Studies On The Crystallization And Modification Of Zeolites In Various Gel Systems, And Catalysis Of Ethanol Dehydration

Zeolites are a kind of microporous crystalline aluminosilicate materials with framework structures.The materials possess some excellent properties,such as uniform distributions of pore size,high surface area and pore volume,good thermal and hydrothermal stability,and easy tuning on the compositions,structures and properties.The materials can be widely used in industrial catalysis,adsorption separation and ion exchange.The synthesis of zeolite is the foundation for further research on properties and applications.Manufacturing zeolites by simple route from some raw materials with low-pollution and low cost is always the hot research points for many zeolite specialists.Therefore,in this thesis,the major research topics are focused on the new process for making some zeolites with wide applications in various sol-gel reactant systems,or for modifying some zeolites with low cost.Chapter 1 reviews the synthesis,modification and application of zeolites.Some synthesis methods of zeolites,such as hydrothermal synthesis,solvent-thermal synthesis and dry gel conversion,as well as the application of organic template, mineralization agent and microwave heating,are briefly introduced.Modification of zeolites,called as the secondary synthesis,can make the materials own some better properties on surface acidity,thermal and hydrothermal stability with methods of ion exchange,framework de-aluminium,replacement of some framework atoms, modification of channels and external surface.The modified zeolites can be applied in adsorption,catalysis and ion exchange,as well as in high-tech areas as advanced materials in recent years.Chapter 2 introduces a novel method for the synthesis of high-silica FER zeolite in THF/H₂O binary vapor phase with vapor phase transport(VPT),and the investigations on the crystallization kinetics and mechanisms in detail as well.Two sections in Chapter 2 introduce the crystallization process of seeded Na₂O-SiO₂-Al₂O₃ dry gel(SDG) in THF/H₂O binary vapor.In the first section,the gel and the products are characterized with XRD,SEM,N₂ adsorption and ¹³C CP MAS NMR for investigating the gel transformation process,and the roles of THF,H₂O and the added seeds are also discussed.In the second section,XRD,FT-IR,SEM,TEM,HRTEM and SAED are used to observe the crystallization process from the macroscopic view, and it has been found that the initial large globoids of SDG composed of nano-scale amorphous globules firstly diminish and disperse into smaller particles with high reaction activity in the beginning of the reaction gradually.FER zeolite crystallites appear at the reaction time of 10.5h,and in the zeolite crystallites further grow in the period from 6h to 35.5h.The reaction rapidly finishes at 36h,and the crystallinity remains till 72h without the structure collapse,which is defined as the re-crystallization stage.In the last section of the chapter,²⁷Al,²⁹Si,¹³C CP MAS NMR,NH₃-TPD and N₂ adsorption are used to investigate the coordination changing of[SiO₄]and[AlO₄]tetrahedrons in dry gel at the molecule level.At the same time, the interactions of template THF molecules with these tetrahedrons and formed zeolite framework are also studied.It is found that THF molecules diffuse from the surface of the gel particles and react with the tetrahedrons,which are then as the centers of FER cages to form the precursors and further construct the zeolite framework.Therefore,the nucleation and the growth process of the zeolite from dry gel can be understood in detail.Chapter 3 introduces the promoting roles of catalytic amount of OP emulsion agent or glycerol in the crystallization process ofFER zeolite as additives in a reactant system of THF-Na₂O-SiO₂-Al₂O₃-H₂O.The target zeolite in the reactant system usually easily co-crystallizes with some impure phases such as MTN,MFI,MOR andα-quartz,leading to that pure FER only crystallizes in a restricted composition region of alkalinity and Si/Al ratio,and a narrow range of reaction temperature and reaction time as well.These limit the scale-up production of FER zeolite.XRD,SEM,²⁷Al, ¹³C CP MAS NMR,XRF and N₂ adsorption are used to characterize the crystallization products in the reactant system with catalytic amount of OP emulsion agent or glycerol,and the obvious promoting role of the additives is found in the zeolite crystallization process,which can lower the reaction temperature,shorten the reaction time and effectively restrict the growth of impure phase.The zeolite products present a form of poly-crystals composed of tiny single crystallites,and possess high surface area and pore volume.STI zeolite is a new natural mineral resource found in Beijing area,with high quality and storage,which is worth to be studied and exploited as useful microporous materials.Because the framework thermal stability of the zeolite is very poor,the detailed research on properties and application is little to be reported.In Chapter 4, the methods of ion exchange and controlled calcinations are used to modify the zeolite framework,and the ultra-stable products with the framework thermal stability higher than 1000℃are recieved.It has been found that the natural zeolite possesses excellent anti-acidity property in hot HCl solutions with different concentrations. Based on this finding,the properties of ion exchange and thermal stabilities of the modified ultra-stable STI zeolite with various cation-types are further studied.The ultra-stable STI zeolite as the basic material has been modified with HCl solution and ammonium hexafluorosilicate solution,respectively,to get the samples with higher Si/Al ratios.The adsorption properties and the framework thermal stability are also determined.All the research works above create a foundation for further industrial application in the adsorption separation,ion exchange and catalysis.Chapter 5 introduces the modification studies of a commercial binder-free ZSM-5 zeolite catalyst,which can be used in the catalysis of diluted ethanol dehydration to prepare ethylene(ETE).The adsorption properties of binder-free and binder-containing ZSM-5 zeolite catalysts are compared,indicating that the former one possesses the excellent properties.The methods of HCl solution treatment,steam treatment,steam combined with HCl solution treatment are taken to modify the framework of binder-free zeolite catalyst,and the reason of the deep de-alumunium and strong tuning on the zeolite framework and surface properties is also revealed. The ETE reactions on the low-silica binder-containing and binder-free zeolite catalysts with SAR of 30 are compared,and it has been found that the latter one possesses the higher reaction activity.Three binder-free zeolite catalysts with different SAR are also studied in the ETE reaction,and the reaction activity lowers as the increase of framework SAR.The low-silica HZSM-5 zeolite catalyst possesses the highest ethylene selectivity and yielding.The modified low-silica binder-free zeolite catalyst prepared with the combined treatment of steam and HCl solution shows the excellent catalysis properties in the ETE reaction.Chapter 6 briefly introduces the research work of coal-based kaolinite transforming to zeolites.The main mineral composition of coal gangue is kaolinte, which is also called coal-based kaolinite,and is the castoff in the excavating process of coal.Without used effectively,the coal-based kaolinite must pollute the environment as a waste.In this chapter,the inorganic directing method is used to synthesize NaY zeolite hydrothermally with the coal-based kaolinite and water glass as the raw materials.The kaolinite transforms to metakaolin after calcination at high temperature,which can be used as the main aluminium source in preparing raw sol-gel materials for the further synthesis.The silica sol and water glass are used as supplementary silica source,respectively,and the crystallite clusters composed of many pure nano-sized NaY zeolite are synthesized.The adsorption properties of the prepared NaY zeolite are better than that of the commercial one synthesized with all chemical raw materials.