Effects Of Preparation Process On The Structure-performance Relationship Of Co₂C-based Catalyst For Direct Production Of Olefins Via Syngas Conversion

Fischer-Tropsch to olefins via syngas?FTO?is one of the most important research areas in C1 chemistry,and has attracted more and more interests among academic and industrial communities.As CO hydrogenation is a structure-sensitive reaction,the catalyst structure including crystalline phase,exposed facets and particle size will greatly affect the catalytic performance.According to previous studies,Co2C nanoprisms can directly convert syngas to olefins with very high selectivity under mild reaction conditions,and its product distribution deviates the traditional ASF model.To precisely control the surface structure of Co₂C nanostructures,it is necessary to investigate the effects of various factors during the preparation process for Co₂C-based FTO catalysts.Herein,effects of preparation process on the structure-performance of Co₂C-based catalyst for direct production of olefins via syngas conversion are investigated in detail to provide useful information for further catalyst optimization.The main research conclusions are listed as following:1.Effects of preparation methods on the structure-performance of Co₂C-based FTO catalystDifferent preparation methods,including precipitation and impregnation with different preparation sequences,were applied to synthesize the CoMn model catalysts.The results showed that different preparation methods significantly affected the interaction between Co and Mn species.When Co and Mn salt were simultaneously precipitated,Mn was in close proximity to the Co species to form CoMn composite carbonate.When Co precursor was precipitated firstly,CoMn composite carbonate was also obtained,although there was still some isolated Co species that did not form composite with Mn.However,when Mn precursor was first precipitated,Mn was completely separated with Co species.For the sample obtained via impregnation method,Co and Mn species could not form CoMn composite oxide and phase separation was observed.During syngas conversion process,those catalysts withMn1-x O phase could gradually evolve intonanoprisms with exposed facets of?101?and?020?,and exhibited high CO conversion,low methane selectivity and high olefins selectivity.The products distribution also had a great negative deviation from the traditional ASF model.However,with spherical morphology carburized from fcc-Co or Co O was observed for the other samples,which presented poor FTO catalytic performance with products distribution obeying the traditional ASF model.2.Effects of calcination temperature on the structure-performance of Co₂C-based FTO catalystCo-precipitated CoMn and inert CNTs-supported CoMn were selected as model catalysts to study the effect of calcination temperature on the structure-performance of Co₂C-based FTO catalysts.The results showed that different calcination temperatures affected the phase content and particle size of Co_xMn_1-xO precursor.As a result,different catalytic performances were observed.For the co-precipitation CoMn catalysts,the lower calcination temperature was not conducive to the decomposition of composite carbonate,with more fcc-Co and less Co_xMn_1-xO precursor after reduction.For the CMN/CNTs catalysts,the higher calcination temperature contributed to the excessive self-reduction capacity,which led to the further reduction of Co_xMn_1-xO precursor,thereby increasing the content of fcc-Co.The morphology of Co₂C carburized from fcc-Co was spherical,which exhibited a poor FTO activity.In addition,the particle size of Co_xMn_1-xO gradually increased with the increase of calcination temperature,which would decrease the carburization rate and led to a poor FTO activity.3.Effects of supports treatment on the structure-performance of Co₂C-based FTO catalystAl₂O₃ was used as the catalyst support to study the effect of supports treatment including heat treatment and surface modification on the structure-performance of Co₂C-based FTO catalysts.For the studied supported catalysts,the strong support-metal interaction hindered the the combination of Co and Mn to form Co_xMn_1-xO precursor,which resulted in lower catalytic activity.By directly increasing the calcination temperature of Al₂O₃ supports,the support-metal interaction could be weakened.Mg Al₂O₄ spinel phase could be formed when the Al₂O₃ support was modified by Mg,which not only effectively improved the catalytic activity,but also inhibited the formation of CH₄ and increased the selectivity of more valuable long-chain olefins.In addition,increasing the calcination temperature of Mg-modified Al₂O₃ supports could further weaken the support-metal interaction,which promoted the combination of Co and Mn to form Co_xMn_1-xO precursor,thereby improving CO conversion and olefins selectivity.