A Review of High-Efficient Synthetic Methods for Zeolite Membranes and Challenges of Their Directional Growth Control

ABSTRACT Zeolite membranes are widely used in separation, catalysis, sensors and other fields because of their good diffusion performance, shape selection and catalysis capabilities. However, the slow synthesis rate of zeolite membranes restricts their industrialization process, and the random orientation seriously affects their performance. Therefore, this paper investigates the methods for accelerating the synthesis rate of zeolites and the roles of different methods was explored. The synthesis conditions of oriented zeolite membranes are also summarized. Microwave method can efficiently synthesize oriented zeolite membranes, but it has the defects of harsh synthesis conditions and high equipment requirements. Using physical methods and chemical methods to introduce hydroxyl radicals, which is also an effective method to improve the synthesis rate of zeolites. However, there is a lack of research on how to realize the directional synthesis of zeolite membranes during this process. Synthesis conditions, template and support properties affect the directional synthesis of zeolite membranes. Therefore, while introducing free radicals, adjusting the composition of the synthetic solution or introducing inhibitors that affect the growth direction of zeolite in the synthetic system can realize the efficient directional synthesis of zeolite membrane, which is another potential method to promote the industrialization of directional zeolite membrane besides microwave method. Graphical Abstract


Introduction
Zeolites are an inorganic crystal materials with regular pore structures formed by the common vertex connection of silicon and an oxygen tetrahedron (TO 4 ). They have a good pore structure, high specific surface area and hydrothermal stability. [1] Zeolite membranes with special morphologies inherit and strengthen the excellent properties of zeolites. Since the large-scale synthesis of zeolite membranes in 1990s, application researches in pervaporation (PV), gas separation, catalysis, sensors and other fields had risen and achieved remarkable results. [2][3][4] For example, a large-scale NaA zeolite membrane PV device was put on the market in 1999. [5] After entering the 21st century, the research of zeolite membranes in different fields has gradually deepened. [6][7][8] The MFI membrane prepared by Tsapatisi exhibited attractive separation characteristics as judged by permeation measurements with butane and xylene isomers. [9] Vilaseca et al. [10] developed semiconductor gas sensors covered with zeolite films (MFI or LTA) to realize the potential for higher selectivity/ sensitivity in gas sensing applications. Moreover, the application range of zeolite membrane is expanding. The use of zeolite molecular adsorption coatings for spacecraft pollution control was proposed by researchers after 2010. [11,12] Zeolite membranes have broad application prospects.
Zeolite membranes are commonly synthesized by conventional hydrothermal methods such as hydrothermal synthesis and secondary growth, which are continuous membranes formed by the symbiotic growth of zeolite crystals. [13] The procedure of traditional hydrothermal zeolite crystals synthesis mainly includes the depolymerization of silicon and aluminum source in alkaline solution and the polymerization into aluminosilicate gel. Then, under the action of template agents (seeds), after a certain period room temperature aging and high temperature (100-200°C) crystallization, zeolite crystals with different pore structures are crystallized. The crystallization process is completed in an autoclave. Finally, the template agent is removed by hightemperature calcination. This procedure is illustrated in Figure 1. The crystal crystallization process mainly includes stages: crystal nucleation and crystal growth. However, long-term nucleation and growth lead to a slow synthesis rate and low industrial production efficiency. [14] Therefore, it is of great practical significance to improve the synthesis rate of zeolite membranes. Additionally, the microstructure of the zeolite membranes, including the crystal size, shape and channel orientation control, seriously affects their performance. [15,16] Considering the anisotropic morphology and pore structure of zeolite crystals, it is important to control the preferred crystal orientation within the membrane for the preparation of high-performance zeolite membranes.
The microwave radiation method converts motion friction between molecules into heat energy, heat energy can be generated from the inside of the heating body, greatly improving the thermal conductivity and causing the gel mixture to dissolve quickly, which greatly improves the reaction rate. [17] A large number of studies have confirmed that preparing zeolites by microwave irradiation has the following characteristics: high reaction speed, uniform product particle size and high crystallinity. [18,19] Moreover, the crystal morphologies can be easily and selectively controlled by rapid and uniform heating with microwave radiation. The microstructure, crystal size, morphology, orientation and membrane thickness of the zeolite membrane were also affected. And twinning crystal growth can be effectively suppressed by the bottleneck effect related to nucleation and the application of single-mode microwave reactor. [20,21] The rapid directional preparation of zeolite membranes can be realized by the microwave radiation method. However, the microwave conditions are harsh, and the microwave conditions affecting the membrane orientation need to be systematically studied. The equipment requirements are high, and the geometry and material of the reactor need to be considered. As a stainless-steel product, a hydrothermal kettle may generate electric sparks when interacting with microwaves, and the autogenous pressure in the kettle increases rapidly under microwave conditions, which results in unsafe conditions. Besides, low energy utilization rate of microwave and the lack of professional microwave equipment make it difficult to use microwave method on a large scale. [22] Introducing free radicals by physical and chemical methods is another means of accelerating the synthesis of zeolites. Physical methods include physical radiation (UV, γ-ray, and ultrasonic), grinding crystal seeds, etc., and chemical methods include the introduction of Fenton's reagent or persulfate. •OH radicals have unique activity compared with OH − ions, which can better stabilize the reaction intermediates and break and form Si-O-Si bonds with a lower energy barrier. [23] They are more conducive to the depolymerization of silicate and the subsequent formation of new Si-O-Si bonds around the template in kinetics and thermodynamics. When applied to the zeolite synthesis system, they have a positive effect on the zeolite synthesis rate. [24] Compared with the microwave method, a free radical accelerator is introduced into the traditional hydrothermal synthesis system, the equipment requirements are low, and the synthesis process is relatively safe. However, there is a lack of research on how to achieve the directional synthesis of zeolite membranes during this process. In the traditional hydrothermal method, using the phase anisotropy of zeolite crystals to adjust the composition of the synthetic solution or introducing inhibitors, cotemplates and substrate modification have been proven to be common factors affecting the orientation of zeolite membranes, which plays an obvious role in the synthesis of oriented zeolite membranes. Therefore, based on the acceleration of free radicals and the universal law of oriented zeolite membrane synthesis, organically combining the two may be another route to efficiently synthesize oriented zeolite membranes. Figure 2 shows the research progress and application of zeolite and zeolite membranes.
The development of a method to improve the synthesis rate of zeolite membranes and prepare zeolite membranes with controllable orientations is still a problem to be solved in the future. In this paper, the role of microwave irradiation in accelerating zeolite synthesis and affecting the zeolite membranes orientation was explored. The method of introducing •OH radicals in the zeolite synthesis system and the mechanism of •OH radicals in accelerating the synthesis rate of zeolite were investigated, and the methods affecting the orientation of zeolite membranes, such as the effects of the synthesis conditions, templates and supports, were also summarized.
This provides a basis for the efficient synthesis of oriented zeolite membranes.

