A method for synthesizing hierarchical pore MFI molecular sieve membrane based on sacrificial seed layer

Abstract

The application discloses a method for synthesizing a hierarchical-pore MFI molecular sieve membrane based on a sacrifice seed layer and belongs to the field of zeolite molecular sieve membrane synthesis. The application successfully constructs a hierarchical-pore MFI molecular sieve membrane with a dense selective top layer and a macroporous transition layer with high porosity on a porous carrier through a surface wet gel secondary growth technology of a sacrifice seed layer. The membrane layer structure and gas separation performance are optimized by adjusting and controlling a synthesis gel composition, an aging time and a crystallization time. The hierarchical-pore MFI molecular sieve membrane has high permeability and excellent selectivity in n-butane / isobutane separation, has good preparation process repeatability and high stability, and is suitable for industrial gas separation application.

Description

Technical Field

[0001] This invention relates to a method for synthesizing zeolite molecular sieve membranes, specifically a method for synthesizing hierarchical porous MFI molecular sieve membranes based on sacrificial seed layers. Background Technology

[0002] Zeolite molecular sieve membranes, due to their molecular-level channels, rigid framework, and tunable surface chemistry, are considered capable of achieving highly selective and stable separation of important industrial gas mixtures such as O2 / N2, C2H4 / C2H6, and n / isobutane. However, existing technologies generally suffer from a trade-off between permeability and selectivity, severely limiting their industrial-scale application. To overcome these bottlenecks, researchers have proposed various performance optimization strategies, among which "microstructure regulation" has proven to be the most effective. Typical methods include simply thinning the separation layer and using nano-zeolite seeds for secondary growth, which can reduce the thickness of FAU-type or CHA-type films to 600 nm or 500 nm, respectively, thereby increasing water vapor permeation flux [Sep. Purif. Technol. 2022, 294, 121177]; or preparing a 250 nm thick ultrafilm using two-dimensional MFI nanosheets with a thickness of about 5 nm to achieve highly selective separation of xylene isomers [Nature 2017, 543, 690–694]. Although the above methods can improve flux, they require the pre-preparation of highly dispersed nanocrystals or two-dimensional nanosheets, which are complex processes, have low yields, rely on expensive surfactants, are difficult to scale up, and have limited versatility.

[0003] Another approach is to introduce amorphous mesopores / macropores into the zeolite separation layer to reduce the effective thickness and shorten the diffusion path. For example, hollow MFI nanocrystals can be constructed first, followed by secondary growth using single-mode microwave heating to obtain a hierarchical porous membrane, which can improve the n-butane permeability while maintaining high selectivity [Angew. Chem. Int. Ed. 2021, 60 (14), 7659–7663]; or a hierarchical porous LTA membrane can be prepared by vapor phase inversion using mesoporous silica microspheres as a hard template, which outperforms conventional membranes in pervaporation of 90wt% ethanol aqueous solution [J. Membr. Sci. 2023, 671, 121391]. However, these techniques still require the additional preparation of "hollow seeds" or "hard templates," which are cumbersome, costly, and prone to introducing defects during template removal.

[0004] In summary, existing technologies suffer from at least the following shortcomings: the preparation process of ultrathin separation layers is complex, costly, and difficult to scale up; the introduction of mesopores / macropores requires the pre-construction of hollow seeds or hard templates, involving multiple steps and large amounts of waste liquid; and there is no simple method to simultaneously form a low-resistance mesopore / macropore transition layer and a dense selective layer during film formation. Therefore, there is an urgent need for a new method that is simple, low-cost, and easy to scale up, capable of simultaneously constructing multi-level porous molecular sieve membranes with controllable thickness without the need for additional templates or surfactants, in order to achieve a synergistic improvement in gas separation efficiency, characterized by "high throughput and high selectivity." Summary of the Invention

[0005] This invention aims to reduce the preparation cost of hierarchical porous MFI molecular sieve membranes and solve the problems of poor reproducibility, complex processes, and low gel utilization in existing technologies. It provides a method for synthesizing hierarchical porous MFI molecular sieve membranes based on a wet gel secondary growth method with a sacrificial seed layer. The method of this invention is simple, highly reproducible, and has a high gel utilization rate. It can directly construct high-performance hierarchical porous MFI molecular sieve membranes on alumina tubes and has good prospects for industrialization.

