Preparation method of heteroatom induced pore and transition metal coordinated carbon molecular sieve gas separation membrane

Abstract

The invention belongs to the technical field of preparation and application of carbon molecular sieve membranes, and discloses a preparation method of a heteroatom induced pore and transition metal coordinated carbon molecular sieve gas separation membrane. According to the heteroatom and transition metal modified carbon molecular sieve, a high-molecular polymer is used as a precursor, an additive containing heteroatoms and a transition metal compound are introduced for blending modification, a mixed matrix membrane containing heteroatoms and transition metal components is prepared, and the mixed matrix membrane is carbonized into a carbon molecular sieve membrane at a certain carbonization temperature and atmosphere. By introducing heteroatom components with different thermal stability, the heteroatom components are partially decomposed in the pyrolysis process, so that the microporous structure of the membrane material is regulated and controlled; meanwhile, the transition metal component exists in the carbon molecular sieve membrane in an atomic or nanoscale form and forms reversible pi coordination with a C = C bond of C2H4, so that the selectivity is improved. The method provides a wide-prospect strategy for developing a high-performance CMS membrane and realizing efficient C2H4 / C2H6 separation.

Description

Technical Field

[0001] This invention belongs to the field of carbon molecular sieve membrane preparation and application technology, and discloses a method for preparing a heteroatom-induced pore and transition metal coordinated carbon molecular sieve gas separation membrane. Background Technology

[0002] Light olefins (such as ethylene C2H4) are basic chemical feedstocks, produced through processes such as hydrocarbon steam cracking, coal / methanol-to-olefins (CTO / MTO), and catalytic cracking. Inevitably, some byproducts, such as ethane (C2H6) (approximately 5%–9%), are present in the crude product. Therefore, further separation is required to obtain high-purity C2H4 for the production of plastics, rubber, coatings, and other materials. Due to the extremely similar physicochemical properties of C2H4 and C2H6, separating these two compounds has always presented significant challenges. The most common separation technology relies on energy-intensive cryogenic distillation, which requires multiple trays and high reflux ratios, resulting in huge energy consumption and serious environmental pollution (e.g., Beibei Lai, John Cahir, Min Ying Tsang, Johan Jacquemin, David Rooney, Barry Murrer, and Stuart L. James, ACS Applied Materials & Interfaces 2021 13 (1), 932-936, DOI: 10.1021 / acsami.0c19044). Some alternative separation methods, such as absorption and adsorption, also have limitations. For example, absorption separation requires frequent replacement of the absorbent, leading to the generation of large amounts of byproducts or pollutants; the stability and recyclability of the adsorbent in adsorption separation also affect separation efficiency and increase operating costs. Therefore, there is an urgent need to develop sustainable, energy-saving, and environmentally friendly separation technologies.

[0003] Membrane-based separation technologies have attracted much attention due to their high energy efficiency, environmental friendliness, and ease of operation. Polymer membranes, with their advantages of ease of manufacture, scalability, and low cost, have become the most commonly used membrane materials. However, polymer membranes generally have low gas permeability, and there is a restrictive balance between membrane permeability and selectivity. Carbon molecular sieve (CMS) membranes are prepared from polymer precursor membranes through a single-step pyrolysis process under an inert atmosphere, exhibiting excellent heat and chemical resistance. CMS membranes possess uniform sub-nanometer-scale pores, a highly interconnected network, and a tunable pore structure, resulting in high gas permeability. However, CMS membranes rely on pore size for gas sieving, and precisely controlling the membrane size to meet specific gas separation requirements presents challenges. Hybrid matrix precursor CMS membranes retain the high permeability of CMS membranes, and blending additives can improve the adsorption selectivity of the carbon membrane. In existing technologies, improving the ethylene / ethane selectivity of membranes by adding transition metals and utilizing the π-π interactions between transition metals and olefins has become a research hotspot (e.g., YH Chu, et al. J. Membr. Sci. 548(2018) 609-620.; Q. Wang, et al. J. Membr. Sci. 621 (2021) 118785.). However, metal addition easily causes membrane pore blockage, and the improvement in membrane selectivity often comes at the cost of membrane permeability. Therefore, this invention simultaneously introduces two functionalized dispersed phases: one is a heteroatom-containing inorganic additive, and the other is a transition metal salt as a dispersant. The heteroatom-containing inorganic additive acts as a pore-forming agent to construct gas mass transfer channels, while the transition metal salt provides active sites for ethylene adsorption. Compared with existing technologies, this invention, through the combination of bifunctionalized dispersed phases, improves both ethylene / ethane selectivity and gas permeability through their synergistic effect. Summary of the Invention

[0004] To address the issues of complex preparation processes and high costs associated with existing mixed matrix precursor carbon molecular sieve membranes, this invention proposes a method for preparing heteroatom-induced pore and transition metal-coordinated carbon molecular sieve gas separation membranes.

