A composite molecular sieve membrane for helium extraction, its preparation method and application

Abstract

This invention discloses a composite molecular sieve membrane for helium extraction, its preparation method, and its application. The membrane comprises, from bottom to top, an α-Al₂O₃ support, a CHA molecular sieve membrane interlayer, and an STT molecular sieve membrane top layer. During preparation, the CHA molecular sieve membrane layer is modified with a suitable concentration of PDDA solution and then immersed and pulled in a seed solution, achieving uniform loading of the STT molecular sieve seed crystals without ammonia rinsing. In this invention, the CHA molecular sieve membrane interlayer not only provides a sieving effect but also acts as a physical barrier to prevent the synthesis solution from eroding the support, reducing aluminum migration and minimizing hydrophilic defects in the framework of the top STT molecular sieve membrane. The dense STT molecular sieve membrane layer further enhances the sieving capacity for small molecule gases. This composite molecular sieve membrane exhibits high separation selectivity for He / CH₄ mixtures and maintains good stability after long-term storage in air, solving the problems of uneven seed loading, susceptibility to aluminum contamination, and poor water resistance in existing technologies.

Description

Technical Field

[0001] This invention belongs to the field of membrane material technology, and particularly relates to a composite molecular sieve membrane for helium extraction, its preparation method and application. Background Technology

[0002] Helium (He), as a strategic rare gas, is indispensable in aerospace, cryogenic superconductivity, and other fields. Membrane separation technology has attracted much attention in the field of helium extraction from natural gas due to its low energy consumption advantage. STT-type molecular sieves have unique intersecting channels of seven-membered and nine-membered rings, with the pore size of the seven-membered rings falling between that of He and CH4, theoretically possessing high potential for precise He / CH4 sieving. However, existing STT molecular sieve membranes are usually grown directly on a rough α-Al2O3 support. To mask the macroporous defects on the support surface, the thickness of the STT membrane layer often needs to be significantly increased, leading to a severe decrease in helium permeability. During the strongly alkaline hydrothermal synthesis of STT, aluminum ions (Al2O3) in the α-Al2O3 support... 3+ A large amount of these molecules will dissolve and participate in the growth of the STT framework, introducing a large number of hydrophilic silanol defects, resulting in poor stability of the membrane in actual working conditions of natural gas containing water vapor.

[0003] To address the aforementioned issues of aluminum dissolution and surface roughness on the support, nesting an intermediate membrane layer between the α-Al₂O₃ support and the STT membrane layer is a promising approach. However, it is generally expected in the art that multilayer membrane composites inevitably increase gas mass transfer resistance, often at the expense of permeability. Therefore, the primary technical problem to be solved by this invention is: how to select and construct an ideal intermediate layer that effectively prevents aluminum ion dissolution and migration from the support from participating in the growth of the STT molecular sieve membrane, while simultaneously breaking the traditional trade-off between multilayer composites and permeability degradation, thus obtaining a composite membrane that combines high flux and high selectivity.

[0004] To address this, the inventors attempted to use CHA molecular sieves, which have high permeability to light gases, as the bottom layer, hoping to prepare a highly selective STT top layer on its relatively smooth surface. However, research revealed significant interfacial growth barriers in the composite of such heterogeneous molecular sieve membranes. High-silica or all-silica CHA molecular sieve membranes have few hydroxyl groups on their surface. Directly applying conventional silane coupling agents (such as APTES) for covalent grafting in existing technologies results in extremely sparse STT seed loading due to the lack of reaction anchors, making it impossible to induce continuous epitaxial growth of the top layer membrane. Furthermore, existing literature ( Comptes Rendus ChimieWhile there are reports in [2016, 19(4), 486-495] of using cationic polymers (such as PDDA) to load seed crystals on the surface of some molecular sieves via electrostatic adsorption, this usually involves long-term immersion modification. This method is prone to large-scale agglomeration and excessive accumulation of seed crystals on weakly negatively charged hydrophobic surfaces. To solve the problem of seed overloading, existing processes have to introduce ammonia water for cumbersome secondary cleaning. This not only easily destroys the uniformity of the seed monolayer, but also causes local seed crystal detachment, which in turn forms fatal intercrystalline pinhole defects in subsequent secondary growth, resulting in a significant loss of gas separation selectivity of the composite membrane.

