Nanosheet zeolites: controlled synthesis, characterization, and advanced catalysis applications

Yankai Bian, Xiaoyang Zhang, Jun Yu* and Weili Dai*
School of Materials Science and Engineering, Nankai University, Tianjin 300350, China. E-mail: yujun@nankai.edu.cn; weilidai@nankai.edu.cn

First published on 6th June 2025

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Wider impact Nanosheet zeolites with two-dimensional structures have emerged as promising candidates to mitigate diffusion limitations of zeolites, attracting considerable research interest in the past few decades. In this review, we provide a systematic overview of recent progress in the synthesis, structural characterization, and catalytic applications of nanosheet zeolites. By elucidating key advancements and outlining the future research directions, this review highlights the pivotal role of zeolite nanosheets as versatile functional materials with significant potential in advancing next generation catalytic technologies. In contrast to prior reviews, we present a more comprehensive discussion, encompassing the synthetic strategies for nanosheet zeolites, structure–property correlations governing their physicochemical characteristics and their catalytic performance towards diverse reaction systems. Additionally, we summarize the latest breakthroughs in nanosheet zeolite catalysts, offering critical insights into emerging opportunities in the field. Therefore, we believe that this review will be of great interest to researchers in the fields of nanosheet zeolites and industrial catalysis.

1. Introduction

The history of zeolites can be traced back to 1756, when Swedish mineralogist Cronstedt discovered a mineral in nature that continuously bubbled upon heating in water, resembling boiling. He named that mineral “zeolite”, meaning “boiling stone.” Following this, early researchers discovered several minerals with zeolite-like properties, such as stilbite, natrolite, and other low-silica zeolites. As the discovery of natural zeolites progressed, researchers began to explore their properties, leading to their application in adsorption, separation, and ion exchange.^1–3 A pivotal moment in zeolite science occurred in 1948, when Barrer synthesized a zeolite that did not exist in nature by transforming known minerals in a high-temperature concentrated salt system through iterative experiments. This breakthrough marked the beginning of artificial zeolite synthesis. Building on Barrer's work, Milton and colleagues used the hydrothermal synthesis method to successfully synthesize various zeolites, including A-type and X-type zeolites. During this period, zeolite synthesis primarily focused on low-silica zeolites. However, the relatively high aluminum content of these zeolites resulted in poor hydrothermal stability, thereby constraining their practical applications. Through continued research and experience accumulation, significant progress has been made in the artificial synthesis of zeolites. A notable development was Barrer's introduction of a novel approach incorporating quaternary ammonium salts into the zeolite synthesis system in 1961. As synthesis techniques for zeolites have advanced, the diversity of zeolite structures has expanded considerably, with the number of known zeolite topologies reaching 255.

Traditionally, zeolites have been extensively utilized in diverse applications, including catalysis, separation processes, and environmental remediation.^4–9 However, the relatively long microporous channels of traditional zeolites limit the mass transfer and diffusion of reactants and products between active sites, particularly in the case of large molecules. This often results in undesirable side reactions, carbon deposition, rapid catalyst deactivation, and other complications. Therefore, effectively reducing diffusion limitations has emerged as a research focus in the controlled synthesis of zeolites. In recent years, the emergence of zeolite nanosheets, characterized by a two-dimensional (2D) morphology, has garnered significant attention owing to their unique structural and functional advantages.^10–18 These nanosheets, typically with a thickness in the order of a few nanometers, offer significantly increased surface areas, more accessible active sites, and improved mass transport properties compared with their bulk counterparts.^19,20

To date, the synthesis of zeolite nanosheets has been the focus of numerous studies, employing methods such as in situ hydrothermal synthesis with organic structure-directing agents,^21–30 additive-assisted synthesis,^32–37 seed-induced synthesis,^38–41 inter-growth synthesis,^42–44 chemical etching,^45,46 swelling, exfoliation, and pillaring^47–53 by post-synthetic to control their structure and properties. The transition to reduced dimensionality not only alters the physical and chemical characteristics of zeolites including their morphology,^54–63 structure,^47,67 porosity,^68,69 and acidity,^70–77 but also imparts unique catalytic behavior, making zeolite nanosheets particularly attractive to realize challenging catalytic transformations, such as methanol conversion,^78–84 cracking,^85–91 isomerization,^92–99 alkylation,^100,101 carbonylation,^102–104 and catalytic oxidation.^105–112

Notably, recent comprehensive reviews have covered the developments in two-dimensional zeolites.^113–115 However, their discussion mainly focused on the qualitative correlations between surface properties and performance, while detailed case studies comparing the catalytic behavior of different nanosheet zeolite structures in specific reaction systems are still lacking. Thus, in the present review, we specifically emphasize the controlled synthesis strategies, advanced structural characterization, and catalytic applications of nanosheet zeolites, providing a systematic summary of the most recent advancements in nanosheet zeolites. The discussion focuses on the impact of nanosheet morphology on catalytic performance and explores the potential of nanosheet zeolites in emerging applications, such as energy conversion and environmental catalysis. By highlighting critical developments and future directions in this field, this review underscores the significance of zeolite nanosheets as multifunctional materials with considerable potential for next-generation catalytic technologies.

2. Synthesis of nanosheet zeolites

Recently, strategies for the synthesis of nanosheet zeolites have been widely reported. Nanosheet zeolites with diverse frameworks, including MFI, MWW, FAU, FER, and MOR, have been successfully fabricated. The synthesis of 2D or nanosheet zeolites can be primarily categorized into two distinct methodologies, as follows: (I) in situ hydrothermal synthesis, which limits crystal growth in specific directions to form lamellar structures via the use of template agents or induced seeds. (II) Post-synthesis treatments, such as chemical etching, pillaring, and exfoliation. In the past few decades, nanosheet zeolites, particularly MFI, MWW, and FER, have achieved industrial-scale production and demonstrated exceptional catalytic performances in various practical applications. More details of their synthesis methods are elaborated in the following section.

2.1 In situ hydrothermal synthesis

2.2 Post-synthetic treatments

In contrast to the above-mentioned synthetic strategy, nanosheet zeolites can also be fabricated through the post-synthetic treatment of zeolite crystals, yielding a diverse range of zeolites with two-dimensional layered structures. The primary synthesis methods include chemical etching, swelling, exfoliation and pillaring by amorphous SiO₂. Post-synthetic treatments are extensively utilized in industry and play a pivotal role in the commercialization and application of zeolites. These treatments enable the formation of zeolites with excellent catalytic and adsorptive properties, including high stability, tailored compositions, and desired acid-site distributions.

3. Physicochemical properties of nanosheet zeolite

Nanosheet zeolites exhibit an ultrathin sheet morphology with a thickness ranging from 2 to 100 nm, displaying unique catalytic properties. Typically, surfactant molecules align along the channels of the MFI framework. Assemblies are formed along the b-axis to create multilayer structures, or adopt random arrangements to form monolayer configurations. These structural variations significantly influence the distribution of active sites. In contrast to the conventional three-dimensional (3D) zeolites with active sites predominantly situated within the micropores, two-dimensional (2D) zeolites predominantly feature active sites located on their external surface, resulting in unique application potential. Consequently, a comprehensive understanding of the structural and chemical characteristics of nanosheets is crucial for optimizing their catalytic performance.