[0005] The technical solution of this invention:

[0006] A method for preparing a heteroatom-induced porous and transition metal-coordinated carbon molecular sieve gas separation membrane, comprising the following steps:

[0007] (1) Dissolve the polymer in organic solvent A and stir until homogeneous to obtain a continuous phase solution with a mass fraction of 2.5-3 wt.%; dissolve the heteroatom-containing inorganic compound B and the transition metal salt C in different organic solvents A, stir until homogeneous to obtain two dispersed phase solutions with a mass fraction of 0.5-5 wt.% respectively;

[0008] (2) Mix the continuous phase solution and the two dispersed phase solutions, stir in the dark for 12 h to obtain a mixed matrix polymer solution; stir, degas by ultrasonication and filter with a needle filter (pore size of 0.4 μm) to remove impurities, and dry in a vacuum oven at 80 °C for 24 h to obtain a polymer precursor film with a thickness of 40-80 μm;

[0009] (3) The polymer precursor membrane is placed in a ceramic boat and fixed with a corundum sheet. Then it is placed in a tube furnace and a protective gas is introduced. It is carbonized at 550-800 °C. After carbonization for a certain time, it is naturally cooled to room temperature to obtain a heteroatom-induced pore and transition metal coordinated carbon molecular sieve gas separation membrane with a thickness of 40-60 μm.

[0010] The organic solvent A is a mixture of two of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF).

[0011] The heteroatom-containing inorganic compound B is at least one or a mixture of two or more of elemental sulfur, sulfur-containing inorganic compounds, and sulfur-containing oligomers; the sulfur-containing inorganic compound is selected from at least one of sodium sulfide, sodium thiosulfate, sodium dithionite, ammonium sulfite, and ammonium thiosulfate; the sulfur-containing oligomer is at least one of polysulfide, polyphenylene sulfide, polysulfone, dithiothreitol dimer, and polythiophene oligomer.

[0012] The transition metal salt C includes at least one of the following: silver salts (including silver nitrate, silver acetate, silver sulfate, silver tetrafluoroborate, silver hexafluorophosphate, or combinations thereof); copper salts (copper acetate, copper nitrate, copper sulfate, or combinations thereof); zinc salts (zinc nitrate, zinc chloride, zinc acetate, zinc sulfate, or combinations thereof); cobalt salts (cobalt nitrate, cobalt chloride, cobalt acetate, cobalt sulfate, or combinations thereof); and iron salts (ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferric formate).

[0013] In step (2), the molar ratio of heteroatom-containing inorganic compound B to transition metal salt C in the mixed matrix polymer solution is 1:1, and the mass fraction of the two dispersed phase solutions in the continuous phase solution is 5 wt.%-30 wt.%.

[0014] In step (3), the temperature is increased from room temperature to 550 °C at a rate of 1-5 °C / min and held for 1 hour; then the temperature is increased to 650-800 °C at a rate of 2-10 °C / min and held for 2-4 hours.

[0015] The heteroatom-induced porosity and transition metal-coordinated carbon molecular sieve gas separation membranes obtained by the above preparation method were tested using the constant volumetric pressure method. The membranes were stabilized under vacuum for 8-12 hours before testing. Subsequently, the gas separation performance of the membranes was tested at 35 °C and 2 bar.

