A method for synthesizing NaA molecular sieve membrane by using wet gel layer crystallization

Abstract

The application discloses a method for synthesizing NaA molecular sieve membrane by using wet gel layer crystallization. The method comprises the following steps: firstly, preparing a wet seed carrier, then, without drying treatment, the wet seed carrier is directly soaked in a gel solution to form a wet gel layer on the surface of the wet seed carrier, and after being taken out, the wet seed carrier is put into a reaction kettle to be heated and crystallized to synthesize high-performance NaA molecular sieve membrane. The preparation process is simple, the utilization rate of the gel solution is high, the preparation reproducibility is good, the prepared NaA molecular sieve membrane has high separation performance, can be applied to large-scale application, and has important industrial application value.

Description

Technical Field

[0001] This invention relates to a method for synthesizing zeolite molecular sieve membranes, specifically a method for synthesizing NaA molecular sieve membranes by crystallizing a wet gel layer. Background Technology

[0002] Organic chemical raw materials are the foundation for the development of various organic chemical products and a major component of modern industrial structures. Dehydration and purification of organic matter is a crucial step in the preparation of organic reagents. Since 1999, when Mitsui Shipbuilding Corporation of Japan established the first pervaporation device using NaA molecular sieve membranes for organic dehydration, NaA molecular sieve membranes have been extensively studied by researchers.

[0003] NaA molecular sieve membranes typically use porous ceramic tubes as carriers, possessing advantages such as uniform pore size (eight-membered ring channels, approximately 0.4 nm pore size), high temperature resistance, good chemical stability, and high mechanical strength. They can recognize minute size / shape differences at the molecular level. Due to the hydrophilicity of NaA molecular sieve membranes, they can be applied to various organic dehydration systems to achieve molecular sieving. The main synthesis methods for NaA molecular sieve membranes include in-situ crystallization and secondary growth. In-situ crystallization requires long synthesis times and multiple attempts, resulting in a long synthesis cycle and poor membrane reproducibility. Secondary growth significantly reduces synthesis time and easily produces dense membranes, but requires a large amount of gel solution, which is used only once, leading to waste of chemical raw materials and high membrane production costs. To reduce membrane production costs and improve gel solution utilization, Cheng et al. [Chinese J.Chem.2003,21:1430-1432] used a vapor phase reforming method to prepare NaA molecular sieve membranes. This method significantly reduces the amount of gel solution used, improves gel solution utilization, and thus lowers membrane production costs. However, the membrane preparation process is relatively complicated, and the need to dry the gel solution can easily lead to poor membrane density, the presence of intercrystalline pores and defects, resulting in low membrane separation performance. Although the vapor phase reforming membrane technology has many problems, it is undeniably an environmentally friendly and green synthetic route.

