Jeon et al. Synthesized monolayer zeolite nanocrystalline layers with high aspect ratio by hydrothermal epitaxy^[30]. They used [bis-1,5 (tripropyl ammonium) pentamethylene diiodide] (dCA) as an Organic structure direction agent (OSDA) and added MFI seeds for gel crystallization. In the initial stage of growth (20 – 40 H), the 30 nm seed slowly grew into 140 nm columnar crystals. The crystal growth changed dramatically over the next few hours. A nanocrystalline layer appears at the edge of the columnar seed and continues to grow around the seed along the (011) plane, forming a 5 nm thick MFI zeolite nanocrystalline layer. The monolayer MFI zeolite nanocrystalline layer synthesized by this method requires the use of expensive OSDA, and the yield is low^[31].

Corma et al. Proposed a dual-template system composed of Hexamethylene (HMI), a commonly used template for the synthesis of MWW-type zeolite, and a second template with surfactant properties to prepare a single/double-layer mixed growth MWW-type DS-ITQ-2 zeolite with an external specific surface area of 300 m²/g^[32]; Rom Román-Leshkov et al. Designed a surfactant-like template and synthesized MIT-1 zeolite containing discrete MWW two-dimensional crystal layers with an external surface area of 500 m²/g by drawing on the concept of multi-quaternary ammonium head surfactant template proposed by Ryoo et al^[33][34]. In the synthesis of B-Ti-MWW zeolite using piperidine as a framework template, Yang et al. Obtained a thin layer of Ti-MWW zeolite with a thickness of about 15 nm (~ 6 layers) by adding a silanization reagent to seal the silicon hydroxyl groups at the crystal layer interface during crystallization, supplemented by acidification treatment^[35]. Yang et al. Also successfully developed a more simple and feasible synthesis route of discrete MWW two-dimensional crystal layer, using HMI as the MWW framework structure directing agent and Dicyclohexylamine (DCHA) with larger size as the auxiliary template for interlayer filling barrier.Controllable single/double crystal layer zeolites (SCM-6, SCM-1) were synthesized by one step. It was found that SCM-1 with double crystal layer structure had higher product purity and stability, and showed great advantages in the liquid phase alkylation of benzene with ethylene in which the crystal layer interface semi-cage structure was the catalytic active region^[36]. Compared with the previously reported preparation methods of zeolite two-dimensional crystal layer, this method uses a cheaper second template, which reduces the synthesis cost, and the simple one-step synthesis process avoids the damage of crystal layer structure caused by the post-treatment process such as secondary exfoliation.

Recently, Wu et al. Reported a multilayer MOR zeolite with a nanosheet thickness of ∼ 10 nm, using a diamine surfactant with benzyl and C₁₆ alkyl chains as the two heads^[37]. The continuous growth of the MOR structure along the B axis is blocked, resulting in a highly exposed 8-membered ring pore. Rimer et al. Reported the preparation of two-dimensional MWW-type zeolite nanocrystalline layer by one-step method (Fig. 3), and the results showed that in the conventional MCM-22 synthesis system, by adding the surfactant Hexadecyl trimethyl ammonium bromide,CTAB) can be used to synthesize disordered two-dimensional MWW materials d8.0-MWW with an average thickness of 3. 5 nm and cross-lamellar growth in one step, and CTA⁺ ions can also modulate the lattice Al atoms, but the effectiveness of CTAB is limited by the Si/Al ratio of the synthesis system (such as Si/Al = 15 ~ 30)^[38].