[0006] According to a first aspect of the present invention, the present invention provides a method for synthesizing hierarchical porous MFI molecular sieve membranes based on sacrificial seed layers, comprising:

[0007] A. Seed layer preparation: MFI molecular sieve seed crystals, which have been calcined to remove the template agent, are coated onto the surface of a porous carrier to form a seed layer;

[0008] B. Gel preparation: Prepare a gel with the following molar composition: XOH: SiO2: TPAOH: H2O = 0.02-0.15:1: 0.24: 20-100, where X is Na or K. The SiO2 used in the gel is a slow-release silicon source, and it is aged for 3-120 hours before use.

[0009] C. Preparation of membrane layer: The dry carrier coated with seed layer is immersed in the aged gel solution. After immersion, it is taken out and a gel layer is uniformly loaded on the surface of the carrier. Then, the carrier is transferred to the reaction vessel and sealed for heating. No additional solvent, gel solution or template agent is added to the reaction vessel. After heating, the carrier is taken out and cleaned, dried and activated to obtain a multi-level porous MFI molecular sieve membrane.

[0010] As a preferred embodiment of the present invention, the porous carrier in step A is a porous alumina carrier, a porous zirconia carrier, or a stainless steel carrier; the pore size of the porous carrier is 0.1-10 μm. The shape of the porous carrier can be selected according to the actual needs of the application scenario, for example, a tubular carrier, a sheet-like carrier, or a hollow fiber carrier.

[0011] Furthermore, in step A, the carrier can be an alumina tube with an outer diameter >10 mm, and its pore size is preferably 0.1-5 μm.

[0012] In a preferred embodiment of the present invention, in step A, the seed crystals are coated using a wiping method. Before the wiping step, the porous carrier is immersed and wetted in an alcohol solvent (such as n-butanol), and the immersion time is preferably 5-30 seconds. After immersion and wetting, the surface of the carrier is wetted and free of droplets.

[0013] Furthermore, the seed crystals mentioned in step A are MFI molecular sieve seed crystals, with a particle size range of 0.1-5 μm and a silicon-to-aluminum ratio greater than or equal to 10. The MFI molecular sieve seed crystals must be pre-treated by calcination to remove the template agent at a temperature of 400-600℃ for 5-10 hours.

[0014] Furthermore, the seed-loaded carrier obtained after seed coating in step A has a seed layer loading of 0.1-1 mg / cm³. 2 .

[0015] As a preferred embodiment of the present invention, the slow-release silicon source used in the gel solution of step B is a silicon source such as silica gel, silica sol or fumed silica.

[0016] As a preferred embodiment of the present invention, the molar ratio of the gel solution in step B is NaOH:SiO2:TPAOH:H2O=0.02-0.15:1:0.24:20-100, preferably 0.02-0.15:1:0.24:40-60.

[0017] As a preferred embodiment of the present invention, the aging of the gel solution in step B is carried out at room temperature to 50°C for 3-120 h.

[0018] As a preferred embodiment of the present invention, in step C, the carrier carrying the seed crystal layer is immersed in the gel solution for 0.01-1 h, and after being taken out, a uniform gel layer is formed on its surface.

[0019] In a preferred embodiment of the present invention, the heating reaction temperature in step C is 150-200℃, and the heating reaction time is 3-48 h. The activation treatment temperature in step C is 400-600℃, and the activation time is 5-10 h.

[0020] According to a second aspect of the present invention, the present invention provides a hierarchical porous MFI molecular sieve membrane prepared by the aforementioned method.

[0021] As a preferred embodiment of the present invention, the multi-level porous MFI molecular sieve membrane simultaneously has a defect-free selective top layer and a low-resistance macroporous transition layer.

