Three-dimensional Ordered Hierachical ZSM-5 As The Catalyst For Methane Non-oxidate Aromatization And Its Coking Behavior During MTO Reaction

The proven reserves of nature gas in the world are so huge that it is very meaningful to use natural gas to produce important chemical raw materials in petrochemical industry.The methane non-oxidate aromatization reaction is an important approach to directly convert natural gas to aromatics which are important chemicals in industrial.Mo/ZSM-5 is the most commonly used catalyst for methane non-oxidate aromatization reaction.In this thesis,we synthesized hierarchical Mo/ZSM-5 catalyst with controllable size and Si/Al ratio using three-dimensionally ordered porous carbon as a hard template and evaluated its catalytic performance with methane non-oxidate aromatization reaction.The results of the reaction were compared with that of the regular Mo/ZSM-5 catalyst prepared through conventional impregnation method.Since the Mo/ZSM-5 catalyst is a kind of bifunctional catalyst,the distance between the Mo species and the acid sites is crucial for its catalyst performance.The influence of the distance between Mo species and acid sites on Mo/ZSM-5 catalyst was further investigated.A core-shell type Mo/ZSM-5@Silicalite-1 catalyst was prepared by epitaxial growth method,where the Mo species were loaded in the epitaxial grown layer of Silicalite-1 zeolite.Thus the distance can be controlled by adjusting the thickness of the MFI zeolite shell.Compared with the catalytic results using regular Mo/ZSM-5.It was concluded that the Mo/ZSM-5@Silicalite-1 catalyst exhibited worse catalytic performance,which might be due to the longer distance between the Mo species and the acid sites in the zeolites.On the other hand,Mo species usually exist in the form of molybdate anion and cannot be introduced into zeolite channels by ion exchange method.Therefore,the loading method of molybdenum also affects the catalyst performance in the Mo/ZSM-5 catalyst.Herein,we used MoS2 and MoO3 as the molybdenum sources to prepare Mo/ZSM-5 catalyst with the solid-state milling method.Characterization results showed that only a small number of Mo species that migrated to the pores of the zeolites and most of the Mo species were mainly located on the surface of the zeolites.As a result,the Mo species were separated from the acid sites,which led to a poorer catalytic performance.In addition,the three-dimensionally ordered hierarchical ZSM-5(3DOH ZSM-5)zeolites also exhibited excellent performance in the methanol to olefins(MTO)reaction.The results showed that the 3DOH ZSM-5 catalyst had a much longer catalytic lifetime than the commercial MFI zeolite catalyst CBV3024E as well as a higher olefins selectivity.The coking behaviors of the 3DOH ZSM-5 catalysts were followed using GC-MS and TGA techniques.It was concluded that the hierarchical pore structure which could accommodate more coke accounts for the prolonged catalyst lifetime.