A method for preparing a benzoxazole-containing polyimide carbon molecular film for ethylene / ethane separation

Abstract

The application belongs to the technical field of membrane preparation and application, and discloses a method for preparing a benzoxazole-containing polyimide carbon molecular membrane for ethylene / ethane separation. The method introduces a novel diamine monomer containing a benzoxazole structure, and polymerizes to obtain a polyimide polymer precursor containing a benzoxazole structure, thereby avoiding the limited conversion rate of thermal rearrangement to generate a benzoxazole structure, and the fact that the thermal rearrangement process causes the molecular chain to break, and further carbonization treatment causes the polymer structure to collapse and the molecular chain to be densely packed. The benzoxazole-containing polyimide carbon molecular sieve membrane has good permeability due to the ordered arrangement of the oxazole ring and the highly twisted rigid structure in the polymer main chain, and in addition, the good planar orientation and intramolecular Π-Π stacking of the polymer are conducive to the selectivity of ethylene / ethane.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of membrane preparation and application, and discloses a method for preparing a benzoxazole-containing polyimide carbon molecular membrane for ethylene / ethane separation. BACKGROUND

[0002] Ethylene is one of the important chemical raw materials in the petrochemical industry, and is also one of the largest chemical products in global consumption and production. The separation process accounts for 50%-75% of the cost of ethylene production, among which the separation of ethylene / ethane is the most difficult, and it is imperative to develop an environmentally friendly, energy-efficient ethylene / ethane separation technology. Membrane separation technology is based on a non-thermal process, without phase change, and has lower investment cost and energy consumption compared with traditional separation technologies such as low-temperature distillation, absorption separation and adsorption separation.

[0003] Membrane materials, as the core component of membrane separation technology, play a decisive role in the feasibility of separation technology. Ethylene / ethane separation membrane materials include organic polymer membranes, facilitated transport membranes, carbon molecular sieve membranes, etc. Organic polymer membranes are based on the solubility-diffusion model, have the characteristics of low cost, easy processing and good flexibility, but there is a Robeson upper limit. Facilitated transport membranes have high selectivity and permeability, but the stability of the carrier and industrial application are more difficult. Carbon molecular sieve membranes (CMS) are formed by pyrolysis of high molecular polymers under special conditions. During the pyrolysis process, SP2 hybrid hexagonal carbon sheets are tightly packed to form unique slit-shaped microporous and ultramicroporous structures, and high selectivity molecular separation is achieved by adjusting the size and shape of the pores (such as literature Fan Y, Wang Q, Cui J, et al. Research progress of microstructure modulation and gas separation performance optimization of carbon molecular sieve membranes [J]. Membrane Science and Technology, 2021, 41(02): 117-126.). The affinity heteroatom can improve the adsorption selectivity of olefins through the interaction between the olefins (such as literature Xu R, Xu X, Wang Y, et al. MOF-derived nanocomposites functionalized carbon molecular sieve membrane for enhanced ethylene / ethane separation [J]. Journal of Membrane Science, 2022, 661: 120890.). CMS has good processing performance, high thermal and chemical stability, and has great potential and value for industrial application. It is the most promising separation material in the current ethylene / ethane separation process (such as literature Salinas O, Ma X, Wang Y, et al. Carbon molecular sieve membrane from a microporous spirobisindane-based polyimide precursor with enhanced ethylene / ethane mixed-gas selectivity [J]. RSC Advances, 2017, 7(6): 3265-3272.).

[0004] Polyimide is a common precursor for the preparation of carbon molecular sieve membrane due to its excellent processability and mechanical properties. In recent years, many polyimides with good chain planarity and in-plane orientation have been reported, such as polyimide polymers containing benzoxazole (PBO) structure (such as Chinese patent CN107207726B). However, the benzoxazole structure in these polymers is mostly formed by high-temperature thermal rearrangement of polyimide polymers with -OH at the ortho position. The conversion rate of PBO structure units is limited during high-temperature thermal rearrangement, and the high-temperature thermal treatment process will cause the decomposition of polymer chains, which is not conducive to the preparation of high-performance carbon molecular sieve membranes.

