| Syngas（CO and H2）obtained from coal,natural gas and biomass could be converted into fuels and chemicals,which is one of the most challenging subjects in the field of C1 chemistry.Olefins including lower olefins(C=2-4,ethylene,propylene,butene)and long-chain olefins(C=5+),are important raw materials in chemical industry.Currently,although methanol to olefins（MTO）technology has been applied in industry,this process is still rather complex and the economic efficiency is not high.Therefore,Fischer-Tropsch to olefins（FTO）via syngas has attracted much attention.Recently,CoMn catalysts have been successfully applied in FTO reaction.It was found that cobalt-manganese complex oxide(CoxMn3-xO4)catalyst afforded high C=2-4 selectivity and inhibited the formation of methane effectively under mild reaction conditions,and the product distribution deviated from the traditional ASF distribution.More specifically,the Co2C nanoprisms with exposed（101）and（020）facets are the active sites for Co-based FTO reaction.However,there are still many scientific problems to be solved:the effect of Mn on the structure and catalytic performance;the effect of catalyst precursor and reaction conditions（such as pressure,temperature,H2/CO ratio,space velocity）;the detailed reaction kinetics of three typical catalysts（Co2C nanoprisms,metallic Co0 and Co2C nanospheres）.In this paper,the above mentioned scientific problems are investigated in detail as following:1.The effect of Mn on FTO reactionThe addition of Mn exhibited a profound effect on the catalyst structure and catalytic performance.With the addition of Mn,both the activity and C=2-4 selectivity increased.However,excessive addition of Mn led to the decrease of both activity and C=2-4 selectivity,and the Co2Mn1 catalyst exhibited the highest activity and C=2-4 selectivity.In addition,it was found that higher content of Co2C nanoprisms in the catalyst was constant with better FTO performance.The product distribution of Co5Mn1,Co3Mn1 and Co2Mn1 catalysts deviated from the traditional ASF plots,while the un-promoted catalyst and the one with very high Mn amount followed the typical ASF distribution with high methane selectivity.However,under higher reaction pressure or temperature,the catalyst with high Mn amount also exhibited promising FTO performance.Therefore,all the studied CoMn catalysts can be used for FTO reaction under suitable conditions,and Mn promoter can promote the generation of Co2C nanoprisms as the active sites for FTO reaction.2.Effect of reduction conditions on FTO reactionThe catalysts reduced under 10%CO-300℃,10%H2-300℃and10%H2-250℃exhibited obvious activation process.When the catalytic performance reached stable,the product distribution deviated from the traditional ASF distribution,and a large amount of Co2C nanoprisms were generated during the reaction process.However,such phenomenon was not observed for the catalysts reduced under CO-300℃,10%H2-350℃and 10%H2-400℃.In contrast,low activity,low C=2-4 selectivity and high methane selectivity were found.In addition,the product distribution obeyed the traditional ASF distribution for the catalysts reduced under CO-300℃,10%H2-350℃and 10%H2-400℃.According to the structure characterization,Co2C nanospheres were detected for the spent catalysts reduced under CO-300℃,10%H2-350℃and 10%H2-400℃.Based on the above results,it was suggested that Co2C nanoprisms were generated under low carbonation rate via CoxMn1-xO,while Co2C nanospheres were formed under fast carbonation rate.3.Effect of reaction conditions of CoMn catalystsThe reaction conditions such as reaction temperature,space velocity,reaction pressure and H2/CO ratio have great influence on the structure and performance of the catalysts.High reaction temperature led to the aggregation and deactivation of the catalyst,and high space velocity caused the further reduction of Co2C,while lower H2/CO ratio was favorable for the formation of long-chain olefins.Accoridng to the study of pressure effects,it was found that under low reaction pressure,the CoMn catalysts afforded low methane selectivity,high C=2-4 selectivity and high olefin/paraffin（O/P）ratio,and the product distribution of hydrocarbon deviated from the traditional ASF plots.Co species mainly existed in the form of Co2C nanoprisms and CoxMn1-xO for the spent catalyst.Under high reaction pressure,the CoMn catalysts exhibited high methane selectivity and oxygenate selectivity.The hydrocarbon product distribution followed the traditional ASF distribution and the Co species in the spent catalyst mainly existed in the form of Co2C nanospheres and metallic Co0.It seemed that the increase of reaction pressure caused the change of the overall catalytic performance from FTO to HAS.In addition,the existence of cobalt species in the catalyst changed greatly by the increase of pressure.Co2C nanoprisms were considered to be the active sites for FTO,while Co2C/Co0 as the due active sites for HAS.4.Dynamics study of several Co-based catalysts for syngas conversionThe dynamics study suggested that the formation rate of olefins was significantly higher than that of paraffins for Co2C nanoprisms,while the opposite trend was found for metallic Co0.No obvious trendency was detected for Co2C nanospheres.Such results were constant with the conclusions that Co2C nanoprisms were the active sites of FTO,while metallic Co0 was the active phase for the typical FTS reaction and Co2C nanospheres were considered as the main deaction reason for Co-based FT catalysts with low activity and high methane selectivity.The reaction orders were also investigated.For Co2C nanoprisms,the CO order was higher than H2 order.For metallic Co0,the H2 order was higher than CO order.For Co2C nanospheres,the H2 order was slightly higher than CO order.It seemed that Co2C nanoprisms tended to produce more olefins with weak ability for second hydrogenation reaction,while metallic Co0 prefered to produce long-chain paraffins and Co2C nanospheres were also prone to generated paraffins with large amount of methane. |
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