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Wieslaw J. Roth, Maksym Opanasenko, Michal Mazur, Barbara Gil,, Jiˇrí ˇCejka, [Takayoshi Sasaki](https://orcid.org/0000-0002-2872-0427)

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Current State and Perspectives of Exfoliated ZeolitesREVIEWwww.advmat.deCurrent State and Perspectives of Exfoliated ZeolitesWieslaw J. Roth,* Maksym Opanasenko, Michal Mazur, Barbara Gil, Jǐrí Čejka,*and Takayoshi SasakiZeolites are highly efficient industrial catalysts and sorbents with microporousframework structures. Approximately 10% of the frameworks, but eventuallyall in the long run, have produced both 3D crystals and 2D layers. The lattercan be intercalated and expanded like all 2D materials but proved difficult toexfoliate directly into suspensions of monolayers in solution as precursors forunique synthetic opportunities. Successful exfoliations have been reportedrecently and are overviewed in this perspective article. The discussionhighlights 3 primary challenges in this field, namely finding suitable 2D zeolitepreparations that exfoliate directly in high yield, proving uniform layerthickness in solution and identifying applications to exploit the uniquesynthetic capabilities and properties of exfoliated zeolite monolayers. Fourzeolites have been confirmed to exfoliate directly into monolayers: 3 withknown structures—MWW, MFI, and RWR and one unknown, bifer with a unitcell close to ferrierite. The exfoliation into monolayers is confirmed by thecombination of 5–6 characterization techniques including AFM, in situ andin-plane XRD, and microscopies. The promising areas of development areoriented films and membranes, intimately mixed zeolite phases, andhierarchical nanoscale composites with other active species like nanoparticlesand clusters that are unfeasible by solid state processes.W. J. Roth, B. GilFaculty of ChemistryJagiellonian UniversityGronostajowa 2, Kraków 30-387, PolandE-mail: wieslaw.roth@uj.edu.plM. Opanasenko, M. Mazur, J. ČejkaDepartment of Physical and Macromolecular ChemistryFaculty of ScienceCharles UniversityHlavova 8, Prague 2 12843, Czech RepublicE-mail: jiri.cejka@natur.cuni.czT. SasakiResearch Center for Materials Nanoarchitectonics (WPI-MANA)National Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adma.202307341© 2023 The Authors. Advanced Materials published by Wiley-VCHGmbH. This is an open access article under the terms of the CreativeCommons Attribution-NonCommercial-NoDerivs License, which permitsuse and distribution in any medium, provided the original work isproperly cited, the use is non-commercial and no modifications oradaptations are made.DOI: 10.1002/adma.2023073411. IntroductionThe subject matter of this article con-cerns direct exfoliation of layered ze-olites leading to formation of disper-sions of monolayered nanosheets insolution. This is a fundamental expec-tation for 2D solids but has not beendemonstrated with zeolites until recently.Zeolites are a special class of poroussolids with framework structures contain-ing uniform pores and channels. Poroussolids are valuable in many industrial pro-cesses due to internal void spaces and ex-tended surface areas, which are benefi-cial in separation and catalytic transfor-mations of organic compounds and in-organic gases.[1,2] The original syntheticporous materials, including various formsof carbon, amorphous silicas, alumina andothers were characterized by polydispersityof pore sizes.[3] The emergence of alumi-nosilicates known as zeolites in the mid-dle of the last century provided crystallineporous solids with periodic structures con-taining discrete pores and channels below1 nm. This represented a breakthroughin fundamental research and industrial applications by enablingshape-selectivity, i.e., discrimination of molecules based on sizeand shape.[4–7] The ensuing research and development resultedin numerous large-scale applications, such as water soften-ing by ion exchange, gas separations, and especially hydrocar-bon conversions to enhance production of fuels, lubricants andmonomers for polymerization, as well as plethora of other ap-plications. The application of zeolites often leads to environ-mental benefits.[8,9] Zeolites gave rise to the concept of molec-ular sieving[4] and influenced the development of other porousmaterials exemplified by pillared layered solids,[10] mesoporousmaterials,[11–13] MOFs,[14] MOPs,[15] and related types of frame-works.The key to uniqueness of zeolites is their framework structurecomposed of oxygen-sharing tetrahedra, TO4, with the central Tatom such as Si, Al or others.[16,17] They are assembled spon-taneously under hydrothermal conditions resulting in differentstructures providing various pores depending on the composi-tion of the synthesis mixture and other conditions such as tem-perature and time.[18] The performance and activity of zeolitescould be modulated by selecting different structures and com-positions to optimize activity toward particular molecules.[19,20]Once produced, zeolite structures were fixed and practicallyAdv. Mater. 2024, 36, 2307341 2307341 (1 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbHhttp://www.advmat.demailto:wieslaw.roth@uj.edu.plmailto:jiri.cejka@natur.cuni.czhttps://doi.org/10.1002/adma.202307341http://creativecommons.org/licenses/by-nc-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadma.202307341&domain=pdf&date_stamp=2023-11-28www.advancedsciencenews.com www.advmat.deimmutable, resulting in a strategy to obtain as many new zeo-lite frameworks as possible for diverse uses. The current numberof recognized zeolite structures is around 250[21] while about 20have been commercialized or made technologically ready.[22] Zeo-lite MWW (MCM-22) patented in 1990 appears to be the last newframework developed for commercial use[23,24] suggesting dimin-ishing returns from the discoveries of new zeolite topologies.New zeolite-based processes are being implemented but they useframeworks known before 1990.[25]The efforts to discover new zeolite structures produced anunexpected breakthrough that the same framework can formboth 3D extended and layered 2D forms.[26–30] Such existenceof 3D and 2D forms with the same structure, except for ter-minations at the surface, is not common so it is significantfundamentally.[31,32] On the practical side, the availability of 2Dzeolite allowed circumventing immutability of the traditional3D zeolite structures and presented the possibility for post-synthesis modifications. Especially valued was the option to ex-pand the structure and increase internal accessibility,[33,34] be-cause the constraints on diffusion and access became invokedas shortcomings-to-be-overcome with the conventional rigid 3Dzeolite crystals.[35] For the layered zeolites the examples of in-tercalation with guest molecules,[36] expansion (swelling) to pro-duce inorganic or organic pillared species,[33,37] layer disorder-ing (delamination)[38] and rearrangements to alternative zeolitestructures have been gradually demonstrated.[39–41] These resultsimplied independence of the layers but the ultimate embodi-ment – exfoliation of the layers, preferably directly, to producelamellar nanosheets/colloids in solution – remained elusive.[29,42]The fundamental and practical significance of this exfoliation ishard to overstate – the definition of layered materials demandssuch exfoliation as a proof of the 2D character and in practice al-lows the use of nanosheets as free gigantic molecules dispersedin a homogeneous liquid phase for combination with othermolecules and entities and deposition on or mixing with solids.This presents an open-ended possibility to use the nanosheets as“extended building blocks” for making intimate nanoscale com-posites and hierarchical materials, many of them unfeasible withthe 3D frameworks and even with 2D zeolite solids. The enor-mous potential of 2D nanosheets dispersed in solution has beenevidenced and practiced for a long time with other classes of lay-ered solids and is particularly exemplified by the 2D aluminosil-icate counterparts of zeolites, clay minerals.[31,43] The direct for-mation of suspensions of dispersed zeolite monolayers in solu-tion has not been well documented and remained elusive untilrecently. The earlier efforts were not direct and relied on pre-expanded, surfactant-swollen layered precursors.[42,44–49] They areoverviewed in more detail in the the section 4. The fundamen-tally expected direct chemical exfoliation resulting in unilamellarnanosheets in solution[50] that could provide versatile substratesfor reactions was confirmed first for the zeolite MWW.[51] It wascarried out with the reagent that has been commonly used forexfoliation of layered metal oxides, namely tetrabutylammoniumhydroxide (TBAOH).[50] This suggests that interlayer chemistryof zeolite nanosheets is not a barrier to exfoliation. Instead, thedeciding factor was the preparation method of the layered zeolite,which suggested crystal or layer intergrowths as the most likelyprimary factors controlling the extent of exfoliation, i.e., the pos-sible yield of dispersed layers in solution. High yield is crucialfor meaningful practical exploitation. Since the first publicationin 2020 there have been several additional reports with well doc-umented cases of such direct exfoliation.[52–54] They show feasi-bility for all layered zeolite frameworks and illustrate the require-ments for conclusive proving of the genuine monolayered natureof the nanosheet in solution, which is not trivial and requires spe-cial approach combining several complementary techniques.The present perspective article reviews the advances and dis-cusses the fundamentals peculiar to such systems and emerg-ing possibilities for development of zeolite materials in new di-rections, including application beyond those demonstrated andpracticed so far. In particular, as mentioned above, the exfoliationenables unconstrained combination of zeolite layers with desiredfunctional components to produce nanoscale hybrids and hierar-chical structures that are unimaginable with the 3D frameworks.The development of zeolite nanosheets in solution showed in-sights that also illustrate challenges and possible shortcomingsthat may have to be overcome. It is hoped that this perspectivewill initiate a roadmap for the development of exfoliated zeolitesas unique materials that can extend the possibilities of using 2Dsolids by providing nanosheets with pores and strong acid cen-ters.2. Explanation of the NomenclatureThe question of nomenclature arises because there are two ba-sic terms describing separation of layers in 2D solids: delami-nation and exfoliation. Specific definitions have been proposedfor differentiation between them but there are discrepancies be-tween different literature sources including incompatible or op-posite meanings.[32,53,55,56] These terms are not applied consis-tently in the literature and have been used interchangeably indiverse situations for both liquid and solid systems.[32,57] Therehave been proposals for strict and uniform usage and differen-tiation between these two terms[55] but they face problems withthe past, i.e., application and sorting out the historical, alreadypublished data, and with the future, buy-in and meaningful com-pliance among scientists, which if agreed in principle may taketime. The problem at hand is naming of the zeolite systems re-viewed herein, i.e., stable dispersions of monolayer nanosheetsin solution.[51,53,55] Exfoliation is the most appropriate and con-venient term to apply for several reasons starting with the needfor differentiation from delamination and delaminated zeolites.The latter concepts are already firmly established in referenceto disorganized layers as final solids with particular interest forcatalysis.