Study On Catalyst And Process For Oxidative Coupling Of Methane With High C₂ Hydrocarbons Selectivity

The oxidative coupling of methane produces ethylene by one-step reaction,bearing the advantage of short conversion process and good economy.However,this process has not yet been industrialized,and the key factor hindering its industrialization is that the catalyst performance is not high enough,and the single-pass yield of C₂ hydrocarbons（ethylene and ethane）is difficult to reach higher than 30%.At present,Na-W-Mn/SiO₂catalyst is still one of the best catalysts for oxidative coupling of methane,and has great potential for industrialization.In this thesis,the Mn-Na₂WO₄/SiO₂ catalysts prepared by different method for oxidative coupling of methane were studied in detail.The preparation method,composition and reaction process conditions of the catalysts were optimized.The affecting factors such as oxidant,diluent,feed gas linear velocity and reaction tube structure on the reaction performance were investigated,and the catalysts were characterized by techniques such as XRD,SEM and MIP.The obtained results were as follows:Compared with the high temperature melting method,the coprecipitation method and the impregnation method,the catalyst prepared by the impregnation method showed good reaction performance.At 800℃and under conditions of CH₄:O₂:H₂O=100:16.7:400,the sum of methane conversion and C₂ hydrocarbon selectivity reached111.4%.Steam was added to the reaction feed,which could significantly improve catalytic activity and C₂ selectivity.Compared with the reaction performed without steam,the CH₄ conversion increased from 23.7%to 29.1%,and the C₂ hydrocarbon selectivity increased from 63.1%to 76.1%.Increasing the linear velocity of the feed gas was beneficial to the selective oxidation of methane and improved the C₂ selectivity.When the catalyst increased from 0.100 g to0.400 g under the conditions of 800℃and CH₄/O₂=3.0,the linear velocity of the feed gas increased from 1.41 cm/s to 5.66 cm/s,and the C₂ hydrocarbon selectivity increased from 30.4%to 51.7%.In the 100 h stability experiment,the CH₄ conversion decreased from the initial 30.7%to 27.9%,and the C₂ selectivity increased from the initial 78.9%to 83.1%.The catalyst activity decreased slowly,and the C₂ selectivity showed a slow increase trend.XRD characterization showed that the crystal phases of SiO₂,Na₂WO₄ and Mn₂O₃could be detected before and after the reaction.MIP characterization showed that the specific surface area of the catalyst decreased with the increase of calcination temperature;SEM characterization showed that there were obvious differences in the morphology of the catalysts before and after the reaction.For the reaction performance of the catalyst,the specific surface area was not the most important factor.The crystal structure of SiO₂,Na₂WO₄ and Mn₂O₃,the particle size and the stacked pores in the catalysts played an important role.