Study Of Fe-Mg Catalyst For Direct Production Of Lower Olefins Via Fischer-Tropsch Synthesis

Lower olefins?C₂^C₄?are the key building blocks of the chemical industry.They are traditionally produced by steam cracking of naphtha mainly.With the diminishing of oil resources,direct producing of lower olefins via Fischer-Tropsch synthesis from syngas derived from coal,or renewable biomass has been an important alternative route.Because of wide applicable temperature rang,low cost,high activity and lower olefins selectivity,iron-based catalyst has attracted wide attention.In this paper,the effects of preparation methods,active phase contents,promoters,calcination conditions and reaction conditions on the FTO performance have been studied on Fe-Mg system catalysts.Catalysts characterization containing H₂-TPR,CO₂-TPD,BET,TG/DTG,XRD/In-situ XRD,FE-SEM and XPS are carried out to investigate physical and chemical properties of the catalysts.The main research contents and results are as follows:Comparing the Fe-Mg catalyst prepared by co-precipitation and incipient wetness impregnation,the results show that stronger interaction betweent active component and MgO in the catalyst prepared by co-precipitation resulted in relatively poor reduction performance,but could obtain finer ?-Fe particle size.FTO performance shows that the catalyst activity of the catalyst prepared by co-precipitation and impregnation are not very different,but the catalyst prepared by co-precipitation has higher selectivity to light olefins and lower paraffins selectivity.After reaching 15wt%,the effect of Fe content on lower olefins selectivity is not obvious.Then,the promotional effects of transition metals of Cr?Mn?Zr?Ti and Zn on the Fe-Mg catalysts were investigated.It's found that Cr and Mn can significantly enhance the lower olefins selectivity,which is related to both of them improving surface basicity of Fe-Mg catalyst.But Cr promoted catalyst has higher CH₄ and light alkanes selectivity.Fe-Mg catalyst modified by Mn promoter can increase the selectivity of lower olefins,while inhibiting the formation of methane and low carbon alkanes.The best FTO performance is obtained when the Mn/Fe molar ratio of Fe-Mn-Mg catalyst is 0.2.But addition of excess Mn can significantly reduce the activity of the catalyst and the selectivity of lower olefins.Among the promoters,addition of Zn promoter significantly reduces the selectivity of lower olefins,which is related to the reduction of the catalyst surface basicity,though increasing catalystic activityTo improve the performance of the catalyst reduction,Cu is used to modifying Fe-Mn-Mg catalyst.When the Cu/Fe molar ratio is 0.02,the catalyst has the best FTO performance.The excess of Cu could reduce the selectivity of the lower olefins,C₂^C₄ olefin to paraffin ratio,and increase methane selectivity.The effects of calcination temperature on the Fe-Mn-Mg catalyst were studied.The results shows that the best calcination temperature was 600 ?.Low calcination temperature can decrease catalytic activity,but higher calcination temperature lead to forming MgFe₂O₄ and Mg₆MnO₈ phases,which could inhibit catalyst reduction and weaken the effect of Mn promoter on the catalyst.At last,the effects of different reaction conditions on the FTO performance of Fe-Mn-Mg catalyst prepared with best calcination temperature were studied.The results shows that as the reaction temperature,the pressure or hydrogen to carbon ratio increases,the selectivity of light olefins decreases.But as the GHSV increasing,the selectivity of light olefins also increases.At optimum reaction conditions?T=320 ?,P=2.0 MPa,H₂/CO=1.5,GHSV=4200 h-1?,the CO conversion is 60.3%,and the selectivity of the lower olefins can reaches 40.7%.