A double-layer CHA molecular sieve membrane and its application in gas separation

Abstract

This invention synthesizes bilayer CHA molecular sieve membranes with different silica-to-alumina ratios on a porous support surface. The surface layer of the low-silica-to-alumina CHA molecular sieve membrane improves gas separation selectivity after ion exchange, while the bottom layer of the SSZ-13 molecular sieve membrane repairs defects in the outer membrane. The molecular sieve membranes of this invention possess advantages such as uniform pore size and good stability. Ion exchange alters the gas adsorption behavior of the molecular sieve membrane, enhancing the sieving effect and significantly improving gas separation performance. It has great application potential in the fields of natural gas decarbonization and natural gas helium extraction.

Description

Technical Field

[0001] This invention relates to a molecular sieve membrane, and more particularly to a CHA molecular sieve membrane and its application in gas separation. Background Technology

[0002] Natural gas is a clean energy source with high calorific value, mainly composed of alkanes, and also contains H2S, CO2, N2, water vapor, and small amounts of rare gases such as He and Ar. CO2 in natural gas reduces fuel calorific value and corrodes pipelines, making decarbonization a crucial production step. Currently, methods for separating CO2 from natural gas mainly include cryogenic distillation and amine absorption. Cryogenic distillation equipment has high investment costs and energy consumption, while amine absorption has limited CO2 load capacity, and the CO2 desorption process increases regeneration costs. Furthermore, He, as a high-value-added gas product, is mainly separated into high-purity forms through cryogenic natural gas processes. However, natural gas fields have low He content, production facilities are small-scale, and the processes are complex and costly. Membrane separation offers advantages such as low energy consumption, continuous operation, and low equipment investment. Although traditional polymer membranes have achieved industrial applications, their lifespan in CO2 separation systems is short due to the easy plasticization of the material. In addition, the separation mechanism based on dissolution-diffusion results in low gas permeability of organic membranes.

[0003] Molecular sieve membranes possess a uniform pore structure, offering unique advantages in gas separation. Compared to molecular sieves such as SAPO-34, low-silica CHA molecular sieve membranes exhibit superior stability; however, the preparation of dense low-silica CHA molecular sieve membranes is challenging, thus presenting numerous obstacles to their application in gas separation processes.

[0004] Therefore, it is urgent to optimize the existing preparation methods of low-silica CHA molecular sieve membranes so that they can meet the requirements for application in gas separation processes. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention designs a double-layer CHA molecular sieve membrane, which enables the double-layer structure of the double-layer CHA molecular sieve membrane to work together to ensure its application in the gas separation process.

[0006] SSZ-13 molecular sieve membranes and low-silica CHA molecular sieve membranes both belong to the CHA configuration. The SSZ-13 / low-silica CHA bilayer molecular sieve membrane provides a large number of ion exchange sites to improve gas separation performance, while the dense SSZ-13 layer can repair the defects of low-silica CHA.

[0007] To address this, the present invention provides a bilayer CHA molecular sieve membrane, the molecular sieve membrane comprising a carrier, a high silica-to-alumina ratio CHA molecular sieve membrane layer, and a low silica-to-alumina ratio CHA molecular sieve membrane layer connected in sequence, wherein the silica-to-alumina ratio of the molecular sieve crystals in the high silica-to-alumina ratio molecular sieve membrane layer is greater than 15, the silica-to-alumina ratio of the molecular sieve crystals in the low silica-to-alumina ratio CHA molecular sieve membrane layer is less than 5, and the low silica-to-alumina ratio CHA molecular sieve membrane layer is exchange-doped with metal cations.

[0008] Preferably, the thickness of the high silica-to-alumina ratio CHA molecular sieve membrane layer is 1-3 μm, and the thickness of the low silica-to-alumina ratio CHA molecular sieve membrane layer is 1-3 μm.

[0009] Preferably, the metal cation is selected from Cs. + Ag + Cu 2+ Zn 2+ One or more of them.

[0010] This invention also provides a method for preparing the above-mentioned double-layer CHA molecular sieve membrane, wherein the high silica-to-alumina ratio CHA molecular sieve layer is SSZ-13 molecular sieve, and the preparation method includes the following steps:

[0011] (1) Prepare SSZ-13 molecular sieves with an average particle size of less than 100 nm; using small-particle-size molecular sieves as seed crystals can induce the preparation of SSZ-13 molecular sieve membranes with thinner membrane layers.