[0022] According to a third aspect of the present invention, the present invention provides the application of the aforementioned hierarchical porous MFI molecular sieve membrane in the separation of mixed gases. The mixed gas contains at least n-butane and isobutane, preferably, the mixed gas is a mixture of n-butane and isobutane. The hierarchical porous MFI molecular sieve membrane provided by the present invention can achieve the separation of n-butane and isobutane.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. This invention utilizes a wet gel secondary growth method based on a sacrificial seed layer to prepare MFI (HMFI) zeolite membranes with a hierarchical porous structure. This method can simultaneously form a defect-free selective top layer and a low-resistance macroporous transition layer, without the need for expensive organic templates or special (e.g., hollow) seed layers. During the crystallization process, OH... - Etching, TPA + Protection and Na + / TPA + (or K) + / TPA + The synergistic effect of competitive adsorption promotes the in-situ transformation of the seed layer, after the removal of the organic template agent, into a macroporous layer, thus establishing a hierarchical structure. Simultaneously, the dissolution of the seeds releases active nuclei, providing a high density of nucleation sites on the surface, promoting the formation and vertical growth of more small crystals, thereby fostering the formation of a dense, continuous layer. Due to this asymmetric structure and defect-free surface, the HMFI membrane achieved a fourfold improvement in the butane isomer separation factor (reaching 74), and a 37% increase in n-butane permeability (n-butane membrane permeation reached 1.2 × 10⁻⁶). -7 mol m -2 pa -1 s -1 The performance of this membrane far exceeds that of traditional MFI membranes. Furthermore, the wet gel layer secondary growth method of this invention can significantly improve the utilization rate of the synthesized gel, reduce the use of raw materials, and decrease waste liquid generation.

[0025] 2. This invention adjusts the Na content in the gel. + (or K) + ), TPA + and OH - The synergistic effect of these mechanisms enables the regulation of the dissolution and growth of the seed layer during the synthesis of molecular sieve membranes, thereby revealing the special function of the seed layer and providing theoretical and practical guidance for the structural optimization of molecular sieve membranes, which is of great scientific significance.

[0026] 3. This invention uses low-activity silica sol (Ludox) or fumed silica as the silicon source. By reducing the crystallization rate of the gel system and utilizing its slow silicon release characteristics, continuous surface crystallization is achieved while etching the seed layer, thereby balancing the etching and growth processes and successfully realizing the controllable preparation of hierarchical porous MFI molecular sieve membranes. Attached Figure Description

[0027] Figure 1 SEM image of the rough alumina tube with seed layer loaded;

[0028] Figure 2 The surface SEM image of a hierarchical porous MFI molecular sieve membrane prepared based on calcined MFI seeds and wet gelation method, with a gel aging time of 48 h and NaOH as the alkali source;

[0029] Figure 3 The image shows a cross-sectional SEM image of a hierarchical porous MFI molecular sieve membrane prepared based on calcined MFI seeds and wet gelation method, with a gel aging time of 48 h and NaOH as the alkali source.

[0030] Figure 4 The surface SEM image of a hierarchical porous MFI molecular sieve membrane prepared based on calcined MFI seeds and wet gelation method, with a gel aging time of 72 h and NaOH as the alkali source;

[0031] Figure 5 The image shows a cross-sectional SEM image of a hierarchical porous MFI molecular sieve membrane prepared based on calcined MFI seeds and wet gelation method, with a gel aging time of 72 h and NaOH as the alkali source.

[0032] Figure 6 The surface SEM image of the hierarchical porous MFI molecular sieve membrane prepared based on calcined MFI seeds and wet gelation method, with a gel aging time of 48 h and KOH as the alkali source;

[0033] Figure 7 The image shows a cross-sectional SEM image of a hierarchical porous MFI molecular sieve membrane prepared based on calcined MFI seeds and wet gelation method, with a gel aging time of 48 h and KOH as the alkali source.

[0034] Figure 8 SEM image of the surface of MFI molecular sieve membrane synthesized by conventional hydrothermal synthesis;

[0035] Figure 9 This is a cross-sectional SEM image of a conventionally hydrothermally synthesized MFI molecular sieve membrane.