[0004] To address the problems of cumbersome preparation process, poor membrane performance, and poor reproducibility in vapor phase conversion, Chao et al. [Chemistry Letters 2006, 35(9): 1056-1057] employed a two-stage vapor phase conversion process involving pre-coating crystallization and secondary growth, and added a template agent to the gel solution to promote membrane crystallization and improve the density of the NaA molecular sieve membrane. Cheng et al. [Adv. Mater. Res. 2014, 864-867: 654-658] also employed a two-stage vapor phase conversion process involving pre-coating crystallization and secondary growth, but optimized the composition of the gel solution, eliminating the use of template agents and ethanol, which improved the density of the NaA molecular sieve membrane to a certain extent and reduced the cost of the gel solution. However, their preparation process was still relatively complex and the synthesis time was long. Based on this, Xiao et al. [J. Mater. Chem. A. 2018, 6: 10484-10489] proposed a direct crystallization method to prepare NaA molecular sieve membranes. After mixing seed crystals and gel solution to form a seed crystal slurry, it is coated onto a support and dried to form a seed crystal layer. Subsequent steps include impregnation with a plugging solution and gel solution to form a wet gel layer on the support. This wet gel layer is then directly placed into a reactor for crystallization without the addition of additional water. Compared to the vapor phase conversion method, the direct crystallization method avoids the formation of pinholes and defects, shortens the synthesis time to 4 hours, and improves membrane formation efficiency. Yang et al. [Micropor.Mesopor.Mater.2022,329:111541] proposed a wet gel conversion method for preparing NaA molecular sieve membranes. First, a dense seed crystal layer is formed on the support using a thermal coating method. The seed-coated support is dried at 80°C, impregnated with water, and dried at room temperature for 1 hour. Then, the seeded support is impregnated with gel solution and placed into a reactor for crystallization without the addition of gel solution. Xiao et al. and Yang et al. prepared NaA molecular sieve membranes using direct crystallization and wet gel conversion methods, respectively. These methods only require forming a single wet gel layer on the outside of the seeding support, replacing the large amount of gel solution needed in hydrothermal synthesis, effectively reducing the waste of chemical raw materials and saving preparation costs. However, the flux of NaA molecular sieve membranes prepared by both methods is lower than that prepared by the traditional secondary growth method. Furthermore, these two methods involve multiple preparation steps, which is not conducive to industrial production and controlling the reproducibility of membrane preparation.

[0005] When preparing NaA molecular sieve membranes using the above methods, the seed layer usually needs to be heated and dried first to promote the bonding force between the seed layer and the support, which is beneficial for the subsequent formation of a dense NaA molecular sieve membrane [J.Membr.Sci.2013,444:513-522]. If the traditional secondary growth method is used to prepare the membrane with a moist seeded support, the separation performance of the obtained membrane is usually lower. It is speculated that this is mainly because the gel solution does not provide sufficient nutrients to the support side when the wet support is inserted into the gel solution, thus leading to a decrease in membrane density.

[0006] Therefore, designing a simple preparation process for NaA molecular sieve membranes with high gel utilization, excellent membrane reproducibility, and high separation performance is an urgent problem to be solved. This will give NaA molecular sieve membranes an advantage in subsequent industrial applications. Summary of the Invention

[0007] The purpose of this invention is to reduce the preparation cost of NaA molecular sieve membranes, overcome the low reproducibility of existing preparation methods, and provide a simple preparation process with high gel utilization and good repeatability to prepare high-performance NaA molecular sieve membranes on rough alumina tubes.

[0008] This invention first provides a method for synthesizing NaA molecular sieve membranes by crystallizing a wet gel layer, comprising:

[0009] A. Immerse the porous carrier in water, then remove it and allow it to air dry at room temperature until the water adsorption capacity of the carrier is 100-170 mg / cm³. 3 A moistened carrier is obtained, and seed crystals are applied to its surface to obtain a moistened seeded carrier.

[0010] B. The moistened seeded carrier is directly immersed in the aged gel solution without drying. After immersion, the carrier is taken out, placed in a reaction vessel, sealed, and heated for reaction. After the reaction is completed, a carrier with a NaA molecular sieve membrane layer grown on the surface is obtained. The carrier is taken out, cleaned, and dried to obtain the NaA molecular sieve membrane.

[0011] As a preferred embodiment of the present invention, the porous support in step A is a porous alumina support, a stainless steel support, or a hollow fiber support; the pore size of the porous support is 0.1-100 μm. Preferably, the shape of the porous support can be selected according to the actual needs of the application scenario. For example, when it is necessary to prepare NaA molecular sieve membrane tubes, a tubular support can be selected as the porous support. Typically, but not limited to, the support in step A can be a rough alumina tube with an outer diameter >10 mm, and its pore size is preferably 0.1-10 μm.

[0012] As a preferred embodiment of the present invention, the immersion time of the porous carrier in water is preferably 5-30 seconds.

