Preparation Of Co And Mn Doped Iron-based Catalysts And Their Catalytic Performance For The FTO Reaction

Olefin is an important raw material for chemical production,mainly from petroleum catalytic cracking.Since China is a"coal-rich,oil-poor,gas-poor"country,thus it is significant to develop Fischer-Tropsch synthesis for the direct preparation of olefins（FTO）from coal to syngas（CO+H₂）,and the key is to develop catalysts with high catalytic activity and olefin selectivity.In this paper,different kinds of Fe-based catalysts were synthesized by low-temperature co-precipitation method,and the effects of the content of active metal Co and promoter Mn on the structure and performance of Fe-based catalysts were investigated.Combined with N₂ physisorption（BET）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,Raman and temperature-program-reduction（TPR）analyses,the effects of different Co/Fe ratios and different Mn contents on the catalyst phase composition,surface area,pore structure characteristics,structural evolution,reduction behavior,phase changes,carbide characteristics and FTO performance.In Fe-Co catalysts,with increasing Co/Fe ratio,the morphology of catalysts evolved from initially smooth and dense bulk to rough and loose structure of the surface,and Co species promoted the dispersion of systems structurally.The Co addition up to20%promoted the reduction ofα-Fe₂O₃ phase effectively.Different activation phase evolution and catalytic performance are exhibited with different Co/Fe values during the activation process.The catalytic performance analysis showed that the activated iron-carbon species corresponded to more production of light olefins（38.0%）when the Co/Fe value is 1/7;when the Co/Fe value is 5/3,the cobalt-iron species promoted each other’s reduction and corresponded to the highest CO conversion（90.1%）;when the Co/Fe value was increased to 1,the selectivity of methane increased to 44.3%.The Fe Mn and Co Mn catalysts with different Mn contents showed significant structural differences,and the incorporation of Mn species increased the surface area of Fe Mn and Co Mn catalysts(147.07 and 152.61 m²·g^-1)and formed regular spherical particles.The catalytic performance analysis showed that the Fe Mn and Co Mn catalysts showed different product selectivity with Mn species incorporation:The catalytic performance tests showed that the Fe Mn and Co Mn catalysts exhibited different product selectivities with the addition of Mn species:Fe Mn catalysts produced more light olefins（up to 40.1%）,but the CO conversion decreased significantly with the increase of Mn content（95.9%→52.4%）;Co Mn catalysts exhibited extremely low selectivity for light olefins（highest only 1.3%）,but alkane selectivity was as high as 46.0%,and the by-product CO₂ selectivity was as low as 19.2%.In Fe-Co-Mn catalysts,the Mn content is fixed and the catalyst morphology shows more spherical shape with the increase of Co/Fe value.with 20%Co content,although the reduction of Fe phase state is improved,the CO conversion decreases due to the decrease of Fe content and massive Mn enrichment on the catalyst surface,which covers the active sites.In situ characterization and catalytic performance analysis showed that:when the cobalt content is less than 20%,the activated species are Fe₅C₂and Fe₃C with the highest selectivity of C₂⁼-C₄⁼（39.3%）;when the cobalt content reaches 20%,the activated phase is dominated by Fe₃C that produces more C₅⁺long-chain hydrocarbons（19.8%）;when the cobalt content is greater than 20%,the activated products are mostly Co₂C with light alkanes C₂⁰-C₄⁰ selectivity up to 15.2%.