Direct production of light olefins from syngas over potassium modified Fe-Mn catalyst

A series of potassium promoted Fe-Mn catalysts for light olefin synthesis from CO hydrogenation were prepared by the co-precipitation-calcination-impregnation method. The impact of potassium promoter on the textural properties, reduction behavior, adsorption of hydrogen, bulk phase composition and their correlation with Fischer-Tropsch synthesis (FTS) performances were emphatically studied. As revealed by N-2 physisorption, the increasing of potassium had no distinct effect on the size of alpha-Fe2O3 and surface area. H-2 temperature-programmed reduction/desorption showed that potassium inhibited the reduction and hydrogen adsorbability of catalysts. CO temperature-programmed desorption showed that potassium enhanced the CO adsorption of catalysts. X-ray photoelectron spectroscopy indicated the enrichment of promoter atoms on the surface of catalysts. The X-ray diffraction and Mossbauer effect spectroscopy results revealed that potassium made carburization easy. After reduction with syngas (H-2/CO = 20) for 48 h, FTS test was performed with the syngas (H-2/CO = 3.5) in the high temperature Fischer-Tropsch synthesis process. The maximum of CO conversion and selectivity to light olefins was noted on increasing K content (2.8 mol% K/Fe), followed by a significant decline at the excessive potassium level. At the point, the selectivity of light olefins and olefin/paraffin (C-2-C-4) was 27.75 mol% and 8.54. The results indicated that potassium promoter could inhibit the water gas shift reaction, suppress hydrogenation ability, which promoted the production of light olefins via suppressing the secondary hydrogenation reaction.