In order to reduce the interlayer condensation of layered zeolite during calcination and improve the overall strength of layered zeolite, structural modification of layered zeolite is usually needed. Two-dimensional layered zeolites templated by long-chain alkyl quaternary ammonium salts are usually supported by interlayer columns to prevent interlayer condensation during calcination^[39]. The interlayer pillared method can only limit the interlayer extension, while the silanol group formed on the scaffold will make the zeolite more hydrophilic and promote the adsorption of polar products, especially the epoxidation products are easy to further hydrolyze into diols under acid catalysis. In addition, most of the resulting silicon pillars are unstable and decomposed by atmospheric moisture after being placed in the environment for several months, and the nanocrystalline layer can easily collapse when the sample is compressed^[40]. It is well known that rotational boundaries are prevalent in MFI zeolites, and there is an overgrowth relationship between the (h00) plane and the (0k0) plane, where a large number of (h00) planes grow epitaxially on the (0k0) plane^[41][42]. The self-supporting structure is realized by the mutual 90 ° rotation growth between the nanocrystalline layers, which eliminates the subsequent expansion and pillared operation, and also prevents the excessive collapse between the layers caused by the lamellar zeolite in the process of removing the template. Through the formation of two-dimensional zeolite with interlayer self-supporting structure, the collapse problem in the calcination process can be effectively solved, and the mesoporous properties of two-dimensional zeolite can be improved through the dense stacking between layers.

At present, part of the means to obtain monolayer zeolite nanocrystalline layers relies on the indirect secondary exfoliation strategy of their ordered (or partially ordered) multilayer zeolite crystal layer stacking structure. Monolayer MFI nanosheets are usually prepared by a multi-step method, in which the multilayer MFI zeolite nanosheets are first exfoliated, and then the unexfoliated particles are removed by centrifugation^[47][48]. Varoon et al. First achieved exfoliation of MFI nanosheet monolayers by melt blending technique^[47⇓~49]. The MFI nanosheet suspension was obtained by first exfoliating the multilayer nanosheets, followed by dissolution, ultrasonic dispersion, and centrifugal purification. Removal of the polymer by calcination or other heat treatment usually results in a significant curling of the sample, but relatively flat nanosheets can be obtained by dissolution and purification. When the nanosheet-polystyrene mixture was sonicated in toluene, the polymer was dissolved by centrifugation and the larger particles were removed, and the monolayer M FI zeolite nanocrystal layer with high purity was obtained. However, this method still has some problems, such as time-consuming and uneconomical, and the generated nanosheets are small and non-uniform in size^[47,48,50]. In addition, MFI nanosheets are difficult to separate from the polymer solution with high concentration, which can easily contaminate the MFI nanosheet solution.

In subsequent work, Zhang et al. Used piranha solution to treat multilayer M FI zeolite exfoliated from polystyrene melt blends to remove organic residues^[50]. Sabnis et al. Isolated multilayer M FI zeolite by dispersing a layered zeolite precursor in polybutadiene followed by short shear or sonication at room temperature^[51]. Guo et al. Repeatedly treated multilayer MFI zeolite with piranha solution to remove the template, and then carried out ultrasonic exfoliation and precipitation purification in water to obtain monolayer MFI zeolite nanocrystalline layer aqueous solution with aspect ratio up to 100^[52]. Ultrasonic treatment is difficult to control the exfoliation process and easily leads to the fragmentation of flakes, resulting in the formation of large-diameter monolayers.

The exfoliation method can also be used for two-dimensional lamella exfoliation of MWW topology. The exfoliation method in organic base system (pH = 12.5 ~ 13.8) developed by Corma as a representative has obtained a high external surface zeolite material named ITQ-2 with a specific surface area of 700 m²/g, which shows its excellent catalytic performance in macromolecular and diffusion-controlled chemical reactions^[29]. However, this method is difficult to control, and many subsequent studies have shown that the zeolite framework structure is easily damaged during swelling and/or ultrasonic treatment due to the presence of strong bases (such as quaternary ammonium bases) and surfactants (such as CTAB) in the system.Amorphous impurities are produced or mesoporous materials such as MCM-41 are formed, and aluminum and other heteroatoms are easily separated from the framework during ultrasonic and acidification treatment, forming framework defects, which not only reduce the acidity of zeolite, but also lead to the reduction of framework active sites^[53⇓~55]. Therefore, Katz and Zones developed a stripping method under mild conditions (pH = 9) in quaternary ammonium salt and CTAB systems containing fluorine and chlorine, and successively realized the effective stripping of MCM-22 (P) in water phase and PPEFER, SSZ-70 and their heteroatom-containing multilayer zeolite crystal layer stacks in organic phase (such as DMF)^[53⇓~55]. Furthermore, Katz, Zone and Ouyang et al. Developed the effective exfoliation of simple inorganic aluminum salts and zinc salts from multilayer superposed structures (ERB-1P, B-SSZ-70) of various boron-containing zeolite crystal layers in acidic system.Tsapatsis and Fan successively developed the exfoliation of swollen MCM-22 (P) by the shear force of polystyrene or end-group modified polybutadiene system to obtain discrete zeolite two-dimensional crystal layers which are easy to prepare membranes, and verified their advantages in diffusion-dominated gas separation or macromolecular reactions^{[56⇓-58][47,51]}.

