| Lower olefins?ethylene,propylene,and butylene?are important raw materials for chemical industry.They are widely used in various fields from packing materials and synthetic textiles to antifreezing agents,solvents,and coatings.At present,lower olefins are mainly produced through oil cracking in industry.China's oil dependence on foreign countries has exceeded the security warning line.To ensure national energy security,it is of strategic importance to develop industrial production from coal.Fischer-Tropsch synthesis to lower olefins?FTO?is a new non-petroleum process,starting from coal-based or biomass-based synthesis gas.In this paper,we fabricated Fe2C-embedded hollow structure nanoreactor and investigated the relationship of the catalyst structure and chemical properties and the performance of the FTO reaction.Organic polymer was coated on SiO2 microspheres,which were used as hard template.After impregnation with Fe precursor,the samples were pyrolyzed at high temperature under an inert atmosphere followed by etching in alkaline solution.The catalysts with hollow structure were successfully prepared?Fe/HCS?and applied to the FTO reaction.The separation and confinement effect of the carbon matrix avoid the sintering and agglomeration of the Fe species,even under weak interaction between the Fe species and the support.The preparation method can effectively shorten the induction period of the catalyst and more importantly,the unique local environment of the carbon matrix leads to the formation and stability of Fe2C at a high temperature,even it was reported as a thermodynamically unfavorable phase.The pyrolysis temperature has a significant effect on the phase state and dispersion of the Fe species.Under the optimal calcination temperature?600 oC?,the Fe/HCS catalyst mainly contains Fe2C crystal phase with good dispersity,which results in the best FTO activity.The amount of per unit time CO converted by per unit mass of iron?FTY?can reach495×10-6 mol CO/gFe·s.By adjusting the size of the hollow sphere and the location of the Fe species,the influence of carrier structure on the production distribution of the FTO reaction was investigated.It has been experimentally confirmed that the structure of the hollow structure have a confinement effect.Specifically,the reactant gases CO and H2 can be enriched in the hollow structure and the enrichment effect is different for the two reactants.As a result,the ratio of H2/CO in the nanoreactor changes,which in turn affects the production distribution of FTO reaction.The enrichment effect of the reactant gas in the nanoreactor changes with the curvature of the hollow structure.As the curvature becomes larger,the CO enrichment ability will increase.Hence,the local H2/CO ratio decrease,so that the secondary hydrogenation reaction is inhibited and the chain growth reaction probability increases.These result in an increase of lower olefins and olefin-paraffin ratio. |
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