3.1 Morphology analysis

In contrast to the typical hexagonal-prismatic or coffin-shaped morphology of the conventional ZSM-5, nanosheet MFI zeolites exhibit diverse morphologies, which are influenced by both the nanosheet architecture and the assembly process. Hu et al. reported the successful synthesis of a high-silica nanosheet MFI zeolite using low-cost raw materials and a bifunctional organic surfactant ([C₁₈H₃₇–N⁺(CH₃)₂–(CH₂)₆–N⁺(CH₃)₂–C₆H₁₃]Br₂), confirming that the obtained nanosheet zeolite exhibits a lamellar stacking morphology, with three-dimensionally intergrown nanosheets.⁵⁵ Ryoo et al. investigated the hierarchically structure-directing effect of multi-ammonium surfactants in the synthesis of MFI nanosheets.^21–23 They synthesized a range of surfactants with a different number of ammonium groups, alkyl tail lengths, and spacer lengths between the ammonium groups. The surfactant molecule must contain at least two ammonium groups to effectively direct the formation of the zeolite structure, alongside the amphiphilicity and micelle packing ability of the surfactant. The –C₃H₆–, –C₆H₁₂–, and –C₈H₁₆– spacers (C_iH_2i) in C_22-iN₂ led to the formation of micrometer-sized bulk crystals, a multilamellar structure, and disordered nanosheets, respectively. Additionally, the thickness of the resulting zeolite nanosheets could be tuned by varying the number of ammonium groups in the surfactant. Using the C_22-6N₂ surfactant, each nanosheet consisted of three five-membered ring layers with a thickness of ∼2 nm, whereas the C_22-6N₃ and C_22-6N₄ surfactants induced the formation of thicker nanosheets with five and more than seven five-membered ring layers, respectively. This is due to the fact that the amino head groups of the surfactant act as structure-directing agents for the zeolite, and the increase in the number of amino groups promotes preferential growth of the zeolite along the b-axis, thereby increasing the nanosheet thickness. The morphology of the different types of nanosheet zeolites is exhibited in Fig. 6.

Fang et al. reported the direct synthesis of hierarchically structured MFI zeolite nanosheet assemblies via a seed-assisted strategy.⁵⁶ The resulting assemblies showed a narrow mesopore distribution, and their morphology could be adjusted from nanosheet stacks to house-of-cards structures by varying the Si/Al ratio to 31. A lower Si/Al ratio led to irregular stacks, whereas a higher Si/Al ratio produced a nanosponge-like morphology. Additionally, the particle size of the obtained zeolite was strongly dependent on the seed content. Increasing the seed content in the gel from 5 to 30 wt% reduced the particle size from 0.8–1.2 μm to ∼500 nm. Zhan et al. synthesized ZSM-5 zeolite nanosheets with thicknesses ranging from ∼4 nm to 20 nm by adjusting the ratio of surfactant to tetraethyl orthosilicate (TEOS), which influenced their pore structure and catalytic performance.

Wattanakit et al. fabricated hierarchical FAU nanosheets via a hydrothermal process, using dimethyloctadecyl-(3-(trimethoxysilyl)propyl) ammonium chloride (TPOAC) as a hierarchical porogen.⁵⁷ They found that the TPOAC content and crystallization temperature played significant roles in determining the morphologies and textural properties of the hierarchical FAU nanosheets. As the TPOAC content increased, the FAU nanosheets transitioned from smooth surfaces to rough spherical crystals, while excessive TPOAC inhibited the self-assembly of the nanosheets. Simultaneously, the microporous specific surface area increased with an increase in the crystallization temperature. Therefore, an appropriate TPOAC content (0.015–0.045 mole fraction) and a high crystallization temperature (85 °C) are crucial for achieving high-crystallinity and uniform FAU nanosheet morphologies. In a distinct approach, Ferdov et al. established the hierarchical assembly of FAU nanosheets by controlling the conditions near the boundary of FAU and EMT zeolite co-crystallization.⁵⁸ FAU-type zeolite nanosheets with a ball-like morphology and a diameter of ∼2 μm were obtained without employing any organic or inorganic morphology-directing templates or agents. In a template-free system, Li et al. synthesized 50–100 nm thick H-mordenite with a nanosheet stack morphology by adjusting the water content.⁵⁹ A decrease in alkalinity (higher H₂O/SiO₂ ratio) promoted the growth and assembly of H-MOR nanocrystals, forming a nanosheet structure.⁶⁰ Dai et al. used γ-aminobutyric acid as a growth modifier to produce plate-like ZSM-5 with a b-axis thicknesses of ∼50 nm. Low pH values facilitated the formation of plate-like ZSM-5 zeolites.⁶¹ The addition of γ-aminobutyric acid as a growth regulator effectively suppressed growth in the [100] direction, while simultaneously promoting oriented growth along the c-axis, thereby yielding a thinner plate-like structure.

Yang et al. developed ultrathin FER nanosheets (designated as SCM-37) through a dual-template strategy using octyl-trimethylammonium chloride (OTMAC) and 4-dimethylaminopyridine (4-DMAP).⁶² The ¹³C MAS NMR results indicated that OTMAC, as an organic template agent, participates in the crystallization process of ultrathin FER zeolites. Its ammonium group is likely located within the pores and cages of the zeolite, synergizing with 4-DMAP to control the morphology and structure of SCM-37. J. D. Rimer et al. reported the synthesis of two-dimensional MWW zeolite nanosheets using a one-step approach.⁶³ They employed the surfactant cetyltrimethylammonium (CTA) as a dual OSDA and exfoliating agent to produce MWW-type layers with an average thickness of 3.5 nm (∼1.5 unit cells). The use of CTA altered the Al sites without affecting the overall Si/Al ratio in the final product. This method offers a straightforward approach for the direct synthesis of 2D MWW-type materials, and is potentially applicable to other layered zeolites.

Ryoo et al. performed TEM, which revealed that each nanosheet is composed of alternating 2 nm-thick layers and 2.8 nm-thick surfactant layers, yielding an overall thickness of about 5 nm.²¹ The formation of both multilayer and single-layer MFI nanosheets was determined by the orientation and arrangement of surfactant molecules within the MFI framework. Qian et al. synthesized MFI zeolite nanosheets with a controllable morphology and interlayer spacing by adjusting the surfactant structure.⁶⁴ TEM images revealed that surfactants with different chain lengths led to distinct nanosheet morphologies of MFI zeolite nanosheets. For instance, templates such as C_6-12-diphe and C_6-6-diphe produced multilayered lamellar structures, each composed of three pentasil zeolite layers separated by surfactant layers. In contrast, the C_6-10-diphe template yielded nanosheets with an irregular and indistinct morphology, lacking clear layer stratification. Furthermore, an increase in the hydrophobic chain length of the surfactant resulted in a proportional increase in the interlayer spacing of the MFI zeolite nanosheets. This phenomenon arises from the geometric parameters of the surfactants, which dictate the formation of varying mesoporous structures. Fig. 7a presents the selected-area electron diffraction (SAED) pattern, exhibiting polycrystalline ring-shaped diffraction features consistent with the characteristic peaks of beta zeolite.²⁶ The TEM (Fig. 7b) image revealed that the beta nanosheet is assembled from ∼16 nm-thick nanosheets, forming a three-dimensional ‘roof-tile’ architecture (∼800 nm). Moreover, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with electron tomography (Fig. 7c) further elucidated the internal ‘roof-tile’ morphology, consisting of interwoven nanosheets with mesoporous regions of low electron density at the core. Rimer et al. used cryogenic transmission electron microscopy (cryo-TEM) to observe the MWW nanosheet.⁶³ Fig. 7f presents the bright-field high-resolution TEM image of randomly oriented MWW nanosheets in the d-MWW_8.0 sample. These nanosheets predominantly exhibit an “edge-on” orientation (with the [001] perpendicular to the electron beam), appearing as narrow ribbon-like features. The enlarged view (Fig. 7g) showed two representative nanosheets, one with a unit-cell thickness and another with a bilayer configuration (two-unit-cell thickness). Fig. 7h reveals the high-resolution image of a planar region with the MWW zeolite crystal structure oriented along the (001) plane. For structural clarification, Fig. 7i superimposes the zeolite framework model onto a localized area of Fig. 7h, where the bright contrast regions are interpreted as the corresponding to zeolitic cage cavities. Fig. 7k and m display the STEM images of an MFI nanosheet⁸ and plate-like MOR,³⁷ respectively. Wannapakdee et al. developed a facile and efficient approach for the synthesis of FAU zeolite nanosheets, utilizing an organosilane surfactant as a structure-directing agent.⁶⁶ TEM analysis revealed that the FAU nanosheets exhibit a multi-layered structure, featuring narrow slits between the layers and surfactant-derived mesoporous cavities. Additionally, variations in the crystallization temperature and surfactant concentration enabled the fine-tuning of the FAU morphology, particle size, crystallite size, and porous structure characteristics.