[57–59] Zeolite delamination has not been concerned ex-plicitly with the dispersion into monolayers in solution and thereis no evidence that the latter have played any but a minor role in it,if at all.[60,61] Aside from this practical reason, the term exfoliationis already often used to describe the process of transforming 2Dsolids to obtain monolayers in solution[62] and in that sense hasbeen reviewed in the seminal review as liquid exfoliation.[32] Thisunderstanding is supported by the definition of exfoliation pro-vided in the Glossary of Clay Science, which says:[56] “exfoliationinvolves a degree of separation of the layers of a host structurewhere units, either individual layers or stacking of several layers,are isotropically dispersed (freely oriented and independent) ina solvent or polymer matrix.” For comparison, in the Glossarydelamination is described as “layer-separation process betweenAdv. Mater. 2024, 36, 2307341 2307341 (2 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 1. Basic types of solution dispersions of layered precursors with examples of reported materials. The systems discussed in this article are in themiddle.the planar faces of adjacent layers of a particle (…) whereby in-tercalation occurs with the introduction of guest material andthe stacking of layers remains.” This definition is consistentwith delaminated zeolites in the as-synthesized form, which usu-ally contain guest species between layers, inorganic, organic, orboth.[61,63,64] The organics are removed by calcination to affordthe final (delaminated) solid.[38] By adding the descriptor “syn-thesized directly” this definition can encompass the materialsobtained in one-pot hydrothermal synthesis like MCM-56[65] orMWW zeolite nanosheets prepared via one-step syntheses.[59] Insummary, the adoption of exfoliation and delamination as advo-cated above is consistent with the prevailing usage of the for-mer and differentiates from the very popular class of delami-nated zeolites. To avoid ambiguities, we also adopted full descrip-tions like “directly exfoliated unilamellar nanosheets in solution”to provide precise meaning to the referenced materials. Directmeans that exfoliation can occur spontaneously upon contact-ing the layered solid with the exfoliation medium, with optionalmild agitation. It is different from exfoliation attempted by pre-expansion with surfactants followed by extrusion with polymers,extraction and purification.[44,45] ‘In solution’ refers to the ho-mogeneous liquid phase, without specifying ‘a solution’ or ‘sol’.It is also needed to differentiate from “exfoliated layers” recov-ered as solids, pure or in mixtures from these solutions. Finally,unilamellar emphasized the absence or minimization of mul-tilayered particles, which for each first case is rigorously ver-ified. The alternatives are polydisperse suspensions shown inFigure 1.[66] This terminology with longer descriptive phrasesmay look cumbersome and like unnecessary repetition but,in our opinion, provides clarity and eliminates ambigu-ity in comparison to other processes and systems involv-ing 2D materials. It is based on precisely defined outcomenot a particular name, which can be understood in vari-ous ways. An alternative precise concept of 1D dissolutionfor exfoliation into monolayers has been proposed recentlybut it is novel and will need time for validation and wideracceptance.[55]Zeolites with a known framework structure are represented bycapital 3-letter codes like MWW and MFI, while the structure ofbifer is unknown so it is written in small letters. MCM-56 andMCM-22 represent various synthetic forms of the zeolite MWW.3. Prior Attempts at Zeolite Exfoliation intoMonolayers in Solution Especially with SurfactantPre-Expanded PrecursorsComplete separation of 2D solids into individual layers has bothfundamental and practical significance. Arguably, the recent re-ports of direct exfoliation of layered zeolites[51–54] producing so-lutions with monolayer nanosheets, represent a watershed, pro-viding zeolite layers with maximally exposed surfaces. Prior tothat, the concepts of delamination and delaminated zeolites, re-ferring to the efforts of layer separation, were focused on the iso-lation and characterization of the final solids, which did not al-low judging the extent of layer separation.[38,67] In contrast, liquidexfoliation,[32] denoting dispersion of monolayers by and into aliquid medium provides complete layer separation as a distinctstage and can be a possible benchmark for assessing both the ef-fectiveness of layer separation and product quality, obtained byother delamination procedures. This role can be useful becauseof the lack of an objective quantitative method of estimating theextent or quality of delamination in a solid state. On the practicalside, successful liquid exfoliation provides nanosheets in solu-tion as reagents for unconstrained combinations with composi-tions of choice, as underscored by the studies on graphene andrelated systems.[68–71]It is reasonable to assume that in the majority of reported pro-cedures of zeolite delamination the presence or role of layers be-ing exfoliated into a liquid phase was minimal. There was also noeffort to explicitly exploit, prove or invoke this crucial stage. Start-ing in 2011,[44] several studies did report preparation and usageof dispersed zeolite monolayers in solution but they were not ob-tained directly. The layered zeolite samples were expanded withAdv. Mater. 2024, 36, 2307341 2307341 (3 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.desurfactants, during or post-synthesis, and processed via multi-step methods. For reasons elaborated below their practical impactwas limited.It is helpful to point out that examples of successful direct ex-foliation of layered zeolites indicate the crucial role of the na-ture of the sample as determined by the procedure of its synthe-sis, namely gel composition and conditions of synthesis. Givena suitable sample, presumed to have low level of intergrowths,the finding of reagents and conditions for exfoliation appears tobe routine. In contrast, the prior studies to separate and disor-ganize zeolite layers were focused on finding reagents and treat-ments for layered zeolite samples at hand, which could have beenpoorly exfoliable from the outset. There were separate efforts todesign direct syntheses for delaminated zeolites, but they werealso focused on the final solid product.[59] There are also classi-cal approaches used for the preparation of layers of other classesof 2D materials (carbons, transition metal oxides, chalcogenides,etc.) but they are not applicable for zeolites. Examples includebottom-up techniques based on vapor phase deposition utilizedfor 2D carbon and transition metal compounds,[72] or microme-chanical cleavage and chemical oxidation methods for grapheneand its derivatives.[73,74]Layered zeolite forms have been obtained with ≈10% ofthe known frameworks and new examples are being addedincrementally.[75] Syntheses by design have been also reported,for example with bifunctional templates for single layers[76,77] andthe top-down approach denoted ADOR.[40,78] There have been re-ports of disordered single-layered materials synthesized directlybut the layers were usually intergrown, which was not favorablefor exfoliation into solution.[76,77]The concept of delaminated zeolites[38] gave rise to many in-vestigations based on preswelling with surfactants[33] followedby the application of stimuli or additional reagents to separatethe layers. In comparison to direct exfoliation reviewed in thisarticle those techniques require several steps and in most caseswere focused on final solids. The possible intermediate exfoli-ated nanosheets in solutions, where in most cases were unlikelyto form or at best at negligible amounts. The exception was thestudy in 2011 involving layered precursors of MWW and MFIwith layers separated by surfactants that were incorporated eitherpost synthesis or as templates during preparation, respectively.[44]These surfactant-swollen composites, expanded by ≈2.5–3 nm,were combined with polymers in a twin-screw extruder, mixedwith appropriate solvent like toluene, and purified by density gra-dient centrifugation to remove multilayered particles. The ob-tained supernatant contained exfoliated nanosheets, coated withsurfactants, which could be an obstacle to interaction in subse-quent syntheses. Nonetheless, it was the first successful demon-stration of exfoliated zeolite monolayers in solution.[44] The exfo-liation was proven by the combination of diffraction (XRD, ED)and microscopy techniques (HRTEM, AFM) validating the struc-ture of the MFI and MWW layers and confirming uniform thick-ness after purification. The verification of single layers in solu-tion did not include techniques like SAXS, in situ and in-planeXRD, which played an important role in the validated directlyexfoliated layers (vide supra).[51,52,54] The dispersed nanosheetswere used to fabricate oriented zeolite films, which after repairto eliminate defects showed effective gas separation of smallmolecules.[44,45,48] This work was notable by demonstrating ze-olite exfoliation and its effectiveness for the fabrication of ori-ented zeolite films but its practical side was limited. The mul-tistep strategy based on a sequence of interlayer expansion, meltblending using a special equipment, dissolution, and purifica-tion has not been conducive to entice broader studies and ap-plicability. Melt blending itself is a well-known, noncostly pro-cedure used for exfoliation of inorganic layered solids.[79] In thepaper on zeolite exfoliation by melt blending, a relatively sim-ple system was used: a corotating twin screw extruder with arecirculation channel utilized under inert atmosphere.[44] How-ever, in comparison to anticipated and subsequently demon-strated direct exfoliation methods rewieved herein, the disadvan-tages of the multistep procedure combined with elevated tem-peratures (200 °C), low yield (5%, later increased to 10%[80]),and layer covering with long surfactant chains presented seriousobstacles.This initial exfoliation approach based on preexpansion of lay-ered zeolites was continued with further refining and improve-ments. One of them was the use of different polymers such aspolystyrene, polybutadiene, polylactic acid, and polyvinylpyrroli-done for dispersion of swollen MWW materials.[46,79,81] Ahydroxyl-terminated polybutadiene (HTPB) was applied withpurely siliceous MWW also preswollen with hexadecyltrimethy-lammonium cation, HDTMA.[82] For layer purification chemi-cal treatments with either acid as a single component or in themixture of sulfuric acid and hydrogen peroxide (the so-called pi-ranha solution) have been applied to decrease the fraction of or-ganics in the final material.[83] From the economic standpoint,zeolite film and membrane applications can tolerate significantcost and labor increases associated with fabrication but the con-cern for sufficient quantity and concentration for potential ap-plications as medium- and large-scale processes remained a ma-jor issue.[84] An example of yield enhancement in the methodinvolving melt blending was the combined purification processthat included the removal of polymer residues by density gradi-ent centrifugation and the exclusion of oligo-layered aggregatesby rate-zonal centrifugation in a multicomponent solvent sys-tem. It allowed to increase the yield of nanosheets from 5 to10%.[80]The use of melt blending exemplifies application of extra force(mechanical) in addition to chemical interaction to enhance layerseparation. It is a common engineering tool, but it is hard topinpoint if and how it seems to be effective. It is possible thatthe major effect is mechanical fracturing, resulting in layer frag-ments separating from the rest. The direct exfoliation of zeolitesreported since 2020, exemplified by ilerite and MWW, have beenaccomplished without and with stirring, respectively. The appliedstirring is not viewed as a surce of additional mechanical force butsimply to accelerate kinetics, which otherwise would have to relyon slow diffusion. Examples of additional mechanical stress canbe found with all layered materials, including those more chem-ically flexible than zeolites, such as metal-organic frameworks[85]and covalent organic frameworks.