Method for preparing large-area oriented molecular sieve mixed matrix membrane based on selective etching

Abstract

This invention discloses a method for preparing large-area oriented molecular sieve mixed matrix membranes based on selective etching. Selective etching adjusts the morphology of MFI molecular sieve crystals to a b-oriented sheet-like material, maximizing the exposure of the (0k0) crystal planes and providing a foundation for the orderly arrangement of nanosheets within the polymer matrix. By utilizing a solution casting strategy and controlling the solvent evaporation temperature, combined with the gel-curing characteristics of Pebax at room temperature, the aggregation of molecular sieve materials is effectively prevented, allowing them to form a planar arrangement within the polymer matrix. This significantly improves the exposure of vertical transport channels, thereby enhancing the membrane's molecular transport efficiency, gas and liquid permeation flux and selectivity, while also strengthening the membrane's stability and service life. This method overcomes the incompatibility between traditional fillers and polymers, successfully preparing large-area flexible mixed matrix MFI molecular sieve membranes and ultrathin mixed matrix membranes, effectively avoiding filler aggregation and non-selective defects.

Description

Technical Field

[0001] This invention belongs to the field of separation membrane preparation technology, and particularly relates to a method for preparing large-area oriented molecular sieve mixed matrix membranes based on selective etching. Background Technology

[0002] To meet the industrial demand for high-efficiency gas separation membranes, existing methods for preparing zeolite-based mixed-matrix membranes employ various techniques. First, selecting suitable polymers and inorganic fillers for the target separation application is a crucial step. Fillers (such as zeolites, metal-organic frameworks, etc.) typically require surface functionalization to improve their compatibility with the polymer matrix, for example, by modifying them with silane coupling agents or other surfactants. The functionalized fillers are then uniformly dispersed in a polymer solution, and the homogeneity of the mixed suspension is ensured through ultrasonic treatment and high-shear stirring. The mixed suspension is then formed into a membrane using methods such as casting, spin coating, or doctor blade coating, and subsequently cured through solvent evaporation or phase inversion.

[0003] Annealing is a common method to further improve membrane performance. Annealing the membrane above the polymer's glass transition temperature (Tg) enhances the interfacial bonding between the zeolite and the polymer matrix. However, annealing can lead to high-density packing of polymer chains, affecting membrane permeability and selectivity. Therefore, rapid cooling after annealing can be employed to maintain high free volume of the polymer chains, thereby improving gas permeability without reducing selectivity.

[0004] Priming methods are also widely used. By coating the surface of the packing particles with a diluted polymer solution, the compatibility between the packing and the polymer matrix is ​​improved, interfacial defects are reduced, and thus the separation performance of the membrane is enhanced. In addition, template methods and in-situ growth methods achieve uniform distribution and tight bonding by introducing template agents into the polymer matrix or directly growing zeolite materials in situ in the matrix, further optimizing the structure and performance of the membrane.

[0005] With the increasing industrial demand for efficient and low-cost gas separation solutions, these technologies, combining knowledge from materials science, chemical engineering, and membrane technology, have driven the application and development of zeolite mixed matrix membranes in fields such as carbon capture and gas purification. However, the following problems still exist:

[0006] Poor adhesion between polymers and fillers leads to non-selective interfacial voids; polymer aging issues or inherent permeability-selectivity trade-offs make it difficult to achieve a balance between high permeability and sufficient selectivity; traditional incompatibility between inorganic fillers and polymers; precipitation or agglomeration of materials during preparation may affect membrane uniformity and performance, thus requiring solutions to filler dispersibility issues. Summary of the Invention

[0007] The purpose of this application is to provide a method for preparing large-area oriented molecular sieve mixed matrix membranes based on selective etching, which overcomes the limitations of traditional polymer membranes and zeolite membranes in gas separation. By combining high-performance CO2-affinity MFI zeolite filler with polyether block polyamide (pebax) polymer, a b-oriented (0k0) mixed matrix membrane that can be prepared over a large area is developed, achieving a balance between high permeability and high selectivity.

