Study On The Modification And Preparation Of Fe-based Catalysts For The Direct Synthesis Of Light Olefins From CO Hydrogenation

Direct synthesis of light olefins from syngas via Fischer-Tropsch Synthesis (FTS), is one of the potential nonpetroleum routes in producing high value-added chemicals such as ethene, propene and butene. However, the product distribution ruled by Anderson-Schulz-Flory (A-S-F) distribution and also the influence of the kinetics and thermodynamics has placed severe limitations on its attractiveness as a commercial operation. The catalysts with high selectivity and yield towards light olefins need improvement during CO hydrogenation. The stability and repeatability of catalyst should be improved and the by-products (CH4, CO2, Cs+, etc.) should be inhibited, in order to further enhance the olefin selectivity and improve the product distribution.Iron-based catalyst has shown great prospects in the production of light olefin, which produced more C2-C4 olefins with lower CH4 selectivity than other catalysts. In our experiment, we propose a new strategy for designing modified Fe-based catalysts by two-step hydrothermal (precipitation) process and solvothermal methods (using ethylene glycol as solvent) to obtain Fe-Zr and Fe-Mn precursors, following potassium modification by impregnation method, in order to improve the dispersion of iron species and decrease the interaction between iron and the promoters (Zr and Mn). The process of two-step Fe catalyst precursors included the following steps. The first step was to prepare Fe2O3 precursor by hydrothermal or precipitation method, and in the second step, the obtained iron precursor was firstly treated by ultrasonic dispersion, which then was added to the mixed aqueous solutions of ZrO(NO3)2-2H2O and urea, and reacted within a dynamic homogeneous reactor. The textural properties, bulk structures, surface composition, and reduction behavior of the catalysts were characterized by SEM、 HRTEM、EDS、XRD、XPS、N2 adsorption-desorption and H2-TPR methods. The effects of preparation methods on the structure and its correlation with catalytic performance during CO hydrogenation were discussed.Studies have shown that the T-Fe/Zr samples prepared by two-step hydrothermal process caused different phase structures of Z1O2, weakened the interaction between iron and zirconia and increased the reducibility of bulk Fe2O3 phase compared with the O-Fe/Zr samples by one-step hydrothermal process. Increasing zirconia content in T-Fe/Zr-K catalysts was beneficial to disperse iron active sites by small and uniform zirconia particles distributed on iron surface. The enrichment of zirconia reduced the secondary hydrogenation ability to improve the olefin selectivity and suppressed the formation of heavy hydrocarbons during CO hydrogenation. The C2-C4 olefin fraction in overall hydrocarbons reached from 38.99 wt% for one-step to 43.84 wt% for two-step catalysts at Fe/Zr molar ratio of 2/1. For Fe-Mn catalysts, the different precipitation procedure could result in different reduction behavior of the catalysts, which can be expressed by the interaction between Fe-Mn:two-step hydrothermal< one-step hydrothermal< two-step precipitation< one-step precipitation. An increase in surface area and decrease in particle size were observed on the samples prepared by precipitation method compared with that of the catalysts by hydrothermal methods. Enrichment of Mn and K could be observed for the catalyst samples by two-step precipitation methods. During CO hydrogenation, high olefin selectivity (O/P) for P-T Fe/Mn-K samples prepared by two-step precipitation method could be obtained, however, high C2=-C4= weight percentage in overall hydrocarbons was observed for M-0 Fe/Mn-K samples for one-step hydrothermal method. The S-Fe/Zr-K sample with hierarchical structure prepared by solvothermal method with too much single ZrO2 particles, showed weaker interaction between Fe and Zr compared with that of H-Fe/Zr-K sample prepared by hydrothermal method. Both of the catalsyts showed high activity with CO conversion> 95% during CO hydrogenation, however, the S-Fe/Zr-K sample showed low olefin selectivity due to the strong secondary hydrogenation reaction.