Study On Fischer-tropsch Synthesis Of Fe-based Catalysts Supported On Carbon Nanotubes

With the growing consumption of oil,a non-renewable resource,and the increasing pressure on environment,the development of an alternative environmental-friendly energy is imminent.Fischer-Tropsch synthesis is a process for the synthesis of liquid fuels that replace petroleum routes with syngas?CO and H₂?under the catalyst and the appropriate reaction conditions.The products of the Fischer-Tropsch synthesis don't contain the aromatic hydrocarbon,nitrogen or sulfur-compounds and other components which are harmful to environment.Terefore,Fischer-Tropsch synthesis has become a hot research topic in recent years.Carbon nanotubes?CNTs?serve as a good catalyst support because of its unique tube structure,high structural regularity and thermal stability.In this paper,Fe/CNT catalyst is prepared by combining Fe-based catalyst with CNT support.The performance of Fischer-Tropsch synthesis is systematically evaluated on a fixed-bed reactor.The effects of carrier and precursor selection,Fe loading,calcination temperature and reaction process conditions are investigated in detail.The promoter modification such as Na?S and Mg are also investigated.The microstructural properties of the catalysts are analyzed by thermogravimetric analysis?TG?,N₂ physical adsorption?BET?,X-ray diffraction?XRD?,H₂-programmed temperature-reduction?TPR?and scanning electron microscopy?SEM?methods.The results show that the Fe/CNT catalyst with CNTs?20-30nm?as the carrier has good performance.It is found that the use of ammonium ferric citrate as precursors contributes to a better dispersion of Fe on the carbon nanotubes to form uniform and smaller grains which could be developed to achieve a better activity and stability of the catalyst.The activity and stability of Fe can be improved by adding much more Fe on the carbon nanotubes to form a uniform grain,and the catalyst activity and the selectivity of the lower olefins are the best when the Fe loading is10%.The high calcination temperature tends to cause the catalyst particles to sinter and decrease the activity of the catalyst.The temperature of 350?is appropriate.At the reaction process conditions T=280?,P=2.0 MPa,GHSV=2500mL/?g·h?,the catalyst performance is the best.The results of the modification of the catalyst show that the activity of the catalyst and the selectivity of the lower olefins can be improved while the formation of CH₄ suppressed by the combination of Na and S additives.When the loading of Na and S is 0.3wt%,the conversion of CO is 94.14%,C₂^=C₄⁼selectivity is 55.82%,the selectivity of methane is14.24%.It is believed that the addition of Mg promoter increases the alkalinity and decreases the activity of the catalyst accelerating the deactivation rate of the catalyst.It also restrains the hydrogenation ability and inhibits the formation of methane,so that the product moves in the direction of heavy hydrocarbon.