Preparation method of natural gas efficient hydrogen / helium extraction fluorinated MOFs mixed matrix membrane

Abstract

The application belongs to the technical field of gas membrane separation, and discloses a preparation method of a fluorinated MOFs mixed matrix membrane for efficient hydrogen / helium extraction from natural gas. Fluorinated MOFs are prepared by introducing -CF3 into the MOFs framework. The introduction of -CF3 reduces the pore size of the MOFs, enhances the molecular sieving effect, and improves the diffusion selectivity of H2(He) / CH4. In addition, -CF3 has an inhibitory effect on CH4 adsorption, which improves the solubility selectivity of H2(He) / CH4 separation. The presence of -CF3 in the fluorinated MOFs can induce the formation of dipole-dipole interaction (C-F … O=C) between the MOFs and the polymer, so that the filler and the polymer have good interfacial compatibility, which is beneficial to the improvement of the filler loading, and avoids the generation of non-selective defects at the interface. The mixed matrix membrane prepared by the method has important significance in the field of efficient hydrogen / helium extraction from natural gas.

Description

Technical Field

[0001] This invention belongs to the field of gas membrane separation technology. It mainly designs a high-efficiency molecular sieve fluorinated MOFs mixed matrix membrane, which reduces the pore size of MOFs molecular sieves, inhibits methane adsorption, improves the interfacial compatibility of the mixed matrix membrane, and enhances the H2 (He) permeability and gas selectivity of the mixed matrix membrane. Background Technology

[0002] With the rapid development of technology, helium and hydrogen are increasingly widely used and indispensable in many fields. In the electronics and semiconductor industry, helium, as a crucial coolant and protective gas, provides a stable and low-temperature environment for delicate processes such as chip manufacturing, ensuring high precision and quality in production. Hydrogen, on the other hand, is emerging in the new energy field and is considered a key component of future clean energy. Its fuel cell technology is gradually becoming a mainstream trend in electric vehicles and other transportation, significantly reducing carbon emissions and improving energy efficiency. However, the global supply of helium and hydrogen currently faces many challenges. Helium resources are extremely unevenly distributed, mainly concentrated in a few countries and regions, such as the United States, and its extraction process is complex and costly. Although hydrogen can be produced through various means, large-scale, efficient, and low-cost production technologies still require further breakthroughs. Traditional hydrogen production methods, such as water electrolysis, are often limited by energy costs and resource distribution. Against this backdrop, extracting helium and hydrogen from natural gas has become a highly promising research and development direction. However, due to the similar properties of H2(He) and CH4, separating H2(He) via traditional physical adsorption processes remains challenging. He and CH4 Due to their different dynamic diameters, membrane-based gas separation technology offers significant advantages for separating H2(He) / CH4 mixtures. Membrane separation technology is a novel and highly efficient separation technology applied in industrial production. Compared with traditional separation methods, it has advantages such as high efficiency and easy coupling. It requires relatively low operating energy during the separation process, making it a highly competitive and popular method. Its core lies in preparing separation membrane materials with high separation performance. Conventional polymer membrane materials exhibit a significant "trade-off" phenomenon, where permeability and selectivity cannot be simultaneously achieved; while inorganic membranes typically offer good separation performance and high stability, but are expensive. By embedding inorganic fillers into a polymer matrix to prepare hybrid matrix membranes, the advantages of both polymers and nanoporous crystalline materials can be fully utilized. Inorganic fillers provide additional gas adsorption sites and preferential diffusion paths, theoretically enabling the simultaneous achievement of high permeability and high selectivity. Metal-organic frameworks (MOFs) are a class of inorganic-organic hybrid materials with porous structures formed by the coordination self-assembly of metal ions and organic ligands. They are a novel type of filler for preparing hybrid matrix membranes, attracting widespread attention due to their well-developed pore structure, high specific surface area, tunable pore size, and ease of functionalization. Theoretically, the porous structure of MOFs in hybrid matrix membranes can enhance gas adsorption and diffusion, thereby improving gas permeability. Secondly, the small pore size of MOFs ensures molecular sieving, contributing to improved gas selectivity. Furthermore, the partially organic properties of MOFs can improve the interfacial compatibility between polymers and MOFs, ensuring reasonable gas selectivity. Therefore, MOF-based hybrid matrix membranes have been widely used in H2(He) / CH4 separation.

