Highly Efficient Synthesis Of High Silica SSZ-13 Zeolites And Its Catalytic Performance In Methanol-to-olefins Reaction

Zeolites commonly synthesized by a conventional hydrothermal crystallization,the use of large quantities of solvents usually results in the generation of mother liquor,which occupies the space of the autoclave,reduces the synthesis efficiency and increases pollution.Therefore,the research of green routes with low pollution and high yield for synthesis of zeolites has been widely concerned.With the in-depth study of molecular sieves,the synthesis methods have gradually diversified,from the traditional hydrothermal synthesis to the dry-gel synthesis,solvent-free system synthesis,solid phase grinding synthesis and other methods,among them,steam-assisted crystallization method,a kind of the dry-gel synthesis route,has attracted wide attention because of its advantages such as less mother liquor discharge,less template dosage,low reactor pressure,high solid yield and environmental protection,at present,a variety of zeolites have been synthesized using the steam-assisted crystallization method.SSZ-13zeolite has a unique pore structure,suitable acidity and good hydrothermal stability,while restricting larger molecule entrance and departure that is critical to overall catalyst performance,it has been commercialized as a catalyst for methanol to olefin reaction.A large amount of expensive organic templates used to direct the hydrothermal synthesis of SSZ-13 zeolite,and the application in the MTO reaction is limited by its large grain size,small specific surface area,high acid content,poor coke resistance and easy deactivation.Therefore,it is of great theoretical significance and application value to seek synthetic methods with controllable grain size,high solid yield,and low template consumption.Based on the above background,the steam-assisted crystallization and ultra-concentrated system methods were adopted to synthesize high silica SSZ-13 zeolites in this work,aiming to decrease template dosage,increase solid yield of product and reduce mother liquor discharge.The main work is as follows:1.High silica SSZ-13 zeolite was synthesized by a ultra-concentrated system method under a low SDA/SiO₂ ratios synthetic system.The influence factors on the synthesis of SSZ-13 zeolites were investigated,such as H₂O/SiO₂ratios,OH^-/SiO₂ ratios and crystallization time,etc.High silica SSZ-13 samples were characterized by XRD,SEM,FT-IR,NH₃-TPD,and N2adsorption-desorption,the catalytic performance of zeolites were evaluated by methanol to olefins catalytic reaction.The results showed that with the decreasing H₂O/SiO₂ ratios from 20 to 3,the hydrothermal crystallization synthesis is gradually converted to ultra-concentration system,the solid yield of SSZ-13 samples significantly increases from 80%to 91%,the crystal size decreases from 2.3 to 1.6?m,and the crystallization rate increases obviously,inaddition,with the decreasing of H₂O/SiO₂ ratios,the catalytic life of the samples synthesized under the ultra-concentration system prolongs,and the highest olefins selectivity have followed the same trend.2.High silica SSZ-13 zeolite was synthesized by an efficient and green steam-assisted crystallization?SAC?method.Some influence factors,such as crystallization time,temperature,SDA/SiO₂ ratios and OH^-/SiO₂ ratios,etc,were investigated.The optimum synthesis conditions were discussed and the properties of the samples were compared with those of samples synthesized by hydrothermal method under the same conditions.The results showed that with the decreasing SDA/SiO₂ ratios from 0.2 to 0.06,the relative crystallinity of SSZ-13 zeolites synthesized by the SAC routes only slightly decreases from98%to 91%,and the solid yield decreases from 95%to 82%.However,the peaks intensity of SSZ-13 zeolites synthesized by the hydrothermal crystallization routes significantly decreases from 98%to 78%with the decrease of SDA/SiO₂ ratios from 0.2 to 0.06,and the solid yield decreases from 79%to67%.The SSZ-13 samples synthesized by steam-assisted method have smaller crystal size,larger specific surface area,higher SiO₂/Al₂O₃ ratio and longer catalytic life than that of SSZ-13-x samples obtained at the same SDA/SiO₂ratios by the hydrothermal crystallization method.3.The drying conditions of gel,the relationship between the amount of water vapor at the bottom and the dry gel powder were discussed,the performance of high silica SSZ-13 samples were characterized,and the catalytic performance were evaluated by methanol to olefins reaction.The results showed that the preparation conditions of dry gel had a remarkable effect on the crystallization process,increasing the gel drying temperature and extending the drying time could accelerate the crystallization rate of SSZ-13 zeolite.It was found that the minimum water content at the bottom of the reactor is necessary to satisfy the saturated vapor pressure under specific conditions,which is related to the volume of reactor and crystallization temperature.Meanwhile,it is necessary to change the volume of water vapor and ensure that dry gel powder can fully contact with water vapor.When the amount of water vapor satisfies a certain saturated vapor pressure and solid-liquid ratio,the crystal size and morphology have little change but the total acid content increase with the further increasing of water vapor amount,and the catalytic lifetime of SSZ-13 zeolites was gradually shortened.