Electron Microscopy Studies of Local Structural Modulations in Zeolite Crystals

Abstract Zeolites are widely used in catalysis, gas separation, ion exchange, etc. due to their superior physicochemical properties, which are closely related to specific features of their framework structures. Although more than two hundred different framework types have been recognized, it is of great interest to explore from a crystallographic perspective, the atomic positions, surface terminations, pore connectivity and structural defects that deviate from the ideal framework structures, namely local structural modulation. In this article, we review different types of local modulations in zeolite frameworks using various techniques, especially electron microscopy (EM). The most recent advances in resolving structural information at the atomic level with aberration corrected EM are also presented, commencing a new era of gaining atomic structural information, not only for all tetrahedral atoms including point vacancies in framework but also for extra‐framework cations and surface terminations.


Introduction
Az eolite framework is formed by corner sharing oxygen atoms in TO 4 tetrahedra, where the Tatom is in general Si or Al. Thef ormula of an aluminosilicate zeolite is generally described as M m+ x/m [(AlO 2 ) x (SiO 2 ) y ]·p H 2 Op er unit-cell or M m+ z/m [Al z Si 1Àz O 2 ]·p H 2 O, where Mi sa ne xchangeable counter cation with av alence of m + ,a nd p H 2 Oi sz eolitic water. Ther ange of x is less than or equal to y and the Si/Al ratio,d efined by y/x (a commonly used parameter in the zeolite community), ranges from infinity for pure silica polymorphs to 1, according to Lçwensteinsr ule [1] ,a lthough af ew exceptions have been reported recently by B. Slater [2] . Natural zeolites,s uch as Sodalite (SOD), Faujasite (FAU), Mordenite (MOR), Cancrinite (CAN), Erionite (ERI), Heulandite/Clinoptilolite (HEU)a nd so on, have rather low framework Si/Al ratios and have been applied for sustainable agricultural usage.
Because of the diverse potential applications of zeolites in the fields of petrochemistry,c atalysis,g as adsorption/separation, etc.,z eolite science has been shifted from natural to synthetic materials in order to create and improve their physicochemical properties,s uch as internal acidity,s electivity,a nd thermal stability through regulation of various structural and compositional parameters.
Although there are currently 252 different framework types identified by the International Zeolite Association (IZA), [3] only some of them are used in industry based on their outstanding properties and low cost in comparison to other alternatives.F or instance, LTA (Si/Al = 1) is used as adetergent (water softener) and desiccant due to its excellent ion exchange and water uptake properties; FAU,which can be crystallized with different Si/Al ratios,named zeolite XorY , can be used for adsorption of water or organic molecules or in fluid catalytic cracking.H igh silica zeolites such as ZSM-5 (MFI)a nd ZSM-11 (MEL), [4] firstly synthesized by Mobil scientists,a re mainly used in the petrochemical industry owing to their high thermal stabilities and specific structural features.B oth structures can be described as the simplest ordered endmembers of the pentasil zeolite family.
In order to understand the physical properties of zeolites in addition to their chemical compositions,structural information such as atomic crystallographic positions,surface terminations, pore connectivity and structural defects should be thoroughly studied. Framework-type structures may be defined in terms of an "ideal structure" by assuming that az eolite is ap olymorph of silica, SiO 2 ,w ith no extraframework cations and by taking the highest symmetry.Here,"modulation" is used to refer to structural deviations from the ideal structure in real space, in terms of type and position of Tatoms and framework symmetry.Such features can be observed as point, columnar and planar modulations by high-resolution scanning/transmission electron microscopy (HR-S/TEM) images or in more specific ways,t hrough the appearance of extra reflections,d iffuse scatterings or intensity changes of reflections in electron diffraction (ED) patterns in reciprocal (momentum) space.