It has also been reported that at room temperature, the layer exfoliation of multilayer (partially) disordered stack MCM-56 with weak crystal layer interaction can be achieved by using a large volume of simple quaternary ammonium strong base, but the similar method for MCM-22 (P) with ordered crystal layer stacking needs to be repeated many times to improve the yield of oligolayer products^[59][60]. It is worth noting that these methods often need to use strong organic bases which may cause framework damage, heteroatom shedding and environmental pollution, or need to operate in organic solvent media to bring environmental problems in the post-treatment.Or it is necessary to add metal salts to introduce impurity residues or cause isomorphous substitution of framework elements, or there are some restrictions on interlayer interaction forces and stacking modes, which leads to the complexity of research and practice.

In recent years, Čejka et al. Selectively removed the interlayer germanium-rich d4r subunit from UTL germanium-silicate in acidic medium to produce a layered zeolite IPC-1P, which was then treated by calcination or interlayer silicification to produce a series of new zeolites (IPC-n) with adjustable interlayer pore size^[61]. This strategy of constructing new zeolites from layered intermediates derived from germanosilicates is called ADOR method, which includes many steps such as Assembly, Disassembly, Organization and Reassembly. This method has been successfully generalized to ITH, IWR, IWW, UOV, and SAZ-1 (Fig. 6)^{[62][63⇓~65][66][67]}. Subsequently, Wu et al. Realized the structural transformation of CIT-13 germanosilicate into ECNU-21 and ECNU-23, two new zeolites, through a layered intermediate in weak alkaline medium, because CIT-13 germanosilicate is rich in Si d4r subunits in acidic medium, in which stable Si-O-Si bonds exist and cannot be transformed^[68,69]. Davis et al. Also reported a CIT-14 zeolite similar to ECNU-23 by post-treatment of CIT-13^[70].

Recently, Tang's group proposed a combination of liquid soft chemical exfoliation and hydrogen peroxide-induced microblasting to prepare highly dispersed porous MWW thin zeolite nanocrystalline layers^[13,14]. According to the method, ethanolamine (ETA) which has stronger hydrogen bonds with a MWW lamellar silicon hydroxyl array and solvent water molecules is used as a stripping agent, and a large amount of oligolamellar MWW zeolite material ETA-OL-MWW rich in two-dimensional sheets is obtained in a zeolite colloidal dispersion, wherein the oligolamellar MWW zeolite material ETA-OL-MWW can not be settled for a long time and has abundant contactable functional alkaline sites. On the basis of this, a series of thin two-dimensional MWW zeolite nanocrystalline layers with good dispersion and interpenetrating graded pore structure were synthesized by adjusting the content of hydrogen peroxide in the synthesis system on the premise of maintaining the high dispersion and structural integrity of the thin MWW zeolite nanocrystalline layer.At the same time, it can also efficiently remove organic templates and construct acidic sites through hydrogen peroxide, further increase the openness of the skeleton structure, and reflect the characteristics of high efficiency, environmental protection and multi-function.

