In Situ Synchrotron Radiation Investigation On The Structural Evolution Of Germanosilicate Zeolites In Gas-Phase ADOR

Zeolites have been widely used in ion exchange,adsorption,separation,and catalysis,due to their unique characteristics of well-defined micropores,special catalytic sites.Up to now,however,only 255 topologically-different structures of zeolites have been identified,which is far from meeting the application requirements.Therefore,the design and synthesis of zeolites with novel structures have important scientific and application significance.The Assembly-Disassembly-OrganisationReassembly(ADOR)method is a new strategy proposed in the last decade to synthesize new zeolites.It can transform parent germanosilicate zeolites into a predictable new structure by selectively disassembling the Ge-rich double-4-ring(d4r)units in the 3D structural framework,and overcomes the limitations of traditional hydrothermal synthesis methods.Although a large number of germanosilicate zeolites are regarded as candidates,traditional ADOR with liquid reactants had been successfully realized in a few specific germanosilicate zeolites yet.In order to solve this problem,our cooperators improved the ADOR method via using hydrochloric acid vapour as reactant,and successfully applied for seveal germanosilicate zeolites.However,the mechanism underlying in this novel gas-phase ADOR process remain unclear hinder further application of this technique.Based on the above reasons,this thesis developed related synchrotron in situ characterization techniques and devices,with which systematically investigated the structural evolution and mechanism of various germanosilicate zeolites during gas-phase ADOR,such as IWW,UTL,UOV,IWR,IWV and ITR.In addition,in situ structural studies of zeolites in high-temperature gas-phase ADOR and hydrothermal synthesis were also carried out in this thesis.This thesis includes the following contents:In chapter one,the scientific background about zeolites was reviewed.Firstly,we introduce the structural characteristics,industrial applications and synthesis methods of zeolites,introduce the four steps of the ADOR method,and briefly summarize the current status of ADOR research.Secondly,we introduce the principles of synchrotron radiation,X-ray diffraction(XRD)and X-ray absorption spectroscopy(XAS),which are used in this thesis.Finally,we introduce the research content of this thesis.In chapter two,the structural evolution of IWW germanosilicate zeolite during hydrochloric acid vapour-assisted gas-phase ADOR was investigated using in situ synchrotron characterization.We designed an in situ device that can precisely control the beginning of gas-phase ADOR reaction,with which we then successfully performed in situ XRD and XAS measurements of IWW in hydrochloric acid vapor.Combined with the quantitative fitting of the XRD and the in situ XAS,we concluded the reaction mechanism of gas-phase ADOR of IWW zeolites.Four key steps including fast hydrolysis of d4r units,diffusion of GeCl4 molecules,self-organisation of Si-rich layers,and layers condensation at high temperature were observed in the gas-phase ADOR process.In chapter three,we applied the gas-phase ADOR method to more germanosilicate zeolites with different pores,such as UTL,UOV,IWR,IWV and ITR.In situ synchrotron XRD was used to study their general mechanisms during the gas-phase ADOR process.The gas-phase ADOR method was only suitable for large or extra-large pore germanosilicate zeolites.The kinetics of the reaction was limited by the diffusion of GeCl4,ang the larger the pore size,the faster the reaction.In chapter four,we introduced the in situ reaction devices of hydrothermal synthesis and high-temperature HCl-assisted gas-phase ADOR process,and explored their application,respectively.We designed an in situ capillary device and then studied the crystal growth and structural evolution of STW zeolites during the hydrothermal synthesis process.And we designed an in situ gas-solid reaction cell which ultilizting high-temperature thermal decomposition of NH4Cl to produce HCl gas,and then studied the structural evolution of UOV zeolites in high-temperature HCl gas.Finally,we gave the empirical references for the in situ device design and experimental operation.The last chapter is a summary and prospect of the works performed in this thesis.