[86] Besides affecting the layers,the application of shear forces (e.g., by rotating screws) or acous-tic cavitation (by sonication) to aid the exfoliation can also im-prove the mobility of molecules (solvent, surfactant) in the inter-layer space, thus increasing the exfoliation efficiency.[79,87] On theother hand, the recent successful exfoliation of zeolites indicatesthat with suitable physical and chemical circumstances, namelyAdv. Mater. 2024, 36, 2307341 2307341 (4 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.decrystals without intergrowth and appropriate medium, the needfor mechanical stimuli diminishes.Although zeolite crystals are typically prepared at pH near12 and higher, still when separated from the synthesis mixturethey may be susceptible to degradation under basic conditions.A matter of some concern in separating zeolite layers by delam-ination and swelling post-synthesis has been the strong basic-ity of the reaction media and the possibility for negative effecton the integrity of zeolite layers, loss of crystallinity, defect gen-eration and possible formation of undesired phases. Reportedsolutions included swelling at room temperature “with layerpreservation,”[88] using lower pH = 9 by swelling in the presenceof fluoride and chloride anions,[64,89] and using water-solublemetal salts of Al or Zn under acidic conditions (pH = 1–3).[58,63]These studies demonstrated the possibility of separating or dis-organizing zeolite layers in a solid but did not show nor soughtthe formation of exfoliated nanosheets in a liquid phase. There isa caveat because the effectiveness of expansion/intercalation oflayered zeolites under mild conditions diminishes with increas-ing Al content. The latter determines acid site concentration andoverall activity and in many catalytic processes its maximizationis desired.[61,90] Based on the analysis of published data[91] it wasshown that swelling of MWW at high pH and elevated temper-ature, which may result in desilication and partial frameworkdegradation, did not diminish catalytic activity.[90] Some litera-ture reports show the use of multilayered aggregates, which canbe obtained by using small zeolite seeds for a bottom-up mem-brane growth.[92,93] This method is not based on exfoliation butprovides films consisting of layers of about 5 nm thickness (largerthan for a single layer) that are not aggregated or intergrown,and thus can be used with almost no limitations for the prepa-ration of ultraselective membranes.[92] Other examples of toler-ance for oligo-layers include MFI systems treated with piranhasolutions,[83] hydrogen peroxide decomposition at elevated tem-perature between MWW layers (up to 180 °C) in a microwave,[94]and simple treatment of MWW with ethanolamine. The lastmethod produced thickness reduction and sufficient uniformityof nanolayer for deposition on paper macroscopic objects viadip-coating for their protection against acidification, aging, andoxidation.[95]Summarizing, prior to the recent reports of direct exfoliationof layered zeolite affording unilamellar nanosheets in solution,this desired outcome was possible only through low yield mul-tistep processing producing layers coated with surfactants. Thisenabled valuable applications for membrane fabrication but wasimpractical for more general uses in syntheses. What was not rec-ognized was that quality of the layered zeolite might be crucial fordirect high yield exfoliation into monolayers. As shown below,such exfoliation of layered zeolites into unilamellar nanosheetsin a homogeneous liquid phase can be achieved by simple soft-chemical treatments with appropriate solid samples. An illustra-tive example is provided by the first case, zeolite MWW, which hasbeen exfoliated only as the MCM-56 preparation.[51] Historically,the most studied form for delamination was the multilayered pre-cursor, MCM-22P, which so far has not shown dispersion intomonolayers in solution by the same procedure as MCM-56.[44,49]This resistance to direct exfoliation may be prevented for funda-mental reasons but maybe an exfoliable MCM-22P form is stillto be found/prepared.4. Proving Monolayered Nature of ZeoliteNanosheets in SolutionThe procedure for direct exfoliation of layered zeolites into mono-layer nanosheets in solution involves mixing solids with the dis-persing solution followed by centrifugation to sediment largeror multilayered particles. The resulting solutions often appearcolloidal-like with translucence/opalescence and are stable with-out visible sedimentation for long periods of time (weeks andmonths). This does not prove exfoliation into monolayers andspecial characterization is necessary to rule out the presence ofmultilayered particles. Preliminary identification such as powderXRD can be carried out by isolating solids by flocculation butmore elaborate techniques are needed to confirm the presenceof truly unilamellar nanosheets in solution. The discussion willfocus first on the methodology for proving monolayers in solu-tion and determining their structure, while the synthetic aspectsare described in the next section. This proving of the presence ofmonolayers in solution and characterization of their structure isbased on a protocol combining the following methods:[51,96]i. SAXS for measuring interlayer d-spacing in solution,ii. in situ XRD confirming mono-dispersity and structure of thelayers in solution,iii. AFM to show distribution of layer thicknesses,iv. in-plane XRD to determine the planar unit cell,v. X-ray diffraction, TEM, and ED to characterize layer struc-ture, andvi. flocculation reaction, e.g., with a surfactant, to show forma-tion of multilayered composites with alternating inorganicand surfactant layers with specific d-spacings.The first 3 methods provide basic, close to complete descrip-tion of the layers in solution. The remaining methods focus onstructure and corroboration of the initial results. Additional sup-plementary techniques can be applied and, needless to say, self-consistency of the results proves successful preparation of zeolitemonolayers in solution.To date, direct exfoliation into monolayers in solution hasbeen proven with four zeolites. Basic structural details andapplied characterization methods are summarized in Table 1.The confirmed exfoliated zeolites are: MCM-56 with the MWWtopology,[51] bifer with unknown structure that may be related toferrierite because of analogous unit cell and synthesis from a gelproducing ferrierite layers,[52] MFI—one of the two most impor-tant zeolites with ≈0.5 nm pores perpendicular to the layers,[54]and ilerite, a layered silicate that is a precursor to zeolite RWR.[53]The extension to other frameworks depends on finding suitablepreparations that are conducive to exfoliation. A detailed reviewof each characterization method is presented below. The firstthree zeolites were exfoliated with TBAOH. Ilerite was exfoli-ated with meglumine (N-methyl-d-glucamine) under milder con-ditions, which may result in differences between products fromthese two approaches. A treatment of ilerite with TBAOH did notresult in exfoliation.[53]The first method, small angle X-ray scattering (SAXS) can re-veal characteristic distances in partially ordered systems.[97] It hasbeen carried out for the MWW (MCM-56) samples treated withaqueous solutions of TBAOH at different concentrations.[51] AAdv. Mater. 2024, 36, 2307341 2307341 (5 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deTable 1. Summary of the methods used to prove direct zeolite exfoliations into solutions of monolayer nanosheets. Sodalite precursor exfoliated viapre-expansion with surfactants is included for comparison.MWW Bifer (unconfirmedstructure)MFI Ilerite (RWR) RUB-15 (sodalite), via HDTMAswelling)Layer thickness [nm]Crystal 2.4 1.9 3.0 (unit cell 2.0) 0.74 0.8Exfoliated as-made calcined 2.52.32.11.83.62.81.38N/A1.140.74Characterization toolSAXS + No No + NoIn situ XRD + + No No NoAFM Statistic Statistic Statistic Individual IndividualIn-plane XRD + + + No NoTEM/ED + + + + +Nanosheet solution + HDTMA reaction + + + No NoFilm by filtration + + + + +series of basal peaks were detected, giving largely expanded in-terlayer spacing of 5.3–6.2 nm (Figure 2) as a function of theTBAOH concentration. These values indicate the interlayer ex-pansion by 2.8–3.7 nm, which is much larger than the size ofTBA+ ion, ≈1 nm. Thus, this behavior can be understood interms of unique interactions, called osmotic swelling, which hasFigure 2. SAXS profiles for MWW samples in aqueous TBAOH solutionsat different concentrations (10, 8, 6 and 4 wt% from a–d). A solution vol-ume to solid weight ratio is 54 cm3 g−1. The interlayer distance observedis 5.3, 5.7, and 6.2 nm for a–c, while no Bragg peak is detected for d(4% TBAOH). Reproduced under terms of the CC-BY license.[51] Copy-right 2020, The Authors, published by American Association for the Ad-vancement of Science.been reported for ion-exchangeable layered host compounds in-cluding clay minerals, layered transition metal oxides and layereddouble hydroxides (LDHs).[98–103]This phenomenon reflects enormous hydration-drivenswelling in smectite clay minerals that has been studied fromas early as 1950s.[98,99] When alkali metal ions such as Li+and Na+ with high hydration energy are incorporated into theclay interlayer gallery, a large volume of water is permeatedto produce a gel-like sample. The interlayer distance is largelyexpanded to over 100 nm. It has long been believed that thisunique reaction is peculiar to a special class of clay minerals.However, in 1998 it was reported that a polycrystalline layeredtitanate underwent massive osmotic swelling upon contactingwith aqueous TBAOH solutions.[100] More recently, accordion-like swelling of platelet crystals of layered metal oxides such astitanates and perovskites was observed directly under opticalmicroscope.[102,103] Unidirectional ≈100-folds expansion of thecrystals into long string-like objects took place in a few sec-onds. Surprisingly, thousands of layers are stably held paralleltogether, having a large volume of the solutions between them,up to ≈100 nm in thickness. The swollen crystals can reversiblygo back to original platelets upon some stimulus, e.g., changein electrolyte concentration or pH. On the other hand, Davidsonet al. recently reported that highly swollen accordion-like crystalsof smectite clays are kinetically stable.[104] Full understanding ofthis intriguing phenomenon needs to await further studies. Thismassive osmotic swelling is induced in various layered metaloxides with a range of amines and organoammonium ions. Thedegree of interlayer expansion is not dependent on the chemicalspecies but is determined solely by their concentration. Theswelling tends to be stable with polar and smaller amines whileopposite with large symmetrical species.[101] On the other hand,LDHs, anionic clays, were found to show similar swelling informamide. The basal spacing as large as 8 nm was observed ata large excess of formamide.[105]The degree of swelling of the layered materials above includ-ing layered zeolites can be controlled by the concentration ofelectrolyte solutions. In general, the swelling is enhanced withdecreasing concentration. In the case of MCM-56 zeolites withAdv. Mater. 2024, 36, 2307341 2307341 (6 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 3. In situ XRD data of the glue-like sample recovered from the col-loidal suspension by the high-speed centrifugation (black line) and thesquare of the structure factor F (red line) calculated based on the MWWstructure. Reproduced under terms of the CC-BY license.