Zn对沉淀铁催化剂结构及其F-T合成性能的影响

采用低温N₂吸附、XRD、MES、CO-TPR和H₂-DTG研究了Zn(100 gFe/x gZn, x=7～100)助剂对 Fischer-Tropsch (F-T) 合成Fe基催化剂的织构性质、还原行为以及相变结构的影响;在H₂/CO=2.0、260 ℃、1.5 MPa和4000 mL/(g·h) 条件下在固定床反应器上考察了Zn助剂含量对Fe基催化剂F-T合成反应活性、烃产物选择性和运行稳定性的影响。研究结果表明,随着Zn含量的增加,氧化态催化剂的物相由α-Fe₂O₃和ZnFe₂O₄逐渐向ZnFe₂O₄和ZnO转变,ZnFe₂O₄在催化剂中优先生成,只有在超出其计量比1∶2之后才有ZnO出现。由于ZnFe₂O₄较为稳定,能够促进催化剂中Fe物相的分散,导致比表面积增加。在还原和反应态催化剂中,ZnFe₂O₄一方面抑制催化剂的过度还原和碳化;另一方面表现为稳定活性相铁碳化物。催化剂的F-T反应性能评价结果表明,纯铁催化剂由于铁碳化物氧化而迅速失活,而Zn助剂催化剂却由于ZnFe₂O₄的稳定作用,活性较为稳定。同时,由于催化剂在反应初相变的影响,导致Zn助剂催化剂的初始烯烃选择性随着Zn含量的增加而增加,在相态稳定之后选择性趋于一致。

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.