Method for preparing intermediate-layer-free ionic liquid gel thin film composite film based on water-soluble sacrificial layer

Abstract

The invention relates to the technical field of preparation of polymer functional materials, in particular to a method for preparing an intermediate-layer-free ionic liquid gel thin film composite membrane based on a water-soluble sacrificial layer, ionic liquid gel is a membrane material with excellent CO2 separation performance, and the gel is subjected to thin film preparation, so that the transfer resistance of the gel can be reduced, and the gas permeation rate can be increased; a precursor solution of a membrane material is prevented from entering a porous supporting layer during preparation of a thin film. According to the preparation method, tough ionic liquid gel is prepared from a cross-linkable polymer under an open condition, and a gel precursor solution is spin-coated on the surface of a sacrificial layer prepared from a water-soluble polymer, so that a novel thin-film composite film only consisting of a porous support layer and a gel separation layer can be obtained; the minimum thickness of the prepared ionic liquid gel separation layer can reach 559nm, and the composite membrane shows excellent CO2 separation performance and good separation stability.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of polymer functional materials, and particularly relates to a method for preparing an ionic liquid gel thin film composite membrane without an intermediate layer based on a water-soluble sacrificial layer. Background Art

[0002] Membrane separation is a new carbon capture technology that has attracted much attention and has many advantages such as simple operation, low energy consumption, low pollution, and easy large-scale application. The flue gas emitted by industrial production is the main source of carbon emissions and is mainly composed of carbon dioxide (CO2) and nitrogen (N2). Compared with the requirements of the membrane performance for the application scenario of flue gas carbon capture, most CO2 separation membrane materials have key problems that need to be solved urgently, such as insufficient CO2 permeability and CO2 / N2 permeation selectivity, poor separation stability, and high cost of membrane materials. Therefore, the membrane material is the core of the development of this technology and determines the feasibility of this technology. Secondly, the thin film preparation of the membrane material to reduce the mass transfer resistance in the separation process is the only way for the separation membrane to move towards high efficiency and large-scale application. Therefore, designing and developing separation membrane materials with excellent separation performance and realizing their thin film preparation are the development trends in this research field.

[0003] Conventional CO2 separation membrane materials include polymer membrane materials, inorganic membrane materials, mixed matrix membrane materials, etc. Each has its own advantages in CO2 separation applications, and at the same time, each has serious defects that restrict its large-scale application in actual production. In recent years, an ionic liquid gel membrane material composed of an ionic liquid and a polymer network has attracted much attention due to its excellent separation performance and stability.

[0004] An ionic liquid is a non-volatile liquid salt with good thermal stability, having high CO2 solubility, high dissolution selectivity relative to other gases, and CO2 diffusion performance far higher than that of traditional polymer materials. The ionic liquid gel can fix a large amount of ionic liquid into a gel state through a small amount of polymer network, which can not only maintain the excellent CO2 separation performance of the ionic liquid but also overcome the defects of the unstable flow state, no mechanical properties, and poor processability of the ionic liquid. The ionic liquid gel membrane is not restricted by problems such as the mutual restriction of permeation performance and selectivity performance and physical aging existing in traditional polymer membrane materials, and the preparation and large-scale production of the ionic liquid gel membrane are simpler than those of inorganic membrane materials. Therefore, the ionic liquid gel membrane has great development significance and good application prospects in the field of separation membranes.

[0005] Under the existing technical conditions, there are still bottlenecks in the thin film preparation of the gel membrane.

[0006] (1) First of all, the mechanical strength and stability of the gel are still relatively poor compared with traditional membrane materials. When the thickness is too low, it is prone to breakage and form defects. Secondly, most of the reported ionic liquid gel membrane materials achieve gelation by in-situ polymerization of monomers in ionic liquids to form polymer networks, which requires a relatively high reaction environment. Most of them need to be carried out in a closed space to avoid the oxidation of free radicals by oxygen, while conventional thin film preparation methods such as spin coating, dip coating, blade coating, roll-to-roll, etc. all need to be operated in an open space. Therefore, the preparation of thin films of ionic gel membranes still faces challenges.

