Study On The Preparations And Applications Of Several Important Zeolite Materials

Zeolite is one kind of inorganic microporous materials widely applied in industrial processes and high tech materials for its excellent performance on ion-exchange,adsorption,shape-selectivity,structure controllability,catalysis and guest accommodation as host materials,etc.Recently,novel methods of synthesis of zeolites and their applications have attracted many attentions.This thesis is focus on the direct synthesis of high silica MOR type(mordenite) zeolite with particular adsorption and catalytic properties,and on MFI type(ZSM-5) zeolite catalysts applied in methanol dehydration processes,and on the preparation and applications of host/guest materials of zeolite/metal oxides.The main objectives are developing novel methods of zeolite synthesis,understanding the complicated formation mechanism and looking for the new applications of zeolite complex materials in fields of catalysis and sensors.Three parts in this thesis are as follows:1) the study of direct synthesis and the formation mechanism of high silica MOR zeolite;2) research on the preparation and catalytic performance of zeolite catalysts in methanol dehydration process in coal chemical industry;3) preparations and applications of metal oxides/zeolites complex materials.(1) The study of direct synthesis and the formation mechanism of high silica MOR zeoliteMOR type zeolite consists of straight channels with 12-membered ring(12-MR) window(0.65×0.70nm) along[001]direction and possesses high thermal and anti-acidic stability.The zeolite is an important aluminosilicate molecular sieve widely used in adsorption separation and catalysis such as cracking,isomerization, and alkylation of hydrocarbons.In this thesis,we report two novel methods for synthesis of the zeolite:dry gel self-transformation in full inorganic system and hydrothermal synthesis in amine-free and fluoride-free ether-containing reactant system.In chapter 2,the dry gel self-transformation(DGST) method of the zeolite is reported.Identifications of the obtained products with powder X-ray diffraction(XRD) indicate that the molar ratio of silica to alumina(SAR) and the alkalinity in the starting dry gel play essential roles in the crystallization of the zeolite with pure phase. Meanwhile,the existence of seedings effectively accelerates and promotes the zeolite crystallization,and prevents the co-crystallization of other impurity phases.The gel composition region for the zeolite crystallization is investigated and expressed with diagram in detail.Rod shape single crystals with size of 1～3×0.2～0.5μm for the as-synthesized zeolite with SAR up to 30 are prepared from DGST with optimum gel compositions.The synthesized products possess perfect framework and channel system,0.18cm³/g of micropore volume,378m²/g of BET surface area and 9.0wt% of benzene adsorption capacity characterized by scanning electron microscope(SEM), high-resolution transmission electron microscope(HRTEM) and selected-area electron diffraction(SAED),²⁷Al magic-angle spinning(MAS) nuclear magnetic resonance(NMR),and adsorption determination.Investigations on the obtained products with XRD prove that the dry gel transformation is a continuous process with three stages possessing different crystallization rates.In the DGST process,the dissociated alkali from dry gel drops down and dissolves in the water that increases the contents of Na₂O and SiO₂ in mother liquid constantly.The results of²⁷Al MAS NMR and ²⁹Si MAS NMR indicate that the state of Al atom is tetrahedron coordination all along.During the crystallization of the zeolite,[AlO₄]tetrahedron preferentially access to the framework of the zeolite,resulting in the formation of the products with low SAR.In the late-stage of the crystallization,SiO₄ tetrahedron structures begin to settle on the zeolite framework.Great deals of Si atoms replace Al atom in the framework and induce re-structuring the framework for forming the zeolite product with high SAR. The SEM images present the change from the aggregated amorphous dry gel to the single rod crystals with perfect appearance.The high silica MOR zeolite exhibits much more excellent hydrothermal stability and solid acid property than that of the low SAR one,implying that the zeolite prepared by the DGST method possess promising application in adsorption and in catalysis.In chapter 3,the synthesis of high-silica MOR zeolite in the temperature range of 140～180℃is investigated in amine-free and fluoride-free Na₂O-Al₂O₃-SiO₂-H₂O reactant system using different alumina sources in the presence of ether,which plays an effectively promoting role in the crystallization and the increment of SAR for the zeolite.SAR and the alkalinity in the starting reactant gel make essential effects on the crystallization of the pure phase zeolite.The crystallization phenomena in the reactants with different alumina sources are similar,except obvious differences in the morphology and SAR of the as-synthesized zeolite.A pure phase MOR zeolite with SAR up to 30 is obtained in the reactant with optimum compositions under a certain reaction condition.Characterized by nitrogen sorption and ²⁷Al MAS NMR,perfect framework and open channel system of the zeolite have been proved.The high silica MOR zeolite exhibits much more excellent hydrothermal stability than that of the low SAR one.