[0005] Therefore, the second key technical problem that this invention urgently needs to solve is: how to overcome the problem of seed adhesion caused by the lack of hydroxyl groups on the surface of CHA molecular sieves, and develop a precise seed loading process that eliminates the cumbersome ammonia washing step, so as to perfectly achieve defect-free high-quality composite at the CHA-STT interface. Summary of the Invention

[0006] To address the aforementioned issues, this invention completely cuts off the erosion path of the strongly alkaline synthesis solution on the underlying aluminum support by nesting a CHA membrane layer between the α-Al2O3 support and the STT membrane layer. This not only solves the problems of single-layer STT membranes being susceptible to aluminum contamination and having poor long-term water vapor stability, but also, with the addition of a CHA membrane layer, the composite molecular sieve membrane not only does not reduce helium permeability, but also achieves a significant improvement in both permeability and He / CH4 separation selectivity, thus completely breaking the performance trade-off in traditional multilayer membrane composites.

[0007] First, the present invention provides a method for preparing a composite molecular sieve membrane for helium extraction, which includes the following steps: (1) α-Al2O3 support with a CHA molecular sieve membrane pre-formed on its surface is subjected to surface charge modification treatment, and then seed crystals are loaded in STT molecular sieve seed solution. After drying, a CHA molecular sieve membrane with STT seed crystals loaded on its surface is obtained; (2) Silicon source, template agent TMAdaOH and water are mixed to obtain a synthesis solution; (3) The CHA molecular sieve membrane with STT seed crystals loaded in step (1) is placed in the synthesis solution of step (2) for hydrothermal synthesis, and an STT molecular sieve membrane top layer is grown on the surface of the CHA molecular sieve membrane; (4) The composite molecular sieve membrane obtained in step (3) is taken out, and the template agent is removed by calcination to obtain the composite molecular sieve membrane for helium extraction.

[0008] Preferably, in step (1), the surface charge modification treatment is performed by immersion in a polydiallyldimethylammonium chloride solution for 1-3 h; and / or, the mass concentration of the polydiallyldimethylammonium chloride solution is 0.5-1.5 wt.%; and / or, the mass concentration of the STT molecular sieve seed solution is 0.5-2.0 wt.%.

[0009] Preferably, in step (1), the seed crystals are loaded using the dip-coating method, and the processing time is controlled to be 5-30 s; and / or, the drying is carried out directly after removal without washing, and the drying conditions are overnight drying in an environment of 50-80 ℃.

[0010] Preferably, in step (2), the molar ratio of SiO2, TMAdaOH and H2O in the synthesis solution is 1:0.1-0.3:40-50; and / or, in step (3), the hydrothermal synthesis temperature is 170-190 ℃ and the synthesis time is 72-120 h; and / or, in step (4), the calcination is carried out in an ozone atmosphere or an air atmosphere, and the calcination temperature is 200-500 ℃.

[0011] Secondly, the present invention provides a composite molecular sieve membrane for helium extraction prepared by the above-mentioned preparation method. The composite molecular sieve membrane includes an α-Al2O3 support as a support, a CHA molecular sieve membrane intermediate layer located on the surface of the support, and an STT molecular sieve membrane top layer located on the surface of the CHA molecular sieve membrane intermediate layer. The CHA molecular sieve membrane intermediate layer is configured as a physical barrier to prevent aluminum species in the α-Al2O3 support from dissolving and migrating to the STT molecular sieve membrane top layer during hydrothermal synthesis, thereby inhibiting the generation of hydrophilic silanol defects in the STT molecular sieve framework.

[0012] Preferably, the intermediate layer of the CHA molecular sieve membrane is an all-silica CHA molecular sieve membrane or an SSZ-13 molecular sieve membrane; and / or, the α-Al2O3 support is in the shape of hollow fiber, tubular or sheet.

[0013] Preferably, the total thickness of the composite molecular sieve membrane is 6-20 μm; and / or, the thickness of the intermediate layer of the CHA molecular sieve membrane is 2-10 μm; and / or, the top layer of the STT molecular sieve membrane is formed by the alternating growth of pseudo-cubic STT molecular sieve crystals with a characteristic size of 5-6 μm.

[0014] Finally, the present invention also provides an application of the above-described composite molecular sieve membrane in helium extraction from a mixed gas containing He and CH4.

