Organic-inorganic composite membrane, preparation method thereof and helium separation method

Abstract

The invention relates to the technical field of gas separation membranes, and discloses an organic-inorganic composite membrane, a preparation method thereof and a helium separation method, the composite membrane is a 6FDA-APAF-BIA / alpha-Al2O3 composite membrane, and the surface of the composite membrane is provided with a microporous structure. The preparation method of the organic-inorganic composite membrane comprises the following steps: S1, preparing a copolymerization product; s2, after the copolymer is dissolved in a solvent, the copolymer is poured on the surface of a supporting body or the surface of the supporting body is coated with the copolymer in a dip coating mode, after the solvent is dried, the temperature is increased for an imidization reaction, then the temperature-increased thermal rearrangement reaction is conducted, and the gas separation membrane is obtained. When the prepared organic-inorganic composite membrane is used for helium separation, the selectivity of He / N2 and He / CH4 mixed gas reaches 85 and 123 respectively. And the film provided by the invention has good film-forming property and wide application prospect and value.

Description

Technical Field

[0001] This invention relates to the field of gas separation membrane technology, specifically to an organic-inorganic composite membrane and its preparation method, as well as a helium separation method. Background Technology

[0002] The statements in this section provide only background information relevant to the disclosure of this application and may not constitute prior art.

[0003] Due to their excellent physicochemical properties and superior gas sieving performance, polyimide membrane materials have been widely studied for the separation of gas pairs such as H2 / CH4, CO2 / CH4, O2 / N2, CO2 / N2, and He / CH4. A wide variety of anhydride and diamine monomers endow polyimide membranes with different separation properties; among them, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) monomer contains a large-volume -CF3 functional group, which can effectively hinder chain rotation and chain stacking, thereby increasing the membrane's He permeability. Besides 6FDA, 2,2'-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF), as a diamine monomer, also contains abundant -CF3 functional groups. Furthermore, APAF's benzene ring contains thermally rearrangeable hydroxyl functional groups at the ortho position, which can be transformed into a rigid benzoxazole structure through thermal rearrangement, inducing the formation of a microporous structure. However, the brittleness of 6FDA-APAF polyimide membranes makes film formation difficult, hindering their practical industrial application.

[0004] According to the relevant literature "Physical aging of thin 6FDA-based polyimide membranes containing carboxyl acid groups. Part I. Transport properties", after aging in air for 1000 hours, the He permeability of spirobisindane polymers and 6FDA-6FpDA-based polyimide membranes decreased by 20-30%. Furthermore, according to the report "Physical aging of ultrathin glassy polymer films tracked by gas permeability", the He permeability of commercial PSF (polysulfone membranes) also decreased by 50%.

[0005] Related reports indicate that the He / CH4 and He / N2 selectivities of the thermally rearranged 6FDA-APAF membrane are only 30 and 19, respectively. However, to obtain 99% pure He industrially through a two-stage membrane process, the He / CH4 selectivity must be at least 54 and the He / N2 selectivity at least 25, which the existing 6FDA-APAF membrane clearly cannot meet.

[0006] Related studies indicate that the introduction of rigid diamine monomers can more effectively increase the gas selectivity of membranes. Lzano et al. introduced rigid 2,2-bis(3-amino-4-hydroxyphenyl)adamantane (ADHAB) into the 6FDA-APAF segment, which effectively increased the He / CH4 selectivity of the membrane to 40 and the He / N2 selectivity to 22, but still could not meet the requirements for industrial applications.

[0007] Existing technology CN118079681A provides an asymmetric gas separation membrane, its preparation method, and its application. By optimizing the casting solution conditions, an asymmetric copolyimide helium gas separation membrane with a selective layer thickness of 300 nm is prepared using a non-solvent-induced phase separation technique. However, this asymmetric structure cannot solve the brittleness problem of the copolyimide membrane, and the copolyimide membrane still has the defect of being difficult to form, hindering its practical industrial application. Summary of the Invention

[0008] The purpose of this invention is to address the brittleness problem of current copolyimide films by providing an organic-inorganic composite film and its preparation method, along with a helium separation method. This application uses an impregnation-coating method to prepare a thinner asymmetric copolyimide composite film on an α-Al2O3 support. Through thermal rearrangement at different temperatures, high He / N2 and He / CH4 selectivity are obtained. At the same time, the prepared film has reduced brittleness and good film-forming properties, which is beneficial for its widespread application.

