Polyimide film composite material, method for preparing same, and use thereof

Abstract

The application relates to the technical field of gas filtering materials, in particular to a polyimide film composite material and a preparation method and application thereof, the composite material is mainly made of polyimide and Zn / g-C3N4, Zn / g-C3N4 accounts for 10-20 wt% of the mass of the polyimide; the preparation method is that the polyimide is stirred in a solvent to obtain a polyimide casting solution, then Zn / g-C3N4 is poured into the polyimide casting solution, stirring is carried out at 60-80 DEG C for 8-12 h, cooling is carried out to room temperature for 1-4 h, the defoaming casting solution is poured on a glass plate, a film with a thickness of 50-80 mu m is scraped out by using a coating machine, the scraped film is dried under vacuum at 100-120 DEG C for 8-12 h, peeling is carried out, annealing is carried out at 200-230 DEG C for 8-12 h, the obtained Zn / g-C3N4 composite polyimide film is placed in a plasma reactor for plasma treatment, and the polyimide film composite material is obtained. The polyimide film composite material can be used for efficient purification of helium, high selectivity is achieved while high flux is met, and high stability is also achieved.

Description

Technical Field

[0001] This invention relates to the field of gas filtration materials technology, and in particular to polyimide membrane composite materials, their preparation methods and applications. Background Technology

[0002] Helium is a rare inert gas that plays an irreplaceable role in defense, advanced medical care, electronics manufacturing, and large-scale scientific facilities. Currently, helium purification primarily relies on natural gas. Natural gas tail gas has a very complex composition; methane, nitrogen, and helium molecules have very similar molecular kinetic diameters, making helium extraction extremely difficult. Existing helium extraction processes are divided into cryogenic and non-cryogenic methods. Cryogenic methods consume more energy, increasing extraction costs. Non-cryogenic methods mainly include pressure swing adsorption (PSA), absorption, and membrane permeation separation, with membrane permeation separation considered the most promising method for helium separation. Therefore, developing membranes that simultaneously offer high throughput and high selectivity for helium extraction is of great significance.

[0003] Membrane permeation separation technology can separate desired gases under relatively mild conditions and requires less energy. However, it is usually limited by the Robeson upper limit, meaning that while the selectivity requirement is met, the flux is typically low, and the selectivity is substandard when the flux is high. Therefore, many researchers have conducted extensive modification work on membrane materials in hopes of breaking through the Robeson upper limit. Patent CN115970523A discloses an ultrathin copolyimide gas separation membrane, which uses a one-step phase inversion method to prepare a polyimide membrane with a selectivity layer thickness of 197 nm. The helium flux is as high as 120 GPU, and it has long stability (>200 h), but its He / N2 and He / CH4 selectivity are not high, only 44 and 67, respectively. Patent CN113731184A prepares a polyimide membrane with MOF and dopamine as functional layers, with a helium flux of 98.2 GPU and a He / CH4 selectivity of 426.96, but its membrane flux is low. Therefore, developing a highly selective, high-throughput helium separation membrane would be of great significance. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a polyimide membrane composite material, a method for preparing the same, and its application, so as to at least solve the problem of poor helium separation effect caused by low membrane flux and poor selectivity in the prior art.

[0005] The present invention solves the above-mentioned technical problems through the following technical means:

[0006] In a first aspect, the present invention provides a polyimide film composite material, the composite material being mainly composed of polyimide and Zn / g-C3N4, wherein the Zn / g-C3N4 accounts for 10-20 wt% of the mass of the polyimide.

[0007] In conjunction with the first aspect, in some embodiments, the zinc in the Zn / g-C3N4 accounts for 10-30 wt% of the mass of g-C3N4.

[0008] Secondly, the present invention provides a method for preparing a polyimide film composite material, comprising the following steps:

[0009] To prepare Zn / g-C3N4, g-C3N4 was weighed and added to a zinc salt solution. The solution was stirred at room temperature for 6–12 h, then placed in an oven at 60–80 °C for 12–24 h. The solution was then transferred to a muffle furnace and heated from room temperature to 300–450 °C at a rate of 5–10 °C / min. The temperature was held for 4–6 h, and after cooling, the solution was ground to obtain Zn / g-C3N4.

