Carbon molecular sieve membranes, their preparation methods and applications

Abstract

A carbon molecular sieve membrane comprises 40–90% by mass of carbon material and 10–60% by mass of molecular sieve, wherein the carbon material is a polymer high-temperature carbonization product, and the molecular sieve is a silica-alumina molecular sieve, a phosphorus-alumina molecular sieve, or a silica-phosphorus-alumina molecular sieve. The carbon molecular sieve membrane has a thickness of 5–60 micrometers. The carbon molecular sieve membrane exhibits high permeation selectivity and can be used for propylene / propane separation.

Description

Technical Field

[0001] This invention relates to a carbon molecular sieve membrane and its preparation method, specifically, a carbon molecular sieve membrane for the separation of olefins and alkanes and its preparation method. Background Technology

[0002] Carbon molecular sieve membranes have attracted widespread attention in the field of gas separation due to their high thermal stability, chemical stability, and permeation selectivity.

[0003] CN101277754A discloses a process for preparing a carbon molecular sieve membrane. First, a mixture of cellulose and hemicellulose is treated with trifluoroacetic acid, and then a metal salt is added before casting into a membrane. After drying, the membrane is carbonized at 500-600℃. The prepared carbon molecular sieve membrane exhibits good separation performance in a CO2 / CH4 separation system and can be regenerated by applying an electric current.

[0004] CN106823821A discloses a method for repairing defects in carbon molecular sieve membranes. The method uses silicone rubber as a raw material, which is mixed with a catalyst, a crosslinking agent, and an organic solvent to prepare a sol. The sol is then coated onto the surface of the carbon molecular sieve membrane to form a film, which is then dried. The catalyst is preferably organotin, the crosslinking agent is preferably tetraethyl orthosilicate, and the organic solvent is preferably isooctane. This method utilizes the excellent permeability of silicone rubber, allowing it to penetrate into the interior of the carbon molecular sieve membrane. The silicone rubber prepared by crosslinking is used to form a film on the surface of the carbon molecular sieve membrane to fill the defects, thereby significantly improving the permeability of the carbon molecular sieve membrane.

[0005] CN108472595A discloses a method for preparing a carbon molecular sieve membrane. First, a polymer precursor is heated to a temperature at which the precursor polymer undergoes pyrolysis to form the carbon molecular sieve membrane. Then, the carbon molecular sieve membrane is cooled to a temperature at which further pyrolysis does not occur. Finally, the carbon molecular sieve membrane is exposed to a conditioned atmosphere composed of target permeable gas molecules. This method can achieve a carbon molecular sieve membrane with an improved combination of selectivity and permeability. CN109070009A discloses an improved method for manufacturing a carbon molecular sieve membrane, wherein a polyimide precursor polymer is pyrolyzed to form the carbon molecular sieve membrane by: heating the polyimide precursor polymer in a furnace to a final pyrolysis temperature of 600°C to 700°C, heating from 400°C to the final pyrolysis temperature at a pyrolysis heating rate of 3 to 7°C / min, and maintaining the final pyrolysis temperature in a non-oxidizing atmosphere for a pyrolysis time of up to 60 minutes. This method can achieve a CMS with an improved combination of selectivity and permeability.

[0006] During the high-temperature carbonization of organic polymers to produce carbon molecular sieve membranes, defects in the carbon molecular sieve membranes can be generated due to gas escape, thereby affecting the permeability / selectivity of the carbon molecular sieve membranes. Existing technologies reduce carbon molecular sieve membrane defects by repairing or selecting special polymer precursor materials, but it is still difficult to effectively suppress the formation of defects during high-temperature pyrolysis. Summary of the Invention

[0007] One objective of this invention is to provide a carbon molecular sieve membrane comprising 60–90% by mass of carbon material and 10–40% by mass of molecular sieve, wherein the carbon material is a polymer high-temperature carbonization product, and the molecular sieve is a silica-alumina molecular sieve, a phosphorus-alumina molecular sieve, or a silica-phosphorus-alumina molecular sieve, and the carbon molecular sieve membrane has a thickness of 5–60 micrometers. This carbon molecular sieve membrane can be used for propylene / propane separation and exhibits high permeation selectivity.