Electron microscopy (EM) provides ag reat opportunity to obtain information of structures in both real and reciprocal space from the same nano-volume of acrystal. In the case of zeolites,the basic principle in forming astructure comes from Zeolites are widely used in catalysis,gas separation, ion exchange,etc. due to their superior physicochemical properties,whicha re closely related to specific features of their framework structures.Although more than two hundred different framework types have been recognized, it is of great interest to explore from acrystallographic perspective,the atomic positions,surface terminations,p ore connectivity and structural defects that deviate from the ideal framework structures,n amely local structural modulation. In this article,w e review different types of local modulations in zeolite frameworks using various techniques,especially electron microscopy(EM). The most recent advances in resolving structural information at the atomic level with aberration corrected EM are also presented, commencing anew era of gaining atomic structural information, not only for all tetrahedral atoms including point vacancies in framework but also for extraframework cations and surface terminations.
arigid TO 4 tetrahedral unit, which is connected to neighboring TO 4 units by corner-sharing oxygen atoms.Arigid-unitmode (RUM) model could be applied to describe amodulated structure (as for example structural transformation with  temperature,i ncluding negative thermal expansion), where  very rigid TO 4 units would be allow to change their relative  configurations of neighboring units through bending or  rotation with respect their shared Oa toms without breaking the T-O-T bonds. [5] However,a pplying this model to modulated zeolite structures is still very challenging. It is too complicated to study modulated structures of zeolites by analysis of powder X-ray diffraction (PXRD) data because of seriously inherent peak overlap and peak broadening induced by the modulation. In the case of MFI and MEL zeolites, at ypical type of modulation can be described as stacking disorder of pentasil-sheets.Perego and co-workers introduced i-type (inversion) and s-type (mirror) stacking descriptions in analysis of the PXRD results, [6] although minor differences might still arise in fitting the fault probability parameter p. Approaches involving HR-TEM and ED should be carried out with cautious treatment of multiple scattering. Nowadays, with the development and implementation of aberration correctors,S/TEM provides detailed structural information of nano-crystals at atomic level.
Although spatial resolution, due to imperfection in the EM lenses,has been amajor limiting factor for along time,it has still been possible to determine stacking sequences from HR-TEM images taken perpendicular to the stacking direction, not only in MFI and MEL [9] but also in other families of polytypes such as the ABC-6 family, FAU and EMT or Beta zeolites.Afew illustrative examples of the power of HR-TEM are presented in Figure 1a.The observation of ERI (a zeolite belonging to the ABC-6 family), with stacking of layers of OFF and SOD,i ss hown in Figure 1a.T he ABC-6 family is one of the most important groups in zeolite science due to their small-medium pore sizes and good thermal stabilities, which have made them in high demand for reactions such as the treatment of pollutants from combustion of vehicles (NO and CO) or reactions relevant to alternative energy sources such as methanol-to-olefin (MTO) conversion. [10] Theinfinite set of ABC-6 zeolites can be obtained by stacking sheets consisting of hexagons,s ix-membered rings (6Rs) of TO 4 tetrahedra, with their centres taking three different positions, A, Bo rCon the projection perpendicular to the sheets ( Figure 1b). In contrast to hard sphere packing, the maximum number of successive stacking of the same type is two (i.e. AA, or BB,o rC C), forming double six-membered rings (D6Rs) and eight-membered rings (8Rs) next to each other. D6Rs,6 Rs and 8Rs are imaged as short line-segments,s mall and large bright dots,r espectively,i nH R-TEM images ( Figure 1a). Therefore,t he corresponding frameworks (marked by red rectangles,g reen dots and blue dots, respectively), can be uniquely determined as shown in Figure 1a.This is the first example in which HR-TEM images provided direct information of the stacking sequence in az eolite. [7] Forc omparison, Figure 1c shows STA-20 (SWY framework), the most recent silicoaluminophosphate with ABC-6 structure,w hich was characterized by spherical aberration (C s )c orrected STEM together with the unit cell schematic model, representing another excellent example of stacking modulation characterization by EM. [8] Qing Zhang received her PhD degree in Laser Optics in 2011 from Institute of Physics in Chinese Academy of Sciences. After ap ostdoctoral fellowshipa tt he Deutsches Elektronen-Synchrotron (DESY) in Germany,s he joined ShanghaiTech University as ar esearch assistant in 2018. She has recently been ar esearch associate working on characterization of various types of porous materials including both imaging and spectroscopy by using aberration-corrected scanning/transmission electron microscopy.