[51] Copyright2020, The Authors, published by American Association for the Advance-ment of Science.MWW layers, the basal peaks shifted toward the smaller angleside with lowering of the TBAOH concentration as shown inFigure 2a–c. The SAXS data presented in Figure 2 represent thehydration-driven expansion (swelling) process of the layered ze-olite MWW, which eventually ends up with separation into unil-amellar nanosheets. In these experiments the swollen samplesof layered zeolites were collected via centrifugation and a glue-like sedimentation was produced and subjected to SAXS analy-sis. The SAXS data were recorded for the sample, which was notexfoliated yet (on its way to it). As can be seen, the interlayer sepa-ration progressively expanded. This is the swelling process wherethe layered structure is maintained, and thus a nematic phase ofexfoliated nanosheets is not formed. The observed shift clearlyindicates the progress of swelling. It is expected that the largerthe interlayer distance, the weaker the interaction between thelayers through the interlayer fluid. Then at the massive swellingthe layers fall apart, facilitated by applying the external force likemechanical agitation. In practice, the basal peaks disappeared ata threshold concentration (Figure 2d), suggesting the loss of theregular layered structure, namely the total exfoliation. A recentreview by Breu explains this phenomenon as ‘1D dissolution.’[55]The samples at low TBAOH concentrations became colloidalsuspensions of layers with translucent appearance. The dis-persed layers were recovered by high-speed centrifugation asglue-like substances and subjected to X-ray diffraction (XRD)measurements at high relative humidity to suppress the drying.The illustration of the unique set up for this measurement wasprovided in the Supporting Information in reference [51], This insitu XRD analysis provides important information about the col-loidal state of the zeolite samples. Figure 3 depicts such typicaldata for the MWW sample, showing a broad and oscillating pro-file peculiar to the material. Importantly, this pattern is closelymatched to the calculated profile as square of the structure factorbased on the MWW layer topology, meaning that the layers scat-ter X-ray individually. In addition, no basal peaks were observed,indicating the absence of a regular stacked structure. All theseresults clearly prove that the multilayered MWW structure wasdisintegrated into colloidal single layers dispersed in aqueous so-Figure 4. Comparison of in situ XRD results for bifer layers with calculatedprofiles for doubled layers with FER and CDO structure and single fer layer.Reproduced under terms of the CC-BY license.[52] Copyright 2021, The Au-thors, published by The American Chemical Society.lutions. Similar analysis has been reported for various layeredmaterials such as clay minerals, layered transition metal oxidesand LDHs.[106–109]The in situ XRD technique is useful not only for confirmingtotal exfoliation but can also help with identifying more compli-cated or not fully known structure of the nanosheets. This is il-lustrated by the material denoted bifer, which was obtained uponmodifying the synthesis of the layered zeolite ZSM-55 by substi-tuting boron by aluminum. The layers in ZSM-55 are ≈0.9 nmthick[30] and are designated fer because they have the topologyof the zeolite ferrierite (FER) but can produce 2 different struc-tures upon topotactic condensation: FER upon reflection in mir-ror planes and CDO through translation.41 The product from themodified sythesis was clearly different from ZSM-55 based onXRD, but was found by in- plane XRD to have a rectangular unitcell (b and c axes) similar to fer (see Table 1). The layer thicknessseemed to be doubled in comparison to fer leading to the desig-nation bifer. The structure of bifer was unknown but for simula-tions such as in situ XRD a model was needed. The structuresCDO and FER were considered. The simulation of theoreticalin situ XRD patterns was carried out for a single fer layer anddouble-layered FER and CDO structures, see Figure 4. The ob-served experimental wavy pattern layer is vastly different fromthe calculated profile for a single fer layer and clearly not accept-able as a possibility. In contrast, the double-layered models gavevery similar profiles resembling the experimental one, provingdoubled layer thickness in bifer. Unambiguous differentiationbetween FER and CDO was not possible. As a new developmentconcerning the identity of bifer, a recent publication reported lay-ered material ECNU-28, which shows XRD pattern very similarto that of bifer.[66] The authors argue that the patterns are differ-ent and propose the SZR topology for ECNU-28. Questions canbe raised about validity of this conclusion and the structure de-termination. This will be discussed as a separate topic below.Adv. Mater. 2024, 36, 2307341 2307341 (7 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 5. Illustrative AFM results – for zeolite MFI layers. Reproduced under terms of the CC-BY license.[54] Copyright 2023, The Authors, published byThe Royal Society of Chemistry.AFM is one of the most powerful tools to characterizenanosheets with ultrathin 2D morphology.[110] Generally, col-loidal nanosheets are deposited on an atomically flat support, e.g.,Si wafer, and probed by cantilever with an appropriate spring con-stant. The deposition from the suspension is facilitated on sur-faces covered with polycations such as polyethyleneimine (PEI).Minimizing overlap of nanosheets is desirable and is possible byappropriate dilution, e.g., 100 times with 1% w/w nanosheet so-lutions. Measurement of the elevation above the background isperformed for individual nanosheet but it is preferred to carryout a more complete statistical distribution of heights over entireviewing area and in several regions (see Table 1).As exemplified by a typical image for the MFI zeolitenanosheets (Figure 5), many 2D objects are detected. The objectshave a very flat terrace with a unique height, revealing the molec-ularly thin feature with a high aspect ratio.AFM data for zeolites MWW, MFI, and bifer showed prevail-ing elevation (layer thickness) approaching 90% and more of thepopulation, corresponding to monolayers with the average thick-ness of 2.5, 3.6, and 2.1 nm, respectively. Ilerite layer thicknesswas 1.38 nm. These values are a few tens of a nanometer greaterthan the crystallographic unit cell of the corresponding 3D solids,i.e., 2.5, 3.0, and 1.9 nm. The difference is explained by hydrationof the layer surfaces, which is commonly observed for variousnanosheets.[111] Lateral sizes are variable reaching widths of theorder of 100 nm. This implies nanosheet fragmentation, whichhas not been examined in detail and may be relevant with timeor require optimization for applications. Particles with a thick-ness above the apparent monolayer values constitute roughly10% or less of the deposited materials and can arise from inciden-tal overlap during deposition, association is solution or simplymultilayer “impurities” present from the beginning. The AFMresults indicate that the obtained suspensions can be consideredmonodisperse with regard to the nanosheet thickness and con-tain essentially monolayers only. There is a small chance that par-ticles deposited on the support are not fully representative of thecontent of the suspension so further validation with other meth-ods is desirable, especially when dealing with a given system forthe first time. In situ XRD described above can be this validatingcharacterization tool.Patterns obtained by the in-plane XRD technique contain onlyin-plane reflections allowing determination of the planar unit celldimensions of the nanosheets. In principle, this information iscontained in powder XRDs but it may be difficult or impossibleto extract due to low quality, peak overlap, unassignable scatter-ing related to thin layers and the impossibility of indexing. Theexamined nanosheets are deposited on a Si substrate and inci-dent X-ray is scanned parallel to it. This can produce sharp peaks,especially when collected with synchrotron radiation (Figure 6),and upon indexing 2D unit cell dimensions can be obtained. Inthe case of MWW, MFI, and bifer, all in-plane peaks were indexedproving purity and consistency with the corresponding 3D frame-work model (known for the first two). The patterns in Figure 6show relatively simple in-plane XRD with all peaks identified,contrasted by complicated powder XRD pattern with some broadand unidentified peaks. The application of in-plane XRD is par-ticularly beneficial with nanosheets of unknown structure like inFigure 6. In-plane XRD for the MWW layers compared with the powderpattern. Reproduced under terms of the CC-BY license.[52] Copyright 2021,The Authors, published by The American Chemical Society.Adv. Mater. 2024, 36, 2307341 2307341 (8 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 7. XRD patterns of layered MWW, bifer, and MFI powders.the case of bifer layers. Thanks to in-plane XRD the planar unitcell was determined to be similar to fer. The structure was notsolved but with known unit cell dimensions, reasonable modelslike FER and CDO could be proposed for simulations.The combined results from in-plane XRD and AFM providereliable information about crystallographic unit cell dimensionsof nanosheets. They allow correlation with expected values for3D frameworks and confirmation of the likely internal structureof the layers. In situ XRD and AFM can confirm monolayerednature of the nanosheets in solution.The primary tool for identifying and characterizing zeolitenanosheets is powder X-ray diffraction (XRD). The XRD patternsfor the nanosheets in the solid state and 3D microcrystals aredifferent but are expected to contain common distinct in-planereflections.[112] The nanosheet XRDs usually contain broad peaksor bands that are hard to assign or identify, and may includenon-Bragg scattering like with MCM-56 and bifer (Figure 7).[65,113] The XRD pattern of MWW nanosheets is one of themost analyzed examples, including theoretical simulation, whichmatch experimental profiles.[44,113–115] The pattern contains rela-tively sharp peaks due to 100, 200, 220, and 310 reflections anda broad band without a valley between 8 and 10° 2𝜃 (CuK𝛼 ra-diation everywhere, except when specified otherwise). The bandis instead of 101 and 102 reflections in the 3D structure, at 8and 10° 2𝜃, respectively, indicating absence of order in the 3rddimension. Appearance of the valley between 8 and 10° 2𝜃 is in-terpreted as partial ordering of the layers in 3D proportional tothe depth of this valley.[65] Both the starting MCM-56 and theexfoliated nanosheets after isolation as a powder exhibit disor-dered patterns. The situation is similar with the bifer layers, i.e.,the original and exfoliated nanosheets show similar XRD pattern,except for the interlayer h00 reflections, which are more distinctwith the latter.[52] The intralayer 0kl peaks are identified basedon the in-plane XRD. The majority of distinct peaks in the XRDof MFI are in-plane so the difference between the patterns fornanosheets and 3D crystals is not pronounced.[54] Summarizing,the XRD patterns of exfoliated nanosheets after isolation confirmtheir basic structure as it was before the treatments. The reduc-tion of XRD pattern quality when going from 3D to 2D frame-works usually precludes direct structure solution. This motivatesadditional verification by ED and TEM.Electron diffraction and TEM complement the powder XRDand other techniques and provide further details about the struc-ture and quality of the layers. The anisotropic shape of the layersfavors planar deposition on the support, frequently allowing “topviews” for examination. The studies of MWW, MFI, and bifershow periodic patterns extending tens of nanometers confirm-ing framework preservation.