[0013] Furthermore, in the moistened seeded carrier obtained after wiping with the seed crystals in step A, the amount of seed crystals is 0.3-0.8 mg / cm³. 2 More preferably, the seed crystals mentioned in step A are NaA molecular sieve seed crystals, and the particle size range of the seed crystals is 0.1-5 μm.

[0014] In a preferred embodiment of the present invention, the molar ratio of the gel solution in step B is Na₂O:SiO₂:Al₂O₃:H₂O = 2-3:2:1:150-500. More preferably, the molar ratio of the gel solution is Na₂O:SiO₂:Al₂O₃:H₂O = 2-3:2:1:200-250. Preferably, in step B, the seeding carrier is immersed in the gel solution for soaking.

[0015] As a preferred embodiment of the present invention, in step B, after the carrier is removed from the gel solution, the excess gel solution on the surface of the carrier is removed, and then it is placed into the reaction vessel.

[0016] In a preferred embodiment of the present invention, the aging time of the gel solution in step B is 3-72 hours. In a preferred embodiment of the present invention, the immersion time of the seed carrier in the gel solution in step B is 0.1-4 hours.

[0017] In a preferred embodiment of the present invention, no solvent, gelling agent, or template agent needs to be added to the reaction vessel in step B. Furthermore, the reaction vessel in step B is either static or rotating.

[0018] As a preferred embodiment of the present invention, the heating reaction temperature in step B is 90-110°C; the heating reaction time is 3-6 hours.

[0019] As a preferred embodiment of the present invention, the drying temperature in step B is 20-60°C and the drying time is 1-6 hours.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. The present invention employs a simple wetting and wiping seed coating method, which enables the seed crystals to fill the carrier pores uniformly and effectively fill large pore defects in the carrier. It also simplifies complex coating methods such as multiple dip coating and seed slurry.

[0022] 2. This invention eliminates the drying step after wetting and wiping the seed crystals. It directly uses a simple immersion method to allow the gel to adhere to the carrier surface, eliminating the complex drying and pore-blocking processes of methods such as vapor phase inversion and reported wet gel crystallization. This prevents the seed crystals from penetrating into the carrier and clogging the carrier pores, thus avoiding low membrane flux. Furthermore, it eliminates the need to add other solvents or gels to the reactor to promote crystallization. Traditional secondary growth methods require the seed crystallization carrier to be continuously immersed in the gel for crystallization. A moist seed crystallization carrier may lead to insufficient nutrient supply from the gel, resulting in poor membrane performance; while a dry seed crystallization carrier may cause pore blockage due to seed migration during immersion in the gel, reducing membrane flux. The method of this invention effectively avoids these problems. This method only requires forming a wet gel layer on the seed crystallization carrier. Since there is no capillary action between the moist seed layer and the carrier, the seed crystals do not penetrate into the carrier pores but only adhere to the carrier surface. Furthermore, during the crystallization process, the moist seed crystals adsorb moisture from the carrier surface, thus preventing further adsorption of water from the gel during immersion in the gel solution. This maintains a suitable gel viscosity and a moderate thickness of gel layer adsorbed on the outer side of the seed crystal layer (providing sufficient nutrients for membrane growth without causing overgrowth and reduced separation performance). Residual moisture inside the carrier facilitates the diffusion of free nutrients from the gel into the carrier, providing the necessary nutrients for membrane growth from the carrier side and increasing membrane density without clogging the carrier pores. During crystallization, the moisture in the gel layer provides saturated vapor pressure within the reactor, increasing the gel layer's viscosity and promoting the formation of a defect-free, dense layer, thereby obtaining a high-flux NaA molecular sieve membrane.

[0023] 3. The NaA molecular sieve membrane prepared in this invention is used for pervaporation separation. Results show that at 75℃, the separation factor of this membrane exceeds 10000 in a 90wt% ethanol-water system, and the membrane flux reaches 4.59 kg / (m³). 2 h). This indicates that the NaA molecular sieve membrane prepared by this method has high separation performance.

