Effect of sulfur on α-Al2O3-supported iron catalyst for Fischer–Tropsch synthesis

• Sulfide is located on surface of metallic Fe particles in a H2-reduced SxFe/α-Al2O3.
• Sulfur inhibits CO dissociation on metallic Fe surface and carburization of Fe species.
• Small amount of sulfur enhances the amount of H2 desorption from iron surface at about 500 K.
• Sulfur modification decreases CO conversion and O/P ratio of the C2–C4 products.
• Sulfur promotes the formation of C2–C4 hydrocarbons.

The effects of sulfur modification (S/Fe = 0.001 ∼ 0.05, molar ratio) on the α-Al2O3-supported iron catalysts for the Fischer–Tropsch synthesis (FTS) were investigated. It was found that the presence of sulfur promoted the reduction of FeO to metallic Fe during the H2 reduction process, but decreased the rates of reduction, carburization and carbon deposition of the catalysts when CO is used as the reducing agent. Both the sulfide (S2−) and sulfate species (SO42-) are found in the SxFe/α-Al2O3 catalysts after H2 reduction and FTS reaction. The sulfide species in the H2-reduced SxFe/α-Al2O3 catalysts are located on the surface of metallic Fe particles. The intensity of H2 desorption peak around 500 K is enhanced by a small amount of sulfur, while an excess of sulfur suppresses the H2 adsorption. The dissociative adsorption of CO on H2-reduced catalysts was also found to be inhibited by the presence of sulfur. The activity of the Fe/α-Al2O3 catalyst can be decreased significantly by the addition of sulfur due to the inhibition of CO dissociation and thus reduce the formation of iron carbide phases under FTS reaction conditions. Resistance to sulfur poisoning of the Fe/α-Al2O3 catalyst was found to improve with increasing the reaction temperature. The presence of sulfur also suppressed the formation of C5+ hydrocarbons and shifted the products to C2–C4 hydrocarbons. At the same time, the olefin to paraffin (O/P) ratio of the C2–C4 hydrocarbons decreased with increasing S/Fe molar ratio. These may have resulted from the increasing of the H/C ratio on the surface of sulfur modified catalyst under FTS conditions.

Incorporation of sulfur into a Fe/α-Al2O3 catalyst decreased the CO conversion, the C5+ selectivities as well as the O/P ratio of the C2–C4 products and promoted the formation of C2–C4 hydrocarbons.Figure optionsDownload high-quality image (195 K)Download as PowerPoint slide