[51,52,54] A compilation of selected im-ages is presented in Figure 8. Electron diffraction of exfoliatedilerite revealed an interesting situation of lowered symmetry ofthe layers in comparison to the 3D framework.[53] The latter iscentrosymmetric and showed specific systematic absences in theED pattern. The center of symmetry arises when the layers arestacked in the crystal but is removed upon exfoliation, resultingin disappearance of the conditions for extinction. The resultantpattern has more visible spots allowing distinguishing 3D zeo-lites from exfoliated monolayers. This is applicable to specificcases where the 3D and 2D forms may exhibit different symmetrybut if so, can be used as additional proof confirming exfoliationinto monolayers.TEM imaging is valuable for quality appraisal and visualizationof pores. All exfoliated types of nanosheets were studied by TEM.An interesting case arose with the mixture of MWW and bifer (ex-amined as a silica pillared sample). Despite the 0.5 nm thicknessdifference it was not possible to distinguish unambiguously be-tween them. In some edged-on views the pores were sufficientlydistinct to allow clear recognition of bifer based on the density ofpores, which for FER and CDO could be similar.Nanosheets in solution can be confirmed by chemical meansby flocculation with cations, especially surfactants. The precip-itated solids show XRD patterns with relatively high intensityand well-defined basal spacings allowing estimation of interlayerspacings. The reaction with excess of a cationic surfactant likeHDTMA (hexadecyltrimethylammonium) stands out as particu-larly illustrative, facile, and useful in providing vital information,including yield estimate of the dispersed nanosheets. The mix-ing of exfoliated layer solutions with surfactants results in im-mediate formation of a white solid, which is analogous to thelayered zeolite swollen with HDTMA-OH. It is generally a mul-tilayered composite with alternating zeolite layers and surfactantbilayers (typically 2.5–3 nm thick with HDTMA, see Figure 8,top right). The three nanosheets, MWW, MFI, bifer, showed XRDwith peaks positions matching those in equivalent materials ob-tained by swelling of the layered precursors. This also indicatesthe prevailing monolayer nature of the nanosheets in solution.Moreover, the intensities of basal peaks appeared augmented incomparison to the intralayer reflections suggesting greater ex-tent and uniformity of interlayer separation than in the swollenones. A possible reason is that upon swelling of the solid notall layers become separated, e.g., due to intergrowth, while withnanosheets in solution the initial state is total separation. The re-action of solutions with exfoliated nanosheets with HDTMA canbe used for a quick (visual) assessment of the amounts of mono-layers in a given preparation. The flocculation is also useful forverifying exfoliation with subsequent samples since it is not prac-tical to validate all solutions by the advanced physical techniquesAdv. Mater. 2024, 36, 2307341 2307341 (9 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 8. Electron microscopy images and SAED of MWW, bifer and MFI layers, MWW-surfactant composites (top right), pillared mixture of MWWand bifer layers (right center) and oriented MFI discs (right bottom). Reproduced under terms of the CC-BY license.[54] Copyright 2023, The Authors,published by The Royal Society of Chemistry.described above. Once a system has been validated by these tech-niques, subsequent preparations can be verified by the reactionwith HDTMA.5. Preparation of Dispersed Nanosheets inSolution by Soft-Chemical ExfoliationLayered zeolites can produce solutions of isolated freenanosheets directly in one step by contacting with moder-ately concentrated solutions, typically a few weight percent, ofdispersing agents like tetrabutylammonium hydroxide, TBAOH.This reagent was effective with three zeolites: MWW, MFI,and bifer. Nanosheet solutions with lower concentration of thehydroxide may be preferred, e.g., in subsequent reactions withother reactants, in which case a two-step method is convenientlyapplied. The first step involves treatment with a more concen-trated TBAOH solution, typically 10%, with stirring for 1–2 h.Centrifugation at 10 000 rpm for 20–60 min results in sedimen-tation of all solids. The obtained clear supernatant is decantedand discarded as it contains little or no dispersed nanosheets.This was ascertained by the addition of HDTMA cations,which did not produce solid as a proof of dispersed layers (seeSection 4). The remaining solid is stirred with excess of water,30–60 w/w, affording solutions of nanosheets with yields upto 70% or more, depending on a zeolite. The concentrationof nanosheets is 1–2% and the typical pH is around 12 butcan be lowered by dialysis or careful acidification. Solutionsapproaching neutral pH, e.g., by being subjected to dialysis, arenot stable and result in slow flocculation. Sonication has beenused to promote layer disorder in the synthesis of delaminatedzeolites but its effect in the process of exfoliation of zeolites hasnot been systematically studied yet.[38,61]The exfoliation of ilerite (RWR) was different and used a meg-lumine solution with pH = 9 with no agitation. In this case,TBAOH and other media proved ineffective.[53] The qualitativedifference observed with ilerite was preservation of the originallayer size, which is valuable and may be the ultimate goal in someapplications. The authors emphasized spontaneous occurrenceof the exfoliation of the layers without additional stimuli, like agi-tation or temperature. These are preconditions for preserving thepristine condition of layers and minimization of fragmentation.Another difference in comparison to the three zeolites treatedwith TBAOH is complete exfoliation of the entire sample, appar-ently without solid residue. This is a sample-dependent behavior,Adv. Mater. 2024, 36, 2307341 2307341 (10 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.dequite rare since the presence of particles that remain unexfoli-ated is typical with the majority of layered materials. The mostobvious explanation for the presence of nonexfoliating particlesis presumed intergrowth but there may be other factors prevent-ing liquid dispersion of the entire sample. Elimination of theseadverse effects that reduce yield and quality may be importantbut for now is hard to approach in a systematic way.The driving force for the exfoliation of 2D solids during theabove treatments is recognized as (repulsive) osmotic swellingand has been recently proposed to be called 1D dissolution as adistinguishing phenomenon.[55] Ideally, it is expected to operatealone without additional stimuli but in practice additional actionslike agitation through stirring for acceleration of the kinetics anddiffusion as well as other enhancers can be useful. Detailed stud-ies of the various effects during exfoliation are needed for betterunderstanding and improvements. The first obvious factor to ex-amine is the type/size of the cation, e.g., using smaller tetraalky-lammonium cations (TPA, TEA, TMA).[116]The critical characteristic of nanosheets is the charge, whichfor zeolite layers is not fixed, in contrast to other layers likeclays and metal oxides,[62] although they may also exhibit variablecharge in some cases.[117] Zeolites are carriers of a fixed chargerelated to acid sites generated by Al in the framework. This fixedcharge is located mostly inside the layers and is compensated bytemplates from the original synthesis, so it is postulated to play alimited role in intercalation, delamination, and exfoliation of ze-olite. If its role was decisive then expansion of zeolites should bepossible by ion exchange at ambient pH, but it usually requireshigh pH, e.g., 12 or more. The basic environment produces de-protonation of silanol groups on the surface and possibly break-ing of Si–O–Si bonds in the framework. The silanols are weaklyacidic so their population and hence the layer charge will decreasewith pH (OH− concentration) in the surroundings. Lowering ofthis charge may result in layer association and other destabilizingphenomena, which presents another topic for inquiry. The den-sity of silanols is determined primarily by the zeolite structureleading to expected differences for various frameworks. Notwith-standing the postulated limited contribution of zeolitic acid sitesto the charge during exfoliation this aspect will have to be verifiedas well.6. Methods of Isolating Exfoliated Layers and TheirCharacterization as Solids with Zeolite PropertiesExfoliated zeolite layers can be viewed as single crystals with highwidth-to-height (aspect) ratio and having the same structure astheir 3D counterparts except for the terminal T-OH moieties onthe surface. Their ability to produce active catalysts depends ondemonstrating that treatments and exfoliation into monolayersand subsequent purification do not degrade the intrinsic zeo-lite acidity. It was shown before for a series of zeolite topolo-gies, namely MWW, PCR, and MFI, by theoretical calculationswith support of the experimental data that transition from 3Dto 2D forms (formation of layered forms, not necessarily exfo-liation) does affect zeolite acidity,[118] slightly lowering not onlyconcentration but also strength of the Brønsted acid sites. On theother hand, Sauer reported, based on calculations for the CHAand FAU frameworks, that relative acid strength in zeolites isgoverned not only by the deprotonation energies (much lowerfor 2D zeolites) but also by interaction energies between the ad-sorbed/protonated molecule and the surface site. These effectswere found to practically cancel out.[119] It is therefore essentialfor the appropriate assessment of acidity to use more than onetechnique to characterize exfoliated and subsequently recoveredlayers.The earlier studies, focused on the preparation of membranesfrom exfoliated layers coated with surfactants, did not report therecovery of the nanosheets for the purpose of evaluating acidity orcatalytic activity. It is often implied in the literature that reassem-bly of zeolite layers could produce hierarchical structure, for ex-ample, with edge-to-face elements, in which intercrystalline voidspaces are connected with intralayer micropores. This should fa-cilitate the transport of reagents and reduce diffusion constraints.An alternative possibility is that a face-to-face reassembly shouldbe preferred, due to high aspect ratio, resulting in, at best, amodest porosity enhancement. To test these possibilities and theproperties like acidity, the nanosheets in solution have been re-covered and converted into the protonic form. This included pu-rification steps such as removal of the residual template andcations (sodium) used in the synthesis, the agents used for ex-foliation, and any other contaminants.In general, colloids and larger particles in solution (includingzeolite monolayers) can be flocculated by chemical coagulation,especially by addition of electrolytes or other reagents causingprecipitation. These agents neutralize the charge of the dispersedparticles destabilizing them and aiding aggregation, which leadsto sedimentation. According to the IUPAC definition, floccula-tion (coagulation, agglomeration) is a process of contact and ad-hesion whereby dispersed particles are held together, leading tophase separation by the formation of precipitates of larger thancolloidal size.[120]The isolation of exfoliated MWW nanosheets[121] was inducedby addition of alcohol or ammonium nitrate and by freeze-drying.The addition of electrolytes causes “salting out” due to increasingionic strength.