[0015] Preferably, the application includes assembling the composite molecular sieve membrane into a membrane module, introducing a mixed gas containing helium and methane into the feed side of the membrane module; and / or, driven by a pressure difference, combining the dual-size sieving effect of the CHA molecular sieve channels and the STT molecular sieve channels, allowing helium to preferentially permeate through the composite molecular sieve membrane to obtain helium-rich gas on the permeate side, while methane is retained on the effluent side.

[0016] Preferably, the volume concentration of helium in the mixed gas on the feed side is 2-50%; and / or, the operating pressure on the feed side is controlled at 0.2-5 MPa; and / or, the operating temperature is 20-100 °C; and / or, after storage in air containing natural moisture for 250-350 days, when the composite molecular sieve membrane separates equimolar He / CH4 mixed gas at an operating pressure of 0.1-0.5 MPa, the He / CH4 separation selectivity remains greater than or equal to 40.

[0017] Compared with the prior art, the present invention has the following beneficial effects: First, this invention innovatively constructs a CHA / STT composite structure with an aluminum ion blocking effect (e.g., Figure 1 (As shown). By nesting a fully silica / high silica CHA molecular sieve membrane as an intermediate physical barrier between the rough α-Al₂O₃ support and the STT top layer, this invention cuts off the erosion path of the strongly alkaline hydrothermal synthesis solution on the underlying aluminum support, effectively blocking aluminum ions (Al₂O₃) in the support. 3+ The dissolution and migration of the membrane into the STT molecular sieve framework is achieved. This structural innovation inhibits the generation of hydrophilic silanol defects in the STT membrane layer from a microscopic mechanism, enabling the prepared composite membrane to maintain high He / CH4 separation selectivity even after being stored in harsh conditions containing natural water vapor for nearly a year (300 days). This solves the problem of existing single-layer STT helium extraction membranes being susceptible to aluminum contamination and having poor water vapor tolerance.

[0018] Secondly, this invention utilizes molecular sieve membrane surface modification and an impregnation-lift seed loading process to provide a seed layer at the heterogeneous interface, thereby preparing a composite molecular sieve membrane. The surface of the CHA molecular sieve membrane is charged with a suitable concentration of cationic polyelectrolyte, and under electrostatic action, STT molecular sieves in a suitable seed solution are adsorbed through impregnation-lift. This suitable concentration modification and short-time contact with a suitable seed solution reduce large-scale agglomeration and excessive accumulation of seed crystals, and avoid steps such as ammonia washing. Therefore, this invention can provide an STT molecular sieve seed layer on the surface of the CHA molecular sieve membrane for STT molecular sieve membrane growth.

[0019] Third, this invention breaks the conventional wisdom of the "game between permeability and selectivity" in traditional multilayer membrane composites, achieving unexpected cascade sieving performance. It is generally expected in the art that increasing the number of membrane layers inevitably leads to increased gas mass transfer resistance and decreased permeability. However, thanks to the defect-free interface resulting from the aforementioned wash-free process, and the repair effect of the amorphous gel formed in the synthesis solution on the microscopic defects of the CHA surface, the STT top layer can achieve continuous epitaxial growth at a relatively thin scale. Under the dual-size cascade sieving effect of the CHA and STT channels, this composite molecular sieve membrane not only does not reduce helium flux due to the addition of the intermediate layer, but also achieves a significant improvement in both helium permeability and He / CH4 separation selectivity, demonstrating extremely high industrial application potential and outstanding technological advancement. Attached Figure Description