Alvaro Mayoral studied chemistry at the University of Alcala (Spain) and obtained his PhD at the University of Birmingham. Since 2018, he is Research Associate Professor at ShanghaiTech University and from 2020; he is aR amon yCajal researcher at INMA-CSIC. He is working on the development and application of new electron microscopy methods for beam sensitive nanoporous solids. He is also interested on metallic nanoparticles and other materials from the nanotechnology perspective,c overing from fundamental aspects up to potential and industriala pplications. Osamu Terasaki worked at Department of Physics, Tohoku Univ,J apan as af aculty member , and stayed at Cambridge Univ (1982)(1983)(1984) and Lund Univ (1988. [3][4][5][6][7][8][9] to start his research on fine structure of zeolites. He was one of the first Research Directors of National CREST project, Japan (1995)(1996)(1997)(1998)(1999)(2000). OT was Prof of Structural Chemistry,S tockholm Univ  Planar modulationsinA BC-6 family.a)HRTEM images of ERI zeolites where OFF and SOD structure can be found with models below the images. Reproducedwith permission. [7] Copyright 1986, Elsevier.b)Projected schematic of ABC-6. c) HR-STEMi mage and the projected structural model of STA-20. Reproducedw ith permission. [8] Copyright 2017, AmericanC hemical Society.6 Rs, 8Rs and D6Rs are marked by green dots, blue dots and red rectangles, respectively.
Both FAU and EMT have ac ommon structural unit, the faujasite sheet, in which all sodalite (SOD)c ages are connected through inversion at the centre of D6R. In FAU and EMT,successive faujasite sheets are related by inversion and mirror,r espectively.F igure 2a shows the intergrowth of EMT within two regions of FAU as the planar (boundary) modulation. [11] Viveka Alfredsson [12] firstly observed the structure of the surface termination (incomplete SOD cages by removing D6R from FAU framework) in zeolites.O f course,t he surface termination is one of the most important structure modulations from ac rystal to vacuum. As for the FAU-type framework, the uniqueness of its large threedimensional (3d) accessible volume (for molecules up to 27 % in contrast to 9.8 %a nd 12.7 %f or MFI and MEL,r espectively) makes it as pecial candidate for "dealumination". As atype of point and framework modulation, dealumination has been developed to increase the strength of aframework, both in terms of internal acidity and thermal stability.Asfar as we know,dealumination of FAU (zeolite-Y) to make it an ultrastable hydrophobic zeolite-Y (USY) while keeping periodic original FAU framework structure was started by Sten Andersson (Lund Univ), Lars Falth (Zeol, later MuntersZeol, Lund), Tetsu Ohsuna and Osamu Te rasaki (Tohoku University) together with To-Soh (Japanese Company) [13] to capture smelly organic compounds at high moisture preventing outlets from the factory of Te tra-Pack (Lund) in 1980's. Observations of structural changes in FAU (Figure 2b) induced by dealumination have been reported by SEM and HR-TEM. Furthermore,o bservations of zonings with different Si/Al ratios in MOR was reported in an oscillatory grown crystal since growth rate of ac rystal depends on the ratio. [14] Therefore,d irect observation at the atomic scale of ap oint modulation in composition during crystal growth or dealumination (including formation of hydroxyl nest) is ar eally challenging research target.