[0007] (2) Secondly, in the conventional preparation of thin films of CO2 separation membranes, the membrane material is prepared on a substrate composed of a porous support layer and a dense intermediate layer. Although the dense intermediate layer can prevent the casting solution from infiltrating into the porous support layer during the thin film preparation process, its relatively large gas mass transfer resistance usually severely restricts the overall CO2 permeation rate and permeation selectivity of the composite membrane.

[0008] Therefore, due to the limitations of the material itself properties or preparation processes, there are few reports on the development and research of ultra-thin ionic liquid gel membranes with high permeation rates. Summary of the Invention

[0009] The purpose of the present invention is to solve the disadvantages existing in the prior art, and to propose a method for preparing a composite membrane of an ionic liquid gel thin film without an intermediate layer.

[0010] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0011] Design a method for preparing a composite membrane of an ionic liquid gel thin film without an intermediate layer based on a water-soluble sacrificial layer, specifically including the following steps:

[0012] S1. Prepare a tough ionic liquid gel under open conditions in contact with air through a crosslinkable polymer;

[0013] S2. Spin-coat the precursor solution of the above ionic liquid gel on a sacrificial layer prepared from a water-soluble polymer;

[0014] S3. After the above steps are completed, heat and dry to form the gel layer, and then attach the porous support layer to the surface of the gel layer;

[0015] S4. Then immerse it in a saturated aqueous solution of ionic liquid to dissolve the sacrificial layer. When the sacrificial layer is dissolved, a new composite membrane with only the porous support layer and the gel separation layer can be obtained.

[0016] Preferably, the tough ionic liquid gel that can be prepared under open conditions can be prepared based on an interpenetrating double polymer network structure (IPN). The IPN gel consists of a physical cross-linked network formed by the crystallization of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), a chemical cross-linked network between poly(ethyl acrylate-co-N-succinimidyl acrylate) (PEA-NSA) and a cross-linking agent, and an ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]).

[0017] Preferably, the synthesis steps of the cross-linkable polymer PEA-NSA are as follows:

[0018] A1. PEA-NSA is synthesized by free radical polymerization. Seal a three-necked round-bottom flask and purge it with N2. Use a syringe to add toluene (18.0 g) into the round-bottom flask, and then add a mixture of monomer ethyl acrylate (EA) (10.0 g) and N-succinimidyl acrylate (NSA) (0.5224 g);

[0019] A2. Dissolve the initiator azobisisobutyronitrile (AIBN) (0.1038 g) in toluene (2.0 g) and add it to the flask;

[0020] A3. Under magnetic stirring, purge the solution with N2 bubbling for 30 minutes, then place the flask in an oil bath at 60 °C and continuously stir and mix for 24 hours using magnetic stirring;

[0021] A4. Drop the reacted solution into n-hexane for precipitation, and dry the obtained precipitate PEA-NSA in an oven at 60 °C for 24 hours to obtain the target polymer.

[0022] Preferably, the preparation steps of the PVDF-HFP / PEA-NSA IPN ionic liquid gel precursor solution are as follows:

[0023] B1. Stir PVDF-HFP (0.64 g) in acetone (9.24 g) until completely dissolved;

[0024] B2. Add PEA-NSA (0.62 g) to the above PVDF-HFP solution and stir and dissolve it with a magnetic stirrer for 1 hour;

[0025] B3. Subsequently, add 5.12 g of the ionic liquid [EMIM][TFSI] to the PVDF-HFP / PEA-NSA / acetone mixture and stir with magnetic stirring for 30 minutes;

[0026] B4. Dissolve 0.02 g of diethylene glycol bis(3-aminopropyl) ether in acetone (1.0 g), and add this mixture to the [EMIM][TFSI] / PVDF-HFP / PEA-NSA / acetone solution. Stir the resulting solution for 30 minutes to obtain the ionic liquid gel precursor solution.

[0027] Preferably, taking the IPN ionic liquid gel as an example, the preparation steps of the composite membrane are as follows:

[0028] C1. Treat the surface of a pre-washed and dried glass plate with air plasma for 5 minutes.