[122] The flocculation with ammonium nitrate wasalso testing the option of replacing sodium in lieu of ion exchangeto generate acidic layers upon calcination. This attempt to elimi-nate the ion exchange step failed, resulting in products with infe-rior properties in comparison to the starting material and show-ing zero acidity. The flocculation with alcohol gave similar prod-ucts. The 3rd tested method of isolation, freeze-drying, known aslyophilization, uses frozen solvent acting as a porogen that cancreate secondary pore system from the voids left upon removalof the solvent.[123] Freeze-drying of the solution with exfoliatednanosheets seemed to have positive influence on the structure ofthe recovered material, which became delicate and fluffy, requir-ing only slight crushing to obtain homogeneous powder.[124]Dialysis was tested as an alternative to ion exchange as the re-quired pretreatment of zeolite catalysts, for purification and re-moval of sodium cations from the solutions with MWW layers.After calcination and burning out of the organics, highly porousand acidic zeolite materials were obtained. The arrangement oflayers in the resulting solid was not affected by dialysis, indi-cated by similar BET and external surface area values, which werecomparable to those of the other flocculated solids obtained bydifferent methods.[121]Original methods of recovering exfoliated layers from the so-lutions were proposed by Breu et al.,[125,126] and included sprayAdv. Mater. 2024, 36, 2307341 2307341 (11 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deTable 2. Representative acidic and textural properties for the starting zeolites and solid products obtained from the exfoliated dispersions by flocculation.Sample BASa) [μmol g−1] LASa) [μmol g−1] SBETb) [m2 g−1] Sextc) [m2 g−1] Vmicrod) [cm3 g−1] Ref.MCM-56 a 1155 99 457 180 0.09Layers from a 649 259 502 220 0.07MCM-56 b 778 97 467 167 0.09Layers from b 557 244 633 309 0.10 [51]MCM-56 before dialysis 669 148 514 225 0.087Layers dialyzed and freeze-dried 598 180 566 171 0.108 [121]MFI original layered 550 147 411 n/a 0.105MFI disc 571 110 447 n/a 0.130 [54]Bifer layers 450 n/a n/a n/a n/a [52]a)Brønsted and Lewis acid sites;b)total BET area;c)external surface area;d)micropore volume from t-plot.coating, doctor blading, or slot die coating. These methods allowpreparation of thin films, and orienting the nanosheets parallelto the substrate, exemplified by the prepared ilerite films, whichproved to be excellent gas barriers.[53]Zeolite nanosheets recovered from solutions have to be testedfor quality and the preservation of zeolitic properties, espe-cially when they are intended for use in catalysis. Since theresulting product is a solid, the structure may be evaluatedby powder XRD, or infrared spectroscopy in the pseudoskele-tal region, both by ATR and transmittance FT-IR after dilu-tion in KBr.[121,127] The critical qualities are porosity and acid-ity, which may be evaluated by the standard methods of lowtemperature gas adsorption and desorption, and sorption of ba-sic probe molecules followed by FT-IR spectroscopy, respec-tively.X-ray diffraction patterns (XRDs) of the flocculated solids in-dicated preservation of the original zeolite structure and revealedspatial disorder with possible layer deformation upon reassemblysuggested by lower scattering intensity.The FT-IR spectroscopy data indicated preservation of theshort-range order. The maxima characteristic of SBU units arepresent in the IR spectra of the crystalline zeolites. In somecases, as shown for MWW zeolites, the content of the crystallinephase can be qualitatively evaluated and even correlated withacidity.[127]A summary of representative results, acid site concentrationand BET surface areas, is presented in Table 2. In general, therecovered nanosheets showed some loss of acid site concentra-tion but not by much, indicating high potential for acid cataly-sis. The concentration of Lewis acid sites increased by up to 2.5times from the low level near 100 μmol g−1. BET areas show mod-erate increases. Overall, the positive outcome is preservation ofthe basic zeolite qualities upon conversion into exfoliated layersand solid recovery. This is confirmed later upon catalytic testingdiscussed in Section 7.7. Preparation of Intimate Mixtures of DifferentZeolites and Composites with Nanoparticles andClustersOne of the most notable benefits of exfoliated zeolite nanosheetsin solution in comparison to the standard 3D crystals lies in thepossibility of using the former to produce molecularly or sub-nanometer intimate composites with other active componentsincluding other zeolites, layered materials, nanoparticles, clus-ters and basically any compound or substance in a suitable formthat can interact with the nanosheets.[54,128] Examples of uniquematerials are illustrated in Figure 9. It should be expected thatthere can be limitations to obtaining useful or desirable prod-ucts due to thermodynamics and compatibility reasons like sol-ubility and separation of phases, etc. An illustrative example isthe reported mixing of two nanosheets resulting in a phase sep-aration driven by so-called depletion effects.[129,130] A contrast-ing example shows that exfoliated zeolite nanosheets of differ-ent topologies can be mixed and retain homogeneous nature.They can be recovered as mixtures of layers and show activitybetter than sum of the parts.52 Notwithstanding possible physicaland chemical limitations, the solutions with zeolite nanosheetsprovide, in comparison to their solid analogues, enormously en-riched latitude in designing nanoscale hybrids and composites.Furthermore, it should be expected that if simple combinationof desired components is not effective, scientists will find waysto circumvent the obstacles. The benefits of bringing togethercomponents of various functionalities are illustrated by the parti-cles of fluid cracking catalysts, which combine intimately severalcomponents with specific activity and roles to play.[131,132] Theidea of combining different zeolites so that each can influencedifferent molecules in the mixture has been also contemplatedbut could not be implemented at a sub-nanometer level due tothe lack of suitable building blocks. Zeolite nanosheets fill thisgap and proof-of-principle examples have been reported includ-ing intimate mixtures of MWW and MFI zeolites,[133] and MWWand bifer layers.[52] Mixtures of MWW zeolite nanosheets andMFI crystals were prepared by cocrystallization, i.e., synthesis ofMFI in the presence of MWW monolayers, and by combinationof MWW monolayers with already synthesized MFI crystals.[133]Zeolite MFI was deliberately chosen to be siliceous, i.e., catalyt-ically inert, so that only the activity originating from the layerscould be evaluated. Basic properties of the mixtures determinedby FT-IR and nitrogen sorption were consistent with proportion-ate contribution from each (active and inactive) component. Cat-alytic evaluation suggested enhancement of activity indicated bythe conversion not diminishing with decreasing content of the ac-tive MWW phase. Detailed discussion is continued in Section 8focused on catalysis.Adv. Mater. 2024, 36, 2307341 2307341 (12 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 9. Increasing diversity of possible products that can be obtained upon progression from 3D frameworks (in the center), through 2D solids (2ndring) to exfoliated monolayers in solution (3rd ring).A variation on the same theme is making mixtures ofnanosheets of different zeolites. This is limited for now becauseonly MWW and bifer nanosheets are readily available in suffi-cient amounts.[52] The composite was obtained by mixing solu-tions of these layers, which did not produce precipitation. Ad-dition of the surfactant HDTMA-Cl resulted in formation of awhite solid with expanded basal spacing consistent with surfac-tant swollen nanosheet assemblies. The treatment with TEOS af-forded pillared layered zeolite containing both MWW and biferlayers. Catalytic testing showed conversion in the mesitylene ben-zylation greater than sum of contribution from each component(based on % content).[52]The last combination of active phases reported so far areMWW layers with Pt nanoparticles.[134] Related previous stud-ies of these systems involved reacting MCM-22P precursor withMWW layers with the surfactant HDTMA and platinum precur-sor in DMF resulting in metal particles below 5 nm that werenot sintering.[135] The preparation with nanosheets in solutioninvolved adding solutions of Pt nanoparticles up to 1% wt/wtPt to zeolite and isolation of solids by freeze-drying. Calcinationcaused some sintering to average ≈10 nm but there was no addi-tional metal particle size increase afterward, even with repeatedhigh temperature regeneration. Catalytic testing is elaborated inSection 8. Silanol groups on the surface of zeolite nanosheetsare able to stabilize metal clusters and prevent their sinteringeven at high temperatures as evidenced for Rh clusters on IPC-1Players.[136]8. Catalytic Activity Alone and in MixturesCatalytic activity and industrial applications are the dominantareas of interest with zeolites hence new advances are exam-ined instantly from the standpoint of potential benefits forcatalysis.[131,137]As a result of this interest, catalytic activity of exfoli-ated nanosheets was evaluated for the zeolite MWW afterisolation from solution and activation by ion exchange andcalcination.[51,52,121] The test reaction was alkylation of mesity-lene with benzyl alcohol as one of the model systems for eval-uating conversion of bulky reactants, which reflects acid siteaccessibility.[138] It has been often used to assess catalytic activ-ity of hierarchical and layered zeolites.[139–142] Its purpose is todistinguish between transformations proceeding exclusively onthe surface of zeolite catalyst (C-alkylation of mesitylene, targetedreaction) and in the pores (O-alkylation of benzyl alcohol, side re-action) as shown in Figure 10. In the C-alkylation, mesitylene isthe largest reacting molecule, too big to enter micropores, andtherefore produces bulky 2-benzyl-1,3,5-trimethylbenzene. TheO-alkylation requires participation of relatively small benzyl al-cohol molecules giving dibenzyl ether, which can be formed onboth the external and internal acid sites. It is a reversible reactionand dibenzyl ether can be consumed providing the product forC-alkylation under appropriate conditions.[143]Typical tests were carried out[51] with 50 mg of the hy-drogen form of a zeolite, 0.1 g of dodecane as the internalAdv. Mater. 2024, 36, 2307341 2307341 (13 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 10. Catalytic activity, conversion versus time, of the exfoliated lay-ered zeolites MWW, bifer and their pillared mixture, after recovery as solidscompared to the parent sample MCM-56 and the 3D form of MWW (MCM-22).chromatographic standard, and 22 ml of mesitylene, acting asboth reagent and solvent, introduced into a 50 ml round-bottomflask. Stirring at >100 rpm was applied. After heating to the reac-tion temperature, 80°C, an aliquot of 0.2 g of benzyl alcohol (lim-iting reagent) was added with a syringe. Samples (250 μl) werewithdrawn after 0.5, 3, 5, 10, 20, 30, 60, 120, 180, 300, and 360min of the reaction time.Catalytic activity of exfoliated MWW layers was evaluated afterrecovery from the solution, ion exchange and calcination. Theconversion of benzyl alcohol in the model reaction was similarto the conversion exhibited by the parent MCM-56, as shown inFigure 10.[121] The latter is composed of disorganized MWW lay-ers, so it is de facto a delaminated MWW with high Al contentprepared by direct synthesis. For completeness, the correspond-ing 3D form of the MWW zeolite was obtained by calcinationof the multilayered precursor MCM-22P and also tested catalyti-cally. It showed much lower conversion as seen in Figure 10. Thecatalysts from exfoliated layers had enhanced textural character-istics in comparison to the parent, MCM-56, but it did not trans-late into higher activity.