[0005] Therefore, a new type of polyimide polymer containing benzoxazole structure is developed by using a molecule containing benzoxazole structure as a diamine monomer and polymerizing with dianhydride. Compared with the prior art, the present application directly introduces a new type of diamine monomer containing benzoxazole structure to prepare a polyimide polymer containing benzoxazole structure, which avoids the limitations of thermal rearrangement to generate benzoxazole structure, and the molecular chain is broken during the thermal rearrangement process. Further carbonization treatment leads to the collapse of polymer structure and the dense accumulation of molecular chains. The new type of polyimide carbon molecular sieve containing benzoxazole prepared by the present application has good separation performance for ethylene / ethane. SUMMARY

[0006] In view of the shortcomings of existing polyimide polymers containing benzoxazole, the present application proposes a new method for preparing polyimide polymers containing benzoxazole.

[0007] Technical scheme of the present application:

[0008] A method for preparing a polyimide carbon molecular sieve containing benzoxazole for ethylene / ethane separation, comprising the following steps:

[0009] (1) Preparation of polyimide precursor containing benzoxazole, which is polymerized from dianhydride monomer A and diamine monomer B;

[0010] The dianhydride monomer A is one of the following structures or a combination of two or more thereof:

[0011]

[0012] The diamine monomer B is one of the following structures or a combination of two thereof:

[0013]

[0014] The steps are as follows:

[0015] Firstly, diamine monomer B is dissolved in solvent A to obtain a solution with a concentration of 15-25 wt.%; after diamine monomer B is completely dissolved, dianhydride monomer A is added under ice water bath and N2 protection, the reactor is taken out from the ice water bath after 2-5 h of reaction, and is kept at 20-35 °C for 12-36 h of reaction; after the reaction is completed, catalyst B and dehydrating agent acetic anhydride are added to the reactor, and the reaction is continued at room temperature for 24-36 h, and the polyamic acid is converted into polyimide; wherein the molar ratio of diamine monomer B to dianhydride monomer A is 1:1; the obtained polyimide is poured into methanol to obtain a filamentous precipitate, and is washed with methanol for multiple times, and then is vacuum dried at 60-100 °C; the polyimide is dissolved in solvent C to obtain a 1-5 wt.% solution, and after complete dissolution, insoluble impurities are removed by filtering through a 0.45 μm PFTE filter; then the polyimide solution is slowly volatilized at room temperature for 12-36 h to obtain a benzoxazole-containing polyimide precursor film, and is dried in a vacuum oven at 100-150 °C for 12-24 h;

[0016] The solvent A is NMP and / or DMAc.

[0017] The catalyst B is pyridine or 3-methylpyridine.

[0018] The solvent C is one or two or more of tetrahydrofuran (THF), dichloromethane (CHCl2), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc).

[0019] (2) Preparation of benzoxazole-containing polyimide carbon molecular membrane

[0020] The benzoxazole-containing polyimide precursor film is placed in a tube furnace, protective gas A is introduced, and carbonization is performed at 500-900 °C; after carbonization for 0.5-3 h, the temperature is lowered to room temperature, and a benzoxazole-containing polyimide carbon molecular membrane is obtained.

[0021] The protective gas A is one of N2, He, and Ar.

[0022] The present application has the following beneficial effects: by using a benzoxazole-containing diamine monomer as a polymerization monomer, the content of benzoxazole structure is increased, and the synthesis steps are simplified; the benzoxazole-containing polyimide carbon molecular membrane has good permeability due to the ordered arrangement and highly twisted rigid structure of the oxazole ring in the polymer main chain; in addition, the good planar orientation and intramolecular Π-Π stacking of the polymer are conducive to the selectivity of ethylene / ethane. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is an FTIR spectrum of a carbon molecular sieve membrane. DETAILED DESCRIPTION

[0024] The following further describes the specific embodiments of the present application in conjunction with the technical solutions.