[0016] The beneficial effects of this invention are as follows: By adding two functionalized dispersed phases, the synergistic effect of these two phases achieves a dual improvement in membrane ethylene permeability and ethylene / ethane selectivity. Specifically, the heteroatom-containing inorganic additive decomposes during the carbonization of the polymer precursor, releasing gases such as S2, H2S, CS2, CO2, H2O, and NH3, thereby forming abundant hierarchical channels within the material. These channels, acting as gas mass transfer pathways, significantly reduce the diffusion resistance of gas molecules, allowing gas to rapidly permeate into the material and fully contact the dispersed active sites. Simultaneously, the transition metal salt, after high-temperature carbonization, forms highly dispersed ethylene adsorption active sites on the pore surface or within the framework. The specific recognition of ethylene by these metal sites (e.g., π-complexation) synergistically improves the selectivity of ethylene relative to ethane. The synergistic effect of both phases prevents the metal active sites from being encapsulated or aggregated, thus avoiding gas mass transfer obstruction and significantly enhancing the ethylene permeation rate and permeation selectivity. Furthermore, the additives used in this invention are inexpensive, and their synergistic combination with transition metal salts can be achieved in a one-step process, which simplifies the process and significantly improves efficiency, providing a novel strategy for the design of high-efficiency ethylene separation membrane materials. Attached Figure Description

[0017] Figure 1 This is a scanning electron microscope image of a cross-section of a mixed matrix carbon molecular sieve membrane.

[0018] Figure 2 It is the EDS distribution of S element in the mixed matrix membrane (before carbonization).

[0019] Figure 3 It is the EDS distribution of sulfur element in a mixed matrix carbon molecular sieve membrane (after carbonization). 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] The continuous-phase polyimide polymer used in this invention is prepared using a traditional chemical imidization (two-step method). First, a certain amount of diamine monomer is dissolved in the organic solvent N-methylpyrrolidone. After complete dissolution, an equimolar amount of dianhydride monomer is added under ice-water bath and N2 protection to obtain a solution with a concentration of 20 wt.%. After stirring the reaction for approximately 3 h, the three-necked flask is removed from the ice-water bath and the reaction continues at room temperature for 24 h. After the reaction is complete, a mixture of pyridine catalyst and acetic anhydride dehydrating agent is added to the reactor using a syringe needle, and the reaction continues at room temperature for 24 h to obtain a polyimide polymerization solution. The polymer solution is poured into methanol to obtain a filamentous precipitate, which is washed three times with methanol and then vacuum dried at 150 °C. A certain mass of the polymer precursor is weighed and dissolved in an organic solvent to obtain a continuous-phase solution with a concentration of 2.5-3 wt.%. A certain mass of inorganic substances and transition metal salts are weighed and dissolved in two organic solvents respectively to obtain dispersed phase solutions with a total concentration of 0.5 wt.%. The continuous and dispersed phase solutions were mixed and stirred for 12 hours to obtain a homogeneous mixed matrix casting solution with a concentration of 3 wt.%. The mixed matrix casting solution was ultrasonically degassed and placed in a petri dish to evaporate naturally and form a film. The film was then vacuum dried at 80 °C to obtain a carbon molecular sieve precursor. The polymer precursor was then placed in a tube furnace and carbonized at 550-800 °C under inert gas protection to form a film.

[0022] Compare with Example 1

[0023] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5 wt.% AgBF4 salt dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0024] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 1.

[0025] Compare with Example 2

[0026] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 2.0 wt.% elemental S dispersed phase solution and a 5 wt.% AgBF4 salt dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0027] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0028] Compare with Example 3

[0029] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with an 8.0 wt.% elemental S dispersed phase solution and a 5 wt.% AgBF4 salt dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0030] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0031] Example 1

[0032] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% elemental S dispersed phase solution and a 5 wt.% AgBF4 salt dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0033] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0034] Example 2

[0035] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% sodium dithionite dispersed phase solution and a 5 wt.% AgBF4 salt dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0036] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0037] Example 3

[0038] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% polythiophene oligomer dispersed phase solution and a 5 wt.% AgBF4 salt dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0039] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0040] Example 4

[0041] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% elemental S dispersed phase solution and a 5 wt.% copper sulfate dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0042] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0043] Example 5

[0044] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% elemental S dispersed phase solution and a 5 wt.% zinc chloride dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0045] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0046] Example 6

[0047] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% elemental S dispersed phase solution and a 5 wt.% cobalt acetate dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0048] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0049] Example 7

[0050] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% elemental S dispersed phase solution and a 5 wt.% ferrous nitrate dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0051] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0052] Example 8

[0053] A 3.0 wt.% continuous phase solution was poured onto a glass plate and mixed thoroughly with a 5.0 wt.% elemental S dispersed phase solution and a 5 wt.% ferric chloride dispersed phase solution. The mixture was then poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain the polymer precursor.

[0054] A 12 mm diameter polymer precursor membrane was placed in a tube furnace, a protective gas was introduced, the temperature was raised to 550 °C and held for 120 min, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Comparative Example 2.