3.2 Structural analysis

Structural characterization of zeolite nanosheets, including their crystallinity, pore size, and surface area, is crucial for elucidating their physical properties. Powder X-ray diffraction (XRD) is widely used to characterize the structural properties of zeolites, particularly their crystallinity. The framework types of zeolites can be reliably distinguished based on their XRD patterns.

In contrast to 3D zeolites, nanosheet zeolites typically exhibit broad diffraction patterns, attributed to the absence of three-dimensional periodicity and disordered interlayer stacking. As shown in Fig. 8a, the 2D structure of the MWW family of zeolites can be identified by their characteristic peaks in the 2θ range of 6–10°, reflecting the interlayer spacing and interlayer orientation.⁶⁷ As the size of the zeolite crystals decreases, their XRD patterns show broader interlayer (l00) diffraction peaks with only the in-plane (hk0) diffraction peaks retained. Meanwhile, a sharp reconfiguration of the 2D layer order in the (h0l) plane occurs, producing sharp peaks. Analogously, Fig. 8b shows the difference in the diffraction peaks between nanocrystal and plate-like ZSM-5. A sharp peak of h0l reflections can be observed, demonstrating that the thickness along the b-axis significantly decreased.

Corma and coworkers investigated the structural evolution of the layered MWW precursor during the post-synthesis process.⁴⁷ MCM-36 was obtained by pillaring the swollen precursor materials, and the XRD pattern of MCM-36 (Fig. 8c) displayed the characteristic peaks of the MWW topological structure. The delamination of the layered precursor of the MCM-22 zeolite to form ITQ-2 produced an almost amorphous XRD pattern, indicating a significant reduction in long-range order. Compared to the MCM-22-type zeolite, the high-angle diffraction peaks of ITQ-2 are broader, consistent with its reduced crystallite size. In summary, MCM-36 retained the key features of the MWW topological structure, with an XRD pattern resembling that of MWW-type zeolites. In contrast, the XRD pattern of ITQ-2 was altered by the delamination of its layered structure.

3.3 Pore analysis

Generally, nanosheet zeolites exhibit a greater mesopore volume and larger external surface area compared to conventional zeolites. Nitrogen (N₂) adsorption–desorption analysis is commonly employed as a reliable technique to determine the pore volume and surface area of zeolitic materials. For example, Qian et al. used the Barrett–Joyner–Halenda (BJH) model to analyze the pore size distribution of their samples.⁶⁵ The synthesized MFI nanosheet samples exhibited an enhanced specific surface area, external surface area, and larger pore volume compared to the commercial ZSM-5 samples with a lower specific surface area but higher micropore area and volume. Furthermore, adjusting the structure of the surfactant, such as varying the hydrophobic chain length, could effectively control the pore size distribution of the layered MFI nanosheets. Similarly, Yan et al. reported that MFI nanosheets exhibited a large specific surface area and abundant mesoporous structure (366–665 m² g⁻¹), which is larger than that of conventional MFI zeolites (293 m² g⁻¹).⁶⁸ In contrast, the traditional MFI zeolite does not exhibit a significant hysteresis loop, consistent with the absence of mesopores. Additionally, Liu et al. synthesized an MFI nanosheet zeolite via the pillaring method, which also exhibited a relatively higher mesopore volume than the conventional ZSM-5.⁶⁹ Moreover, the pore size distribution of nanosheet zeolites varies significantly with their synthesis method, where the intercalation method is more efficient in generating mesoporous structures, whereas the dual-template approach produces more microporous structures.

Limtrakul et al. further found that the porous structural features of layered FAU nanosheets can be modulated by their synthesis conditions, including surfactant content and crystallization temperature. Specifically, as the surfactant content increases (TPOAC/Al₂O₃ ratio from 0.01 to 0.04), the specific surface area and pore volume of the samples gradually decreased (from 734 to 566 m² g⁻¹, and from 560 to 407 cm³ g⁻¹, respectively).⁶⁶ Corma et al. demonstrated that the nanosheet zeolite obtained by the delamination process (ITQ-2) exhibited higher total and mesoporous surface areas than both pillared MCM-36 and layered precursor MCM-22 samples.⁴⁷

3.4 Acid analysis

The introduction of trivalent heteroatoms (e.g., Al³⁺, Fe³⁺, Ga³⁺, and B³⁺) into the zeolite framework generates a negative charge. When a proton balances the negative charge, a Brønsted acid site is formed within the zeolite framework. Dehydration of the Brønsted acid sites can convert them into Lewis acid sites. The acidic characteristics of zeolite catalysts, such as acid type, density, and strength, significantly impact the catalyst activity and product selectivity in various chemical reactions. Therefore, the effective characterization of zeolite acidity is crucial for understanding the structure–activity relationship of zeolite catalysts. To this end, numerous techniques have been developed to determine the acidity of zeolites both quantitatively and qualitatively.

Brønsted acidity in zeolites originates from their bridging Si–OH–Al groups, whereas Lewis acidity arises from their three-coordinated framework Al species, rather than tetrahedrally coordinated Al. The acidic properties of zeolites (e.g., strength and density) can be effectively characterized by ammonia temperature-programmed desorption (NH₃-TPD), where weak acid sites correspond to desorption peaks at 373–673 K, while strong acid sites are assigned to the peaks in the range of 673–1073 K. Complementarily, the type of acidity can be analyzed by Fourier-transform infrared spectroscopy (FTIR). The Lewis acid sites (LAS) can be detected at 1454, 1487, and 1625 cm⁻¹, and the Brønsted acid sites (BAS) can be observed at 1487, 1540, and 1634 cm⁻¹.⁷⁰ Furthermore, ammonia (NH₃)-assisted ¹H MAS NMR is employed to quantitatively analyze both the BAS and LAS concentrations. The coordination environment of aluminum (Al) within the zeolite framework can be investigated via ²⁷Al MAS NMR. The peak at 55 ppm corresponds to tetrahedrally coordinated Al atoms integrated into the zeolite framework, while the peak at 0 ppm is associated with extra-framework Al species.⁷¹ The acidity properties can be enhanced by increasing the crystallinity of the zeolite (at the same Si/Al ratio), given that higher crystallinity results in more internal-framework Al sites and fewer external-framework Al sites. The internal-framework Al sites contribute more to the acid strength due to their better tetrahedral geometry.