Microwave irradiation accelerates the synthesis rate of zeolite membranes
In the early 1990s, microwave technology began to be applied to zeolites synthesis, and a series of zeolites were prepared by researchers under microwave irradiation. [25,26] They found that the introduction of microwaves significantly increased the synthesis rate of zeolites. The process of synthesizing zeolites by this method is similar to the traditional hydrothermal method, but the difference is that in the traditional method, heat is transmitted from the outside to the inside. Under microwave conditions, heat energy is generated from the inside, which causes the atoms in the lattice to have higher mobility, greatly reduces the activation energy of the reaction, and improves the crystal nucleation rate in the induction period and crystal growth rate in the crystallization period. [18,27] In addition, microwave irradiation accelerates the dissolution of reactants (precursors or gels), resulting in a high concentration of nuclei. Subsequently, the reactants are rapidly consumed by the crystallization of nuclei, and zeolite crystals with narrow and uniform particle sizes can be formed on the surface of the support. In a system containing fluorine and seeds, Kim et al. [28] shortened the induction period through the joint action of microwave radiation and fluorine ions, thus increasing the speed of the crystallization process and rapidly synthesizing β zeolite. Li and colleagues [29] used the fast and instantaneous heating of a microwave to make the gel more uniform and easier to dissolve at the molecular level, producing more nuclei in a relatively short time and shortening the synthesis time of the zeolite phase. Highly crystalline and pure mordenite was obtained within 6 h, which improved the purity and specific surface area of the zeolites. Vichaphund et al. [18] treated ZSM-5 crystals by microwave irradiation, the crystallinity reached 100%, and the crystallization time was only 18 h. At the same time, rapid microwave heating promoted the dissolution of the precursor gel, resulting in rapid homogeneous nucleation and the formation of smaller crystals with a narrow particle size distribution. The grain size was smaller than or equal to 1 μm. Therefore, compared with conventional hydrothermal synthesis, the synthesis of zeolites by microwave radiation has many advantages, such as a short synthesis time, a narrow particle size distribution and high purity. [30] Microwave irradiation is not only an effective method to accelerate the rate of zeolites synthesis, but also an important way to synthesize high-quality zeolite membranes. In recent years, microwave-assisted in-situ crystallization and microwave-assisted secondary growth have been widely used to prepare zeolite membranes. Different from the low production efficiency of zeolite membranes caused by the large temperature gradient and low heating rate in the traditional heating method, microwaves provide fast and uniform heating, and the thermal gradient is small, which causes the zeolite membrane to form on the support surface quickly and evenly. It has been shown to be an effective method to significantly shorten the synthesis time. Xu et al. compared different models for the synthesizing zeolite membranes under conventional heating and microwave heating conditions. The role of microwave in zeolite membrane synthesis was explained. [27,31] The effect of microwave in the synthesis of zeolites can be divided into two parts: the "thermal effect" and the "microwave effect." The formation of a uniform and thin zeolite membrane resulted from both the "thermal effect" and "microwave effect," while the fast formation of zeolite membrane was mainly caused by the "microwave effect." The "microwave effect" means the changes of the characteristic of the substance in the microwave field. In the synthesis of zeolites, the "microwave effect" mainly refers to the change of the characteristic of water in the microwave field. Microwave irradiation destroys the hydrogen bridges between the water molecules by ion oscillation and water dipole rotation to result in isolated "active" water. The "active" water has a high activation energy and has a higher potential to dissolve the gel than normal water. The synthetic gel can be easily dissolved by the "active" water. Therefore, the synthesis of zeolite was promoted. The "thermal effect" refers to the fast and uniform heating of microwave. Because of the fast and uniform heating and the easy dissolution of the gel layer, a great number of zeolite nucleated and grew on the support surface simultaneously and uniformly. Therefore, uniform and small zeolite crystals can be synthesized. As a result, thin zeolite membrane can be obtained, and so the formation of a thin and uniform zeolite membrane by microwave heating was caused by both the "thermal effect" and "microwave effect." At present, many researchers have accelerated the synthesis of different zeolite membranes by microwave radiation, and the performance of the obtained zeolite membranes has been significantly improved. Xu et al. [31] successfully synthesized NaA zeolite membranes with a membrane thickness of approximately 4 μm on a porous α-Al 2 O 3 support by microwave heating, and the synthesis time was only 15 minutes, which is 10 times faster than that of conventional heating. The permeability coefficient was 3-4 times that of the conventionally heated NaA zeolite membrane. Zhou et al. [32] prepared T-type zeolite membranes by microwave-assisted hydrothermal synthesis, which required 30 h for conventional heating at 373 K. However, under microwaveassisted synthesis conditions, the nucleation rate at the interface between the seed layer and the nutrient-rich synthetic solution was accelerated, and the synthesis time was shortened to 9 h. The synthesized crystals were a&b-oriented membranes, and the performance of the membranes was improved, with higher hydrothermal stability and acid stability. Sun et al. [33] repeatedly synthesized highly (h0 h) oriented tubular silicalite-1 zeolite membranes by microwave heating, and the synthesis time was 1/5 that of conventional heating. There were few intermembrane defects and high n-butane permeability.

Microwave irradiation promotes the directional growth of zeolite membranes
A large number of studies have confirmed that microwave irradiation can not only accelerate the synthesis of zeolite membranes, but also affect the microstructure, crystal size, morphology, orientation and thickness of zeolite membranes. With microwave irradiation, depending on the bottleneck effect caused by the rapid temperature increase and the rapid and uniform growth of crystals, not only can zeolite membranes with good reproducibility and fewer intermembrane defects be formed but the crystal morphology can also be easily and selectively controlled, effectively suppressing twinning and preparing oriented zeolite membranes. [33][34][35] The bottleneck effect of microwave was explained by the P.M.Slangen's research. [36] Through the study of microwave synthesis NaA zeolite crystals, they found that the rearrangement of synthetic mixtures to yield nuclei is the bottleneck in a microwave synthesis. Specifically, the importance of aging in microwave synthesis was proved. A large number of crystal nuclei were produced in the aging process of synthetic solution, and the formation of crystal nuclei became the bottleneck in the process of microwave synthesis zeolite crystals. Relying on the unique bottleneck effect of microwave irradiation, zeolite bulk phase nucleation is effectively inhibited, effectively avoiding the formation of twins. Liu et al. [37] exploited the bottleneck effect of microwave synthesis to suppress twin growth and rapidly synthesized high b-oriented MFI membranes without a-oriented twins. In their subsequent study, they showed that by employing microwave heating, the unexpected out-of-plane twin growth was suppressed, while the epitaxial growth rate was increased, and a well-intergrown, highly b-oriented MFI membrane was formed within 2 hours. [38] Wang et al. [39] synthesized a highly oriented MFI zeolite membrane with chitosan and microwaveassisted seed growth. The large number of amino and hydroxyl groups in chitosan adsorb TPA + and zeolite precursors. Compared with conventional heating, microwave heating ensures crystal growth along the in-plane direction. A continuous and thin pure b-oriented MFI zeolite membrane with a thickness of approximately 600 nm was prepared in an ultradilute synthetic solution.
Although the preferred orientation is usually desirable, most zeolite membranes currently available are randomly oriented. The factors affecting the membrane orientations with microwave irradiation were investigated. Not only the importance of aging on the formation of zeolite membranes has been confirmed, but also the important influence of crystallization conditions on the oriented formation of zeolite membranes has been found. [40,41] Li et al. [34] showed microwave-assisted aging significantly affected the film continuity, and a diagram of film continuity synthesis conditions was obtained  [34] Reproduced by permission of American Chemical Society.
( Figure 3), which may be helpful to prepare continuous b-oriented MFI zeolite films on various substrates. The relationship between the film continuity and microwave-assisted aging conditions was shown in the lower part of the figure. The upper part of the figure shown the relationship between the film continuity and crystallization conditions. Moreover, zeolite crystals with different preferred orientations (CPO) or morphology can also be synthesized under microwave assistance by controlling temperature and power. Motuzas et al. [42] showed that microwave heating significantly activated the growth of the membrane at very short synthesis time and low temperature. The silicaliye-1 membranes prepared at 453 K, the more obvious was c-orientation (c-axis-CPO), in which defects seem to be easier to form (Figure 4(a)). For the silicaliye-1 membranes prepared at medium temperature (433 K) with an oblique crystallographic preferred orientation (101-CPO) (Figure 4(b)). The performances of the membranes were evaluated. An oblique crystallographic preferred orientation (101-CPO) was found whose N 2 permeance reached 1.5 at 294 k × 10 −6 mol m −2 S −1 Pa −1 at 294 K, and the c-oriented membranes (c-axis-CPO) have very low ideal selectivity. Vichaphund et al. [18] studied the formation of ZSM-5 zeolite under different power. At low microwave power  [42] Reproduced by permission of Elsevier.
(400 W), the particles was spherical in shape and agglomerated with size less than 5 μm. When increasing microwave power from 600 to 1000 W, the ZSM-5 crystals were more cubic in shape with small size less or equal than 1 μm. Madhusoodana et al. [35] synthesized ZSM-5 zeolite membranes, which showed less orientation at 25% and 50% power, but a greater degree of preferred orientation at 100% power. Therefore, different microwave conditions may affect the orientation of the membranes. Oriented zeolite membranes obtained under microwave irradiation is affected by many factors. Except for the microwave synthesis conditions, the use of microwave reactor also has a certain effect on their orientation, which will be discussed in section 2.3.
The microwave method has a positive effect on the synthesis of zeolites, which can accelerate the collision of polar molecules, destroy the structures of reactants and promote the transfer of the zeolite phase. However, due to the high crystallization rate obtained by microwave heating, the product may be in a less stable phase. [19] For example, Xia et al. [43] promoted direct phase transfer from amorphous zeolites to EMT zeolites by microwave heating. EMT zeolite with high crystallinity were obtained by microwave heating at 353 K for 30 min. However, Yang et al. [44] used microwave heating at 453 K to crystallize SPAO-5 membranes in situ on porous α-Al 2 O 3 substrates with continuity and well-intergrown, but the orientation was random, and most of the grains lost their shape. Therefore, exploring and selecting the appropriate microwave synthesis conditions for the preparation of high-quality oriented zeolite membranes is very important.
Although the preparation of zeolites by microwave method has been widely studied, the research on microwave synthesis zeolite membranes, especially the oriented zeolite membranes, is still in its infancy. To prepare high quality zeolite membranes, it is of great significance to understand their mechanism fundamentally. In the current investigation, Li et al. [45] synthesized NaA membrane by in situ aging-microwave heating method, the whole formation process of its synthesis was characterized by employing a series of characterization methods including gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance/ Fourier transform infrared spectroscopy (ATR/FTIR), and gas permeation. And the formation mechanism of LTA zeolite membrane was proposed (as shown in Figure 5).
In addition to the research on the formation mechanism of LTA membrane by Li et al., most other researchers speculated on the mechanism of microwave accelerating zeolites synthesis and affecting their orientation [32,34,46] : (1) Microwave radiation breaks the hydrogen bridge between water molecules by ion oscillation and water dipole rotation and produces isolated activated water. The pair of active water molecules and OH groups have a better ability to dissolve gels than ordinary water, accelerating the synthesis of crystals. (2) The rapid and instantaneous heating achieved by microwaves causes the gel to be easier to dissolve and more uniform and generates high-concentration nuclei in a short time, which accelerates the crystallization of zeolites. (3) The high supersaturation of the gel and fast and uniform growth achieved with microwave irradiation may account for the formation of the highly oriented layer. (4) The bottleneck effect related to nucleation with microwave irradiation can effectively suppress twinning, which is beneficial to the formation of highly oriented zeolite membranes. These summarized points and the comparison model of zeolite membranes synthesized by microwave heating and conventional heating are shown in Figure 6. Although these researchers have explained the role of microwave in zeolite membranes synthesis, their formation mechanism has not been systematically studied. At present, there are few reports on the formation mechanism of zeolite membranes under microwave irradiation, and the growth mechanism of zeolite crystals with microwave irradiation is not clear. Therefore, the systematic characterization of the synthesis process to further explore their mechanism is of great significance for the preparation of high-quality zeolite membranes under microwave irradiation. . Schematic illustration of the proposed formation mechanism of LTA zeolite membrane synthesized by "in situ aging-microwave heating" method. [45] Reproduced by permission of Elsevier.