[0029] C2. Subsequently, use a spin coater to coat an aqueous solution of a water-soluble polymer (such as sodium poly(4-styrenesulfonate) (PSS-Na)) with a mass concentration of 30 wt% on the glass plate under the conditions of a rotation speed of 3000 rpm and a duration of 1 minute.

[0030] C3. Dry the glass plate coated with the PSS-Na layer on a hot plate at 120 °C for 5 minutes, and then treat its surface with air plasma for 10 seconds to increase the wettability of the IPN ionic liquid gel precursor solution on its surface.

[0031] C4. Then, use a spin coater to coat the IPN ionic liquid gel precursor solution on the surface of the PSS-Na layer under the conditions of a certain rotation speed (3000 - 7000 rpm) and a duration of 1 minute, and heat the resulting coating in an oven at 60 °C for 24 hours to form an IPN ionic liquid gel layer.

[0032] C5. Paste a hydrophobic polytetrafluoroethylene (PTFE) porous membrane on the IPN ionic liquid gel layer.

[0033] C6. Subsequently, immerse the glass plate coated with the PSS-Na layer and the gel layer in an aqueous solution saturated with [EMIM][TFSI].

[0034] Preferably, after completing step C6, that is, when the sacrificial layer dissolves in water, the PTFE porous substrate and the ionic liquid gel layer will automatically peel off from the glass plate and remain adhered to each other, that is, a composite membrane composed of a porous PTFE-based membrane and a PVDF-HFP / PEA IPN ionic liquid gel layer is obtained. Heat the obtained composite membrane in an oven at 100 °C for 24 hours to finally obtain a dry ionic liquid gel composite membrane without an intermediate layer.

[0035] Preferably, the thickness of the ionic liquid gel separation layer can be adjusted based on the following method:

[0036] D1. As the rotation speed of the spin coater increases during the spin coating of the gel layer, the thickness of the gel layer of the obtained composite film decreases significantly. When the spin coating speed increases from 3000 rpm to 7000 rpm, the thickness of the gel layer decreases from 15.9 μm to 9.3 μm. Therefore, the thickness of the gel layer can be significantly regulated by adjusting the spin coating speed.

[0037] D2. As the concentration of the gel precursor solution decreases, the thickness of the gel layer of the obtained composite film decreases significantly. When the spin coating speed is fixed at 1000 rpm and the concentration of the precursor solution ((polymer + crosslinker + ionic liquid) / total mass of the solution × 100%) decreases from 62.5 wt% to 4.17 wt%, the thickness of the gel layer decreases from 19.5 μm to 559 nm. Therefore, the thickness of the gel layer can be significantly regulated by adjusting the concentration of the gel precursor solution.

[0038] A method for preparing a thin film of an ionic liquid gel material based on an interpenetrating double polymer network structure proposed by the present invention has the beneficial effects as follows:

[0039] 1. By introducing a water-soluble sacrificial layer, the present invention overcomes the problem that the precursor solution easily penetrates into the pores of the base film during the preparation of the composite film without an intermediate layer, and successfully prepares a new composite with only a porous base film and a gel separation layer;

[0040] 2. The composite film with a gel separation layer thickness of 559 nm prepared by the present invention shows excellent CO2 separation performance, with a CO2 permeation rate of 1325 GPU (1 GPU = 3.35×10 -10 mol / (m 2 ·s·Pa)) and a CO2 / N2 permeation selectivity of 29;

[0041] 3. The composite film can withstand a transmembrane pressure difference of 200 kPa and an operating temperature of 70 °C, and can continuously operate for at least 200 hours while maintaining high separation performance, indicating good environmental tolerance and stability of the composite film. Description of the Drawings

[0042] Figure 1 It is a comparison diagram of the traditional composite film structure and the composite film developed by the present invention.