Fig. 9a shows the NH₃-TPD analysis of plate-like ZSM-5. All the samples exhibit peaks corresponding to weak and strong acid sites. The areas of these desorption peaks are consistent with the aluminum content, reflecting the acid site density. Pyridine adsorption infrared spectroscopy (Py-FTIR) identifies the acid types (Fig. 9b), revealing that all the samples contain Brønsted acid sites and a small amount of Lewis acid sites. Quantitative analysis indicates that the total acid amount of the samples is in the range of 361 to 388 μmol g⁻¹, with the Brønsted acid sites being the dominant type.^72,73 Wattanakit et al. investigated the effect of the synthesis precursors.⁷⁴ The use of pure silica nanobeads as the precursor produced a significantly higher intensity at 0 ppm compared to aluminosilicate nanobeads. This suggests that hierarchical ZSM-5 nanospheres synthesized from aluminosilicate nanospheres incorporate more aluminum species into the zeolite framework, forming Brønsted acid sites, rather than remaining in the extra-framework state.

In the case of heteroatom-substituted zeolites, their acid properties are determined by their heteroatom species. Yan et al. evaluated the acid strength of isomorphous MFI nanosheet zeolites (MFI (M), M = Al, Ga, and Fe).⁷⁵ Compared to the ZSM-5 zeolite, the MFI-Ga zeolite exhibited a Brønsted-to-Lewis acid site ratio. However, the acidity and acid strength of the Fe-MFI zeolite were relatively low, resulting in an inferior catalytic performance towards the catalytic cracking of n-dodecane relative to ZSM-5. Dai et al. further investigated the acid strength and density of the plate-like H[Ga]MFI zeolite using TMPO-assisted ³¹P MAS NMR spectroscopy.⁷⁶ As shown in Fig. 8c, the primary signal in the ³¹P MAS NMR spectrum at 64–83 ppm corresponds to the TMPO probe molecules bound to the Brønsted acid sites with varying acid strengths. Additionally, the weak signal at 47–49 ppm likely originates from the Lewis acid sites, extra-framework Ga species, or small GaO_x clusters within the zeolite pores.

In contrast to the three-dimensional structure, the nanosheet structure is more likely to enhance the acid sites and improve their accessibility. Choi et al. employed 2,6-di-tert-butylpyridine as a probe to quantitatively measure the accessibility of the acid sites in TNU-9 and BEA zeolites via infrared spectroscopy.⁷⁷ The extinction coefficient for the characteristic peak corresponding to the interaction between 2,6-di-tert-butylpyridine and Brønsted acid sites was determined. The results showed that the dealumination treatment markedly improved the accessibility of the external acid sites. Similar results were observed in other zeolite samples, such as MCM-22, ITQ-2, ZSM-5, and AS-824. Hensen et al. evaluated the total and external acidity of the MFI zeolite using FTIR with pyridine and 2,4,6-collidine as probe molecules.⁷³ The Brønsted acid sites in the nanosheet zeolites exhibit comparable strength to the bulk H-ZSM-5, but the BAS concentration on the external (mesoporous) surface was higher. In addition, single-layer zeolites possess higher external BAS and silanol group concentrations than multi-layer zeolites.

The ultrathin morphology of nanosheet zeolites not only significantly increases their external surface area (reaching up to ∼50% of the total) but also shortens the diffusion paths. Consequently, a higher proportion of acid sites (particularly Lewis acid sites) becomes accessible compared to bulk zeolites. These features greatly promote the processing of bulky molecules. For instance, methanol conversion over ZSM-5 nanosheets achieved enhanced conversion along with significantly reduced coking. The introduction of mesoporosity through pillaring further improved the mass transport.⁷⁸ Further details on the applications of nanosheet zeolites are discussed in the following section.

4. Catalytic applications of nanosheet zeolites

Nanosheet zeolites, with their crystal size in one dimension reaching the nanometer scale or even the thickness of a single or a few unit cells, dramatically shortening the diffusion paths. Furthermore, this morphological feature endows nanosheet zeolites with a more open pore structure and better accessibility to acid sites. Owing to these structural advantages, nanosheet zeolites exhibit excellent catalytic performances in diverse reactions, such as methanol conversion, cracking, carbonylation, isomerization, alkylation, and oxidation. Table 1 summarizes representative examples of their applications across various reactions.

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4.1 Methanol conversion

Methanol can be converted into hydrocarbons to produce fuel via the methanol-to-hydrocarbons (MTH) process. Based on the product distribution, the MTH reaction can be categorized into three types including methanol-to-olefins (MTO), methanol-to-gasoline (MTG), and methanol-to-aromatics (MTA). Initially, methanol undergoes dehydration to form dimethyl ether (DME) over the Brønsted acid sites. The methanol and DME mixture reaches an equilibrium state, and subsequently transforms into hydrocarbons via further dehydration.⁷⁹ Nanosheet zeolites, with their shortened diffusion paths and increased external acid site exposure, significantly enhance the reaction kinetics and suppress coke formation during methanol conversion. Their unique structure facilitates the faster access of reactants to the active sites, improving the selectivity towards light olefins and aromatics.^80,81

The MTO reaction, producing ethylene and propylene, widely utilizes SAPO-34 with a CHA framework as a highly efficient catalyst owing to its high selectivity for light olefins (over 80% C₂–C₄ olefins). Nevertheless, the commercial SAPO-34 catalysts suffer from rapid deactivation due to coke accumulation within their micropores. This limitation can be mitigated by employing SAPO-34 nanosheets. Yu et al. found that nanosheet SAPO-34 with a thickness of 20 nm exhibited an extended catalyst lifetime along with reduced coke deposition rate in the MTO reaction.⁸² A decrease in crystal size improved the mass transfer, thereby reducing coke deposition. Moreover, SAPO-34 and SAPO-18 share similar double 6-membered ring layer structures, which facilitate the intergrowth of the two frameworks and enhance the catalyst performance. Wu et al. reported that an 11-nm-thick nanosheet MOR zeolite achieved markedly improved ethylene selectivity (42.1%) in the MTO reaction compared to conventional bulk MOR crystals (3.3%).²⁵ The nanosheet mordenite catalyst featured highly exposed (010) crystal planes and 8-membered ring channels, not only enhancing the accessibility of methanol molecules to these channels, but also increasing the diffusion rate of ethylene molecules along the b-axis, leading to heightened ethylene selectivity.