[0010] To prepare a Zn / g-C3N4 composite polyimide membrane, polyimide was added to a solvent and stirred at 60–80°C for 8–12 h to obtain a polyimide casting solution. Zn / g-C3N4 was then poured into the polyimide casting solution and stirred at 60–80°C for 8–12 h. After cooling to room temperature for degassing for 1–4 h, the degassed casting mixture was poured onto a glass plate and coated with a film of 50–80 μm thickness using a coating machine. The coated film was then dried under vacuum at 100–120°C for 8–12 h, peeled off, and annealed at 200–230°C for 8–12 h. The resulting Zn / g-C3N4 composite polyimide membrane was then subjected to plasma treatment in a plasma reactor to obtain the polyimide membrane composite material.

[0011] In conjunction with the second aspect, in some embodiments, the preparation method of the g-C3N4 is as follows: the carbon nitride precursor is placed in a muffle furnace and heated from room temperature to 500-550°C at a rate of 5-10°C / min, held at that temperature for 4-6 hours, and then cooled and ground in the furnace to obtain g-C3N4.

[0012] In conjunction with the second aspect, in some embodiments, the carbon nitride precursor is any one of melamine, urea, and cyanamide.

[0013] In conjunction with the second aspect, in some embodiments, the concentration of the zinc salt solution is 1-10 wt%, the zinc salt is any one of zinc nitrate, zinc acetate, and zinc chloride, and the zinc in the Zn / g-C3N4 accounts for 10-30 wt% of the mass of g-C3N4.

[0014] In conjunction with the second aspect, in some embodiments, the polyimide is Matrimid 5218 or P84, the solvent is N-methylpyrrolidone or N,N-dimethylacetamide, and the polyimide accounts for 10-30% of the solvent mass.

[0015] In conjunction with the second aspect, in some embodiments, the mass ratio of the polyimide to Zn / g-C3N4 is 1:(0.15 to 0.2).

[0016] In conjunction with the second aspect, in some embodiments, the plasma treatment process is as follows: using helium as the working gas, under the conditions of a discharge frequency of 10–30 kHz, a discharge voltage of 10–20 kV, and a discharge time of 5–10 min, the Zn / g-C3N4 composite polyimide membrane is treated with helium plasma.

[0017] Thirdly, the present invention provides the application of the above-mentioned polyimide membrane composite material in the separation and extraction of helium.

[0018] The polyimide membrane composite material of this invention is made by combining Zn / g-C3N4 with a polyimide membrane and then subjecting the membrane surface to helium plasma treatment. The two-dimensional layered structure of Zn / g-C3N4 significantly improves the membrane flux, and the subsequent plasma treatment enhances the molecular sieving effect and improves helium selectivity. This polyimide membrane composite material can be used for the efficient purification of helium, achieving both high flux and high selectivity, as well as high stability.

[0019] Tests showed that the polyimide membrane composite material of the present invention exhibits strong helium permeability, reaching 180 GPU, while the permeability to CH4 and N2 is less than 1 GPU. Furthermore, in the selective gas mixture test, the plasma-modified Zn / g-C3N4 composite polyimide membrane showed a helium selectivity of up to 363 in the He / N2 system and up to 402 in the He / CH4 system. Attached Figure Description

[0020] Figure 1 This is a stability test chart of the polyimide film composite material prepared in Example 2.

[0021] Figure 2 This is a stability test chart of the polyimide film composite material prepared in Example 5.

[0022] Figure 3 This is a stability test chart of the polyimide film composite material prepared in Comparative Example 3. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Unless otherwise specified in the following examples, the conditions are as per standard conditions or the manufacturer's recommendations. Raw materials, equipment, or instruments whose manufacturers are not specified are all commercially available products.