In addition, columnar structural modulation in LTL was observed by Tetsu Oshuna [15] (Figure 2c). Thed ata were recorded using aJEM-4000EX operated at 400 kV.F igure 2c shows the HR-TEM image of LTL along [001] with columnar faults marked by yellow arrows.
In general, these few examples illustrate the richness of zeolite science from as tructural perspective and how advanced EM can help to understand these materials in ways that other methodologies cannot do.I nt his review,w e describe various structural modulations in zeolite frameworks from an EM perspective.P romising results on aberration corrected STEM are also presented as the first step to directly observe Ta toms,o xygen bridges and even single extraframework cations,w hich is an essential future direction for characterization of structural modulations at the atomic level.

Structural Modulations in Zeolites
2.1. Basic structure of pentasil family and planar modulation in MFI/MEL MFI and MEL zeolites are ordered end-members within the pentasil family.T hey display different types of modulations,a ss hown below.T he differences between the two structures in projection are so small that the intergrowths between them have mainly been studied by electron diffraction, based on the stacking description model between i-type and s-type. [16] Several years later,i tw as possible to directly observe the position and type of symmetry elements in the projected structures from HR-TEM images of MFI and MEL through great advances in high-quality crystal syntheses. [17] Thef rameworks can be described based on the pentasilunit, pentasil-chain and pentasil-sheet shown in Figure 3. The pentasil-unit has characteristic features:eight five membered rings (5Rs) with symmetry element of 4 1 m2w ith the units joined through edges to form ap entasil-chain with left-or right-handed chirality along the 4 1 -axis.T hese left-and righthanded chains are connected through mirror symmetry to form the pentasil-sheet, which is the basic and common structural unit of MFI and MEL.  [11b] Copyright 1995, Elsevier.b )HRTEM and SEM images for faujasite before and after dealumination. Reproduced with permission. [13] Copyright1 994, American Chemical Society.c )Columnarm odulationi nL TL which coincides with the schematic model and simulated image. Reproduced with permission. [15] Copyright 2004, Wiley-VCH.  (Figure 5e). TheF ourier filtered image using the 000 and four 110 reflections from the FD shows ah azy contrast running parallel to the (100) and (010) planes.O nt his basis, am odel of the chain-type related planar modulation in the framework of B-MEL projected along the [001] direction is schematically illustrated in Figure 5g using the La nd R pentasil-chains.A ssuming that the pentasil chain is ag rowth unit on MEL crystals,amodel for the growth process along (100) surfaces of the B-MEL crystal can be proposed as shown in (Figure 5h).
"Planar-modulation models" can be further extended from [010] projection of MEL with ab oundary parallel to (100) and (001) planes.The boundary can be either acommon shared plane for (100) (Modulation 1, as in FAU/EMT, Figure 2a)ornarrow extra boundary regions (grey bands) to form smooth framework connections (001) (Modulation 2) as shown in Figure 6. Theperiodic existence of this planar fault can be commensurate or incommensurate with respect to the basic lattice.I ncommensurate structure modulation in zeolites was first observed in SSZ-24 (AFI-type framework), in which atomic positions are incommensurately modulated along the c-axis.I tw as analysed by RUMm odel. [18] Besides, SSZ-57 with possible incommensurately modulated structure is demonstrated as follow through Modulation 2.
