Design And Synthesis Of Amphiphobic/hydrophilic Iron Based Catalysis With Tuning Product Distribution Of Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis(FTS)is one of the important ways to produce high value-added chemicals from non-petroleum routes.Based on the energy characteristics of China's "rich coal,lack oil and miss gap",the development of Fischer-Tropsch synthesis and chemical processing technology is of great significance for the balanced use of national resources,alleviating the shortage of China's petroleum resources and ensuring national energy strategy security.Fe-based catalysts have attracted much attention due to their low cost,high catalytic activity,wide temperature range,and high olefin selectivity.However,Fe-based catalyst exhibited higher water gas shift(WGS)activity,resulting in the formation of a large amount of CO2.Meanwhile,the hydrocarbon products is limited by the Anderson-Schulz-Flory(ASF)distribution.Its application remains scant due to wide hydrocarbon distribution followed by ASF limitation.Improvement on the selectivity of target products and inhibition of by-products such as CO2 are still a research hotspot and technical difficulty in this process.Olefins are the primary product in FTS.Secondary reactions such as hydrogenation,isomerization and chain growth will happen by readsorption of primary olefins during FTS,resulting in a decrease in olefin selectivity.Also,it is known that the formation of CO2 is related closely to activity of Fe catalyst for the WGS.Therefore,regulating the iron catalyst activity for secondary reaction of olefins and for the WGS reaction would tune the product distribution of FTS.Hydrophobic or hydrophilic properties of catalyst surface can change the adsorption behaviors of products of FTS such as light olefins and water and hence affect their distribution.Catalysts with amphiphobicity will act well for both inhibiting the WGS reaction to produce CO2 and improving olefin selectivity.Based on this,Fe-based catalysts with amphiphobic and hydrophilic properties were designed,combined with N2 physical adsorption,XRD,SEM-EDX,TEM,contact angle,TG-DSC,FT-IR,XPS,H2-TPR,CO-TPD measurements,the physical and chemical properties of the catalysts before and after reaction and its catalytic performances in CO hydrogenation were extensively studied.,Structure-activity relationship of the modified catalyst is discussed in detail..The results are as follows.(1)Fe3O4@SiO2-PFTS catalyst with amphiphobic properties was prepared by hydrothermal method,stober method,and silylation reaction.The effects of Fe3O4,Fe3O4@SiO2 and Fe3O4@SiO2-PFTS catalysts on CO hydrogenation and product distribution were investigated.The results showed that modified Fe3O4@SiO2 and Fe3O4@SiO2-PFTS catalysts exhibited lower CO hydrogenation activity compared with that of Fe3O4.The amphiphobic surface of Fe3O4@SiO2-PFTS reduced evidently the production of CO2 from 44%of selectivity to 12%ascribed to suppression against the water gas shift(WGS)reaction.After modification by coating SiO2 and PFTS,the pore size of the catalyst becomes smaller and the steric hindrance effect increases,resulting in the enrichment of the primary hydrocarbon products on the surface of the catalyst,which then intensified the secondary reaction of primary olefins and decreased the selectivity.The catalysts showed high structure stability with time on stream of 120 h and it still had strong hydrophobicity.(2)Fe3O4/HEC catalyst by hydroxyethyl cellulose(HEC)modification has a significantly enhanced hydrophilicity compared with that of Fe3O4.The results showed that hydrophilicity modification has great influence on the product distribution.Without modification by alkaline additives,the olefin selectivity on Fe3O4/HEC catalyst was greatly improved,and the C2=-C4=/C20-C40(O/P)value is significantly increased from 0.58 to 4.79.Also,the heavy hydrocarbon content is increased from 14.14%to 43.27%during CO hydrogenation.the selectivity of CH4 over Fe3O4/HEC catalyst decreased by 13.14%,and the selectivity of C2=-C4= was increased by 13.59%.It can be concluded that hydrophilic modified catalyst is beneficial to the formation of light olefins and increase of chain propagation.The preparation method of the catalyst is simple and reproducible.The catalyst maintains good stability within 120h.