[0012] (2) The SSZ-13 molecular sieve prepared in step (1) is coated on the carrier as a seed crystal and hydrothermally synthesized in the first casting solution to form an SSZ-13 molecular sieve membrane layer.

[0013] (3) Continue to hydrothermally synthesize a low silica-alumina ratio CHA molecular sieve membrane layer on the surface of the SSZ-13 molecular sieve membrane layer prepared in step (2) in the second casting solution; and calcine the hydrothermally synthesized membrane to remove the template agent;

[0014] (4) The membrane obtained in step (3) is placed in a metal salt solution and ion exchange is carried out at a certain temperature. After washing and drying, a double-layer CHA molecular sieve membrane is obtained.

[0015] Preferably, the solvent for the metal salt solution in step (4) is water and / or an organic alcohol solvent, and the concentration of the metal salt solution is 0.01 to 2 M.

[0016] Preferably, the ion exchange temperature is room temperature to 200°C, the ion exchange time is 2 to 10 hours, the number of ion exchange cycles is 1 to 3, and the sample drying temperature is 60 to 200°C.

[0017] Preferably, in step (2), the seed concentration of SSZ-13 molecular sieve is 0.4-0.6wt%, the ratio of the first casting solution is 10-30NaOH:105SiO2:100-120Al(OH)3:10-30TMAdaOH:4000-5000H2O, the synthesis temperature is 150-170℃, and the synthesis time is 4-8h; in step (3), the ratio of the second casting solution is 28SiO2:0.2-1Al2O3:3-8CC (choline chloride):10-12Na2O:1000-2000H2O.

[0018] Preferably, in step (3), the SSZ-13 molecular sieve membrane layer to be hydrothermally synthesized is subjected to hydrothermal synthesis of a low silica-to-alumina ratio CHA molecular sieve membrane layer without calcination to remove the template agent; the calcination conditions in step (3) are a calcination temperature of 200-500℃ and a calcination atmosphere of air, oxygen, or ozone. The template agent of the SSZ-13 molecular sieve membrane layer has a stabilizing effect on the molecular sieve framework and can prevent the alkaline hydrothermal conditions during the secondary synthesis process from damaging the underlying SSZ-13 molecular sieve membrane structure.

[0019] The present invention also provides an application of the above-mentioned double-layer CHA molecular sieve membrane in the separation of CO2 mixed gas, wherein the CO2 mixed gas contains one or more of He, H2, N2, CH4, and C2H6.

[0020] The present invention also provides an application of a double-layer CHA molecular sieve membrane in the separation of He-containing mixed gases, wherein the He-containing mixed gas contains one or more of CO2, H2, N2, CH4, and C2H6.

[0021] Compared to existing technologies, this invention synthesizes bilayer CHA molecular sieve membranes with different silica-to-alumina ratios on a porous support surface. The surface layer of the low-silica-to-alumina CHA molecular sieve membrane improves gas separation selectivity after ion exchange, while the bottom layer of the SSZ-13 molecular sieve membrane repairs defects in the outer membrane. The molecular sieve membrane of this invention possesses advantages such as uniform pore size and good stability. Ion exchange alters the gas adsorption behavior of the molecular sieve membrane, enhancing the sieving effect and significantly improving gas separation performance. It has great application potential in the fields of natural gas decarbonization and natural gas helium extraction. Attached Figure Description

[0022] Figure 1 SEM images of the surface (a) and cross-section (b) of the bilayer CHA molecular sieve membrane;

[0023] Figure 2 Cs of bilayer CHA molecular sieve membranes under different pressures +The He / CH4 separation performance before and after ion exchange was tested at 25℃ with argon purging and equimolar He / CH4 feed. Solid and hollow dots represent data before and after ion exchange, respectively.

[0024] Figure 3 For Cs + The enrichment of He(a) and CO2(b) in simulated natural gas was measured using a double-layer CHA molecular sieve membrane before and after exchange. The test temperature was 25℃, the feed pressure was 0MPa, and the initial feed composition was 0.45% CO2, 0.075% He, 0.93% N2, 0.23% C2H6, and the remainder was CH4. Detailed Implementation

[0025] Example 1

[0026] This embodiment uses the following steps to prepare a double-layer CHA molecular sieve membrane.