[0008] According to a first aspect of the embodiments of this application, a method for preparing a large-area oriented molecular sieve mixed matrix membrane based on selective etching is provided, comprising:

[0009] (1) Synthesis of sheet-like MFI molecular sieve crystals: A mixture with a molar ratio of 0.15 TPAOH:1 TEOS:1.6 NH4F:15~300 H2O was added to a homogeneous reaction oven, heated and rotated to obtain sheet-like MFI molecular sieve crystals. The template agent was removed by calcination. Through rapid optimization of the synthesis conditions, the generation of twins and co-occurrences in the sheet-like MFI material was reduced.

[0010] (2) Crystallization adjustment of sheet-like MFI molecular sieve crystals: The sheet-like MFI molecular sieve crystals are placed in TPAOH solution, and after etching, the sheet-like MFI molecular sieve crystals are subjected to tip ultrasonication and centrifugation to obtain molecular sieves without twins and crosses. The MFI molecular sieve crystals are adjusted to be b-oriented sheet-like materials by selective etching.

[0011] (3) Preparation of mixed matrix membrane: Prepare polymer solution and molecular sieve solution, mix the polymer solution and molecular sieve solution and pour into petri dish, let stand and dry overnight and vacuum dry to obtain mixed matrix membrane. Combine solution casting and gel solidification technology to realize the directional self-assembly of b-axis oriented molecular sieve material in polymer.

[0012] (4) Post-treatment of the mixed matrix membrane: The mixed matrix membrane was soaked in deionized water, then peeled off from the culture dish and dried to obtain a large-area, flexible and stable self-supporting molecular sieve mixed matrix membrane.

[0013] Further, in step (1), the temperature is heated to 100~120℃, so that the MFI crystals tend to grow along the a-axis and c-axis and exhibit a relatively fast crystallization rate. The rotation speed is 40-80 rpm. The sheet-like MFI molecular sieve crystals are placed in a tube furnace at 500-550℃ for calcination for more than 10 hours to remove the organic template agent in the molecular sieve pores.

[0014] Further, in step (2), a 0.18-0.23 M TPAOH solution is used to selectively act on the defect sites of the MFI crystal. The etching temperature is 90-120℃, the etching time is 4-24h, and the tip ultrasonic time is 5-10 minutes.

[0015] Further, in step (3), the polymer is selected from pedax1047, pedax1657, pedax4533, and pedax2533. After mixing the polymer solution and the molecular sieve solution and pouring them into the petri dish, the overnight drying temperature is 50-70°C. This avoids the sheet material from drying completely before the directional assembly is completed, forming a disordered film, and avoids the solvent inside the film from not being completely removed and clogging the pores due to the low temperature.

[0016] In one possible implementation, in step (3), a porous polymer substrate is attached to the surface of the solution. The porous polymer substrate can be polyvinylidene fluoride (PVDF), polycarbonate (PCTE) membrane, polytetrafluoroethylene (PTFE) membrane, polyethersulfone (PES) membrane, nylon membrane, etc. Ultrathin membranes have low transport resistance during separation, so the separation efficiency is higher. By preparing an ultrathin mixed matrix molecular sieve membrane on a porous polymer substrate, the difficulties in post-processing can be reduced, and the preparation is more convenient.

[0017] According to a second aspect of the embodiments of this application, a large-area oriented molecular sieve mixed matrix membrane is provided, which is prepared by the method described in the first aspect.