[0025] The preparation method of the polyimide film composite material of the present invention includes the following steps:

[0026] (1) Preparation of Zn / g-C3N4

[0027] The carbon nitride precursor was placed in a muffle furnace and heated from room temperature to 500–550°C at a rate of 5–10°C / min, held for 4–6 hours, cooled, and ground to obtain g-C3N4. A zinc salt solution with a mass fraction of 1–10 wt% was prepared and poured into the prepared g-C3N4. The mixture was stirred at room temperature for 6–12 hours, then dried in an oven at 60–80°C for 12–24 hours. It was then placed in a muffle furnace and heated from room temperature to 300–450°C at a rate of 5–10°C / min, held for 4–6 hours, cooled, and ground to obtain Zn / g-C3N4. The carbon nitride precursor was any one of melamine, urea, or cyanamide, and the zinc salt was any one of zinc nitrate, zinc acetate, or zinc chloride.

[0028] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0029] Polyimide was added to the solvent in small batches and stirred at 60–80°C for 8–12 hours to obtain a polyimide casting solution. The prepared Zn / g-C3N4 was then added to the polyimide casting solution and stirred at 60–80°C for 8–12 hours. After cooling, the solution was placed in a vacuum oven for degassing for 1–4 hours. The degassed casting solution was then poured onto a glass plate and coated with a 50–80 μm film using a coating machine. The coated film was then dried under vacuum at 100–120°C for 8–12 hours. After peeling off the film, it was annealed at 200–230°C for 8–12 hours to obtain a Zn / g-C3N4 composite polyimide film. The polyimide is either Matrimid 5218 or P84, and the solvent is either N-methylpyrrolidone (NMP) or N,N-dimethylacetamide (DMAc). The mass of the polyimide accounts for 10 to 30 wt% of the mass of the solvent.

[0030] (3) Preparation of plasma-modified Zn / g-C3N4 composite polyimide membrane

[0031] A Zn / g-C3N4 composite polyimide membrane was placed in a plasma reactor. Helium was used as the working gas. Under the conditions of a discharge frequency of 10–30 kHz, a discharge voltage of 10–20 kV, and a discharge time of 5–10 min, the Zn / g-C3N4 composite polyimide membrane was treated with helium plasma to obtain a plasma-modified Zn / g-C3N4 composite polyimide membrane, i.e., a polyimide membrane composite material.

[0032] Example 1

[0033] The preparation method of the polyimide film composite material in this embodiment is as follows:

[0034] (1) Preparation of Zn / g-C3N4

[0035] 20g of melamine was placed in a muffle furnace and heated from room temperature to 500℃ at a rate of 5℃ / min, and held for 4 hours. After cooling, it was ground to obtain g-C3N4. A zinc nitrate solution with a mass fraction of 1wt% was prepared. 1.72g of g-C3N4 was poured into 50g of zinc nitrate solution and stirred at room temperature for 6 hours. Then it was placed in an oven and dried at 60℃ for 12 hours. After that, it was placed in a muffle furnace and heated from room temperature to 300℃ at a rate of 5℃ / min, and held for 4 hours. After cooling, it was ground to obtain Zn / g-C3N4 with zinc accounting for 10wt% of the mass of g-C3N4.

[0036] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0037] 2g of Matrimid 5218 was poured into 20g of NMP in small batches and stirred at 60℃ for 8h to obtain Matrimid 5218 casting solution. Then, 0.2g of Zn / g-C3N4 was poured into the Matrimid 5218 casting solution and stirred at 60℃ for 8h. After cooling to room temperature, it was placed in a vacuum oven for degassing for 1h. The degassed casting solution was then poured onto a glass plate and coated with a 50μm film using a coating machine. The coated film was then dried under vacuum at 100℃ for 8h. After peeling off the film, it was annealed at 200℃ for 8h to obtain a Zn / g-C3N4 composite polyimide film.

[0038] (3) Preparation of plasma-modified Zn / g-C3N4 composite polyimide membrane

[0039] A Zn / g-C3N4 composite polyimide membrane was placed in a plasma reactor. Using helium as the working gas, the Zn / g-C3N4 composite polyimide membrane was treated with helium plasma under the conditions of a discharge frequency of 10 kHz, a discharge voltage of 10 kV, and a discharge time of 5 min, to obtain a plasma-modified Zn / g-C3N4 composite polyimide membrane, i.e., a polyimide membrane composite material.

