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Fabrication and Application of MFI Zeolite Nanosheets

Xiuli Shao , Siqi Wang , Xuan Zhang , Jun Li , Ningning Wang , Zheng Wang , Zhongyong Yuan

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (12) : 2651 -2666.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (12) : 2651-2666 DOI: 10.7536/PC220413
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Fabrication and Application of MFI Zeolite Nanosheets
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Abstract

MFI zeolite nanosheets have tremendous application potential in adsorption, separation and catalysis fields, which has become one of the hot topics in control synthesis and application of MFI zeolite due to its open framework structure, large external surface, optimized surface acidity, highly accessible acid sites and excellent molecular mass transfer properties. This review focuses on the synthesis mechanism and template types by in-situ hydrothermal synthesis and post-synthesis, as well as the influencing factors of thickness, lamellar spacing and orderliness in depth on the manufacturing and utilization of MFI zeolite nanosheets. The development of MFI zeolite nanosheets with low economic cost and suitable for mass production, as well as its application in the preparation of ultrathin zeolite membranes, catalysis of organic macromolecular reactions, and the preparation of metal catalysts supported by MFI zeolite nanosheets, are the main future research directions.

Contents

1 Introduction

2 Synthesis method of MFI zeolite nanosheets

2.1 In-situ hydrothermal

2.2 Post-synthesis

3 Applications of MFI zeolite nanosheets

3.1 Synthesis and separation application of ultrathin membrane

3.2 Catalytic conversion of organic compounds

3.3 Confinement synthesis and application of metal catalysts within MFI zeolite nanosheets

4 Conclusion and outlook

Graphical abstract

Keywords

MFI zeolite / nanosheets / in-situ hydrothermal synthesis / post-synthesis / catalytic performance

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Xiuli Shao, Siqi Wang, Xuan Zhang, Jun Li, Ningning Wang, Zheng Wang, Zhongyong Yuan. Fabrication and Application of MFI Zeolite Nanosheets. Progress in Chemistry, 2022, 34(12): 2651-2666 DOI:10.7536/PC220413

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1 Introduction

Zeolite molecular sieve is a crystalline hydrated aluminosilicate material with special framework and regular pore structure. At present, 255 kinds of zeolite molecular sieves with topological structures have been synthesized[1]. Among them, MFI topology molecular sieves have been studied most intensively. MFI zeolite has a three-dimensional framework composed of zigzag channels (0.55 × 0.51 nm) along the a axis and straight channels (0.56 × 0.53 nm) along the B axis, which are crossed with each other. It has high hydrothermal stability, adjustable acidity and excellent shape selectivity, and is widely used in petrochemical, fine chemical, environmental protection, gas adsorption and other fields[2⇓⇓~5]. However, the long microporous channels of traditional MFI zeolites limit the mass transfer and diffusion of reactants and products between active centers, especially when macromolecules are involved, which often leads to side reactions, carbonization, rapid catalyst deactivation and other phenomena[6⇓~8]. Therefore, how to effectively reduce the diffusion limitation is an important research direction in the controlled synthesis of MFI molecular sieves[9⇓~11].
In 2009, Ryoo et al. First synthesized a nanosheet-structured MFI molecular sieve with single cell thickness and well-matched hierarchical pores, which promoted the controlled synthesis and application research of nanosheet-structured MFI molecular sieve[12]. Previous studies have confirmed that the wider (010) crystal plane and thinner nanosheets of nanosheet-structured MFI molecular sieves can provide a larger proportion of the outer surface and effectively shorten the molecular diffusion path, thus overcoming the mass transfer and diffusion limitations of traditional microporous MFI molecular sieves. In addition, the cross arrangement of nanosheets can make the active sites fully exposed, and there are a variety of surface defects, which is conducive to modulating the surface acidity and adsorption capacity of molecular sieves, improving the accessibility of acidic sites, and effectively improving the catalytic reaction performance and catalyst stability[13⇓~15].
The structural characteristics and unique advantages of nanosheet MFI molecular sieves have attracted wide attention of researchers at home and abroad[16⇓⇓⇓⇓~21]. In this paper, the latest research progress in the synthesis and application of nanosheet-structured MFI molecular sieves was systematically summarized, focusing on the formation mechanism of nanosheet-structured MFI molecular sieves by in-situ synthesis and post-treatment methods, the types of templates, and the factors affecting the thickness, spacing and ordering of nanosheets.The main research directions in the future are to develop a new synthesis strategy of nanosheet-structured MFI molecular sieves with low economic cost, which can be used for large-scale production, and to apply it to the preparation of ultra-thin molecular sieve membranes, catalysis of organic macromolecular reactions, and the preparation of sheet-supported metal catalysts.

2 Ynthesis method of nano sheet structure MFI molecular sieves

2.1 In-situ hydrothermal method

The in-situ hydrothermal synthesis method is to design and synthesize a corresponding quaternary ammonium salt surfactant as a structure directing agent (template agent) to restrict the growth of crystals in a certain direction to form lamellar structure crystals.The key is to maximize the interaction between the template and the molecular sieve precursor and reduce the phase separation during the crystallization of the molecular sieve. A large number of experimental studies have shown that the structure of the template plays a key role in the formation, morphology, structure, thickness and ordering of the nanosheets[22~23]. At present, the template agents used in the synthesis of nanosheet-structured MFI molecular sieves mainly include mono-/di-/polyquaternary ammonium salts and mono-/di-/polyquaternary ammonium salts containing double benzene rings (the molecular structure of which is listed in Table 1).They all contain hydrophilic ammonium groups that guide the formation of MFI molecular sieve microporous structure and hydrophobic groups such as carbon chains or benzene rings that inhibit the growth of molecular sieve in the b-axis direction. When they guide the formation of MFI molecular sieve with nanosheet structure, they follow different mechanisms and obtain different morphologies. Therefore, by using different synthesis methods and templates with different structures, the controlled synthesis of MFI molecular sieves with different layer spacing, layer thickness and order can be realized.