Microwave reactors
In the preparation of zeolite membranes by microwave method, microwave reactor is used, which also has a great impact on their synthesis. According to the difference of microwave operating modes in the resonant cavity, microwave reactors are divided into single-mode and multi-mode microwave reactors. [47,48] Single-mode means that there is only one mode and one microwave distribution, which can easily realize uniform heating and achieve ideal repeatability. [49] Multi-mode is that there are multiple modes in a cavity, which has some defects in heating uniformity and repeatability. [48] The formation of zeolite membranes is affected by different microwave waveform modes. Liu et al. [38] found that there is a large microstructure difference between MFI membranes prepared by single-mode microwave heating and traditional/multi-mode microwave heating. B-axis-oriented MFI membranes with a few twins were grown by single-mode microwave heating epitaxy, which were not affected by the substrate surface properties. This was achieved precisely because single-mode microwave heating has obvious advantages in microwave field uniformity and intensity compared with multi-mode microwave heating. In their subsequent studies, [50] they also confirmed the role of single-mode microwave-assisted secondary growth in the preparation of oriented zeolite membranes. Hierarchical MFI zeolite membranes comprising higher degree of (h0 h) preferentially oriented ultrathin (approximately 390 nm) selective top layers and porous intermediate layers on porous α-Al 2 O 3 substrates were fabricated. The separation factor (SF) of equimolar n-/ i-butane mixture through the membrane reached as high as 44.4 with a n-butane permeance of 3.3 × 10 −7 mol −1 m −2 s −1 Pa −1 . At the same time, they proved that due to the advantages of single-mode microwave heating in Figure 6. Mechanism of zeolite membranes microwave synthesis and comparison model of zeolite membranes synthesized by microwave heating and conventional heating microwave field uniformity and intensity, the reaction temperature and time required to form a good symbiotic MFI zeolite membrane were reduced. [50] A sub-100 nm thick b-oriented MFI layer can be successfully obtained at 100°C for 2 h. [51] Besides, when zeolites are synthesized by microwave method, the geometry of the reactors also has a certain impact on the crystallization rate of zeolites. Panzarella et al. [52,53] showed that in a microwave synthesis system, the geometry of the reactors needs to be considered, and the relative synthesis rates of zeolites synthesized in different reactors are different. Using a microwave oven with a larger reactor (33 mm) results in a shorter induction time and faster crystallization than using a microwave system smaller reactor (11 mm), and the microwave synthesis rate is higher. Furthermore, it is also very important to consider the material of the reactors. In terms of their safety performance, metal utensils, such as iron, aluminum, and stainless steel, cannot be used in microwave ovens, and hydrothermal reactors are a stainless steel products. Consequently, heating the hydrothermal reactor in a microwave oven produces electric sparks, and microwaves are reflected, which damages the furnace body. Moreover, the self-generated pressure in the reactor rises rapidly under microwave conditions, which also results in unsafe conditions. At present, the microwave equipment commonly used is still household microwave ovens or modified microwave ovens, as professional equipment is unavailable, and the utilization rate of microwave energy is low (shown in Table 1). Therefore, the harsh synthesis conditions, the lack of professional microwave equipment and the low energy utilization are still the problems to be solved for the efficient preparation of directional zeolite membranes by microwave irradiation.
The influence of microwave reactors and other factors may restrict the industrial application of microwave synthetic zeolite membranes. Up to now, some progress has been made in the industrial scale-up of NaA zeolite membranes. Li's research group [27] has realized the scale-up of NaA zeolite membranes by combining hydrothermal synthesis and microwave heating technology, and a pilot scale vapor permeation dewatering installation based on NaA zeolite membranes has been set up. And then a 30,000 tons per year ethanol/water separation industrial demonstration unit was built in 2016. [66] However, they believe that microwave synthesis zeolite membranes is still in the early stage, and there is still a certain distance to realize the industrial production of directional zeolite membranes by microwave method. Microwave conditions, reactor design, and mass and heat transfer effects may hinder their application scope and scale-up. Firstly, due to the difference of microwave radiator, output power and reactor used, compare the experimental results obtained from various laboratories is difficult, which seriously affects the reproducibility of microwave synthesis. [67] Meanwhile, because some mechanisms of microwave heating are not very clear, it is difficult to accurately predict and design the reaction conditions and equipment, resulting in relatively few microwave equipment that can be used for the reaction. Secondly, the penetration of microwave in various materials is limited, and the current microwave reaction tanks are relatively small. [68] How to ensure the mass and heat transfer of the reaction system is the key to realize the scale-up test. For example, the increase of precursor volume has a negative effect on the reaction rate. [69] In order to make the reaction go smoothly, the effect of mass and heat transfer of reaction raw materials must be ensured. Moreover, the stability and safety protection of large-scale microwave equipment are also problems that need to be solved. Thirdly, the industrial production of most zeolites is a batch process. Considering the large volume involved, the industrial application of microwave is facing challenges. The expansion of microwave continuous synthesis to the industrial level is obviously complicated because of reactor engineering and microwave application device engineering. [67] In addition to the above problems, the problems of microwave frequency, dielectric properties of materials, uniform distribution of microwave field and energy utilization need to be further studied and solved to realize the industrialization of zeolite membranes. [70] In conclusion, the rapid and uniform heating of microwave, the bottleneck effect related to nucleation and application of single-mode microwave reactor play a positive role in the efficient synthesis of oriented zeolite membranes, which have the advantages of short synthesis time, high purity, good membrane reproducibility and high orientation. However, at present, there is a lack relevant research on the membranes-forming mechanism of zeolite membranes synthesized by microwave method. It is of great significance to systematically explore the synthesis process by using various characterization methods. The microwave irradiation method faces many problems, such as harsh microwave conditions, low energy utilization, high equipment requirements, lack of professional microwave equipment, and certain risks. Its application in the synthesis of directional zeolite membranes is still in the laboratory stage, and has many shortcomings. The development and design of microwave equipments is still a challenge for the rapid preparation of oriented zeolite membranes by microwave method.

