A method for preparing a fluorinated Trog base carbon molecular sieve membrane with high gas separation performance

Abstract

The present invention belongs to the technical field of membrane preparation and application, and discloses a method for preparing a fluorinated Trog base carbon molecular sieve membrane with high gas separation performance. The fluorinated Trog base carbon molecular sieve membrane with high gas separation performance uses a Trog base polymer as a substrate. A fluorinated Trog base polymer is obtained by introducing a fluorinated diamine monomer during synthesis, and then a carbon molecular sieve membrane is prepared through a further specific pyrolysis process. The present invention successfully customizes a carbon molecular sieve membrane with a specific porosity by adding fluorinated units to the main chain of the Trog base base polymer through pyrolysis, and has significant hydrogen / helium cross-selectivity and high propylene / propane selectivity. The present invention has broad application prospects 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 method for preparing a fluorine-containing Trog base carbon molecular sieve membrane with high gas separation performance. Background Art

[0002] Helium is the second most abundant element in the universe and its reserves on Earth are limited. Due to its wide application in military, scientific research, medicine, superconducting experiments and other fields, helium has become a rare strategic resource. Membrane technology has been used to separate helium and hydrogen from natural gas reservoirs, but this process is still very challenging because hydrogen and helium The kinetic diameter difference is less than The boiling point difference is less than 16K.

[0003] Light olefins are essential synthetic feedstocks for the petrochemical industry, and the separation and purification of their polymer-grade products are key steps in the petrochemical production process. However, existing technologies for propylene / propane separation, such as cryogenic distillation, have been energy-intensive and economically expensive. Alleviating the severe energy consumption of propylene / propane separation has been identified as one of the seven major chemical separations that could transform global energy use.

[0004] In recent years, membrane separation technology has received increasing attention in gas separation processes such as hydrogen recovery, oxygen enrichment, and carbon dioxide capture. Carbon molecular sieve (CMS) membranes are formed by pyrolysis of polymer membranes. Compared with polymer membranes, it has many advantages, including high thermal stability, excellent chemical resistance, and great potential for highly selective and cost-effective gas separation. The pyrolysis process involves the gradual decomposition, fragmentation, and aromatization of polymer chains to form "carbon chains" and "carbon plates", leading to the formation of ultra-micropores. The high carbon content and strong carbon-carbon (CC) bonds of the carbon molecular sieve membrane ensure excellent structural integrity and stability. The bimodal pore size distribution of the carbon molecular sieve membrane further improves the permeability and selectivity of the membrane. This is achieved through micropores (7- ) in the rapid gas diffusion and less than This is achieved by combining the high-efficiency screening function of ultra-micropores.

[0005] At present, in terms of helium / hydrogen separation, the selectivity of carbon molecular sieve membranes is generally in the range of 0.24-1.52 (Chong Yang Chuah, Tae-Hyun Bae, Polyimide-derived carbon molecular sieve membranes for advanced gas separation: From membrane development to pilot-scale operations [J]. Sep. Purif. Technol, 2023, 320, 124114). However, carbon molecular sieve membranes with a selectivity of 10.5 require complex and dangerous fluorine / nitrogen post-treatment processes (Qi Wu, Lu Liu, Yang Jiao, Zhenyuan Li, Ju Bai, Prof. Xiaohua Ma, Prof. Shuangjiang Luo, Prof. Suojiang Zhang, Precise Helium Sieving from Hydrogen Using Fluorine-Decorated Carbon HollowFiber Membranes, Angew. Chem. Int. Ed, 2024, 63(33), e202400688). The development of reverse-selective helium-hydrogen separation membranes is crucial, as hydrogen with a larger kinetic diameter is more permeable than helium with a smaller kinetic diameter. These membranes offer several advantages over conventional helium-selective membranes. For example, hydrogen-selective membranes can significantly reduce separation costs because the unwanted hydrogen is more permeable and remains at a very low pressure on the permeate side. However, the desired helium remains on the high-pressure side, saving on helium compression costs and further reducing separation costs.

[0006] On the other hand, most of the carbon molecular sieve membrane materials used for propylene / propane separation are polyimides, and have shown promising propylene-propane separation performance, but the application of carbon molecular sieve membranes belonging to self-polymerized microporous polymers in propylene / propane separation has not been fully explored (Giuseppe Genduso, Wojciech Ogieglo, Yingge Wang, Ingo Pinnau[J], Carbon molecular sieve gas separation materials and membranes: A comprehensive review, J. Membr. Sci., 2024, 699, 122533).