[0018] The technical solutions provided by the embodiments of this application may include the following beneficial effects:

[0019] As can be seen from the above embodiments, this application utilizes selective etching to adjust the morphology of MFI molecular sieve crystals into b-oriented sheet materials, thereby maximizing the exposure of (0k0) crystal planes and providing a basis for the directional arrangement of nanosheets within the polymer matrix. By employing a solution casting strategy and controlling the solvent evaporation temperature, and leveraging the gel-curing property of Pebax at room temperature, the aggregation of molecular sieve materials is prevented, enabling the sheet-like molecular sieve materials to achieve planar alignment within the polymer matrix. This maximizes the exposure of vertical transport channels, providing the shortest molecular transport path, improving gas or liquid permeation flux and selectivity, while also enhancing membrane performance and stability. Consequently, it results in longer lifespan and higher gas or liquid separation efficiency in practical applications. The method improves the selectivity of polymers during separation by loading permeable and sieving molecular sieves between polymers, thus overcoming the inherent problem of high permeability but low selectivity of polymers. Utilizing the advantages of two-dimensional sheet materials, the adhesion between the polymer and the membrane is increased, reducing interfacial porosity. This also solves the incompatibility between the polymer matrix and inorganic nanomaterials, successfully preparing self-supporting large-area flexible mixed matrix MFI molecular sieve membranes, as well as ultrathin mixed matrix molecular sieve membranes prepared on porous polymer matrices. This avoids the defects of filler aggregation and non-selectivity, and prevents delamination, demonstrating the versatility of this method and providing a high-performance solution for the separation of various gases and liquids.

[0020] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0021] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0022] Figure 1 The images shown are scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns of the synthesized sheet-like MFI crystals in Example 1, where a) is a scanning electron microscope image of the synthesized sheet-like MFI material, b) is the molecular sieve size and thickness statistics obtained by optimizing the molar ratio of water and silica in this invention, and c) is an XRD pattern of the synthesized MFI material.

[0023] Figure 2 This is a schematic diagram illustrating the preparation of the hybrid matrix membrane in Example 1;

[0024] Figure 3 These are the casting solutions with different mass percentages and the corresponding mixed matrix films in Example 1;

[0025] Figure 4The images show a top view and a cross-sectional electron micrograph of the mixed matrix membrane in Example 1, where a) is a top view of the membrane with a loading of 65%, and bd) are cross-sectional images of the membrane at different magnifications.

[0026] Figure 5 This is a schematic diagram illustrating the preparation of the hybrid matrix membrane in Example 2;

[0027] Figure 6 This is an electronic image of the mixed matrix film on the polymer substrate in Example 2;

[0028] Figure 7 The images show X-ray diffraction patterns of 65% MFI loading in different polymer matrices in Example 3. Detailed Implementation

[0029] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application.

[0030] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application. As used in this application and the appended claims, the singular forms "a," "an," "the," and "the" are intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0031] Example 1: Preparation of self-supporting large-area oriented molecular sieve hybrid matrix membrane

[0032] (1) Synthesis of sheet-like MFI molecular sieve crystals: A mixture with a molar ratio of 0.15 TPAOH (tetrapropylammonium hydroxide): 1 TEOS (tetraethoxysilane): 1.6 NH4F (ammonium fluoride): 15~300 H2O (pure water) was added to a homogeneous reaction oven, heated to 100~120℃ and rotated at a speed of 40-80 rpm to obtain sheet-like MFI molecular sieve crystals, and the template agent was removed by calcination;

[0033] To prepare plate-like MFI crystals with large size and relatively small crystal thickness, we conducted experiments at a TEOS / H2O molar ratio of 90. We observed that the lack of mechanical stirring during the aging stage led to abundant twinning, resulting in orthogonal staggered growth of all crystals after crystallization. However, stirring the aging precursor solution significantly reduced twinning. Inspired by this, a rotational synthesis strategy was employed to reduce twinning during crystallization.

[0034] In one embodiment, 8.598 g TPAOH (25%), 14.605 g TEOS (tetraethoxysilane), and 4 g water were added to a homogeneous reaction oven containing polytetrafluoroethylene (PTFE) and rotated at 60 rpm. The homogeneous reaction oven was a rotatable reactor containing five PTFE cubes with sides of 3 mm for stirring to ensure system homogeneity. Other heat-resistant materials that do not react with the reaction system could also be used. The reactor rotation speed during the reaction was 40-80 rpm. Based on temperature optimization, between 100 and 120 °C, MFI crystals tended to grow along both dimensions (a and c axes) and exhibited a relatively fast crystallization rate. Therefore, at 120 °C, after 12 hours of reaction, fortunately, the number of twins was significantly reduced after synthesis. This was attributed to rotational interference hindering silicon absorption on the crystal plates, thereby preventing staggered twin growth. The resulting powder was calcined in a tube furnace at 500-550℃ for more than 10 hours, with air continuously introduced during the process to remove the organic template agent from the molecular sieve pores.