Free radical accelerated synthesis of zeolites
In addition to the use of microwave radiation to accelerate the synthesis rate of zeolites, hydroxyl radicals are another proven effective method to improve the synthesis of zeolites. In 2016, Yu's research group [71] first introduced •OH radicals into the zeolite synthesis system, and based on the [SiO 2 (OH)-O-SiO 3 ]Na 5 calculation model, the •OH-accelerated zeolite crystallization NaA [71] In situ UV radiation at 298 K NaA [72] In situ γ-ray irradiation at 20°C -The crystallization time of NaA zeolites is shortened to 18 h The adsorption capacity of CO 2 is 6 times higher than that of non irradiated NaA zeolites γ-ray source, stainless steel autoclave T [73] In situ   [139] In situ N-propylammonium (PA) and n-ethylhexamethylene tetraammonium bromide High silicon MFI zeolite membrane with a b-axis preferred orientation Not tested substrate MFI [159] In Porous glass TS-1 [160] In situ Polyethylene oxide monolayer (PEOM) used as a substrate template A highly preferred (010) orientation and a small amount of inplane deformation Not tested (Continued)  [161] In situ A chitosan-modified substrate introduced a large number of amino and hydroxyl groups B-axis-oriented continuous membrane, with a uniform grain size in the shape of a coffin, and average dimensions of approximately 5 μm in length, 3 μm in width, and 0.4 μm in thickness It has competitive a reaction rate and high selectivity for alcohol products A [162] Seed method Modification of support by cationic polymer polydimethylpropyl ammonium chloride (PDDA) Type A zeolite membrane with high (200) orientation and 1 mm thickness is repeatedly formed, which is pure, dense and well adhered H 2 /N 2 permeation selectivity is 4.80.
MFI [152] In situ SiO 2 coating Columnar b-oriented MFI crystal with good symbiotic behavior, and a thickness of approximately 2-3 mm.
Not tested.
Zirconia (YSZ) hollow fiber (flat/curved) MFI [163] Seed method Aluminum free yttrium stabilized zirconia hollow fiber Thin layer with a thickness of approximately 3 μm It has high selectivity for ethanol and water. The separation factor of 5 wt% ethanol/water at 60°C is 47.
MFI [150] In situ SiO 2 coating b-oriented MFI membranes of approximately 500 nm Not tested.
Porous stainless steel MFI [136] Seed method Mesoporous SiO 2 layer and covalently connected with 3-chloropropyltrimethylsilane b-oriented MFI membrane with good symbiosis Not tested. mechanism was elucidated through theoretical calculations. At present, the synthesis rate of ZSM-5, silicalite-1, NaA, SAPO-34 and other molecular sieves has been significantly improved by the action of free radicals (shown in Table 2). However, the •OH radicals in the system usually have very short lifetimes, so appropriate methods are needed to introduce •OH radicals. [79] The common methods for introducing free radicals include physical methods and chemical methods. [80]

Physical methods
The common physical methods for introducing free radicals are the physical radiation method and activated seeds method. Among them, the physical radiation method includes ultraviolet radiation, γ-ray radiation and ultrasonic treatment. By applying physical radiation to the synthetic precursor solution or the reaction kettle into which the synthetic liquid is placed, free radicals are generated in the synthesis system, and the synthesis rate of zeolite is improved. However, this method requires additional radiation equipment and has high requirements for experimental equipment. The zeolite seed crystals are activated by grinding to accelerate the synthesis of zeolites.

UV radiation.
Ultraviolet (UV) radiation is a common physical method used to produce hydroxyl radicals. [81] UV radiation can be combined with hydrogen peroxide, ozone or other chemical initiators to effectively produce hydroxyl radicals. [82,83] Formulas (1)-(4) show the generation of hydroxyl radicals by ozone under the action of ultraviolet light. [84,85] Moreover, because the UV treatment method occurs at low temperature, UV/ O 3 treatment can also minimize the generation of cracks and defects during template removal, which can be used to prepare high-quality zeolites. [86] Furthermore, Feng et al. [71] accelerated the crystallization rate of zeolites through UV radiation and showed that hydroxyl radicals play an important role in the nucleation stage in the crystallization process. They formed highly crystalline NaA zeolites in 40 hours, while it took 55 hours under non-UV conditions. The ultraviolet radiation method has been proven to accelerate the synthesis of zeolites, but compared with the normal hydrothermal method, stricter experimental conditions and experimental equipment are needed.

γ-ray radiation.
Gamma rays can also accelerate the synthesis of zeolites by radiating the hydrothermal synthesis system to produce free radicals. [87] γ-rays can penetrate stainless-steel autoclaves to reach internal water and produce a very high concentration of •OH in a fraction of a microsecond, which cannot be achieved by ultraviolet light. Cheng et al. [72] first reported that γ-rays were introduced into the synthesis of zeolites, and the results showed that the introduction of γ-rays into the synthesis system greatly accelerated the crystallization process of NaA, NaY, silicalite-1 and ZSM-5 molecular sieves, and the performance of the zeolites was significantly improved. The prepared NaA zeolite has a larger pore volume and specific surface area, and the adsorption capacity of pure CO 2 is increased by 6 times (from 1.98 wt% to 12.83 wt%); ZSM-5 exhibited more brønsted acid sites(the substance that can provide hydrogen protons), which significantly enhanced the mass transfer and catalytic performance of MTG (methanol to gasoline technology). Figure 7 shows the rapid synthesis mechanism of NaA zeolites under γ-ray irradiation.

Ultrasonic treatment.
Ultrasonic treatment is another physical radiation method used to accelerate zeolite synthesis. In the ultrasonic radiation treatment process, a large number of shock waves release energy during the bubble collapse process in a liquid, which helps produce free radicals through the thermal cracking of water molecules. Water molecules and oxygen molecules in cavitation bubbles are excited and dissociated at the same time to form reactive species, such as hydrogen atoms, hydroxyl radicals and superoxide radicals, [88,89] as shown below in Formulas (5) -(8): The bubble collapse caused by ultrasound provides a large energy density for crystallization. [90] In the sonochemical reaction, H 2 O, as a proton donor, forms a hydrogen bond with the oxygen atom of SiOH. Using these strong oxidants and reducing agents, the formation of Si-O-Si and Si-O-Al bonds can be completed in the reaction medium ( Figure 8). In the alkaline environment, the dominant silicate species are anions. The first step is the formation of a Si-O-Si bond between two molecules and the second step is the removal of H 2 O molecules from the dimer species. In another mechanism, the most stable species in strong alkali solution are Si(OH) 4 − and anion Al(OH) 4 − . The two species combine to form an Si -O-Al bond (Figure 8(a)). [91] For the formation of Si-O-Si bond, the most stable species in strong alkali solution are Si(OH) 4 − , two Si(OH) 4 − species combine with five coordination species through transition state to form Si-O-Si bond. They form free radicals through ultrasonic cavitation, and then remove water molecules (Figure 8(b)). [92] With ultrasonic treatment, many zeolites have been rapidly synthesized by researchers. Lee et al. [93] prepared 4A zeolites under ultrasonic conditions, which have a lower crystallization temperature and shorter crystallization time. Tiffany et al. [94] carried out ultrasonic treatment on zeolites in the aging stage before hydrothermal synthesis, which accelerated the synthesis of the zeolite RHO and shortened the synthesis time to 2 days (8 days for conventional hydrothermal treatment). Moreover, with the ultrasonic method, uniform nucleation of crystals can occur, and the crystallization time at room temperature can be greatly shortened. Bose et al. [92] rapidly synthesized a DDR zeolite membrane by using an ultrasonic-mediated method with uniform nucleation and greatly shortened the crystallization time at room temperature.

Activated seeds
The activation seed method is another physical method different from radiation that produces free radicals. Among the common methods of zeolite synthesis, the seed method effectively decouples the nucleation and growth of zeolites. [13] At the same time, the structure-directing effect and nuclei source effect of crystal seeds bring higher relative crystallinity and a lower waste discharge. [95] When Liu et al. used the seed method to assist in the synthesis of zeolites, the crystal seeds could also improved the synthesis rate and crystallinity of zeolites Figure 8. The proposed mechanism for the formation of aluminosilicate dimer (a). [91] and aluminate dimer (b). [92] Reproduced by permission of Elsevier and Royal Society of Chemistry. and significantly reduced the amount of water and template agent. [95,96] By ball milling or grinding the crystal seed, the silicon dioxide Si-O-Si bond breaks during the grinding process, which can produce silicon-based free radicals, namely, surface nonbridged oxygen hole centers (NBOHC,:Si-O•) and E' centers (Si•). [97,98] They can easily react with water to form •OH, thereby accelerating the synthesis of zeolites. Compared with ordinary seeds, the activated seeds react with water to produce additional •OH, which can also enhance the crystallization effect. Zhang et al. [75] accelerated the synthesis of β zeolites by grinding seeds in the absence of organic templates, and in the presence of suitable excited seeds, the crystallization time was significantly shortened by 30 h, which was a 30% reduction. Cheng et al. [76] obtained free radical crystal seeds by grinding or heating to accelerate the synthesis of a silicalite-1 zeolite. The morphology of heated crystal seeds is almost the same as that of normal crystal seeds, but the ground seeds are gathered together due to the grinding effect. In this study, Si• was not found in the E' center, which may be oxidized by oxygen to form Si-O At the same time, their research found that the longer the seed crystal is ground, the better the crystallization effect of the seed crystal. Increasing of the grinding time may break all the Si-O-Si bonds in the seed crystal, resulting in the collapse of the skeleton structure of the seed crystal and loss of its own structural guidance so that the target product zeolite cannot be synthesized. [76]

Chemical method
The chemical method is different from the physical method. It has low equipment requirements and does not require the use of additional equipment. A free radical initiator is added to the synthetic solution to produce free radicals. For example, Fenton's reagent is introduced to produce hydroxyl radicals by the Fenton reaction or persulfate is used, which is also a common method to produce free radicals.