[0009] The technical solution of the present invention is as follows:

[0010] An organic-inorganic composite membrane, the surface of which has a microporous structure and a polymer layer with a repeating unit structure as shown below:

[0011]

[0012] Where m and n are taken from any integer between 1 and 20; the thickness of the membrane is 3 to 12 μm.

[0013] Preferably, the organic-inorganic composite membrane is a composite membrane in which a polymer layer-copolyimide gas separation membrane is loaded on the surface of a support.

[0014] The support is a porous ceramic material. Preferably, the porous ceramic material is α-Al₂O₃.

[0015] This application also provides a method for preparing an organic-inorganic composite membrane as described above, specifically including the following steps:

[0016] Step S1: The copolymerization reaction was carried out by heating 2,2'-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, a non-hydroxydiamine monomer, and 4,4'-(hexafluoroisopropylidene) phthalic anhydride in an organic solvent. After washing and precipitation, the copolymer product was obtained.

[0017] Step S2: After the copolymer is dissolved in the solvent, it is poured onto the surface of the support. After the solvent is dried, the temperature is raised to carry out an imidization reaction, followed by a thermal rearrangement reaction to obtain a gas separation membrane.

[0018] According to a preferred embodiment, the molar ratio of 2,2'-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, non-hydroxydiamine monomer and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride in step S1 is 10:5-15:15-30.

[0019] According to a preferred embodiment, the copolymerization reaction in step S1 includes a first reaction stage and a second reaction stage. The conditions for the first reaction stage are: in an inert atmosphere, at 0-10°C for 5-30 hours; the conditions for the second reaction stage are: reflux reaction at 150-200°C for 5-12 hours, and an azeotropic agent is added to remove water after the reaction is completed.

[0020] Preferably, in step S2, the concentration of the copolymer dissolved in the solvent is 10-20 wt.%.

[0021] Preferably, in step S2, the imidization reaction conditions are: 200-300℃ for 1-5 hours, with a heating rate of 0.5-3℃ / min, and the reaction is carried out in an inert atmosphere. The working temperature for imidization is: when the temperature rises to 250℃, the degree of imidization approaches 94%.

[0022] Preferably, in step S2, the conditions for the thermal rearrangement reaction process are: 350-450℃ for 0.5-2h, with a heating rate of 0.5-3℃ / min.

[0023] Preferably, the conditions for the thermal rearrangement reaction process are: 350-450℃ for 0.5-1.5h, with a heating rate of 0.5-3℃ / min.

[0024] This application also provides a helium separation method, comprising the following steps:

[0025] Step S1: The raw material gas is passed through an organic-inorganic composite membrane as described above. The helium in the raw material gas is separated by passing through the organic-inorganic composite membrane.

[0026] Preferably, the feed gas is natural gas.

[0027] The components in the raw gas are either He / N2 or He / CH4.

[0028] Preferably, the feed pressure of the raw gas through the copolyimide gas separation membrane as described above is 0.1 MPa to 0.6 MPa.

[0029] Compared with existing technologies, the advantages of this invention are:

[0030] 1. An organic-inorganic composite membrane and its preparation method. This application synthesizes and prepares a 6FDA-APAF-BIA copolyimide composite membrane containing benzo[a]heterocyclic rings, and performs thermal rearrangement treatment at 350℃~450℃ to obtain a poly(benzimidazole-co-imide) membrane with a microporous structure. As the thermal rearrangement temperature increases, the FFV and d-spacing values ​​of the polymer membrane gradually increase, indicating the formation of micropores. The He permeability is also greatly improved. Compared with the precursor membrane, the separation membrane obtained by thermal rearrangement at 450℃ has a 2.1-fold increase in He permeability and a 1.4-fold increase in selectivity.

[0031] 2. An organic-inorganic composite membrane and its preparation method and helium separation method, wherein the TR450-BIA membrane obtained by thermal rearrangement treatment of 6FDA-APAF-BIA copolyimide composite membrane at 450℃ has a He / N2 and He / CH4 mixing selectivity of 92 and 141, respectively. The mixing selectivity of the membrane is much higher than that of traditional cellulose acetate membrane and polysulfone membrane.

[0032] 3. A method for preparing an organic-inorganic composite membrane. In this application, a 6FDA-APAF-BIA selective layer of about 7 μm is coated on α-Al2O3 by dip-coating and followed by thermal rearrangement treatment to obtain a 6FDA-APAF-BIA / α-Al2O3 composite membrane. The He permeability of this composite membrane is 24 GPU, which is an order of magnitude higher. The mixed selectivity of He / N2 and He / CH4 only decreased by 13%, which confirms the feasibility of the strategy of improving He permeability by reducing the membrane thickness.