[0020] Figure 1 A schematic diagram of the structure of a composite molecular sieve membrane for helium extraction provided by the present invention; Figure 2 A schematic diagram of a process for separating a mixed gas containing helium and methane, provided for the present invention; Figure 3 (a) is a planar SEM image of the CHA molecular sieve membrane of Example 1 of the present invention. Figure 3 (b) is a planar SEM image of the CHA molecular sieve membrane obtained by direct impregnation and pulling of STT molecular sieve in Comparative Example 2 of the present invention. Figure 3 (c) is a planar SEM image of the polydiallyldimethylammonium chloride modified CHA molecular sieve membrane after impregnation with STT molecular sieve in Example 1 of the present invention. Figure 3 (d) is a planar SEM image of the CHA molecular sieve membrane after grafting STT molecular sieve with 3-aminopropyltriethoxysilane in Comparative Example 3 of the present invention. Figure 4 The XRD patterns of the composite molecular sieve membranes of the present invention (synthesis time of Comparative Example 6: 24 h, Synthesis time of Comparative Example 7: 48 h, Synthesis time of Example 1: 72 h, Synthesis time of Example 6: 96 h, Synthesis time of Example 7: 120 h) are shown below. Figure 5 Planar SEM image (a) and cross-sectional SEM image (b) of Comparative Example 6 of the present invention after a synthesis time of 24 h. Planar SEM image (c) and cross-sectional SEM image (d) of Comparative Example 7 with a synthesis time of 48 h; Planar SEM image (e) and cross-sectional SEM image (f) of Example 1 with a synthesis time of 72 h; Planar SEM image (g) and cross-sectional SEM image (h) of Example 6 with a synthesis time of 96 h; Planar SEM image (i) and cross-sectional SEM image (j) of Example 7 with a synthesis time of 120 h. Figure 6The graphs show the helium separation performance of the composite molecular sieve membrane for helium extraction in Example 7 of the present invention and the CHA molecular sieve membrane in Comparative Example 1 for a mixed gas containing helium and methane under different pressures. Figure 7 The graph shows the helium separation performance of the composite molecular sieve membrane for helium extraction in Example 7 of the present invention for a mixed gas containing different concentrations of helium and methane. Figure 8 The graphs show the helium separation performance of the composite molecular sieve membrane for helium extraction in Example 7(a) and Comparative Example 8(b) after storage in air for different times. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0022] Example 1 The preparation of the composite molecular sieve membrane in this embodiment includes the following steps: (1) Preparation of CHA molecular sieve membrane on α-Al2O3 hollow fiber carrier: CHA molecular sieve membrane was prepared according to the preparation method described in Example 1 of invention application CN120771739A.

[0023] (2) Loading STT molecular sieves onto the surface of CHA molecular sieve membrane: The two ends of the CHA molecular sieve membrane on the hollow fiber carrier were sealed by wrapping with polytetrafluoroethylene tape. The CHA molecular sieve membrane was held with tweezers and placed in a 0.5 wt.% polydiallyldimethylammonium chloride (PDDA) solution for 2 h. Then, the CHA molecular sieve membrane was held with tweezers and placed in a seed solution (the seed crystals were ball-milled STT molecular sieves, and a 1 wt.% STT molecular sieve seed solution was prepared) for immersion and lifting treatment for 15 s. The CHA molecular sieve membrane coated with seed crystals was dried overnight at 60 °C.

[0024] (3) Preparation of synthesis solution: The synthesis solution was prepared according to the molar ratio of 1SiO2:0.2TMAdaOH:44H2O. First, deionized water was added to the polytetrafluoroethylene bottle, then TMAdaOH was added, stirred for 5 min, silica sol was slowly added dropwise, and the mixture was stirred and aged for 10 h to obtain the synthesis solution.

[0025] (4) Hydrothermal synthesis: The CHA molecular sieve membrane loaded with STT seed crystals was placed vertically in the synthesis solution and hydrothermal synthesis was carried out at 180 °C for 72 h in a stainless steel high-pressure reactor.

[0026] (5) Take out the composite molecular sieve membrane, wash it with deionized water until neutral, dry it, and then calcine it at 200°C for 4 days in an ozone atmosphere. Before testing, place the composite molecular sieve membrane after removing the template agent in an oven at 60°C.

[0027] Example 2 Example 2 is basically the same as Example 1, except that the PDDA concentration in step (2) is 1%.

[0028] Example 3 Example 3 is basically the same as Example 1, except that the PDDA concentration in step (2) is 1.5%. Example 4 Example 4 is basically the same as Example 1, except that the molar ratio in step (3) is 1SiO2:0.3TMAdaOH:44H2O.

[0029] Example 5 Example 5 is basically the same as Example 1, except that the molar ratio in step (3) is 1SiO2:0.1TMAdaOH:44H2O Example 6 Example 6 is basically the same as Example 1, except that the synthesis time in step (4) is 96 h.

[0030] Example 7 Example 7 is basically the same as Example 1, except that the synthesis time in step (4) is 120 h.

[0031] Comparative Example 1 Comparative Example 1 directly uses the CHA molecular sieve membrane generated in steps (1) and (5) of Example 1.

[0032] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that in step (2), the PDDA solution was not impregnated, and the STT molecular sieve was directly loaded onto the CHA molecular sieve membrane.

