Crosslinked polyimide carbon molecular sieve membrane for propylene / propane separation and preparation method thereof

Abstract

The invention belongs to the technical field of membrane preparation and application, and discloses a cross-linked polyimide carbon molecular sieve membrane for propylene / propane separation and a preparation method of the cross-linked polyimide carbon molecular sieve membrane. A diamine structure with a plurality of crosslinkable carboxyl groups is introduced into a polyimide precursor polymer, and the precursor membrane is subjected to crosslinking treatment before the carbon molecular sieve membrane is prepared. Through decarboxylation cross-linking treatment, close packing of carbon chains in the carbonization process is effectively inhibited, so that more free volumes are formed in the carbon molecular sieve membrane, and the gas permeability is remarkably improved. Meanwhile, the method enables the carbon molecular sieve membrane to have higher graphite carbon content, enhances the pi-pi interaction between the membrane material and propylene molecules, promotes the selective permeation of the propylene molecules, and further greatly improves the separation selectivity of propylene / propane. The carbon molecular sieve membrane has a wide application prospect in the field of gas separation.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane preparation and application, and discloses a crosslinked polyimide carbon molecular sieve membrane for propylene / propane separation and a preparation method thereof. Background Art

[0002] As one of the basic raw materials for the three major synthetic materials in modern polymers, propylene is an extremely important energy resource and industrial chemical raw material. Propylene is mainly produced by petroleum cracking, and propane impurities will be generated during this process, which need to be separated before use. The similar physical properties of propylene and propane make the separation of propylene / propane one of the most important and challenging separation processes in the petrochemical industry. Therefore, achieving efficient and low-energy consumption separation of propylene and propane is of great significance to the global chemical industry. Gas membrane separation technology has been proven to be a promising technology for propylene / propane separation, with advantages such as low energy consumption, no phase change, and small floor area.

[0003] Carbon molecular sieve membranes have a bimodal pore size distribution, that is, they have both micropores and supermicropores. Larger micropores have better gas permeability due to their high adsorption coefficient, while supermicropores provide excellent size screening performance for carbon molecular sieve membranes. The coexistence of these two pore structures enables carbon molecular sieve membranes to have both high permeability and high selectivity. In addition, carbon molecular sieve membranes have good thermal stability and chemical stability, and are one of the most promising separation membrane materials (such as the literature Swaidan R J, Ma X, Pinnau I. Spirobisindane-based polyimide as efficient precursor of thermally-rearranged and carbon molecular sieve membranes for enhanced propylene / propane separation [J]. Journal of Membrane Science, 2016, 520: 983-989.). However, due to the limitation of the structure of its precursor membrane, problems such as chain segment accumulation and pore size collapse occur during the carbonization process, and the prepared carbon molecular sieve membrane has a low selective adsorption ability for propylene, resulting in its inability to have ideal propylene / propane separation performance. To improve the above problems, the present invention introduces a diamine monomer with multiple carboxyl groups into the precursor polymer membrane. The introduction of carboxyl groups makes the carbon molecular sieve membrane have a larger free volume, and two carboxyl groups can serve as crosslinking sites to crosslink the precursor membrane by decarboxylation crosslinking to further improve the separation performance of the carbon molecular sieve membrane for propylene / propane. Summary of the Invention

[0004] Aiming at the problem of insufficient separation performance of existing carbon molecular sieve membranes for propylene / propane, the present invention provides a cross-linked polyimide carbon molecular sieve membrane and a preparation method thereof.

[0005] The present invention provides a cross-linked polyimide carbon molecular sieve membrane for propylene / propane separation and a preparation method thereof. A diamine monomer containing multiple carboxyl groups is introduced into the polyimide precursor polymer, and the carboxyl groups in its structure can be used as cross-linking sites to perform decarboxylation cross-linking on the polyimide precursor polymer membrane before carbonization. The cross-linking process can effectively increase the free volume in the carbon molecular sieve membrane and enhance the π-π interaction between it and propylene, thereby improving both the propylene permeability and the propylene / propane selectivity.