[0024] 4. Compared with the NaA molecular sieve membrane preparation methods reported at home and abroad, this invention simplifies the preparation process and uses a more dilute gel solution, which can effectively improve the utilization rate of the gel solution, reduce the membrane preparation cost, and avoid a large waste of the gel solution; it has the advantages of low cost, high reproducibility, and high membrane flux. Attached Figure Description

[0025] Figure 1 SEM image of the outer surface of the rough alumina tube;

[0026] Figure 2 SEM image of the outer surface of the seeded roughened alumina tube;

[0027] Figure 3SEM image of the surface of the NaA molecular sieve membrane synthesized by heating reaction;

[0028] Figure 4 Cross-sectional SEM image of the NaA molecular sieve membrane synthesized by heating reaction;

[0029] Figure 5 SEM image of the surface of NaA molecular sieve membrane synthesized on a dry seeding support;

[0030] Figure 6 Cross-sectional SEM image of NaA molecular sieve membrane synthesized on a dry seeding support. Detailed Implementation

[0031] To better illustrate the method for synthesizing NaA molecular sieve membranes by heating a wet gel layer, and the advantages of using a wet seed layer to prepare high-throughput NaA molecular sieve membranes, some examples of molecular sieve membrane synthesis are given below. However, the present invention is not limited to the examples listed.

[0032] Example 1

[0033] A. Preparation of seed layer

[0034] A rough alumina tube carrier with a pore size of 5μm and an outer diameter of 12mm was used. Figure 1 The carrier was vertically immersed in deionized water for 10 seconds, then removed and allowed to air dry at room temperature. Once no free water flowed from the carrier surface, the water adsorption capacity of the carrier was 137 mg / cm³. 3 Then, NaA molecular sieve seeds with a particle size of 0.8 μm are repeatedly wiped to obtain a moist seeded support (such as...) with a relatively dense seed layer. Figure 2 (As shown), the seed crystal amount is 0.52 mg / cm³. 2 .

[0035] B. Preparation of the film

[0036] Dissolved aluminum source was added dropwise to a stirred silicon source to obtain a gel solution with a molar ratio of Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:200. The gel solution was stirred and aged at room temperature for 6 hours. The resulting moist seeded support was directly immersed in the gel solution for 0.5 hours without any drying. After removal, excess gel solution was removed from the surface of the support, and it was vertically placed in a stainless steel reactor with a polytetrafluoroethylene liner. Synthesis was carried out at 100°C for 4 hours. After the reaction, the membrane tube was removed, washed, soaked overnight, and dried at 40°C for 2 hours. The membrane was then analyzed by scanning electron microscopy. Figure 3 The film layer is observed to be dense and well-connected, with no obvious pinholes or other defects. The cross-section shows a film thickness of 2.8 μm. Figure 4 The membrane was used for pervaporation testing to separate 90 wt.% ethanol from water, and the results are shown in Table 1. The membrane flux was 4.59 kg / (m²).2 The separation factor is over 10,000, indicating that the membrane is dense. This is significantly higher than that of conventional NaA molecular sieves (3.60 kg / (m³). 2 The NaA molecular sieve membrane prepared by this method has a 27% higher membrane flux and a water content of over 99.97% in the permeate. This not only meets the requirements for pervaporation alcohol-water separation but also helps to increase the membrane dehydration efficiency.

[0037] Example 2

[0038] Similar to Example 1, the molar ratio of the gel solution in step B is Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:150. The membrane was used for pervaporation testing, and the separation results are shown in Table 1. The membrane flux was 4.14 kg / (m²). 2 The separation factor reached over 10,000, indicating that the membrane is dense. This is compared to conventional NaA molecular sieves (3.60 kg / (m²)). 2 The NaA molecular sieve membrane prepared by this method has a 15% higher membrane flux and a water content of over 99.95% in the permeate. This not only meets the requirements for pervaporation alcohol-water separation but also helps to increase the membrane dehydration efficiency.