[0033] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the CHA molecular sieve membrane was grafted with STT molecular sieve using (3-aminopropyl)triethoxysilane (APTES). The specific steps are as follows: (1) Using hexane as a solvent, STT molecular sieve seed crystals are covalently bonded to APTES. The specific steps are as follows: First, dry the STT molecular sieve to completely remove moisture. Weigh 0.2 g of STT molecular sieve seed crystals and place them in a three-necked flask. Add 20 mL of hexane and sonicate for 1 min. Then add about 0.1-0.2 g of APTES and sonicate again for 1 min. Heat the mixture to 110 °C using an oil bath and react for 3 h. Use argon gas as a protective gas. Install a condenser to reflux the hexane solvent. Add a rod rotor to make the reaction more uniform. Separate the molecular sieve from the solvent by centrifugation. Wash it continuously with hexane to remove excess APTES and then place it in a 60 °C oven.

[0034] (2) Using hexane as a solvent, the CHA molecular sieve membrane is covalently linked to (2,3-epoxypropoxy)propyltrimethoxysilane (KH-560). The specific steps are as follows: clean the surface of the CHA molecular sieve membrane with ethanol, wrap both ends with polytetrafluoroethylene tape, and dry it in a 60 ℃ oven for later use. Place the CHA molecular sieve membrane in a three-necked flask, add hexane to submerge the membrane, and then add about 0.2-0.5 g of (2,3-epoxypropoxy)propyltrimethoxysilane. Sonicate for 1 min. After setting up the experimental apparatus, heat the mixture to 110 ℃ using an oil bath and react for 3 h. Use argon gas as a protective gas and install a condenser to reflux the hexane solvent. After the reaction is complete, rinse the ultrasonically sonicated CHA molecular sieve membrane with hexane multiple times to remove excess silane coupling agent, and then dry the CHA molecular sieve membrane in an oven.

[0035] (3) Using hexane as a solvent, the CHA molecular sieve membrane and the STT molecular sieve are covalently bonded. The specific steps are as follows: wrap the two ends of the previously used CHA molecular sieve membrane with polytetrafluoroethylene tape, then place it in a three-necked flask, add hexane to submerge the molecular sieve membrane, then add the STT molecular sieve, and sonicate for 1 min. After setting up the experimental apparatus, heat to 110 ℃ using an oil bath, react for 6 h, introduce argon gas as a protective gas, install a condenser to reflux the hexane solvent, and add a rod rotor to make the reaction more uniform. After the reaction is complete, remove the molecular sieve membrane.

[0036] Comparative Example 4 Comparative Example 4 is basically the same as Example 1, except that the PDDA concentration in step (2) is 0.05 wt.%. Comparative Example 5 The method of helium extraction in Comparative Example 5 is basically the same as that in Example 1, except that the PDDA concentration in step (2) is 2 wt.%. Comparative Example 6 The method of helium extraction in Comparative Example 6 is basically the same as that in Example 1, except that the synthesis time in step (3) is 24 h.

[0037] Comparative Example 7 The method of helium extraction in Comparative Example 7 is basically the same as that in Example 1, except that the synthesis time in step (3) is 48 h.

[0038] Comparative Example 8 Comparative Example 8: STT molecular sieve membranes were prepared directly on α-Al₂O₃ support. The specific steps are as follows: (1) The preparation method of STT molecular sieve membrane grown on α-Al2O3 hollow fiber support is as follows: the two ends of the hollow fiber support are sealed by wrapping with polytetrafluoroethylene tape, and the support is held with tweezers and placed in the seed solution (the seed crystal is ball-milled STT molecular sieve, and 1 wt.% STT molecular sieve seed solution is prepared) for immersion and pulling treatment for 15 s; the seed-coated support is dried overnight at 60 ℃. Preparation of synthesis solution: the synthesis solution is prepared according to the molar ratio 1SiO2:0.2TMAdaOH:44H2O. First, deionized water is added to the polytetrafluoroethylene bottle, then TMAdaOH is added, stirred for 5 min, silica sol is slowly added dropwise, and the mixture is stirred and aged for 10 h to obtain the synthesis solution.

[0039] (2) Hydrothermal synthesis: The support loaded with STT seed crystals was placed vertically in the synthesis solution and hydrothermally synthesized at 180 °C for 72 h in a stainless steel high-pressure reactor. The STT molecular sieve membrane was removed, washed with deionized water until neutral, dried, and then calcined at 200 °C for 4 days under an ozone atmosphere. The STT molecular sieve membrane after template removal was placed in an oven at 60 °C before testing.