In order to maintain the ordered mesoporous structure of multilayer MFI zeolite, it is usually necessary to support the zeolite lamellae. At present, the commonly used pillared method is to disperse two-dimensional zeolite in alkoxide liquid after expanding the interlayer space in the swelling process, and then hydrolyze the alkoxide in alkaline aqueous solution to form inorganic oxide pillars, which are finally inserted into the pillared precursor^[25]. Two-dimensional zeolite can increase the surface area and acid sites by layering, while pillared can increase the distance between layers and further improve the accessibility of external acid sites. Meanwhile, the pillared treatment before calcination significantly increased the total specific surface area and total pore volume of the zeolite^[71]. However, the total pore volume of the Al-rich sample is slightly lower due to the penetration of TEOS between the layers. By interlayer silanization using silane monomer in acidic medium, two additional silicon can be inserted in the interlayer pore, thus enlarging the pore size.

TS-1 is the preferred catalyst for the epoxidation of C = C double bond with hydrogen peroxide, which not only has good catalytic performance, but also is a new type of green catalyst. TS-1 has a pore size of 5.55 Å and has high epoxidation activity for small molecular olefins such as propylene, 1-hexene and cyclopentene, but it is difficult for the bulky olefin reactant like terpene to enter the pore and contact the active site in the pore, which is a major challenge for epoxidation^[78,79]. Compared with traditional three-dimensional zeolites, two-dimensional layered zeolites have higher specific surface area and shorter diffusion distance, which makes it possible to overcome diffusion limitation.

Na et al. Synthesized TS-1 disordered nanosheets with a mesoporous diameter of 6.3 nm by using bis-quaternary ammonium surfactant [C₁₆H₃₃-N⁺(CH₃)₂-zC₆H₁₂-N⁺(CH₃)₂-C₆H₁₃](OH⁻)₂（C_16-6-6(OH)₂） as a structure-directing agent^[80]. Compared with the traditional TS-1, the Ti sites on the outer surface of the nanosheets showed high activity. In the epoxidation of cyclohexene and cyclooctene, the conversion of cyclooctene was high (15.3% for TS-1 nanosheets and 0.6% for conventional TS-1 after 60 ° C and 2 H). In the epoxidation of 1-hexene, the catalytic performance of TS-1 nanosheets before and after fluorination was lower than that of traditional bulk microporous TS-1 zeolite, indicating that the catalytic activity of external Ti sites was lower than that of internal Ti sites. Wang et al. Synthesized multilayer TS-1 nanosheets (LTS-1) with C_22-6-6(OH)₂ replacing C_16-6-6(OH)₂ as a structure-directing agent^[81]. In the epoxidation reaction, the mesoporous nature and ultrathin crystal structure of LTS-1 make its catalytic activity much higher than that of microporous TS-1, Ti-Beta, Ti-MWW, and comparable to that of mesoporous Ti-MCM-41.

On this basis, P Přech et al. Synthesized layered TS-1 zeolite using C_18-6-6(OH)₂ as a structure-directing agent^[74]. Layered TS-1 zeolite was intercalated with a mixture of TEOS and TBOT as the supporting medium. In the epoxidation of cyclooctene, the catalytic performance of the titanium-containing columnar TS-1 catalyst (16.9% yield) was higher than that of the titanium-free columnar TS-1 catalyst (3.5% yield). However, the addition of titanium species makes the surface area of TS-1 columnar structure smaller than that of pure TEOS columnar structure, and the resulting product lacks long-range ordered layered structure. Wilde et al. Prepared the same layered TS-1 as Na et al., and used pure silica or a silica/titania mixture to support the layered TS-1^[82]. In the epoxidation of methyl oleate, the presence of octahedrally coordinated Ti in the SiO₂-TiO₂ column would lead to the failure of decomposition of H₂O₂, resulting in higher catalytic performance of layered TS-1 and columnar TS-1 containing SiO₂. In the epoxidation of high molecular weight substrates with H₂O₂, tetrahedrally coordinated Ti centers are more important than partial porosity for obtaining high activity and selectivity. In addition, due to the continuous reaction of epoxy compounds, the oligomers formed will block the active center, resulting in catalyst deactivation. Columnar TS-1 and conventional microporous TS-1 can be regenerated by roasting, while layered TS-1 can cause partial interlayer collapse during roasting, so the reproducibility of layered TS-1 is poor.