[0035] The size and morphology of the obtained product were confirmed by scanning electron microscopy, and the crystallinity of the product was confirmed by X-ray diffraction. Figure 1 As shown in a), this step can yield an ultra-large sheet-like MFI crystal with a size of 5.2 × 1.65 µm and a thickness of 150 ± 10 nm. Figure 1 Figure b) shows the effect of the optimized H2O / TEOS ratio of this application on thickness and size. The higher the water content in the reactants, the larger the crystal size and the thicker the crystal. Figure 1 c) shows that the material obtained in this step has very good crystallinity. The plate-like MFI crystals overcome the traditional incompatibility between inorganic fillers and polymers, mainly because the plate-like material has a large and flat outer surface, which allows it to adhere very well to the polymer. The plate-like MFI crystals are usually accompanied by twinning and symbiosis by conventional synthesis methods, which inhibits the application of this material in separation membranes. In the preparation process, a rotational synthesis strategy is used to reduce the formation of twins during crystallization.

[0036] (2) Crystallization adjustment of plate-shaped MFI molecular sieve crystals: The plate-shaped MFI molecular sieve crystals are placed in 0.18-0.23 MTPAOH solution, and the plate-shaped MFI molecular sieve crystals are etched at 90-120℃ for 4-24h and then subjected to tip sonication for 5-10 minutes to obtain molecular sieves without twins and crosses; the samples are collected by centrifugation at 5000 rpm.

[0037] By selectively etching, the morphology of MFI molecular sieve crystals is adjusted to a b-oriented plate-like material, maximizing the exposure of the (0k0) crystal plane. To obtain highly b-oriented crystals, this invention employs a selective etching strategy to remove twins using a diluted TPAOH solution with a concentration ranging from 0.18 to 0.23 M for heterogeneous etching. This solution selectively targets defect sites in the MFI crystals. This invention explores the effects of etching time and tip ultrasound on the morphology of MFI crystals (plate-like and coffin-like crystals). By varying the etching time in the TPAOH solution, we observed the changes in the morphology of plate-like MFI crystals with increasing etching time. Etching was performed at 90°C for different times (4 hours, 8 hours, 12 hours, 18 hours, and 24 hours). In the initial stage, the plate-like crystals were not etched. When weak etching was performed at 90°C for 4-8 hours without sharp ultrasonic treatment, the morphology of the twins remained unchanged. However, after a few seconds of electron beam irradiation, a large hole appeared, indicating that the crystallinity of the crystals was weaker than before, showing the potential to separate the twins from the etched crystals. Subsequently, weak etching without destroying the shape could be observed for 12-18 hours. Expectedly, by combining 5-10 minutes of sharp ultrasonic treatment with 18 hours of etching, the twins could be separated from the plate without destroying the shape of the plate. Due to the weak crystallinity of the twins and co-occurrences, the free hydroxyl groups in the alkaline solution can etch the crystalline areas. TPAOH is selective and can spontaneously and selectively etch the weakly crystallinity areas, removing the twins and cross-sections. Therefore, the resulting material can always be stacked in a beta-oriented order, thus enabling the preparation of oriented films.

[0038] The molecular sieve obtained in this step is an MFI type mesoporous molecular sieve with a pore size of 0.56-0.58 nm.

[0039] (3) Preparation of mixed matrix membrane: Prepare a 1%-5% pedax polymer solution and an 8% molecular sieve solution by mass fraction. Mix the polymer solution and the molecular sieve solution. The mass fraction of molecular sieve in the mixed matrix solution is determined according to the requirements (the maximum can be configured to 65%). The higher the mass fraction, the stronger the sieving effect and the higher the selectivity. Take 0.1g-2g and pour it into a glass petri dish. After standing, place it in an oven at 50-70℃ for overnight drying, and then vacuum dry for 2 hours to obtain the mixed matrix membrane.