[0040] Example 2

[0041] The preparation method of the polyimide film composite material in this embodiment is as follows:

[0042] (1) Preparation of Zn / g-C3N4

[0043] 20g of urea was placed in a muffle furnace and heated from room temperature to 550℃ at a rate of 10℃ / min, and held for 6 hours. After cooling, it was ground to obtain g-C3N4. A zinc acetate solution with a mass fraction of 10wt% was prepared. 5.9g of g-C3N4 was poured into 50g of zinc acetate solution and stirred at room temperature for 12 hours. Then it was placed in an oven and dried at 80℃ for 24 hours. Then it was placed in a muffle furnace and heated from room temperature to 450℃ at a rate of 10℃ / min, and held for 6 hours. After cooling, it was ground to obtain Zn / g-C3N4 with zinc accounting for 30wt% of the mass of g-C3N4.

[0044] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0045] 6g of P84 was poured into 20g of DMAc in small batches and stirred at 80℃ for 12h to obtain P84 casting solution. Then, 1.2g of Zn / g-C3N4 was poured into the P84 casting solution and stirred at 80℃ for 12h. After cooling to room temperature, it was placed in a vacuum oven for degassing for 4h. The degassed casting solution was then poured onto a glass plate and coated with an 80μm film using a coating machine. The coated film was then dried under vacuum at 120℃ for 12h. After peeling off the film, it was annealed at 230℃ for 12h to obtain a Zn / g-C3N4 composite polyimide film.

[0046] (3) Preparation of plasma-modified Zn / g-C3N4 composite polyimide membrane

[0047] A Zn / g-C3N4 composite polyimide membrane was placed in a plasma reactor. Helium was used as the working gas. The Zn / g-C3N4 composite polyimide membrane was treated with helium plasma under the conditions of a discharge frequency of 30kHz, a discharge voltage of 20kV, and a discharge time of 10min to obtain a plasma-modified Zn / g-C3N4 composite polyimide membrane, i.e., a polyimide membrane composite material.

[0048] Example 3

[0049] The preparation method of the polyimide film composite material in this embodiment is as follows:

[0050] (1) Preparation of Zn / g-C3N4

[0051] 20g of cyanamide was placed in a muffle furnace and heated from room temperature to 550℃ at a rate of 5℃ / min, and held for 6 hours. After cooling, it was ground to obtain g-C3N4. A zinc chloride solution with a mass fraction of 1wt% was prepared. 2.38g of g-C3N4 was poured into 50g of zinc chloride solution and stirred at room temperature for 12 hours. Then it was placed in an oven and dried at 80℃ for 24 hours. Then it was placed in a muffle furnace and heated from room temperature to 450℃ at a rate of 10℃ / min, and held for 6 hours. After cooling, it was ground to obtain Zn / g-C3N4 with zinc accounting for 10wt% of the mass of g-C3N4.

[0052] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0053] 6g of P84 was poured into 20g of DMAc in small batches and stirred at 80℃ for 12h to obtain P84 casting solution. Then, 1.2g of Zn / g-C3N4 was poured into the P84 casting solution and stirred at 80℃ for 12h. After cooling to room temperature, it was placed in a vacuum oven for degassing for 4h. The degassed casting solution was then poured onto a glass plate and coated with an 80μm film using a coating machine. The coated film was then dried under vacuum at 120℃ for 12h. After peeling off the film, it was annealed at 230℃ for 12h to obtain a Zn / g-C3N4 composite polyimide film.

[0054] (3) Preparation of plasma-modified Zn / g-C3N4 composite polyimide membrane

[0055] A Zn / g-C3N4 composite polyimide membrane was placed in a plasma reactor. Helium was used as the working gas. The Zn / g-C3N4 composite polyimide membrane was treated with helium plasma under the conditions of a discharge frequency of 30kHz, a discharge voltage of 20kV, and a discharge time of 10min to obtain a plasma-modified Zn / g-C3N4 composite polyimide membrane, i.e., a polyimide membrane composite material.

