Preparation And Performance Study Of Carbon-encapsulated Iron-based Catalyst For Fischer-tropsch Synthesis

Converting coal-derived synthesis gas to ultra-pure liquid fuels and other chemical products via Fischer-Tropsch synthesis?FTS?is an important alternative route for energy development.Compared with Co and Fe catalyzed low-temperature FTS process,FTS catalyzed by Fe-based catalyst under a higher temperature condition?High-Temperature Fischer-Tropsch,HT-FT?has a longer industrial chain and more multiple product types.With the production of gasoline,diesel and other clean oil products,other high value-added chemical products such as?-olefins also can be produced.And supported Fe-based HTFT catalysts are currently hot research topics.Due to the strong metal-support interaction?SMSI?between Fe oxides and traditional oxide supports?SiO₂,?-Al₂O₃,etc.?,it is not conducive to the subsequent reduction and carbonization of Fe oxides,which will result in a poor catalytic activity.In contrast,carbon supported Fe-base catalysts always received a higher catalytic activity.It can be attributed to the inert property of carbon materials,which wouldn't generate SIMI.However,due to the too weak interaction between Fe nanoparticles?NPs?and carbon,it is easier suffer from a deactivation caused by sintering of Fe NPs,especially for these promoted with Na or K promoter and under a high temperature and high pressure conditions.However,these Na and K promoters are the key factor for regulating the catalytic performance of Fe-based FTS catalysts.In addition,the carbon material surface is hydrophobic,which is unfavorable to the dispersion of the metal species on it,directly.Usually,the surface acidification pretreatment of carbon materials is necessary,prior to the use of carbon materials,but it is environmentally unfriendly.In this paper,we focus on the problem of Fe specie hardly disperse on carbon and the carbon supported Fe-based FTS catalysts easily suffer from deactivation caused by sintering of Fe NPs.Here,Na promoted carbon-encapsulated Fe-based catalysts and the K promoted N-doping carbon encapsulated catalysts with a three dimensional structure were respectively prepared with glucose and chitosan as carbon source,and the CS was used as support.The introduction of glucose was aimed at the promotion of the dispersion of Fe species,and the carbon-encapsulated structure was in order to improving the catalytic stability.And the mian research contents and conclusions are as follows:?1?CS served as support.A series carbon-encapsulated catalysts Fe-Na@C/CS promoted with Na were prepared via glucose co-impregnation and carbonization strategy.The effect of encapsulated carbon amount on the HT-FT catalytic performance was also investigated.The introduction of glucose significantly increased the dispersion of Fe and Na.During the preparation of catalyst,glucose acted as“Nanoglue”enhanced the interaction between Fe species and CS.With the increase of amount of glucose,the XRD diffraction peaks intensity and particles size of Fe oxides both decreased gradually.And the results of SEM characterization showed that the Fe NPs were encapsulated in the carbon matrix carbonized from glucose and coated on the external surface of CS.The CS favored the dispersion of Fe-glucose composite by supplying its external surface,which prevented the formation of large Fe-C bulk.?2?The characterization results of used catalysts as well as catalytic performance results suggested that the carbon encapsulated structure can inhibit the migration and agglomeration of Fe NPs during the reaction.The increase of coating carbon amount can improve the catalytic activity and stability.When the encapsulated carbon amount increased to 40%,and Na amount is 1%,the catalyst FTY value can reach to 1710.8?mol·g_Fe^-1·s^-1,C₅₊selectivity can attain62.8%,with only 8.0%CH₄,and without losing any activity during the reaction of 50 h.?3?K promoted three dimensional structure N-doping carbon coated Fe-based catalyst Fe-K@NC/CS with high activity and stability for HT-FT was prepared by unidirectional freezing technology with CS as building units and chitosan as nitrogen source and carbon source.IR spectra showed that the-NH₂and-OH functional groups in chitosan can coordinate with Fe?III?.Benefiting from the coordination effect,the ferric nitrate wouldn't precipitate out with ice crystals during unidirectional freezing,which resulted in a well encapsulation of the Fe NPs in the N-doping carbon layer.Owing to the introduction of chitosan,the catalysts opposed a smaller and narrow distribution of Fe NPs.The texture properties results showed that the introduction of unidirectional freezing improved the external area of catalysts and benefited the dispersion of Fe-Chitosan,which can inhibite the formation of large Fe-C bulk.?4?Performance evaluation results showed that the introduction of promoter K and N-doping carbon encapsulated structure both can improve the FTS activity.Although the catalyst Fe-K/CS promoted by K possess a high initial activity,it is prone to inactivation.However,the catalyst Fe-K@NC/CS under the synergistic effect of K and encapsulated structure,the catalytic activity and stability are both promoted significantly,and without losing too much activity during the reaction of 50h.The XRD results of used catalysts exhibited that the catalyst Fe/CS still possess a Fe₃O₄ diffraction peak even after reaction,however,it is absence for catalyst Fe@NC/CS.These results suggested that the doping N can promoted the carbonization of Fe species and the catalytic activity.