[0006] The technical solution of the present invention:

[0007] A cross-linked polyimide carbon molecular sieve membrane for propylene / propane separation, the precursor polymer of the cross-linked polyimide carbon molecular sieve membrane has the following repeating unit structure:

[0008]

[0009] Among them, the dianhydride monomer A is one or a combination of two or more of the following structures:

[0010]

[0011] The diamine monomer B is any one of the following structures:

[0012]

[0013] The diamine monomer C is any one of the following structures:

[0014]

[0015] Among them, m = 1-4; n = 4-1.

[0016] A preparation method of a cross-linked polyimide carbon molecular sieve membrane for propylene / propane separation, comprising the following steps:

[0017] (1) Preparation of polyimide precursor polymer by chemical imidization method: In an inert atmosphere D, diamine monomer B and diamine monomer C are dissolved in solvent E in a certain proportion. After complete dissolution, dianhydride monomer A is added, where the total molar ratio of diamine monomer to dianhydride monomer is 1:1. React for 2 - 5 hours under stirring in an ice - water bath, then remove the ice - water bath and continue to react at room temperature for 10 - 20 hours to form polyamic acid. Then, a catalyst and a dehydrating agent are added to the reaction system and continue to react for 15 - 25 hours to form polyimide. Finally, the completed reaction solution is poured into methanol to obtain a white filamentous polymer. The obtained white filamentous polymer is washed 5 - 6 times with methanol and dried in a vacuum oven to obtain the polyimide precursor polymer;

[0018] (2) Preparation of polyimide precursor polymer film by solvent evaporation method: Dissolve the polyimide precursor polymer in solvent F to prepare a solution with a mass fraction of 2 - 3 wt.%. Stir for 12 - 36 hours to completely dissolve the solute. Filter the solution using an organic filter membrane to remove insoluble impurities, then pour the filtered solution into a petri dish, dry it at 60 - 80 °C and transfer it to a vacuum oven until the solvent completely evaporates to obtain the polyimide precursor polymer film;

[0019] (3) Preparation of cross - linked polyimide precursor polymer film: Put the polyimide precursor polymer film into a tubular furnace and carry out a cross - linking process in a nitrogen atmosphere to obtain a cross - linked polyimide precursor polymer film;

[0020] (4) Preparation of cross - linked polyimide carbon molecular sieve membrane: Put the cross - linked polyimide precursor polymer film into the tubular furnace again and carry out a carbonization process under the protection of a nitrogen atmosphere to obtain a cross - linked polyimide carbon molecular sieve membrane with a smooth surface and uniform thickness.

[0021] The dianhydride monomer A is one or a combination of two or more of the following structures:

[0022]

[0023] The diamine monomer B is any one of the following structures:

[0024]

[0025] The diamine monomer C is any one of the following structures:

[0026]

[0027] Where m = 1 - 4; n = 4 - 1;

[0028] The inert atmosphere D is high - purity nitrogen or high - purity argon;

[0029] The solvent E described above is N-methylpyrrolidone or N,N-dimethylformamide.

[0030] The catalyst described above is pyridine or 3-methylpyridine, and its dosage is 0.1-0.3 g / mmol of the dianhydride monomer A;

[0031] The dehydrating agent described above is acetic anhydride, and its dosage is 5-15 times that of the catalyst;

[0032] The solvent F described above is one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

[0033] The crosslinking temperature described above is 300-450 °C, and the crosslinking time is 0.5-24 hours.

[0034] The carbonization temperature described above is 500-900 °C, and the carbonization time is 1-8 hours.

[0035] Advantages of the present invention: Aiming at the existing problems, the present invention provides a new idea for improving the separation performance of polyimide carbon molecular sieve membranes for propylene / propane. A diamine structure containing multiple crosslinkable carboxyl groups is introduced into the precursor polymer, and decarboxylation crosslinking pretreatment is carried out before carbonization. Decarboxylation crosslinking can inhibit the close packing of carbon chains and pore structure collapse formed during carbonization, which results in more free volume in the prepared carbon molecular sieve membrane, thereby enhancing its gas permeability. On the other hand, the carbon molecular sieve membrane after decarboxylation crosslinking has a higher graphite carbon content to strengthen the selective adsorption of propylene by the carbon molecular sieve membrane and promote the penetration of propylene molecules, thereby enhancing its propylene / propane separation selectivity. Description of the Drawings

[0036] Figure 1 It is the thermogravimetric test result of the precursor membrane in the example. Detailed Embodiments

[0037] The following further illustrates the detailed embodiments of the present invention in conjunction with the drawings and technical solutions.

