Research On Methanol-to-olefins Reaction Catalyzed By Metal Cations Modified SAPO Molecular Sieve

The methanol-to-olefins(MTO)reaction is an important non-petrochemical route for the production of light olefins.It is of great significance to enhance the selectivity of ethylene and light olefins in the MTO reaction.With the application of SAPO molecular sieves modified with metal cations,the product distribution in the MTO reaction was adjusted,and enhanced selectivity to ethylene and light olefins were efficiently achieved in this thesis.Together with the analysis of the diffusion property variation and coke formation during the reaction process,the relationship of "metal cation incorporation-reaction environment modification-adjustment of reaction and diffusion property"is revealed.The SAPO-34 catalysts were modified with zinc,copper,cobalt,nickel,magnesium and calcium cations by two-step ion exchange process,and higher selectivity to light olefins were achieved over the zinc and copper cations modified SAPO-34 catalysts.Then the SAPO-34 catalysts were modified with zinc and copper cations by different approaches including two-step ion exchange process,template-assisted ion incorporation and alcoholic ion exchange process.The zinc and copper cations are enriched in the shell layer near the external surface of the zinc and copper cations modified SAPO-34 catalysts.The enrichment of metal cations in the shell layer near the external surface has great impact on the catalytic performance of MTO reaction of metal cations modified SAPO-34 catalysts.the higher enrichment of metal cations in the shell layer,the higher selectivity to ethylene and ratio of ethylene to propylene in the MTO reaction during the intial reaction period.For the SAPO-34 catalysts modified with zinc cations by the two-step ion exchange process,the distribution of zinc species was comprehensively analysed,and the diffusion property of zinc cations modified SAPO-34 catalysts and the evolution of coking species in MTO reaction over zinc cations modified SAPO-34 catalysts were characterized via light alkane and light olefins as probing molecules.The relationship of "metal cations incorporation-reaction environment modification-adjustment of reaction and diffusion property" was established.In the MTO reaction,enhanced selectivity to ethylene and light olefins were achieved,which was attributed to the modification of diffusion property and the promotion of ethylene generation associated with the zinc cations incorporation in SAPO-34 catalysts.The SAPO-18 molecular sieve was also modified with zinc cations,and the catalytic performance of MTO reaction over zinc cations modified SAPO-18 catalyst was investigated.The zinc cations modification effectively shifts the products distribution from propylene as the main product to comparable production of ethylene and propylene.which testifies the feasibility of zinc cations modification on the improvement of catalytic performance of MTO reaction over SAPO molecular sieves.The preparation of zinc cations modified SAPO-34 catalyst was directly realized in one step by the adoption of template-assisted ion incorporation method.The incorporation of zinc cations modifies the CHA cavity and adjusts the diffusion property of the catalysts,which increases the diffusion limitation for products with relatively larger molecular size.The incorporation of zinc cations also promotes the formation of lower methylbenzenes that favors the ethylene generation.The adjustment of diffusion property and coking species leads to enhanced selectivity to ethylene and light olefins at the begining reaction period of MTO reaction.The template-assisted ion incorporation process acts as an effective and convenient method of catalyst modification for the adjustment of product distribution in the MTO reaction.