Super-crosslinked polyimide-based carbon molecular sieve gas separation membrane and preparation method thereof

Abstract

The invention belongs to the technical field of membrane preparation and application, and discloses a super-crosslinked polyimide-based carbon molecular sieve gas separation membrane and a preparation method thereof. The preparation method comprises the following steps: by taking polyimide as a precursor polymer, realizing super-crosslinking of a precursor based on Friedel-Crafts alkylation reaction, and then carbonizing under a certain pyrolysis condition to obtain the carbon molecular sieve gas separation membrane. According to the invention, alkyl bridge bonds are introduced between precursor polymer chains through super-crosslinking, and the alkyl bridge bonds are pyrolyzed to form pores in the carbonization process, so that the porosity of the membrane is greatly improved, and the separation performance of the carbon membrane, especially ethylene permeability, can be effectively improved. In addition, compared with a traditional cross-linking method of a carbon film precursor, the super-cross-linking method based on the Friedel-Crafts alkylation reaction does not need strong acid as a catalyst, the requirement for functional groups on a precursor polymer is low, the application range is wide, and a microporous network can be introduced into the polymer precursor under the mild condition. The preparation method has a wide application prospect in the field of gas separation membranes.

Description

Technical Field

[0001] The invention belongs to the technical field of membrane preparation and application, and discloses a super-crosslinked polyimide-based carbon molecular sieve gas separation membrane and a preparation method thereof. Background Art

[0002] Ethylene is an important basic raw material for petrochemical industry. In order to obtain high-purity ethylene, ethylene needs to be separated from other cracking products such as ethane. In industry, cryogenic distillation technology is mainly used to separate ethylene / ethane. However, due to the very similar physical properties of ethylene / ethane, traditional cryogenic distillation towers need to be operated under extremely harsh conditions to separate high-purity ethylene, which consumes a lot of energy. Gas separation membrane technology has become an emerging gas separation technology with its advantages of no phase change, low energy consumption, small footprint, and easy integration. It has great development potential in the field of efficient separation of olefins / alkanes. Therefore, it is urgent to develop gas separation membrane materials with high ethylene / ethane separation performance.

[0003] The key to membrane separation technology is the membrane. So far, the membrane materials reported for ethylene / ethane separation include polymer membranes, mixed matrix membranes, MOF membranes, carbon molecular sieve membranes, etc. Among them, carbon molecular sieve membranes (abbreviated as carbon membranes) can effectively identify the tiny shape and size differences between ethylene and ethane with their unique rigid pore structure, while obtaining considerable selectivity and permeability, standing out among many membrane materials. Carbon molecular sieve membranes are carbon-based nanoporous membranes obtained by pyrolyzing polymers under controllable conditions.

[0004] In order to develop a carbon membrane with excellent ethylene / ethane separation performance, the separation performance of the carbon membrane can be effectively improved by adjusting the pyrolysis scheme and designing the structure of the precursor polymer (such as the literature Xu R, Hou M, Wang Y, et al. High-performance carbon molecular sieve membrane for C2H4 / C2H6 separation: molecular insight into the structure-property relationships [J]. Carbon, 2023, 201: 24-36). In addition, cross-linking the precursor or increasing the cross-linking structure can also improve the separation performance of the carbon membrane, especially the ethylene permeability (such as the literature Liu Z, Qiu W, Quan W, et al. Advanced carbon molecular sieve membranes derived from molecularly engineered cross-linkable copolyimide for gas separations [J]. Nature Materials, 2023, 22 (1): 109-116.). However, since the cross-linking treatment of traditional carbon membrane precursors is mostly limited to polymers with special functional groups or using strong acids as catalysts, the selection range of precursors is greatly limited and the production cost is high.