High silica SSZ-57 zeolite (*SFV)was synthesized in 2003 at Chevron using the same synthesis procedure of MEL.SSZ-57 shows some similarities with boron loaded MEL (B-MEL) in terms of:( i) morphology,( ii)4-fold rotation symmetry along the c-axis and (iii)similar diffuse streaks in the SAED pattern along [001] incidence (Figure 5d and Figure 7c). The structure of SSZ-57 was solved by Baerlocher et al. [19] in 2011 from high-quality single-crystal (2 mm 2 mm 8 mm) synchrotron X-ray diffraction. They studied the framework structure in 4-dimensional space (corresponding to Figure 6,  Modulation 2) using Superflip algorithm. Thes olution was related to ZSM-11 (MEL)b ut commensurately modulated along the c-axis (P4m2, a = b = 20.091 , c = 110.056 )with modulation vector q = 0.125 c*, yielding as tructure with twelve membered ring (12R): ten membered ring (10R) ratio of 1:15. They also discussed the disordered structure as amodulated one,which is highly scholarly treated model that could explain the experimental diffraction and HR-TEM images. [20] TheSAED pattern of SSZ-57 taken along [100] is shown as an inset in Figure 7a,which is very similar to that of MEL taken along [100] (Figure 4) except for the existence of aclear modulation along the c*-axis (an enlarged pattern of 00l is shown in the orange rectangle). Baerlocher et al. [19] proposed an idealized model of SSZ-57 (Figure 7e)involving an eightfold superstructure formed by 8 MEL cells and two "connections" (four ring) as au nit cell. We noted, however,t hat periods of the contrast modulation observed in Figure 7a show afew irregularities marked by white arrows.Due to the introduction of the "connection", the unit cell of SSZ-57 is larger than 8times that of MEL along the c-axis.The spacing of the 00l superstructure reflections (Figure 7a)i sa pproximately 1/8 of the c*. Although the incommensurate modulation should be taken into consideration, this idealized model gives ag ood explanation of the SAED pattern of SSZ-57.
Theenlarged HR-TEM image from the white rectangle in Figure 7a shows two domains marked MEL(L) and MEL(R) together with (but not so well resolved due to possible overlap) ab and between them that would correspond to the blocked 12Rs;f or easier interpretation the model of the framework is overlaid in yellow (Figure 7b). Theperiod of the large bright dots in MEL (L) and MEL (R) along c-axis (corresponding to the 10Rs) is given by c M ,while the period in the central part is given by c m ,which is slightly larger than c M .
To further interpret the HRTEM image,astructural disorder in SSZ-57 was introduced, described by misplacement of 12R columns in the stacking of the pentasil sheets. Figure 7d shows how the two pentasil sheets stack along the a-axis,1 2R columns (marked by black rectangles) blocking each other to be 10R and ad istinct projected structure is produced in the overlapping region. Different intervals of large pores in the HRTEM images (c M and c m in Figure 7b) can be explained by the insertion of the "connections" and overlapping 12Rs in this model.   [19] and the schematic of super structure. The blocked twelve membered ring are formed by overlappingoft wo twelve membered rings columns marked by black rectangles. e) b-c plane of the idealized structure model of single layer,w hich forms asuper structure consisting of 8 MEL cells and the connection part. Reproduced with permission. [20] Copyright 2011, Elsevier.

Angewandte Chemie
Here,asimplified model with limited disorder was found to be coincident with the domain in the HRTEM image.T he disorder would be more complicated in the macroscopic crystal as it may occur in two dimensions (a-axis and b-axis) due to the symmetry.T hus the stacking disorder along the aaxis would appear also along the b-axis.T he diffraction pattern of SSZ-57 along [001] (Figure 7c)r eveals homogeneous but weak diffuse streaks through hk0 reflections (h, k = 2n + 1) and extended along the a*a nd b*r eciprocal axes, similar to that for B-MEL (Figure 5d). Diffraction patterns for the SSZ-57 structure with random disorder simulated by aM onte Carlo algorithm verify this structure model, as the results give an extremely good match to the single crystal Xray diffraction pattern, allowing the probability of faults to be determined in the calculated model. [19] Thus,b yu sing advanced crystallographic techniques and EM analysis,q uasi-ideal SSZ-57 with an incommensurately modulated structure along c-axis was successfully observed. Thep ore geometry is modified toward an ew absorption behaviour from the structural modulation such as at hree-dimensional 10R channel system with large isolated 12R pockets,w hich may provide many advantages in applications of zeolites.

