Synthesis And Catalytic Performance Of Iron-based Catalysts For The Production Of Light Olefins From Syngas

Alkenes are widely used in chemical industries and are well known as high valuable chemicals.Among them,the lower olefins?C2-C4 olefins?are even the cornerstone and symbol of petrochemical industry and occupy an important status in the national economy;they are mainly obtained from the cracking of naphtha in commercial production.With the decrease of petroleum resources and the increasing prominent of environmental problems,the non-oil route which produces lower olefins from the syngas based on coal,natural gas and biomass has been attracted much attention.This thesis focuses on the catalytic performance of iron-based catalysts in FTO synthesis from the aspects of structure and alkaline of support,structure and acid of molecular sieve and the electronic effect of Mn²⁺,so as to explore the clear structure-activity relationship between the catalyst structure/electronic properties and the selectivity of olefins/lower olefins.The main work is as follows:?1?Mg is introduced into Fe-based catalysts as a typical alkaline promoter,,which could significantly enhance the adsorption of CO,suppress secondary hydrogenation process and thus increase the selectivity of olefins.Meanwhile,alkality of MgO with various exposing crystal surfaces is also studied,MgO-c?MgO?100??and MgO-ns?MgO?111??are used as the supports and a series of Fe/MgO catalysts with well-defined exposed crystal planes are synthesized by impregnation?IM?,deposition-precipitation?DP?and ultrasonic impregnation methods?UI?,namely Fe/MgO-ns-IM,Fe/MgO-ns-DP,Fe/MgO-ns-UI and Fe/MgO-c-UI catalysts.Many different effects on the structure and catalytic performance of iron catalyst was observed.Compared with Fe/MgO-c-UI catalyst,the Fe/MgO-ns-UI catalyst had higher activity,O/P ratio and selectivity of lower olefins.This may be because the?100?surface of MgO cubes ending with alternant Mg²⁺/O^2-ions provides medium basic sites,while the?111?surface of MgO nanosheets ending with O^2-ions possesses strong basic sites.In addition,it was found that the strong basicity sites of MgO supports could be remained during the ultrasonic impregnation process.?2?The SAPO-34 molecular sieve has strong Br?nsted acid sites and its pore diameter is about 0.43 nm,this pore size is equivalent to the kinetic diameter of ethylene?0.39 nm?and propylene?0.45 nm?,and is beneficial to the mass diffusion of short chain hydrocarbon.In this work,we use a in-situ hydrothermal synthesis method and a mechanical mixing method to prepare Fe₃C@SAPO-34 catalysts of the core-shell structure and the Fe₃C+SAPO-34 catalyst,respectively.The effects of molecular sieve structures on the selectivity of lower olefins were studied.Compared with Fe3C@C catalysts,the Fe3C@SAPO-34 catalysts and Fe₃C@C+SAPO-34 catalysts could effectively restrain the production of long chain hydrocarbons,significantly improve the selectivity of lower olefins,however,the selectivity of methane was increased at the same time.This may be due to the acid catalytic cracking of molecular sieve.In addition,the Fe₃C@C+SAPO34 catalyst had higher O/P ratio and higher selectivity of lower olefins than Fe₃C@SAPO-34catalyst.It could be attributed to the smaller kinetic diameter of H₂ than that of CO,H₂ is easier to get into the shell and reach the active surface,resulting in the high H₂/CO ratio on the surface of the active center in the Fe₃C@SAPO-34 catalyst,and thus reducing the O/P ratio and the selectivity of lower olefins.Moreover,in the core-shell structured Fe₃C@SAPO-34 catalyst,the acid center of molecular sieve is closer to the active phase for Fischer-Tropsch synthesis,and it is easier to promote the hydrogenation of olefins and the cracking of long chain hydrocarbons.?3?Manganese has complex structure and electronic effect that can decrease the methane selectivity and promote the formation of olefins.In this work,the effects of Mn²⁺ion on the structure of iron-based catalyst and the catalytic performance of Fischer-Tropsch synthesis were studied.The charaterazation results indicated that in Fe₃C-MnO@C catalyst,Mn was mainly existed in the form of MnO.Low state of Mn²⁺ion is benefit to increase the electron density of iron species,inhibit the generation of methane and improve the selectivity of lower olefins.In addition,The Fe₃C-MnO@C catalyst can achieve the high selectivity of lower olefins?50%?under a higher CO conversion?9.9%?by adjusting the reaction temperature and the space velocity of syngas.