[0040] Performance Characterization use Figure 2 The process involved using a composite molecular sieve membrane to test the separation selectivity of a gas mixture containing equimolar amounts of helium and methane. The feed flow rate of the mixed gas was controlled at 100 mL / min. Argon gas at a flow rate of 1.15 mL·min⁻¹ was used to purge the permeation side, and the feed pressure was controlled at 0.2 MPa by a back pressure valve.

[0041] Characterization results from Figure 3 (a) It can be seen that the plane of the CHA molecular sieve membrane prepared in Example 1 is composed of numerous particles with distinct edges; Figure 3 (b) It can be seen that in Comparative Example 1, only a few seed crystals are scattered on the surface of the unmodified film; Figure 3 (d) In Comparative Example 2, the surface seed coverage of the APTES-grafted film was also relatively small; while Figure 3(c) In Example 1, the surface of the PDDA-modified membrane was completely covered by the seed crystals, and no CHA molecular sieve crystals with obvious edges were observed. This indicates that the surface of the PDDA-modified CHA molecular sieve membrane is more suitable for coating with STT molecular sieve seed crystals, which is beneficial for the subsequent secondary growth to prepare a high-quality membrane layer.

[0042] Table 1

[0043] a : ×10 -8 mol·m -2 ·s -1 Pa -1 ; b : ×10 -10 mol·m -2 ·s -1 .

[0044] As shown in Table 1, compared to Comparative Example 1 (CHA molecular sieve membrane), Example 1 exhibits better He / CH4 separation performance, with selectivity increasing from 9 to 28. Examples 2 (PDDA concentration of 1 wt.%) and 3 (PDDA concentration of 1.5 wt.%) show similar He / CH4 separation performance to Example 1. Examples 4 (1SiO2: 0.3 TMAdaOH) and 5 (1SiO2: 0.1 TMAdaOH) demonstrate some He / CH4 separation performance, with selectivities of 17 and 14, respectively. This is related to the changes in membrane growth conditions and the initial performance of the CHA molecular sieve membrane. Comparative Example 2 (unmodified CHA molecular sieve membrane directly loaded with seeds) and Comparative Example 3 (grafted seeds using APTES grafting method) show selectivities of 10.5 and 11, respectively, indicating that these two methods have no significant effect on the preparation of composite molecular sieve membranes. Comparative Examples 4 (PDDA concentration of 0.05 wt.%) and 5 (PDDA concentration of 2 wt.%) show that both insufficient and excessive PDDA can lead to the formation of poorly sparse or stacked seed layers, resulting in poor growth of molecular sieve membranes.

[0045] from Figure 4 It can be seen that with the extension of crystallization time, the intensity of the STT characteristic diffraction peaks (2θ = 8.1°, 8.5°, 9.4°, 10.7°) in Examples 1, 6, 7 and Comparative Examples 6, 7 monotonically increases, while the intensity of the CHA characteristic peaks decreases accordingly, confirming that the STT molecular sieve crystal phase has been successfully epitaxially grown on the surface of the CHA molecular sieve membrane, obtaining an STT@CHA bilayer molecular sieve membrane. Figure 5It can be seen that after 24 h of crystallization, most of the surface of the CHA molecular sieve membrane is covered with amorphous gel, making it difficult to observe the disc-shaped STT molecular sieve seeds. A membrane layer of about 8 μm is formed, slightly thicker than the initial CHA molecular sieve membrane thickness (6 μm). PXRD did not detect a significant STT diffraction signal, which means that during the 24 h synthesis time under this formulation, some silicate species in the synthesis solution are in the induction phase, covering the STT molecular sieve seeds or CHA molecular sieve membrane, and some silicate species are used to repair the CHA molecular sieve framework. Extending the crystallization time to 48 h, the amorphous gel on the surface of the CHA molecular sieve membrane shrinks, and the STT molecular sieve seeds begin to absorb and consume the amorphous gel for growth, with the molecular sieve membrane layer thickness reaching about 10 μm. Further extending the synthesis time to 72 h, the corresponding molecular sieve membrane layer exhibits typical pseudo-cubic STT molecular sieve characteristics, with a size of about 5 μm and a membrane layer thickness of 20 μm. With prolonged synthesis time (96 h and 120 h), the size of STT molecular sieves increased to 6 μm, and the inter-crystal growth became more efficient.