Dou et al. demonstrated that the nanosheet MFI zeolite displayed an excellent catalytic performance in the MTP reaction, with high propylene selectivity (51.0%), high propylene/ethylene ratio (12.1), and long catalyst lifetime (240 h).⁵⁵ Recently, Dai et al. synthesized plate-like MFI zeolites via a fluoride-assisted approach, achieving a remarkably long service life of 750 h in the MTP reaction.⁷² The significant improvement in stability originated from the unique morphological features of the zeolite. As mentioned above, the plate-like ZSM-5 possessed a higher crystal surface aspect ratio, resulting in a higher surface fraction of (010) than the conventional form. Therefore, methanol is more likely to reach the acidic sites of MFI nanosheets through the straight channels. In addition, the use of sheet-like ZSM-5 catalysts can shorten the reactant diffusion path, effectively suppressing coke formation and improving the stability and recyclability of the catalyst. Dai et al. designed plate-like H[Ga]MFI zeolites with controllable acidity, which exhibited a high performance in methanol-to-propylene (MTP) reactions.⁷⁶ As illustrated in Fig. 9, the catalyst lifetime of the plate-like H[Ga]MFI zeolite initially increased, and then declined with a decrease in BAS density (Fig. 10b–f). Notably, the H[Ga]MFI-150 catalyst with an optimal BAS density of 0.10 mmol g⁻¹ demonstrated the longest catalyst lifetime of 252 h (Fig. 10d). In contrast, a slight reduction in BAS density from 0.10 to 0.07 mmol g⁻¹ in the H[Ga]MFI-200 catalyst led to a pronounced decrease in catalyst lifetime from 252 h to 43 h (Fig. 10e). This underscores the critical importance of precisely modulating the BAS density in plate-like H[Ga]MFI zeolites for optimizing their stability as catalysts in methanol-to-propylene conversion. In the case of H[Ga]MFI catalysts with comparable acid strength, the BAS density exerted a significant influence on the product distribution. As the BAS density decreased from H[Ga]MFI-50 to H[Ga]MFI-200, the selectivity towards ethylene and aromatics declined sharply (Fig. 10b–e and g), whereas the selectivity towards long-chain alkenes (e.g., C₅–C₈ alkenes) increased substantially. This phenomenon is likely attributed to the enhanced olefin-based cycle facilitated by H[Ga]MFI catalysts with a lower BAS density. Simultaneously, some long-chain alkenes may diffuse out of the MFI channels, contributing to the detected products. A direct comparison of the C₄–C₈ alkene selectivity (associated with the olefin-based cycle) and aromatic selectivity (linked to the aromatic-based cycle) across catalysts with varying BAS densities is presented in Fig. 10h. Apparently, the aromatics selectivity correlated positively with the BAS density, whereas the C₄–C₈ alkenes selectivity exhibited the inverse relationship.

Niaei et al. reported the synthesis of M-substituted (M: Mn, Ce, W) MFI zeolite nanosheets and their catalytic performance towards the methanol-to-propylene reaction.⁸⁰ The results showed that the W-substituted MFI nanosheets exhibited the best performance, achieving complete methanol conversion with a propylene selectivity of 55.7%, total light olefin selectivity of 88% and catalyst lifetime of 81 h. Luo et al. explored the influence of two different types of ZSM-5 crystals as seeds on the morphology, pore structure, acidity, and defect sites of synthesized ZSM-5 zeolites, and investigated their catalytic performance in the MTP reaction.⁸³ The result showed that the ZSM-5 samples derived from hydrothermally treated spherical seeds exhibited a longer lifetime and higher propylene selectivity compared to that from plate-like seeds. This is because the hydrothermally treated samples possess suitable acidity, accelerated diffusion rate, and reduced framework defects. Kim et al. investigated the influence of MFI nanosheet thickness and Si/Al ratios on the MTP reaction.⁸⁴ The ultrathin ZSM-5 nanosheet (2.5 nm) exhibited better catalytic activity and propylene selectivity compared to the 7.5-nm-thick ZSM-5 nanosheet and commercial ZSM-5 zeolites. The optimal performance was observed at an Si/Al ratio of 500, which is attributed to the appropriate acid density and facile diffusion of molecules in the ZSM-5 nanosheet zeolites. Ryoo et al. found that the MFI nanosheets exhibited a longer catalytic lifetime in the methanol-to-gasoline reaction relative to traditional MFI zeolites.²¹ In the MTG reaction, the catalytic conversion rate curve of MFI nanosheets over time is significantly better than that of traditional MFI zeolites. This is because the coke on MFI nanosheets is primarily deposited on their external surface (mesopores) rather than in their internal micropores. The coke in the internal micropores is more effective at causing catalyst deactivation, while external coke has a relatively smaller obstruction to diffusion.

4.2 Catalytic cracking

The fluid catalytic cracking (FCC) process is pivotal refining technology for converting heavy feedstocks into lighter and more valuable products, such as gasoline, diesel, and petrochemical feedstocks. Aluminosilicate zeolites have been extensively employed in FCC units due to their unique structural properties, which are a key factor in achieving high activity and light olefin selectivity. However, the cracking of long-chain hydrocarbons inevitably leads to coke formation. The accumulation of coke can block the active sites of zeolites, thereby deactivating the catalyst, but nanosheet zeolites can efficiently mitigate this problem. In catalytic cracking, nanosheet zeolites mitigate secondary reactions and coke deposition owing to their ultrathin morphology, high external surface area, and improved mass transport properties. These structural advantages lead to a higher reaction rate and improved product selectivity, especially towards light olefins.^85,86

In the catalytic cracking of long-chain alkanes, the crucial factors influencing the product selectivity are the textural properties and acidity of zeolites. Liu et al. reported the performance of pillared ZSM-5 nanosheet zeolites in n-decane catalytic cracking for the production of light olefins.^87,88 The pillared structure stabilized the hierarchical porosity and preserved the interlayer mesopores, which facilitated product diffusion and restricted secondary reactions that consume light olefins. The pillared ZSM-5 nanosheets displayed a remarkable light olefin selectivity of up to 37.8% at 500 °C, with an n-decane conversion of ∼92%, nearly twice as high as that of the conventional ZSM-5. Moreover, the pillared structure exhibited exceptional anti-coking stability, with the deactivation rate of the conventional ZSM-5 zeolite reaching 68.24%, which was more than 11-times higher than that of the ZSM-5 nanosheets (5.81%). Recently, they synthesized pillared nanosheet H-ZSM-5 for the catalytic cracking of supercritical n-dodecane. Compared to the parent nanosheet H-ZSM-5 zeolite, the pillared nanosheet zeolite presented a higher surface area, mesoporous volume, and Brønsted acid site accessibility, which led to an improved catalytic performance with a conversion of 76.8%, TOF value of 130.92 s⁻¹, and deactivation rate of 9.11%. Xiao et al. synthesized aluminosilicate ITH zeolite using a cationic oligomer as the organic template.⁸⁹ In the vacuum gas oil cracking reaction, the Al-ITH zeolite enhanced the propylene and butene yields without increasing the yield of saturated propane and butanes, thereby improving the olefin/paraffin ratio in the C₃ and C₄ fractions.

The large external surface area and strong acidity of zeolites provide abundant active sites for the reaction of large-molecule compounds. Nanosheet zeolites also exhibit an excellent performance in cracking large molecules. Wu et al. developed ECNU-7P, an MWW-type zeolite with a layered porous structure, which was prepared via the self-assembly of the CTAB surfactant and zeolite precursors.⁹⁰ As shown in Fig. 11a, compared to the traditional MCM-22, Al-ECNU-7 exhibited about threefold higher conversion in cracking 1,3,5-triisopropylbenzene (TIPB), together with a significantly slower deactivation rate during the reaction process. Moreover, Al-ECNU-7 also delivered a superior cracking performance in terms of product distribution. Dai et al. synthesized plate-like ZSM-5 zeolites with a reduced b-axis thickness. As shown in Fig. 11b, in the cracking reactions of large hydrocarbon molecules such as propyl-benzene and 1,3,5-triisopropylbenzene, the plate-like ZSM-5 zeolites showed obvious advantages, which are attributed to the improved accessibility to their acid sites and reduced diffusion limitations.⁹¹