2.1.1 Single template method

Bonilla and Chaikittisilp et al. First systematically studied the effects of quaternary ammonium salt monomers and bis-quaternary ammonium cations on the morphology of MFI zeolites, and found that MFI zeolites synthesized with different templates had different lengths in the a-axis, b-axis and c-axis directions[24,25]. Among them, (C3H7)3N+-(C5H10-N+(C3H7)3(diquat-C5) showed two completely different growth behaviors for MFI molecular sieve. When the system supersaturation is high, the obtained crystal has no obvious crystal face and the growth rate along the B axis is the fastest, while when the system supersaturation is low and there is an obvious crystal face, the growth rate along the B axis is extremely slow, and thinner MFI molecular sieve nanosheets are obtained. This work has a certain guiding significance for the design and synthesis of MFI molecular sieves with nanosheet structure.
Ryoo et al. Innovatively designed and synthesized a series of double-headed quaternary ammonium surfactants containing long-chain alkyl groups [CmH2m+1-N+(CH3)2-(CH2)i-N+R2-CnH2n+1]Br2(m=6~22,i=3, 6, 8, n = 1 – 6), and used them as templates to synthesize MFI molecular sieves with two morphologies of ordered stacked multilayer (MI-MFI) and randomly stacked monolayer (UI-MFI) with a thickness of only 2 nm in the b-axis direction in one step[12]. In this process, the two-headed quaternary ammonium surfactant forms micelles in the water phase, in which one quaternary ammonium cation is located in the framework and the other is located at the orifice of the outer surface, leading to the formation of the MFI molecular sieve framework structure, so that the crystal grows along the a-c plane. The long chain alkyl group extends along the B axis direction, which promotes the formation of mesopores and restricts the further growth of the crystal along the B axis direction. This innovative study provides a new direction for the research and application of MFI molecular sieves with nanosheet structure prepared by using single/double/multiple quaternary ammonium surfactants as templates. At present, there are many reports on the successful preparation of MFI molecular sieves with nanosheet structure by using this series of surfactants[26⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~39].
The reported results show that the thickness, interlayer spacing and ordering of nanosheet MFI molecular sieves can be controlled by changing the values of m, I, n and the type of R group: (1) The CmH2m+1 of alkyl side chain (i.e. The value of m) and the length of terminal carbon chain (i.e. The value of n) affect the structure and spacing of nanosheets. When m = 10, 12, 16, 18 and 22, respectively, the alkyl side chain is long enough to form a layered micelle structure through self-assembly, which prevents the crystal from continuing to grow in the ammonium region, and the multi-layered MFI molecular sieve with a layer spacing of 5. 2 ~ 6.1 nm is obtained[35]. When m ≤ 8, because the alkyl side chain is too short, the surfactant is embedded into the micropores, so that the crystal grows continuously to obtain a thicker MFI molecular sieve. With the increase of the terminal carbon chain length (i.e., the n value), the hydrophobic interaction is enhanced, the dissolution and hydration are weakened, the ammonium group and the silica-alumina species are not easy to contact, and the function of guiding the formation of the nanosheet structure MFI molecular sieve is weakened. In addition, the volume of the template increases with the increase of n, while the matching degree with the microporous structure of MFI zeolite decreases. (2) The length of the gap (I value) affects the order of the nanosheets. When I = 3, the surfactant molecules will undergo Hoffman elimination to form small molecules at high temperature, which can not lead to the formation of MFI nanosheets[36]; When I = 6, the two NH 4 + at both ends of the carbon chain are surrounded by Si-OH, which can lead to the formation of MFI multilamellar structure. However, when I = 8, the carbon chain length is too long, one of the NH 4 + cannot be surrounded by Si-OH, and the mobility of the C22 alkyl side chain connected to it increases, which cannot lead to the MFI multi-sheet structure, but only to the MFI single-layer nanosheet structure. (3) When the R group is CH3- or C2H5-, the nanosheet structure can be formed, and when the carbon chain length is increased to C3H7- or n-C4H9-, the nanosheet structure cannot be formed. (4) The number of ammonium groups affects the MFI nanosheet thickness and the stability of the molecular sieve[37]. At least two ammonium groups are needed to synthesize the MFI nanosheet, wherein the ammonium group directly connected with the CmH2m+1 of the long alkyl side chain is stacked together with the alkyl side chain and positioned on the outer surface of the nanosheet in the crystallization process,As a linker of long chain alkyl and internal ammonium group, it does not participate in the synthesis of molecular sieve framework, but only plays a stabilizing role, while the internal ammonium group provides a high enough degree of molecular freedom to guide the formation of the internal framework of nanosheets. These multilayer MFI nanosheets are orderly arranged along the B axis, so that uniform mesopores are formed between adjacent nanosheets, but the ordered mesopores will disappear after calcination. Based on the unique structure-directing effect of bis-quaternary ammonium salts, Ryoo et al. Further designed and synthesized multi-quaternary ammonium salts 18-Nn-18, 22-Nn-22(n=3, 4, 5) (see Table 1 for specific molecular formulas).Ordered mesoporous MFI molecular sieve nanosheets with pore walls as crystals were synthesized by using the template. It was found that the thickness of the nanosheets increased with the increase of the number of ammonium groups due to the multi-ammonium structure, which was beneficial to the growth of the molecular sieve along the b-axis direction[38,39].
By introducing aromatic hydrocarbon groups such as phenyl, biphenyl and naphthyl at the end of the hydrophobic chain of amphiphilic molecules, Che Shunai et al. Developed a series of bifunctional structure-directing agents with strong self-assembly ability, which contain hydrophilic ammonium groups, mutable carbon chains and rigid double benzene rings or naphthyl groups.While providing stable lamellar micelle π-π interactions, the structure was adjusted to match the MFI molecular sieve framework geometrically[40][41⇓~43][44]. Among them, the relatively rigid aromatic hydrocarbon groups inhibit the growth of crystals, and the non-covalent intermolecular force of π-π stacking between adjacent benzene rings promotes the orderly arrangement of molecules, thus forming regular, stable MFI molecular sieve nanosheets with a new mesoscopic structure, and the sheet structure is not easy to collapse after calcination[11,44,45]. It was found that the thickness, interlayer spacing and order of MFI zeolite could be controlled by changing the aromatic group and carbon chain length of quaternary ammonium surfactant: (1) The aromatic group and carbon chain length affect the binding energy between the template and silicon species, thus affecting the morphology of MFI zeolite[41,44]. When monoquaternary ammonium salt C22-6, phenyl monoquaternary ammonium salt CPh-10-6, and bisphenyl monoquaternary salt CPh-Ph-4-6 w ere used as templates, the binding energy with silicon species was higher than that of the synthesized conventional MFI zeolite, and the conventional MFI zeolite morphology was preferentially formed. The reason is that when CPh-10-6 is used as a template, the π-π bond stacking effect is too weak to form a bilayer that inhibits the growth of molecular sieves in the b-axis direction, thus presenting a conventional morphology. When CPh-Ph-4-6 is used as a template, the distance between the benzene ring and the ammonium group is too close to inhibit the stacking effect of π-π bonds, and the ordered layered structure cannot be formed. When asymmetric bis-quaternary ammonium salt C22-6-6, bis-phenyl mono-quaternary ammonium salt CPh-Ph-m-6(m=6, 8, 10) and symmetric phenyl bis-quaternary ammonium salt BCPh-n-6-6(n=4, 6, 8, 10, 12) are used as templates, due to the introduction of biphenyl groups,The binding energy of the MFI molecular sieve synthesis system is significantly reduced, the MFI molecular sieve nanosheet morphology appears preferentially, and the π-π bond stacking interaction between the biphenyl groups makes the hydrophobic alkyl chain micelle located between the adjacent nanosheets to stabilize the sheet structure[41,44]. Therefore, when the monoquaternary ammonium salt is used as the base molecule, there must be more than two benzene rings and an alkyl chain with more than six carbon atoms to form an orderly stacked sheet structure. (2) The carbon chain length changes the arrangement parameters of the template, and then affects the sheet order, thickness and texture properties of the nanosheet-structured MFI molecular sieve[42⇓~44]. When bisphenyl monoquaternary ammonium salt CPh-Ph-m-6(m=6, 8, 10) was used as the template, the ordering of the nanosheet-structured MFI zeolite increased with the increase of m, and the interlamellar spacing increased (4. 4 ~ 4.8 nm). When symmetrical phenyl bis-quaternary ammonium salts BCPh-m-6-n(m,n=4, 6, 8, 10, 12) were used as templates, the interlayer spacing (4.37 ~ 5.52 nm) and thickness of the synthesized MFI nanosheets increased with the increase of m value[43]; By changing the n value, the stacking morphology of the nanosheet MFI zeolite can be modulated, thus controlling its texture properties, and the order of the MFI nanosheet decreases with the increase of n value[43]. On this basis, azobenzene was introduced into the hydrophobic end group, and the straight cylindrical mesoporous MFI nanosheets were prepared by σ bond torsion, which was completely different from the above experiments[45]. The quaternary ammonium salt structure-directing agent not only can be used for synthesizing the ZSM-5 molecular sieve with a nanosheet structure, but also can be used for synthesizing the TS-1 molecular sieve nanosheet with a single unit thickness[46⇓~48].
Although the above research work has achieved remarkable research results, the complex, expensive and unrecoverable templates designed and synthesized by the laboratory are used in the synthesis process, which restricts the industrial application of MFI molecular sieves with nanosheet structure. Therefore, it is the current research direction to develop new synthetic strategies to effectively construct nanosheet-structured MFI zeolites with low economic cost, less environmental damage, and large-scale synthesis and application.

