Effects of Zn promoter on the structure and Fischer-Tropsch performance of iron catalyst

The effects of Zn (100 gFe/x gZn, x=7～100) on the textural properties, reduction behavior and structural changes during reduction and reaction of the Fe-Zn catalysts were studied by using N₂ physical adsorption, X-ray diffraction (XRD), Mssbauer spectroscopy (MES), H₂ differential thermogravimetric analysis (H₂-DTG) and CO temperature-programmed reduction (CO-TPR). The F-T performances of the catalysts were investigated in a fixed-bed reactor under the conditions of H₂/CO=2.0, 260 ℃, 1.5 MPa and 4 000 mL/(g·h). The results show that, with the increase of Zn content in catalysts, the phases of catalysts transform from α-Fe₂O₃ and ZnFe₂O₄ to ZnFe₂O₄ and ZnO, where ZnO appears just beyond the stoichiometry of Zn/Fe (1∶2). The presence of ZnFe₂O₄ plays an important role in catalysts. In the fresh catalysts, ZnFe₂O₄ can enhance the dispersion of Fe phase and thus enlarge the surface area. For the reduced and used catalysts, ZnFe₂O₄ not only inhibits the excessive reduction and carbonization, but also improves the stability of the iron carbide phase. F-T tests show that the fast deactivation was observed in the un-promoted catalyst, but the catalytic activity was stabilized by ZnFe₂O₄ in Zn-promoted catalysts. In addition, high olefin selectivity is observed in Fe-Zn catalysts which gradually decreases to same content as that of un-promoted catalyst probably due to the change of phases at the beginning of reaction.