[0008] A second objective of this invention is to provide a method for preparing the aforementioned carbon molecular sieve membrane, comprising the following steps:

[0009] (1) A polymer, molecular sieve and solvent are mixed to obtain a casting solution, a polymer membrane is prepared and a film-forming treatment is performed to obtain a polymer membrane;

[0010] (2) Place the polymer membrane obtained in step (1) in a crosslinking solution, perform crosslinking treatment, wash and dry to obtain a crosslinked polymer membrane;

[0011] (3) Place the cross-linked polymer film obtained in step (2) in air for stabilization treatment;

[0012] (4) The product processed in step (3) is carbonized in an inert gas atmosphere at a pressure of 0.15 to 0.8 MPa.

[0013] A third objective of this invention is to provide a method for separating a mixture of propylene and propane gases using the carbon molecular sieve membrane described in this invention or a carbon molecular sieve membrane prepared using the method of this invention.

[0014] The method for preparing carbon molecular sieve membranes provided by this invention involves adding molecular sieves to a casting solution, followed by membrane formation and cross-linking. Carbonization can then be performed at a relatively low temperature and under certain pressure, which helps reduce gas escape and inhibits the formation of defects in the carbon molecular sieve membrane. The carbon molecular membrane prepared by this method is used for propylene / propane separation and exhibits high propane / propylene permeation selectivity. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a permeability evaluation device. Detailed Implementation

[0016] In the preparation of carbon molecular sieve membranes, this invention introduces molecular sieves into the casting solution, forming a polymer membrane containing molecular sieves. After cross-linking treatment, due to the catalytic effect of the molecular sieves, the polymer can be converted into carbon materials at a relatively low temperature, reducing gas escape during carbonization. Simultaneously, the carbonization process is carried out under certain pressure, further reducing gas escape and effectively suppressing the formation of defects in the carbon molecular sieve membrane. The carbon molecular sieve membrane prepared using the method described in this invention exhibits high propylene / propane separation performance.

[0017] The carbon molecular sieve membrane provided by this invention comprises 60-90% by mass of carbon material and 10-40% by mass of molecular sieve, preferably 70-80% by mass of carbon material and 20-30% by mass of molecular sieve. The carbon material is a high-temperature carbonization product of a polymer, and the polymer is selected from polyvinylidene fluoride, polyimide, cellulose acetate, ethyl cellulose, or polyacrylonitrile, preferably polyvinylidene fluoride. The molecular sieve is a silica-alumina molecular sieve, a phosphorus-alumina molecular sieve, or a silica-phosphorus-alumina molecular sieve, preferably a silica-alumina molecular sieve, more preferably a type A molecular sieve, and more preferably, the cation of the type A molecular sieve is Na. + K + Ca 2+ The thickness of the carbon molecular sieve membrane is 5 to 60 micrometers, preferably 8 to 25 micrometers.

[0018] The method for preparing carbon molecular sieve membranes provided by this invention includes the following steps:

[0019] (1) A polymer, molecular sieve and solvent are mixed to obtain a casting solution, a polymer membrane is prepared and a film-forming treatment is performed to obtain a polymer membrane;

[0020] (2) Place the polymer membrane obtained in step (1) in a crosslinking solution, perform crosslinking treatment, wash and dry to obtain a crosslinked polymer membrane;

[0021] (3) Place the cross-linked polymer film obtained in step (2) in air for stabilization treatment;

[0022] (4) The product processed in step (3) is carbonized in an inert gas atmosphere at a pressure of 0.15 to 0.8 MPa.

[0023] The method for preparing carbon molecular sieve membrane provided by the present invention includes (1) a film-forming step, wherein the casting solution comprises 5-20% by mass of a polymer and 3-10% by mass of a molecular sieve; the polymer is polyvinylidene fluoride, polyimide, cellulose acetate, ethyl cellulose or polyacrylonitrile, preferably polyvinylidene fluoride; the molecular sieve is aluminosilicate molecular sieve, aluminosilicate molecular sieve or aluminosilicate molecular sieve, preferably aluminosilicate molecular sieve; the solvent is at least one of tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, and chloroform; the film-forming treatment temperature is 60-150°C and the time is 0.5-5 hours.