[0038] Example 1: Add 12 ml of N-methylpyrrolidone to a three-necked flask. The three-necked flask is purged with nitrogen for 2 hours before use to remove potential moisture. Weigh 4 mmol of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (DBd) and put it into the solution, and continuously stir until the solid is completely dissolved. Weigh 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) in an equimolar amount to the diamine monomer and add it to the reaction solution. Keep stirring and let the reaction system react in an ice-water bath for 3 hours, then remove the ice-water bath and continue to react at room temperature for 21 hours to form polyamic acid. Add 0.42 ml of the catalyst pyridine and 4.17 ml of the dehydrating agent acetic anhydride and continue to react for 24 hours to convert the polyamic acid into polyimide. Pour the reaction-terminated solution into a methanol solution to obtain a white filamentous precipitate. After washing with methanol 6 times, filter and dry it to obtain the precursor polyimide 6FDA-DBd.

[0039] Weigh 0.3 g of the above-synthesized 6FDA-DBd polymer and dissolve it in 9.7 g of N,N-dimethylformamide to prepare a casting solution with a mass fraction of 3 wt.%. Stir for 36 hours until it is completely dissolved, then filter it. After ultrasonic degassing, pour it into a petri dish, volatilize the solvent at 60 °C, and dry it under vacuum at 80 °C to obtain the 6FDA-DBd precursor polymer membrane.

[0040] Put the above-prepared polymer membrane into a tubular furnace and carry out cross-linking at 400 °C / 2 h and carbonization at 550 °C / 2 h under the protection of a nitrogen atmosphere to obtain a carbon molecular sieve membrane with 6FDA-DBd cross-linked at 400 °C.

[0041] Example 2: Put the 6FDA-DBd precursor polymer membrane prepared in Example 1 into a tubular furnace and carry out cross-linking at 300 °C / 2 h and carbonization at 550 °C / 2 h under the protection of a nitrogen atmosphere to obtain a carbon molecular sieve membrane with 6FDA-DBd cross-linked at 300 °C.

[0042] Example 3: Put the 6FDA-DBd precursor polymer membrane prepared in Example 1 into a tubular furnace and carry out cross-linking at 350 °C / 2 h and carbonization at 550 °C / 2 h under the protection of a nitrogen atmosphere to obtain a carbon molecular sieve membrane with 6FDA-DBd cross-linked at 350 °C.

[0043] Example 4: Put the 6FDA-DBd precursor polymer membrane prepared in Example 1 into a tubular furnace and carry out cross-linking at 450 °C / 2 h and carbonization at 550 °C / 2 h under the protection of a nitrogen atmosphere to obtain a carbon molecular sieve membrane with 6FDA-DBd cross-linked at 450 °C.

[0044] Example 5: Add 12 ml of N-methylpyrrolidone into a three-necked flask. The three-necked flask is purged with nitrogen for 2 hours before use to remove potential moisture. Weigh 4 mmol of 2,3,5,6-tetraamino-1,4-benzenedicarboxylic acid and put it into the solution, and continuously stir until the solid is completely dissolved. Weigh 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with an equimolar amount of the diamine monomer and add it to the reaction solution. The synthesis method is the same as that in Example 1.

[0045] Weigh 0.3 g of the polymer synthesized above and dissolve it in 9.7 g of N,N-dimethylformamide to prepare a casting solution with a mass fraction of 3 wt.%. Stir for 36 hours until it is completely dissolved, then filter it. After ultrasonic degassing, pour it into a petri dish, volatilize the solvent at 60 °C, and dry it under vacuum at 80 °C to obtain a precursor polymer membrane.

[0046] Put the polymer membrane prepared above into a tubular furnace and carry out cross-linking at 450 °C for 0.5 h and carbonization at 900 °C for 1 h under the protection of a nitrogen atmosphere to obtain a cross-linked carbon molecular sieve membrane.