Intrinsic structural modulation in unit cell of IMF
Another good example of structural modulation in the pentasil family is the IM-5 zeolite with IMF framework type. Because of heavily overlapped reflections and extra peaks from impurity phases in the PXRD pattern, the structure of IM-5 remained unsolved for almost 10 years after its discovery.The structure of IM-5 was reported in 2007 by Baerlocher et al., [21] using an ewly developed charge-flipping structuresolution algorithm combining PXRD data and TEM data, and by Sun et al. [21b] from TEM data in 2010.
Three-dimensional ED data were collected from nano single-crystals of IM-5 by Ruan. [22] Tw os ets of as eries of selected area electron diffraction (SAED) patterns were collected by tilting ac rystal around the b*-and c*-axes, observing mirror symmetries perpendicular to their axes.F or following discussions,s ome of them are shown in Figure 8a. Ther eciprocal planes perpendicular to c*-axis have diffuse intensities (Figure 8a)w hile diffuse streaks are observed by asection with Ewald sphere in Figure 8aand 8b.However, all 0k0 reflections are sharp without streaks along the b*-axis, which is the most heavily modulated direction. Based on these observations,the modulation can be explained as astructural unit perpendicular to the b-axis.The main components of the modulation along b (b*) at [100] incidence,w here multiple scattering is enhanced, are the 060 and 080 reflections, indicated by orange and black arrows in Figure 8b Possible space groups were obtained from observed 3delectron diffraction data: Cmc2 1 (36), C2cm (40, standard setting Ama2), and Cmcm (63). Among them, only Cmcm has ac entre of inversion. In this case,t he phase of the crystal structure factor for the reflections could be either 0orp,ifthe reference origin is taken at the inversion centre,w hile for Cmc2 1 and C2cm,t he phases could deviate from 0o rp. Therefore,Baerlocher et al. and Sun et al. [21] took Cmcm after considering the apossibility of C2cm. However,itwas found that few strong reflections in the FD of IMF along [100] incidence,largely deviate from 0orp in an independent work of Ruan. [22] In particular,t he relative phases to the origin could change with increase of crystal thickness through multiple scattering. ED patterns of IM-5 (model from IZA with Cmcm space group) were simulated for different thickness (Figure 9; phase information is given in Table 1), showing great differences between the kinematic condition and ar ather thin sample.A dditionally,p hase information derived from HRTEM images is affected by experimental conditions,including deviation of electron incidence from an exact crystal zone axis.I ns uch circumstance,i ti sd ifficult to determine the space group by phase information from the HRTEM images.
Unambiguous determination of the local structural modulation was further proved from HR-TEM images at the atomic level. Theprocessed HRTEM images of IMF obtained by Ruan, [22] by enhancing the signal to noise ratio (SNR) without imposing any symmetry constraints along [001] and [100] incidences are shown in Figure 10. In both images,   Table 1. Dynamical diffraction patterns are simulated by eMap using the multi-slice method based on structure data (using reflections with resolution up to 1.67 )taken from Baerlocher et al. [21a] Angewandte Chemie Minireviews contrast modulations are seen along the b-axis with different periods (marked by arrows in Figure 10 a,b,r espectively), which corresponds to the previous discussion on ED patterns (Figure 8b). Theo bserved modulations are equal to b/2, which means that there is no actual super lattice in the crystals but the intrinsic modulated repeated units in one unit cell of IMF.W eb elieve this is the reason that Sun and co-workers call them pseudo three-fold super-lattices.A sf or [001] incidence,t he projected structure shows similarities to the MFI framework, which consists of pentasil chains (sheets for IMF)a nd the modulated contrasts appear every three pentasil chains,ahalf b.
Thes chematic models based on Baerlocherss tructure solution is inserted showing good match with the observed images with both FDs as insets.