2.1.2 Double template method

In the synthesis process of the nanosheet-structured MFI molecular sieve, a small molecular quaternary ammonium salt and a single/double/multi-head long-chain quaternary ammonium salt are purposefully matched, namely a dual-template method, so that the self-pillared nanosheet-structured MFI molecular sieve with an interlayer structure can be obtained in one step. In the dual-template method, the long-chain template (such as cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium toluenesulfonate (CTATOS), C22-6-6, etc.) Directs the sheet structure of MFI zeolite, while the small-molecule template (such as tetrapropylammonium hydroxide (TPAOH), tetrapropylammonium bromide (TPABr), tetrabutylammonium hydroxide (TBAOH)) directs the formation of microporous MFI zeolite framework between the sheet structure. It was found that the addition of small molecular template could effectively inhibit the self-assembly of long-chain template and [SiO4] while directing the formation of MFI zeolite nanocrystals, while the addition of long-chain template could hinder the diffusion of nanocrystals, resulting in the aggregation of nanocrystals into small spherical particles and attachment to the surface of nanosheets. Emdadi and Rani et al. Used C22-6-6 and TPAOH as dual templates to obtain a series of lamellar MFI molecular sieves with varying layer thickness and interlayer mesopore regularity (Fig. 1)[49,50]. After calcination, both the external surface area and mesopore volume were higher than those of the sheet zeolite prepared only by the single-template method, and its morphology and porosity could be controlled by changing the n(C22-6-6)∶n(TPAOH) ratio in the initial gel. In addition, CTAB and TPABr were used as dual templates to directly synthesize morphology-controlled nanosheet MFI molecular sieves by adjusting the growth kinetics, aging conditions and the molar ratio of the initial gel[51⇓~53]. Chen Hongli et al. Used CTATOS and TBAOH as dual templates to successfully synthesize nanosheet titanium silicalite (MTS-2) by reducing the size of the primary structural unit of the molecular sieve to make the matching degree between the nanocrystal and the intermediate phase higher, and the catalytic activity was significantly improved in the reaction involving macromolecules[54].