[0027] (1) Preparation of small-particle-size SSZ-13 molecular sieve seeds

[0028] Small-particle-size SSZ-13 molecular sieves (particle size <100nm) were synthesized using the FAU molecular sieve crystallization method. These molecular sieves were used as seed crystals for preparing SSZ-13 molecular sieve membranes. The specific synthesis steps and scheme are as follows: The synthesis solution ratio was 1.0SiO2:0.036Al2O3:0.4TMAdaOH:5H2O. The FAU molecular sieves and TMAdaOH were stirred and mixed evenly. The water content of the synthesis solution was controlled by evaporation or additional water addition. After stirring for 4 hours, the mixture was transferred to a hydrothermal synthesis reactor and hydrothermally synthesized at 175℃ for 14 days. The product was centrifuged and washed until neutral, dried in an oven at 100℃, and calcined at 580℃ to remove the template agent.

[0029] (2) Preparation of high-silica SSZ-13 molecular sieve membrane

[0030] α-Al₂O₃ hollow fibers with a pore size of 400 nm, prepared in the laboratory, were used as the carrier. A 0.5 wt% seed solution was prepared using the synthesized SSZ-13 molecular sieve. Seed crystals were coated onto the carrier surface using the dip-coating method. The synthesis solution formulation was 20NaOH:10⁵SiO₂:10⁵Al(OH)₃:20TMAdaOH:4400H₂O. NaOH, TMAdaOH, and H₂O were added to a PTFE cup, and the mixture was thoroughly stirred and placed in a 90℃ water bath until all Al(OH)₃ was dissolved. The mixture was then cooled to room temperature, and silica sol was added, followed by aging for 6 hours. The synthesis solution was poured into a liner containing the coated carrier and placed in an autoclave. The mixture was then allowed to stand at 160℃ for 6 days for synthesis. After synthesis, the samples were washed and dried for later use.

[0031] (3) Preparation of low silica-alumina ratio CHA molecular sieve membranes

[0032] The synthesized SSZ-13 molecular sieve membrane was immersed in 1.5 wt% CHA seed solution for dip-coating. Choline chloride (CC) was used as a template agent. The low silica-to-alumina ratio CHA molecular sieve membrane layer was synthesized using the formula reported in the group. The synthesis solution ratio was 28SiO2:0.58Al2O3:4.7CC:11.78Na2O:1260H2O. A certain amount of sodium aluminate, sodium hydroxide, and choline chloride were weighed into a stirring flask, a certain amount of water was added, and the mixture was stirred until the solid was completely dissolved. A certain amount of 30 wt% silica sol was slowly added, and the mixture was aged at room temperature for 2 hours. The synthesis solution was poured into the liner containing the SSZ-13 molecular sieve membrane and placed in a stainless steel synthesis kettle for hydrothermal synthesis for 12 hours. After crystallization, the membrane was removed, washed, dried, and calcined in an ozone atmosphere at 200℃ to remove the template agent. (4) Ion exchange of double-layer CHA molecular sieve membrane

[0033] The prepared bilayer CHA molecular sieve membrane was subjected to ion exchange using cesium acetate as the ion source. To avoid structural damage to the outer CHA molecular sieve membrane with a low silica-to-alumina ratio under hydrothermal conditions, ethanol was used as the solvent to prepare a 0.1 mol·L⁻¹ solution. -1 A cesium acetate solution. Place the membrane at the bottom of the flask and add 0.1 mol·L⁻¹. -1 A cesium acetate solution was prepared, and a condenser was attached to the top of the flask. The mixture was refluxed at 75°C for 5 hours, and the above steps were repeated several times. The ion-exchanged sample was then air-dried at room temperature and dried in a 60°C oven for later use.

[0034] Example 2

[0035] (1) SEM characterization

[0036] The membrane sample prepared in Example 1 was characterized by SEM, such as... Figure 1 As shown, the low silica-to-alumina ratio CHA molecular sieve crystals on the surface are plate-like and exhibit good cross-growth. The cross-section shows a distinct bilayer membrane structure with a thickness of approximately 3.5 μm. The surface low silica-to-alumina CHA molecular sieve membrane has a thickness of 1.5 μm, while the bottom SSZ-13 molecular sieve membrane has a thickness of 2 μm.

[0037] (2) Investigation on the effect of calcination on SSZ-13 molecular sieve membrane (membrane layer without ion exchange treatment)

[0038] The effect of whether the template agent was removed from the bottom SSZ-13 molecular sieve membrane on the performance of the final prepared bilayer CHA molecular sieve membrane was investigated. The template agent was removed by calcination of the bilayer membrane sample in an ozone atmosphere at 200℃.

[0039] Table 1. CO2 / CH4 separation performance of double-layer CHA molecular sieve membrane after ozone calcination.

[0040]

[0041] a The template agent in the bottom SSZ-13 molecular sieve membrane was not removed. b The template agent has been removed from the bottom SSZ-13 molecular sieve membrane.