[0040] To ensure optimal separation efficiency, this invention develops a suitable strategy to achieve the directional alignment of nanosheets within a polymer matrix. Specifically, by adjusting parameters such as the concentration of the polymer solution and the solvent evaporation temperature, the nanosheets gradually self-align according to the lowest energy configuration, forming a planar structure. Furthermore, the gelation properties of the Pebax polymer at room temperature effectively prevent the aggregation of nanosheets, ensuring consistent directional alignment within the polymer matrix, thereby forming an ideal molecular separation structure and improving membrane performance and application potential.

[0041] The purchased granular Pebax polymer (1047, 4533, or 2533, etc.) was dissolved in n-butanol and refluxed at 90°C for 8 hours. The polymer solution had a mass percentage of 1-5%. The etched molecular sieve was prepared into an 8% solution using ethanol and water in a mass ratio of 7:3 and ultrasonically dispersed. The polymer and molecular sieve solutions were then mixed to prepare solutions with molecular sieve-to-polymer mass ratios (i.e., molecular sieve loading) of 0%, 3.5%, 10%, 20%, 30%, 45%, 55%, and 65%, respectively. 0.1-2g of each solution was poured into glass petri dishes, allowed to stand for 1 hour, and then dried overnight in an oven at 50-70°C to obtain a mixed matrix membrane. Figure 2 As shown. Then, vacuum drying is performed for 2 hours to remove residual solvent from the oven and to prevent the inhalation of evaporating organic solvents. The solvent evaporation temperature should not be too high (not >70℃), otherwise the sheet material will be completely dried before it has completed its directional assembly, easily leading to the formation of a disordered membrane. If the evaporation temperature is too low (not <50℃), the solvent inside the membrane cannot be completely removed, clogging the pores. This step solves the problem of inorganic material precipitation and aggregation in the polymer. The polymer pedax used easily forms a gel at room temperature (curing within 5 minutes), therefore, inorganic materials will not precipitate or agglomerate during the preparation of the mixed matrix membrane.

[0042] The polyether block polyamide (pebax) mentioned therein is 1047, 1657, 4533, 2533, etc. In specific implementation, the solvent is selected according to the type of pebax selected. For example, pebax 1657 can also be a mixed solvent of ethanol and water.

[0043] (4) Post-treatment of the mixed matrix membrane: After soaking the mixed matrix membrane in deionized water for 1-2 minutes, peel it off from the glass culture dish and dry it in an oven at 100°C for 12 hours to remove the solvent and water remaining in the molecular sieve channels;

[0044] Soak the dried mixed matrix membrane in deionized water for 1-2 minutes, peel the mixed matrix membrane off the glass with tweezers, and place it on a clean glass container to dry in an oven at 100℃ for 12 hours. The resulting self-supporting mixed matrix membrane is shown below. Figure 3 As shown. The morphology and cross-sectional images of the dried hybrid matrix film were characterized using scanning electron microscopy, as shown. Figure 4 As shown, Figure 4 a) shows good compatibility between the sheet material and the polymer, with no defects. Figure 4 Figures b) to d) show cross-sectional views of the membrane, revealing a layered stacking morphology, confirming that the membrane prepared by the above method is highly β-oriented. In this embodiment, the preparation of the self-supporting membrane overcomes the limitations imposed by the membrane substrate, and this self-supporting membrane can meet the requirements of industrial applications; the size of the self-supporting membrane can be customized according to the equipment size.