[0056] Example 4

[0057] The preparation method of the polyimide film composite material in this embodiment is as follows:

[0058] (1) Preparation of Zn / g-C3N4

[0059] 20g of melamine was placed in a muffle furnace and heated from room temperature to 550℃ at a rate of 5℃ / min, and held for 6 hours. After cooling, it was ground to obtain g-C3N4. A zinc chloride solution with a mass fraction of 1wt% was prepared. 2.38g of g-C3N4 was poured into 50g of zinc chloride solution and stirred at room temperature for 12 hours. Then it was placed in an oven and dried at 80℃ for 24 hours. Then it was placed in a muffle furnace and heated from room temperature to 300℃ at a rate of 5℃ / min, and held for 6 hours. After cooling, it was ground to obtain Zn / g-C3N4 with zinc accounting for 10wt% of the mass of g-C3N4.

[0060] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0061] 4g of Matrimid 5218 was poured into 20g of NMP in small batches and stirred at 80℃ for 12h to obtain Matrimid 5218 casting solution. Then, 0.6g of Zn / g-C3N4 was poured into the Matrimid 5218 casting solution and stirred at 80℃ for 12h. After cooling to room temperature, it was placed in a vacuum oven for degassing for 4h. The degassed casting solution was then poured onto a glass plate and coated with a 50μm film using a coating machine. The coated film was then dried under vacuum at 120℃ for 12h. After peeling off the film, it was annealed at 230℃ for 12h to obtain a Zn / g-C3N4 composite polyimide film.

[0062] (3) Preparation of plasma-modified Zn / g-C3N4 composite polyimide membrane

[0063] A Zn / g-C3N4 composite polyimide membrane was placed in a plasma reactor. Helium was used as the working gas. The Zn / g-C3N4 composite polyimide membrane was treated with helium plasma under the conditions of a discharge frequency of 30kHz, a discharge voltage of 20kV, and a discharge time of 10min to obtain a plasma-modified Zn / g-C3N4 composite polyimide membrane, i.e., a polyimide membrane composite material.

[0064] Example 5

[0065] The preparation method of the polyimide film composite material in this embodiment is as follows:

[0066] (1) Preparation of Zn / g-C3N4

[0067] 20g of melamine was placed in a muffle furnace and heated from room temperature to 550℃ at a rate of 5℃ / min, and held for 6 hours. After cooling, it was ground to obtain g-C3N4. A zinc chloride solution with a mass fraction of 5wt% was prepared. 7.95g of g-C3N4 was poured into 50g of zinc chloride solution and stirred at room temperature for 12 hours. Then it was placed in an oven and dried at 80℃ for 24 hours. Then it was placed in a muffle furnace and heated from room temperature to 300℃ at a rate of 5℃ / min, and held for 6 hours. After cooling, it was ground to obtain Zn / g-C3N4 with zinc accounting for 15wt% of the mass of g-C3N4.

[0068] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0069] 4g of Matrimid 5218 was poured into 20g of NMP in small batches and stirred at 80℃ for 12h to obtain Matrimid 5218 casting solution. Then, 0.6g of Zn / g-C3N4 was poured into the Matrimid 5218 casting solution and stirred at 80℃ for 12h. After cooling to room temperature, it was placed in a vacuum oven for degassing for 4h. The degassed casting solution was then poured onto a glass plate and coated with a 50μm film using a coating machine. The coated film was then dried under vacuum at 110℃ for 12h. After peeling off the film, it was annealed at 230℃ for 12h to obtain a Zn / g-C3N4 composite polyimide film.

[0070] (3) Preparation of plasma-modified Zn / g-C3N4 composite polyimide membrane

[0071] A Zn / g-C3N4 composite polyimide membrane was placed in a plasma reactor. Helium was used as the working gas. The Zn / g-C3N4 composite polyimide membrane was treated with helium plasma under the conditions of a discharge frequency of 30kHz, a discharge voltage of 20kV, and a discharge time of 10min to obtain a plasma-modified Zn / g-C3N4 composite polyimide membrane, i.e., a polyimide membrane composite material.

[0072] Comparative Example 1

[0073] The preparation method of the polyimide film material in this embodiment is as follows:

[0074] 2g of Matrimid 5218 was poured into 20g of NMP in small batches and stirred at 60℃ for 8 hours to obtain Matrimid 5218 casting solution. After cooling to room temperature, the solution was placed in a vacuum oven for degassing for 1 hour. The degassed casting solution was then poured onto a glass plate and coated with a 50μm film using a coating machine. The coated film was then dried under vacuum at 100℃ for 8 hours. After peeling off the film, it was annealed at 200℃ for 8 hours to obtain a polyimide film.

