Highly Selective Synthesis Of Olefins Over Cobalt-based Fischer-tropsch Catalysts

Energy reserves in china are characterized by coal-rich,oil-poor,and gas-lean.At present,coal-based energy consumption route will lead to increasingly prominent environmental problems.Syngas obtained from coal,coal bed methane or shale gas is not only able to produce clean fuel and high value-added chemicals through Fischer-Tropsch Synthesis?FTS?,but also can effectively alleviate the environmental problems caused by the utilization of coal resources,which has been widely concerned.Olefins are important chemical intermediates.With the improvement of national living standards,the demand for olefins is also growing.Currently,the production of olefins by Fischer-Tropsch synthesis has become a research hotspot.Iron-based and cobalt-based catalysts are common catalytic systems for FTS.Compared with iron-based catalysts,cobalt-based catalysts have the advantages of high conversion,low water-gas shift activity,and inability to deactivate,but their easy hydrogenation ability makes the synthesis of olefins more difficult.In the current research of cobalt-based catalysts for olefins,the mechanism of the promoter manganese?Mn?to facilitate olefin formation is not clear enough.The relationship between the phase state and crystal facet of cobalt and the catalytic performance is not fully understood.And the research on the cobalt-based catalyst system for light olefins is not comprehensive enough.Solving these problems has important guiding significance for highly selective synthesis of olefins over cobalt-based Fischer-Tropsch catalysts.?1?In this paper,a series of catalysts were prepared by incipient wetness impregnation.The effect of Mn promoter on the catalytic performance over 15Co/SiO₂ catalyst was investigated.H₂-TPR shows that Mn inhibits the reduction of cobalt oxides.In situ XPS exhibits that Mn decreases the electron density of d orbital in Co atoms to promote the adsorption and dissociation of CO.XRD and HRTEM methods confirmed that the metallic Co?111?gradually evolves into Co₃C?101?during FTS reaction by introducing Mn.And Co₃C?101?has low catalytic activity with favoring olefins,especially for lower olefins,but its formation depends on the operating conditions.Therefore,changing the operating conditions such as the reaction temperature and the reaction pressure can efficiently adjust olefin distribution and an excellent selectivity of 74.2%in total olefins is obtained.?2?A reduction carbonation reduction?RCR?pretreatment method was used to flexibly control the phase state and crystal facet of cobalt over 15Co/SiO₂ and 15Co3.7Mn/SiO₂catalysts,and the corresponding catalytic performances were studied.For 15Co/SiO₂ catalyst,Co₃O₄?311?is always firstly reduced to metallic Co?111?followed by carburization with CO to form Co₂C?111?.However,second reduction over Co₂C?111?results in Co?101?and Co?002?.Co₂C?111?shows a negligible activity with undesired high selectivity to methane.And RCR pretreatment improves the selectivity to total olefins over 15Co/SiO₂ catalyst because Co?101?and Co?002?display easier formation of olefins than Co?111?.For15Co3.7Mn/SiO₂ catalyst,the addition of Mn only alters the evolution of Co₂C?111?into Co?101?and Co?100?in the secondary reduction process.Because Co?101?and Co?100?are kept almost unchanged and Co₃C?101?is not formed during FTS reaction,RCR pretreatment exhibits a great improvement in the stability of 15Co3.7Mn/SiO₂ catalyst but reduces the selectivity to total olefins.?3?Cobalt-based catalysts were supported by two types of materials?carbon materials and non-carbon materials?,and the effects of Na and Mn promoter on different supported cobalt-based catalysts were investigated.Compared with non-carbon materials,carbon materials are more suitable for the synthesis of lower olefins to support cobalt-based catalysts.For 30Co10Mn0.4Na/AC-2 catalyst,the selectivity to lower olefins could reach 33.3%by adjusting the reaction conditions.For carbon supported cobalt-based catalysts,Na promotes the non-dissociative adsorption of CO and increases the selectivity of CO₂.The Mn promotes the dissociation of CO to facilitate more cobalt carbide.For non-carbon supported cobalt-based catalysts,Na inhibits hydrogen adsorption,leading to reduce the catalytic activity.Mn also can promote dissociation of CO.But metallic Co phase state remains during FTS reaction under the same condition.Moreover,the selectivity of CO₂ over the cobalt-based catalysts is obviously affected by the reaction temperature,especially when the reaction temperature is higher than 240°C,as well as the supports and alkali promoter.