Direct Conversion of Syngas to Light Olefins through Fischer- Tropsch Synthesis over Fe-Zr Catalysts Modified with Sodium

Fe-Zr-Na catalysts synthesized by coprecipitation and impregnation methods were implemented to investigate the promoting effects of Na and Zr on the iron-based catalyst for high-temperature Fischer-Tropsch synthesis (HTFT). The catalysts were characterized by Ar adsorption-desorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, CO temperature-programmed desorption, H-2 temperature-programmed desorption, X-ray photoelectron spectroscopy, and Mossbauer spectroscopy (MES). The results indicated that Na changed the active sites on the catalyst surface for the CO and hydrogen adsorption, owing to the electron migration from Na to Fe atoms, which resulted in an enhanced CO dissociative adsorption and a decrease in hydrogen adsorption on the metallic Fe surface. The decreased H/C ratio on the catalyst surface accounted for the increased chain propagation and weakened hydrogenation of light olefins. Besides, Na could also facilitate the carbonization of catalysts and protect the iron carbide against oxidation, which provides more active sites for HTFT reaction and is beneficial to the C-C coupling. Zr could decrease the hematite crystallite size and stabilize the active phase to improve the HTFT activity. At an optimal Na loading of 1.0 wt %, the Fe-Zr-1.0Na catalyst exhibited the highest light olefin selectivity of 35.8% in the hydrocarbon distribution at a CO conversion of 95.2%.

Automated summary

In high-temperature Fischer-Tropsch synthesis, the promoting effects of Na and Zr on Fe-based catalysts are mainly reflected in enhanced CO dissociative adsorption, reduced hydrogen adsorption, and increased light olefin selectivity. Na can facilitate the carbonization of catalysts and protect iron carbide, providing more active sites for C-C coupling, while Zr can improve HTFT activity by decreasing hematite crystallite size and stabilizing the active phase.