[0033] 4. An organic-inorganic composite membrane and its preparation method, along with a helium separation method, show that the permeability of He, N2, and CH4 in the composite membrane increases monotonically with increasing temperature, indicating that the diffusion process of gas molecules is active diffusion. The activation energy for He diffusion is 9.5 kJ / mol. -1 The concentration of polybenzimidazole film is lower than 21.0 kJ / mol. -1 This indicates that He requires a lower energy barrier to overcome through the 6FDA-APAF-BIA / α-Al2O3 membrane;

[0034] 5. An organic-inorganic composite membrane and its preparation method and helium separation method, 6FDA-APAF-BIA / α-Al2O3 composite membrane, in the pressure range of 0.1MPa-0.6MPa, with the increase of feed pressure, the He permeability remains unchanged, and the selectivity only decreases slightly. At the pressure of 0.6MPa, the selectivity of He / N2 and He / CH4 is still 62 and 92, respectively, which is much higher than the 54 of industrial applications, and has good pressure resistance.

[0035] The separation performance of the 6FDA-APAF-BIA / α-Al2O3 composite membrane disclosed in this application not only exceeds the upper limit of Robeson, but also surpasses that of high-performance polymer materials such as Poly(PFMD), TR-PBOI, PORAs, and PIM.

[0036] 6. An organic-inorganic composite membrane, in long-term testing, the membrane after thermal rearrangement shows a slight decrease in gas permeability (e.g., He decreases by only 12%), and the membrane after thermal rearrangement is resistant to aging, which has significant advantages compared with the prior art. Attached Figure Description

[0037] Figure 1 These are the infrared characterization data of the precursor BIA film, TR350-BIA film, TR400-BIA film and TR450-BIA film, where (a) is the transformation of the rearrangement structure; (b) is the infrared spectrum.

[0038] Figure 2 These are the wide-angle X-ray diffraction (WXRD) patterns of the precursor BIA film and the same series of thermal rearrangement films;

[0039] Figure 3 This is the TG curve of the precursor BIA membrane;

[0040] Figure 4 Figure (a) shows the single-component gas permeability and ideal selectivity of the TR450-BIA membrane. (a) shows the single-component gas permeability of the TR450-BIA membrane, with the blue line graph representing permeability in Barrer and the magenta line graph representing permeance in GPU. (b) shows the ideal selectivity of the TR450-BIA membrane, with the orange bar graph representing Knudsen selectivity.

[0041] Figure 5 These are SEM images of the 6FDA-APAF-BIA / α-Al2O3 composite membrane prepared in Example 2, where (a) is a surface view of the membrane and (b) is a cross-sectional view of the membrane.

[0042] Figure 6Figure 2 shows the single-component separation performance of the 6FDA-APAF-BIA / α-Al2O3 composite membrane prepared in Example 2. In Figure 2, (a) shows the single-component gas permeability, with the blue line representing the composite membrane prepared in Example 2 and the magenta line representing the TR450-BIA self-supporting membrane prepared in Example 1. Figure 2 shows the ideal selectivity of the composite membrane prepared in Example 2, with the orange bar chart representing the Knudsen selectivity.

[0043] Figure 7 The figure shows the effect of temperature on the separation performance of the mixed gas. Figures (a) and (b) show the effect of temperature on the He / N2 separation performance and the Arrhenius temperature-dependent He and N2 permeability in the 6FDA-APAF-BIA / α-Al2O3 composite membrane prepared in Example 2. Figures (c) and (d) show the effect of temperature on the He / CH4 separation performance and the Arrhenius temperature-dependent He and CH4 permeability. The test pressure was 0.1 MPa.

[0044] Figure 8 The effect of pressure on the separation performance of He / N2(a) and He / CH4(b) mixed gases in the 6FDA-APAF-BIA / α-Al2O3 composite membrane was investigated at a test temperature of 25℃.

[0045] Figure 9 This is a graph showing the results of comparing the separation performance of the 6FDA-APAF-BIA / α-Al2O3 composite membrane for He / N2 (a) and He / CH4 (b) mixed gases with other polymers at the Robeson upper boundary. Detailed Implementation

[0046] The specific embodiments listed in this invention are merely examples, and the invention is not limited to the specific embodiments described below. For those skilled in the art, any equivalent modifications and substitutions to the embodiments described below are also within the scope of this invention. Therefore, all equivalent transformations and modifications made without departing from the spirit and scope of this invention should be covered within its scope. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. All reagents or instruments whose manufacturers are not specified are commercially available conventional products. To better illustrate this invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this invention can be practiced even without certain specific details. In other embodiments, methods, means, equipment, and steps well known to those skilled in the art are not described in detail in order to highlight the main points of this invention.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Unless otherwise specified, all units used in this specification are International Standard Units (SI), and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors unavoidable in industrial production.