Fenton
The Fenton method uses Fe 3+ and Fe 2+ as catalysts to decompose hydrogen peroxide in an acidic medium to produce hydroxyl radicals, [99] as shown below in Equations (9) and (10): The Fenton reaction has mild conditions and convenient operation, but it has a narrow pH range. [100] The hydroxyl radical produced by Fenton's reagent can greatly shorten the induction period of the zeolite and reduce the apparent activation energy of crystal growth, thus accelerating the crystal growth process and promoting the crystallization of zeolites. Since H 2 O 2 self-decompose at higher temperatures, it decompose into •OH radicals at temperatures no higher than 80°C. Moreover, the aging of the amorphous gels increase the number of nuclei at low temperature, which thus was in favor of shortening the induction time of zeolite synthesis and accelerating the crystallization process. [101] Therefore, the promotion of Fenton's reagent at lower temperatures is stronger than that at higher temperatures. Gou et al. [23] accelerated the crystallization process of the nucleation and growth of Y zeolite by introducing Fenton's reagent. Depending on the temperature, the induction period was shortened by 1-8 hours. Although the use of Fenton's reagent as a free radical initiator causes the doping of heteroatomic iron ions and ferrous ions, which affects the crystallization of zeolites, some studies have proven that the introduction of Fe heteroatoms has a less negative effect than the acceleration of hydroxyl radicals. [23]

Persulfate(S 2 O 8 2-
) is also a common hydroxyl radical initiator used to promote the crystallization of zeolites. [102] With persulfate as an additive in hydrothermal synthesis, S 2 O 8 2easily generates free radicals over a wide pH range (SO 4 − •), [77,103] and the generated SO 4 − • free radicals can further generate •OH to accelerate zeolite synthesis, as shown below in Equations (11), (12) and (13): The research shows that the introduction of persulfate not only improves the synthesis rate of zeolites but also enhances the crystallization effect of zeolites and reduces the use of template agents. Zhang and his colleagues [104] synthesized NaA zeolite by pyrolyzing of sodium persulfate at a certain temperature to produce hydroxyl radicals. Unlike the case with no persulfate, the main phase in the early stage was sodalite, and then, pure sodalite was quickly generated. While improving the synthesis efficiency of zeolites, the phase selectivity of the zeolite was also improved, and a purer and better crystallized zeolite was obtained. Cheng et al. [77] shortened the synthesis time of silicalite-1 from 24 h to 16 h in the presence of persulfate and significantly reduced the use of the template. Only half of the organic template was used to synthesize a highly crystalline silicalite-1 zeolite. In addition, using sodium persulfate as a hydroxyl radical initiator, silicate polymerization generates more active silicon radicals through interaction with •OH. Therefore, due to the tetrahedral coordination of silicon in the skeleton, more silicon species penetrate the zeolite skeleton. With increasing the crystallization time, the proportion of Si (3Si, 1Al) increases, which promotes the synthesis of zeolites with a high silicon aluminum ratio. [78] Sodium persulfate as an •OH initiator also provides an effective strategy for the synthesis of aluminum rich zeolites. [105] Relying on the free radical initiator sodium persulfate to promote the dissociation of Si-O-Si bonds and the polymerization of Si-O-Si/Al bonds in neutral medium, mesoporous aluminosilicate with high aluminum content is obtained, and the content of Al can be adjusted by changing the amount of sodium persulfate. It has been reported that SO 4 2has the same effect as S 2 O 8

2-
, which provides a basis for the acceleration of zeolite synthesis with sulfate ions. The introduction of hydroxyl radicals by physical and chemical methods to accelerate the synthesis rate of zeolites has been confirmed by most researchers. Whether the introduction of free radicals has a certain impact on the morphology and orientation of zeolite crystals, the SEM images were analyzed. Figure 9(a) shown the NaA zeolite particles obtained by Cheng et al. under the gamma ray of 6.10 kGy/h. [72] With the extension of irradiation time, the spherical phase changes to cubic crystal. The NaA zeolite with small spherical crystals was obtained in the presence of sodium persulfate by Zhang et al [104] (Figure 9(b)). Figure 9(c-d) shown MFI zeolite crystals formed after Figure 9. a.) NaA zeolite, [72] b.) NaA with small spherical crystals, [104] c.) silicalite-1 crystals with the uniform spherical aggregates of 100 nm nanocrystals, [77] d.) ZSM-5 crystals with spherical sample, average particle size is 81 nm, [78] e.) cross sectional view of DDR zeolite membrane layer with a thickness of approximately 20-25 μm. [92] Reproduced by permission of Elsevier and Royal Society of Chemistry.
introducing free radicals, all of which are spherical. [77,78] Figure 9(e) shown the cross-sectional view of the DDR membrane with the aid of ultrasound. [92] It can be seen that the introduction of free radicals can form zeolite crystals with good crystallization and small size. However, whether free radicals have a certain effect on their morphology and orientation in this system has not been found in these references. Therefore, it is not clear whether free radicals will affect the orientation of zeolite crystals.

Mechanism of hydroxyl radicals in the zeolite synthesis system
Aiming at the problem of whether the directional zeolite membranes can be synthesized in the free radical system, the mechanism of hydroxyl radical in accelerating the synthesis of zeolites has been investigated. In some aluminosilicate molecular sieves, Si(OH) 4 and Al(OH) 4 − forms are the main reactants that form zeolites. In an alkaline environment, two stable Si(OH) 4 and Al(OH) 4 − species can be condensed to form a fivecoordinated intermediate with a Si-O-Al bond. OH − ions can increase the coordination between tetrahedral silicon and pentahedra or octahedra under alkaline conditions to weaken and destroy the Si-O-Si bond. Hydroxyl radicals have higher activity than OH − ions and can better stabilize the reaction intermediates. Moreover, theoretical calculations show that •OH is more conducive to the fracture of Si-O-Si bonds in the system than OH − due to its unique activity. [106,107] •OH only needs 16.7 kJ/mol energy to catalyze the fracture of the Si-O-Si bond in quartzite, while 79.1 kJ/mol is required under traditional OH-ion conditions. Studies have shown that the introduction of hydroxyl radicals into the system can accelerate the nucleation and growth of zeolites, especially in the nucleation stage of zeolites. However, due to the dark environment of the hydrothermal kettle and the short lifetime of hydroxyl radicals, the specific mechanism through which •OH radicals affect zeolite synthesis is not clear, and there are few reports on this aspect. In the literature, researchers usually use the indirect method to determine free radicals, commonly use electron paramagnetic resonance (EPR) technology to capture hydroxyl radicals and use density functional theory (DFT) to carry out theoretical calculations. To date, some researchers have studied the condensation mechanism of SiO 4 units and SiO 4 or AlO 4 units to Si-O-Si and Si-O-Al after introducing •OH radicals through the simulation of Si(OH) 3 ONa and Al(OH) 4 Na monomers [108] or the calculation of an [SiO 2 (OH)-O-SiO 3 ]Na 5 model. [71] As shown in Figure 10, Wang et al. [108] simulated Si(OH) 3 ONa and Al(OH) 4 Na monomers and deeply understood their mechanism through DFT calculations. The condensation reaction of Si(OH) 3 ONa or Al(OH) 4 Na monomers with the formation of Si(OH) 3   ratio in the synthesis process. Under very basic conditions, Feng et al. [71] dissolved a gel by breaking Si-O-Si bonds and studied the attack of OH − or OH with a highly deprotonated [SiO 2 (OH)-O-SiO 3 ]Na 5 gel model. The formation process of Si-O-Si bond is similar to that of Wang. Therefore, the introduction of free radicals not only accelerates the synthesis rate of zeolites and shortens their synthesis time, but is also conducive to the synthesis of high-silica zeolite. However, because •OH radicals promote zeolite synthesis by promoting the fracture and reconstruction of Si-O-Si bonds, the lack of a silicon source for SAPO zeolites hinders the application of this strategy. [109] However, if the silicon source in the system is increased, more free radical initiators are needed to depolymerize the Si-O bond, and too many free radical initiators reduce the crystallinity of zeolites and form amorphous silica. Through the sonochemical method, Askari et al. [110] found that a fully crystallized small SAPO-34 crystal can be formed in only 1.5 h, and the prepared sample has a high silicon-aluminum ratio. However, its skeleton is unstable, and a considerable part of the SAPO-34 crystal phase is transformed into the dense phase of the AlPO4 structure, i.e., calcite. Zhou et al. [111] also studied the effect of hydroxyl radicals on the synthesis of the SAPO-34 molecular sieve, indicating that the presence of hydroxyl radicals can significantly shorten the induction period of SAPO-34 and accelerate the crystallization process. However, free radicals are conducive to the formation of silicon (0Al) islands, which change the chemical environment of silicon atoms in the crystallization process. Therefore, the crystallization mechanism of SAPO-34 is different in the presence of •OH free radicals.
In summary, in the zeolite synthesis system, especially in the synthesis of silicon aluminum molecular sieves, the introduction of hydroxyl radicals through physical methods (physical radiation, activating crystal seeds, etc.) or chemical methods (Fenton's method, S 2 O 8 2-, etc.) is more conducive to the fracture and formation of Si-O-Si bonds, which can effectively accelerate the synthesis of zeolites. Moreover, it also plays a certain role in reducing the amount of template or preparing a high-silica zeolite. The physical radiation method can produce free radicals through radiation, which requires more rigorous equipment and experimental conditions than the chemical method. The chemical method can be conducted in the original hydrothermal kettle, and the equipment requirements are low. However, the existing research shows that researchers have improved the synthesis rate of zeolites by introducing free radicals, but there is also a lack of directional control of zeolite membranes.
Through the investigation on the mechanism of hydroxyl radical accelerating the synthesis of zeolites, it is found that hydroxyl radical mainly accelerates the synthesis of zeolite by accelerating the fracture and formation of Si-O-Si bonds. Whether this system can be organically combined with the synthesis of directional zeolite membranes, it is necessary to systematically investigate the directional preparation of zeolite membranes and explore the universal law of the preparation of directional zeolite membranes. Finally, a path to realize the efficient synthesis of directional zeolite membranes may be found.