[0003] Among polymers used to prepare H2(He) / CH4 separation mixed matrix membranes, hexafluorodianhydride (6FDA)-based polyimide (PI) with large -CF3 groups on its molecular chain exhibits better H2(He) / CH4 separation performance. The rigid, non-coplanar structure of the 6FDA-based PI polymer chain increases steric hindrance, reduces intermolecular forces and chain packing density, and has a larger free volume, significantly improving the permeation of H2 and He. Furthermore, the strong electron-withdrawing property of the -CF3 group weakens the electron cloud density of the strongly negatively charged benzene ring, resulting in a lower binding energy between the fluorinated segments and the positively charged CH4 surface. This weakens the interaction between the polymer and CH4, inhibiting the dissolution and adsorption of CH4 molecules in the polymer, thus further improving selectivity. In addition, segments with -CF3 as side groups have greater difficulty overcoming rotational energy barriers, further suppressing the diffusion of larger-sized CH4 molecules. However, the H2 / CH4 separation performance of the fluorinated 6FDA-based PI mixed matrix membrane still exhibits certain trade-offs. Researchers attribute this to two reasons: First, the interfacial compatibility between MOFs and polymers leads to insufficient MOF loading, hindering performance improvement, and weak interfacial compatibility easily results in non-selective defects at the MOF-polymer interface. Second, the flexibility of the MOF framework structure causes a "breathing effect," resulting in MOF pore sizes much larger than the theoretical sieve pore size. Therefore, improving the interfacial compatibility between MOFs and polymers and suppressing the "breathing effect" of MOFs are crucial for enhancing their gas permeability and selectivity.

[0004] Currently, the core methods for improving the interfacial compatibility between MOFs and polymers are surface modification and in-situ synthesis. Metal ion replacement, ligand post-replacement, and the rational design and functionalization of ligands can effectively regulate the pore size of MOFs. Ding Baisuo proposed a method (CN202310874036.1) to prepare a COOH-PI / NH2-ZIF-8 mixed matrix membrane by carboxylating PI (COOH-PI) and simultaneously amino-functionalizing ZIF-8 (NH2-PI). This method improves the interfacial compatibility of the mixed matrix membrane; however, due to the still relatively large pore size of NH2-ZIF-8, the He / CH4 selectivity of the prepared mixed matrix membrane is not high. Wang Shaofei (CN202410955428.5) proposed a Pd@MOF gel hybrid matrix membrane and its preparation method for improving H2 transport and separation efficiency. This membrane is formed by uniformly distributing Pd on ZIF-67 gel to form Pd@ZIF-67 gel. The combination of Pd and ZIF-67 gel effectively reduces the pore size of ZIF-67 gel, allowing for more efficient screening of hydrogen molecules and inhibiting methane passage. Subsequently, he introduced Pd@ZIF-67 gel into polyimide (P84) and self-polymerizing microporous polymer (PIM-1) to prepare a hybrid matrix membrane. However, the prepared Pd@ZIF-67 gel / P84 hybrid matrix membrane exhibited low permeability, and the Pd@ZIF-67 gel / PIM-1 hybrid matrix membrane showed low selectivity.

[0005] To address the aforementioned shortcomings and deficiencies, the present invention aims to design a method for preparing a high-efficiency molecularly sieved fluorinated MOF mixed matrix membrane. Summary of the Invention

[0006] The purpose of this invention is to design a method for preparing a high-efficiency molecular sieve fluorinated MOF mixed matrix membrane. By introducing -CF3 groups to inhibit ligand rotation and reduce the pore size of MOFs, the diffusion rate of CH4 is suppressed, improving the H2(He) / CH4 diffusion selectivity. Furthermore, the introduction of -CF3 inhibits CH4 adsorption, improving the H2(He) / CH4 dissolution selectivity. Simultaneously, -CF3 induces dipole-dipole interactions between the filler and the polymer, improving interfacial compatibility and further enhancing the permeability and selectivity of the mixed matrix membrane.