[0025] The benzoxazole-containing polyimide polymer used in the present application is prepared by a conventional two-step method (chemical imidization). First, 1-20 mmol of benzoxazole-containing diamine monomer is dissolved in 2-40 ml of N-methyl pyrrolidone, and after complete dissolution, an equimolar amount of dianhydride monomer is added under ice water bath and N2 protection. After about 3 h, the reactor is taken out of the ice water bath and kept at a certain temperature for 24-36 h. After the reaction is completed, a certain amount of 0.1-2 ml of pyridine and 1-20 ml of acetic anhydride mixture is added to the reactor by a syringe needle, and the reaction is continued at a certain temperature for 24-36 h to convert the polyamic acid into polyimide. The polymer is poured into methanol to obtain a filamentous precipitate, and washed with methanol for multiple times, and then dried in a vacuum oven at 150°C.

[0026] In Comparative Example 1, 20 mmol of diamine monomer 3,3-diaminobiphenyl not containing benzoxazole is dissolved in 40 ml of N-methyl pyrrolidone (NMP), and after complete dissolution, an equimolar amount of dianhydride monomer 6FDA is added under ice water bath and N2 protection. After about 3 h, the reactor is taken out of the ice water bath and kept at a certain temperature for 24 h. After the reaction is completed, a certain amount of 2 ml of pyridine and 20 ml of acetic anhydride mixture is added to the reactor by a syringe needle, and the reaction is continued at a certain temperature for 24 h to convert the polyamic acid into polyimide. The polymer is poured into methanol to obtain a filamentous precipitate, and washed with methanol for multiple times, and then dried in a vacuum oven at a certain temperature.

[0027] 3 g of polyimide not containing benzoxazole is dissolved in 100 ml of NMP, and after stirring at room temperature for 24 h. After standing and degassing for 24 h, the solution is poured onto a glass plate, dried at 60°C, and then dried in a vacuum drying oven at 80°C for 24 h.

[0028] About 1 g of the obtained film is placed in a tube furnace, a protective gas is introduced, heated to 550°C, and then naturally cooled to obtain the carbon molecular sieve membrane of Comparative Example 1.

[0029] In Example 1, 10 mmol of diamine monomer 2-(4-aminophenyl)-5-aminobenzoxazole is dissolved in 20 ml of NMP, 5 mmol of 6FDA and 5 mmol of ODPA are added, and the synthesis is carried out in the same manner as in Comparative Example 1.

[0030] 0.3 g of benzoxazole-containing polyimide is dissolved in 10 ml of NMP, and after stirring at room temperature for 24 h. After standing and degassing for 24 h, the solution is poured onto a glass plate, dried at 60°C, and then dried in a vacuum drying oven at 80°C for 24 h. About 0.1 g of the obtained film is placed in a tube furnace, heated to 550°C, and then naturally cooled to obtain the carbon molecular sieve membrane of Example 1.

[0031] Example 2: 10 mmol of the diamine monomer 2-(4-aminophenyl)-5-aminobenzoxazole was dissolved in NMP, and 10 mmol of 6FDA was added. The synthesis method was the same as that of Control Example 1.

[0032] Dissolve 0.3g of benzoxazole-containing polyimide in 10ml of NMP and stir at room temperature for 24h. Allow to stand for 24h to remove bubbles, then pour the solution onto a glass plate, dry at 60℃, and then dry in a vacuum drying oven at 80℃ for 24h. Place approximately 0.2g of the resulting membrane into a tube furnace, heat to 550℃, and then allow to cool naturally to obtain the carbon molecular sieve membrane of Example 2.

[0033] Example 3: 0.6 g of benzoxazole-containing polyimide was dissolved in 20 ml of NMP and stirred at room temperature for 24 h. After standing for 24 h to remove bubbles, the solution was poured onto a glass plate, dried at 60 °C, and then dried in a vacuum drying oven at 80 °C for 24 h. Approximately 0.2 g of the resulting membrane was placed in a tube furnace, heated to 700 °C, and then allowed to cool naturally to obtain the carbon molecular sieve membrane of Example 3.

[0034] Table 1. Ethylene and ethane permeability and selectivity of carbon molecular sieve membranes prepared in comparative examples and examples.