[0005] To this end, the present invention intends to achieve the crosslinking treatment of the precursor by a hypercrosslinking method based on Friedel-Crafts alkylation reaction, which does not require strong acid as a catalyst, has low requirements on the functional groups on the precursor polymer, and has a wide range of applications. Under relatively mild conditions, a microporous network is introduced into the polymer precursor, and the alkyl bridge bonds between the precursor polymer chains are pyrolyzed into pores during the carbonization process, which greatly improves the porosity of the membrane and can effectively improve the separation performance of the carbon membrane. The hypercrosslinked polyimide-based carbon molecular sieve gas separation membrane prepared by the present invention has good separation performance for ethylene / ethane. Summary of the invention

[0006] The invention provides a super cross-linked polyimide-based carbon molecular sieve gas separation membrane and a preparation method thereof.

[0007] The technical solution of the present invention:

[0008] A hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane, which uses polyimide as a precursor polymer, realizes hyper-crosslinking of the precursor based on Friedel-Crafts alkylation reaction, and then carbonizes under pyrolysis conditions to obtain a carbon molecular sieve gas separation membrane; the carbon molecular sieve membrane has a unique bimodal pore structure consisting of micropores and ultramicropores;

[0009]

[0010] A method for preparing a hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane comprises the following steps:

[0011] (1) Preparation of polyimide film

[0012] The polyimide precursor is formed by polycondensation of dianhydride monomer and diamine monomer;

[0013] The dianhydride monomer is one or a combination of two or more of the following structures:

[0014]

[0015] The diamine monomer is one or a combination of two or more of the following structures:

[0016]

[0017] Dissolve the diamine monomer in solvent A, stir and dissolve, add an equal molar amount of dianhydride monomer, add solvent A to obtain a solution with a concentration of 10-25wt%; slowly heat up to 50-80°C, react for 1-3h; add catalyst B and entrainer C, slowly raise the temperature to 150-250°C, react for 6-24h, and obtain polyimide; pour the obtained polyimide into methanol to obtain a filamentous precipitate, wash it with methanol for multiple times, and then vacuum dry it at 60-150°C;

[0018] Dissolve polyimide in solvent D to obtain a 1-5 wt.% solution, filter and ultrasonically degas after complete dissolution, and then cast a film; then slowly evaporate at 30-80° C. for 12-36 hours, and then dry in a vacuum oven at 120-150° C. for 12-24 hours to obtain a polyimide film;

[0019] (2) Preparation of hypercrosslinked polyimide membrane

[0020] The crosslinking agent E and the catalyst F are mixed in equal proportions in the solvent G, and stirred until completely dissolved to obtain a brown solution; the reaction solution is heated to 60-90°C under condensation reflux conditions; the polyimide membrane is placed in the heated reaction solution according to a molar ratio of the crosslinking agent E to the polyimide monomer of 1-100:1, and allowed to stand for 1-5 hours, and then naturally cooled to room temperature, and the super-crosslinked polyimide membrane is washed with methanol until the filtrate is clear and colorless, and the super-crosslinked polyimide membrane is placed in a vacuum oven and dried for 24 hours to obtain a super-crosslinked polyimide membrane;

[0021] (3) Preparation of hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane

[0022] The hyper-crosslinked polyimide membrane is placed in a tubular furnace, a protective gas H is introduced, carbonized at 500-900° C., and naturally cooled to room temperature to obtain a hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane.

[0023] In step (1), solvent A is m-cresol, NMP or DMAC; catalyst B is isoquinoline or benzoic acid; entrainer C is toluene or 1,2-dichlorobenzene or o-xylene; solvent D is ethylene dichloride, DMF, DMAC or tetrahydrofuran.

[0024] In step (2), the molar ratio of the crosslinking agent E to the catalyst F is 1:0.5-20, the crosslinking agent E is dimethoxymethane, 1,2-dimethoxyethane, 2,2-dimethoxypropane, 1,2-dimethoxypropane, ethylene dichloride or carbon tetrachloride; the catalyst F is aluminum chloride, anhydrous ferric chloride, zinc chloride or tin tetrachloride; and the solvent G is ethylene dichloride or carbon tetrachloride.

[0025] In step (3), the protective gas H is nitrogen or argon.