Control of the growth direction of zeolite membranes
The improvement of zeolite membranes performance mainly depends on the optimization of their microstructure, including the crystal size, shape, grain boundary and channel orientation control. Among them, the crystal orientation has a strong impact on their performance. [112] A zeolite membrane with a preferred crystal orientation shows better performance than a zeolite membrane without the preferred crystal orientation in separation and sensing performance, corrosion resistance and dielectric properties. Therefore, it is very important to prepare zeolite membranes with preferred orientations. An introductory membrane figure is shown in Figure 11, explaining orientation (a and b) and the importance of orientation. Researchers have used the competitive growth model to explain the formation of an ordered zeolite membrane structure with crystallographic orientation (Figure 12). A crystal starts to grow from a crystal nucleus or a seed with a disordered structure. Each anisotropic crystal, grows rapidly along the direction of fastest crystal growth (i.e., the long axis), while a crystal growing along other directions has lower growth speed or may even stop growing because of interactions with the faster growing crystal. The final result is that the crystal grows on the surface of the support perpendicular to its long axis and finally occupies the dominant position. [113] However, membrane orientation control is not limited to the vertical direction of the channel. A suitable method was found to prepare zeolite membranes with any desired orientation and it is widely used in any desired direction. [114,115] At present, AEL, [116] MEl [114] and MFI [117] zeolite membranes with preferred orientations have been studied.
In situ hydrothermal crystallization and secondary growth methods are commonly used for the synthesis of oriented zeolite membranes except microwave method. In in situ hydrothermal synthesis, the support is directly placed in the mother liquor of zeolite synthesis, which is simple and easy to realize. The secondary growth method can effectively decouple the nucleation and growth of crystals, and allows easier control of the oriented growth and microstructure of zeolite membranes. At the same time, it is less affected by synthetic solutions and has wide operating conditions. It is a more effective method for preparing oriented zeolite membranes and controlling their thicknesses. For both the in situ hydrothermal crystallization method and the secondary growth method, oriented zeolite membranes can be obtained by adjusting the composition of the synthetic solution and modifying the substrate surface. Besides, the secondary growth method can also be realized by controlling the orientation of the seed and the direction of the depositing seed layer. In short, many factors affect the orientation of the zeolite membrane. In this section, we investigate and summarize the common factors that affect the orientation of zeolite membranes, such as the composition of the synthetic solution, the template and support properties (shown in Table 3).

Figure 12.
Schematic diagram of the competitive growth mechanism. [113] Reproduced by permission of Elsevier.

Effect of the synthetic solution compostition
The synthetic solution of a zeolite membrane includes silicon and aluminum sources, silicon aluminum ratio, alkali and water as a solvent. Adjusting the composition of the synthetic solution affects the orientation of the zeolite membrane.
Zeolite membranes are usually synthesized in alkaline solution. The appropriate alkalinity can ensure that the zeolite membrane has good crystal intergrowth and small intercrystalline gaps, [118] and can adjust the surface morphology and crystal orientation of the membrane to improve the membrane performance. Soydas et al. [119] studied the effect of alkali concentration on the morphology of MFI zeolite membranes, and successfully synthesized oriented MFI zeolite membranes on a seeded α-Al 2 O 3 disk. The results showed that the crystal orientation was sensitive to the OH − /Si ratio of the mixture. When OH − /Si ≤ 0.64, a membrane with a preferential orientation formed, and the length diameter ratio of the crystal increased with decreasing synthetic alkalinity. At low alkali concentrations, it presented a (h0 h)/c-axis orientation, showing the ideal selectivity of H 2 /n-C 4 H 10 and CH 4 /n-C 4 H 10 . Liu et al. [120] prepared an MFI zeolite membrane by changing the alkalinity (OH − /Si) and water content (H 2 O/Si) of a synthetic liquid without a template. When OH − /Si was 0.56 and H 2 O/Si was 270, the MFI zeolite membrane was continuous and had a preferred b-axis orientation, and the membrane thickness was approximately 4 μm.
The introduction of alkali will inevitably lead to the introduction of alkali metal ions, which also has an impact on the membrane. For example, alkali metal Na + ions can accelerate the nucleation and growth of crystals and membranes and reduce intermembrane defects. Wang et al. [121] studied the control of Na + on an MFI membrane. A continuous highly b-oriented MFI membrane can be obtained at medium OH − /Si values (0.32) and low Na + / TPA + values (less than 0.31). However, with an increasing Na + ion content, the membrane orientation gradually changes from a b-axis orientation to a random orientation. Further research by Xu [122] showed that when the Na + ion concentration continued to increase, a large amount of sodium ions retarded nucleation and led to the formation of gel particles. The zeolite crystals in the membrane layer became thicker, and the crystals changed from a random orientation to a c-axis orientation. The separation factor of ethanol aqueous solution could be increased to 76. Additionally, increasing the water content of the synthetic solution and reducing the supersaturation degree in the synthetic solution are conducive to the in-plane growth of zeolite crystals, reduce the epitaxial growth rate of zeolite seeds, and reduce the thickness of the membrane. For example, Lu [123] obtained an MFI zeolite membrane with a highly preferred b-axis orientation in an ultradilute solution by a secondary growth method. Studies have shown that this is because the increase in the amount of water inhibits the crystallization and nucleation of the synthesis solution, and the nutrients in the synthesis solution are all used for seed epitaxial growth, which inhibits the twinning of the a-axis orientation. This method is applicable not applicable to highly oriented MFI zeolite membranes. Li et al. [124] also used the secondary growth method to prepare zeolite beta membranes with controllable orientation on randomly oriented seed layers by adjusting the H 2 O/SiO 2 ratio. When H 2 O/SiO 2 was 4, a β zeolite membrane with a preferential (h0 l) orientation could be obtained, while when H 2 O/SiO 2 was 7, the zeolite membrane had a c-axis orientation and separation factors of 36.5 and 32.4 for 1 and 5 wt% n-butanol/water mixtures, respectively. This phenomenon was also confirmed by the preferentially oriented MEL molecular sieve membrane prepared in dilute solution by Dong et al. [114] However, too much water not only reduces the supersaturation degree of the synthesis solution, but may also affect the growth of crystals, resulting in slow growth.
Adding appropriate additives or inhibitors (such as NH4 + , and starch) to the synthesis solution can also regulate the orientation of the membrane and inhibit the formation of twins to obtain the directional zeolite membrane. As shown in Figure 13, the addition of NH4 + can be used as a secondary growth regulator, which has a certain regulatory effect on the crystallization process. [125] The results show that the strong interaction of silicate-NH4 + is Figure 13. Formation diagram of twins on the seed layer of a synthetic solution with different NH4 + contents. [125] Reproduced by permission of Royal Society of Chemistry stronger than that of TPA-NH 4 + , which can effectively inhibit nucleation in the secondary growth process. Therefore, it can promote the b-axis oriented growth of the MFI zeolite membrane, and then the b-oriented MFI zeolite membrane can be obtained. This phenomenon has also been confirmed by other researchers. Banihashemi et al. [126] prepared an MFI zeolite membrane with a highly preferred b-axis orientation and tight filling by adding (NH 4 ) 2 SO 4 or H 2 SO 4 to adjust the pH value and ammonium salt to inhibit nucleation in bulk solution, and this synthesie was performed without a template on a stainless steel plate coated with crystal seeds. By adding cheap starch and urea to aluminosilicate gel, flaky and chain-like ZSM-5 crystals with adjustable b axis lengths were also successfully synthesized by Xiao's research group. [127] They found that the urea additive dispersed and adsorbed mainly on the (010) surface and inhibited the growth of the ZSM-5 zeolite along the b axis, and a thin sheet of the ZSM-5 molecular sieve was obtained. The condensation of hydroxyl groups in starch lead to the accumulation of crystals along the (010) plane (b-axis direction) to form ZSM-5 chains. The catalytic test on changing m-xylene to p-xylene shows that the chains of ZSM-5 crystals with a long b-axis length have high p-xylene selectivity and m-xylene conversion. Besides, with the addition of additives such as graphene oxide(GO), [128] 1,2-dihydroxybenzene (C 6 H 6 O 2 ), [129,130] and polyacrylamide hydrogel (C-PAM), [131] oriented zeolite membranes were also obtained.