[0047] Example 6: Add 12 ml of N-methylpyrrolidone into a three-necked flask. The three-necked flask is purged with nitrogen for 2 hours before use to remove potential moisture. Weigh 2 mmol of 3,5-diamino-1,2-benzenedicarboxylic acid and 2 mmol of 2,4,6-trimethyl-1,3-benzenediamine and put them into the solution, and continuously stir until the solid is completely dissolved. Weigh 4 mmol of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and add it to the reaction solution. The synthesis method is the same as that in Example 1.

[0048] Weigh 0.3 g of the polymer synthesized above and dissolve it in 9.7 g of N,N-dimethylformamide to prepare a casting solution with a mass fraction of 2 wt.%. Stir for 36 hours until it is completely dissolved, then filter it. After ultrasonic degassing, pour it into a petri dish, volatilize the solvent at 60 °C, and dry it under vacuum at 80 °C to obtain a precursor polymer membrane.

[0049] Put the polymer membrane prepared above into a tubular furnace and carry out cross-linking at 300 °C for 24 h and carbonization at 500 °C for 8 h under the protection of a nitrogen atmosphere to obtain a cross-linked carbon molecular sieve membrane.

[0050] Example 7: Add 12 ml of N-methylpyrrolidone into a three-necked flask. The three-necked flask is purged with nitrogen for 2 hours before use to remove potential moisture. Weigh 2 mmol of 6,6'-diamino-3,3'-methylenedibenzoic acid and 2 mmol of 3,3'-oxydianiline and put them into the solution, and continuously stir until the solids are completely dissolved. Weigh 4 mmol of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and add it to the reaction solution. The synthesis method is the same as that in Example 1.

[0051] Weigh 0.3 g of the above-synthesized polymer and dissolve it in 9.7 g of N,N-dimethylformamide to prepare a casting solution with a mass fraction of 2 wt.%. Stir for 36 hours until it is completely dissolved, then filter it. After ultrasonic degassing, pour it into a petri dish, volatilize the solvent at 60 °C, and dry it under vacuum at 80 °C to obtain a precursor polymer membrane.

[0052] Put the above-prepared polymer membrane into a tubular furnace, and carry out cross-linking at 450 °C for 12 h and carbonization at 500 °C for 4 h under the protection of a nitrogen atmosphere to obtain a cross-linked carbon molecular sieve membrane.

[0053] Comparative Example 1: Add 12 ml of N-methylpyrrolidone into a three-necked flask. The three-necked flask is purged with nitrogen for 2 hours before use to remove potential moisture. Weigh 4 mmol of 3,5-diaminobenzoic acid (DABA) and put it into the solution, and continuously stir until the solid is completely dissolved. Weigh 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in an equimolar amount to the diamine monomer and add it to the reaction solution. The synthesis steps are the same as those in Example 1, and thus the precursor polyimide 6FDA-DABA is obtained.

[0054] Weigh 0.3 g of the above-synthesized 6FDA-DABA polymer and dissolve it in 9.7 g of N,N-dimethylformamide to prepare a casting solution with a mass fraction of 3 wt.%. Stir for 36 hours until it is completely dissolved, then filter it. After ultrasonic degassing, pour it into a petri dish, volatilize the solvent at 60 °C, and dry it under vacuum at 80 °C to obtain a 6FDA-DABA precursor polymer membrane.

[0055] Put the polymer membrane into a tubular furnace and carry out carbonization at 550 °C for 2 h under nitrogen protection to obtain a 6FDA-DABA carbon molecular sieve membrane.

[0056] Comparative Example 2: Put the polymer membrane in Example 1 into a tubular furnace and carry out carbonization at 550 °C for 2 h under nitrogen protection to obtain a 6FDA-DBd carbon molecular sieve membrane.

