toluene,xylene and other aromatic hydrocarbons.(3)GC-MS was used to analyze the composition of the oil samples obtained from the tandem catalytic hydrogenation of CO2 by K-Fe3O4/Ni-ZSM-5.Aromatics are the main products in the oil samples of K-Fe3O4/Ni-ZSM-5(27,ie)or Ni-ZSM-5(100),reaching 60.78%and 69.09%of the total oil samples,respectively.In the total aromatics of Ni-ZSM-5(27,ie),light aromatic hydrocarbons accounted for 50.48%,while the aromatics ofNi-ZSM-5(100)are dominated by heavy aromatics,which account for 73%of the total aromatics. |
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