[0046] Table 2

[0047] Table 2 shows the equimolar He / CH4 gas separation performance of the composite molecular sieve membranes in Examples 1, 6, and 7, and Comparative Examples 6 and 7. When the synthesis time was 24-48 h, the He / CH4 separation selectivity of the molecular sieve membrane was 20, and the He permeability was 1.2 × 10⁻⁶. -8 mol·m -2 ·s -1 ·Pa -1 The prepared molecular sieve membrane exhibited He / CH4 separation selectivity comparable to that of the high-performance CHA molecular sieve membrane (20.6). With an increase in crystallization time of 72 h, the selectivity improved to 28, and the average He permeability was 1 × 10⁻⁶. -8 mol·m -2 ·s -1 ·Pa -1 Further extending the crystallization time to 120 h increased the He / CH4 separation selectivity of the molecular sieve membrane to 34, while maintaining the He permeability at 1.5 × 10⁻⁶. -8 mol·m -2 ·s -1 ·Pa -1 .

[0048] Figure 6This diagram shows the helium separation performance of the composite molecular sieve membrane for helium extraction in Example 7 and the CHA molecular sieve membrane in Comparative Example 1 for a mixed gas containing helium and methane under different pressures. The separation performance of the composite molecular sieve membrane for helium extraction in Example 7 and the CHA molecular sieve membrane in Comparative Example 1 for a He / CH4 mixed gas under different pressures was tested. At 0.2 MPa, the He permeability of the STT@CHA molecular sieve membrane was 9.39 × 10⁻⁶. -9 mol·m -2 ·s -1 ·Pa -1 As the pressure increased to 5 MPa, the He permeability decreased to 5.1 × 10⁻⁶. -9 mol·m -2 ·s -1 ·Pa -1 Comparing the He / CH4 mixed gas separation performance of the CHA molecular sieve membrane, it was found that when the pressure increased from 0.2 MPa to 5 MPa, the He permeability of the STT@CHA molecular sieve membrane decreased to 5.1 × 10⁻⁶. - 9 mol·m -2 ·s -1 ·Pa -1 The decrease was 45%; the He permeability of the CHA molecular sieve membrane decreased from 1.68 × 10⁻⁶. -8 mol·m -2 ·s -1 ·Pa -1 Decreased to 6.81×10 -9 mol·m -2 ·s -1 ·Pa -1 The decrease was 59%. Furthermore, the CH4 permeability of the CHA molecular sieve membrane decreased from 8.2 × 10⁻⁶ at 0.2 MPa. -10 mol·m -2 ·s -1 ·Pa -1 8.8 × 10⁸ when increased to 5 MPa -10 mol·m -2 ·s -1 ·Pa -1 The He / CH4 separation selectivity decreased from 21 to 7; the CH4 permeability of the STT@CHA molecular sieve membrane decreased from 3.0 × 10⁻⁶ at 0.2 MPa. -10 mol·m -2 ·s -1 ·Pa -1 1.92 × 10⁻⁶ when the pressure drops to 5 MPa -10 mol·m -2 ·s -1 ·Pa -1The He / CH4 separation selectivity decreased from 40 to 26.

[0049] Figure 7 This is a graph showing the helium separation performance of the composite molecular sieve membrane for helium extraction in Example 7 of the present invention for a mixed gas containing different concentrations of helium and methane. From... Figure 7 It can be seen that as the proportion of He in natural gas gradually increases from 2% to 50%, the He permeability increases from 1.03 × 10⁻⁶. -8 mol·m -2 ·s -1 ·Pa -1 Slightly increased to 1.21 × 10 -8 mol·m -2 ·s -1 ·Pa -1 CH4 permeability from 2.86 × 10 -10 mol·m -2 ·s -1 ·Pa -1 Increased to 3.14 × 10 -10 mol·m -2 ·s -1 ·Pa -1 The selectivity for He / CH4 separation increased from 37 to 38, while the selectivity and permeability remained relatively stable.