[0055] Table 1. Comparative Examples and Examples: Ethylene and ethane permeability and selectivity of carbon molecular sieve membranes

[0056]

[0057] By comparing the effects of additive type and dosage on membrane performance, gas permeability tests were conducted under the same carbonization temperature and other test conditions, and the results are shown in Table 1. Compared with Control Example 1, the membrane permeability was significantly improved after adding inorganic additives. The main reason is that heteroatoms volatilize during carbonization, forming a large number of micropores in situ, thereby promoting gas transport within the membrane. However, there is an optimal range for the amount of inorganic additives; too low or too high a dosage is not conducive to improving membrane selectivity. Example 1 (optimal ratio of elemental S to AgBF4): The membrane permeability and selectivity were both optimal. This is attributed to two aspects: firstly, the transition metal (Ag) + The enhanced π-coordination between the π electrons of ethylene and ethane improves the ethylene adsorption selectivity, thereby optimizing the ethylene / ethane separation selectivity. Secondly, the introduction of appropriate micropores accelerates ethylene permeation, giving the membrane excellent permeation performance. Examples 2 and 3 (inorganic additives: sodium dithionate and polythiophene oligomer): The membrane permeability is improved, and the selectivity is also high. Examples 4-8 (transition metal additives: Cu) 2+ Zn 2+ Co 2+ Fe2+ Fe 3+ Different transition metals exhibit π-interactions with ethylene, allowing the membrane to achieve high permeability while maintaining high selectivity. In summary, the introduction of heteroatoms effectively mitigates the adverse effects of metal atom aggregation on membrane permeability by generating additional pore structures; simultaneously, transition metals selectively adsorb C2H4 through π-coordination. This synergistic effect results in dual-doped mixed-matrix CMS membranes possessing both excellent permeability and selectivity.

Claims

1. A method for preparing a heteroatom-induced pore and transition metal coordinated carbon molecular sieve gas separation membrane, characterized in that, The steps are as follows: (1) Dissolve the high molecular polymer in the organic solvent A and stir uniformly to obtain a continuous phase solution with a mass fraction of 2.5-3 wt.%; dissolve the inorganic substance containing heteroatoms B and the transition metal salt C in different organic solvents A, respectively, and stir uniformly to obtain two kinds of dispersed phase solutions with a mass fraction of 0.5-5 wt.%; (2) Mix the continuous phase solution and the two kinds of dispersed phase solutions, and stir in the dark for 12 h to obtain a mixed matrix polymer solution; after stirring, ultrasonic defoaming and filtration with a needle filter to remove impurities, dry in a vacuum oven at 80 °C for 24 h to obtain a polymer precursor film with a thickness of 40-80 μm; (3) Place the polymer precursor film in a porcelain boat and fix it with a corundum sheet, then place it in a tube furnace, introduce a protective gas, and carbonize at 550-800 °C for a certain period of time, and then naturally cool to room temperature to obtain a heteroatom-induced pore and transition metal-coordinated carbon molecular sieve gas separation membrane with a thickness of 40-60 μm.

2. The preparation method according to claim 1, characterized in that, the organic solvent A is a mixture of two of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide and tetrahydrofuran; the inorganic substance containing heteroatoms B is at least one or a mixture of two or more of elemental sulfur, sulfur-containing inorganic substances and sulfur-containing oligomers; the sulfur-containing inorganic substance is at least one selected from sodium sulfide, sodium thiosulfate, sodium dithionite, ammonium sulfite and ammonium thiosulfate materials; the sulfur-containing oligomer is at least one selected from polythioether, polyphenylene sulfide, polysulfone, dithiothreitol dimer and polythiophene oligomer; the transition metal salt C is at least one selected from silver salts, copper salts, zinc salts, cobalt salts and iron salts.

3. The preparation method according to claim 1, characterized in that, in step (2), the molar concentration ratio of the inorganic substance containing heteroatoms B to the transition metal salt C in the mixed matrix polymer solution is 1:1, and the mass fraction of the two kinds of dispersed phase solutions in the continuous phase solution is 5 wt.%-30 wt.%.

4. The preparation method according to claim 1, characterized in that, in step (3), the temperature is raised from room temperature to 550 °C at a rate of 1-5 °C / min, and maintained for 1 hour; then the temperature is raised to 650-800 °C at a rate of 2-10 °C / min, and maintained for 2-4 hours.

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