Co-precipitated Fe-Zr catalysts for the Fischer-Tropsch synthesis of lower olefins (C2O ∼ C4O): Synergistic effects of Fe and Zr

Abstract

Synthesis of lower olefins using syngas (H2/CO) in a fixed-bed reactor under 1.0 MPa, WHSV = 3 NLH2/CO·gFe-1·h-1 (WHSV = 3 NLH2/CO·gcat-1·h-1 for Zrp), H2/CO = 2 was investigated over a series of Fe-based catalysts with Zr as the promoter. The catalysts were prepared by co-precipitation (Fe and Zr) and K was impregnated into the precipitate. The prepared catalysts with different Zr/Fe mol ratio (Zr0, Zr0.2, Zr1, Zr2 and Zrp) were characterized by N2 adsorption/desorption, X-ray diffraction (XRD), Raman spectroscopy (Raman), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). Synergistic effects of Fe and Zr on the nanostructure, textural, surface property and catalytic performance were analyzed. Characterization of the catalyst showed that the introduction of ZrO2 is in favor of the formation of mesopores due to the inhibition between Fe and Zr on each other′s crystal growth, increasing α-Fe2O3 dispersion and the proportion of surface reduction to bulk reduction, which makes the catalysts easily be reduced by H2 and CO. However, the interaction between Fe and Zr significantly suppresses the formation of θ-Fe3C and carbon deposition of catalysts. The optimal Fe/Zr mole ratio of ZrX catalysts is found to be 1/1 due to the synergistic effect of Fe and Zr, the improved distribution of α-Fe2O3 and the abundant defects on α-Fe2O3 in the catalyst. In the case of Zr1, the CO conversion and the selectivity toward lower olefins reached 82.1% and 34.6% at 270 °C. Adequate Zr, acting as a block of chain growth and weakening the adsorption of light olefins on active sites, is essential to improve the light olefins selectivity.