[0035]

[0036] Compared to the carbon molecular sieve membrane without benzoxazole structure, such as Control Example 1, the carbon molecular sieve membranes containing benzoxazole structure, such as Examples 1 and 2, exhibited higher permeability and selectivity at the same carbonization temperature. This is related to the strong π-π interaction of the benzidine structure. The presence of PBO weakens the strong π-π stacking between molecules, resulting in a large number of micropores in the carbon molecular sieve membrane and improved permeability. When different dianhydride monomers were mixed in proportion and copolymerized with benzoxazole diamine, the membrane obtained in Example 1 showed higher permeability and selectivity than that in Control Example 1, indicating that the membrane containing PBO structure has better permeability and selectivity for C2H4 / C2H6 separation. When all the dianhydride monomers used were 6FDA, a large number of trifluorine groups were decomposed during carbonization, resulting in a large number of micropores; therefore, the membrane in Example 2 had the highest permeability. The performance of Examples 2 and 3 shows that as the carbonization temperature increases, the membrane permeability decreases while the selectivity increases. This is related to the contraction of discontinuous micropores in the membrane caused by the increased carbonization temperature.

[0037] Depend on Figure 1 It is evident that most functional groups disappeared after carbonization of both types of membranes. This is related to the decomposition of polymer functional groups at higher carbonization temperatures. Compared to the benzoxazole-free carbon molecular sieve membrane CMS-6FDA-HAB, the benzoxazole-containing carbon molecular sieve membrane CMS-6FDA-PBO showed a significant decrease in the oxazole structure at 1100 cm⁻¹. -1 CO and 1558cm-1 The asymmetric stretching vibration peak at C=N structure indicates that the structure is not decomposed at 550℃ carbonization, which is related to the good thermal stability of the aromatic oxazole structure.

Claims

1. A method for preparing a benzoxazole-containing polyimide carbon molecular film for ethylene / ethane separation, characterized by, The steps are as follows: (1) Preparation of benzoxazole-containing polyimide precursor, which is polymerized from dianhydride monomer A and diamine monomer B; The dianhydride monomer A is one of the following structures or a combination of two or more thereof: ； The diamine monomer B is one of the following structures or a combination of two thereof: The steps are as follows: First, the diamine monomer B is dissolved in solvent A to obtain a solution with a concentration of 15-25 wt.%; after the diamine monomer B is completely dissolved, the dianhydride monomer A is added under ice water bath and N2 protection, and after 2-5 h of reaction, the reactor is taken out of the ice water bath and kept at 20-35℃ for 12-36 h of reaction; after the reaction is completed, the catalyst B and the dehydrating agent acetic anhydride are added to the reactor, and the reaction is continued at room temperature for 24-36 h, and the polyamic acid is converted into polyimide; wherein the molar ratio of the diamine monomer B to the dianhydride monomer A is 1:1; the obtained polyimide is poured into methanol to obtain a filamentous precipitate, which is washed with methanol for multiple times, and then vacuum dried at 60-100℃; the polyimide is dissolved in solvent C to obtain a 1-5 wt.% solution, which is completely dissolved and filtered through a 0.45 µm PFTE filter to remove insoluble impurities; then the polyimide solution is slowly volatilized at room temperature for 12-36 h to obtain a benzoxazole-containing polyimide precursor film, which is dried in a vacuum oven at 100-150℃ for 12-24 h; (2) Preparation of benzoxazole-containing polyimide carbon molecular film The benzoxazole-containing polyimide precursor film is placed in a tube furnace, and protective gas A is introduced, and carbonization is carried out at 500-900℃, and after 0.5-3 h of carbonization, the temperature is lowered to room temperature, and a benzoxazole-containing polyimide carbon molecular film is obtained.

2. The method of claim 1, wherein, The solvent A is NMP and / or DMAc.

3. The method of claim 1, wherein, The catalyst B is pyridine or 3-methylpyridine.

4. The method of claim 1, wherein, The solvent C is one of tetrahydrofuran, dichloromethane, N,N-dimethylformamide, N,N-dimethylacetamide, or a mixture of two or more thereof.

5. The method of claim 1, wherein, The protective gas A is one of N2, He, and Ar.

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