2.1.3 Seed induction method

The seed induction method, that is, the interaction between the precrystallization precursor seed or sub-nanometer crystal and the template to induce the synthesis of molecular sieve, can effectively eliminate phase separation, accelerate the crystallization rate and shorten the crystallization time.Reducing the amount of organic template, reducing the synthesis cost and environmental pollution, which is in line with the concept of green chemistry, is one of the directions of attention in the field of synthesis of nanosheet MFI molecular sieves[55⇓⇓⇓~59][60⇓⇓~63]. For the first time, Tsapatsis et al. Used 30 nm MFI grains as seeds to synthesize single-layer MFI molecular sieve nanosheets with a thickness of only 5 nm and a width of several microns in one step through epitaxial growth, thus achieving a major breakthrough in the synthesis of single-layer MFI molecular sieve nanosheets[64,65]. Our group first reported a new method for the preparation of nanosheet ZSM-5 zeolite catalyst by using a simple and commercial cationic surfactant cetyltrimethylammonium bromide (CTAB) as a structure-directing agent[66]. In this method, a multilayer nanosheet-structured ZSM-5 molecular sieve single crystal with a length of 2 μm and a regular morphology was successfully prepared by using the synergistic structural guidance of CTAB and a small amount of pure silicon molecular sieve seed gel (Fig. 2). The catalyst is formed by orderly stacking benzene annular two-dimensional nanosheets with the thickness of 70 to 100 nm along the b-axis direction, and shows high olefin selectivity in the methanol to olefins catalytic reaction. The results of density functional theory calculation show that the adsorption energy of CTAB molecule is-0. 41 eV, and the long chain alkyl group (length of 2. 17 nm) of CTAB not only acts as a structure-directing agent to occupy the straight channels of the molecular sieve, but also can be stably adsorbed on the surface of the nanosheet, and the hydrophobic long chain alkyl group can inhibit the growth and ordered stacking of the nanosheet, which is the key to the formation of the guiding sheet structure. This study opens up a new way for the preparation of nanosheet ZSM-5 zeolite, and lays a theoretical and experimental foundation for the industrial application of nanosheet ZSM-5 zeolite catalysts in macromolecular catalytic reactions.

2.2 Post-processing method

In recent years, based on the diversity and plasticity of two-dimensional layered molecular sieves, chemical etching, interlayer swelling, pillared support, layer exfoliation and "intercalation" have been used.Can not only maintain that basic structural unit and the layere structure of the molecular sieve and prevent the nano sheets from bee connected into a three-dimensional crystal structure due to the Si-O-Si bond generated by roasting, but also enable the two-dimensional layere molecular sieve to have large pore size and high external specific surface area[20].

2.2.1 Chemical etching

Chemical etching is a process in which a specific atomic layer in a three-dimensional layered crystal is purposefully etched away, leaving behind a target two-dimensional nanosheet structure[67⇓~69]. Prech et al. Used NH4F to preferentially extract the defect sites of ZSM-5 molecular sieve nanocrystals, and then etched the rest of the crystal layer by layer. The holes after etching were interconnected to form a ZSM-5 molecular sieve single crystal with a nano-lamellar house of cards morphology, which showed high catalytic activity and stability in ethanol hydrogenation[70]. Liu et al. Used different concentrations of TPAOH solution to etch MFI molecular sieves of different sizes anisotropically, and prepared single-layer MFI nanosheets with a b-axis thickness of 25 nm[71]. The method is easy to operate, and can realize the batch preparation of single-layer nanosheets with different thicknesses by adjusting the grain size of the precursor and the concentration of TPAOH, but the concentration and addition range of the TPAOH solution are narrow, and even a slight difference will lead to incomplete etching or incomplete nanosheet structure. In addition, for high-silica molecular sieves, due to fewer defect sites, a small amount of aluminum distributed in the framework is not easy to remove, and it is difficult to form nanosheets by etching. Etchant plays a key role in the chemical etching process. Therefore, efficient, environmentally friendly and low-cost etchant can effectively promote the chemical etching method to the large-scale controllable preparation of two-dimensional nanosheets.

2.2.2 Layer stripping

Three-dimensional layered crystals are formed by the combination of corresponding two-dimensional nanostructured lamellae with weak interaction force, so some molecules or ions can enter the interlayer of layered crystals under certain conditions.The lamellar crystal produces local strain to increase the interlayer spacing and reduce the interaction force between the lamellae, which leads to the reduction of the exfoliation energy of the lamellar crystal and the easy exfoliation of the corresponding two-dimensional nanosheets under suitable conditions. Tsapatsis et al. Swelled the multilayer Silicalite-1 molecular sieve with polystyrene and dissolved it in toluene, and a large number of polystyrene molecules were filled between the layers, so that the Silicalite-1 molecular sieve had a large volume expansion in the b-axis direction, and then the acoustic cavitation and acoustic shear force induced by ultrasonic waves were used to promote the exfoliation and dispersion of nanosheets in the toluene solution, resulting in a monolayer nanosheet with a thickness of about 3.4 nm[72]. On this basis, Tsapatsis et al. Used H2SO4-H2O2 solution to treat the multilayer MFI molecular sieve and exfoliated it into monolayer nanosheets with a thickness of 3.2 nm, which had good dispersion in water[73]. Cho et al. Used polyvinylpyrrolidone (PVP) as a stripping agent and dispersant to exfoliate multilayer Silicalite-1 molecular sieve nanosheets into monolayer nanosheets through the strong penetration effect of PVP on multilayer nanosheets[74]. The above study found that ultrasonic assistance helps the swelling agent molecules to penetrate the layered structure, expand the distance between adjacent nanosheets, and reduce the interaction between layers, thus exfoliating two-dimensional monolayer nanosheets. This method lays a theoretical and experimental foundation for the preparation of oriented, ultrathin zeolite membranes.