[0075] Comparative Example 2

[0076] The preparation method of the polyimide film material in this embodiment is as follows:

[0077] 2g of Matrimid 5218 was poured into 20g of NMP in small batches and stirred at 60℃ for 8 hours to obtain Matrimid 5218 casting solution. After cooling to room temperature, the solution was placed in a vacuum oven for degassing for 1 hour. The degassed casting solution was then poured onto a glass plate and coated with a 50μm film using a coating machine. The coated film was then dried under vacuum at 100℃ for 8 hours. After peeling off the film, it was annealed at 200℃ for 8 hours to obtain a polyimide film.

[0078] A polyimide film was placed in a plasma reactor, with helium as the working gas. The polyimide film was treated with helium plasma under the conditions of a discharge frequency of 10 kHz, a discharge voltage of 10 kV, and a discharge time of 5 min, to obtain a plasma-modified polyimide film.

[0079] Comparative Example 3

[0080] The preparation method of the polyimide film composite material in this embodiment is as follows:

[0081] (1) Preparation of Zn / g-C3N4

[0082] 20g of melamine was placed in a muffle furnace and heated from room temperature to 500℃ at a rate of 5℃ / min, and held for 4 hours. After cooling, it was ground to obtain g-C3N4. A zinc nitrate solution with a mass fraction of 1wt% was prepared. 1.72g of g-C3N4 was poured into 50g of zinc nitrate solution and stirred at room temperature for 6 hours. Then it was placed in an oven and dried at 60℃ for 12 hours. After that, it was placed in a muffle furnace and heated from room temperature to 300℃ at a rate of 5℃ / min, and held for 4 hours. After cooling, it was ground to obtain Zn / g-C3N4 with zinc accounting for 10wt% of the mass of g-C3N4.

[0083] (2) Preparation of Zn / g-C3N4 composite polyimide film

[0084] 2g of Matrimid 5218 was poured into 20g of NMP in small batches and stirred at 60℃ for 8 hours to obtain Matrimid 5218 casting solution. Then, 0.2g of Zn / g-C3N4 was poured into the Matrimid 5218 casting solution and stirred at 60℃ for 8 hours. After cooling to room temperature, it was placed in a vacuum oven for degassing for 1 hour. The degassed casting solution was then poured onto a glass plate and coated with a 50μm film using a coating machine. The coated film was then dried under vacuum at 100℃ for 8 hours. After peeling off the film, it was annealed at 200℃ for 8 hours to obtain a Zn / g-C3N4 composite polyimide film, i.e., a polyimide film composite material.

[0085] The membrane materials prepared in Examples 1-5 and Comparative Examples 1-3 were tested for their sieving capacity, as follows:

[0086] The membrane materials prepared in Examples 1-5 and Comparative Examples 1-3 were placed in a membrane cell, and their He separation performance was tested at room temperature. The inlet gas contained He, CH4, and N2 at atmospheric pressure, and the permeate side was purged with argon gas for 20 min at a flow rate of 20 mL / min. The test results are shown in Table 1.

[0087] Table 1:

[0088]

[0089] Table 1 shows that the plasma-modified Zn / g-C3N4 composite polyimide membrane exhibits strong helium permeability, reaching 180 GPU, while its permeability to CH4 and N2 is less than 1 GPU. Furthermore, in the mixed gas selectivity test, the plasma-modified Zn / g-C3N4 composite polyimide membrane shows a helium selectivity of up to 363 in the He / N2 system and up to 402 in the He / CH4 system. In addition, combined with the test results of the comparative examples, it can be seen that plasma treatment of ordinary polyimide membranes can also significantly improve helium selectivity and permeability, but the effect is not as significant as that of the plasma-modified Zn / g-C3N4 composite polyimide membrane. Using an untreated Zn / g-C3N4 composite polyimide membrane can also significantly improve helium selectivity and permeability, even better than the plasma-treated ordinary polyimide membrane, but still far inferior to the plasma-modified Zn / g-C3N4 composite polyimide membrane. In summary, the plasma-modified Zn / g-C3N4 composite polyimide membrane of the present invention exhibits high selectivity and high permeability for helium and can be applied in the separation and purification of helium.