In order to maintain the long-range ordered structure of TS-1 nanosheets, Wu et al. Synthesized columnar TS-1 zeolite by TEOS pillared method using quaternary ammonium bromide salt C_22-6-6 as a template and C₆DN as a precursor to provide basicity^[76]. Phenol hydroxylation and cyclooctene epoxidation reactions were performed on conventional TS-1 (C-TS-1), layered TS-1 (M-TS1), and columnar TS-1 (P-TS-1). The conversion of phenol increased with reaction time over P-TS-1 and M-TS-1 zeolite catalysts. Due to the low concentration of Ti sites in P-TS-1 zeolite, the phenol conversion of C-TS-1 and M-TS-1 was higher. In the epoxidation of cyclooctene in the presence of hydrogen peroxide, the conversion of P-TS-1 zeolite was much higher than that of M-TS-1 and C-TS-1 zeolite due to the limitation of the outer surface. P-TS-1 has the advantages of uniform mesoporous structure and long-range ordered structure, which has more significant advantages in promoting macromolecular reactions. When the epoxide is formed, it is easier to diffuse from the outer surface into the uniform mesopores, thus inhibiting the possibility of epoxide ring opening and improving the epoxide selectivity.

Layered TS-1 nanosheets (HTS-1) with good interlayer stability and unique house-of-cards-like structure were synthesized by using Bola type surfactant BC_Ph-12-6-6 as structure-directing agent^[12]. Because HTS-1 has mesopores on the external surface and a larger external surface area, its titanium active sites are more accessible to the macromolecular reactants during the epoxidation of macrocyclic olefins (cyclohexene and cyclooctene) than the traditional monomicroporous TS-1 (CTS-1) catalyst and the microporous TS-1 (MTS-1) synthesized directly from a commercial organosilane surfactant (TPOAC). Wang et al. Prepared layered TS-1 (LTS-1) with stable interlayer mesopores by one-pot method using Bola-type surfactant BC_Ph-12-6-6 and TPAOH as dual templates^[46]. The dual-template synthesized LTS-1 has a highly extended structure, which makes it easier to obtain a high-performance titanium active site dominated by TiO₆ species. In cyclooctene epoxidation, LTS-1 showed better performance than TS-1 with BC_Ph-12-6-6 as a single template for directional demixing. The LTS-1 sample synthesized by the dual-template method showed high TiO₆ content and good catalytic performance for the epoxidation of cyclooctene.

Liu et al. Used a hydrothermal method to synthesize hierarchical ZSM-5 nanosheets (Hi-ZSM-5) with intracrystalline mesopores and honeycomb structure^[83]. The catalytic performance of Hi-ZSM-5 was better than that of C-ZSM-5 in the alkylation of toluene with 1,3,5-trimethylbenzene. This is due to the fact that the layered structure increases the acid site concentration and enhances the alkylation efficiency and diffusion rate of toluene. In addition, Hi-ZSM-5 has a low carbon deposition rate and a long catalytic lifetime because it combines the characteristics of mesoporous and layered crystal structures, which facilitates the transport of coke precursors from the zeolite.

Bian et al. Synthesized ZSM-5 nanosheet single crystals with a thickness of 100 nm by the seeded orientation method^[84]. During the alkylation of benzene and ethylene, the layered ZSM-5 nanosheets with a b-axis thickness of 100 nm exhibited an ethyl selectivity of 94. 8%, a stable benzene conversion of 44. 0%, and a significant deactivation of the large particle ZSM-5 catalyst. Wei et al. Synthesized a pillared layered MFI molecular sieve by VPP process^[25]. Compared with the two-dimensional layered zeolite without pillared treatment, the columnar zeolite prepared by VPP method not only retains the layered structure of zeolite, but also retains the acidity of zeolite, and also improves the catalytic effect of trimethylbenzene alkylation with benzyl alcohol. Naranov et al. Synthesized a layered MFI zeolite with mesoporous structure by using surfactant C_22-6-6 as structure-directing agent, zeolite seed and a small amount of perchloric acid as promoter^[85]. MFI nanosheets exhibited very high catalytic activity for toluene alkylation compared to conventional ZSM-5 zeolite. Traditional ZSM-5 has only intracrystalline micropores, which is not suitable for acid-catalyzed processes of macromolecules, so its selectivity for most products is low (3%).