[0026] Beneficial effects of the present invention: The hyper-crosslinking method based on Friedel-Crafts alkylation reaction provided by the present invention does not require a strong acid as a catalyst, has low requirements on the functional groups on the precursor polymer, has a wide range of applications, and introduces a microporous network into the polymer precursor under relatively mild conditions. In the carbonization process of the hyper-crosslinked polymer, the alkyl bridge bonds between the precursor polymer chains are pyrolyzed into pores, which greatly improves the porosity of the membrane and can effectively improve the separation performance of the carbon membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM cross-sectional view of the hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane in Example 1. DETAILED DESCRIPTION

[0028] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0029] The polyimide polymer used in the present invention is prepared by a one-step method. 1 mmol of diamine HAB is dissolved in 7 ml of m-cresol, and after stirring and dissolving, 1 mmol of dianhydride 6FDA is added, and 1 ml of m-cresol is added, and the temperature is slowly raised to 70°C, and the reaction is carried out for 2 hours, and 0.37 ml of catalyst isoquinoline and 3.5 ml of azeotropic agent toluene are added. The temperature is slowly raised to 190°C, and the reaction is carried out for about 12 hours. The obtained polyimide 6FDA-HAB is poured into methanol to obtain a filamentous precipitate, and washed with methanol for multiple times, and then vacuum dried at a certain temperature.

[0030] Comparison example:

[0031] Take 0.2g of polyimide 6FDA-HAB and dissolve it in 10ml of dichloroethane. After it is completely dissolved, filter and ultrasonically degas, and then pour the casting solution onto a glass plate. Then, slowly evaporate it at room temperature for 36 hours, and then dry it in a vacuum oven at 50°C for 24 hours to obtain a polyimide film.

[0032] The polyimide film is not subjected to any treatment and is directly placed in a tube furnace. A protective gas is introduced and the temperature is raised to 550° C. according to a certain heating program and then naturally cooled to room temperature.

[0033] Example 1

[0034] Take 0.2g of polyimide 6FDA-HAB and dissolve it in 10ml of dichloroethane. After it is completely dissolved, filter and ultrasonically degas, and then pour the casting solution onto a glass plate. Then, slowly evaporate it at room temperature for 36 hours, and then dry it in a vacuum oven at 50°C for 24 hours to obtain a polyimide film.

[0035] Dimethoxymethane and ferric chloride are mixed in dichloroethane at a ratio of 1:1, and stirred until completely dissolved to obtain a brown solution; the reaction solution is heated to 60°C under condensation reflux conditions; according to a molar ratio of dimethoxymethane to polyimide monomer of 60:1, a certain amount of 6FDA-HAB membrane polyimide membrane is cut and placed in the above solution, allowed to stand for 4 hours, and then naturally cooled to room temperature, the polymer membrane is washed with methanol until the filtrate is clear and colorless, and the membrane is placed in a vacuum oven and dried for 24 hours to obtain a hyper-crosslinked polyimide membrane.

[0036] 0.1 g of a hyper-crosslinked polyimide film was placed in a tubular furnace, a protective gas was introduced, the temperature was raised to 550° C. according to a certain heating program, and the temperature was naturally lowered to room temperature.

[0037] Example 2

[0038] Take 0.15g of polyimide 6FDA-HAB and dissolve it in 10ml of dichloroethane. After it is completely dissolved, filter and ultrasonically degas, and then pour the casting solution onto a glass plate. Then, slowly evaporate it at room temperature for 36 hours, and then dry it in a vacuum oven at 50°C for 24 hours to obtain a polyimide film.

[0039] Dimethoxymethane and ferric chloride are mixed in dichloroethane at a ratio of 1:1, and stirred until completely dissolved to obtain a brown solution; the reaction solution is heated to 60°C under condensation reflux conditions; according to a molar ratio of dimethoxymethane to polyimide monomer of 30:1, a certain amount of 6FDA-HAB membrane polyimide membrane is cut and placed in the above solution, allowed to stand for 4 hours, and then naturally cooled to room temperature, the polymer membrane is washed with methanol until the filtrate is clear and colorless, and the membrane is placed in a vacuum oven and dried for 24 hours to obtain a hyper-crosslinked polyimide membrane.