[0039] Example 3

[0040] Similar to Example 1, the molar ratio of the gel solution in step B was Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:250. The membrane was used for pervaporation testing, and the separation results are shown in Table 1. The membrane flux was 4.13 kg / (m²). 2 The separation factor reached over 10,000, indicating that the membrane is dense. This is compared to conventional NaA molecular sieves (3.60 kg / (m²)). 2 The NaA molecular sieve membrane prepared by this method has a 15% higher membrane flux and a water content of over 99.95% in the permeate. This not only meets the requirements for pervaporation alcohol-water separation but also helps to increase the membrane dehydration efficiency.

[0041] Comparative Example 1

[0042] A. Preparation of seed layer

[0043] The carrier was vertically immersed in deionized water for 10 seconds, then removed and allowed to air dry at room temperature. Once no free water flowed from the carrier surface, the water adsorption capacity of the carrier was determined to be 137 mg / cm³. 3 NaA molecular sieve seeds with a particle size of 0.8 μm were repeatedly rubbed to obtain a moist seeded carrier, which was then dried at 60 °C for 0.5 h to obtain a dry seeded carrier.

[0044] B. Preparation of the film

[0045] Dissolved aluminum source was added dropwise to a stirred silicon source to obtain a gel solution with a molar ratio of Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:200. The gel solution was stirred and aged at room temperature for 6 hours. The resulting dried seeded support was directly immersed in the gel solution for 0.5 hours without any drying. After removal, excess gel solution was removed from the surface of the support, and it was vertically placed in a stainless steel reactor with a polytetrafluoroethylene liner. Synthesis was carried out at 100°C for 4 hours. After the reaction, the membrane tube was removed, washed, soaked overnight, and dried at 40°C for 2 hours. Figure 5 The membrane surface is dense and well-connected, with no obvious defects. The cross-section shows a membrane thickness of approximately 3.4 μm. Figure 6 The prepared NaA molecular sieve membrane was used for pervaporation testing, and the separation results are shown in Table 1. The membrane flux was 3.59 kg / (m²). 2 The separation factor reached over 10,000, indicating that the membrane is dense. The membrane flux was comparable to that of conventional NaA molecular sieves (3.60 kg / (m²). 2 h)) is equivalent, with no improvement. It only meets the requirements for pervaporation alcohol-water separation.

[0046] Comparative Example 2

[0047] A. Preparation of seed layer

[0048] The carrier was vertically immersed in deionized water for 10 seconds, then removed and allowed to air dry at room temperature. Once no free water flowed from the carrier surface, the water adsorption capacity of the carrier was determined to be 137 mg / cm³. 3 Then, NaA molecular sieve seeds with a particle size of 0.8 μm were repeatedly rubbed to obtain a moist seeded support, which was then dried at 60 °C for 0.5 h to obtain a dry seeded support. The dry seeded support was vertically immersed in deionized water for 10 s, removed, and then placed at room temperature to dry for 0.5 h to obtain a moist seeded support.

[0049] B. Preparation of the film

[0050] Dissolved aluminum source was added dropwise to a stirred silicon source to obtain a gel solution with a molar ratio of Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:200. The gel solution was stirred and aged at room temperature for 6 hours. The resulting moist seeded support was directly immersed in the gel solution for 0.5 hours without any drying. After removal, excess gel solution was removed from the support surface, and the support was vertically placed in a stainless steel reactor with a polytetrafluoroethylene liner. Synthesis was carried out at 100°C for 4 hours. After the reaction, the membrane tube was removed, washed, soaked overnight, and dried at 40°C for 2 hours. The membrane surface was dense and had good intergrowth, with no obvious defects. The membrane thickness was approximately 2.7 μm. The prepared NaA molecular sieve membrane was used for pervaporation testing, and the separation results are shown in Table 1. The membrane flux was 3.48 kg / (m²). 2The separation factor reached over 10,000, indicating that the membrane was dense. The membrane flux was slightly lower than that of conventional NaA molecular sieves (3.60 kg / (m²). 2 h)). Only meets the requirements for pervaporation alcohol-water separation.