[0024] The method for preparing carbon molecular sieve membrane provided by the present invention includes step (2) as a crosslinking treatment step, wherein the crosslinking liquid comprises 10-40% by mass of a crosslinking agent, 5-30% by mass of an inorganic alkali and 30-85% by mass of a dispersant, wherein the crosslinking agent is p-phenylenediamine, p-phenylenediamine or hexamethylenediamine, the inorganic alkali is sodium hydroxide or potassium hydroxide, and the dispersant is methanol or ethanol; the mass ratio of the crosslinking liquid to the polymer membrane is 5-20:1; the washing solvent is methanol or ethanol, preferably the same as the dispersant, and the number of washing times is 2-5; the crosslinking treatment temperature is 20-100°C and the time is 2-12 hours, and the drying treatment temperature is 50-100°C and the time is 5-24 hours.

[0025] In the preparation method of carbon molecular sieve membrane provided by the present invention, in step (3), the stabilization treatment temperature is 160-250℃ and the time is 0.5-5 hours.

[0026] In the preparation method of carbon molecular sieve membrane provided by the present invention, in step (4), the inert gas atmosphere is nitrogen, argon or helium; the heating rate is 0.5-5℃ / min, preferably 0.8-3℃ / min; the carbonization treatment temperature is 300-500℃, preferably 320-420℃, and the time is 1-8 hours.

[0027] The carbon molecular sieve membrane described in this invention, or the carbon molecular sieve membrane prepared by the method of this invention, is suitable for gas separation, especially for the separation of propylene / propane. The permeability of a single-component gas is determined using a constant-pressure gas permeation device at an operating pressure of 0.3 MPa and an operating temperature of 30°C. A schematic diagram of the constant-pressure gas permeation device is shown below. Figure 1 As shown, the device consists of a membrane cell, pressure gauge A, flow meter, etc. The gas to be tested is introduced into the membrane cell, and the pressure on the gas inlet side is adjusted to 0.3 MPa. After 2 hours, the time t seconds required for 0.5 mL of gas to flow out on the permeate side is recorded. The effective membrane area is 8.04 cm². 2 The membrane thickness is 1 cm. The gas permeability P is calculated using the following formula, in barrers.

[0028]

[0029] In the formula, 3 represents the transmembrane pressure difference in bar, and 75 represents that 1 bar equals 75 cmHg.

[0030] When the gas to be tested is propylene, P is the permeability of propylene; when the gas to be tested is propane, P is the permeability of propane.

[0031] The propylene / propane permeability selectivity is the ratio of their permeabilities; the higher the permeability selectivity, the better the separation effect.

[0032] The invention is further illustrated by the following examples, but the invention is not limited thereto.

[0033] The method for determining the thickness of the carbon molecular sieve membrane in this invention is as follows: a thickness gauge is used to measure the thickness at the four right-angle vertices and the center of a square, and the average value of the membrane thickness at these five locations is taken.

[0034] In this invention, the mass fraction of carbon material and molecular sieve in carbon molecular sieve membrane is calculated as follows: From the polymer mass fraction a1, molecular sieve mass fraction a2 and polymer membrane mass m1 in step (1) casting solution, it can be known that the molecular sieve mass m2 in polymer membrane is m1 × (a2 / (a1+a2)). The molecular sieve mass remains unchanged during carbonization. The mass of carbon molecular sieve membrane is m3. Then, the molecular sieve mass fraction a3 in the obtained carbon molecular sieve membrane is m2 / m3, and the carbon material mass fraction a4 is (m3-m2) / m3.

[0035] The invention is further illustrated by the following examples, but the invention is not limited thereto.

[0036] Example 1

[0037] (1) Polyvinylidene fluoride and NaA molecular sieve were added to the solvent N,N-dimethylacetamide to prepare a casting solution containing 15% by mass of polyvinylidene fluoride and 4.7% by mass of NaA molecular sieve. The casting solution was poured onto a glass plate and a polymer film was formed by scraping with a 300μm doctor blade. The film was then treated at 80℃ for 3 hours.

[0038] (2) Place 0.8 g of the polymer film obtained in step (1) into a crosslinking solution prepared with 1.6 g of p-phenylenediamine, 1.5 g of sodium hydroxide and 5 g of methanol, and let it stand at 40°C for 6 hours for crosslinking treatment. Then wash it with methanol 3 times and dry it at 80°C for 12 hours.

[0039] (3) Place the cross-linked polymer film obtained in step (2) in the air and stabilize it at 190°C for 1.5 hours.

[0040] (4) The product from step (3) is carbonized in a nitrogen atmosphere at a pressure of 0.3 MPa and at a heating rate of 0.5 °C / min, and then heated to 350 °C for 6 hours. The carbon is then cooled to room temperature in a nitrogen atmosphere to obtain carbon molecular sieve membrane A.