[0048] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0049] Example 1: An organic-inorganic copolyimide composite film and its preparation method

[0050] A method for preparing a copolyimide gas separation membrane specifically includes the following steps:

[0051] Step S1: Ortho-hydroxy copolyimide was synthesized using a two-step azeotropic imidization method.

[0052] In a three-necked round-bottom flask, the hydroxydiamine monomer 2,2'-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF, 10 mmol) and the non-hydroxydiamine monomer 5-amino-2-(4-aminobenzene)benzimidazole (BIA, 10 mmol) were added, followed by the addition of 35 mL of N-methylpyrrolidone (NMP) and stirring to dissolve both monomers. After complete dissolution, 20 mmol of 4,4'-(hexafluoroisopropylidene)phthalic anhydride (6FDA) was added to the round-bottom flask to initiate the copolymerization reaction.

[0053] The reaction process is divided into two stages: In the first stage, the reaction mixture is stirred vigorously for 16 hours under argon protection to generate pale yellow polyamic acid, and the reaction temperature is controlled at 5℃; In the second stage, the polyamic acid solution is heated to 180℃ and reacted for 8 hours using a Dean-Stark reflux apparatus. During the heating process, 10 mL of o-xylene is added to the solution as an azeotropic agent to remove the water byproduct generated after the reaction.

[0054] After the reaction, the transparent, pale yellow polyamic acid solution turned into a brownish-black polyimide solution. The polymer solution was poured into a washing solution of methanol and water (1:3), and vigorous stirring was performed to precipitate filamentous polymer. The washing process was repeated three or more times. The washed polymer was then transferred to a vacuum oven and dried at 150°C for 24 hours. The resulting copolymer was named 6FDA-APAF-BIA.

[0055] Step S2: Preparation of the gas separation membrane:

[0056] Step S2.1: Preparation of self-supporting membrane: The 6FDA-APAF-BIA polymer was dissolved in NMP and stirred vigorously for 12 hours to form a homogeneous solution of 15 wt.%. After filtration through a 0.45 μm PTFE membrane, a fixed mass of the solution was poured into flat-bottomed culture dishes. The culture dishes containing the cast solution were transferred to a vacuum oven with a set temperature gradient: 80 °C for 2 hours, 100 °C for 2 hours, 120 °C for 6 hours, and 150 °C for 6 hours, allowing the solvent to evaporate slowly. After complete solvent evaporation, the precursor membrane was peeled off from the culture dish, yielding a pale yellow transparent membrane with a thickness of approximately 60 μm. To further remove residual solvent and fully imidize, the membrane was placed in a tube furnace maintained at 250 °C for 3 hours, with a heating rate of 1 °C / min and argon gas as a protective gas.

[0057] Step S2.2: Thermal rearrangement: The self-supporting membrane obtained in the previous step was transferred to a tube furnace under Ar atmosphere protection and heated to 450℃ at a heating rate of 1℃ / min, and held for 1 hour to obtain the rearranged membrane. To facilitate comparison of the effect of thermal rearrangement temperature on membrane performance, the thermal rearrangement temperature was also adjusted to 350℃ and 400℃, thus obtaining polymer membranes at three different rearrangement temperatures. The original self-supporting precursor membrane prepared from polymer 6FDA-APAF-BIA without thermal rearrangement treatment is denoted as BIA. The membranes obtained after thermal rearrangement treatment of BIA at 350℃, 400℃, and 450℃ are respectively denoted as TR350-BIA, TR400-BIA, and TR450-BIA. Experiments showed that the color of the membrane material gradually deepened with increasing heat treatment temperature.

[0058] The properties of the prepared BIA, TR350-BIA, TR400-BIA, and TR450-BIA membranes were characterized, and the results are as follows:

[0059] The number-average molecular weight of the 1.6 FDA-APAF-BIA polymer is 1.38 × 10⁻⁶. 4 g mol -1 The weight-average molecular weight is 2.95 × 10⁻⁶. 4 g mol -1 According to the polydispersity index, the polymer molecular weight distribution is uniform.