Effect of templates
A template agent (SDA), also known as a structure directing agent, is essential for the synthesis of zeolite. Templates play a key role in the structure and properties of zeolite channels. They can act as shape improvers by promoting the growth rate along the respective crystal axis, so it important to use an appropriate structure directing agent to control the growth of the membrane or design a specific SDA to achieve the expected performance. [132] Moreover, it has been reported that structure-directing agents have preferred orientations in zeolite synthesis, and can be used to regulate the orientation of zeolite membranes. [133] The orientation of zeolite membranes can be adjusted by changing the SDA for seed formation and secondary growth by the secondary growth method. [134] In Choi's study, tetrapropylammonium hydroxide (TPA), a structure-directing agent used to form a b-oriented membrane for seed formation, was used for secondary growth, while the trimer N, N-bis(tripropylaminohexamethylene)-N,N-dipropylammonium cation (TPA) forming a b-oriented membrane for secondary growth was used to synthesize MFI seed crystals, which changed the orientation of the membrane, and continuous and uniform a-oriented MFI membranes were obtained. Similarly, Chaikittisilp et al. [135] developed a new steam-assisted crystallization method (SAC), in which only the structure guiding agent (TPA + ) was coated on the silicon wafer matrix, and two out-of-plane a-&b-orientations MFI zeolite membranes were obtained through the transformation of the TPA +mediated layer on the silicon surface. However, the flux of the a-oriented MFI membrane is lower due to the higher curvature along the a-axis of the sinusoidal channels aligned perpendicular to the scaffold (1.2 times that of the straight channels along the b-axis). Moreover, Mabande et al. [136] used 3-chloropropyltrimethoxysilane as a coupling agent and the trimer TPA as a structure-guiding agent to prepare b-oriented MFI membranes on porous stainless steel supports precoated with smooth mesoporous layers. The membrane has high coverage and good symbiosis.
Moreover, the use of cotemplates plays an important role in the synthesis of highly oriented zeolite membranes, and different cotemplates have different effects on the orientation of zeolite membranes. The use of a cotemplate improves the prepared membrane symbiosis and results in fewer defects. Kim et al. [137] controlled membrane growth by using an appropriate structuredirecting agent. In the case of the dimer C5-TPA, the secondary growth of the a-oriented MFI seed layer led to epitaxial growth, thus maintaining the a-plane orientation. The triple growth in the presence of the trimer TPA improved symbiosis. Yoon et al. [115] prepared a perfect b-oriented MFI membrane on a b-oriented seed monolayer with TEAOH and (NH 4 ) 2 SiF 6 as cotemplates, which have very high separation factors. At the same time, a-oriented Si-BEA membranes were obtained by combining TEA + with F − . Among them, F − is used to promote the secondary growth of crystals, but zeolite crystals prepared with F − ions as mineralizers at neutral pH contain small defects. Nevertheless, a b-axis oriented MFI zeolite membrane could be synthesized with TPA + F as a cotemplate, which had fewer defects, and the membrane had high separation performance. [138] In addition to the above cotemplate, the use of n-propylammonium (PA) and n-ethylhexamethylene tetraammonium bromide also played a cotemplate role in the process of crystal formation and growth. Using the cotemplate, Yu et al. [139] also synthesized a high silicon MFI zeolite membrane with a b-axis preferred orientation.
In addition, thanks to the design of structure directing agent, many research progress have been made in nanosheets of zeolite, which also provides a new idea for the synthesis of oriented zeolite membranes. [140] Choi et al. [141] designed a long-chain di-quaternary ammonium-type surfactant (C22-6-6), used its guiding effect to limit the growth of MFI zeolite in the b-axis orientation, and obtained ZSM-5 nanosheet films with a thickness of only 2.0 nm. This is due to the formation of micelles of di-quaternary ammonium-type surfactants in the aqueous phase, and the formation of microporous channels and zeolite skeleton structure guided by quaternary ammonium hydrophilic groups, so that the crystals grow along the a-c plane. The long chain of hydrophobic group extends along the b-axis direction, and finally zeolite nanosheets with b-axis growth restriction were presented. [141,142] By depositing zeolite nanosheets on the support, directional zeolite membranes can be induced to form after secondary growth. Tsapatsis et al. [143] reported the study of depositing thin oriented nanosheet seed layer for secondary growth to form dense b-oriented membrane with a thickness of 80 nm. Subsequently, they used Langmuir-Schaeffer deposition method to uniformly deposit zeolite nanosheet monolayers on silicon wafers. After secondary growth, symbiotic b-oriented membranes with a thickness of less than 12 nm were obtained. [144] Simultaneously, it is proved that the directional zeolite membrane prepared by this method delivered a stable performance in the separation of the xylene isomers with an SF of 30 and corresponding p-xylene permeance of 5.1 × 10 −7 mol m −2 s −1 Pa −1. [145] In the use of zeolite nanosheets to induce the formation of oriented zeolite membranes, the problems of complex and costly synthesis of template agents used for the nanosheets synthesis and the uniform and oriented deposition of nanosheets on supports need to be urgently solved in the future.