2.2.3 Column bracing

Pillar support is a process of intercalating guest molecules between the layers of layered materials, which can not only maintain the ordered structure of layered materials, but also introduce chemical functions other than the inherent characteristics of host materials. At present, the column support of nanosheet-structured MFI molecular sieves is mainly to separate the layers by intercalating organic compounds or partially filling the gaps between nanosheets with thermally stable inorganic "columns". These "columns" serve to maintain the separation between the layers, while the adsorbed swelling agent creates an interconnected pore structure by decomposing the resulting voids[19].
Ryoo et al. filled tetraethyl orthosilicate (TEOS) between the sheets, and formed "silica columns" after hydrolysis, drying, and calcination. The mesoporous structure of the "pillared" nanosheet MFI molecular sieve is more stable, and has a larger BET surface and pore volume than the molecular sieve before treatment (Fig. 3A)[75]. Tsapatsis et al. Synthesized a house-of-cards structure self-supported molecular sieve formed by 90 ° interactive growth of MFI/MEL nanosheets in one step, in which MFI is the main structure, and MEL is interspersed between MFI nanosheets to play the role of "pillar". Compared with the molecular sieve "pillar" by TEOS method, its acid density is increased by 15% (Fig. 3B)[76,77]. The method directly synthesizes the self-pillared layered molecular sieve, does not need to carry out swelling treatment on the precursor, avoids the problems of structure collapse and the like in the swelling process, and shows good application prospect. Wei and Tsapatsis et al. Intercalated TEOS into the layers of MFI molecular sieve by vapor phase conversion, and converted it into a stable SiO2 column after calcination to form a self-pillared two-dimensional layered MFI molecular sieve[78][79]. The TEOS intercalation efficiency is up to 100%, and only a small amount of water needs to be added, and product separation is not required after synthesis, which greatly reduces the waste of liquid and protects the environment (Fig. 3C). These nanosheet MFI molecular sieves have high stability through "pillared", and the mesoporous pores generated by the surfactant tail are still retained even after calcination. In addition, the mesoporous structure after "pillared" provides new sites for loading transition metals, and there are abundant Si-OH groups on the surface of the sheet, which provides a basis for silanization to change the surface properties or grafting some groups and metals with rich catalytic ability[48].
To sum up, the in situ hydrothermal method can effectively and accurately control the thickness, interlayer spacing and ordering of the nanosheet-structured MFI molecular sieve. Among them, the single template method has better dispersion in the synthesis system, more flexible in the adjustment of the sheet structure and thickness, and stronger universality, and can be applied to the synthesis of various two-dimensional nanosheet molecular sieves such as TS-1. The dual-template method controls the morphology and porosity of the molecular sieve by changing the molar ratio of the two templates in the initial gel, increases the specific surface area, enhances the stability, and improves the catalytic performance. However, the appropriate proportion of the two templates, the synergistic mechanism, and the mechanism of guiding the nucleation and growth process of molecular sieves are not clear, and the research on their catalytic activity, structural advantages and structure-activity relationship is also relatively lacking. In addition, there is often a competitive relationship between the mesoporous template and the microporous template, which may lead to phase separation during the synthesis process to obtain a mixture of the two materials. The seed induction method effectively synergizes the structure-directing effect of the template and the seed induction effect, has simple steps and easy operation, conforms to the concept of green synthesis, and provides a new path for the industrial preparation of the nanosheet MFI molecular sieve. The post-treatment synthesis strategies, such as chemical etching, layer exfoliation and pillar support, can not only maintain the basic structural units and layered structure of nanosheet-structured MFI molecular sieves, but also make them have large pore size, high external specific surface area and high stability, thus showing excellent catalytic and adsorption properties. However, most of the in-situ synthesis and post-treatment methods use long-chain multi-head quaternary ammonium salts as templates, and the complex synthesis process and high cost limit the industrial application of MFI zeolites with nanosheet structure. Although the use of simple template or no template has been reported in the literature, it is difficult to synthesize on a large scale because of the time-consuming preparation process and strict synthesis conditions. Therefore, whether the scale-up synthesis of the above synthesis strategies can be further realized is the key to realize its industrial application, which is of great significance to promote its industrial application process.

3 Application of MFI Molecular Sieve with Nanosheet Structure

3.1 Synthesis and Separation Application of Ultrathin Zeolite Membrane

B-axis oriented ultrathin MFI zeolite membranes have been widely used in isomeric mixture separation, gas separation, seawater desalination and other fields, and have become a hot research topic in recent years. How to prepare b-axis oriented, dense and defect-free ultrathin MFI zeolite membranes at low cost and on a large scale is the main research direction at present. Compared with the traditional MFI molecular sieve grains, the two-dimensional MFI molecular sieve nanosheets are easy to prepare the molecular sieve membrane, and the smaller the thickness of the two-dimensional nanosheets is, the thinner the prepared molecular sieve membrane is, which is more beneficial to the diffusion of molecules in the straight pore channel and the selective separation of smaller species. Tsapatsis et al. Exfoliated the nanosheet-structured MFI molecular sieve into two-dimensional nanosheets with high crystallinity and a b-axis thickness of about 3.4 nm, and coated them on a porous α-Al2O3 substrate by phase interface self-assembly method, and successfully prepared a dense and uniform thickness MFI molecular sieve membrane with a p-xylene/o-xylene separation factor of 40 – 70 (Fig. 4A)[80]. Nair et al. Deposited two-dimensional MFI molecular sieve nanosheets with a length of 2 μm and a b-axis thickness of 5 nm on a α-Al2O3 hollow fiber substrate by vacuum filtration method, and then obtained a defect-free highly selective MFI molecular sieve membrane with a b-axis thickness of 500 nm (Fig. 4 B) after two-step hydrothermal treatment, which had a permeation flux as high as 2000 – 3000 GPU and an n-butane separation factor of 35 – 300[81]. Therefore, two-dimensional MFI molecular sieve nanosheets were uniformly deposited on macroporous alumina, porous silicon fibers, monocrystalline silicon substrates and other carriers by filtration-deposition dispersion, coating and other methods to form a nanosheet coating, which was used as the best carrier for silicate reactants, while promoting their mutual crosslinking growth to prepare almost defect-free b-axis oriented ultrathin MFI molecular sieve membranes. In addition, the stripped molecular sieve two-dimensional monolayer nanosheets can be directly prepared into a b-axis oriented ultrathin molecular sieve membrane without secondary growth, thereby avoiding high temperature and energy consumption. The molecular sieve membrane prepared by the method effectively reduces the diffusion resistance of molecules along a straight pore channel in the B axis direction, shows extremely high selectivity, permeation flux and desalination rate in the aspects of separation of isomer mixtures (such as n-butane/isobutane and p-xylene/o-xylene), purification of salt-containing wastewater and the like,It will play an important role in energy saving and emission reduction, efficient conversion of resources and environmental protection, and open up a new way for the application and development of two-dimensional MFI molecular sieve nanosheets in the field of membrane separation[60,81][64,71,80,82][83⇓~85].