[0060] 2. The membrane structure of BIA was analyzed by Fourier transform infrared spectroscopy (FTIR). Figure 1 It can be seen that for the BIA series membranes, at 1780cm -1 (1) shows a symmetrical imide carbonyl absorption band (C=O), at 1330 cm⁻¹. -1(3) These peaks, representing the stretching vibrations of the CNC groups in the imide, appeared in both the precursor BIA, TR350-BIA, TR400-BIA, and TR450-BIA films, indicating the formation of the polyimide structure. Furthermore, for the TR350-BIA, TR400-BIA, and TR450-BIA films, the peak at 1658 cm⁻¹... -1 (2) A new peak appeared, which is the C=N stretching vibration, indicating the formation of the benzoxazole ring in the thermal rearrangement.

[0061] 3. Figure 2 The WXRD patterns of the BIA series membranes are shown. The d-spacing value of the BIA membrane is... As the thermal rearrangement temperature increases, the d-spacing value increases monotonically, with the TR450-BIA membrane reaching its highest d-spacing value. This result suggests that thermal rearrangement forms a microporous structure, and the thermal conversion rate increases with increasing rearrangement temperature (the thermal conversion rates of TR350-BIA, TR400-BIA and TR450-BIA are 82.8%, 92.1% and 96.3%, respectively), with a more dense pore distribution.

[0062] 4. The free volume fraction (FFV) of a polymer membrane typically reflects its gas permeability. The density and free volume fraction (FFV) data for BIA series membranes are as follows:

[0063]

[0064] a: FFV values ​​are calculated based on a 100% heat conversion rate.

[0065] The data above shows that the FFV value of the BIA membrane is 0.149, which is significantly lower than that of the 6FDA-APAF polyimide membrane (0.211). This indicates that the benzimidazole structure on the polymer chain of the 6FDA-APAF-BIA membrane makes the membrane structure more compact. After heat treatment at 350℃, its FFV increases to 0.154, indicating that thermal rearrangement generates more free volume elements. As the thermal rearrangement temperature increases to 450℃, its FFV value increases by 24%. This result shows that higher temperatures enhance the degree of thermal rearrangement, thereby inducing the formation of more micropores. This result is consistent with... Figure 2 The trend of WXRD in the data is consistent.

[0066] 4. Thermogravimetric (TG) curves are used to analyze the degree of thermal rearrangement and stability of membrane materials. Figure 3It can be seen that the BIA membrane underwent significant degradation within the temperature range of 350℃ to 450℃. This is due to the release of CO2 caused by chain rearrangement. The 6FDA-APAF-BIA membrane degraded by 5.3%, and a clear inflection point occurred around 510℃, indicating that the polymer backbone began to degrade. The maximum degradation rate was around 560℃. For the 6FDA-APAF-BIA membrane, the polymer completely degraded when the temperature reached 787℃.

[0067] 5. The thermal conversion rate (C / %) of rearrangement membranes effectively reflects the degree of thermal rearrangement of polymer membranes and is closely related to gas separation performance. Generally, membranes with higher thermal conversion rates have higher gas permeability than those with lower thermal conversion rates. Therefore, based on the ratio of actual weight loss to theoretical weight loss after thermal rearrangement, the thermal conversion rate at different temperatures can be obtained. The thermal conversion rate of the precursor BIA membrane at 350℃ is 82.8%. As the thermal rearrangement temperature increases, the thermal conversion rate of the polymer increases monotonically. Under rearrangement conditions at 450℃, the C value of the TR450-BIA membrane reaches 96.3%, far exceeding the 76% of the 6FDA-APAF-ADHAB (ADHAB = 2,2-bis(3-amino-4-hydroxyphenyl)adamantane) membrane and the 60% of the 6FDA-APAF-DAM (DAM = 2,4,6-trimethylm-phenylenediamine) membrane. However, the thermal conversion rate of the TR450-BIA membrane does not exceed 100%, indicating that no degradation of the polymer chain backbone occurred during the rearrangement process.

[0068] 6. To further evaluate the effect of thermal rearrangement temperature on gas separation performance, the BIA series membranes were tested for the separation of He / CH4 and He / N2 mixed gases at 25℃ and 0.1MPa. The test results are as follows:

[0069]

[0070] The data shows that the He permeability of the BIA precursor membrane is 72 Barrer, and the selectivity for He / CH4 and He / N2 mixed gases is 101 and 72, respectively. As the thermal rearrangement temperature gradually increases to 450℃, the He permeability increases to 152 Barrer, nearly doubling. The FFV value of the membrane material also increases from 0.149 to 0.202. This is due to the free volume elements (micropores, narrow channels, etc.) generated by the thermal rearrangement. The traditional PBI polymer has a He permeability of only 2.67 Barrer. By introducing large-volume side-group functional groups, the He permeability is increased to 10.1 Barrer. At 450℃, the He / CH4 and He / N2 selectivity of the TR450-BIA membrane also increase to 141 and 92, respectively, which is 1.4 times that of the precursor membrane. This is because the thermal rearrangement produces a rigid rod-shaped benzoxazole heterocyclic structure that hinders greater CH4 and N2 diffusion.