[0057] Table 1 Propylene, propane permeability and selectivity of the carbon molecular sieve membranes prepared in comparative examples and examples

[0058]

[0059] Compared with Comparative Example 1, the carbon molecular sieve membrane prepared in Comparative Example 2 has higher propylene permeability and propylene / propane selectivity. This is because the carbon molecular sieve membrane containing polycarboxylic diamine monomers has a higher free molecular volume and a more uniform pore size distribution. Compared with Comparative Example 2, the carbon molecular sieve membrane in Example 3 has undergone a crosslinking process at 350 °C, and it has increased propylene permeability and propylene / propane selectivity, but the improvement amplitude is small. This is because the decarboxylation crosslinking is not completely carried out. As Figure 1 shown, there is still a decomposition peak in the membrane after crosslinking at 350 °C / 2 h before 500 °C, corresponding to the anhydride structure formed during the crosslinking process. The propylene permeability of the carbon molecular sieve membrane in Example 1 has increased by 108%. This benefits from the fact that decarboxylation crosslinking can inhibit the close packing of carbon chains formed during the carbonization process, which results in more free volume in the prepared carbon molecular sieve membrane, thereby enhancing its gas permeability. The propylene / propane selectivity has also been improved because the crosslinking process increases the graphite carbon content in the carbon molecular sieve membrane, strengthens the π-π interaction between it and propylene, promotes the permeation of propylene molecules, and thus enhances its propylene / propane separation selectivity.

Claims

1. A cross-linked polyimide carbon molecular sieve membrane for propylene / propane separation, characterized in that: The precursor polymer of the cross-linked polyimide carbon molecular sieve membrane has the following repeating unit structure: Wherein, the dianhydride monomer A is one or a combination of two or more of the following structures: The diamine monomer B is any one of the following structures: The diamine monomer C is any one of the following structures: Among them, m=1-4; n=4-1.

2. A method for preparing a cross-linked polyimide carbon molecular sieve membrane for propylene / propane separation, characterized in that: The following steps are involved: (1) preparing a polyimide precursor polymer by a chemical imidization method: in an inert atmosphere D, dissolving a diamine monomer B and a diamine monomer C in a solvent E according to a proportion, and then adding a dianhydride monomer A after the diamine monomer and the dianhydride monomer are completely dissolved, wherein the molar ratio of the diamine monomer to the dianhydride monomer is 1:1; reacting in an ice-water bath under stirring conditions for 2-5 hours, removing the ice-water bath, and continuing to react at room temperature for 10-20 hours to generate polyamic acid; then adding a catalyst and a dehydrating agent to the reaction system and continuing to react for 15-25 hours to form a polyimide; finally, pouring the reaction solution into methanol to obtain a white filamentous polymer, washing the obtained white filamentous polymer with methanol for 5-6 times, and drying in a vacuum oven to obtain a polyimide precursor polymer; (2) preparing a polyimide precursor polymer film by a solvent volatilization method: dissolving the polyimide precursor polymer in a solvent F to prepare a solution with a mass fraction of 2-3 wt.%, stirring for 12-36 hours to completely dissolve the solute; filtering the solution with an organic filter membrane to remove insoluble impurities, then pouring the filtered solution into a culture dish, drying it at 60-80° C., and then transferring it into a vacuum oven until the solvent is completely volatilized to obtain a polyimide precursor polymer film; (3) Preparation of a cross-linked polyimide precursor polymer film: placing the polyimide precursor polymer film in a tube furnace and performing a cross-linking process in a nitrogen atmosphere to obtain a cross-linked polyimide precursor polymer film; (4) Preparation of cross-linked polyimide carbon molecular sieve membrane: The cross-linked polyimide precursor polymer membrane is placed in the tubular furnace again and carbonized under the protection of a nitrogen atmosphere to obtain a cross-linked polyimide carbon molecular sieve membrane with a smooth surface and uniform thickness.

3. The preparation method according to claim 1, characterized in that: In step (1), The inert atmosphere D is high-purity nitrogen or high-purity argon; The solvent E is N-methylpyrrolidone or N,N-dimethylformamide; The catalyst is pyridine or 3-methylpyridine, and the amount used is 0.1-0.3 g / mmol of dianhydride monomer A; The dehydrating agent is acetic anhydride, and its usage is 5-15 times of the catalyst.

4. The preparation method according to claim 1, characterized in that: In step (2), The solvent F is one of N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide or a mixture of two or more thereof.

5. The preparation method according to claim 1, characterized in that: In step (3), The cross-linking temperature is 300-450° C., and the cross-linking time is 0.5-24 hours.

6. The preparation method according to claim 1, characterized in that: In step (4), The carbonization temperature is 500-900° C., and the carbonization time is 1-8 hours.

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