Controlled Synthesis And Catalytic Performance Of Co_xMn_1-xO For Fisher-tropsch Synthesis

Lower olefins are the cornerstone of the chemical industry,obtained from naphtha cracking or alkanes dehydrogenation usually.The increasing demand for lower olefins and the gradual depletion of crude oil resources urges scientists to find new ways to obtain lower olefins through non-petroleum methods.Through the methanol to olefin process（MTO,Methanol To Olefin）,the synthesis gas can be effectively produced through methanol to produce lower olefins.However,with complicated production process,the reaction conditions of MTO process is severe.Fischer-Tropsch synthesis could convert synthesis gas to high chemical value-added hydrocarbons.The direct production of lower olefins（FTO,Fischer-Tropsch To Olefins）through the Fischer-Tropsch synthesis process can further reduce costs and energy consumption.As new type of Fischer-Tropsch synthesis of lower olefins in recent years,the Co Mn Na catalytic system can break through the distribution of ASF and convert synthesis gas to produce lower olefins directly,promoting the selectivity of lower olefins up to 60%.better understanding of Co Mn Na catalytic system is expected to guide the development of high-efficiency FTO catalysts.However,the complex components of catalyst are accompanied by complicated phase change process during reaction.One of the key points to understand the Co Mn Na catalytic system is to understand its phase evolution and changes in catalytic behavior during reaction.In this paper,in order to clarify the relationship between the structure and performance,Co_xMn_1-xO nanoparticle which is important intermediate in this catalytic system,were directly prepared by thermal decomposition method to simplify the structural evolution process.The main research content of this article has the following three points:（1）Aiming at the catalytic behavior of Co Mn Na catalytic system accompanied by the phenomenon of complex structure evolution,the key intermediate Co_xMn_1-xO formed in the structural reconstruction of the catalytic system was directly prepared by thermal decomposition in this paper.Through XRD,TEM and other characterization methods to characterize its structure,it is found that the material has crystal structure consistent with Co O and Mn O.Because of the coexisting of two elements in the material,the lattice spacing of Co_xMn_1-xO is between Co O and Mn O.with cobalt and manganese are evenly dispersed in materials,element contact degree is maximized.Through Fischer-Tropsch synthesis performance characterization,Under the same reaction conditions,Co_xMn_1-xO catalyst exhibits best FTO performance compared to other catalysts composed of same chemical element and different cobalt-manganese contact degrees.with CO conversion reached 17.2%and the selectivity of lower olefins was up to 66.5%,Co_xMn_1-xO catalyst was not obviously deactivated within 160 h.To explore the factors that affect catalytic performance,model catalysts with different degrees of cobalt-manganese contact degree were constructed.The results show that the degree of contact of cobalt-manganese elements in the Co Mn Na catalytic system strongly affects the catalytic performance.（2）On the basis of Co_xMn_1-xO nanoparticles with clear crystal structure,the phases of catalysts at different stages are tracked and analyzed.The results show that after the reduction treatment of Co_xMn_1-xO,part of Co²⁺is reduced to Co⁰and escapes the granular phase,the x value of Co_xMn_1-xO decreases with the lattice spacing increases.In the subsequent catalytic reaction,Co⁰ will be carbonized and transformed into Co₂C active phase which have excellent lower olefin selectivity.It is worth noting that the XRD pattern of spent catalyst shows that the reduction of cobalt in Co_xMn_1-xO not only occurs during reduction process,still further reduced during reaction process.There is no catalytic activity Co₂C phase in the spent unreduced Co_xMn_1-xO catalyst,which implies that Co_xMn_1-xO with Co⁰ on the surface will be further reduced during reaction.Above view was comfirmed by in-situ diffuse reflection Fourier transform infrared spectroscop.（3）The best optimal cobalt-manganese contact state achieved by the Co_xMn_1-xO structure,by adjusting the raw material ratio of the catalyst cobalt-manganese element,the x value of Co_xMn_1-xO is precisely controlled and chemically controlled.Characteristics and catalytic performance are tested.The results show that it is feasible to adjust the x value of Co_xMn_1-xO by altering the ratio of raw materials.The increase content of Mn leads to an increase difficulty of reduction,and the Co⁰ formed after reduction exhibits smaller particle size.The catalytic performance test showed that under the same reaction conditions,the chain growth probability of the catalyst with higher Mn content was lower,indicating that the amount of cobalt-manganese contact interface can directly affect the product composition of the catalyst.