[0071] In comparison, the 6FDA-APAF polyimide membrane exhibits a He / CH4 selectivity of only 30 and a He / N2 selectivity of 19. The 6FDA-APAF-BIA membrane also shows higher selectivity. The base(TB) copolyimide membrane exhibits a He / CH4 and He / N2 selectivity of 20, indicating that the synergistic effect of the BIA unit and thermal rearrangement significantly enhances the membrane's selectivity.

[0072] 7. Because the TR450-BIA membrane exhibited the highest He permeability and He / N2 and He / CH4 selectivity, these two membranes were selected for single-component gas permeability testing. To evaluate the permeation performance of different gases in the membrane, the single-component permeability of the following gases was tested: He (0.255nm), H2 (0.289nm), CO2 (0.33nm), N2 (0.364nm), and CH4 (0.38nm), under test conditions of 0.1MPa and 25℃.

[0073] from Figure 4 As shown in the small figure (a), the permeability gradually decreases with the increase of the gas molecule dynamic diameter, indicating that the gas permeation in these two membranes is based on a strong size sieving effect. The single-component He permeability of the TR450-BIA membrane is 160 Barrer, slightly higher than the He permeability of the mixed gas, which is mainly attributed to the competitive adsorption of the mixed components.

[0074] While thickness-dependent permeability (measured in Barrers) values ​​can effectively reflect the inherent properties of polymers, in practical industrial applications, thickness-independent He permeability (measured in GPUs) is a more effective indicator of gas productivity. The He permeability value is calculated based on the actual thickness of the membrane material. Figure 4 (The magenta broken line in the small figure of (a)) shows that the He permeability of the TR450-BIA membrane is 2.67 GPU. At room temperature, the He permeability of the PBDI polymer membrane prepared by interfacial polymerization reaches 7.5 GPU. At a test temperature of 100°C, the He permeability is 40 GPU. This is mainly due to the large thickness of the self-supporting membrane (the thickness of the self-supporting membrane prepared in this example is about 60 μm).

[0075] Figure 4Figure (b) shows the ideal selectivity of He for other gases. The ideal selectivities of He / CH4 and He / N2 for the TR450-BIA membrane are 141 and 95, respectively, which is attributed to the densely packed benzimidazole rings hindering the diffusion of large CH4 and N2 molecules. Due to the absence of competitive adsorption from other components, the ideal selectivities are slightly higher than those for He / CH4 and He / N2 in the mixed components. The ideal selectivities for He / N2 and He / CH4 gases are significantly higher than the Knudsen selectivity.

[0076] Compared with the BIA membrane and the TR450-BIA membrane, the membrane after thermal rearrangement showed a slight reduction in gas permeability (e.g., He decreased by only 12%), which is a significant advantage over the prior art without thermal rearrangement.

[0077] Example 2: Preparation of 6FDA-APAF-BIA / α-Al2O3 composite membrane

[0078] Although the TR450-BIA self-supporting membrane prepared in Example 1 achieved higher He / CH4 and He / N2 separation selectivity of 141 and 92, respectively, its He permeability without thickness was only 2.67 GPU. In this example, we consider improving the He permeability of the membrane by reducing the membrane thickness.

[0079] Step S1 of the preparation method of the 6FDA-APAF-BIA / α-Al2O3 composite membrane in this application is the same as that in Example 1. The specific steps of step S2 are as follows:

[0080] Step S2.1: Prepare a 10 wt.% NMP solution of the 6FDA-APAF-BIA polymer, stir vigorously for 2 days, and then sonicate to completely dissolve it. The coating method is dip-coating. Specifically, the bottom-sealed α-Al2O3 support is inverted and immersed in the pre-prepared solution for 30 seconds, then placed in a fume hood for 0.5 hours to defoam and avoid defects. Solvent evaporation is performed according to the same method as in Example 1. After solvent evaporation, to further remove residual solvent and fully imidize, the membrane is placed in a tube furnace maintained at 250°C for 3 hours, with a heating rate of 1°C / min and argon gas introduced as a protective gas.

