Study On Na-W-Mn/Ti-SBA-15 Catalyst For Oxidative Coupling Of Methane

Oxidative coupling of methane(OCM)is an effective way in converting natural gas directly into ethane and ethylene.It is of great significance to commercialize this technology,which will change the whole petrochemical industry.Na-W-Mn/SiO2 catalyst is one of the most promising catalysts for OCM due to its superior catalytic performance and good high-temperature stability.However,low yield and high reaction temperatures are still two major hurdles needed to be overcome.In recent years,it has been found that for Na-W-Mn catalysts,different silica support materials have a significant effect on the catalytic performance at lower temperatures and SBA-15 molecular sieves as carriers have better OCM catalytic performance than SiO2.The addition of Ti can effectively decrease the reaction temperature of OCM.Thus,in this dissertation,a series of Na-W-Mn-loaded catalysts with different supports(Ti-SBA-15,TS-1,Ti-MWW and SiO2)have been prepared and tested in the oxidative coupling of methane at different reaction temperatures.The results show that Na-W-Mn/Ti-SBA-15 catalyst has better performance at lower temperatures.On one hand,the addition of Ti effectively activates methane at lower temperatures and improves the migration of active components from the bulk phase to the surface of catalysts.One the other hand,Ti-SBA-15 also increases the concentration of surface active phases and surface active oxygen species.Meanwhile,in order to further study the effects of Ti and W,different contents of Ti and W have been investigated in Na-W-Mn/Ti-SBA-15 catalysts.It is found that the change of Ti content does not have a significant effect on the active phase of the catalyst.The proper amount of Ti is beneficial to the migration of active components to the surface and improve the performance of the catalyst.However,too high content of Ti will cover the active components of the catalyst and decrease the rate of W activation of methane,which has an adverse impact on the catalytic performance.The regulation of the W content causes the catalyst to exhibit different reaction performance at higher temperatures and lower temperatures.At higher temperatures,too high or too low W content is detrimental to the formation of the active crystalline phases like Na2WO4 and Mn2O3,which decreases the activity of the catalyst.At lower temperatures,the ability of the catalyst to activate CH4 is the key to its catalytic ability.The catalyst has better activity when the Ti/W ratio is 1 or 4.The prepared Na-W-Mn/Ti-SBA-15 catalyst with Si/Ti ratio of 30 and Ti/W ratio of 4 has better catalytic performance at 700? with 19.4%of CH4 conversion and 10.9%of C2 yield.