[0040] 0.1 g of a hyper-crosslinked polyimide film was placed in a tubular furnace, a protective gas was introduced, the temperature was raised to 550° C. according to a certain heating program, and the temperature was naturally lowered to room temperature.

[0041] Table 1 shows the ethylene / ethane permeability and selectivity of the carbon molecular sieve membranes prepared in the control examples and examples.

[0042]

Claims

1. A hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane, characterized in that: Using polyimide as a precursor polymer, hyper-crosslinking of the precursor is achieved based on Friedel-Crafts alkylation reaction; then, carbonization is continued under pyrolysis conditions to obtain a carbon molecular sieve gas separation membrane; the carbon molecular sieve membrane has a unique bimodal pore structure consisting of micropores and ultramicropores; 2. A method for preparing a hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane, characterized in that: The steps include: (1) Preparation of polyimide film The polyimide precursor is prepared by polycondensation of dianhydride monomer and diamine monomer, specifically as follows: dissolving the diamine monomer in solvent A, stirring and dissolving, adding an equal molar amount of dianhydride monomer, and supplementing solvent A to obtain a solution with a concentration of 10-25wt%; slowly heating to 50-80°C, reacting for 1-3h; adding catalyst B and co-boiling agent C, slowly raising the temperature to 150-250°C, reacting for 6-24h, and obtaining polyimide; pouring the obtained polyimide into methanol to obtain a filamentous precipitate, washing it with methanol for multiple times, and then vacuum drying it at 60-150°C; dissolving the polyimide in solvent D to obtain a 1.5wt.% solution, filtering and ultrasonic degassing after complete dissolution, and then casting a film; then slowly volatilizing at 30-80°C for 12.36h, and then drying it in a vacuum oven at 120-150°C for 12.24h to obtain a polyimide film; (2) Preparation of hypercrosslinked polyimide membrane The crosslinking agent E and the catalyst F are mixed in equal proportions in the solvent G, and stirred until completely dissolved to obtain a brown solution; the reaction solution is heated to 60-90°C under condensation reflux conditions; the polyimide membrane is placed in the heated reaction solution according to a molar ratio of the crosslinking agent E to the polyimide monomer of 1-100:1, and allowed to stand for 1-5 hours, and then naturally cooled to room temperature, and the super-crosslinked polyimide membrane is washed with methanol until the filtrate is clear and colorless, and the super-crosslinked polyimide membrane is placed in a vacuum oven and dried for 24 hours to obtain a super-crosslinked polyimide membrane; (3) Preparation of hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane The hyper-crosslinked polyimide membrane was placed in a tube furnace, a protective gas H was introduced, carbonized at 500-900°C, and naturally cooled to room temperature to obtain a hyper-crosslinked polyimide-based carbon molecular sieve gas separation membrane.

3. The preparation method according to claim 2, characterized in that: In step (1), The dianhydride monomer is one or a combination of two or more of the following structures: The diamine monomer is one or a combination of two or more of the following structures:

4. The preparation method according to claim 2, characterized in that: In step (1), The solvent A is m-cresol, NMP or DMAC; the catalyst B is isoquinoline or benzoic acid; the azeotroping agent C is toluene or 1,2-dichlorobenzene or o-xylene; and the solvent D is dichloroethane, DMF, DMAC or tetrahydrofuran.

5. The preparation method according to claim 2, characterized in that: In step (2), The molar ratio of the crosslinking agent E to the catalyst F is 1:0.5-20, the crosslinking agent E is dimethoxymethane, 1,2-dimethoxyethane, 2,2-dimethoxypropane, 1,2-dimethoxypropane, ethylene dichloride or carbon tetrachloride; the catalyst F is aluminum chloride, anhydrous ferric chloride, zinc chloride or tin tetrachloride; and the solvent G is ethylene dichloride or carbon tetrachloride.

6. The preparation method according to claim 2, characterized in that: In step (3), The protective gas H is nitrogen or argon.

Images