Synthesis Of Nanosized Alumino/Germanosilicate Zeolites In Concentrated Gel Systems And Their Catalytic Properties

Because of their superior catalytic and adsorption properties,zeolites have been widely used in petrochemical,coal and fine chemical industries.Synthesizing zeolites with new topologies and developing novel synthesis strategies are two main focuses in the field of zeolites.The micropores in zeolites exhibit“molecular sieving”effect,leading to a unique shape-selectivity for the reactants,products,and transition states in catalytic reactions.Besides the channel structures,the morphology and size of the zeolite crystals also have an important effect on the diffusivity in reactions.Moreover,it has been proved that nanosized zeolites with larger external surface and shorter diffusion path length show better catalytic activities in diffusion limitated reactions.In order to promote the catalytic performance of zeolites,considerable efforts have been devoted to improve inherent diffusion limitations of zeolites by tailoring their crystal morphology,size and porosity.Conventionally,zeolites are synthesized by hydrothermal methods where water content excerts an important influence on the morphology and size of zeolites.In contrast in the concentrated gel systems,zeolites are synthesized with only a small amount of water?H₂O/T molar ratio?10,T=Si,Ge,Al,etc.?,which is conducive to the formation of crystal nuclei.Therefore,nanosized zeolites would be more favored in the concentrated gel systems.Besides the decrease of crystal sizes,synthesis of zeolites with hierarchically porous structures can also afford better mass transport efficiency and catalytic activity.Currently,destructive strategies such as demetallization and recrystallization,constructive methods such as using hard templates,soft templates,supramolecular templates,as well as self-assembly methods have been utilized to generate the hierarchical zeolites.However,it is noted that these strategies mostly involve the complicated post-modifications and additional mesopore-directing agents.Hence,it is highly desirable to develop a cost-effective method to synthesize nanosized hierarchical zeolites.In this thesis,we used different quaternary ammonium salts as the structural template agents in concentrated gel systems to synthesize different nanosized aluminosilicate and germanosilicates.In addition,we studied the influence of different synthesis parameters on the morphologies,sizes and catalytic properties of the as-prepared materials.The main results in this thesis are as follows:1.Nanosized hierarchical zeolites are desirable for the reduction of diffusion limitations in catalysis.Herein,we report a cost-effective strategy to synthesize nanosized germanosilicate ITQ-21 zeolites with abundant intracrystalline meso/macropores upon modulating zeolite crystallization.With quaternary ammonium N-cyclohexyl-N,N-dimethylcyclohexanaminium hydroxide as the organic structure-directing agent,nanosized hierarchical ITQ-21 zeolites have been successfully prepared in a concentrated gel system free of any mesoporogen or seed crystals.A possible mechanism based on aggregation followed by ripening process is proposed.The as-prepared nanosized hierarchical Al-ITQ-21 zeolites show good catalytic activity for the acetal reaction of converting bulky substrates.This work provides a facile route upon modulating crystallization to generate intracrystalline mesopores in zeolite catalysts.2.Based on the“acceleration of nucleation”strategy,we have synthesized a series of different nanosized aluminosilicate MTW zeolites.With seed-assisted route,the size and morphology of samples can be changed.The water content,Si/Al molar ratios and alkalinity are proved to be crutial to the formation of nanosized MTW zeolites.Powder X-ray diffraction,N₂ adsorption-desorption analyses,NH₃-TPD,scanning electron microscopy,transmission electron microscopy,elemental analyses and solid state MAS NMR analyses have been used for characterizations.In addition,through the catalytic reaction of making lactide and PLA from lactic acid,the influence of Si/Al molar ratios and crystal sizes on MTW zeolites have been investigated.3.Silicogermanate?JU110?with an interrupted open-framework has been synthesizedbyhydrothermalmethodusing1,1'-?1,4-phenylenebis?methylene??bis?1-methylpyrrolidin-1-ium?hydroxide as an organic structure-directing agent?OSDA?.Silicon and fluoride anions are introduced to the concentrated-gel synthesis system,and the influence of different synthetic parameters on the influence of the synthesis is discussed.The structure of JU110 is characterized by using rotation electron diffraction?RED?and high-resolution powder X-ray diffraction.JU110 crystallizes in the space group Fm2m?No.42?with a=13.9118?2??,b=18.2981?3??and c=32.7799?6??.The structure is constructed by the sti layers found in the STI framework that are pillared by D4R/Ge₇ units to form a large cavity,showing 10-ring opening along[100]and 9-ring opening along[110].