[0036] Figure 10 The image shows the surface SEM image of an MFI molecular sieve membrane prepared using uncalcined MFI seeds and wet gelation method with a gel aging time of 48 h and NaOH as the alkali source.

[0037] Figure 11 The image shows a cross-sectional SEM image of an MFI molecular sieve membrane prepared using uncalcined MFI seeds, wet gelation method, gel aging time of 48 h, and NaOH as the alkali source.

[0038] Figure 12 The image shows the surface SEM image of an MFI molecular sieve membrane prepared by calcining MFI seeds and wet gelation method with a gel aging time of 48 h, using TEOS as the silicon source and NaOH as the alkali source.

[0039] Figure 13 The image shows a cross-sectional SEM image of an MFI molecular sieve membrane prepared using calcined MFI seeds and a wet gelation method with a gel aging time of 48 h, and TEOS as the silicon source and NaOH as the alkali source. Detailed Implementation

[0040] To better illustrate the method for synthesizing hierarchical porous MFI molecular sieve membranes from sacrificial seed layers and the advantages of the membranes produced, some examples of molecular sieve membrane synthesis are given below, but the present invention is not limited to the examples listed.

[0041] Example 1

[0042] A. Preparation of seed layers

[0043] An α-Al₂O₃ tube with an outer diameter of 12 mm, an inner diameter of 8 mm, and a length of 5 cm was ultrasonically cleaned with deionized water and ethanol for 15 min and then dried at 80 °C. Subsequently, it was immersed entirely in n-butanol (≥99%) at room temperature for 10 s, removed, and allowed to stand vertically for 1–2 min to maintain surface wetness without obvious droplets. 0.1 g of MFI seed crystals, calcined at 550 °C for 3 h, was weighed and placed in a clean petri dish. Wearing powder-free nitrile gloves, the seed crystals were picked up with the fingertip and rubbed evenly along the axial direction of the wetted support surface three times to form a continuous and uniform seed layer. Figure 1 Finally, the seed-coated support is naturally dried at room temperature and 50% relative humidity for 30 seconds, and then dried in an oven at 60℃ for 2 hours. It can then be directly used for subsequent hydrothermal synthesis of hierarchical porous MFI membranes.

[0044] B. Preparation of the gel solution

[0045] The gel molar ratio for synthesizing the HMFI membrane was 1 SiO2:0.24 TPAOH:0.12 NaOH:42 H2O. The specific steps were as follows: Weigh 1.56g TPAOH, 0.15g NaOH and 4.81g Ludox colloidal silica, add 16.65g deionized water, stir and age at room temperature for 48h to obtain the gel solution.

[0046] C. Preparation of the film layer

[0047] An α-Al₂O₃ support with a seed layer was immersed in the above gel solution for 60 seconds. After removal, a uniform layer of gel was deposited on the surface of the support. The gel-coated support was then placed vertically in a reaction vessel lined with polytetrafluoroethylene and crystallized at 170°C for 24 hours. After the reaction, the membrane tube was removed, rinsed with deionized water until neutral, and dried at 60°C overnight. Finally, it was calcined at 500°C for 10 hours to remove the template agent, yielding an HMFI molecular sieve membrane. Figure 2 The cross-section revealed a total film thickness of ~5.9 μm and a dense layer thickness of ~2.5 μm. Figure 3 The membrane was used for the separation of equimolar amounts of n-isobutane, and the separation results are shown in Table 1. At room temperature and an absolute pressure of 0.15 MPa, the membrane achieved a separation factor of 74, and the n-butane membrane permeation reached 1.2 × 10⁻⁶. -7 mol m -2 pa -1 s -1 .

[0048] Example 2

[0049] Similar to Example 1, the aging time of the gel solution in step B was extended to 72 hours at room temperature. The prepared membrane ( Figure 4 The separation performance of n-butane was tested, and the results are shown in Table 1. At room temperature and an absolute pressure of 0.15 MPa, the n-butane permeation rate was 1.8 × 10⁻⁶. -7 mol m -2 pa -1 s -1 The separation factor is 42. Figure 5 The selected layer thickness of the membrane shown is 1.5 μm. This is because extending the gel aging time generates more crystal nuclei in the gel, thus altering the membrane thickness. Therefore, by adjusting the gel aging time, the thickness of the top selected layer can be changed, thereby controlling the membrane's n- and isobutane separation performance.