[0007] The technical solution of the present invention:

[0008] A method for preparing a fluorinated MOFs hybrid matrix membrane for high-efficiency hydrogen / helium extraction from natural gas includes the following steps: First, fluorinated MOFs with different degrees of fluorination were prepared by introducing -CF3 into the MOF framework via a hydrothermal reaction. The introduction of -CF3 reduced the pore size of the MOFs, enhanced molecular sieving, and improved the H2 / CH4 diffusion selectivity. Furthermore, -CF3 inhibited CH4 adsorption, further improving the dissolution selectivity of H2 / CH4 separation. Subsequently, fluorinated MOFs of different proportions were doped into 6FDA-based PI polymers to prepare the hybrid matrix membrane. The presence of -CF3 in the fluorinated MOFs can induce dipole-dipole interactions (CF3-CF3) between the MOFs and the polymer. … O=C) ensures good interfacial compatibility between the filler and the polymer, facilitating increased filler loading while preventing non-selective defects at the interface, thus guaranteeing the permeability of the mixed matrix membrane and improving its selectivity. The specific steps include:

[0009] Step 1: Imidazole and trifluoromethyl imidazole as ligands are dissolved in methanol, and then triethylamine is added. The mixture is stirred at room temperature until completely dissolved to prepare mixed solution A. Metal salt is dissolved in methanol and stirred at room temperature until dissolved to prepare mixed solution B. Mixed solution B is added to mixed solution A, and the mixture is stirred for a certain time. Then, it is transferred to a reaction vessel and reacted for a certain time. After centrifugation, washing, and drying, fluorinated MOFs powder is obtained.

[0010] Step 2: Dissolve and disperse the dried PI polymer in a mixed solution of DMAC and THF, and stir for a certain time to prepare a uniform PI solution; disperse the fluorinated MOF powder in a mixed solution of DMAC and THF, and stir for a certain time to prepare a uniform MOF suspension.

[0011] Step 3: The MOF suspension is then added to the PI solution to prepare a mixed solution. The mixed solution is then sonicated and stirred at room temperature for a certain time to obtain a membrane solution. Finally, the membrane solution is cast into a PTFE plate and dried to obtain a fluorinated MOF mixed matrix membrane.

[0012] In step 1, the imidazole is 2,-methylimidazolium or / and benzimidazole, the trifluoromethyl imidazole is 2-(trifluoromethyl)-1H-imidazolium or / and 2-(trifluoromethyl)benzimidazole, and the metal salt is cobalt nitrate hexahydrate or / and zinc nitrate hexahydrate; the molar ratio of imidazole to trifluoromethyl imidazole is 3:1 to 0:4, and the molar ratio of ligand to metal salt is 4:1.

[0013] In step 1, mixed solution B is added dropwise to mixed solution A, stirred at room temperature for 6 hours, and then transferred to a reaction vessel for 12 hours of reaction. The solvent used for washing is methanol, and the centrifugation rate during washing is 10,000 rpm. The drying time is ≥12 hours, and the drying conditions are 80°C under vacuum.

[0014] In step 2, the PI polymer is a 6FDA-based PI polymer, which is one or more of 6FDA-TFMB, 6FDA-DAM, 6FDA-DABA, 6FDA-BD, and 6FDA-FDA. The drying time is 12 hours, and the drying conditions are 120°C under vacuum. The concentration of the PI solution is 3.55 wt%, and the mass ratio of DMAC to THF is 8:2 to 6:4. The PI solution is stirred at 50°C for 4 hours.

[0015] In step 3, the mass fraction of fluorinated MOFs in the mixed solution is 10–30 wt%, the ultrasonic time is 10 min, and the membrane solution drying temperature is 50 °C.