[0051] Comparative Example 3

[0052] A. Preparation of seed layer

[0053] The carrier was vertically immersed in deionized water for 10 seconds, then removed and allowed to air dry at room temperature until no free water flowed on the carrier surface and the water content of the carrier was 137 mg / cm³. 3 Then, NaA molecular sieve seeds with a particle size of 0.8 μm were repeatedly rubbed to obtain a moist seeded support. This was then dried at 60 °C for 0.5 h to obtain a dry seeded support. One section of the dry seeded support was plugged with a rubber stopper, and deionized water was injected into the tube cavity for 10 s. The water was then removed from the tube cavity, and the tube was left to dry at room temperature for 0.5 h to obtain a moist seeded support.

[0054] B. Preparation of the film

[0055] Dissolved aluminum source was added dropwise to a stirred silicon source to obtain a gel solution with a molar ratio of Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:200. The gel solution was aged at room temperature for 6 hours. The resulting moist seeded support was directly immersed in the gel solution for 0.5 hours without any drying. After removal, excess gel solution was removed from the support surface, and the support was vertically placed in a stainless steel reactor with a polytetrafluoroethylene liner. Synthesis was carried out at 100°C for 4 hours. After the reaction, the membrane tube was removed, washed, soaked overnight, and dried at 40°C for 2 hours. The membrane surface was relatively dense with no obvious defects. The membrane thickness was approximately 2.8 μm. The prepared NaA molecular sieve membrane was used for pervaporation testing, and the separation results are shown in Table 1. The membrane flux was 3.93 kg / (m²). 2 The separation factor was 5778. Although the membrane flux was higher than that of conventional NaA molecular sieves (3.60 kg / (m²), the separation factor was still 5778. 2 However, the separation factor is slightly low, and intercrystalline pores still exist, which can only basically meet the requirements for pervaporation alcohol-water separation.

[0056] Comparative Example 4

[0057] Similar to Example 1, the molar ratio of the gel solution in step B is Na₂O:SiO₂:Al₂O₃:H₂O = 2.2:2:1:100. The membrane was used for pervaporation testing, and the separation results are shown in Table 1. The membrane flux was 3.50 kg / (m²). 2 The separation factor was 7666. The membrane flux was slightly lower than that of conventional NaA molecular sieves (3.60 kg / (m²). 2h)), while the separation factor is slightly low, only meeting the requirements for pervaporation alcohol-water separation.

[0058] The table below shows the pervaporation test results of the NaA molecular sieve membranes synthesized in Examples 1-3 and Comparative Examples 1-4. The feed solution was a 90 wt.% aqueous ethanol solution, and the test temperature was 75°C.

[0059] Table 1 Membrane pervaporation separation performance

[0060] <![CDATA[Total flux (kg / (m 2 h))]]> Separation factor Example 1 4.59±0.2 >10000 Example 2 4.14±0.2 >10000 Example 3 4.13±0.2 >10000 Comparative Example 1 3.65±0.2 >10000 Comparative Example 2 3.48±0.2 >10000 Comparative Example 3 3.93±0.2 5778 Comparative Example 4 3.50±0.2 7666

