Study On The Catalyst Of Methane Dehydroaromatization Reaction

The main component of natural gas and shale gas is methane,which is abundant in China.In recent years,how to convert methane into chemical products with higher value is one of the hot research fields of scientific researchers.Methane dehydroaromatization,as a key technology that can directly convert methane into aromatic products,has attracted much attention.Because of its high conversion and aromatic selectivity,Mo/HZSM-5 catalyst is one of the most widely studied catalysts now.However,low stability and rapid deactivation of Mo/HZSM-5 catalysts severely restrict the further development and application of methane dehydroaromatization reaction.This thesis designed a Mo-based HZSM-5 zeolite catalyst with a completely new structure by embedding the active component Mo into the HZSM-5 zeolite.As a result,the efficient and stable Mo-embeded HZSM-5(Mo@HZSM-5)catalyst was obtained through in-situ hydrothermal synthesis method.This thesis first explored the optimal synthesis formula and crystallization conditions of the Mo@HZSM-5 catalyst,which were Si:Al:TPABr:NH4F:H2O=1:0.025:0.4:0.23:11.2(molar ratio)and 180?for 72 h respectively.In addition,the effects of the proportion of each raw material(template,mineralizer and solvent)and crystallization time on the performance and activity of the catalyst were also studied.By analyzing the related characterization and reaction evaluation of the Mo@HZSM-5 catalyst and Mo/HZSM-5 catalyst,we explored the differences in structure and properties of the two catalysts.Evaluation of the activity of the two catalysts was carry out at 700?,2000 mL/(gcat·h)and 0.1 MPa,we found that the activity and stability of Mo@HZSM-5 catalyst and the selectivity of benzene were significantly higher than those of Mo/HZSM-5 catalyst.After 9 hours of reaction,Mo@HZSM-5 catalyst still maintained a methane conversion rate of nearly 5%,an aromatics selectivity of 77%,and a benzene selectivity of 44%.By analyzing XPS and TEM results,we demonstrated that the Mo species were mainly located on the outer surface of the zeolite in Mo/HZSM-5 catalyst,while those were mainly embedded inside the zeolite in Mo@HZSM-5 catalyst.TG-DTA analysis proved that the aromatic-type coke in used Mo@HZSM-5 catalyst was significantly less than used Mo/HZSM-5 catalyst.Therefore,we made a conclusion that compared with Mo/HZSM-5 catalyst,Mo@HZSM-5 catalyst had a stronger shape-selective effect so as to suppress the formation of aromatic-type coke,which is the main reason for the high activity and stability of Mo@HZSM-5 catalyst.In addition,N2 adsorption-desorption and NH3-TPD characterization analysis showed that compared with Mo/HZSM-5 catalyst,Mo@HZSM-5 catalyst had higher specific surface area,better pore size distribution,and more Br(?)nsted acid sites,which also explained why the Mo@HZSM-5 catalyst had higher activity and stability.