Study On Modification Of ZSM-35 Zeolite And Its Isomerization Properties Of 1-Butene

As an important organic chemical raw material,isobutene is widely used in industrial production.In the process of isomerization of 1-butene,ZSM-35 zeolite showed excellent catalytic performance due to its unique pore structure,good hydrothermal stability and suitable acid site distribution.In this reaction,the catalytic performance of zeolite depends on the distribution state of framework Al atoms,which determines the number and distribution of active sites in the zeolite,and then affects the reaction path and changes the distribution of reaction products.In this study,by analyzing the number and distribution of Al atoms in the framework of ZSM-35 zeolite,combined with a micro-fixed bed evaluation reaction,to explore the influence of acid sites on the catalytic isomerization of 1-butene skeletal,providing a theoretical basis for the construction of a new and efficient isomerization catalyst.In the first part of this thesis,based on the characterization of ammonia temperature-programmed desorption（NH₃-TPD）and pyridine infrared（Py-IR）,the distribution of acid sites on ZSM-35 zeolites with different Si/Al ratios was further analyzed through UV-visible diffuse reflectance spectroscopy（UV-vis DRS）.Combined with the evaluation reaction of 1-butene skeletal,the influence of the number and distribution of acid sites on the isomerization performance of zeolite was explored.The results showed that the active site（Br?nsted acid site）of 1-butene skeletal isomerization reaction was mainly distributed in the intersection cavity of ZSM-35 zeolite,and the catalytic stability of zeolites was related to the number and distribution of acid sites,and the zeolite showed best isomerization stability when there was an appropriate amount of Br?nsted acid site in the intersection cavity.In addition,when the density of Br?nsted acid sites in zeolites is high,a large amount of carbon deposition will be produced in the reaction process,causing rapid deactivation of the catalyst.In the second part of this thesis,the influence of acid modification on the acid sites and isomerization performance of zeolites was investigated.It was found that appropriate acid modification could selectively remove some of the framework aluminum atoms,reduce the content of acid sites in the molecular sieve,and improve the stability of the catalyst without deeply destroying the crystal structure of the molecular sieve.After modification with oxalic acid,the zeolites showed higher yield of isobutene at the initial stage of isomerization reaction,but it contained a large number of Lewis acid sites and the catalyst deactivates quickly.However,after the modification of nitric acid and citric acid,an appropriate amount of acid sites remained in the zeolites,and the citric acid treatment made the acid sites more inclined to distribute in the intersection cavity of the zeolite,resulting in higher isobutene yield and selectivity during the reaction process.Finally,based on the preference of citric acid treatment as the modification method,the effects of citric acid concentration and acid treatment order on the isomerization performance were further investigated.It was found that H-ZSM-35 zeolite modified with 1.0 mol/L citric acid exhibited the best isomerization performance of 1-butene.Compared with the original ZSM-35 zeolite,the acid sites in modified zeolite were reduced,and the retained acid sites were more likely to be distributed in the intersection cavity,resulting in significantly improved isomerization stability of 1-butene.Under the same conditions,citric acid treatment of Na-ZSM-35 resulted in severe dealumination of the zeolite,and the proportion of acid sites in the eight-membered ring increased,resulting in rapid deactivation of the catalyst during the isomerization of 1-butene.