New advances
With the implementation of spherical aberration (C s ) correctors coupled with more and better electron detectors, lateral resolution is significantly improved for such beam sensitive materials.V ery recently,P rashant et al. [23] have succeeded in the synthesis of very well controlled MFI nanosheets with al ateral area of 200 nm 200 nm and uniform thickness of only 3.2 nm along the b-axis.T he authors found that one to few unit cells intergrowths of MEL domains were inserted along the a-axis in the MFI framework and extended along the c-axis.B ased on ultrahigh resolution C s -corrected STEM data using an annular dark field detector (ADF), they identified the planar modulation distribution of these MEL domains,a nd showed that af raction of nanosheets have as ignificant amount of MEL content ( % 25 %b yv olume) while the majority of nanosheets are purely MFI.T his work combined traditional SAED patterns collection and analysis with state-of-art EM observations,i mage treatment and data analysis (Figure 11). In here,anintergrowth of a MEL band was observed between the two MFI domains,which are in mirror relation to each other through the MEL band perpendicular to the a-axis (Figure 11 a). They are marked with circles or belts in different colours.T he corresponding FD is shown in Figure 11 b. Ar egion corresponding to the diffuse streaks is enlarged and show inset corresponding to the 102 reflection (h + l = 2n + 1), which indicates the presence of finite domains of MFI trapped between MEL layers along the adirection. Acloser observation of the framework is shown in Figure 11 c, with the two frameworks denoted and the model superimposed.
Thed evelopment of C s -corrected STEM with various detectors,such as annular bright-field (ABF), ADF and highangle annular dark-field (HAADF) detectors,gives enhanced contrast from light elements and medium atomic number Zatoms to heavy Z-atoms,r espectively.F urthermore,c uttingedge characterization of hetero-atom or vacancies in the framework of zeolites with atomic resolution electron microscopes combined with advanced analytical methods plays Table 1: Phases and amplitudes of electrons for different reflections of IM-5, at 300 kV,c alculated by eMap. Structural data is taken from Baerlochere tal. [21a] (Space group Cmcm,unit-cell parameters a = 1.430 nm, b = 5.679 nm, c = 2.029 nm).

ReflectionsK inematical [a]
Dynamical [b]  [a] The origin of lattice is set at the inversion center. [ b] Phase value for dynamical scattering is not relative to the 000 beam but the incident beam for all thickness.  Reproducedw ith permission. [23] Copyright 2020, Springer Nature. more essential roles in the structural characterization of zeolites,e specially for modulated structures that are largely related to the catalytic activities.
With the intention of utilizing the power and potential of advanced EM in the observation and analysis of nanoporous materials,w ep resent data obtained from MFI crystals as af irst step toward the characterization of point modulation. In this case,v ery thin areas were investigated, and the Si atoms of the framework were clearly differentiated by both ADF and ABF imaging. Figure 12 shows C s -corrected MFI images along the b-axis using both detectors.ADF and ABF can be differentiated as they produce reversed contrast images,w ith ABF similar to conventional TEM imaging.T he ADF has been more widely used as this detector is more readily available,i mage acquisition is easier and the signal is sensitive to atomic number of the elements, [24] which makes it suitable for analysing metals in light supports as zeolites. [25] On the other hand, ABF is more sensitive to aberrations in the microscope and thus data acquisition is more challenging;h owever,i t provides complementary information on light compounds, such as light cations and oxygen bridges.F igure 12 a-c shows the atomic-resolution ADF data of the MFI framework in the [010] orientation (atoms appear in white), while the reversed contrast is obtained from the ABF detector ( Figure 12 d-f). Because of the low SNR, images were Wiener filtered (Figures 12 aa nd 12 df or ADF and ABF respectively) allowing direct visualization of the four types of rings:5 Ra, 5Rb,6 Ra nd 10R. To further extend the information limit, images were symmetry averaged by p1, just translational averaging by unit cell vectors a and c (Figure 12 b,e) and p2gg (Figure 12 f), which are the projected symmetry along the [010] orientation for Pnma. We can see particularly good match with the projected framework of MFI (Figure 12 g). Despite the excellent data reported in recent years by different groups,w ec onsider that spatial resolution should be further increased to be able to discuss point defects as point modulations at least at the atomic level shown in Figure 12. Figure 12 hs hows an enlargement of the p2gg micrographs,w hich clearly reveal the pentasil chains described earlier, and in the case of the ABF image,t he visualization of oxygen bridges between Si atoms.