[0007] Trog base polymers are a type of self-polymerizing microporous polymer. Carbon molecular sieve membranes derived from their pyrolysis exhibit a hydrogen / helium selectivity of 2.37, but a hydrogen permeability of only 202.4 barrer. Furthermore, the propylene / propane separation performance of Trog base-based carbon molecular sieve membranes has not been explored. This study aims to explore the intrinsic propylene / propane performance of Trog base-based carbon molecular sieve membranes and further develop Trog base polymer carbon molecular sieve membranes with high hydrogen / helium and propylene / propane separation performance. Summary of the Invention

[0008] To address the shortcomings of existing carbon molecular sieve membranes, the present invention provides a method for preparing a fluorinated Trog base carbon molecular sieve membrane with high gas separation performance. To achieve this objective, the present invention provides a method for preparing a fluorinated Trog base carbon molecular sieve membrane. By introducing trifluoroisopropyl groups into the backbone of a Trog base polymer and then pyrolyzing it, a customized carbon molecular sieve membrane with a specific micropore distribution is produced, capable of highly efficient hydrogen / helium separation while also achieving propylene / propane separation performance.

[0009] The technical solution of the present invention:

[0010] A method for preparing a fluorine-containing Trog base carbon molecular sieve membrane with high hydrogen / helium separation performance comprises the following steps:

[0011] (1) Preparation of fluorotrog base polymer: Monomer A and monomer B are added to a dimethoxymethane solution; trifluoroacetic acid is slowly added dropwise to the system under ice-water bath conditions; the reaction mixture is stirred at 25°C, and ammonia is added to terminate the reaction. The obtained product is washed with solvent C multiple times, redissolved with solvent D, reprecipitated with methanol, and filtered and dried to obtain a fluorotrog base polymer;

[0012] (2) Preparation of fluorotrog base precursor film: dissolve the fluorotrog base polymer in chloroform, stir, and introduce the casting solution into a glass watch glass. After complete natural evaporation, dry under vacuum to obtain a polymer film.

[0013] (3) Preparation of fluorinated Trog base carbon molecular sieve membrane: The polymer membrane is placed in a tubular furnace, and a protective gas E is introduced. The carbonization is carried out at 550°C-800°C, and the temperature is maintained for 1-3 hours and then cooled to room temperature to obtain a fluorinated Trog base carbon molecular sieve membrane.

[0014] Furthermore, monomer A is 4,4'-diamino-3,3'-dimethylbiphenyl, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-diaminooctafluorobiphenyl or 3,3-bis(trifluoromethyl)-[1,1-biphenyl]-4,4-diamine; monomer B is 4,4'-(hexafluoroisopropylidene)diphenylamine or 3,3',5,5'-tetramethylbenzidine; solvent C is water or methanol; solvent D is one or a mixture of two or more of chloroform, N-methylpyrrolidone, and N,N-dimethylformamide; and protective gas E is nitrogen, argon or helium.

[0015] Furthermore, in step (1), the molar ratio of monomer A to monomer B is 1:0.2-0.5; the molar ratio of monomer A to dimethoxymethane is 1:4-6; the molar ratio of monomer A to trifluoroacetic acid is 1:20-30; the stirring time is 48-168h; and the volume ratio of ammonia water to trifluoroacetic acid is 5:1.

[0016] Furthermore, in step (2), the mass fraction of the casting solution is 1.0-20.0 wt.%; and the stirring time is 12-48 h.

[0017] Beneficial effects of the present invention: The preparation method of the present invention forms a carbon molecular sieve membrane with a specific micropore distribution by introducing trifluoroisopropyl into the main chain of the Trog base polymer and performing pyrolysis, which has high hydrogen permeability and high hydrogen / helium selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 BET spectra of fluorinated and non-fluorinated Trog base carbon molecular sieve membranes. DETAILED DESCRIPTION

[0019] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0020] The fluorotrog base carbon polymer membrane used in the present invention is prepared by adding 8.492 g of 4,4'-diamino-3,3'-dimethylbiphenyl and 3.340 g of 4,4'-(hexafluoroisopropylidene)diphenylamine to 20 mL of a dimethoxymethane solution. An ice-water bath is then added to the system, followed by the slow dropwise addition of 85 mL of trifluoroacetic acid. The reaction mixture is stirred at 25°C for 96 hours, and 500 mL of aqueous ammonia is added to terminate the reaction. The resulting product is washed multiple times with aqueous solution, redissolved in chloroform, reprecipitated with methanol, filtered, and vacuum-dried at 80°C to obtain a fluorotrog base polymer.

[0021] Gas permeability test: The permeability test of the carbon molecular sieve gas separation membrane in the present invention adopts the constant volume variable pressure method, and the test temperature is 35°C.