[0081] Step S2.2: Then transfer it to a tube furnace under Ar atmosphere protection, heat it to 450℃ at a heating rate of 1℃ / min, hold for 1h, and obtain the thermally rearranged composite membrane.

[0082] Related performance tests

[0083] 1. SEM image of the 6FDA-APAF-BIA / α-Al2O3 composite membrane prepared in Example 2 is shown below. Figure 5As shown, the membrane surface is smooth and no defects were found. According to its cross-sectional view, the selective layer thickness of the membrane material is 7 μm, and it is tightly bonded to the α-Al2O3 support; no interface defects were observed.

[0084] 2. From Figure 6 As shown in Figure (a), the single-component permeability of the composite membrane decreases with increasing gas molecule kinetic diameter, exhibiting a significant drop between H2 and N2. The He permeability of the composite membrane is 25.3 GPU, which is 10 times that of its corresponding TR450-BIA self-supporting membrane. This is due to the membrane thickness decreasing from 60 μm to 7 μm, a reduction of approximately 9 times, a thickness that is difficult to achieve with the 6FDA-APAF-BIA membrane of Example 1. This effectively reduces the path of gas through the selective membrane layer. The membrane thickness can be controlled by shortening the immersion time of the inverted α-Al2O3 support in the pre-prepared NMP solution and by adjusting the NMP solution concentration. Shortening the immersion time or reducing the concentration allows for the preparation of a thinner composite membrane, while increasing the immersion time or concentration allows for the preparation of a thicker composite membrane.

[0085] The performance of 6FDA-APAF-BIA / α-Al2O3 composite membranes with different thicknesses was tested at 0.1 MPa, as shown in the table below.

[0086]

[0087] As can be seen from the table above, the membrane selectivity is optimal when the membrane thickness is 7 μm.

[0088] from Figure 6 As shown in Figure (b), the ideal selectivities for He / N2 and He / CH4 of the composite membrane are 85 and 123, respectively, far exceeding the Knudsen selectivities (He / N2 is 2.56, He / CH4 is 2). The selectivities are essentially consistent with the TR450-BIA self-supporting membrane of Example 1, indicating that the thinner selective layer prepared by the dip-coating method did not produce defects. According to Scholes et al., to concentrate 1%-3% He in a nitrogen removal unit (NRU) to 70%, the He / CH4 selectivity in a two-stage membrane system needs to reach 55, and the He / N2 selectivity needs to reach 25. The composite membrane prepared in Example 2 of this application has a selectivity far exceeding this standard, meeting the requirements for industrial applications.

[0089] 3. At 25℃ and 0.1 MPa, the He permeability is 25.3 GPU, and the selectivity of the He / N2 and He / CH4 mixed gases are 85 and 123, respectively, slightly lower than the single-component He permeability and ideal selectivity. This is due to the competitive adsorption of CH4 and N2. As the temperature increases from 25℃ to 180℃, the permeability of He, N2, and CH4 all show a monotonically increasing trend. Figure 7The smaller figures (a) and (c) illustrate the active diffusion of the gases. At 180 °C, He permeability reaches a high of 89.3 GPU. Due to the more significant increase in N2 and CH4 permeability, the selectivity gradually decreases, with He / N2 and He / CH4 selectivities dropping to 40 and 50, respectively. The apparent activation energies of N2 and CH4 are 14.1 kJ / mol. -1 and 15.3 kJ mol -1 ( Figure 7 (See smaller figures in (b) and (d)). This is because the molecular dynamic diameter of CH4 is larger than that of N2. Conversely, the diffusion activation energy of He in the He / CH4 mixture is 9.5 kJ mol. -1 Slightly higher than 9.4 kJ mol in the He / N2 system. -1 When gas molecules are transported in micropores, rapidly diffusing gas molecules are always delayed and restricted by slowly diffusing gas molecules. This result indicates that CH4 molecules have a stronger influence on He diffusion, which is attributed to the easier adsorption of CH4. Although the 6FDA-APAF-BIA membrane contains a benzimidazole ring that promotes chain stacking, its He diffusion activation energy is still lower than that of the 6FDA-6FpDA membrane (12 kJ / mol). -1 It exhibits lower energy barrier, thanks to the lower film thickness of the 6FDA-APAF-BIA / α-Al2O3 composite film.