Tian et al. Synthesized two kinds of ZSM-5 pillared nanosheets by dual-template method (DZN-2) and TEOS pillared method (PZN-2)^[86]. The dual-template method (DZN-2) retained more mesopores and had better channel connectivity than the TEOS method (PZN-2). Compared with layered and massive MFI zeolites, columnar MFI zeolites have more mesopores, which is beneficial to the rapid diffusion of light olefins in the catalytic cracking of n-decane. It can also effectively inhibit secondary reactions such as hydrogen transfer and oligomerization, and has high selectivity for light olefins. Compared with PZN-2 and parent ZN-2 catalyst, DZN-2 catalyst showed the highest selectivity to low olefins at 500 ℃, reaching 37.8%, and the conversion of n-decane was about 92%. The low olefin selectivity of PZN-2 catalyst prepared by TEOS column (28.2%) is higher than that of ZN-2 catalyst (21.2%), but the conversion is lower due to the introduction of Si column which reduces the Brønsted acidity of PZN-2.

Zhang et al. Used solid silica and dense gel at 373 K by adding ammonium fluoride, using fluoride ions instead of hydroxide ions as mineralizer, and tetrapropylammonium bromide (TPABr) as a single template^[87]. The dissolution of solid silica is promoted by highly electronegative fluoride ions in neutral fluoride medium. ZSM-5 zeolite nanosheets with a rich mesoporous structure were synthesized using ion-pair （F⁻-TPA⁺） as a structure-directing agent. It has higher surface area and pore volume. Compared with the traditional ZSM-5 zeolite, the ZSM-5 layered zeolite has higher propylene and butene selectivity in the methanol to propylene reaction. In addition, ZSM-5 layered zeolite can inhibit the deposition of coke and prolong the service life of the catalyst.

Feng et al. Prepared ZSM-5 zeolite with hexagonal lamellar structure and low acid content by a relatively simple hydrothermal crystallization method using TPAOH as template and glucose as additive, and the thickness of b-axis orientation was 220 nm^[88]. At the same time, the acid content and acid strength of b-oriented ZSM-5 zeolite were further reduced by phosphorus modification with ammonium phosphate as modifier. Compared with conventional ZSM-5, multilayer ZSM-5 has shorter b-axis mass transfer distance, abundant multistage pore structure and suitable surface acidity, and has higher propylene selectivity, longer catalyst life and excellent coke resistance for methanol to olefins reaction.

Although the use of additives can alleviate the cost pressure of template, it also inhibits the growth of crystals in other directions, resulting in small crystal particles and complex separation problems. Meng et al. Proposed a new method for the synthesis of ZSM-5 zeolite by dual-template method^[89]. ZSM-5 nanoflake zeolite was synthesized using diethylamine (DEA) and inexpensive quaternary ammonium salt surfactant C₁₆H₃₃-[N⁺-methylpiperidine]（C₁₆MP） as mesoporous agents. In the methanol to olefins reaction, its catalytic performance is similar to that of the zeolite nanocrystal layer catalyst synthesized with the expensive bis-quaternary ammonium salt template. ZSM-5 zeolite is highly mesoporous and crystalline, and its microporous space, a small amount of silica groups, and the efficient utilization of Brønsted acid sites on the outer surface all increase the lifetime of the zeolite.

Chen et al. Used a simple and low-cost method to synthesize ZSM-5 zeolite nanosheets, and further shortened the synthesis time to 4 d^[90]. They used the traditional surfactant CTAB as the template, replaced the organic quaternary ammonium salt template TPAOH with ZSM-5 seeds, and added ZSM-5 seeds to the synthetic gel to guide the formation of ZSM-5 structure. This method effectively avoided the phase separation caused by CTAB competition, and prepared ZSM-5 zeolite nanosheets with large specific surface area, large mesopore volume and narrow intercrystalline mesopores. Layered ZSM-5 exhibits higher stability and propylene selectivity than conventional microporous ZSM-5 in methanol to propylene reaction.