[0043] Figure 2 It is a schematic diagram of the preparation process of a high-performance ionic liquid gel composite film without an intermediate layer, where PSS-Na is the water-soluble polymer sodium poly(4-styrenesulfonate). Detailed Embodiments

[0044] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0045] A method for preparing a thin film of an ionic liquid gel material based on an interpenetrating double polymer network structure, specifically including the following steps:

[0046] S1. Develop a tough ionic liquid gel that can be prepared under open conditions based on an interpenetrating double network (IPN) structure;

[0047] The gel is composed of a physical cross-linked network formed by the crystallization of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), a chemical cross-linked network between polyethyl acrylate-co-N-succinimidyl acrylate (PEA-NSA) and a cross-linking agent, and an ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]);

[0048] Synthesis of cross-linkable polymer PEA-NSA:

[0049] PEA-NSA is synthesized by free radical polymerization. Seal a three-necked round bottom flask and purge it with N2. Use a syringe to add toluene (18.0 g) into the round bottom flask, and then add a mixture of ethyl acrylate (EA) (10.0 g) and N-succinimidyl acrylate (NSA) (0.5224 g). Dissolve the initiator azobisisobutyronitrile (AIBN) (0.1038 g) in toluene (2.0 g) and add it to the flask. Under magnetic stirring, purge the solution with N2 bubbling for 30 minutes. Then place the flask in an oil bath at 60 °C and heat it, and continuously mix it with magnetic stirring for 24 hours. Precipitate the obtained reaction solution in n-hexane. The obtained precipitate is PEA-NSA. Dry it in an oven at 60 °C for 24 hours to obtain the dried target polymer.

[0050] Preparation of PVDF-HFP / PEA-NSA IPN ionic liquid gel precursor solution:

[0051] Stir PVDF-HFP (0.64 g) in acetone (9.24 g) until it is completely dissolved. Add PEA-NSA (0.62 g) to the above PVDF-HFP solution and stir to dissolve it with a magnetic stirrer for 1 hour. Subsequently, add 5.12 g of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) to the PVDF-HFP / PEA-NSA / acetone mixture and stir magnetically for 30 min. Dissolve 0.02 g of diethylene glycol bis(3-aminopropyl) ether in acetone (1.0 g) and add it to the [EMIM][TFSI] / PVDF-HFP / PEA-NSA / acetone solution, and stir the resulting solution for 30 minutes to obtain an ionic liquid gel precursor solution. The precursor solution can be heated in an oven at 60 °C for 24 hours to form a tough ionic liquid gel.

[0052] The physical cross-linking of PVDF-HFP can endow the gel with excellent mechanical properties, and the good compatibility between the PEA-NSA chemical cross-linking network and the ionic liquid can endow the gel with good ionic liquid retention ability to prevent ionic liquid leakage. Since the gel is prepared entirely from hydrophobic raw materials, it has good underwater stability. As follows Figure 2 shown.

[0053] S2. Spin-coat the precursor solution of the above IPN ionic liquid gel on the water-soluble sacrificial layer: Treat the surface of a pre-washed and dried glass plate with air plasma for 5 minutes. Subsequently, use a spin coater to coat an aqueous solution of sodium poly(4-styrenesulfonate) (PSS-Na) with a mass concentration of 30 wt% on the glass plate under the conditions of a rotation speed of 3000 rpm and a duration of 1 minute, and dry the glass plate coated with the PSS-Na layer on a hot plate at 120 °C for 5 minutes. Then treat the surface of PSS-Na with air plasma for 10 seconds to increase the wettability of the IPN ionic liquid gel precursor solution on its surface. Then use a spin coater to coat the IPN ionic liquid gel precursor solution on the surface of the PSS-Na layer under the conditions of a certain rotation speed (3000 - 7000 rpm) and a duration of 1 minute, and heat the resulting coating in an oven at 60 °C for 24 hours to form an IPN ionic liquid gel layer.

[0054] S3. After the above steps are completed, heat and dry to form the gel layer, and then paste a hydrophobic polytetrafluoroethylene (PTFE) porous membrane on the IPN ionic liquid gel layer.