Influence of supports
Zeolite membranes are divided into symmetric membranes (supported membranes) and asymmetric membranes (self-supporting membranes). In practical applications, zeolite membranes are usually grown on supports. Common supports are α-Al 2 O 3 , glass, silicon wafer, stainless steel, etc., and these substrates can not only be used as supports of synthetic molecular sieve membranes, but also play the roles of second silicon and aluminum sources. [146] However, the inherent roughness of porous substrates is detrimental to the directional growth of zeolite crystals on the substrate surface. Taking the preparation of zeolite membranes by the seed method as an example, after seed deposition on a rough surface, the accumulation and orientation of seed crystals are not as good as those on a smooth support surface. Huang et al.'s research [116] shows that when using zeolite seeds to prepare zeolite membranes on silicon and porous alumina, the membranes are synthesized by randomly oriented seed layers, or grow in unfavorable gels, resulting in no or only weakly oriented crystals. Therefore, the surface roughness of the support plays a key role in determining whether the directional preparation of the zeolite membrane can be achieved. Therefore, the directional nucleation and growth of zeolite crystals can be regulated by changing the properties of the substrate surface to form uniformly oriented zeolite membranes. [147] When a zeolite membrane is synthesized by the in situ method, the surface of the support is improved by applying a coating to make the surface smooth, or when the seed crystal method is used, the surface of the seed crystal is passivated to protect the crystal from nucleation and growth along the out-ofplane direction. These methods can be used to obtain highly oriented zeolite membranes. Hrabanek et al. [148] optimized the direct hydrothermal synthesis method through the combination of monomer (TEOS) and colloidal (TOSIL) silica and synthesized preferentially oriented b-silicalite-1 zeolite membranes on different supports (silicon wafer, porous stainless steel, nonporous stainless steel, etc.). Their research shows that a b-oriented silicalite-1 zeolite membrane was obtained on a porous stainless-steel support, coated with a TiO 2 layer and modified by tetrabutyl titanate as a coupling agent. The membrane has obvious symbiosis. The study confirmed that this is because when preparing the b-axis oriented MFI zeolite membrane by the in situ method, a TiO 2 coating was deposited on the support surface, which provided a smooth surface with high TiO-OH density for the growth of the b-axis-oriented MFI crystal and promoted the continuous and uniform diffusion of the synthetic solution on the support. [149] Therefore, the crystal was oriented and nucleated, the growth of the zeolite crystals was accelerated, and finally the b-axisoriented membrane formed. Apart from TiO 2 coatings, SiO 2 coatings have the same effect and are another commonly used coating. [150,151] Zhang et al.- [152] showed that the SiO 2 mesoporous buffer layer was precoated on porous supports, such as α-Al 2 O 3 supports, which acted as a polishing layer. The prepared MFI crystals were b-axis-oriented and had good symbiotic behavior, while the membranes obtained without a SiO 2 coating were randomly oriented. While zeolite membranes can be synthesized by the seed method, an MFI zeolite membrane with a highly preferred b-orientation can be prepared by coating a Au/Pt passivation seed crystal surface. [153] This is because applying Au/Pt to passivate the crystal surface can prevent the crystal from undergoing nucleation and growth along the out-of-plane direction and allow the crystal to grow along the in-plane direction. In subsequent research, Li et al. [154] further showed that passivating the (0k0) surface of a b-oriented seed layer through a Au/Pt coating prevented the attachment of new nuclei and inhibited the growth of twins during seed growth. The performance of the membrane was evaluated, and the maximum separation factor of p-xylene/ o-xylene was approximately 4.
The geometric structure and chemical properties of the substrate surface also play an important role in the formation, crystal orientation and properties of zeolite membranes. In other words, the orientation of zeolite membranes grown on substrates is controlled by the chemical properties of the substrate surface. Therefore, by depositing polymers rich in hydroxyl and carboxyl groups on the substrate surface, oriented zeolite membranes can be synthesized by modifying the substrate surface. This is because based on the deposition of reaction groups on the substrate surface, these reaction groups can be coupled with matching groups on zeolite crystals, which are used as covalent linkers to enhance the bonding strength and interact with each other. For example, hydrogen peroxide and carboxymethyl chitosan (CMC) solution were used to modify the substrate surface, and hydroxyl, carboxyl, ammonia and ether bonds were successfully planted on the substrate surface. The substrate surface was rich in functional groups, which enhanced the interaction between the substrate surface and submicron crystals to form a continuous b-oriented MFI zeolite membrane. [117] Similarly, the use of chemical modifiers such as polypropylene and polyvinyl alcohol has also achieved satisfactory results. Elyassi et al. [155] used polyacrylic acid and ethylene glycol as a polymer interlayer to increase the adhesion of crystal seeds on the support to synthesize a b-axis-oriented silicalite-1 zeolite with good growth and a thickness of 1 μm. The performance test shows that the pervaporation separation factor of the 5 wt% ethanol and water mixture is as high as 85, and the total flux is 2.1 kg/(m 2 • h). Zhang et al. [156] modified the surface of porous, rough alumina with cheap polyvinyl alcohol (PVA) to make it smooth and rich in hydroxyl groups and then used 3-chloropropyl trimethylsilane (3CP-TMS) as a molecular linker. The MFI molecular sieve monolayer with a tight arrangement and preferential b-axis orientation was obtained by the action of "laminated ultrasound." In short, if the surface roughness of the substrate is reduced and the substrate surface is chemically modified, the preparation of high-quality directional zeolite can be significantly improved.
In summary, adjusting the composition of the synthesis solution and the template agent and changing the properties of the substrate are ways to improve the synthesis of oriented zeolites. Taking the most widely studied MFI zeolite membrane as an example, Figure 14 shows the crystals orientation control diagram. The low alkali concentration, the increase in water content, Figure 14. Schematic diagram of MFI zeolite crystals orientation control and the use of a cotemplate or trimmer template are beneficial to the synthesis of b-oriented MFI zeolite membranes, while the alkali metal in the synthesis solution is beneficial. The content of Na + ions is beneficial to the growth of c-oriented MFI zeolite membranes. Regarding the substrate properties, a smooth substrate surface or the use of polymers to improve the chemical properties of the substrate to make it rich in hydroxyl and carboxyl groups is more conducive to the combination of oriented zeolite membranes and zeolite seeds on the support to obtain oriented zeolite membranes. In addition, we found that NH 4 + ions and starch, as orientation additives, are good reagents for obtaining oriented zeolite membranes. Therefore, the use of inhibitors that can regulate the orientation of zeolite crystals may promote the acquisition of highly oriented zeolite membranes. Based on the above-obtained laws affecting the orientation of zeolite membranes, combining it with hydroxyl radicalpromoted zeolite synthesis may be another way to achieve efficient synthesis of oriented zeolite membranes.
At the same time, in this process, it is very important to explore their mechanism for the efficient synthesis of directional zeolite membranes. In addition to the membrane-forming mechanism of LTA zeolite membrane under microwave-assistance described in section 2.2, Yamazaki et al. [157] and koegler et al. [158] proposed the formation mechanism of A zeolite membrane and MFI zeolite crystal respectively. The MFI zeolite crystal orientation growth model proposed by koegler et al. [158] At the beginning of synthesis, the silica gel was deposited on the support to form a thin, low density silica surface. TPA + from the solution will be attracted to the surface, at which point nucleation will take place. Crystals will grow by consuming the gel. An extra orientation mechanism is now provided by the underlying support, yielding an almost perfectly oriented coating. Nucleation and growth take place at the interface between gel and liquid, which is the only region where silicon source and template exist at the same time. Orientation is caused by the preferential growth of the ac-plane of the crystallites in the plane of the substrate surface. The anisotropic morphology of zeolite crystal promotes the deposition, and its maximum area plane is parallel to the support plane, resulting in preferential orientation.
The universal law of oriented zeolite membrane synthesis is one of the objectives of this paper, but unfortunately, this universal law has not been put forward in any reference found. Based on the analysis of the factors affecting the membranes orientation and the existing membranes forming mechanism, the universal law is tried to be found: 1) Promote the formation of target oriented zeolite crystals by adjusting the synthesis conditions, such as alkalinity, water content, template, and adding inhibitors. 2) The directional deposition of zeolite crystals on the support surface, which can be achieved by support modification. 3) Prevent the nucleation and growth of crystals along the out-of-plane direction to inhibit the generation of twins. However, due to the different synthesis ratios or methods used in each laboratory, the regular synthesis conditions still need to be further systematically explored, and the types of templates and inhibitors with different orientations also need to be designed and developed. Moreover, in order to get a more accurate universal law of the formation of oriented zeolite membranes, the crystal heterogeneity, the crystal growth kinetics and the adjustment mechanism of membrane orientation need to be further explored to provide theoretical basis.

Conclusion
Zeolite and zeolite membranes are widely used in catalytic adsorption and other fields, especially oriented zeolite membranes, which have excellent performance because of their orientation and good application prospects. Aiming at the problem of how to quickly prepare oriented zeolite membranes, this paper investigated the effect of the microwave method for the rapid synthesis of oriented zeolite membranes, investigated the mechanism through which hydroxyl radicals accelerate zeolite synthesis, and summarized the factors that affect the orientation of zeolite membranes. The following conclusions were drawn and the future development direction was clarified.
(1) Microwave irradiation produces isolated active water, and a high heating rate can improve the zeolite synthesis rate. At the same time, the rapid and uniform heating of the microwave method plays a positive role in the efficient synthesis of oriented zeolite membranes. However, high equipment requirements, harsh synthesis conditions, low energy utilization rate, lack of professional microwave equipment, and industrialization difficulties are still the problems faced by microwave method to efficiently synthesize oriented zeolite membranes. Therefore, studying the formation mechanism of oriented zeolite membranes under microwave conditions to clarify their formation process, and developing professional microwave equipment to improve the energy utilization rate during synthesis and relax the synthesis conditions are the future research focuses on the efficient synthesis of oriented zeolite membrane using microwave method.
(2) Hydroxyl radicals, which have lower energy for the fracture and formation of Si-O-Si bonds than OH − , are a common method used to improve the zeolite synthesis rate. Hydroxyl radicals can be introduced by physical methods such as irradiation (ultraviolet, gamma rays, and ultrasound), grinding of seed crystals, and chemical methods such as the introduction of Fenton's reagent, persulfate, etc. However, the application of free radicals is currently limited to improving the synthesis rate of zeolites, and application studies in the preparation of oriented zeolite membranes are lacking. Therefore, focusing on the changes of zeolites morphologies after the addition of free radicals, and studying whether the synthesis of oriented zeolite membranes in free radical systems can be achieved by adjusting the composition of synthesis solution or introducing inhibitors is another development direction. The key point to realize this path is to deeply explore the mechanism of •OH free radicals accelerating the formation of zeolite and their influence on the crystal growth morphology, and to find the relationship between free radicals and the synthesis of oriented zeolite membrane.
(3) The efficient synthesis of oriented zeolite membranes is still a problem to be solved in the future. The oriented zeolite membranes were synthesized by adjusting the composition of synthetic solution, using templates and inhibitors, and modifying the substrate. However, further systematic studies on the synthesis conditions of oriented zeolite membranes to reveal the universal laws of oriented zeolite membrane synthesis, and the design and development of template and inhibitor types affecting different orientations are still important to promote the synthesis of high-quality oriented zeolite membranes. Moreover, various characterization tools for the systematic characterization of zeolite membranes formation need to be investigated to clarify their formation process and mechanisms. This will facilitate the discovery of the link between oriented zeolite membranes and free radical accelerated zeolite synthesis. Meanwhile, the further clarification of the crystal heterogeneity, the crystal growth kinetics, and the adjustment mechanism of membrane orientation will also provide a theoretical basis for the combination of oriented zeolite membranes with free radicals. This is an indispensable way to achieve efficient synthesis of oriented zeolite membranes in the future.

Disclosure statement
No potential conflict of interest was reported by the author(s).