3.2 Catalytic conversion of organic compounds

MFI zeolite with nanosheet structure has many advantages, such as open sheet structure, large external surface area, short microporous transport path, rich pore structure, high utilization of active sites, diverse surface defects, tunable surface adsorption properties and accessible acid sites.It can effectively prevent the occurrence of side reactions, improve its catalytic reaction activity and anti-carbon deposition deactivation performance, and become an ideal catalyst for the synthesis of organic compounds[86]. Table 2 lists the comparison of the catalytic performance of the nanosheet MFI molecular sieve for the synthesis of organic compounds.

3.2.1 Friedel-Crafts alkylation to aromatics

Friedel-Crafts alkylation reaction is one of the important methods for the industrial production of aromatic hydrocarbons with high added value. MFI molecular sieve catalyst is the main catalyst for Friedel-Crafts alkylation reaction because of its strong acidity, shape-selective and high hydrothermal/chemical stability. However, Friedel-Crafts alkylation liquid phase catalytic conversion mainly occurs on the outer surface of microporous molecular sieve catalyst, and the reactant and product molecules are relatively large and can hardly enter the micropores for adsorption and diffusion, resulting in low reactant conversion and catalyst utilization of traditional MFI molecular sieve. In addition, with the increase of the contact time between the reactants and the catalyst, the alkylation products will further undergo side reactions such as polyalkylation, isomerization, transalkylation, dealkylation and polymerization. Therefore, increasing the external surface area and pore size of MFI molecular sieve catalyst to improve the molecular diffusion and mass transfer performance is the main research direction at present.
Liu et al. Used nanosheet MFI molecular sieve to catalyze the alkylation of toluene and 1,3,5-trimethylbenzene, and the 1,3,5-trimethylbenzene conversion (61.9%) and xylene selectivity (68.3%) were higher than those of traditional MFI molecular sieve[51]. Duan Qianqian et al. Found that the conversion rate of Friedel-Crafts alkylation of phenol and tert-butyl alcohol on MFI molecular sieve nanosheets was nearly 6 times higher than that of traditional microporous MFI molecular sieve[87]. Liu et al. Found that the pseudo-first-order rate constant for the alkylation of 1,3,5-trimethylbenzene and benzanol on free-standing nanosheet MFI zeolite was increased by two orders of magnitude compared with that on traditional microporous MFI zeolite[88,89]. Titration of the number of accessible acid sites on the outer surface by 2,6-di-tert-butyl peroxide (DTBP) molecules showed that 30% and 45% of the acid sites were accessible in the "pillared" MFI and free-standing zeolites, respectively, which was much higher than that in the traditional microporous MFI zeolites (~ 2%). It is because of these more accessible acidic sites that the above nanosheet-structured MFI molecular sieves exhibit excellent alkylation catalytic performance. In recent years, researchers have developed liquid phase aromatization routes to produce aromatics (benzene, toluene, xylene) from ethylene and ethane. When MFI molecular sieve with nanosheet structure is used as catalyst, the conversion of reactants, aromatics selectivity and catalyst life can be significantly improved[90,91][92]. Therefore, the nanosheet MFI molecular sieve lays a foundation for further research and industrial production of Friedel-Crafts alkylation to aromatics, and is expected to become an efficient catalyst for aromatics production.