[0061] The experimental results above indicate that a moist seed layer is a key factor in preparing high-performance membranes. Dry NaA seed layers are hydrophilic; when the seed carrier is immersed in the gel solution, it preferentially adsorbs water from the gel on the outer side of the seed layer, leading to increased gel concentration and viscosity at the interface. This results in a thicker gel layer on the outer side of the seed carrier, hindering gel penetration into the carrier's interior. Because the thicker gel layer provides more nutrients, it leads to excessive membrane growth and a slightly lower membrane flux (as shown in Comparative Example 1). In contrast, undried seed layers lack capillary action, so the seeds do not penetrate into the carrier pores and only adhere to the carrier surface. Furthermore, during the coating process, the moist seed layer adsorbs water from the carrier surface, thus preventing further water adsorption during immersion in the gel solution. This maintains a suitable gel viscosity and a moderate thickness of the gel layer adsorbed on the outer side of the seed layer (providing sufficient nutrients for membrane growth without causing excessive growth and reducing separation performance). The residual moisture inside the carrier facilitates the diffusion of free nutrients from the gel into the carrier, providing the necessary nutrients for membrane growth during subsequent crystallization. This increases membrane density without clogging the carrier pores, resulting in a high-flux NaA molecular sieve membrane (Example 1). However, when the seed layer is dried and then re-coated with water from the outside of the tube, small-sized seeds can enter the carrier due to capillary action. Subsequent coating with gel solution, while achieving a moist seed layer, allows the small seeds inside the carrier to induce membrane growth during subsequent crystallization. These seeds absorb nutrients from the inside of the carrier, grow within the pores, and clog the pores, causing a significant decrease in membrane flux (Comparative Example 2). When water is applied to the inside of the dried seed layer, the water diffuses from the inside out, preventing the seeds from penetrating into the carrier. However, the diffusion of water causes seed movement, reducing the seed layer coverage and resulting in larger pores on the seed layer surface. Pinholes were observed on the membrane surface, and exposed support could be seen in some areas, but no transition layer was observed on the support side, resulting in a separation factor of only 5778 (Comparative Example 3). This indicates that directly coating the wet seed crystal layer with the gel solution without drying is key to preparing high-performance NaA zeolite membranes, forming a membrane layer of moderate thickness without clogging the support pores. However, the composition of the gel solution also has a significant impact on membrane flux. For example, in Comparative Example 4, when the water content in the gel solution is low, the gel viscosity is high, resulting in a thicker gel layer adhering to the seed crystal layer. This also hinders the diffusion of free nutrients in the gel, leading to a thicker membrane layer (3.5 μm) and reduced membrane flux.

[0062] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for synthesizing NaA molecular sieve membranes by crystallization of a wet gel layer, characterized in that... include: A. Immerse the porous carrier in water, then remove it and allow it to air dry at room temperature until the water adsorption capacity of the carrier is 100-175 mg / cm³. 3 A moistened carrier is obtained, and seed crystals are coated on its surface to obtain a moistened seeded carrier; the pore size of the porous carrier is 0.1-100 μm; the seed crystal content in the obtained moistened seeded carrier is 0.3-0.8 mg / cm³. 2 The seed crystals are NaA molecular sieve seed crystals, and the particle size range of the seed crystals is 0.1-5 μm; B. The moistened seeded carrier is directly immersed in the aged gel solution without drying for 0.1-4 hours; the molar ratio of the gel solution is Na2O:SiO2:Al2O3:H2O=2-3:2:1:150-500; after immersion, the carrier is taken out, placed in a reaction vessel, sealed, and heated for reaction, without adding solvent, gel solution, or template agent to the reaction vessel; after the reaction, a carrier with a NaA molecular sieve membrane layer grown on its surface is obtained, which is taken out, cleaned, and dried to obtain the NaA molecular sieve membrane.

2. The method according to claim 1, characterized in that... The porous carrier in step A is a porous alumina carrier, a stainless steel carrier, or a hollow fiber carrier.

3. The method according to claim 1, characterized in that... The molar ratio of the gel solution in step B is Na2O:SiO2:Al2O3:H2O = 2-3:2:1:200-250.

4. The method according to claim 1, characterized in that... The aging time of the gel solution in step B is 3-72 hours.

5. The method according to claim 1, characterized in that... In step B, the reactor is either in a static or rotating state.

6. The method according to claim 1, characterized in that... The heating temperature in step B is 90-110°C. o C; The heating reaction time is 3-6 hours.

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