To sum up, two-dimensional zeolite is more conducive to substrate contact with active sites because of its larger specific surface area, more exposed acidic sites and smaller mass transfer resistance.The diffusion efficiency of the product is improved, the carbon deposition rate is reduced, the selectivity of the product is improved, and the catalytic life is prolonged, so the two-dimensional zeolite can play a great advantage in the field of catalysis.

MMM composed of layered structured packing with high aspect ratio, perpendicular to the diffusion path of gas through the polymer matrix, has high gas separation performance. Two-dimensional zeolites are considered to be an emerging material due to their barrier properties, and have therefore been extensively studied as fillers for MMMs in gas separation applications.

Two-dimensional zeolite not only has the high aspect ratio of oriented growth zeolite, but also has a short diffusion path for selective separation of chemical components^[98]. Cao et al. Successfully prepared layered ultrathin ZSM-5 zeolite by seeded secondary growth method for pervaporation desalination^[99]. They successfully achieved high salt rejection (up to 24% NaCl content) with porous alumina-supported ZSM-5 zeolite membranes, and observed that the lateral size of zeolite nanosheets increased with the time of seed secondary growth during the synthesis. The lateral size of zeolite nanosheets increased from an average of 2. 15 μm at 3 d to an average of 2. 75 μm at 4 d, while the thickness did not change significantly. The large aspect ratio of zeolite nanosheets in the laminated film significantly reduces the intergranular (i.e., internanosheet) boundary openings in the surface and significantly lengthens the internanosheet channels to minimize the penetration of liquid brine into the permeate-side surface.

Min et al. Prepared 2D M FI zeolite nanosheets on porous α-alumina support for butane isomer separation^[100]. It was found that the nanosheet suspension concentration affected the thickness of the 2D MFI zeolite nanosheet coating on the α-alumina support, thereby affecting the n-butane permeability. The increase in the thickness of the 2D MFI nanosheets brings additional resistance to the passage of n-butane through the membrane. Although the films prepared after the primary growth had no obvious cracks or defects, the films did not show significant selectivity for n-butane/isobutane isomer separation due to the presence of voids between the grown nanosheets. In order to enhance the molecular sieving mechanism and cover the voids of the two-dimensional zeolite nanosheets after the primary seed growth, they used the hydrothermal method for secondary growth and tertiary growth, and the resulting membrane had good separation performance of n-butane/isobutane isomers, with a separation factor of 42 and a n-butane Permeance of 382 GPU. Compared with the primary seeded two-dimensional zeolite nanosheets, the secondary and tertiary grown nanosheets showed a greatly reduced n-butane permeability and a significantly improved separation factor, which may be due to the selective separation of isomers by the molecular sieving mechanism. Gou et al. Used 2D MFI zeolite to prepare polydimethylsiloxane (PDMS)/2D MFI zeolite nanosheet MMM for n-butane/isobutane isomer separation applications^[52]. In their study, the permeability and selectivity of n-butane/isobutane isomers were improved simultaneously. PDMS intercalated with 1 wt% 2D MFI zeolite has an n-butane permeability of 15 615 barrer and an n-butane/isobutane separation factor of 15.6, which are 541% and 524% higher, respectively. The 2D MFI nanosheets formed tortuous paths in the PDMS polymer matrix. In addition, the MFI membrane is selective for n-butane^[101]. The two-dimensional MFI nanosheets contain 10-ring channels along the b-axis, which facilitate the transport of n-butane on the PDMS polymer matrix. At the same time, isobutane passes through the MMM along the tortuous path formed by the zeolite nanosheets in the polymer matrix. This resulted in a significant increase in the permeability of n-butane and only a slight increase in the isobutane permeability. In addition, the continuous phase of PDMS fills the gap between the two-dimensional MFI nanosheets, effectively improving the separation factor of n-butane/isobutane. Kim et al. Used C_22-6-6 as a template to grow layered (2D) MFI sheets on the surface of bulk MFI crystals^[102]. The structure and morphology of this material is referred to as "block-MFI-layer-MFI" (BMLM) material, which, in combination with polyimide, enables fabrication of composite membranes with enhanced CO₂ permeability and good selectivity for CO₂/CH₄ gas separation. Fig. 9a shows the morphology of pure MFI particles, and Fig. 9b ~ e shows the morphology of BMLM structure growing on the surface of MFI particles. Recently, Wang's group successfully prepared a two-dimensional zeolite LTA material with a thickness of only 1. 38 nm by template method using kaolin as raw material for CO₂ capture^[103]. Microscopic characterization showed that the conventional cubic structure of zeolite LTA was transformed into a two-dimensional sheet structure, and the specific surface area was also increased. In addition, in the comparative experiments of CO₂ separation at different temperatures, the two-dimensional sheet structure with larger specific surface area can capture more CO₂ than the traditional three-dimensional zeolite, and its CO₂/CH₄ selectivity is 76.78.