3.2.2 Methanol to hydrocarbon reaction

MFI zeolite catalyst showed excellent product selectivity in methanol to hydrocarbons (MTH) reaction. However, when the traditional MFI molecular sieve is used to catalyze MTH reaction, the yield of aromatics is high, and the catalyst is easy to be deactivated by carbon deposition, so it needs frequent regeneration and is easy to cause environmental pollution. Therefore, inhibiting the formation of aromatics is the main research direction for developing green and efficient molecular sieve catalysts for MTH reaction. It was found that the introduction of mesopores into MFI zeolite could significantly improve the mass transfer and diffusion performance, increase the coking volume, inhibit carbon deposition, and prolong the catalyst life. In addition, if the size of the molecular sieve crystal is reduced to the size of a single unit cell, the diffusion of molecules can be maximized, which effectively shortens the residence time of multi-methylbenzene intermediates in the sheet, thereby inhibiting the occurrence of dealkylation secondary reactions, improving the proportion of C3/C2 products, and reducing the selectivity of aromatic products[93,94].
The coke distribution of the deactivated catalyst for MTH reaction was studied, and it was found that the carbon deposition on the nano-lamellar MFI zeolite had a special crystal face selectivity[95]. When the thickness of the nanosheet is small, the carbon deposition mainly occurs on the (010) plane, and the carbon deposition on the (100) plane is suppressed. When the thickness of the nanosheet is large, different degrees of carbon deposition will occur on each crystal plane. Therefore, the smaller the nanosheet thickness, the better the MTH reactivity and the longer the catalyst lifetime[96].
Choi et al. Used the synthesized single-layer MFI molecular sieve nanosheets for methanol to gasoline (MTG) catalytic reaction. Because of the small molecular size of methanol,There was no significant difference in the initial catalytic activity between the traditional MFI zeolite and the single-layer MFI zeolite nanosheets, but with the reaction going on, the carbon deposition and the selectivity of aromatics on the single-layer MFI zeolite nanosheets decreased significantly, and the catalyst life increased by 3 to 4 times[12]. Yanjun Gong and Kim et al. Used MFI molecular sieve nanosheets to catalyze the methanol to propylene (MTP) reaction, and the selectivity of propylene and light olefins ( C 2 = ~ C 4 =) was as high as 48.92% and 80.87%, and the lifetime was as long as 302 H[61,97⇓ ~99]. The excellent catalytic performance and long catalyst life of the nanoflake structure MFI molecular sieve are attributed to the large accessible external surface area, short diffusion path and high mesoporosity of the ultrathin nanoflake molecular sieve,The product molecules are easy to diffuse out of the micropores, secondary reactions such as aromatization and hydrogen transfer are effectively inhibited, the selectivity of aromatic hydrocarbons is reduced, and the selectivity of low-carbon olefin products is improved. In addition, the carbon deposition precursor formed in the micropores can quickly migrate to the outer surface to form external carbon deposition by means of a large number of mesopores and a short diffusion path, thereby reducing the influence of the internal carbon deposition on the contact of reactants with active sites, improving the carbon accommodation capacity of the nanosheet-structured MFI molecular sieve, and reducing the deactivation rate.
MFI molecular sieves with nanosheet structure not only show excellent performance in the Friedel-Crafts alkylation and methanol conversion reactions mentioned above, but also are widely used in the cracking of some hydrocarbons, Beckmann reaction, catalytic cracking of aromatic compounds and synthesis of macromolecular compounds[100⇓~102][27,103][40,104][12,48,105,106]. However, Xu Dongdong et al. Applied the nanosheet MFI molecular sieve to MTH reaction after hydrothermal treatment at 600 ℃, and the methanol conversion was lower than that of the traditional ZSM-5 molecular sieve treated under the same conditions[41]. The catalytic activity of nanosheet MFI zeolite decreased by ~ 25% under hydrothermal treatment at 500 ℃. It can be seen that the lamellar structure of nano-lamellar MFI molecular sieve is thin, and hydrothermal treatment will destroy the lamellar structure, which is not conducive to the stability of the ordered mesoporous structure. In addition, the nanosheet MFI zeolite showed good recycling catalytic performance in Beckmann condensation and formylation of trimethylbenzene and benzyl alcohol, but its crystallinity and acidity decreased greatly after multiple recycling[57⇓⇓~60]. Therefore, the structural stability of nanosheet MFI zeolite is a key problem to be solved in its application.

3.3 Preparation and Application of Metal Catalysts Supported on MFI Molecular Sieve with Nanosheet Structure

In supported metal catalysts, there is a strong interaction between the support and the metal component, which directly affects the morphology, size, electronic structure and dispersion of the metal particles and clusters on the support, thereby affecting the catalytic efficiency of the metal nanoparticles[107]. It has been found that the encapsulation of small metal clusters or metal atoms, especially sub-nanometer metal clusters, into molecular sieve carriers with confinement effect for catalytic reactions, namely "nano-confinement catalysis", can restrict the migration and agglomeration of nanoparticles, thus showing excellent catalytic performance and ultra-high stability[108⇓⇓⇓⇓⇓⇓~115]. Due to its large outer surface and abundant Si-OH defects, MFI molecular sieves with nanosheet structure can effectively disperse metal precursor species, thus significantly improving the confinement effect and dispersion of metal active components between layers and pores, which has become an ideal support for the synthesis of monometallic, bimetallic nanoparticles, sub-nanoclusters or monatomic supported metal catalysts in recent years[116⇓~118].

3.3.1 In situ synthesis

The in-situ synthesis method is not limited by the pore size of the molecular sieve because the encapsulation process of the metal species is synchronous with the crystallization process of the molecular sieve,Most metal cations are easily precipitated as hydroxides in alkaline synthetic gels, resulting in the formation of large particles on the surface of molecular sieve crystals and even the separation of metal species from the molecular sieve support. In addition, metal clusters are prone to serious aggregation in molecular sieve precursors due to their high surface energy. Therefore, limiting the aggregation or rapid precipitation of metal species is the key to encapsulate ultrasmall metal clusters in the framework of molecular sieves in situ, and researchers have adopted metal-amine complexes to overcome this problem[119⇓~121]. Wang Ning et al. Used metal complexes [M(NH2CH2CH2NH2)2]2+(M=Pd, Co, Ni) as raw materials to prepare nano-metal cluster and sub-nano-metal cluster catalysts[119]. Due to the greatly reduced size of metal clusters and the synergistic effect of bimetals, the Pd-cluster molecular sieve catalyst exhibits excellent catalytic activity and efficient hydrogen production for formic acid decomposition, which opens up new prospects for the practical application of hydrogen storage materials such as fuel cells. yuan Zhongyong et al. Used the electrostatic interaction between Pt and Zn complexes and molecular sieve gel to uniformly encapsulate ultra-small Pt and Zn bimetallic nanoclusters in the pores of Silicalite-1 (S-1) molecular sieve.The catalyst showed good catalytic performance in propane dehydrogenation (PDH), with propane conversion of 45. 3% and propylene selectivity of more than 99%, and no sintering of Pt and Zn bimetallic nanoclusters was observed after a long time of reaction at 550 ℃[120].
Based on the research and development of in-situ encapsulation of metal clusters and the synthesis of nanosheet-structured MFI zeolites, researchers have encapsulated metal clusters Fe and Ni into the interlayer or pore during the synthesis of nanosheet-structured MFI zeolites to further improve their catalytic performance and stability[122~123][124]. Hensen et al. Synthesized in situ a nanosheet-stacked Fe/ZSM-5 zeolite supported metal catalyst by adding a bis-quaternary ammonium salt surfactant to the traditional Fe/ZSM-5 synthesis gel.The number of active sites (aggregated Fe species) for catalyzing the decomposition reaction of N2O is low, while the number of active sites (monomeric Fe species) for catalyzing the oxidation of benzene to phenol is high, so the catalyst shows high catalytic activity for the oxidation of benzene to phenol[122,123]. In addition, due to the short diffusion path of coke or coke precursor to the outer surface of molecular sieve nanosheets and the fact that coke is mostly deposited on the mesopores, the formation of carbon deposits and the occurrence of secondary reactions are greatly reduced, and more than half of the initial activity is still maintained after 24 H of reaction. Li Baoshan et al. Added NiSO4·6H2O in the in-situ synthesis process, so that Ni species were loaded on the surface of MFI molecular sieve nanosheets through the formation of — O — Si — O — Ni — O chemical bonds to play a "pillared" role, effectively preventing the formation of new Si — O — Si bonds in adjacent nanosheets and preventing the collapse of nanosheet structure[124]. During the reduction process, the Ni species were reduced to highly dispersed Ni nanoclusters (< 1.6 nm), and the tetralin selectivity was up to 100% and the naphthalene conversion was up to 84.9% in the naphthalene hydrogenation reaction. In addition, the addition of NiSO4·6H2O affects the nanosheet thickness and morphology.

