Fe-based Bifunctional Catalysts For Direct Conversion Of Syngas To Lower Olefins

Lower olefins are basic building blocks in the chemical industry,which are produced normally by steam cracking of naphtha.Currently,direct conversion of syngas to lower olefins via Fischer-Tropsch synthesis is a green process.Recently,how to improve the selectivity of lower olefins while maintaining the high activity of catalyst is an urgent problem.From the perspective of improving catalyst activity,alkaline earth metal（Ca as an example）was introduced on the basis of FeK/Al₂O₃ catalyst to study its mechanism of action,which will clarify its promoter property.Then,in order to improve the selectivity of lower olefins,the Fe-based catalyst was coupled with zeolite.The the structure-function relationship of bifunctional catalysts will be studied from the aspects of preparation of bifunctional catalysts,shape-selectivity,spatial confinement,catalysis cracking.We prepared supported FeKCa/Al₂O₃ catalyst on basis of FeK/Al₂O₃ catalyst.A maximum of CO conversion,FTY and FTY(C⁼_2-4)and better stability is achieved at1.0 wt.%Ca.Further increasing Ca loading resulted in a decline in activity due to covering active sites.XPS,Raman and H₂-TPR show that the electron property of catalyst has not been changed by Ca promoter.In addition,the results of XRD,TEM and CO chemisorption show that Ca promotes dispersion of iron species thus leads to the much higher catalytic performance.Notably,the nearly constant TOF values and product distribution before and after addition of Ca,which indicates that Ca mainly plays the role of structural promoter.The FTO catalyst FeKS/Al₂O₃ previously studied by our group and SAPO-34zeolite were mixed to prepare bifunctional catalysts,and the catalytic cracking of long-chain hydrocarbons by acidic site was used to improve the selectivity of lower olefins.The optimum process condition,the optimal loading method and mass ratio of the two components were confirmed.SAPO-34 with 0.55 mmol/gcat strong acid showed the best catalytic performance:the selectivity of low olefins was significantly improved under similar CO conversion.This is due to the catalytic cracking of long chain hydrocarbons by the strong acid sites.Excessive amount of strong acid tends to aromatization and polymerization of olefins,which increases the selectivity of C₅₊.We synthesized a micron-sized catalyst with a core（FeMnK/Al₂O₃ powder）-shell（Silicalite-2）structure by combination of Str?ber method and hydrothermal crystallization method.The encapsulated catalyst with S2 zeolite shell showed an excellent catalytic activity for improving selectivity of lower olefins meanwhile suppressing long-chain hydrocarbons formation,which benefits from the shape selectivity of S2 microporous structure.The alkaline treatment can remove residual Si O₂,which significantly weakens the diffusion limitation and promoting the reduction,resulting in an improvement of the catalytic performance.In order to give full play to the confinement effect and eliminate the diffusion restriction,we prepared the hollow zeolite encapsulating iron particle catalyst.The hollow catalyst Fe@S2 has been synthesized by two steps which involves a one-pot hydrothermal synthesis and a recrystallization accompanied by hydrolysis.The results show that the iron particle size is～4 nm.Raman and UV/vis spectra proved that Fe inserted the framework of S2 and then was removed to form Fe particles.However,the Fe was not completely removed from framework,and there was a strong acid site in Fe@S2.H₂-TPR results showed that the presence of hollow shell promoted the reduction of Fe species.After eliminating the diffusion limitation,lower olefins selectivity of Fe@S2 was significantly improved,which ascribes to the shape-selectivity,confinement effect and catalytic cracking of S2 shell.In addition,the FTY_olefins value had obvious advantages over the supported catalysts and still showed a significant increase after 30 h.