Recently, Azizi et al. Used molecular dynamics to simulate the CH₄/N₂ separation of MFI molecular sieve nanocrystal layers with dimensions of 38 38 Å × 38 Å × 20 Å and pore size of 4.44 Å^[104]. Their simulations showed that the MFI zeolite nanocrystalline layer is highly selective for N₂ molecules compared to CH₄ molecules. The MFI zeolite nanocrystalline layer exhibits stronger van der Waals interactions with the CH₄ molecules than the N₂ molecules. Due to this interaction, the N₂ molecule can smoothly pass through the MFI nanosheet, while the CH₄ is either trapped in the pore of the MFI nanosheet or unable to pass through it. When the system is simulated at high temperature and high pressure, a small amount of CH₄ molecules can also permeate through the MFI nanosheets due to the increase of transmembrane driving force, which overcomes the van der Waals interaction between CH₄ molecules and the pores of MFI nanosheets. MFI molecular sieves, which can be exfoliated into nanosheets, exhibit good separation performance in pervaporation desalination and can separate n-butane/isobutane isomers by molecular sieving mechanism, and also have high potential in CO₂ gas separation applications.

The nanosheet structure shows good barrier performance, which can improve the selectivity of the separated gas. Wolf et al. Concluded in their review that layered nanoparticles are more effective than isodimensional or elongated nanoparticles in reducing relative permeability^[105]. However, the effect of adding layered nanoparticles cannot be easily anticipated due to other factors, which may disturb the curvature of the modification, adsorbability, polymer matrix-modified chains, generation of interfacial phases, voids and cracks, etc., leading to the unexpected behavior of MMM. In addition, no general pattern has been observed on selective transport for gas separation applications and it is difficult to predict.

To sum up, the advantage of traditional zeolite is that the porous interconnected structure of zeolite increases the diffusion coefficient of gas molecules through the membrane; Some zeolites (e.g., SAPO-34) have very small pores that can distinguish CO₂ from other larger gas molecules by molecular sieving. However, the shortcomings of traditional zeolite can not be ignored. If there is interfacial incompatibility between the polymer matrix and the filler, the pores of the zeolite filler will be blocked, resulting in reduced permeability; The difference of physical properties between zeolite and polymer coating solution may lead to the precipitation of zeolite and the formation of non-selective channels by the migration of particles to the surface, which affects the gas selectivity.

Two-dimensional zeolites have an extremely high aspect ratio compared to typical conventional zeolites; And that nano-sheet fill has a blocking characteristic, so that larger gas molecule can be limited to move through a tortuous path formed by the nano-sheets, and the require small gas molecules are allowed to rapidly diffuse; In addition, two-dimensional zeolites have great potential in the field of separation membranes because of their good film-forming characteristics; Recent research on various two-dimensional materials is developing rapidly. Of course, there are still many challenges in the application of two-dimensional zeolite in separation membranes, such as controlling the ideal orientation of nanocrystalline layers in MMM, disturbing the polymer matrix, etc; In addition, some nanocrystalline layers (such as MFI zeolite nanocrystalline layer) require long time and complex process to synthesize, making the commercialization progress slow.