3.3.2 Post-modification method

After the organic template is removed by calcining the MFI molecular sieve at high temperature, the vacant pore channels or cages can accommodate the introduced metal cations. The common methods for introducing metal cations into molecular sieve support are ion exchange, impregnation and adsorption/decomposition of organometallic compounds. In this process, the metal precursor is uniformly filled into the pores through the molecular sieve crystals, and after reduction, metal clusters are formed inside the molecular sieve. At present, monometallic Ni, Pt, Pd and bimetallic Pd-Pt, Rh-Ru have been successfully dispersed in the channels, nanosheet surfaces or interlayers of nanosheet-structured MFI molecular sieves in the form of ultra-small nanoparticles or nanoclusters by the post-modification method, showing excellent catalytic performance and lifetime in various reactions[125][126,127][128][129][130]. Liu Guozhu and Pan Ying et al. Embedded Pt nanoclusters (4. 3 nm) into monolayer nanosheets (2. 9 nm) through the electrostatic adsorption of ammonium ions in bis-quaternary ammonium surfactant. The Pt nanoclusters played a "pillared" role, which well protected the mesopores between layers and showed good dispersion and high stability after high temperature treatment[126,127]. Wang Ning et al. used the impregnation method to uniformly disperse ultra-small monometallic Rh, Ru nanoparticles and bimetallic Rh-Ru sub-nanoclusters (0.7 ~ 0.78 nm) in the five-membered ring sinusoidal channels of the nanosheet-structured MFI molecular sieve (Fig. 5), and the hydrogen production rate in the boron-ammonia hydrolysis process was almost the same as that of the monatomic catalyst, which was 6 times that of the traditional MFI molecular sieve-supported Rh catalyst[130]. 1 H MAS NMR studies of ultra-small monometallic and subnanometallic clusters show that isolated Si-OH defects may be responsible for the immobilization of subnanometallic clusters, and that with the increase of isolated Si-OH defects, metal particles and nanoclusters are more dispersed and smaller in size. In the above post-modification process, because some gas molecules remain in the molecular sieve skeleton, the metal precursor solution is not easy to fill the whole micropores of the molecular sieve, so that in the reduction and reaction process, excessive metal is loaded on the outside of the molecular sieve and further aggregated to form larger nanoclusters or nanoparticles.

4 Conclusion and prospect

(1) Based on the complexity and limitations of the synthesis process and template of nanosheet-structured MFI molecular sieves, it is of great significance to further realize the scale-up synthesis of existing synthesis strategies for promoting its industrial application process. At the same time, it is the future research direction to develop new synthesis strategies of MFI molecular sieves with multilayer, monolayer, self-supporting or ultra-thin two-dimensional nanosheet structures, which are economical, less environmentally harmful, and can be synthesized on a large scale and applied in industry.
(2) Based on the minimization of b-axis thickness and the maximization of a-c crystal plane, two-dimensional MFI molecular sieve nanosheets can easily realize the preparation of ultra-thin and oriented molecular sieve membranes.The prepared molecular sieve membrane shows extremely high selectivity, permeation flux and desalination rate in the aspects of separation of isomer mixture, purification of salt-containing wastewater and the like, and plays an important role in energy conservation, emission reduction, efficient conversion of resources and environmental protection. Therefore, the future research direction is to design and prepare ultrathin and oriented molecular sieve membranes through two-dimensional nanosheet MFI molecular sieves.
(3) Based on the advantages of open lamellar structure, large external surface area, short microporous transport path and accessible acid sites, nanolamellar MFI molecular sieves are widely used in Friedel-Crafts, Beckmann, methanol conversion, aromatic compound catalytic cracking and macromolecular compound synthesis reactions, and show excellent catalytic activity and carbon deposition deactivation resistance. Therefore, the study of the structure-property relationship of MFI molecular sieves with nanosheet structure is not only very important to improve their catalytic performance, but also can be further extended to other macromolecular organic reactions.
(4) Based on the large external surface area and abundant Si-OH defects, the MFI molecular sieve with nanosheet structure is used as a carrier, which can effectively disperse metal precursor species, significantly improve the confinement effect and dispersion of metal active components between layers and pores, thus forming ultra-small metal nanoparticles and effectively improving metal utilization. Therefore, it is also an important development direction in the future to prepare monometallic, bimetallic nanoclusters, sub-nanoclusters or monatomic supported metal catalysts using MFI molecular sieves with nanosheet structure as carriers.

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Funding

National Natural Science Foundation of China(22169015)

Natural Science Foundation of Ningxia(2021AAC02003)

Leading Talent Project of Ningxia(KJT2016001)

First-rate Discipline Construction Project of Ningxia(NXYLXK2017A04)

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