[0090] The membrane stability of the polyimide membrane materials prepared in Examples 2, 5, and Comparative Example 3 was tested as follows:

[0091] The prepared membrane material was placed in a membrane cell, and the He separation performance of the membrane material was tested at room temperature. The inlet gas contained He, CH4 and N2, and the pressure was 0.2 MPa. The permeate side was purged with argon gas for 20 min at a flow rate of 20 mL / min.

[0092] The test results of the membrane material in Example 2 are shown below. Figure 1 The test results for the membrane material in Example 5 are shown below. Figure 2 The test results for the membrane material in Comparative Example 3 are shown below. Figure 3 . Figures 1-3 The data show that the plasma-modified Zn / g-C3N4 composite polyimide membrane of the present invention operates smoothly and has high membrane stability.

[0093] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention. Technical aspects, shapes, and structures not described in detail in this invention are all well-known technologies.

Claims

1. A polyimide membrane composite material for helium separation, characterized in that, The composite material is mainly composed of polyimide and Zn / g-C3N4, wherein Zn / g-C3N4 accounts for 10-20 wt% of the mass of polyimide; The zinc content in the Zn / g-C3N4 is 10-30 wt% of the mass of g-C3N4; A method for preparing polyimide film composite materials includes the following steps: To prepare Zn / g-C3N4, g-C3N4 was weighed and added to a zinc salt solution. The solution was stirred at room temperature for 6–12 h, then placed in an oven at 60–80 °C for 12–24 h. The solution was then transferred to a muffle furnace and heated from room temperature to 300–450 °C at a rate of 5–10 °C / min. The temperature was held for 4–6 h, and after cooling, the solution was ground to obtain Zn / g-C3N4. To prepare a Zn / g-C3N4 composite polyimide membrane, polyimide was added to a solvent and stirred at 60–80°C for 8–12 h to obtain a polyimide casting solution. Zn / g-C3N4 was then poured into the polyimide casting solution and stirred at 60–80°C for 8–12 h. After cooling to room temperature for degassing for 1–4 h, the degassed casting mixture was poured onto a glass plate and coated with a film of 50–80 μm thickness using a coating machine. The coated film was then dried under vacuum at 100–120°C for 8–12 h, peeled off, and annealed at 200–230°C for 8–12 h. The resulting Zn / g-C3N4 composite polyimide membrane was then subjected to plasma treatment in a plasma reactor to obtain the polyimide membrane composite material.

2. The polyimide membrane composite material for helium separation according to claim 1, characterized in that, The preparation method of g-C3N4 is as follows: the carbon nitride precursor is placed in a muffle furnace and heated from room temperature to 500-550°C at a rate of 5-10°C / min, held at the temperature for 4-6 hours, and then cooled and ground in the furnace to obtain g-C3N4.

3. The polyimide membrane composite material for helium separation according to claim 2, characterized in that, The carbon nitride precursor is any one of melamine, urea, or cyanamide.

4. The polyimide membrane composite material for helium separation according to claim 3, characterized in that, The zinc salt solution concentration is 1-10 wt%, and the zinc salt is any one of zinc nitrate, zinc acetate, and zinc chloride. The zinc in the Zn / g-C3N4 accounts for 10-30 wt% of the mass of g-C3N4.

5. The polyimide membrane composite material for helium separation according to claim 4, characterized in that, The polyimide is Matrimid 5218 or P84, the solvent is N-methylpyrrolidone or N,N-dimethylacetamide, and the polyimide accounts for 10-30% of the solvent mass.

6. The polyimide membrane composite material for helium separation according to claim 5, characterized in that, The mass ratio of the polyimide to Zn / g-C3N4 is 1:(0.15~0.2).

7. The polyimide membrane composite material for helium separation according to claim 6, characterized in that, The plasma treatment process is as follows: using helium as the working gas, under the conditions of a discharge frequency of 10-30kHz, a discharge voltage of 10-20kV, and a discharge time of 5-10min, helium plasma is used to treat the Zn / g-C3N4 composite polyimide film.

8. The application of the polyimide membrane composite material according to any one of claims 1-7 in the separation and extraction of helium.

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