Preparation And Characterization Of Germanosilicate And High-silica Beta Zeolites

Zeolites have a wide range of applications in petrochemical and other industrial fields due to their regular pore structure,rich and diverse framework composition,and high thermal stability.Molecular sieves are usually prepared in the presence of organic templates,which leads to environmental pollution and high costs.Therefore,the development of a green,environmentally friendly,and low-cost synthesis strategy is one of the most important topics in the field of zeolite synthesis.In this thesis,a steaming-assisted conversion（SAC）method is employed to prepare a highly stable STW-zeotype germanosilicate zeolite with N,N-diethylethylenediamine（DEEDA）structural directing agent.Compared with the traditional hydrothermal method,the mild conditions in steaming-assisted synthesis can significantly increase the crystallinity and intra-framework Al content of products,but reduce the amount of organic templates.An array of characterization techniques was used to study the influence of water on the structure of the products during the steaming-assisted synthesis of germanosilicate zeolites.It was found that the produced framework gradually transformed from Ge O₂to STW-zeotype structure,and finally to the compound of Ge O₂,STW and MFI structure during the steaming-assisted synthesis.In addition,the trace amount of H₂O in the reactants was favorable for the hydrolysis,rearrangement,and condensation of inorganic source,especially for preferentially activating Ge species,which adjusts and even determines the structure of the final product to a certain extent.Beta（*BEA）zeolite has a three-dimensional micrstructure with a twelve-membered micropore,is an important industrial catalytic material.The acidity and stability of Beta zeolite can easily be adjusted by changing the framework composition,such as Si/Al ratio.A relative high Al content in Beta normally leads to the poor acidic strength and structural stability.On the other hand,the micropore structure limits the diffusion and transportation of reactants,intermediates,and products,therefore inhibits the applications in large molecule involved reactions.Therefore,the synthesis of high-silica Beta zeolite with a hierarchical pore structure has always been one of the hot topics in the field of zeolite synthesis.To reduce the waste liquid and high cost caused by the utilization of large amount of organic templates,we adopt a seed-assisted strategy to strengthen the structure-directing effect of the Beta precursors,optimize the preparation of the precursors,enhance their organic-inorganic interaction,and promote the generation of more primary structural building units in the early stage of crystallization.At the same time,the content of silicon species involved in the resulting structure is increased,the hydrolysis of those inorganic sources is accordingly adjusted,and the framework elements are quickly consumed.With assistance of seeds,the rearrangement,cross-linking and condensation of framework elements are optimized to prepare Beta zeolite with a high Si content.The optimal Si/Al ratio of reactants for the preparation of Beta zeolite in this thesis is ranged from 40 to 50.The obtained material has a large amount of interparticle mesoporous structure,in which Al species mostly exist in the form of four coordination,and Si species are mostly in Q4and Q3,the total amount of both which is as high as 94%.The obtained Beta nanoparticle aggregates have strong acidic strength,of which the strong B acid is dominant,with a B/L ratio of 5.85 at high desorption temperature of 723 K in Py-IR experiment;thanks to excellent mesoporous structures and acid properties.The Beta aggregates show high cracking activity in the LDPE catalytic cracking reaction,and the reaction temperature corresponding to the maximum cracking rate is 392℃.