In-situ Synthesis Of Metal-embeded Porous Carbnon Sphere And Their Performance Of The Catalytic Hydrogenation

Light olefins?C₂-C₄ olefins,including ethylene,propylene,butylenes?are widely used as key building blocks in the chemical industry.The increase in global demand for light olefins coupled with the regional diversification of carbon raw materials brings about opportunities for emerging technologies to produce them.As one of the most promising,synthesis gas?a mixture of CO and H₂?derived from natural gas,coal,and renewable biomass,which can be used to produce light olefins via Fischer-Tropsch to olefins reactions?FTO?process.Iron-based catalysts are commonly applied in FTO reaction,due to they are favorable for light olefins production,low cost and high activity for water-gas shift reaction?WGS?and mechanical stability.In this thesis,iron-manganese?FeMn?catalyst was prepared by co-precipitation method and used for converting synthesis gas to light olefins.The catalyst activity and C₂-C₄⁼selectivity were optimized by surfactant pretreatment,adding surfactant content and optimizing the amount of Mn promoter.The structure,the dispersion of active components and surface chemical states were characterized by XRD,N₂-physisorption,H₂-TPR,CO₂-TPD,TEM,and XPS measurements to reveal the relationship between the catalyst structure and its catalytic performance.And obtaining the main conclusions like below:Surfactant pretreatment had a great effects on catalyst surface areas,pore volume and pore size,crystal orientation,grain size,and the dispersion of the?-Fe₂O₃ or MnO_x particles,which leading to the different catalytic performance.The hexadecyl trimethyl ammonium bromide?CTAB?was selected as an ideal surfactant to modulate the catalytic performance of FeMn catalyst and enhance the light olefins selectivity in the context of the experiment.Here,a series of iron-based catalysts were synthesized with different CTAB addition.And it is found that when adding the CTAB content,the agglomeration of catalyst particles was dramatically alleviated,the average particle size was reduced from 18.7 nm to 4.79 nm,the catalyst special surface areas and catalyst surface basicity increased as well.Moreover,larger amount of MnO_x species dispersed on the surface of?-Fe₂O₃ upon the addition of CTAB.In consequence,C₂⁼-C₄⁼selectivity significantly increased from 30%to 55.45%.Mn could enlarge special surface areas,increase catalyst surface basicity and so on.It is observed that moderate Mn amount?Mn/Fe=0.1,mol/mol?with optimum CTAB addition?CTAB/Fe=0.8,mol/mol?,C₂⁼-C₄⁼selectivity over FeMn catalyst could maintain at high level of about 61%at TOS=15⁴0 h under the industrially relevant conditions(320°C,1.0 MPa,H₂/CO ratio of 1.5?v/v?,GHSV=4200 h^-1).Even more attention is to C₂⁼-C₄⁼selectivity was still as high as 50.38%at TOS=65 h.It is found that special iron carbides had a close relationship with the catalytic activity and the FTO selectivity.For example,Fe₃C and Fe₇C₃ formed during the reaction,which were favorable for the increase of C₂⁼-C₄⁼production?selectivity,61%?,while the formed?-Fe₅C₂favored C₅₊hydrocarbons and oxygenates formation?selectivity,40.8%?with relative lower C₂⁼-C₄⁼selectivity?30.16%?.