Earlier in this review,w em entioned that the best orientation to study the MFI/MEL modulation is [001],a sit is the orientation in which the 5Rs are connected differs depending on the framework (Figure 4). In this sense,o nly Oshunaswork [17c] has shown this modulation experimentally for zeolite MEL.A tt hat time,t he spatial resolution was limited by aberrations in the lenses and if these modulations occur locally in ashort range,they could be unnoticed. Based on atomic resolution observation along this projection (Figure 13), the structures can now be studied from membered rings to membered rings.F igure 13 ac orresponds to the C scorrected STEM ABF micrograph of MFI along [001];t he two areas marked by yellow and red rectangles were further studied. From the FDs (Figure 13 b,c) we infer some variations respect to the ideal FD (Figure 4). FDs obtained from both regions show significant differences:w hile the red rectangle can be indexed as MFI,the yellow region does not strictly match either MFI or MEL.M agnified data are presented in Figure 13 d. Theblue circles in Figure 13 dmark the local modulations in this crystal, which are formed as ar esult of co-existence along the c-axis of units from both MFI and MEL;therefore,the direction of the interconnected 5Rs cannot be distinguished as both orientations are overlaid along the same columns.From Figure 13 eand 13 f, this can be clearly appreciated. In Figure 13 e, aperfect, or nearly perfect transition between MEL and MFI is visualized, for ab etter understanding the model is presented below,w here MEL units appear in green. On the other hand, in Figure 13 f, four units of the interconnected 5Rs are depicted, three of which correspond to pure MFI (blue), while the other is associated with the coexistence of both MFI and MEL along the same column (blue and green in the model).  These results,w hich were obtained from at ypical MFI zeolite,r aise the possibility that modulations between MFI and MEL may be more common than it has been reported. However,the fact that they occur at alocal level suggests the possibility that they have not been noticed in some cases.

Conclusions
In this Minireview,w eh ave summarized the relevant modulated structures in af ew types of zeolites,f rom the simplest end-members of the pentasil family MFI/MEL to SSZ-57 with interesting pseudo super lattice and the most puzzling structure IM-5. These examples illustrate avariety of local structural modulations including planar modulation, surface termination and intrinsic modulation in the framework, together with recent observations that reveal planar intergrowths of MFI and MEL at the atomic level.
With the most advanced C s corrected EM techniques in combination with other characterization techniques,s uch as X-ray diffraction and mathematical algorithms,itisnowadays possible to trace more precisely the local modulation in the framework, not only involving heavy atoms,b ut also light elements or even single heteroatoms.T he direct observation of all framework atoms of Na-LTA and atomic level substitution of Si by Fe in Fe-MFI has already been reported. [26] To locally characterize modulated structures of zeolites at the atomic level, the key challenges from the experimental point of view are:( i) how to overcome electron beam sensitivity of zeolite crystals in order to acquire more precise data, (ii)how to obtain highly crystalline and thin specimens with clean surfaces,( iii)how to obtain precise/reproducible 3d-structural information through diffraction and imaging and (iv) how to develop amethodology to obtain 3d-structure solution including local structure modulations such as point modulations.
Finally,inthe near future,special efforts are expected be achieved to distinguish Si and Al directly and also to observe vacancy/point defects in af ramework called "hydroxy-nest" at the atomic level, which will facilitate the study of modulated point defects in the framework of structures that are related to dealumination processes,s ynthesis of mesoporous materials and so on.