[0022] Comparison example:

[0023] 10.615 g of 4,4'-diamino-3,3'-dimethylbiphenyl was added to 20 mL of dimethoxymethane solution. An ice-water bath was then added, followed by the slow dropwise addition of 85 mL of trifluoroacetic acid. The reaction mixture was stirred at 25°C for 96 hours, and then quenched by the addition of 500 mL of aqueous ammonia. The resulting product was washed several times with aqueous solution, redissolved in chloroform, reprecipitated with methanol, filtered, and dried under vacuum at 80°C to obtain a fluorotrope-free polymer.

[0024] 0.2 g of the obtained polymer was added to chloroform solvent to prepare a casting solution with a mass fraction of 2.0 wt.%. After stirring for 48 hours, the casting solution was introduced into a glass watch glass, evaporated naturally for 24 hours, and then dried under vacuum conditions for 48 hours.

[0025] The obtained film was taken to a size of about 4 cm 2 The mixture was placed in a tubular furnace, and protective gas N2 was introduced. The temperature was raised to 550°C and maintained for 2 hours. After the temperature was naturally cooled to room temperature, the carbon molecular sieve membrane of Comparative Example 1 was obtained.

[0026] Example 1:

[0027] Add 8.492 g (40 mmol) of 4,4'-diamino-3,3'-dimethylbiphenyl and 3.340 g (10 mmol) of 4,4'-(hexafluoroisopropylidene)diphenylamine to 20 mL of dimethoxymethane solution. Slowly add 85 mL of trifluoroacetic acid dropwise in an ice-water bath. The synthesis and cleaning procedures are the same as those in Control Example 1.

[0028] 0.2 g of the obtained polymer was added to chloroform solvent to prepare a casting solution with a mass fraction of 2.0 wt.%. After stirring for 48 hours, the casting solution was introduced into a glass watch glass, evaporated naturally for 24 hours, and then dried under vacuum conditions for 48 hours.

[0029] The obtained film was taken to a size of about 4 cm 2 The mixture was placed in a tubular furnace, and N2 protective gas was introduced. The temperature was raised to 550° C. and maintained for 2 h. The carbon molecular sieve membrane of Example 1 was obtained after the temperature was naturally lowered to room temperature.

[0030] Example 2:

[0031] Add 6.369 g of 4,4'-diamino-3,3'-dimethylbiphenyl and 6.680 g of 4,4'-(hexafluoroisopropylidene)diphenylamine to 20 mL of dimethoxymethane solution. Slowly add 85 mL of trifluoroacetic acid dropwise in an ice-water bath. The synthesis and cleaning procedures are the same as those in Control Example 1.

[0032] 0.2 g of the obtained polymer was added to chloroform solvent to prepare a casting solution with a mass fraction of 2.0 wt.%. After stirring for 48 hours, the casting solution was introduced into a glass watch glass, evaporated naturally for 24 hours, and then dried under vacuum conditions for 48 hours.

[0033] The obtained film was taken to a size of about 4 cm 2 The mixture was placed in a tubular furnace, and protective gas N2 was introduced. The temperature was raised to 550°C and maintained for 2 hours. The carbon molecular sieve membrane of Example 2 was obtained after the temperature was naturally lowered to room temperature.

[0034] Example 3:

[0035] Add 5.308 g of 4,4'-diamino-3,3'-dimethylbiphenyl and 8.350 g of 4,4'-(hexafluoroisopropylidene)diphenylamine to 20 mL of dimethoxymethane solution. Slowly add 85 mL of trifluoroacetic acid dropwise in an ice-water bath. The synthesis and cleaning methods are the same as those in Control Example 1.

[0036] 0.2 g of the obtained polymer was added to chloroform solvent to prepare a casting solution with a mass fraction of 2.0 wt.%. After stirring for 48 hours, the casting solution was introduced into a glass watch glass, evaporated naturally for 24 hours, and then dried under vacuum conditions for 48 hours.

[0037] The obtained film was taken to a size of about 4 cm 2 The mixture was placed in a tubular furnace, and protective gas N2 was introduced. The temperature was raised to 550°C and maintained for 2 hours. The carbon molecular sieve membrane of Example 3 was obtained after the temperature was naturally lowered to room temperature.

[0038] Example 4:

[0039] 0.2 g of the polymer obtained in Example 1 was added to chloroform solvent to prepare a casting solution with a mass fraction of 2.0 wt.%. After stirring for 48 h, the casting solution was introduced into a glass watch glass, evaporated naturally for 24 h, and then dried under vacuum conditions for 48 h.

[0040] The obtained film was taken to a size of about 4 cm 2 The mixture was placed in a tubular furnace, and protective gas N2 was introduced. The temperature was raised to 700°C and maintained for 2 hours. The carbon molecular sieve membrane of Example 4 was obtained after the temperature was naturally lowered to room temperature.