[0016] The beneficial effects of this invention are as follows: This invention designs a method for preparing a highly efficient molecular sieving fluorinated MOF mixed matrix membrane. The introduction of -CF3 reduces the pore size of the MOF, enhances the molecular sieving effect, and improves the diffusion selectivity of H2(He) / CH4. Furthermore, -CF3 has a certain inhibitory effect on CH4 adsorption and dissolution, further improving the dissolution selectivity of H2(He) / CH4 separation. The presence of -CF3 in the fluorinated MOFs can induce the formation of dipole-dipole interactions (CF3) between the MOFs and the polymer. … O=C) ensures good interfacial compatibility between the filler and the polymer, which is beneficial for increasing the filler loading capacity, while avoiding the generation of non-selective defects at the interface, ensuring the permeability of the mixed matrix membrane, and improving its selectivity. Attached Figure Description

[0017] Figure 1 The diagram shows the pore size distribution of MOFs before and after fluorination.

[0018] Figure 2 The images are scanning electron microscope (SEM) images of MOFs mixed matrix membranes before and after fluorination. (a) shows the MOFs mixed matrix membrane before fluorination, and (b) shows the MOFs mixed matrix membrane after fluorination.

[0019] Figure 3 The graph shows the gas separation performance of the MOFs mixed matrix membrane for H2(He) / CH4 before and after fluorination. Detailed Implementation

[0020] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0021] Example 1:

[0022] A method for preparing a high-efficiency molecularly sieved fluorinated MOF mixed matrix membrane includes the following steps:

[0023] Step 1: Dissolve 0.3712 g of a 0:4 mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole as a ligand in 15 ml of methanol solution, then add 15 μl of triethylamine and stir at room temperature until completely dissolved to prepare mixed solution A. Dissolve 0.14 g of cobalt nitrate hexahydrate in 25 ml of methanol solution and stir at room temperature until dissolved to prepare mixed solution B. Add solution B dropwise to solution A and stir at room temperature for 6 h. Transfer to a reaction vessel and stir at 120 °C for 12 h. Centrifuge and wash with methanol as the washing solvent at a centrifugation rate of 10000 rpm. Then dry to prepare fluorinated ZIF-9 powder (FZIF-9) for ≥12 h under vacuum at 80 °C.

[0024] Step 2: Dry the 6FDA-TFMB polymer under vacuum at 120°C for 12 hours. Then, dissolve and disperse 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir at 50°C for 4 hours to prepare a 3.55 wt% 6FDA-TFMB polymer solution. Weigh 0.043 g of FZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir to obtain a uniformly dispersed MOF suspension.

[0025] Step 3: Disperse the obtained MOF suspension in a 6FDA-TFMB polymer solution, sonicate the mixed solution, and then stir at room temperature for 12 hours. Finally, cast the above membrane solution into a PTFE plate and dry the membrane solution at 50°C to obtain a 20wt% fluorinated MOF-based mixed matrix membrane (20wt% FZIF / PI).

[0026] Example 2:

[0027] A method for preparing a high-efficiency molecularly sieved fluorinated MOF mixed matrix membrane includes the following steps:

[0028] Step 1: Dissolve 0.3712 g of a mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole (molar ratio 0:4) as a ligand in 15 ml of methanol solution. Then add 15 μl of triethylamine and stir at room temperature until completely dissolved to prepare mixed solution A. Dissolve 0.14 g of cobalt nitrate hexahydrate in 25 ml of methanol solution and stir at room temperature until dissolved to prepare mixed solution B. Add solution B dropwise to solution A and stir at room temperature for 6 h. Transfer to a reaction vessel and stir at 120 °C for 12 h. Centrifuge and wash with methanol as the washing solvent at a centrifugation rate of 10000 rpm. Then dry to prepare fluorinated ZIF-9 powder (FZIF-9). Drying time ≥ 12 h and drying conditions are 80 °C under vacuum.