[0042] As shown in Table 1, the CO2 / CH4 separation selectivity of the bilayer membrane prepared by removing the template agent SSZ-13 molecular sieve membrane after calcination was less than 40, indicating that the preparation of the low-silica CHA layer under alkaline hydrothermal conditions would damage the structure of the bottom SSZ-13 molecular sieve membrane. In contrast, the bilayer membrane prepared by retaining the template agent SSZ-13 molecular sieve membrane exhibited a higher CO2 / CH4 separation selectivity (100±20), consistent with the separation performance of the single-layer SSZ-13 molecular sieve membrane, indicating that the template agent has a stabilizing effect on the molecular sieve framework and can prevent the damage to the structure of the bottom SSZ-13 molecular sieve membrane under alkaline hydrothermal conditions during the secondary synthesis process. Its CO2 permeability did not change significantly compared to the single-layer SSZ-13 molecular sieve membrane, indicating that the gas mass transfer resistance did not increase significantly with the increase of the bilayer membrane thickness.

[0043] (3) Application of membranes after ion exchange

[0044] Application of double-layer CHA molecular sieve membrane in CO2 mixed gas separation

[0045] Cs was tested using a CO2 mixture system. + Separation performance of the membrane before and after ion exchange (Table 2). For the CO2 / CH4 system, CO2 permeability decreased by 22.5% after ion exchange, while CH4 permeability decreased by 93.6%, and the CO2 / CH4 selectivity increased from 121.5 to 146. A similar phenomenon was observed in the CO2 / N2 system, where CO2 permeability decreased by only 7.1% after ion exchange, while N2 permeability decreased by 35%, and the CO2 / N2 separation selectivity increased by 30%. This is because CO2 can trigger a "gating effect" to pass through the molecular sieve channels, while CH4 and N2 have difficulty passing through Cs. + The occupied octagon enters the cage, thus improving separation selectivity.

[0046] Table 2 Cs of the double-layer CHA molecular sieve membrane + Separation performance of CO2 mixture before and after exchange a

[0047]

[0048] a The test conditions were: temperature 25℃, feed pressure 0.1 MPaG.

[0049] In reality, natural gas contains a small amount of CO2. A double-layer CHA molecular sieve membrane, before and after ion exchange, was used to concentrate ultra-low concentration CO2 in a simulated natural gas field (0.45% CO2, 0.075% He, 0.93% N2, 0.23% C2H6, the remainder being CH4). + After exchange, the CHA molecular sieve membrane can increase the concentration of low-concentration CO2 in the mixed gas from 0.45% to 21.5%, a 60% increase in concentration compared to the unexchanged membrane. This confirms the feasibility of using ion-exchanged bilayer CHA molecular sieve membranes for natural gas decarbonization.

[0050] Application of double-layer CHA molecular sieve membrane in the separation of He mixed gas

[0051] Figure 2 This demonstrates the separation performance of equimolar He / CH4 under different pressures. After ion exchange, the He permeability decreased from 10... -8 Reduced to 10 -9 mol·m -2 ·s -1 ·Pa -1 CH4 permeability increased from 10 -10 Reduced to 10 -11 mol·m -2 ·s -1 ·Pa -1 The He / CH4 separation performance was improved by 8%. The significant reduction in gas permeability is due to the improvement in Cs after ion exchange. + Occupying the center of the eight-membered ring increases the resistance of gas molecules passing through the molecular sieve channels; ion exchange reduces the pore volume of the CHA molecular sieve membrane, enhancing its sieving effect on He / CH4 and slightly improving the He / CH4 separation performance after exchange. When the pressure increases from 0 MPa to 0.6 MPa, the selectivity of the un-ion-exchanged membrane decreases by 48.5%, while the selectivity of the ion-exchanged membrane decreases by 38.3%, indicating that the effect of pressure on He / CH4 separation performance weakens after ion exchange.

[0052] In reality, natural gas contains gases such as He, N2, and light hydrocarbons. The double-layer CHA molecular sieve membrane before and after ion exchange is used to simulate ultra-low concentration He concentration in a natural gas field (0.45% CO2, 0.075% He, 0.93% N2, 0.23% C2H6, the remainder being CH4). Figure 3The un-ion-exchanged bilayer CHA molecular sieve membrane increased the He concentration in the mixed gas from 0.075% to 1.0%, a 13-fold increase. After ion exchange, the He concentration on the permeate side of the bilayer CHA molecular sieve membrane was 1.2%, a 16-fold increase compared to the initial He concentration in the mixed gas, indicating that the bilayer CHA molecular sieve membrane increased the He concentration by 20% after ion exchange. This confirms the feasibility of using the ion-exchanged bilayer CHA molecular sieve membrane for helium extraction from natural gas.