[0090] 4. Considering that high pressure is required as the driving force for He extraction from natural gas, the pressure resistance of membrane materials is crucial for practical industrial applications. At room temperature, the changes in the He / N2 and He / CH4 mixed separation performance of the 6FDA-APAF-BIA / α-Al2O3 composite membrane within a pressure range of 0.1 MPa–0.6 MPa were evaluated. The results are shown below. Figure 8 At a feed pressure of 0.1 MPa, the He permeability was 25.1 GPU. With increasing pressure, the He permeability remained constant, while the permeabilities of N2 and CH4 increased slightly, leading to a gradual decrease in selectivity. This was due to the competitive adsorption of N2 and CH4 molecules in the binary mixture. At a pressure of 0.6 MPa, the He / N2 selectivity decreased by 24% compared to the test pressure of 0.1 MPa, with a selectivity of 62, still meeting the standard for industrial applications (>25). The He / CH4 selectivity decreased by 23%, with a selectivity of 92 at 0.6 MPa, also significantly higher than the 54 required for industrial applications. This good pressure resistance indicates that the polymer selective layer coated on the alumina surface is defect-free, as confirmed by its SEM images. Figure 5 This has been verified.

[0091] 5. To compare the separation performance of this membrane with that of high-performance polymer membranes reported in the literature, the 2008 Robeson upper boundary lines for He / N2 and He / CH4 were plotted respectively. Figure 9 For He / N2 separation, the membrane performance under different pressures (0.1MPa-0.6MPa) far exceeded the upper limit, with selectivity higher than membrane materials such as PIM, Nafion 117, and PBI, and permeability exceeding that of PBDI, CA, Poly (PFMD), and TR-PBOI membranes. Compared to self-supporting thermal rearrangement membranes, permeability was improved by nearly an order of magnitude, mainly due to the reduction of membrane thickness from 60μm to 7μm. For He / CH4 separation, the selectivity exceeded 100, and He permeability was improved by reducing membrane thickness, bringing its separation performance close to the upper limit and exceeding that of membranes such as CA, PBI, TR-PBOI, and PORAs.

[0092] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

Claims

1. An organic-inorganic composite membrane, characterized in that, The composite membrane has a microporous structure on its surface, which has a polymer layer with a repeating unit structure as shown below: Where m and n are any integers between 1 and 20.

2. The organic-inorganic composite membrane according to claim 1, characterized in that, The organic-inorganic composite membrane is a composite membrane in which a polymer layer is loaded on the surface of a support, and the support is a porous ceramic material; the thickness of the membrane is 3 to 12 μm.

3. The organic-inorganic composite membrane according to claim 2, characterized in that, The porous ceramic material is α-Al2O3.

4. A method for preparing an organic-inorganic composite membrane according to any one of claims 1 to 3, characterized in that, Specifically, the steps include the following: Step S1: The copolymerization reaction was carried out by heating 2,2'-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, a non-hydroxydiamine monomer, and 4,4'-(hexafluoroisopropylidene) phthalic anhydride in an organic solvent. After washing and precipitation, the copolymer product was obtained. Step S2: After the copolymer is dissolved in the solvent, it is poured onto the surface of the support or dipped and coated onto the surface of the support. After drying the solvent, the temperature is raised to carry out an imidization reaction, followed by a thermal rearrangement reaction to obtain a gas separation membrane.

5. The method for preparing an organic-inorganic composite membrane according to claim 4, characterized in that, In step S2, the conditions for the thermal rearrangement reaction are: 350-450℃ for 0.5-2h, with a heating rate of 0.5-3℃ / min.

6. The method for preparing an organic-inorganic composite membrane according to claim 5, characterized in that, The temperature of the thermal rearrangement reaction process is 450°C.

7. The method for preparing an organic-inorganic composite membrane according to claim 4, characterized in that, In step S2, the concentration of the copolymer dissolved in the solvent is 10-20 wt.%.

8. The method for preparing an organic-inorganic composite membrane according to claim 4, characterized in that, In step S2, the conditions for the imidization reaction are: 200-300℃ for 1-5 hours, heating rate of 0.5-3℃ / min, and the reaction is carried out in an inert atmosphere.

9. A helium separation method, characterized in that, The separation of helium from feed gas using the organic-inorganic composite membrane as described in any one of claims 1 to 3 specifically includes the following steps: Step S1: The raw material gas is passed through an organic-inorganic composite membrane according to any one of claims 1 to 3, wherein the helium in the raw material gas passes through the organic-inorganic composite membrane, and the remaining gases are blocked.

10. A helium separation method according to claim 9, characterized in that, The feed pressure of the raw gas through the organic-inorganic composite membrane in step S1 is 0.1 MPa to 0.6 MPa.

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