[0029] Step 2: Dry the 6FDA-TFMB polymer under vacuum at 120°C for 12 hours. Then, dissolve and disperse 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir at 50°C for 4 hours to prepare a 3.55 wt% 6FDA-TFMB polymer solution. Weigh 0.057 g of FZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir to obtain a uniformly dispersed MOF suspension.

[0030] Step 3: The obtained MOF suspension was dispersed in a 6FDA-TFMB polymer solution, and the mixed solution was ultrasonically treated, followed by stirring at room temperature for 12 hours. Finally, the above membrane solution was cast into a PTFE plate and dried at 50°C to obtain a 25wt% fluorinated MOF-based mixed matrix membrane (25wt% FZIF / PI).

[0031] Example 3:

[0032] A method for preparing a high-efficiency molecularly sieved fluorinated MOF mixed matrix membrane includes the following steps:

[0033] Step 1: Dissolve 0.3712 g of a 2:2 mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole as a ligand in 15 ml of methanol solution, then add 15 μl of triethylamine and stir at room temperature until completely dissolved to prepare mixed solution A. Dissolve 0.14 g of cobalt nitrate hexahydrate in 25 ml of methanol solution and stir at room temperature until dissolved to prepare mixed solution B. Add solution B dropwise to solution A and stir at room temperature for 6 h. Transfer to a reaction vessel and stir at 120 °C for 12 h. Centrifuge and wash with methanol as the washing solvent at a centrifugation rate of 10000 rpm. Then dry to prepare fluorinated ZIF-9 powder (F2ZIF-9) for ≥12 h under vacuum at 80 °C.

[0034] Step 2: Dry the 6FDA-TFMB polymer under vacuum at 120°C for 12 hours. Then, dissolve and disperse 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir at 50°C for 4 hours to prepare a 3.55 wt% 6FDA-TFMB polymer solution. Weigh 0.043 g of FZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir to obtain a uniformly dispersed MOF suspension.

[0035] Step 3: Disperse the obtained MOF suspension in a 6FDA-TFMB polymer solution, sonicate the mixed solution, and then stir at room temperature for 12 hours. Finally, cast the above membrane solution into a PTFE plate and dry the membrane solution at 50°C to obtain a 20wt% fluorinated MOF-based mixed matrix membrane (20wt% F2ZIF / PI).

[0036] Comparative Example 1:

[0037] A method for preparing a high-efficiency molecularly sieved MOF mixed matrix membrane, the specific process includes the following steps:

[0038] Step 1: Dissolve 0.3712 g of a mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole (molar ratio 4:0) as a ligand in 15 ml of methanol solution. Then add 15 μl of triethylamine and stir at room temperature until completely dissolved to prepare mixed solution A. Dissolve 0.14 g of cobalt nitrate hexahydrate in 25 ml of methanol solution and stir at room temperature until dissolved to prepare mixed solution B. Add solution B dropwise to solution A and stir at room temperature for 6 h. Transfer to a reaction vessel and stir at 120 °C for 12 h. Centrifuge and wash with methanol as the washing solvent at a centrifugation rate of 10000 rpm. Then dry to prepare ZIF-9 powder. The drying time is ≥12 h and the drying conditions are 80 °C under vacuum.

[0039] Step 2: Dry the 6FDA-TFMB polymer under vacuum at 120°C for 12 hours. Then, dissolve and disperse 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir at 50°C for 4 hours to prepare a 3.55 wt% 6FDA-TFMB polymer solution. Weigh 0.043 g of ZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF with a mass ratio of 8:2. Stir to obtain a uniformly dispersed MOF suspension.

[0040] Step 3: Disperse the obtained MOF suspension in a 6FDA-TFMB polymer solution, sonicate the mixed solution, and then stir at room temperature for 12 hours. Finally, cast the above membrane solution into a PTFE plate and dry the membrane solution at 50°C to obtain a ZIF-9 fluorinated MOF-based mixed matrix membrane with a mass fraction of 20 wt% (20 wt% ZIF / PI).