[0053] The above experiments demonstrate that the bilayer CHA molecular sieve membrane exhibits excellent membrane density, significantly improving the separation selectivity of CO2 and He mixed systems after ion exchange, and significantly increasing the concentration factor for ultra-low concentrations of CO2 and He. Combined with the stability of the molecular sieve membrane in acidic gases, this provides a new approach for natural gas decarbonization and helium extraction from natural gas.

Claims

1. A method for preparing a double-layer CHA molecular sieve membrane, characterized in that... The molecular sieve membrane comprises a carrier, a high-silicon-to-aluminum ratio (HSA) molecular sieve membrane layer, and a low-silicon-to-aluminum ratio (HSA) molecular sieve membrane layer connected in sequence. The HSA molecular sieve membrane layer has a HSA ratio greater than 15, while the HSA molecular sieve membrane layer has a HSA ratio less than 5. The HSA molecular sieve membrane layer is exchange-doped with metal cations. The HSA is SSZ-13, and the HSA is low-silicon CHA. The thickness of both the HSA and HSA molecular sieve membrane layers is 1-3 μm. The preparation method includes the following steps: (1) Prepare SSZ-13 molecular sieve with an average particle size of less than 100 nm; (2) The SSZ-13 molecular sieve prepared in step (1) is coated on the carrier as a seed crystal and hydrothermally synthesized in the first casting solution to form an SSZ-13 molecular sieve membrane layer. (3) Continue to hydrothermally synthesize a low-silicon-aluminum ratio CHA molecular sieve membrane layer on the surface of the SSZ-13 molecular sieve membrane layer prepared in step (2) in the second casting solution; and calcine the hydrothermally synthesized membrane to remove the template agent. The ratio of the second casting solution is 28SiO2:0.2-1Al2O3:3-8choline chloride:10-12Na2O:1000-2000H2O; (4) The membrane obtained in step (3) is placed in a metal salt solution and ion exchange is performed at a certain temperature. After washing and drying, a double-layer CHA molecular sieve membrane is obtained. The double-layer CHA molecular sieve membrane is used for the separation of CO2 mixed gas or He-containing mixed gas.

2. The method for preparing the double-layer CHA molecular sieve membrane according to claim 1, characterized in that... The metal cation is selected from Cs. + Ag + Cu 2+ Zn 2+ One or more of them.

3. The method for preparing the double-layer CHA molecular sieve membrane according to claim 1, characterized in that... The solvent for the metal salt solution in step (4) is water and / or an organic alcohol solvent, and the concentration of the metal salt solution is 0.01 to 2 M.

4. The method for preparing the double-layer CHA molecular sieve membrane according to claim 1, characterized in that... The ion exchange temperature is room temperature to 200℃, the ion exchange time is 2 to 10 hours, the number of ion exchange cycles is 1 to 3, and the sample drying temperature is 60 to 200℃.

5. The method for preparing a double-layer CHA molecular sieve membrane according to claim 1, characterized in that... In step (2), the seed concentration of SSZ-13 molecular sieve is 0.4-0.6wt%, the ratio of the first casting solution is 10-30NaOH:105SiO2:100-120Al(OH)3:10-30TMAdaOH:4000-5000H2O, the synthesis temperature is 150-170℃, and the synthesis time is 4-8h.

6. The method for preparing a double-layer CHA molecular sieve membrane according to claim 1, characterized in that... In step (2), the SSZ-13 molecular sieve membrane layer synthesized by hydrothermal synthesis was not subjected to calcination to remove the template agent before hydrothermal synthesis of low silica-alumina ratio CHA molecular sieve membrane layer; in step (3), the calcination conditions were a calcination temperature of 200-500℃ and a calcination atmosphere of air, oxygen, and ozone.

7. The application of the double-layer CHA molecular sieve membrane obtained by the preparation method of claim 1 in the separation of CO2 mixed gases, characterized in that, The CO2 mixture contains one or more of He, H2, N2, CH4, and C2H6.

8. The application of the bilayer CHA molecular sieve membrane obtained by the preparation method of claim 1 in the separation of He-containing mixed gases, characterized in that, The He-containing mixed gas contains one or more of CO2, H2, N2, CH4, and C2H6.

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