[0041] The thickness, composition, propylene and propane permeation fluxes and permeation selectivity of carbon molecular sieve membrane A are shown in Table 1.

[0042] Example 2

[0043] Carbon molecular sieve membrane B was prepared according to the method in Example 1, except that (1) the mass fraction of NaA molecular sieve in the casting solution was 7.6% by mass.

[0044] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane B are shown in Table 1.

[0045] Example 3

[0046] The carbon molecular sieve membrane C was prepared according to the method of Example 1, except that (1) the mass fraction of NaA molecular sieve in the casting solution was 3.2% by mass.

[0047] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane C are shown in Table 1.

[0048] Example 4

[0049] The carbon molecular sieve membrane D was prepared according to the method in Example 1, except that in step (1), the mass fraction of polyvinylidene fluoride in the casting solution was 11% by mass, and the solvent was tetrahydrofuran.

[0050] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane D are shown in Table 1.

[0051] Example 5

[0052] The carbon molecular sieve membrane E was prepared according to the method in Example 1, except that the film formation temperature in step (1) was 120℃, and the carbonization pressure in step (4) was 0.6MPa and the carbonization temperature was 330℃.

[0053] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane E are shown in Table 1.

[0054] Example 6

[0055] The carbon molecular sieve membrane F was prepared according to the method in Example 1, except that in step (4), the temperature was increased to 400℃ and carbonized for 4.5 hours at a heating rate of 1.5℃ / min.

[0056] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane F are shown in Table 1.

[0057] Example 7

[0058] Carbon molecular sieve membrane G was prepared according to the method in Example 1, except that the NaA molecular sieve in step (1) was replaced with KA molecular sieve, and the crosslinking agent in step (2) was p-phenylenediamine.

[0059] The thickness, composition, propylene and propane permeation fluxes and permeation selectivity of carbon molecular sieve membrane G are shown in Table 1.

[0060] Example 8

[0061] The carbon molecular sieve membrane H was prepared according to the method in Example 1, except that the NaA molecular sieve in step (1) was replaced with CsX molecular sieve.

[0062] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane H are shown in Table 1.

[0063] Comparative Example 1

[0064] Carbon molecular sieve membrane I was prepared according to the method in Example 1, except that NaA molecular sieve was not added to the casting solution in step (1).

[0065] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane I are shown in Table 1.

[0066] Comparative Example 2

[0067] The carbon molecular sieve membrane J was prepared according to the method in Example 1, except that in step (4), the carbonization temperature was 550℃.

[0068] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane J are shown in Table 1.

[0069] Comparative Example 3

[0070] The carbon molecular sieve membrane K was prepared according to the method in Example 1, except that in step (4), the carbonization treatment pressure was atmospheric pressure.

[0071] The thickness, composition, propylene and propane permeation flux and permeation selectivity of carbon molecular sieve membrane K are shown in Table 1.

[0072] Table 1

[0073]

[0074] As shown in Table 1, in Comparative Example 1, when preparing carbon molecular sieve membrane I, no molecular sieve was added to the casting solution, resulting in low permeation flux for both propylene and propane, and no permeation selectivity, thus failing to achieve propylene / propane separation. In Comparative Example 2, when preparing carbon molecular sieve membrane J, the carbonization temperature was higher, and the thickness, carbon content, and propylene / propane permeation selectivity of the resulting carbon molecular sieve membrane J were all lower than those of the carbon molecular sieve membranes in Examples 1-8 of this invention, indicating that more gas escaped during the carbonization process, resulting in more defects in the carbon molecular sieve membrane. In Comparative Example 3, when preparing carbon molecular sieve membrane K, the carbonization pressure was atmospheric pressure, and the thickness, carbon content, and propylene / propane permeation selectivity of the resulting carbon molecular sieve membrane K were all lower than those of the carbon molecular sieve membrane A in Example 1 of this invention, indicating that atmospheric pressure carbonization resulted in more gas escape than pressurized carbonization, resulting in more defects in the carbon molecular sieve membrane.

