Study On Anaerobic Conversion Of Methane To C₂₊ Chemicals

Methane is abundant in reserves and inexpensive,and has the commercial potential of catalytic conversion to more valuable chemicals.Driven by national"dual carbon"goals,direct,nonoxidative conversion of methane has attracted much attention due to its high utilization of carbon atoms.Since reaction is thermodynamic limited,metals such as Mo,Fe and Zn are usually required as active center of the catalyst to participate in methane activation process.Metal Fe has become a potentially viable active metal species due to its friendly price and readily available raw materials,but conventional supported iron-based catalysts have serious problems of carbon deposition,making it difficult to industrialization.Therefore,this thesis prepared HZSM-5 catalyst by one-step hydrothermal synthesis method,and embedded metal Fe and second metal in it to explore the catalytic performance of bimetallic iron-based molecular sieve catalyst on the nonoxidative conversion of methane towards C₂₊chemicals.The main content is as follows:Firstly,taking Fe-Na metal as an example,the differences in structure and reactivity of supported and embedded bimetallic molecular sieves were explored in detail.The characterizations by XRD and SEM showed that both catalysts have typical ZSM-5 molecular sieve structure,the process of catalyzing the nonoxidative conversion of methane towards C₂₊chemicals was significantly different.Under the reaction condition of 1 atm,750℃and 2000m L/(g_cat?h),the deactivation rate of the conventional supported Fe-Na/HZSM-5catalyst was faster,and the methane conversion rate of the embedded Fe-Na@HZSM-5 was higher and more stable.Continue to adjust the Si/Al ratio of the molecular sieve,the loadings of the two metals and the impregnation sequence to optimize the preparation process of Fe-Na@HZSM-5.Finally,the Si/Al ratio was determined to be 80,the loadings of Fe metal and Na metal were 5 wt%and 0.5 wt%.Besides,the method of impregnating Na metal first was more favorable for the activity and stability of the reaction of nonoxidative conversion of methane to C₂₊chemicals.Under the reaction conditions of 1 atm,750°C and 2000 m L/(g_cat?h),the highest conversion of 5Fe-0.5Na@HZSM-5 catalyst was 8.2%,and the methane conversion was still above 4%after 8 hours of reaction.The conversion rate of methane decreased with the decrease of the reaction temperature.Under the reaction conditions of 700°C,the highest conversion rate of methane was 4.2%,and the catalyst can react stably for more than 25 hours.In addition,the iron-based molecular sieve catalysts were modified with different metal promoters.Six kinds of 5Fe-0.5X@HZSM-5 catalysts（X=Na,Ga,Co,Zn,Au or Pt）were prepared by a combination of sub-impregnation method and hydrothermal synthesis method.Evaluation of the activity of all samples were carried out at 750℃.The samples were analyzed by TG,XRD,NH₃-TPD,H₂-TPR and N₂ adsorption and desorption tests.Experiments showed that additional element can change the dispersibility of the active iron metal,have varying degrees of influence on the acidity and reducibility of the catalyst,and also affect the performance of catalyzing the anaerobic conversion of methane to C₂₊chemicals.The addition of Na contributed to the improvement of the activity and stability of the catalyst,and the highest conversion rate of methane reached 8.2%.Ga-promoted catalyst increased the initial conversion of methane to 11.8%,but the deactivation process of the catalyst was accelerated at the same time.The addition of Zn and Co significantly improved the reaction stability and the conversion only decreased by about 1%after 8 hours of reaction.The addition of precious metal Au had little effect on the conversion of CH₄,while the addition of Pt improved the interaction between the molecular sieve carrier and the active metal components,and the selectivity of benzene in the product increased,up to71.3%.