[0050] Example 3

[0051] Same as Example 1, except that in step B, the NaOH in the gel solution component is replaced with KOH. The prepared membrane ( Figure 6 , Figure 7 The n-butane / isobutane separation performance was tested, and the separation results are shown in Table 1. At room temperature and an absolute pressure of 0.15 MPa, the n-butane permeation was 1.9 × 10⁻⁶. -7 mol m -2 pa -1 s -1 The separation factor is 27. Therefore, K... +This can also lead to the transformation of the seed layer into a macroporous transition layer, thereby forming a hierarchical porous membrane structure and achieving excellent n- and isobutane separation performance, confirming the universal role of alkali metal cations in the sacrificial seed layer strategy.

[0052] Comparative Example 1

[0053] A. The preparation of the seed layer is the same as in Example 1.

[0054] B. Gel preparation

[0055] Add 3.02g TEOS and 1.41g TPAOH to 44.83g deionized water to prepare a synthesis solution with a molar ratio of TEOS: TPAOH:H2O = 1:0.2:200. Stir at room temperature for 4 h to allow TEOS to be fully hydrolyzed and obtain a gel solution.

[0056] C. Preparation of the membrane

[0057] The porous alumina support loaded with the seed layer was then immersed in the synthesis solution, removed, and placed in a stainless steel autoclave with a polytetrafluoroethylene liner for hydrothermal crystallization at 170°C for 24 hours. After crystallization, the membrane sample was removed, rinsed with deionized water until neutral, dried at 60°C for 12 hours, and then calcined at 500°C for 10 hours to remove the template agent, yielding a continuous and dense MFI-type molecular sieve membrane. Figure 8 From cross-sectional SEM ( Figure 9 It can be seen that there is no hierarchical porous layer and the total membrane thickness is 6.8 μm, which is similar to that of hierarchical porous MFI molecular sieve membranes. At room temperature and an absolute pressure of 0.15 MPa, the n-butane permeation is 0.5 × 10⁻⁶. -7 mol m -2 pa -1 s -1 The separation factor is 20. MFI molecular sieve membranes prepared by traditional methods lack a macroporous layer, resulting in greater resistance when n-butane passes through, and the seed layer cannot generate active nuclei to promote the compaction of the top membrane layer.

[0058] Comparative Example 2

[0059] A. The preparation of the seed layer is the same as in Example 1. The difference is that this example uses an uncalcined seed layer containing an organic template agent (TPA). + MFI zeolite was used as a seed crystal.

[0060] B. The preparation of the gel solution is the same as in Example 1.

[0061] C. The preparation of the membrane is the same as in Example 1.

[0062] SEM images of the prepared MFI film surface are shown below. Figure 10 As shown. The cross-sectional SEM is as follows. Figure 11As shown, there is no hierarchical porous layer and the total film thickness is 5.7 μm, similar to that of hierarchical porous MFI molecular sieve membranes. This is because the seed crystal contains a template agent, which stabilizes its structure and makes it less susceptible to etching to form a macroporous layer. Figure 8 The prepared film exhibits uneven crystal size and numerous defect sites on its surface. This is because the seed crystals were not etched, thus failing to release active nuclei to promote surface film densification and crystal size uniformity. At room temperature and an absolute pressure of 0.15 MPa, the n-butane permeation rate was 0.4 × 10⁻⁶. -7 mol m -2 pa -1 s -1 The separation factor was 23, further confirming that the absence of a macroporous layer increases the resistance to n-butane passage, and that the non-dense surface layer affects the selectivity of n-isobutane separation.

[0063] Comparative Example 3

[0064] A. The preparation of the seed layer is the same as in Example 1.

[0065] B. The preparation of the gel solution is the same as in Example 1. The difference is that the silicon source used is tetraethyl orthosilicate (TEOS), an organosilicon source, instead of silica sol.