4.3 Isomerization

Isomerization is a crucial catalytic process in the petrochemical and fine chemical industries, involving the rearrangement of molecular structures without altering their atomic composition. In industrial applications, isomerization enhances the octane number of gasoline, produces valuable branched alkanes, and converts linear alkenes into more reactive isomers. Traditional zeolite catalysts, such as ZSM-5 and beta, have been widely employed in isomerization reactions due to their shape-selectivity and acidity. However, their microporous nature often limits the diffusion of bulky molecules, leading to lower activity and rapid deactivation. Nanosheet zeolites provide superior diffusion and accessibility to confined acid sites, which is especially beneficial for the isomerization of bulky molecules. Their highly open structure effectively minimizes pore blockage and allows better control over reaction pathways, leading to improved catalytic efficiency.^92,93

Choi et al. evaluated the performance of acetylene aromatization over ITQ-2 and the traditional microporous MCM-22 zeolite.⁹⁴ Compared to MCM-22, ITQ-2 produced more C₃–C₈ aliphatic hydrocarbons than aromatic hydrocarbons. This arises from the larger external surface area of ITQ-2, which shortened the reactant residence time and promoted the formation of aliphatic hydrocarbons. Besides, ITQ-2 exhibited superior resistance to deactivation than MCM-22. Wattanakit et al. demonstrated that Ga-modified HZSM-5 nanosheets significantly boosted the catalytic performance in C5 hydrocarbon aromatization, with the n-pentane conversion and BTEX (benzene, toluene, ethylbenzene, and xylene) selectivity exceeding 40% and 45%, respectively.⁹⁵ The layered structure facilitates the migration and dispersion of Ga species, promoting the formation of tetrahedral Ga species (up to 43%) and enhancing the catalytic activity. Ryoo et al. investigated the effect of the zeolite crystal thickness on the n-heptane hydro-isomerization over the Pt/MFI zeolite.⁹⁶ The results showed that reducing the zeolite crystal thickness to the nanoscale markedly improved the selectivity for branched isomers, which is attributed to the shorter diffusion path lengths, allowing the branched products to escape before undergoing cracking.

FER zeolites are widely employed in the isomerization of n-butene to isobutene due to their sheet-like morphology. Xu et al. analyzed the catalytic performance of FER nanosheets with sizes in the range of 2 μm to 100 nm,⁹⁷ revealing that nano-scale FER presented better stability than micro-scale FER during the reaction due to its larger external surface area and reduced diffusion limitations. Nano-scale FER exhibited a coking amount of 10.6% after 72-h reaction, whereas micro-scale FER showed only 6.6% after 48-h reaction, revealing the superior coke resistance of nano-scale FER. Similarly, adjusting the morphology of the FER nanosheets could enhance their performance. Dai et al. reported the synthesis of nanorod-like FER zeolites.⁹⁸ The nanorod-like FER zeolite exhibited a significantly reduced diffusion path along the c-axis, resulting in a larger external surface area and better accessibility to acid sites compared to its micron-sized counterpart. These improved properties endowed the nanorod-like FER zeolite with high selectivity and low deactivation rates in n-butene isomerization. In situ FTIR spectroscopy (Fig. 12d) revealed that the conventional FER (FER-F-0) exhibited two additional characteristic absorption bands at 1650 and 1460 cm⁻¹ compared to nanorod-like FER (FER-F-0.2). The peak at 1650 cm⁻¹ is assigned to π-coordinated butene/oligomeric olefin species, while the band at 1460 cm⁻¹ corresponds to the C–H vibrations of CH₂/CH₃ groups, indicating the formation of highly branched aromatic compounds on the FER-F-0 catalyst. These bulky aromatic species rapidly block the accessibility of the acid sites to 1-butene molecules, thereby leading to catalyst deactivation.

Isomerization is also crucial in biomass conversion to obtain the target products. Tsapatsis et al. reported that the single-unit-cell Sn-MFI zeolite nanosheet displayed a superior catalytic performance in glucose and lactose isomerization reactions,⁹⁹ with Sn sites uniformly distributed and exclusively located at the framework sites. As illustrated in Fig. 12e, the self-pillared Sn-MFI nanosheet achieved ∼65% fructose (FRU) yield from glucose (GLU) in ethanol solvent, outperforming traditional USY and Al-BEA. In the conversion of lactose (LAC) to lactulose (LACTU), the self-pillared Sn-MFI nanosheet also demonstrated an excellent catalytic performance, achieving 31% LAC conversion and 97% LACTU selectivity in methanol solvent, significantly surpassing traditional Sn-BEA and Sn-MCM-41. These exceptional performance stems from the uniform distribution of framework Sn sites, which exhibit both Lewis and Brønsted acidity, as well as unique nanosheet structural features, providing efficient diffusion channels and accessibility to active sites for large molecular substrates.

4.4 Alkylation

Alkylation and acylation are vital organic synthesis reactions widely used in the petrochemical, fine chemical, and pharmaceutical industries. The choice of catalyst is critical for improving the catalytic efficiency, selectivity, and environmental impact of these processes. Therefore, a balance between the Brønsted/Lewis acid ratio must be achieved to obtain the optimal catalytic performance. The nanosheet architecture of zeolites enables acid site accessibility and minimized steric constraints, which are crucial for the alkylation of aromatic hydrocarbons. These structural advantages deliver superior activity and selectivity, while simultaneously improving the catalyst stability under industrial conditions.

Wattanakit et al. fabricated hierarchical nano-spherical ZSM-5 zeolites using uniform aluminosilicate (AS) nanobeads as the starting material.⁷⁴ This approach enabled better control of the distribution of Al in the ZSM-5 framework. AS nanospheres provided both silicon and aluminum sources, ensuring that most of the Al species were uniformly embedded into the zeolite framework in tetrahedral coordination. The uniform Al distribution generated a higher density of Brønsted acid sites, which facilitated efficient ethylene transfer from ethanol dehydration to the adjacent acid sites, thereby promoting the alkylation reaction. Consequently, the AS-derived hierarchical ZSM-5 nanosheets presented an improved benzene conversion (60%) and ethylbenzene selectivity (62%) compared to the conventional ZSM-5.

Fang et al. synthesized hierarchically structured MFI nanosheet assemblies, which demonstrated an extraordinary catalytic performance in the benzylation of benzyl alcohol with mesitylene.¹⁰⁰ As shown in Fig. 13b, compared to the conventional ZSM-5, the layered MFI zeolite nanosheet showed a higher benzyl alcohol conversion and excellent cyclic stability, maintaining a conversion of 70.1% even after 6 cycles. In contrast, the conventional ZSM-5 zeolite was nearly deactivated after 6 cycles. Notably, the selectivity towards 2-benzyl-1,3,5-trimethylbenzene increased with a decrease in the Si/Al ratio, highlighting the pivotal role of the zeolite acidity in governing the product selectivity. Moreover, the spatial distribution of Al species determined both the reaction activity and product selectivity.

Rimer et al. employed the surfactant cetyltrimethylammonium (CTA) as both a dual OSDA and exfoliating agent to produce MWW-type layers with an average thickness of 3.5 nm, termed H-d-MWW. As shown in Fig. 13d, the obtained H-d-MWW exhibited enhanced catalytic activity compared to the three-dimensional MWW (MCM-22) and 2D layers prepared by post-synthesis exfoliation (ITQ-2).⁶³ During the Friedel–Crafts alkylation reaction, the conversion of benzyl alcohol over H-d-MWW was significantly faster than that on other catalysts. The comparison of the turnover numbers indicated that d-MWW showed similar catalytic activity to MCM-22. Simultaneously, the post-synthesis exfoliation process used to prepare 2D nanosheets, such as ITQ-2, introduced substantial structural defects, including extra-framework aluminum and distorted aluminum sites, which significantly compromised their catalytic performance.

