Co-baesd Catalysts Prepared By Complex-impregnation Method For Fischer-tropsch Synthesis

FT synthesis is the core technology for converting non-oil-based carbon-containing resources such as coal,natural gas,and biomass into high value-added chemicals and clean liquid fuels,it is significantly importance for achieving oil substitution and safeguarding national energy security.The supported cobalt-based catalysts are excellent FT synthesis catalysts with high activity,high stability,and low CO2 selectivity.Improving the density of metallic cobalt?Co0?on the surface of the catalyst effectively is the key to obtaining high dispersion cobalt-based catalysts.However,high-dispersion small-particle Co species and oxide carriers tend to form strong interactions with each other,inhibiting the reduction of cobalt species and the formation of highly active FT synthesis catalyst.Therefore,the rational design of catalysts to increase the dispersion of metallic cobalt and break the problem of “dispersion and reduction dependence” in traditional catalysts is of great theoretical significance and application value to obtain high activity and high stability FT synthesis catalysts.In view of the advantages of organic complexing agents in increasing the dispersity of metal Co in supported metal catalysts,amino acid chelate were introduced into the impregnating solution to coordinate with Co2+ to form metal complexes to avoid the migration,aggregation,growth of Co species and the strong interaction with the carrier during the process of drying,calcination and reduction of the catalyst in this paper.The effect of structure of amino acids,the method of introduction of amino,the ratio of glycine to Co2+ and the carrier structure and other factors to the activity of FT synthesis and its product selectivity were systematically studied.The mechanism of the above factors on the dispersion,distribution and reducibility of Co was elucidated Combined with BET,XRD,XPS,H₂-TPR and other characterization methods.The main research contents and conclusions are as follows:?1?In order to investigate the structure of chelating agents on the dispersion of cobalt particles,the cobalt particle size,and catalyst reduction behavior,a series of Co/SiO₂ catalysts were prepared with glycine,serine,and citric acid as chelating agents,respectively.The results show that the introduction of glycine greatly increased the FT synthesis activity of the catalyst,which was mainly attributed to improved dispersion of metallic Co.In order to further improve the FT reaction activity of the catalyst,a variety of Co/SiO₂ catalysts were prepared by changing the introduction method of the chelating agents,using Co?OH?2 as a precursor and glycine as a chelating agent?complex impregnation method II?.The Co/SiO₂ catalyst prepared by this method has slightly lower reactivity than the Co/SiO₂ catalyst prepared by the complex impregnation method I.However,the Co/SiO₂ catalyst prepared by the complex impregnation method II exhibits higher stability,which may be related to the uniform distribution of its Co particle size.?2?A series of Co/SiO₂ catalysts were prepared by using complex-impregnation method II with SiO₂ as carrier and glycine as chelating agent.The effects of the molar ratio of glycine to Co2+ ions and Co loading on its FT synthesis reactivity were studied.The results showed that the Co dispersion of Co/SiO₂ catalyst was high when the Co loading was 7%,the molar ratio of glycine to Co2+ was 3:1,and the highest FT synthesis activity and stability were exhibited.The main reason may be that when the ratio of glycine to Co2+ is less than 3:1,the complexing agent required for the six-coordinated structure of Co2+ is not sufficient,resulting in the existence of many forms of the cobalt complex of glycine in the impregnating solution,the intensity of Co-species interact with the carrier is different,which results in a non-uniform size distribution of Co;when the glycine ratio is relatively high,excessive glycine may lead to a small amount of incompletely removed carbon species on the catalyst surface,resulting in a lower FT synthesis reactivity.?3?A series of 7% Co/SBA-15 catalysts were prepared using SBA-15 as the carrier and Co?NO₃?2·6H₂O as the precursor of cobalt.The effects of the preparation method,the structure of the chelate,and the ratio of the chelating agent to Co2+ on the crastal size and the reduction behavior of the catalyst were investigated.The results showed that compared with the amorphous SiO₂ carrier,the SBA-15 carrier with larger specific surface area was more conducive to the dispersion of the active component.Among them,the Co/SBA-15 catalyst prepared with citric acid as the chelating agent has the highest FT synthesis reaction activity and stability.Different from Co/SiO₂ catalysts prepared by complex impregnation method II,Co-SBA-15 catalysts prepared by complex impregnation method I increase gradually with the molar ratio of glycine to Co2+,and the FT reaction activity of catalysts gradually increases,and When the glycine and Co2+ mol ratio is 5:1,the activity of FT synthesis of Co/SBA-15 catalyst is the highest.