Study On Catalytic Materials For Nonoxidative Conversion Of Methane At Low Temperature

With the increasing proved reserves and exploitation of natural gas and shale gas in China,it is of great significance to convert abundant natural gas and shale gas resources into high value-added chemicals efficiently.Direct,nonoxidative conversion of methane only uses cheap and abundant natural gas and shale gas as raw materials to produce high value-added aromatic products and hydrogen that are convenient for transportation and storage.It is an ideal direct conversion path for methane.However,most of the current related research is conducted under high temperature conditions（typically700～1100℃）,resulting in severe carbon deposition,rapid deactivation,poor stability,and high reaction energy consumption of the catalysts,which greatly limit its industrial application.In this study,a novel bifunctional catalyst for the nonoxidative conversion of methane was designed and prepared by physically mixing the Pt or Ni based supported catalyst modified by promoters with Mo supported molecular sieve catalyst in a certain proportion.This type of catalysts overcame the thermodynamic limitation and reduced the reaction temperature of nonoxidative conversion of methane through an efficient two-step relay catalytic reaction,effectively suppressed the carbon accumulation on the catalyst,and achieved a stable conversion of methane to aromatics and hydrogen at a lower temperature.At the same time,the effects of the addition of metal promoters Ga,Zn,and Sn and their loading amounts on methane conversion activity,product distribution,and catalyst stability were further investigated.The physical mixed bifunctional catalysts were characterized by XRD,SEM,NH₃-TPD,H₂-TPR,N₂adsorption desorption tests,and TGA-DTA.At a reaction temperature of 550℃and a reaction space velocity of 3000m L/g_cat·h,the conversion of methane over the 1Zn3Pt/Al₂O₃-6Mo/HZSM-5catalyst was 2.57%after a long reaction time of 30 hours,and the selectivity of aromatic products was higher than 98%.No significant deactivation was observed.Various characterization results indicate that these physical mixed catalysts effectively avoid catalyst sintering,maintains acidity,inhibits the generation of carbon deposits on the catalyst,and achieves stable conversion of methane to aromatics at low temperatures.Combining the reaction data and characterization results,this paper proposes the reaction mechanism of nonoxidative conversion of methane at low temperature over the new physical mixed bifunctional catalysts.The methane conversion reaction occurs on different components of the physical mixed catalyst,and different components of the catalyst have different functions.CH₄is first activated and dehydrogenated on Pt or Ni metal catalysts to generate CH_Xspecies,which are spontaneously coupled to form C₂Hy species.These CH_Xand C₂Hy species continue to oligomerize,cyclize,and dehydrogenate to form aromatic products on 6Mo/HZSM-5 catalysts,prompting the reaction equilibrium to continuously shift towards the products.In addition,under lower temperature（500℃）and higher space velocity(6000 m L/g_cat·h),the nonoxidative conversion of methane to aromatic hydro-carbons and hydrogen was achieved on promoter modified5Ni/Al₂O₃-6Mo/HZSM-5 catalysts.The characterization results show that physical mixing operations and the addition of promoters do not affect the crystal structure and acidity of the catalyst;Adding a certain amount of metal promoters can effectively control the product distribution and inhibit the occurrence of carbon deposition on the catalyst;Carbon nanotubes were synthesized while generating high value aromatic hydrocarbons and hydrogen.The new physical mixed bifunctional catalyst designed and prepared in this thesis not only opens up a promising pathway for the low-temperature nonoxidative conversion of methane to aromatics and hydrogen,but also provides a new catalyst design idea and construction strategy for further development of efficient catalysts for the nonoxidative conversion of methane.