Claims

1. A method for preparing a carbon molecular sieve membrane, comprising the following steps: (1) A polymer, molecular sieve and solvent are mixed to obtain a casting solution, a polymer membrane is prepared and a film-forming treatment is performed to obtain a polymer membrane; (2) Place the polymer membrane obtained in step (1) in a crosslinking solution, perform crosslinking treatment, wash and dry to obtain a crosslinked polymer membrane; (3) Place the cross-linked polymer film obtained in step (2) in air for stabilization treatment; (4) The product processed in step (3) is carbonized in an inert gas atmosphere, wherein the carbonization pressure is 0.15 to 0.8 MPa; The crosslinking liquid comprises 10-40% by mass of a crosslinking agent, 5-30% by mass of an inorganic base, and 30-85% by mass of a dispersant. The crosslinking agent is p-phenylenediamine, p-phenylenediamine, or hexamethylenediamine, the inorganic base is sodium hydroxide or potassium hydroxide, and the dispersant is methanol or ethanol.

2. The method according to claim 1, characterized in that... (1) In step 1, the casting solution contains 5-20% by mass of polymer and 3-10% by mass of molecular sieve.

3. The method according to claim 1, characterized in that... (1) In step, the polymer is polyvinylidene fluoride, polyimide, cellulose acetate, ethyl cellulose or polyacrylonitrile; the molecular sieve is aluminosilicate molecular sieve, aluminosilicate molecular sieve or aluminosilicate-phosphorus molecular sieve; the solvent is at least one of tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide or chloroform.

4. The method according to claim 3, characterized in that... (1) In step, the polymer is polyvinylidene fluoride; the molecular sieve is a silica-alumina molecular sieve.

5. The method according to claim 1, characterized in that... (1) In the step, the film-forming treatment temperature is 60-150℃ and the time is 0.5-5 hours.

6. The method according to claim 1, characterized in that... (2) In step 2, the mass ratio of the crosslinking liquid to the polymer film is 5 to 20:

1.

7. The method according to claim 1, characterized in that... (2) In step 2, the solvent used for washing is methanol or ethanol, and the number of washing cycles is 2 to 5.

8. The method according to claim 1, characterized in that... (2) In the crosslinking treatment, the temperature is 20-100℃ and the time is 2-12 hours, and the drying treatment temperature is 50-100℃ and the time is 5-24 hours.

9. The method according to claim 1, characterized in that... (3) In the stabilization process, the temperature is 160-250°C and the time is 0.5-5 hours.

10. The method according to claim 1, characterized in that... (4) In step 4, the inert gas atmosphere is nitrogen, argon or helium.

11. The method according to claim 1, characterized in that... (4) In step 4, the heating rate of the carbonization process is 0.5 to 5 °C / min.

12. The method according to claim 11, characterized in that... (4) In step 4, the heating rate of the carbonization process is 0.8 to 3 °C / min.

13. The method according to claim 1, characterized in that... (4) In the carbonization process, the temperature is 300-500℃ and the time is 1-8 hours.

14. The method according to claim 13, characterized in that... (4) In step 4, the carbonization temperature is 320-420℃.

15. A carbon molecular sieve membrane prepared according to any one of claims 1 to 14, comprising 60 to 90% by mass of carbon material and 10 to 40% by mass of molecular sieve, wherein the carbon material is a polymer high-temperature carbonization product, the molecular sieve is a silica-alumina molecular sieve, a phosphorus-alumina molecular sieve, or a silica-phosphorus-alumina molecular sieve, and the carbon molecular sieve membrane has a thickness of 5 to 60 micrometers.

16. The carbon molecular sieve membrane according to claim 15, characterized in that... The polymer is polyvinylidene fluoride, polyimide, cellulose acetate, ethyl cellulose, or polyacrylonitrile.

17. The carbon molecular sieve membrane according to claim 15, characterized in that... The polymer is polyvinylidene fluoride.

18. The carbon molecular sieve membrane according to claim 15, characterized in that... The molecular sieve is a silica-alumina molecular sieve.

19. The carbon molecular sieve membrane according to claim 18, characterized in that... The silica-aluminum molecular sieve is a type A molecular sieve.

20. The carbon molecular sieve membrane according to claim 19, characterized in that... The cation of the type A molecular sieve is Na. + K + Ca 2+ .

21. The carbon molecular sieve membrane according to claim 15, characterized in that... The thickness of the carbon molecular sieve membrane is 8–25 micrometers.

22. A method for separating a mixture of propylene and propane gases using a carbon molecular sieve membrane prepared by any one of claims 1 to 14 or using a carbon molecular sieve membrane according to any one of claims 15 to 21.

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