[0041] Example 5:

[0042] 0.2 g of the polymer obtained in Example 1 was added to chloroform solvent to prepare a casting solution with a mass fraction of 2.0 wt.%. After stirring for 48 h, the casting solution was introduced into a glass watch glass, evaporated naturally for 24 h, and then dried under vacuum conditions for 48 h.

[0043] The obtained film was taken to a size of about 4 cm 2The mixture was placed in a tubular furnace, and protective gas N2 was introduced. The temperature was raised to 800°C and maintained for 2 hours. The carbon molecular sieve membrane of Example 5 was obtained after the temperature was naturally cooled to room temperature.

[0044] The hydrogen / helium separation performance of the control example and examples 1, 2, and 3 at 35°C and 2 Bar is shown in Table 1. By comparing the control example 1 and example 1, and example 2 and example 3, we can find that as the degree of fluorination increases, the hydrogen permeability and selectivity of the Trog base carbon molecular sieve membrane are simultaneously improved.

[0045] The hydrogen / helium separation performances of Examples 1, 4, and 5 at 35°C and 2 Bar are shown in Table 2. By comparing Example 1, Example 2, and Example 3, we can find that as the pyrolysis temperature increases, the hydrogen permeability of the Trog base carbon molecular sieve membrane decreases and the selectivity further improves.

[0046] The propylene / propane separation performance of the control example and examples 1, 2, and 3 at 35°C and 4 Bar is shown in Table 3. By comparing the control example 1 and example 1, and example 2 and example 3, we can find that as the pyrolysis temperature increases, as the degree of fluorination increases, the propylene permeability and selectivity of the Trog base carbon molecular sieve membrane are simultaneously improved.

[0047] Table 1 shows the hydrogen / helium permeability and selectivity of the carbon molecular sieve membranes prepared in the control example and Example 1, Example 2 and Example 3.

[0048]

[0049] Table 2 shows the hydrogen / helium permeability and selectivity of the carbon molecular sieve membranes prepared in the control example, Example 4 and Example 5.

[0050]

[0051] Table 3 shows the propylene / propane permeability and selectivity of the carbon molecular sieve membranes prepared in the control example, Example 1, Example 2 and Example 3.

[0052]

Claims

1. A method for preparing a fluorine-containing Trog base carbon molecular sieve membrane with high gas separation performance, characterized in that: The following steps are involved: (1) Preparation of fluorotrog base polymer: Monomer A and monomer B are added to a dimethoxymethane solution; trifluoroacetic acid is slowly added dropwise to the system under ice-water bath conditions; the reaction mixture is stirred at 25°C, and ammonia is added to terminate the reaction. The obtained product is washed with solvent C several times, redissolved with solvent D, reprecipitated with methanol, and filtered and dried to obtain a fluorotrog base polymer; (2) Preparation of fluorotrog base precursor film: dissolve the fluorotrog base polymer in chloroform, stir, and introduce the casting solution into a glass watch glass. After complete natural evaporation, dry under vacuum conditions to obtain a polymer film. (3) Preparation of fluorinated Trog base carbon molecular sieve membrane: Place the polymer membrane in a tubular furnace, introduce protective gas E, and carbonize it at 550-800 °C. Maintain the temperature for 1-3 h and then cool it to room temperature to obtain a fluorinated Trog base carbon molecular sieve membrane. The monomer A is 4,4'-diamino-3,3'-dimethylbiphenyl, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-diaminooctafluorobiphenyl or 3,3-bis(trifluoromethyl)-[1,1-biphenyl]-4,4-diamine; The monomer B is 4,4'-(hexafluoroisopropylidene)diphenylamine or 3,3',5,5'-tetramethylbenzidine.

2. The method for preparing a fluorine-containing Trog base carbon molecular sieve membrane according to claim 1, wherein: The solvent C is water or methanol.

3. The method for preparing a fluorine-containing Trog base carbon molecular sieve membrane according to claim 1, wherein: The solvent D is one of chloroform, N-methylpyrrolidone, and N,N-dimethylformamide, or a mixture of two or more thereof.

4. The method for preparing a fluorine-containing Trog base carbon molecular sieve membrane according to claim 1, wherein: The protective gas E is nitrogen, argon or helium.

5. The method for preparing a fluorine-containing Trog base carbon molecular sieve membrane according to claim 1, wherein: In step (1), the molar ratio of monomer A to monomer B is 1:0.2-0.5; the molar ratio of monomer A to dimethoxymethane is 1:4-6; the molar ratio of monomer A to trifluoroacetic acid is 1:20-30; the stirring time is 48-168 h; and the volume ratio of ammonia water to trifluoroacetic acid is 5:

1.

6. The method for preparing a fluorine-containing Trog base carbon molecular sieve membrane according to claim 1, wherein: In step (2), the mass fraction of the casting solution is 1.0-20.0 wt.%; and the stirring time is 12-48 h.

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