Electron Microscopy Study Of Local Structure In Zeolite Crystals

Zeolites are porous inorganic crystals which have been widely used as catalysts,absorbents,or ion-exchangers due to their abundant catalytically active centers,distinct shape selectivity and outstanding hydrothermal stability.Ideal zeolite frameworks are constructed by TO₄ tetrahedra connecting through the corner-sharing oxygen atoms,forming the ordered multi-dimensional channel systems.In order to functionalize the zeolite crystals for their applications in industrial production or other emerging fields,the synthetic zeolites are usually modulated by introducing framework defect,guest species or tuning the element distribution,internal/external surface.These“deviations”from the ideal zeolitic frameworks,referred to as local structure,usually accompany with the modulation of geometric structure and electronic structure,including the change of pore size and connectivity,incorporation of new catalytically active center,or modification of chemical environment on surface,which can further improve the performance and broaden the application of zeolite materials.The performance of a material depends on its structure,therefore,a thorough understanding of the fine structure of zeolite crystals,especially their local structure within the crystal,is the key to rationally control their physical and chemical properties.Conventional X-ray crystallography methods have encountered difficulties in characterizing the local structure in zeolite crystals because of the requirement of large single crystal in single-crystal X-ray diffraction（SCXRD）or the limited one-dimensional averaged diffraction information provided by powder X-ray diffraction.Electron microscopy is the most powerful technique to study the local structure in zeolite crystals as it enables to detect the long/short-range periodic information from electron diffraction in the reciprocal（momentum）space,atomic positions information by direct imaging in the real space,and the chemical or bonding information from spectroscopy in the energy space,utilizing the strong interaction between electrons and matters.However,characterization of zeolite crystal by electron microscopy is challenging due to the electron-beam sensitivity of zeolite frameworks.In this thesis,utilizing the emerging transmission electron microscopy（TEM）techniques including atomically resolved spherical aberration corrected（C_s-corrected）scanning transmission electron microscopy（STEM）imaging and the three-dimensional electron diffraction（3D ED）,comprehensive studies of local structures in zeolite crystals are carried out to elucidate their function in practical applications and the process of rational synthesis combining the diffraction,imaging,and spectroscopy.The research work is presented in three sections:1.Structure solution and framework defect analysis of synthetic zeolite.A new UTL-type germanosilicate,UTL-DBU,has been synthesized using a commercially available superbase 1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）as an organic structural direction agent（OSDA）.The d4r-pillared-layer framework structure was confirmed by ab initio structure solution from 3D ED and C_s-corrected STEM annular dark field（ADF）imaging.According to the high-resolution transmission electron microscopy（HRTEM）images of UTL-DBU along[010]incidence,there exists framework defect,micro-twinning,in the crystal parallel to the crystallographic bc plane.The appearance of random twin planes introduced a disordered shift of（1/3c+1/2b）to the stacking layers.Diffuse scattering generated by the stacking disorder was observed in the reconstructed reciprocal space by 3D ED data and the selected area electron diffraction（SAED）.The corresponding simulated SAED pattern matched well with the experimental patterns,which have confirmed the micro-twinning faulted model.The micro-twinning model have been further proved by the HRTEM image along[101]incidence where intergrowth of[101]and[301]were observed.Complementary to these observations,edge-dislocation-like planar defect were also detected,indicating a different growth speed along different crystallographic directions and the coalescence of crystallites during the crystal growth process.The high-resolution images taken in a low electron dose condition revealed the atomic stacking manner of these planar defects.2.TEM methodological study for locating the guest species in zeolite crystals.Three kinds of partially order or disorder guest species including exchange metal cation,OSDA,and metal nanoparticle（MNP）in zeolite frameworks have been studied by TEM technique.（a）All the K⁺cations in an LTL-type zeolite were quantitatively located through 3D ED.Direct imaging by C_s-corrected STEM annular bright field（ABF）was also performed,where all the framework atoms and K⁺cations were resolved,including a K⁺cations located only 0.8?to another oxygen atom,which is close to the resolution limit of STEM imaging under these conditions.Similar evaluation was performed to another zeolite Cu-T,which contains an intergrowth of OFF-and ERI-types framework.Not only the exchange Cu²⁺cations but also the ratio of intergrowth frameworks can be quantitatively refined through 3D ED.The Cu²⁺cations on the interface of intergrowth enabled the excellent performance in ammonia selective catalytic reduction of NO_x.（b）3D ED experiment was performed to directly located the OSDA（TPA⁺）in silicalite-1 zeolite with a rigid-body refinement strategy.The TPA⁺were found to be in the intersection of two 10-ring channels which is similar to the results by SCXRD analysis.It is highlighted that using a crystal that is a million times smaller than that for SCXRD is possible to resolve the host–guest interaction in zeolite framework.（c）Combining the ultramicrotomy to prepare wedge-shape thin specimen and multi-frame photographing to improve the signal-to-noise ratio,we succeeded in obtaining high-resolution STEM ADF images that can resolve the zeolite framework and the position of MNPs at the same time with clear elemental contrast.A depth sectioning using STEM ADF imaging have also been performed to study the three-dimensional distribution of MNPs.3.Modulating local structure in zeolite crystals for the rational synthesis of high-performance catalyst.Point defects that related to the internal surface structure（vacancy）and atom distribution（hetero-metal-atom）in zeolite crystals are crucial to explain their reaction mechanism or catalytic performance.An MSE-type titanosilicate zeolite was synthesized by rationally regulating point vacancies through a three-step post-processing route.Electron microscopy analysis was performed to obtain direct structural evidence for the evolution of point defects including atomic site vacancies and the incorporated Ti atoms.First,the qualitative determination by SAED and the quantitative refinement by 3D ED of vacancy in the starting material YNU-2P indicated that vacancies concentrate on the site T7/T8.C_s-corrected STEM quantitative differential phase contrast（q DPC）imaging directly showed the random distribution of T7/T8 vacancies across the whole crystal.After the steaming process to stabilize the framework,intracrystalline mesopores were observed in YNU-2ST by STEM imaging and electron tomography.The vacancy-originated hydrolysis process and the defect healing of the silica framework was inferred by the observation of a mesopore concentric to the T7/T8 vacancy position in a STEM low-angle annular dark field（LAADF）image of the vacancy-free YNU-2ST.In the final step,titanium atoms were incorporated to the vacancy related T7/T8 and T1/T2 sites as reveal by the atomic-resolution LAADF images of Ti-YNU-2.The resulting material,Ti-YNU-2,exhibited superior performance in the catalytic hydroxylation of phenol due to the“entropy trap”forming by the silanol groups（residual vacancies）in crystal near to the titanium active sites.This study provided insights into the roles of local structure in zeolite for the rational design of catalyst with desired arrangement of catalytically active sites.