Surface Modification Of Fe-based Catalyst And Its Effect On The Product Distribution In Fischer-Tropsch Synthesis

The production of basic chemicals such as olefins,oxygenated compounds and other chemicals from syngas produced from coal,petroleum and biomass,is of great interest from both an academic and industrial point of view.Recently,the direct production of lower olefins from syngas through Fischer-Tropsch synthesis,so-called Fischer-Tropsch to olefins(FTO)process,has attracted wide interests from many research groups.As is known that the product distribution of FT reactions follows the Anderson-Schulz-Flory(ASF)equation,therefore,the main problems need to be solved are to control the product distribution,improve the selectivity of light olefins and reduce the by-product formation.Compared with Co and Ru based catalysts,Fe based catalysts have attracted more and more attention due to their low cost,high olefin selectivity and high reactivity.The key scientific problems to be solved are the regulation of product distribution,the inhibition of secondary reaction of olefins,and the improvement of selectivity of target products.The olefin selectivity is significantly affected by secondary reactions,such as secondary hydrogenation,isomerization and cracking.In addition,the strong water gas shift reaction(WGS)activity of Fe based catalyst results in high CO2 selectivity.In this study,based on the surface modification of Fe based catalyst for FT synthesis,mechanical mixing method and hydrothermal coating method are used to realize the composite of hydrophilic precursor and Fe active component.By changing the surface affinity of Fe based catalyst,the reabsorption of primary olefin and the adsorption and diffusion behavior of H2O are improved to realize the improvement of product distribution.Combined with olefin adsorption desorption,N2 adsorption and desorption,XRD,SEM,TEM,contact angle(CA),TG-DSC,FT-IR,XPS,H2-TPR and CO-TPD measurements,the catalytic performances of the catalysts were studied and the influence of surface modification and mechanism of surface modification on product distribution of Fe based catalyst were explored.The main research contents and results are as follows:(1)The hydrophilic modified Fe3O4-CTS catalyst was prepared by mechanical grinding with chitosan(CTS)as promoter.The results showed that the size of active component affected the catalytic performance of the catalyst.Compared with Fe3O4,the catalytic performance of the catalyst changed obviously,and the hydrophilicity of the catalyst was enhanced over Fe3O4-CTS-500 catalyst samples.Fe3O4 particles are uniformly attached to the surface of CTS microspheres without agglomeration,and the structure is intact and the thermal stability is good.The selectivity of light olefins is significantly improved and the C2O-C40 products are reduced without adding other additives.The catalysts showed good stability within 120 h.The methane and C2O-C40 selectivity of Fe3O4-CTS-500 catalyst samples are slightly higher than that of Fe3O4-CTS-50 catalyst samples,while the C5+selectivity is reduced.The C2=-C4=selectivity of Fe3O4-CTS-500 catalyst samples is 35.24%,and the O/P value is 2.15.Catalysts with small particle size is beneficial to inhibit the secondary reaction and promote the generation of heavy hydrocarbons.(2)Polyimide(PI)modified Fe3O4@PI and Fe3O4-PI catalyst samples were prepared by hydrothermal-coating and ball milling methods,respectively.The effect of PI modification on the product distribution of Fe-based catalysts in Fischer-Tropsch synthesis was investigated.The catalyst samples were characterized by SEM,TEM,XRD,H2-TPR,CO-TPD,XPS,TG and contact angle measurements.The results showed that Fe3O4,Fe3O4@PI and Fe3O4-PI were all spherical with uniform particle size,and Fe3O4@Pl particles were smaller.PI modification promoted the reduction and hydrophilicity of Fe3O4.For Fe3O4@PI sample,PI was uniformly coated on Fe3O4 surface,which has good thermal stability.Compared with Fe3O4 and Fe3O4-PI samples,CO adsorption was promoted on Fe3O4@PI with coated structure.During CO hydrogenation,compared with Fe3O4,the catalytic activity of Fe3O4@PI and Fe3O4-PI decreased,and the secondary hydrogenation ability was inhibited,resulting in an increase of olefin selectivity.For Fe3O4@PI sample,the olefin selectivity was enhanced significantly with an olefin to alkane ratio in C2-C4 fraction increased from 0.50 to 2.15.Also,PI modification with suitable content was beneficial to the formation of C5+ hydrocarbons.