Preparation And Characterization Of High Performance ZSM-5 Zeolite For Methanol To Olefins

In recent years,with the rise of coal chemical industry,ZSM-5 zeolite has been widely used in methanol-to-olefins,methanol-to-aromatics,methanol and toluene methylation-to-para-xylene,alkane aromatization and other reactions,especially in the fixed-Bed methanol-to-propylene industrial plant.Acidity,A1 distribution,particle size,defect sites and texture of the ZSM-5 zeolite are the keys to the development of MTP catalysts.For ZSM-5 zeolites,the diffusion rate through straight channels is superior to sinusoidal channels.Therefore,it is of great significance to study modified ZSM-5 zeolite with a thinner thickness in b-axis direction.The location and distribution of the aluminum framework also significantly influence the accessibility of the acid sites and stereo-hindrance effect in the pores.Quantification of the framework Al distribution and location of the ZSM-5 zeolite helps to accurately understand catalyst activity,selectivity,and carbon deposition which may lead to catalyst deactivation.Addition of urea in the synthesis gel system can make the morphology of the zeolite gradually change from spherical to sheet form.Effects of urea content on the physicochemical properties of synthetic samples were characterized by XRD,BET,NH3-TPD and so on.The methanol-to-olefin reaction was investigated as a model reaction.The results showed that for samples with a Si/Al ratio of 50 or 150,the catalytic life and propylene selectivity of the sample increases gradually with the increase of urea content;while at the Si/Al of is 108,the sample shows the opposite performance,indicating that it is very important to match the Si/Al ration and the particle size of ZSM-5 zeolite to improve the catalytic performance of the zeolite.Seed crystals were added to the urea-added zeolite synthesis system.Nanosheet ZSM-5 zeolite as synthesized by seed induction method,is investigated to study its physical,chemical properties and MTP reaction performance.The evaluation results show that the nanosheet ZSM-5 zeolite greatly reduces the diffusion path along the b-axis and improves the diffusion performance.The catalytic lifetime is as long as 365 h,and the propylene selectivity is up to 50.77%.These results indicate that much better catalytic performance than that of conventionally synthesized sheet-shaped or spherical morphology ZSM-5 zeolites.The physicochemical properties and catalytic properties of zeolite before and after hydrothermal treatment were also compared in this study.The physicochemical property characterization results show that hydrothermal treatment will cause a slight decrease in the crystallinity of the zeolite.Hydrothermal treating also improves the pore structure,reduces the acidity of the zeolite,and eliminates the zeolite defects,especially the internal silicon hydroxyl.The catalytic performance data shows that with hydrothermal treatment,the service life and the propylene selectivity of the samples were greatly improved.With 29Si MAS NMR,27Al MAS NMR,XPS,Co(?)UV-vis-DRS,SEM,and XRF characterization was used to study Al distribution and its location in the ZSM-5 zeolites with different morphology.Characterization results indicate that the two kinds of morphologies of ZSM-5 zeolites had more framework Al atoms siting at the intersection of the channels;framework Al of the sheet ZSM-5 zeolite is almost always in the form of isolated aluminum,which accounts for 96%-98%of the total aluminum.In the spherical ZSM-5 zeolites S-40/80,there are only 67%-73%of isolated Al,i.e.,paired Al account for only 27%-33%.At low Si/Al ratios,the sheet zeolite shows the characteristics of lean surface aluminum and rich internal aluminum species,while the spherical zeolite shows the rich surface aluminum and lean internal aluminum species.