[0066] C. The preparation of the membrane is the same as in Example 1.

[0067] SEM images of the prepared MFI film surface are shown below. Figure 12 and cross-sectional SEM, such as Figure 13 As shown, a dense film layer failed to form, and the seed layer did not transform into a macroporous transition layer. This is because using highly reactive TEOS as a silicon source leads to an excessively rapid crystallization rate in the synthesized gel, resulting in a large number of free crystals in the gel that cannot continuously grow to form a dense film layer. Furthermore, the balance between the etching and recrystallization processes of the seed layer is disrupted, thus preventing the formation of a macroporous transition layer. Because the prepared film does not form a continuous film layer, n-butane / isobutane separation cannot be achieved.

[0068] Table 1. Separation performance of n-butane / isobutane membranes

[0069]

[0070] This invention utilizes a sacrificial seed layer and wet gel secondary growth method to obtain MFI molecular sieve membranes with a hierarchical porous structure under the same crystallization temperature and time. Compared with MFI molecular sieve membranes prepared by conventional hydrothermal synthesis via secondary growth method:

[0071] (1) The n-butane flux increased by 3-4 times, and the separation factor increased by 2-4 times;

[0072] (2) The thickness of the selective layer can be adjusted by simply changing the gel aging time, thereby achieving simultaneous optimization of gas flux and separation selectivity;

[0073] (3) This method does not require the preparation of special seed layers, transition layers or post-processing. The process is simple and has good repeatability. It is suitable for industrial tubular carriers with an outer diameter ≥12 mm and can be directly scaled up to continuous preparation.

[0074] 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 hierarchical porous MFI molecular sieve membranes based on sacrificial seed layers, characterized in that, Includes the following steps: A. Seed layer preparation: MFI molecular sieve seeds, after calcination to remove the template agent, are coated onto the surface of a porous carrier to form a seed layer; in step A, the seed layer is prepared by wiping the surface of the carrier, which is wetted and free of droplets, with MFI molecular sieve seeds after calcination to remove the template agent, and the seed layer loading is 0.1-1 mg / cm²; the particle size of the MFI molecular sieve seeds is 0.1-5 μm, and the silicon-to-aluminum ratio is greater than or equal to 10; the calcination temperature of the seed layer is 400-600℃, and the time is 5-10 hours; B. Gel preparation: Prepare a gel with a molar composition of XOH:SiO2:TPAOH:H2O=0.02-0.15:1:0.24:20-100, where X is Na or K. The SiO2 used in the gel is a slow-release silicon source. After aging for 3-120 hours, it is ready for use. C. Preparation of membrane layer: The dry carrier coated with seed layer is immersed in the aged gel solution. After immersion, it is taken out and a gel layer is uniformly loaded on the surface of the carrier. Then, the carrier is transferred to the reaction vessel and sealed for heating. No additional solvent, gel solution or template agent is added to the reaction vessel. After heating, the carrier is taken out, and after cleaning, drying and activation treatment, a multi-level porous MFI molecular sieve membrane is prepared.

2. The method according to claim 1, characterized in that, The porous carrier is a porous alumina carrier, a zirconium oxide carrier, or a stainless steel carrier, with a pore size ranging from 0.1 to 10 μm.

3. The method according to claim 1, characterized in that, The aging temperature of the gel solution in step B is from room temperature to 50°C, and the time is 48-72 hours.

4. The method according to claim 1, characterized in that, The slow-release silicon source used in the gel solution of step B is silica gel, silica sol, or fumed silica.

5. The method according to claim 1, characterized in that, In step C, the soaking time of the dried porous carrier coated with the seed layer in the gel solution is 0.01-1 h.

6. The method according to claim 1, characterized in that, In step C, the heating temperature is 150-200℃ and the reaction time is 3-48 h; the activation temperature is 400-600℃ and the activation time is 5-10 h.

7. A hierarchical porous MFI molecular sieve membrane prepared by the method according to any one of claims 1-6.

8. The application of the hierarchical porous MFI molecular sieve membrane of claim 7 in the separation of n-butane / isobutane.

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