(2) Research on the preparation and catalytic performance of zeolite catalysts in methanol dehydration process in coal chemical industryDimethyl ether and propylene are widely applied in chemical industry as important chemicals.ZSM-5 zeolite possesses outstanding catalytic performance in methanol dehydration due to its particular porous and acid properties.In chapter 5.1,a series of binder-free ZSM-5 catalysts and a binder-containing catalyst are prepared and characterized with XRD,XRF,²⁷Al MAS NMR,N₂ sorption and ammonia TPD.The catalytic activity and selectivity in the dehydration of crude methanol to DME are evaluated in a fixed-bed reactor for the catalysts.The outstanding structural characters such as high zeolite contents,sufficiently open channels and richness in mesopores have been proved on these binder-free catalysts. The influence of the solid-acidity,which closely related to the framework SAR of the catalysts,on the catalytic properties has been discussed.A binder-free catalyst with low framework SAR has been selected for in-depth research.The advantages of high activity and selectivity in wide range of operating temperature,non-sensitivity to water contents in the feed,long life-time and easiness to regeneration make the binder-free catalysts with low SAR good potential in industrial applications.The reason for its excellent performance of the catalyst is discussed.In chapter 5.2,the influences of the chemical characteristics of ZSM-5 zeolite catalysts to its catalytic performance in the reaction of methanol transforming to propylene(MTP) are studied,and a catalyst with super high SAR(>1000) presenting excellent catalytic capacity is prepared.High silica ZSM-5 zeolite(SAR>300) and silicalite-1(without Al) are chosen for comparison.The catalytic evaluation results indicate that the framework SAR of the catalysts plays essential role in selectivity of propylene and ratio of propylene to ethylene(PE).When the dehydration occurs on the catalyst with SAR less than 100,the reaction is incomplete with amounts of high olefins and aromatics as major products.Increasing the SAR of the catalyst to about 400,the conversion of methanol achieves to 100%and the selectivity of propylene significantly elevates to 40%.In the situation of super high SAR catalyst,the selectivity of propylene further increases to 45%.However,the full-silica catalyst exhibits non-selectivity to olefin but high degree of selectivity to methane.High silica catalyst and super high silica catalyst are post-treated to obtain dealuminated catalysts with full-silica framework,in which the trace Al located in the micropores or the supercages of the zeolites.The fact that the dealuminated catalysts present high conversion of methanol and selectivity to olefin implies the important role of Al whether its chemical states are framework or extra-framework in the MTP reaction.(3) Preparations and applications of metal oxides/zeolites complex materialszeolites are excellent materials to provide wide spaces for assembly and encapsulation of guest materials such as metal oxides,organic functional materials due to its huge surface areas and pore volumes.In chapter 7,Ti,Sn and Li cantaining metal oxides/zeolites complex metarials are prepared,and the applications on photocatalysis and gas sensors are studied.In chapter 7.1,nano TiO₂,one kind of metal oxide with high melting point,is prepared on the surfaces of ZSM-5 zeolites with various SAR by impregnation and calcinations under the guidance of de-wetting effect.The experimental results characterized by XRD,TEM indicate that the SAR of zeolite significantly influences the crystal phase of TiO₂ assembled.The amount of rutile phase increases and anatase decreases with elevating the SAR of the zeolite constantly.Nano TiO₂ particles less than 10nm are indetectable by XRD form on the surface of the zeolite at low loadings. With increasing the loadings,TiO₂ particles grow to large scale and form nano rod shape single crystallites with the size of 200×15nm.Temperature of calcinations in preparing is another important factor for influencing the states of TiO₂ in complex materials.When the temperature lowers to 400℃,no characteristic diffraction peaks of TiO₂ appear.Followed by increasing the temperature to 500℃,TiO₂ grow up with anatase as dominate phase.When the temperature rises to 600℃,the phase of TiO₂ nanoparticles transfer from anatase to rutile constantly and the particle size enlarges. The photocatalytic performance of TiO₂/zeolite complex material is evaluated by phenol photodegradation.The results reveal that the material with small particle size and more anatase phase of TiO₂ present outstanding photocatalytic capability.In chapter 7.2,STI,FER and FAU(Y) zeolites are used as matrix to prepare SnO₂ nano complex materials by impregnation of SnCl₂ solution and subsequently calcination at certain temperatures.XRD and TEM are used to characterize the crystalline phase,the morphology,the particle size and the agglomerative state of the formed nano-materials.SnO₂ nanorods with size of 12×60 nm form on the surface of Y zeolite with low SAR.Increasing the SAR of the zeolite to 20,the particle sizes of SnO₂ grown on the zeolite significantly decrease to less than 10 nm.The complex material based on Y zeolite with high SAR presents excellent sensitivity and repeatability to hydrogen in ppm concentration.On the surface of STI zeolite,SnO₂ nanoparticles about 10-20 nm in size grow up and exhibit high hydrogen sensitivity and repeatability.Unfortunately,SnO₂ aggregates in size of 50-300 nm form on FER zeolites instead of nano particles spread uniformly.The resistance of SnO₂/FER zeolite material is even equal to that of pure FER zeolite,presenting even no hydrogen sensitivity.In chapter 7.3,LiCl/FER zeolite guest/host material is prepared by assemble the guest LiCl into the channels of host FER zeolites with thermal dispersion method.By elevating the calcination temperature,an interaction between the guest and the host occurs and induces a phase transformation of the complex material.Characterization with XRD,FT-IR and TG/DTA/DTG,the process of phase transformation is proved to commence at the temperature of 500℃with the product of LiAlSi₃O₈.With further elevating the temperature,β-quartz appears at the temperature of 800℃as the primary product in the process of phase transformation and the relative content of which increases with decreasing the loadings of LiCl in the guest/host materials.