[121] These results show a very importantpoint, that nanosheets exfoliated into solution and recovered canmaintain high activity despite accompanying harsh treatmentsthat can cause degradation of quality. The second conclusion isthat disorganization of the layers, which is embodied by exfolia-tion and reassembly, does not by itself result in enhanced activityof a layered zeolite above that of the parent. To improve the per-formance, additional elements like incorporation of other com-ponents or different processing may be essential.An example illustrating such activity uplift is the pil-lared MWW material obtained from exfoliated nanosheets.It was flocculated with the surfactant cation HDTMA(hexadecyltrimethylammonium) and pillared with TEOS(tetraethylorthosilicate).[144] The activity, conversion versustime (not presented in this paper), in the test mesitylene benzy-lation reaction was greater than that of the initial nonexfoliatedzeolite MCM-56.[144] The conditions of pillaring, in particularhigh pH during flocculation and avoiding excess of the pillaringagent, were crucial for such enhanced activity and suggestfurther detailed studies.Section 7 alluded to the fact that mixed zeolite catalysts con-taining 50% of zeolite layers as a sole active component mayshow the same activity as pure layers (100%). This was observedwith the mixtures prepared from MWW layers and siliceous MFI.They were tested in the reaction of mesitylene with benzyl alco-hol. The conversion with catalysts containing 50% of inert MFIwas the same as with pure MWW layers, so was apparently notdiminished by dilution of the active component to one half. Fur-ther reduction of the content of MWW did cause reduction of theconversion rate. A physical mixture of the starting MWW parentand MFI crystals was also less active. This retention of activityupon dilution of active zeolite layers can be exploited to include inthe mixture another active component with additional or comple-menting functionality. The outcome may be synergistic enhance-ment of activity or creation of a new type of activity in terms ofkinetics and products. A similar situation was observed with themixture of layers – MWW and bifer as described above. It wasobtained by mixing solutions of both types of nanosheets, floccu-lation with surfactant and pillaring with TEOS. Subsequent cat-alytic testing showed conversion in the mesitylene benzylationthat was higher than the sum of contributions from each com-ponent (based on % content). This reveals the potential of suchmixed systems for activity enhancement or modification.Another example of composites obtained from exfoliated ze-olites is illustrated by the platinum activated MWW layers withup to 1% of Pt nanoparticles.[134] The catalysts were obtained bymixing solutions of nanosheets and Pt nanoparticles and isola-tion by freeze-drying. The test reaction was hydrogenation of 3-nitrotoluene. The observed activity was comparable to a commer-cial sample of 1% Pt deposited on alumina obtained from a ven-dor. The top activity was reached already at 0.3% Pt and levelledoff with increasing Pt content.The results of catalytic activity with the illustrated mixedsystems underscore the potential for exploiting exfoliatednanosheets in combinations with other components. The pos-sibilities are practically unlimited, unlike with 3D zeolites andeven 2D solids, allowing intimate combination with any ingre-dients, layers, nanoparticles, etc. There is little precedence withdesigning such systems because this option was unavailable untilexfoliated zeolite layers emerged. Attractive combinations seemto be mixing medium pore layers like MWW with larger pore ze-olites (FAU, beta) or mesoporous materials, active nanoparticleslike ceria or titania. They can be readily combined with exfoliatedlayers and afford dual reactivity.9. Preparation of Oriented Films and Membraneswith Exfoliated ZeoliteExfoliation provides zeolites in a novel form, different from thestandard polycrystalline powders that are generally used as-madewithout substantial alteration of the macrostructure. The avail-ability of nanosheets in solution allows the design of new materi-als and applications that have been impossible or unimaginableAdv. Mater. 2024, 36, 2307341 2307341 (14 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deFigure 11. XRD identification of oriented MFI films based on comparisonwith powder and disorganized layers. The 020 label refers to the peak ofthe 3D MFI, which is also exhibited in the disc pattern and confirms planarorientation of the layers.with the conventional zeolite crystals. There are not many con-crete ideas for that, mainly because the lack of suitable precur-sors has not encouraged contemplation of how they could beused to exploit the unique zeolite properties: active and porouslayers. Zeolite layers in solution present the problem of availabil-ity in large quantities and sufficient concentrations, which makesthem less competitive for traditional catalytic applications, whichrequire high volumes. However, they may be suitable for lowvolume high value applications exemplified by membranes.[145]Earlier studies with dispersed zeolite layers obtained via the sur-factant pre-expansion route, showed their usefulness for prepa-ration of oriented membranes. The obtained membranes con-tained defects, especially after calcination, but they could becured, and were tested for gas separations. The membranes pre-pared with zeolites MFI, MWW, and SOD showed promisingperformances.[44,45]The primary advantage of the present nanosheets obtained bydirect exfoliation is availability and easy preparation – a solid sam-ple can be converted in a few hours in 2 easy steps into a solu-tion with exfoliated layers that can be immediately used for thepreparation of films. Simple filtration was shown to afford filmsand transparent self-standing discs that can be ion exchanged andcalcined to activate pores and acid sites.[51,54] XRD patterns con-firmed strict orientation expected for high aspect ratio particlesby showing only purely interplanar reflection with two Miller in-dices equal to 0. As-synthesized and calcined films showed dif-ferent basal spacings. The former was greater than the crystallo-graphic unit cell of the corresponding 3D framework. This appar-ent expansion is readily rationalized by surface termination withsilanols and additional molecules intercalated between layers –water and to a smaller extent organics. Films made of MFI lay-ers presented an intriguing and more complex situation, whichis attributed to the fact that the layer thickness was 1.5 times ofthe unit cell—3.0 nm versus 2.0 nm for the corresponding b unitaxis. As a results 2 sets of 0k0 reflections can be observed in theXRD, see Figure 11. Those based on the unit cell include a promi-nent 020 peak at 9° 2𝜃. A typical zeolite MFI XRD pattern shows adoublet with peaks at 8 and 9° 2𝜃 but the former originates fromreflections with nonzero h or l indices. These latter reflections areabsent in the XRD of obtained films, which proves the b orien-tation. Standard MFI does not show low angle XRD peaks belowthe 8°–9° 2𝜃 doublet. In contrast, the MFI films show additionalreflections at lower 2𝜃 angles that appear as orders of the basalspacing slightly greater than 3.0 nm and 6.0 nm for surfactantprecipitated MFI layers. The latter indicates swollen-like MFI lay-ers. Calcination results in contraction in both cases to ≈2.8 nmd-spacing indicated by two asymmetric peaks at 2.8 and 1.4 nm.This contraction below the nominal layer thickness is also char-acteristic for laterally disorganized zeolite layers, which can pro-duce upon calcination materials called subzeolites.[146] Their for-mation can be explained by uneven surfaces of zeolite layers withhigh points and troughs. The high points (representing limitsof the layer thickness) can end up in valleys reducing the appar-ent interlayer repeat. The studies on producing zeolite films anddiscs from directly exfoliated zeolite nanosheets in solution havenot been extended beyond the preparation and characterization.Meaningful testing for applications such as gas sieving, possi-ble catalysis by oriented zeolites, etc. require advanced dedicatedequipment and preparation. They are simply beyond technicalcapabilities in a conventional laboratory setting but are expectedto find interest in due time.This potential of using exfoliated nanosheets to produce gasseparation zeolite membrane is illustrated by a recent study re-porting hydrogen purification membrane prepared from a lay-ered zeolite denoted ECNU-28.[66] As-synthesized ECNU-28 ma-terial was first treated with an acid to remove the organic tem-plate and then dispersed in DMF. The suspension containedboth mono- and multilayered nanosheets but produced a work-ing membrane after filtration and defect repair. The structuralcharacterization and identity of ECNU-28 raise serious doubtsfor several reasons. ECNU-28 and bifer have similar XRD pat-terns and unit cells, which suggests identical or similar struc-ture. Minor differences in the XRD patterns can be attributed totypical variation of the XRDs of layered materials depending onthe preparation and treatments. The questionable aspects includethe proposed swollen structure of ECNU-28 with 0.81 nm thickinorganic layers, separated by 1.53 nm organic interlayer contain-ing vertical decamethonium templates and the overall structureidentification as the SZR zeolite. The proposed swollen structureis inconsistent with the determined relatively low organic con-tent of 18% which should be much higher, closer to 50%, for atruly swollen precursor. All other data (XRD, TEM, AFM) suggestdoubled layer thickness, comparable to bifer. The proposed SZRstructure is based on qualitative comparison of calculated and ex-perimental XRD and ED patterns, the latter obtained with a poly-crystalline sample. Doubts about the structure assignment canbe also raised based on the synthesis—the SZR framework is noteasy to synthesize and so far has been obtained only with potas-sium in the gel,[147] which is not used in this case. The topologyof bifer is unknown so it is important to continue investigationof the structure of ECNU-28 and its relation to bifer.10. Perspectives Beyond Traditional ZeoliteApplicationsZeolite nanosheets straddle two structurally and frequentlychemically different classes of solids: 3D porous materials andAdv. Mater. 2024, 36, 2307341 2307341 (15 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.de2D ultrathin layers. Summarizing the above data and discussionthere are two basic takeaways pertinent to their practical poten-tial. First, unmodified zeolite nanosheets are usually not muchbetter catalytically than conventional 3D zeolites and in particulardo not produce significantly more porous structures without ad-ditional help. Second, these nanosheets readily produce orientedfilms, which can be competitive in special applications that cantolerate increased cost of preparation. Regarding the first, zeolitenanosheets are unlikely replacement of catalysts in the conven-tional large-scale processes. However, as illustrated above and inFigure 9, they can be used to produce nanohybrid materials thattheir 3D counterparts cannot. This, in combination with the pos-sibility of easy and reliable production of films can be useful notonly for membrane fabrication as already demonstrated but alsoin catalysis on surfaces and by thin films. This method can sup-ply new types of materials with well-defined surfaces that can dif-fer for each zeolite topology. For starters, this can be a valuableresearch tool of reaction mechanisms because of precise knowl-edge of the surface that can be modified through surface charge,silanol density, heteroatoms, etc. Hence, investigation of modelreactions can provide insights that may be obfuscated with poly-crystalline samples, which do not allow much control over ex-posure of particular crystal faces. The potential learnings mayallow tuning of surface properties and consequently reactivity.From the perspective of zeolite formation mechanism and sta-bility, the films can be used as substrates for immobilization andinvestigation of guest molecules, e.g., various structure directingagents. Going further, as nanosheets allow unhindered combi-nation with other entities like clusters, nanoparticles and otherlayers, the resulting hybrid precursors can be also deposited onsupports and investigated. Needless to say, the nonzeolite com-ponents can have alternative activity, e.g., redox, photocatalytic,etc., and result in multifunctional catalysts. Nanoparticles of ti-tania, ceria and other metal oxides have proven catalytic activitythat can complement zeolites.Looking from side of the 2D solids, zeolite nanosheets canbe considered for the various applications unrelated to catal-ysis. The examples include protective covering (anticorrosion,flame retardant, etc.),[55] design of functional composite materi-als via LbL (layer by layer) deposition,[50] and bio-med applica-tions like drug delivery.