[0041] The pore size of the nanoparticles before and after fluorination was characterized by... Figure 1 It can be seen that the introduction of -CF3 reduced the pore size of FZIF-9 by 11% compared to ZIF-9. Scanning electron microscopy images show obvious interfacial voids in the 20 wt% ZIF / PI film prepared in Comparative Example 1, indicating poor interfacial compatibility. In contrast, no voids were found at the interface of the 20 wt% FZIF / PI film prepared in Example 1, suggesting that the introduction of -CF3 significantly altered the interfacial compatibility. Figure 3As shown, compared with the 20wt% ZIF / PI mixed matrix membrane prepared in Comparative Example 1, the 20wt% FZIF / PI mixed matrix membrane prepared in Example 1 has better H2(He) / CH4 separation performance, and the H2(He) / CH4 selectivity is much higher than that of the 20wt% ZIF / PI mixed matrix membrane prepared in Comparative Example 1. This is because the introduction of -CF3 inhibits the diffusion and dissolution of methane, thereby improving the H2(He) / CH4 selectivity.

Claims

1. A method for preparing a high-efficiency hydrogen / helium extraction fluorinated MOFs hybrid matrix membrane from natural gas, characterized in that, The steps are as follows: Step 1: Imidazole and trifluoromethyl imidazole, or imidazole containing trifluoromethyl as a ligand, are dissolved in methanol, followed by the addition of triethylamine. The mixture is stirred at room temperature until completely dissolved to prepare mixed solution A. A metal salt is dissolved in methanol and stirred at room temperature until dissolved to prepare mixed solution B. Mixed solution B is added to mixed solution A, and the mixture is stirred for a certain time. The mixture is then transferred to a reaction vessel and reacted for a certain time. After centrifugation, washing, and drying, fluorinated MOFs powder is obtained. The imidazole is 2-methylimidazolium or / and benzimidazole, the trifluoromethyl imidazole is 2-(trifluoromethyl)-1H-imidazolium or / and 2-(trifluoromethyl)benzimidazole, and the metal salt is cobalt nitrate hexahydrate or / and zinc nitrate hexahydrate. Step 2: Dissolve and disperse the dried 6FDA-based PI polymer in a mixed solution of DMAC and THF, and stir for a certain time to prepare a uniform PI solution; disperse the fluorinated MOF powder in a mixed solution of DMAC and THF, and stir for a certain time to prepare a uniform MOF suspension. Step 3: The MOF suspension is then added to the PI solution to prepare a mixed solution. The mixed solution is then sonicated and stirred at room temperature for a certain time to obtain a membrane solution. Finally, the membrane solution is cast into a PTFE plate and dried to obtain a fluorinated MOF mixed matrix membrane.

2. The preparation method according to claim 1, characterized in that, In step 1, the molar ratio of imidazole to trifluoromethyl imidazole is 3:1 to 0:4, and the molar ratio of ligand to metal salt is 4:

1.

3. The preparation method according to claim 1, characterized in that, In step 1, mixed solution B is added dropwise to mixed solution A, stirred at room temperature for 6 hours, and then transferred to a reaction vessel to react for 12 hours.

4. The preparation method according to claim 1, characterized in that, In step 1, the solvent used for washing is methanol, and the centrifugation rate during washing is 10,000 rpm; the drying time is ≥12 h, and the drying conditions are 80 ℃ under vacuum.

5. The preparation method according to claim 1, characterized in that, In step 2, the 6FDA-based PI polymer is one or more of 6FDA-TFMB, 6FDA-DAM, 6FDA-DABA, 6FDA-BD, and 6FDA-FDA. The drying time is 12 hours, and the drying conditions are 120 °C under vacuum.

6. The preparation method according to claim 1, characterized in that, In step 2, the concentration of the PI solution is 3.55 wt%, the mass ratio of DMAC to THF is 8:2 ~ 6:4, and the PI solution is stirred at 50 ℃ for 4 h.

7. The preparation method according to claim 1, characterized in that, In step 3, the mass fraction of fluorinated MOFs in the mixed solution is 10 ~ 30 wt%, the ultrasonic time is 10 min, and the membrane solution drying temperature is 50 ℃.

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