Limtrakul et al. developed a simple and efficient method to prepare self-pillared FAU zeolite nanosheets.⁶⁶ In the benzylation of toluene with benzyl chloride, the conventional FAU zeolite catalyst exhibited limited initial activity (8.78% conversion within 6 h), whereas the FAU nanosheet catalyst achieved a 3–4-times enhancement in conversion under identical conditions. This is because the conventional FAU zeolite has microporous characteristics, which limit the diffusion path for reactions involving large molecules. Yang et al. reported a novel method for the direct synthesis of partially and fully delaminated MWW-type zeolites (SCM-1).¹⁰¹ In the liquid-phase alkylation of benzene with ethylene, the SCM-1 nanosheets displayed nearly a two-fold increase in ethylene conversion compared to the conventional MCM-22 zeolite. Remarkably, the conventional MCM-22 zeolite suffered significant activity loss after 64-h reaction, while SCM-1 maintained a stable ethylene conversion with less adsorbed organic matter and carbon deposition. This enhanced catalytic performance is attributed to the improved mass transfer properties provided by the delaminated nanosheet structure.

4.5 Carbonylation

The carbonylation reaction involves introducing carbon monoxide (CO) into organic molecules, replacing their substituents (e.g., alkyl, hydrogen, etc.) with carbonyl (CO) groups. Typically, dimethyl ether (DME) carbonylation to methyl acetate (MA), followed by hydrogenation is a novel route for producing ethanol. In carbonylation reactions, the diffusion limitations of conventional zeolites are alleviated by the open framework of nanosheet structures. This facilitates the accelerated migration of reactants and intermediates, thereby boosting the turnover frequency and selectivity for carbonylated products.

Yang et al. synthesized the hierarchical nanosheet H-ZSM-35 zeolite, which outperformed conventional ZSM-35 in DME carbonylation.¹⁰² Li et al. developed a templating agent-free strategy to synthesize H-mordenite (H-MOR) nanosheet assemblies,⁶⁰ which exhibited high catalytic activity and stability in the carbonylation of DME to MA. They found that decreasing the basicity of the synthesis gel by dilution with water promoted the growth of H-MOR nanocrystals, which assembled into nanosheet bundles. The resulting expanded external surface area was beneficial for the mass transfer of the reactants and products, suppressing the formation of hard coke in the micropores. Shen et al. fabricated MOR zeolite nanosheets with a thickness of 20–40 nm.¹⁰³ During the carbonylation of dimethyl ether, the DME conversion reached 36.7% after 4 h on the nanosheets, versus merely 14.6% after 6 h on the micro-sized sample. The marked enhancement is ascribed to the shortened diffusion distances in the 8-MR and 12-MR channels, which accelerated molecular transport and improved the accessibility of the acid sites to the reactant molecules. More recently, they also studied the identification of the active sites on a plate-like mordenite (MOR) zeolite for DME carbonylation. As shown in Fig. 14a, the results show that the T3 site in the 8-MR channel serves as the active center, exhibiting unprecedented activity.³⁷

Tsubaki et al. reported the development of a carbonylation catalyst, Al-RUB-41 zeolite, featuring a unique nanosheet structure, which remarkably enhanced the DME carbonylation efficiency.¹⁰⁴ As shown in Fig. 14b, the distinctive nanosheet structure of Al-RUB-41, with the 8-MR channels oriented perpendicular to the thin sheets, facilitated the efficient mass transfer of the reactants and products, achieving >95% MA selectivity, which outperformed the commercial H-MOR and H-ZSM-35 catalysts. The deactivation of the Al-RUB-41 catalyst is attributed to its external surface acid sites, where larger coke precursor species (e.g., alkylated naphthalene) preferentially accumulate, rather than within the 8-MR channels. Coating the Al-RUB-41 with a silica layer (Al-RUB-41@SiO₂) effectively removed its external acid sites, thereby preventing the formation of hard coke precursors and enhancing the long-term stability and efficiency of the catalyst.

4.6 Oxidation

The application of zeolites in oxidation reactions is typically coupled with metal components. Although conventional microporous ferro-silicate and titano-silicate zeolites demonstrate high efficiency in oxidation processes, their microporous structure can induce diffusion limitations, resulting in rapid coke deposition. Nanosheet zeolites exhibit superior accessibility to redox-active sites and improved dispersion of metal species due to their high external surface area, which facilitates electron and mass transport during oxidation reactions, thereby demonstrating higher activity and suppressed deactivation.^105,106

Gao et al. evaluated the catalytic performance of Cu–ZSM-5 nanosheets for the decomposition of N₂O.¹⁰⁷ The results showed that the Cu–ZSM-5 nanosheets were quite stable in a 50 h stream-online test, maintaining ∼80% conversion at 475 °C without deactivation. In contrast, the N₂O conversion over conventional Cu–ZSM-5 decreased gradually from the initial 74% to 54% after 50 h. The CO-IR characterization showed that the Cu⁺ species in the spent Cu–ZSM-5 nanosheets remained almost unchanged compared to the fresh Cu–ZSM-5, while the amount of Cu⁺ species in the spent conventional Cu–ZSM-5 decreased obviously. This is likely due to the oxidation of a portion of Cu⁺ by O₂ generated from N₂O decomposition at 475 °C. The O₂-TPD profiles revealed that oxygen desorption from the Cu sites on the Cu–ZSM-5 nanosheets occurred more easily than on the conventional Cu–ZSM-5, which further enhanced the N₂O decomposition reaction. Hensen et al. synthesized Fe/ZSM-5 nanosheet zeolite catalysts for the oxidation of benzene to phenol using N₂O as the oxidant.¹⁰⁸ By decreasing the Fe content and b-direction crystal thickness of the MFI nanosheets, consecutive phenol reactions were suppressed, improving the catalyst stability. As illustrated in Fig. 15a, the Fe/ZSM-5 nanosheets, with 0.24 wt% Fe content and a crystal size of ∼3 nm in the b-direction, achieved a phenol yield of 185 mmol g⁻¹ in 24 h, the highest observed yield. Compared to bulk Fe/ZSM-5, the nanosheet catalyst showed significantly reduced deactivation due to the absence of diffusion limitations and lower Fe agglomeration. For instance, the phenol yield of Fe/ZSM-5 nanosheets was 6.0 mmol g⁻¹ h⁻¹ after 24 h, while the bulk Fe/ZSM-5 only produced 0.6 mmol g⁻¹ h⁻¹ phenol yield.

Zhang et al. developed a sandwich-structured Pt@ZSM-5 nanosheet catalyst, prepared by controllably intercalating Pt nanoparticles (∼4.3 nm) between ZSM-5 single-layer sheets.¹⁰⁹ The Pt nanoparticles confined between the nanosheet layers displayed better dispersion and higher thermal stability compared to conventional Pt/ZSM-5 catalysts during toluene combustion. Although the Pt@ZSM-5 catalyst showed slightly higher coke formation (1.36 wt%) than Pt/ZSM-5 (1.13 wt%), its large external surface area and stabilized mesopores enhanced its anti-coking ability and coke tolerance.

