Study Of Mo-based Catalysts For Dehydro-aromatization Of Methane

In this thesis, the influences of different preparation methods, HZSM-5 zeolite with different Si/Al, pretreatment methods of the support, the addition of promoter Fe and the addition of a small amount of H2 in methane feed were systemically investigated. The results show that the Mo/HZSM-5(DT) catalyst prepared by impregnation method, with Si/Al=25 in HZSM-5 and dehydration pretreatment, has better catalytic performance, because Mo species properly distribute on the internal and external surface of the catalyst and the acid strength is moderate for the reaction. After 32 hours time on stream, the methane conversion and benzene yield still remained at 8% and 6%. The further addition of promoter Fe into the catalyst led to a significant increase in methane conversion and the selectivity to benzene. In this case, the methane conversion and benzene yield increase by 18% and 52% respectively compared with those obtained from Mo/HZSM-5(DT) catalyst.The influence of different dealumination degree of HZSM-5 support upon the acid types and acid strength of the corresponding catalysts were investigated by characterization measurement, the relation between the acidic property of the catalysts and the catalytic performance was also elucidated. The results show that the pretreatment with different degree of dealumination exerts great influence on the acidic property of the support and the catalysts, the catalysts with moderate Bronsted acid sites and acid strength have better capability of removing carbonaceous deposit, so the activity and stability of the catalysts enhanced greatly.The types of the carbonaceous deposit as well as the catalyst deactivation were studied by XPS, TPO and TPH. It's concluded that there are at least there types of carbonaceous deposit on the surface of the catalyst: Mo2C and/or Mo2OxCy, graphitic carbon on the external surface of the zeolite, hydrogen-poor pregraphitic-type carbon on the internal and external surface of the zeolite, the amount of the last two types of carbonaceous deposit increase during the methane dehydro-aromatization, gradually cover the active sites and the external surface of the catalysts, finally leading to thedeactivation of the catalysts.