Preparation Of Iron Carbides And Its Performance Of Fischer-Tropsch Synthesis To Light Olefins

Light olefins（including ethylene,propylene and butene）are the most basic raw materials for petrochemical production,and the basis for the production of other organic chemicals.At present,the method of producing light olefins can be divided into two routes:one is petroleum route,the other is non-petroleum route.However,the resource of non-renewable petroleum is limited,so it is imperative to develop a new non-oil based process to producing light olefins.The direct synthesis of light olefins via Fischer-Tropsch（FTO）has been attracted more and more attention.The key of FTO process is the development of economical and efficient catalysts.Iron-based catalysts are used widely for FTO owing to their numerous advantages such as low price,moderate activity for the Water-Gas Shift（WGS）reaction,a wide range of applicable temperature and larger space for controlling the ratio of hydrogen to carbon monoxide,etc.Iron carbide has been proved to be the active phase of iron-based Fischer-Tropsch Synthesis（FTS）,but various phases of iron carbides（?-Fe₅C₂,?-Fe₃C,?-Fe₂C and Fe₇C₃ et.al）have been found in the FTS process.There is no clear and uniform conclusion on the relationships between the active phases and the products selectivity of different iron carbide phases in FTS.A series of iron carbide catalysts were prepared through a simple solid-state reaction-carburization route,there is no situ activation treatment before the catalysts were evaluated in a fixed-bed reactor.The relationships between different iron carbide phases and the light olefinss electivity were investigated,including the preparation of iron carbide,modification of promoters and the change of carburization procedures.The structure-activity relationship of the catalysts was further characterized by XRD,SEM,TEM,XPS,TPH,CO/CO₂-TPD and N₂-physisorption,etc.And the main conclusions were summarized as follows:（1）The Fe₅C₂ catalyst can be successfully prepared through a simple solid-state reaction-carburization route at room temperature.Under the reaction conditions at 320?C,1 MPa,H₂/CO=1.5 and GHSV=6000 h^-1,the CO conversion and the selectivity to light olefins of the catalyst were 95.34%and 36.48%,respectively,showing higher activity and stability.The addition of Mn improved the light olefins selectivity,the selectivity to light olefins on Mn-0.02 catalyst was 41.32%,and the O/P（the molar ratio of olefins to paraffins in C₂-C₄ range）was 2.09.While Zn promoter reduced light olefins selectivity by about 10%,and the selectivity to C₅⁺hydrocarbons improved obviously.In addition,it was found that the conversion of CO decreased with the content of Mn increasing at the conditions of 320?C,1 MPa,H₂/CO=1.5 and GHSV=12000 h^-1,and the selectivity to light olefins increased first and then decreased.When the molar ratio of Mn/Fe was 0.035,the selectivity to light olefins was 55%.（2）It was found that the catalysts with mixed iron carbide phases of Fe₃C and Fe₇C₃ could be successfully prepared by modification of iron oxalate with surfactants.The selectivity to light olefins and C₅⁺hydrocarbons increased with the addition of CTAB,and the O/P ratio increased.For the iron oxalate-0.05CTAB catalyst,the O/P ratio and selectivity to C₅⁺hydrocarbons were 2.49 and 42.21%,respectively.（3）The carburization procedures of iron oxalate were studied.It was found that,（1）The different iron carbide phases could be obtained with the increase of carburization temperature in CO atmosphere,and the thickness of carbon layer increased.Fe₅C₂ and Fe₃C phase co-existed when the carburization temperature is 360?C.（2）The carburization of iron oxalate in syngas atmosphere（H₂/CO=1.5）at different temperature showed that,there was no relationship between the carburization temperature and the main phase,and it was Fe₅C₂.The Fe₅C₂ phase was not changed and oxidized after evaluation.（3）In order to control the thickness of carbon layer on the surface,the stepwise residence temperature programmed carburization method was used.It was found that this method can not only reduce the thickness of carbon layer effectively（from 9 nm to about 5 nm）,but also slow down the transition of iron carbide phase.