[0045] Example 2: Preparation of ultrathin hybrid matrix molecular sieve membranes on polymer substrates

[0046] (1) Synthesize sheet-like MFI molecular sieve crystals according to step (1) in Example 1;

[0047] (2) Adjust the crystal state of the sheet-like MFI molecular sieve crystals according to step (2) in Example 1;

[0048] (3) Prepare the mixed matrix membrane according to step (3) in Example 1, such as Figure 5 As shown, after pouring the prepared mixed matrix solution into a glass petri dish, a porous polymer substrate was attached to the surface of the mixed matrix solution, and the dish was dried in an oven at 50-70℃ for 12 hours, followed by drying under vacuum for 12 hours. The resulting product is shown below. Figure 6 As shown. The polymer substrate can be polyvinylidene fluoride (PVDF), polycarbonate (PCTE) membrane, polytetrafluoroethylene (PTFE) membrane, polyethersulfone (PES) membrane, nylon membrane, etc.

[0049] (4) Perform post-treatment of the mixed matrix membrane according to step (4) in Example 1.

[0050] Example 3: Preparation of b-oriented self-supporting hybrid matrix molecular sieve membranes using different polymers

[0051] (1) Synthesize sheet-like MFI molecular sieve crystals according to step (1) in Example 1;

[0052] (2) Adjust the crystal state of the sheet-like MFI molecular sieve crystals according to step (2) in Example 1;

[0053] (3) Prepare the mixed matrix membrane according to step (3) in Example 1, wherein the polymer used is replaced, and the replaceable polymers include but are not limited to pebax 1657, pebax 1074, pebax 4533 and pebax 2533.

[0054] (4) Post-processing of the mixed matrix membrane was performed according to step (4) in Example 1. The orientation of the dried membrane was characterized by X-ray diffraction, such as... Figure 7 As shown, it exhibits high β-orientation, demonstrating excellent compatibility between the sheet-like molecular sieve material and different polymer matrices.

[0055] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein.

[0056] It should be understood that this application is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope.

Claims

1. A method for preparing large-area oriented molecular sieve mixed matrix membranes based on selective etching, characterized in that, include: (1) Synthesis of sheet-like MFI molecular sieve crystals: A mixture with a molar ratio of 0.15 TPAOH:1 TEOS:1.6 NH4F:15~300 H2O was added to a homogeneous reaction oven, heated and rotated to obtain sheet-like MFI molecular sieve crystals, and calcined to remove the template agent; (2) Crystallization adjustment of plate-shaped MFI molecular sieve crystals: The plate-shaped MFI molecular sieve crystals are placed in TPAOH solution, etched, and then subjected to tip sonication and centrifugation to obtain molecular sieves without twins and crosses. (3) Preparation of mixed matrix membrane: Prepare polymer solution and molecular sieve solution, mix the polymer solution and molecular sieve solution and pour into petri dish, let stand and then dry overnight and vacuum dry to obtain mixed matrix membrane; (4) Post-treatment of the mixed matrix membrane: After soaking the mixed matrix membrane in deionized water, peel it off from the culture dish and dry it.

2. The method according to claim 1, characterized in that, In step (1), heat to 100~120℃ and rotate at 40-80 rpm.

3. The method according to claim 1, characterized in that, In step (1), the sheet-like MFI molecular sieve crystals are calcined in a tube furnace at 500-550℃ for more than 10 hours.

4. The method according to claim 1, characterized in that, In step (2), a 0.18-0.23 MTPAOH solution is used.

5. The method according to claim 1, characterized in that, In step (2), the etching temperature is 90-120℃ and the etching time is 4-24h.

6. The method according to claim 1, characterized in that, In step (2), the tip ultrasound time is 5-10 minutes.

7. The method according to claim 1, characterized in that, In step (3), the polymer is selected from pedax1047, pedax1657, pedax4533, and pedax2533.

8. The method according to claim 1, characterized in that, In step (3), after mixing the polymer solution and the molecular sieve solution and pouring them into a petri dish, the porous polymer substrate is attached to the surface of the solution.

9. The method according to claim 1, characterized in that, In step (3), the overnight drying temperature is 50-70℃.

10. A large-area oriented molecular sieve mixed matrix membrane, characterized in that, It is prepared by the method according to any one of claims 1-9.

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