Ma et al. reported the synthesis of hierarchical titanium silicalite-1 (TS-1) nanosheets,¹¹⁰ which exhibited a unique house-of-cards-like structure with stable interlayer mesopores and macropores, enhancing the accessibility of the active Ti sites for the epoxidation of bulky cyclic olefins (e.g., cyclohexene and cyclooctene). The catalytic performance of the TS-1 nanosheets surpassed that of the conventional microporous and mesoporous TS-1 catalysts due to their larger external surface area and improved mass transfer. Additionally, post-fluoride treatment further increased their hydrophobicity and catalytic activity. Ryoo et al. synthesized titanosilicate MFI nanosheets with a single-unit-cell thickness as efficient oxidation catalysts for bulky molecular epoxidation, using peroxides such as H₂O₂ or t-butyl hydroperoxide.¹¹¹ The conversion of the bulk TS-1 catalyst for large-ring olefins was very low because these large molecules could not enter the 10-membered ring pores of TS-1. In contrast, the nanosheet-like TS-1 exhibited relatively high catalytic activity, given that its external Ti sites were sufficient to catalyze the epoxidation of these large-ring olefins. More recently, Dai et al. constructed plate-like TS-1 zeolites using L-lysine as a growth modifier.³⁶ As shown in Fig. 15e, in comparison to the traditional TS-1 zeolite and plate-like TS-1 prepared in fluorine-containing media, the plate-like TS-1 synthesized with L-lysine displayed a superior catalytic performance towards the 1-hexene epoxidation reaction. The addition of L-lysine effectively inhibited the aggregation of the titanium species, increasing the Ti content within the framework, and thereby enhancing the catalytic activity. Moreover, the plate-like TS-1 zeolite synthesized with L-lysine maintained a stable catalytic performance over five reuse cycles, demonstrating excellent stability. Zhu et al. demonstrated that Ti-beta zeolite nanosheets (Ti-beta-NS) showed an improved catalytic performance in cyclohexene epoxidation.²⁶ As illustrated in Fig. 15h, Ti-beta-NS exhibited higher activity, 1,2-cyclohexanediol selectivity (increased by 13.3–15.0%) and hydrogen peroxide utilization efficiency than the conventional Ti-beta catalyst. These enhancements originated from its unique nanosheet structure, which improved the internal diffusion efficiency and alleviated mass transfer limitations. The oriented distribution of strong acid sites was also favorable for the consecutive reaction steps in the reaction mechanism.

Wu et al. developed a novel titanosilicate catalyst with an MWW structure (Ti-MWW), and applied it to the liquid-phase ammoximation of cyclohexanone.¹¹² The Ti-MWW achieved >99% cyclohexanone conversion with high oxime selectivity under the optimized conditions. Compared to TS-1 and Ti-MOR, Ti-MWW tended to oxidize hydroxylamine to NO_x compounds, which affected hydroxylamine formation and the cyclohexanone oxime formation.

5. Summary and perspective

In conclusion, nanosheet zeolites have garnered remarkable progress in the past few decades. The synthesis strategies of nanosheet zeolites can be broadly classified into in situ hydrothermal synthesis and post treatment methods. The in situ hydrothermal can effectively and precisely control the layer thickness, interlayer spacing, and structural ordering of nanosheet zeolites. OSDA-based methods employ quaternary ammonium surfactants, which direct the formation of zeolite frameworks, while restricting crystal growth along specific directions to obtain nanosheets. Additive-assisted synthesis involves utilizing commercially available additives (e.g., urea) to regulate crystal growth, introduce mesoporosity, and reduce the crystal thickness, offering a cost-effective alternative to OSDA-based methods. Seed-induced synthesis accelerates crystallization by adding seed crystals to a template-free gel, promoting rapid growth and high-quality nanosheets. However, the layered precursor may undergo condensation during the calcination process. Several post-synthesis strategies, such as chemical etching, swelling, exfoliation, and pillaring, can not only preserve the fundamental structural units and layered framework of the nanosheets but also endow them with large pore sizes, high external surface areas, and enhanced stability.

These methods are widely used in industrial applications to optimize the zeolite properties, including morphology, pore structure, acid-site, and catalytic performance. Nanosheet zeolites with ultrathin morphologies (2–100 nm) possess unique structures and high external surface areas, enhancing the accessibility to their active sites. Their synthesis relies on surfactant-directed methods, in which the variations in surfactant composition, structure, and crystallization conditions significantly influence their morphology, interlayer spacing, and thickness. The structural properties of nanosheet zeolites can be analyzed through techniques such as XRD, which highlights their reduced crystallinity and unique diffraction patterns. N₂ adsorption–desorption studies reveal that nanosheet zeolites present larger mesopore volumes and specific surface areas compared to traditional zeolites. Their acidic properties, which play a pivotal role in their catalytic performance, are assessed via NH₃-TPD, FTIR, and solid-state NMR. Nanosheet structures enhance the accessibility to the Brønsted acid sites and silanol groups on their external surface, with the acid strength and type determined by the framework composition and crystallinity. Nanosheet zeolites also demonstrate tailored catalytic properties through heteroatom substitutions (e.g., Al, Ga, and Fe), optimizing their activity and selectivity in diverse reactions. Also, their ultrathin morphology with nanoscale confinement in one dimension endows them with superior catalytic efficiency in methanol conversion, cracking, isomerization, alkylation, carbonylation, and oxidation reactions, owing to the shortened diffusion paths and enhanced acid site accessibility.

However, despite the advantages of nanosheet zeolites over conventional zeolites, several challenges are encountered in their synthesis, including high cost, time-consuming processes, and the reliance on complex multi-step organic reactions for the preparation of SDAs. In this case, the seed-induced approach has emerged as a simpler and more cost-effective alternative, potentially gaining broader adoption. To advance this field, future studies should focus on optimizing the crystal growth parameters to precisely control the nanosheet thickness, lateral dimensions, and chemical composition, producing nanosheet zeolites with desirable properties. Moreover, small molecules (e.g., urea) can be used to inhibit the crystal growth in specific directions, promoting the formation of nanosheets. Furthermore, the fabrication of nanosheet zeolites utilizing cost-effective and sustainable precursors, such as silica derived from agricultural waste, presents a viable and environmentally benign approach. This strategy not only reduces production costs but also aligns with the principles of green chemistry, offering a promising alternative for the synthesis of advanced zeolitic materials. Additionally, the synthesis of zeolite nanosheets containing redox heteroatoms remains limited compared to aluminosilicate or pure silica zeolite nanosheets. Given the importance of metal-based catalysts in industrial processes, incorporating heteroatoms into nanosheet frameworks can significantly enhance the catalytic performance of metal-based catalysts.

Looking ahead, the design of affordable bifunctional templates and the use of renewable sources for nanosheet zeolite synthesis should be prioritized. In situ characterizations coupled with computational studies provide valuable insights into the nanosheet formation mechanisms and reaction pathways. From a technological perspective, applying nanosheet zeolites in catalytic membrane reactors can revolutionize the catalysis industry, offering breakthroughs in the energy and fine chemical industries. To achieve this, scaling up the industrial synthesis and applications of nanosheet zeolites are critical to realize their full potential. To date, nanosheet zeolite catalysts have demonstrated considerable potential at the laboratory scale; however, their large-scale industrial application remains in its infancy. Although some industrial-scale production methods have been reported for materials (e.g. using 20 L reactors),³⁵ the majority of nanosheet catalysts are still under development. Thus, future research should focus on establishing commercially viable synthesis routes and verifying their long-term stability to facilitate their practical implementation.

Conflicts of interest

The authors declare no competing interests.

Data availability

We declare that the data supporting the findings of this review are available within the paper. Should any raw data files be needed in another format, they are available from the corresponding author upon reasonable request.

Acknowledgements

This work was partially supported by the National Key R&D Program of China (2023YFB4103204) and the Fundamental Research Funds for the Central Universities (023-63253174).

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