[0050] Figure 8 The graphs show the helium separation performance of the composite molecular sieve membrane for helium extraction in Example 7 of the present invention and Comparative Example 8 after storage in air for different times, in a mixed gas containing helium and methane. Figure 8 As can be seen, the composite molecular sieve membrane of Example 7 maintained good separation performance after long-term storage in air, specifically after 300 days of storage, when used to test a mixed gas containing helium and methane. In contrast, the STT molecular sieve membrane of Comparative Example 8 showed a selectivity of only 3 after 7 days, indicating a loss of separation performance.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a composite molecular sieve membrane for helium extraction, characterized in that, Includes the following steps: (1) Prepare a CHA molecular sieve membrane on the surface of an α-Al2O3 support, perform charge modification treatment on the surface of the CHA molecular sieve membrane, load seeds in an STT molecular sieve seed solution, and dry to obtain a CHA molecular sieve membrane with STT molecular sieve seeds loaded on the surface; (2) Mix silicon source, template agent TMAdaOH and water to obtain a synthesis solution; (3) Place the CHA molecular sieve membrane with STT seeds obtained in step (1) in the synthesis solution of step (2) for hydrothermal synthesis, and grow the top layer of the STT molecular sieve membrane on the surface of the CHA molecular sieve membrane; (4) Take out the composite molecular sieve membrane obtained in step (3), calcine to remove the template agent, and obtain the composite molecular sieve membrane for helium extraction.

2. The preparation method according to claim 1, characterized in that, In step (1), the surface charge modification treatment is performed by immersion in polydiallyldimethylammonium chloride solution for 1-3 h; and / or, the mass concentration of the polydiallyldimethylammonium chloride solution is 0.5-1.5 wt.%; and / or, the mass concentration of the STT molecular sieve seed solution is 0.5-2.0 wt.%.

3. The preparation method according to claim 1, characterized in that, In step (1), the seed crystals are loaded using the dip-coating method, and the processing time is controlled to be 5-30 s; and / or, the drying is carried out directly after removal without washing, and the drying conditions are overnight drying in an environment of 50-80 ℃.

4. The preparation method according to claim 1, characterized in that, In step (2), the molar ratio of SiO2, TMAdaOH and H2O in the synthesis solution is 1:0.1-0.3:40-50; and / or, in step (3), the hydrothermal synthesis temperature is 170-190 ℃ and the synthesis time is 72-120 h; and / or, in step (4), the calcination is carried out in an ozone atmosphere or an air atmosphere, and the calcination temperature is 200-500 ℃.

5. A composite molecular sieve membrane for helium extraction prepared by the preparation method according to any one of claims 1-4, characterized in that, The composite molecular sieve membrane includes an α-Al2O3 support as a support, a CHA molecular sieve membrane intermediate layer located on the surface of the support, and an STT molecular sieve membrane top layer located on the surface of the CHA molecular sieve membrane intermediate layer. The intermediate layer of the CHA molecular sieve membrane is configured as a physical barrier to prevent aluminum species in the α-Al2O3 support from dissolving and migrating to the top layer of the STT molecular sieve membrane during hydrothermal synthesis, thereby reducing the generation of hydrophilic silanol defects in the STT molecular sieve framework.

6. The composite molecular sieve membrane according to claim 5, characterized in that, The intermediate layer of the CHA molecular sieve membrane is an all-silica CHA molecular sieve membrane or an SSZ-13 molecular sieve membrane; and / or, the α-Al2O3 support is in the shape of hollow fiber, tubular or sheet.

7. The composite molecular sieve membrane according to claim 5 or 6, characterized in that, The total thickness of the composite molecular sieve membrane is 6-20 μm; and / or, the thickness of the intermediate layer of the CHA molecular sieve membrane is 2-10 μm; and / or, the top layer of the STT molecular sieve membrane is formed by the alternating growth of pseudo-cubic STT molecular sieve crystals with a characteristic size of 5-6 μm.

8. The application of a composite molecular sieve membrane as described in any one of claims 5-7 for helium extraction in a mixed gas containing He and CH4.

9. The application according to claim 8, characterized in that, The application includes assembling the composite molecular sieve membrane into a membrane module, introducing a mixed gas containing helium and methane into the feed side of the membrane module; and / or, driven by a pressure difference, combining the dual-size sieving effect of the CHA molecular sieve channels and the STT molecular sieve channels, allowing helium to preferentially permeate through the composite molecular sieve membrane to obtain helium-rich gas on the permeate side, while methane is retained on the effluent side.

10. The application according to claim 8 or 9, characterized in that, The volume concentration of helium in the mixed gas on the feed side is 2-50%; and / or, the operating pressure on the feed side is controlled at 0.2-5 MPa; and / or, the operating temperature is 20-100℃; and / or, after storage in air containing natural water vapor for 250-350 days, when the composite molecular sieve membrane separates equimolar He / CH4 mixed gas at an operating pressure of 0.1-0.5 MPa, the He / CH4 separation selectivity remains greater than or equal to 40.

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