[148] In this regard, they are in competi-tion with many established and already extensively studied other2D solids, but have different structures and additional featuresalready mentioned—pores and active sites.[32,75,149]The applications of 2D solids often invoke special optical, elec-tric, electronic and magnetic properties for exploitation in vari-ous situations and devices.[150] Zeolite nanosheets do not possesssuch special characteristics but may be used in combination withthese materials for example as insulators or separators betweenphysically active layers. The special trait of zeolite layers, namelyinternal porosity, may be exploited in a way that the the other 2Dsolids cannot offer. This could be exploited for introduction in thepores of selected ions or moieties capable of imparting specificproperties: optical, magnetic, electronic, electric conductance asalternatives to other layers with these qualities. It may even bepossible to vary the magnitude of particular effects by changingthe amount of introduced species in the pores. The pores in thelayers can also be used as “dynamic” gas barriers, so named be-cause they can prevent not only diffusion through but also ad-sorb external molecules. Applications as sensing devices are alsoattractive for zeolite films.In summary, zeolite nanosheets are promising objects to studyfor applications specific to zeolites and 2D solids with a particularfocus on the exploitation as oriented films in catalysis and spe-cial devices. The trait that distinguishes zeolite nanosheets fromother 2D solids, i.e., the presence of internal pores and exchange-able sites, may be a particular fulcrum toward valuable uniqueapplications.Since zeolite nanosheets in solution were not available untilrecently there has been very little attention or contemplation oftheir possible uses beyond generic, already mentioned applica-tions as both zeolites and layered materials. The enabled capa-bilities for thin zeolite film fabrication have been unavailablehence there is little prior art or even conceptual planning forthe design of viable schemes. Specific uses can only be specula-tive, especially because the mentioned classes of materials pro-vide abundant prior art with proven usages and benefits. Thepursuit of practical application requires expanding knowledgeabout preparation and properties of exfoliated zeolite nanosheets.The primary objectives are expanding the exfoliation to otherframeworks and understanding synthesis parameters that influ-ence formation of intergrowth and thus affect the amount ofnanosheets that can exfoliate directly. There are many aspects ofthe exfoliation and the nanosheet properties that are little known,like distribution of lateral sizes, quality preservation, associa-tion upon standing and possibility for using different exfoliationagents and solvents. Each of these parameters may be importantin specific instances so they deserve systematic examination.Summarizing, readily available exfoliated zeolite nanosheetsare novel, so there was no time to document applications andshow their importance except by inference from other inorganicnanosheets. There is already evidence of their value for syn-thesizing oriented discs both porous[44,48] and impermeable[53]by choosing different frameworks. The importance of inor-ganic nanosheets is underscored by frequently published re-views on topics including uses in catalysis,[151] biomedicalapplications,[148,152] to make heterostructures[153] and functionaldevices.[154] As was emphasized, zeolite nanosheets exhibit newfunctionalities to exploit: strong catalytic activity and layer poros-ity. They can be instantly considered for catalysis and compar-ison with clays, e.g., in biomedical applications. Specific useswill depend on particular needs and testing, which are hard topredict. What is particularly promising about exfoliated zeolitenanosheets is easy preparation and genuine versatility for thepreparation of nanoscale composites because of availability inthe dispersed liquid state. This opens enormous opportunitiesin comparison to the reliance on solid systems as substrates forsyntheses.AcknowledgementsThis work was financed with the funds from the National Science Cen-tre Poland, grant no. 2020/37/B/ST5/01258 (W.J.R, B.G.). M.M. and J.C.thank the Czech Science Foundation for funding this research throughthe ExPro project (19–27551X) and OP VVV “Excellent Research Teams,”project no. Z.02.1.01/0.0/0.0/15_003/0000417 − CUCAM from Minister-stvo Školství, Mládez ̌e a Tělovýchovy. M.O. thanks Ministry of Education,Youth and Sports of Czech Republic for funding through ERC_CZ projectLL 2104. T.S. acknowledges Japan Science and Technology Agency (JST),Adv. Mater. 2024, 36, 2307341 2307341 (16 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deGrant No. JPMJCR17N1 (CREST) and Japan Society for the Promotion ofScience (JSPS), Grant No. P21036. The work was also supported by Minis-terstvo Školství, Mládez ̌e a Tělovýchovy as ERDF/ESF project TECHSCALE(Nos. CZ.02.01.01/00/22_008/0004587).Conflict of InterestThe authors declare no conflict of interest.Keywordsbifer, chemical exfoliation, ilerite, nanocomposites, ultrathin zeolitenanosheets, zeolite films, zeolites MFI and MWWReceived: July 24, 2023Revised: September 15, 2023Published online: November 28, 2023[1] P. Van Der Voort, K. Leus, E. De Canck, Introduction to Porous Mate-rials, Wiley, New York, NY, USA, 2019.[2] J. Rouquerol, F. Rouquerol, P. Llewellyn, G. Maurin, K. S. W. 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Bruce, D.O’Hare), Inorganic Materials, Wiley, New York, NY 1997, pp. 172-254.[150] R. Ma, T. Sasaki, Acc. Chem. Res. 2015, 48, 136.[151] T. A. Shifa, F. Wang, Y. Liu, J. He, Adv. Mater. 2019, 31, 1804828.[152] A. K. Gaharwar, L. M. Cross, C. W. Peak, K. Gold, J. K. Carrow, A.Brokesh, K. A. Singh, Adv. Mater. 2019, 31, 1900332.[153] M. Gobbi, E. Orgiu, P. Samorì, Adv. Mater. 2018, 30, 1706103.[154] R. Ma, T. Sasaki, Adv. Mater. 2010, 22, 5082.Wieslaw J. Roth is a professor at the Faculty of Chemistry of the Jagiellonian University in Krakow,Poland. He worked at Mobil R&D in Paulsboro and ExxonMobil R&E in Clinton, New Jersey, for21 years prior to joining the Faculty in 2012. He received the D. W. Breck Award in 1994 for the co-discovery of ordered mesoporous materials, Thomas Alva Edison Award in 2008 for the patent andcommercialization of mesoporous materials, and the FEZA Cronsted Awards in 2020, jointly with J.Cejka, for the contributions to the development of layered zeolites.Adv. Mater. 2024, 36, 2307341 2307341 (19 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deMaksym Opanasenko received his MSc degree in Chemistry from the Lomonosov Moscow StateUniversity. He continued his PhD study at Pisarzhevsky Institute of Physical Chemistry (the NationalAcademy of Sciences of Ukraine) and obtained his PhD degree in porous polymeric and compositematerials in 2011. In 2011 Maksym joined the group of Prof. Čejka at Heyrovsky Institute of PhysicalChemistry, ASCR. From 2021 Maksym has been holding an Associate Professor position at the Facultyof Science, Charles University in Prague. Currently, he is focused on the design of new nanostructuredinorganic and hybrid materials and their application in catalysis.Michal Mazur is an Assistant Professor and a head of Electron Microscopy Laboratory at the Depart-ment of Physical and Macromolecular Chemistry, Charles University, Prague, Czechia. He obtainedhis PhD in chemistry from Charles University in 2016. Afterward, he held a post-doctoral position atthe University of St Andrews, Scotland. Michal has been investigating novel approaches to synthesisof zeolites, focusing on the use of 2D layered precursors and functionalization of zeolites with metalnanoparticles. His main interest is characterization of zeolitic materials by advanced electron mi-croscopy and diffraction techniques.Barbara Gil is a full Professor and the Head of Zeolite Chemistry Group at the Faculty of Chemistry ofthe Jagiellonian University. She has specialized in the synthesis and characterization of micro- andmesoporous materials. Her main areas of interest are the classical 3D zeolites and the new class oflamellar 2D zeolites, MOFs and ordered silica mesoporous materials. Her research combines IR spec-troscopy with catalysis and drug delivery. She is an expert in quantitative characterization of acidic andredox centers in general and in environmentally important reactions. She is a co-author of over 140scientific articles.Jǐrí Čejka is a professor at the Department of Physical and Macromolecular Chemistry at the Faculty ofScience of the Charles University. He obtained his PhD at the Czechoslovak Academy of Sciences in1988. In 2016 he joined the Faculty of Science where he is leader of the research group focused on thesynthesis, characterization and application of zeolites and other porous materials in adsorption andcatalysis. He is a principal investigator of projects supported by the Czech Science Foundation andHorizon Europe. He received the FEZA Cronsted Awards in 2020, jointly with W. J. Roth.Adv. Mater. 2024, 36, 2307341 2307341 (20 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.dewww.advancedsciencenews.com www.advmat.deTakayoshi Sasaki is a principal investigator of Research Center for Materials Nanoarchitectonics (WPI-MANA) at the National Institute for Materials Science (NIMS) in Japan. He obtained his PhD in chem-istry from the University of Tokyo in 1985. Since 1980, he has been working for the National Institutefor Research in Inorganic Materials (NIRIM, now NIMS), Japan. In 2009, he was appointed as a NIMSfellow. He has been investigating layered transition metal oxides and hydroxides and their delami-nated 2D nanosheets.Adv. Mater. 2024, 36, 2307341 2307341 (21 of 21) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2024, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202307341 by National Institute For, Wiley Online Library on [08/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advmat.de Current State and Perspectives of Exfoliated Zeolites 1. Introduction 2. Explanation of the Nomenclature 3. Prior Attempts at Zeolite Exfoliation into Monolayers in Solution Especially with Surfactant Pre-Expanded Precursors 4. Proving Monolayered Nature of Zeolite Nanosheets in Solution 5. Preparation of Dispersed Nanosheets in Solution by Soft-Chemical Exfoliation 6. Methods of Isolating Exfoliated Layers and Their Characterization as Solids with Zeolite Properties 7. Preparation of Intimate Mixtures of Different Zeolites and Composites with Nanoparticles and Clusters 8. Catalytic Activity Alone and in Mixtures 9. Preparation of Oriented Films and Membranes with Exfoliated Zeolite 10. Perspectives Beyond Traditional Zeolite Applications Acknowledgements Conflict of Interest Keywords