[0055] S4. Immerse the glass plate coated with the PSS-Na layer and the gel layer in an aqueous solution saturated with the dissolution of [EMIM][TFSI]. When the sacrificial layer dissolves in water, the PTFE porous substrate and the ionic liquid gel layer will automatically peel off from the glass plate and remain adhered to each other. After drying the obtained composite film in an oven at 100 °C for 24 hours, a composite film composed of a porous PTFE-based membrane and a PVDF-HFP / PEAIPN ionic liquid gel layer as shown in Figure 1 (b) is obtained, successfully overcoming the resistance of the intermediate layer of the traditional thin film composite membrane to gas mass transfer.

[0056] Since PSS-Na has good compatibility with multiple ionic liquids with high affinity for CO2, this thin film preparation method is expected to be extended to the thin film preparation of other ionic liquid gels, that is, this new thin film preparation method is expected to become a general strategy for the thin film preparation of ionic liquid gels.

[0057] As mentioned above, only the preferred specific embodiments of the present invention are described, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A method for preparing an ionic liquid gel thin film composite membrane without an intermediate layer based on a water-soluble sacrificial layer, characterized in that: It includes the following steps: S1. Prepare a tough ionic liquid gel under open conditions in contact with air through a crosslinkable polymer; S2. Spin-coat the precursor solution of the above ionic liquid gel onto a sacrificial layer prepared from a water-soluble polymer; S3. After the above steps are completed, heat and dry to form the gel layer, and then attach a porous support layer to the surface of the gel layer; S4. Then immerse it in a saturated aqueous solution of an ionic liquid to dissolve the sacrificial layer. When the sacrificial layer is dissolved, a novel composite membrane with only a porous support layer and a gel separation layer can be obtained.

2. The method for preparing an ionic liquid gel thin film composite membrane without an intermediate layer based on a water-soluble sacrificial layer according to claim 1, wherein The tough ionic liquid gel that can be prepared under open conditions can be realized based on an interpenetrating double polymer network structure (IPN). This IPN gel consists of a physical crosslinking network formed by the crystallization of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), a chemical crosslinking network between polyethyl acrylate-co-N-succinimidyl acrylate (PEA-NSA) and a crosslinking agent, and an ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]).

3. The IPN ionic liquid gel according to claim 2, wherein: The synthesis steps of the crosslinkable polymer PEA-NSA are as follows: A1. PEA-NSA is synthesized by free radical polymerization. Seal a three-necked round bottom flask and purge it with N2. Use a syringe to add toluene (18.0 g) into the round bottom flask, and then add a mixture of monomer ethyl acrylate (EA) (10.0 g) and N-succinimidyl acrylate (NSA) (0.5224 g); A2. Dissolve the initiator azobisisobutyronitrile (AIBN) (0.1038 g) in toluene (2.0 g) and add it to the flask; A3. Under magnetic stirring, use N2 to bubble and purge the solution for 30 minutes, then place the flask in an oil bath at 60 °C and heat it, and continuously stir and mix it with magnetic stirring for 24 hours; A4. Drop the reacted solution into n-hexane for precipitation, and dry the obtained precipitate PEA-NSA in an oven at 60 °C for 24 hours to obtain the target polymer.

4. The IPN ionic liquid gel according to claim 2, wherein The preparation steps of its precursor solution are as follows: B1. Stir PVDF-HFP (0.64 g) in acetone (9.24 g) until it is completely dissolved; B2. Add PEA-NSA (0.62 g) to the above PVDF-HFP solution and stir and dissolve it with a magnetic stirrer for 1 hour; B3. Subsequently, add 5.12 g of the ionic liquid [EMIM][TFSI] to the PVDF-HFP / PEA-NSA / acetone mixture and stir it with magnetic stirring for 30 minutes; B4. Dissolve 0.02 g of diethylene glycol bis(3-aminopropyl) ether in acetone (1.0 g) and add this mixture to the [EMIM][TFSI] / PVDF-HFP / PEA-NSA / acetone solution, and stir the obtained solution for 30 minutes to obtain the ionic liquid gel precursor solution.

5. The method for preparing an ionic liquid gel thin film composite membrane without an intermediate layer based on a water-soluble sacrificial layer according to claim 1, wherein: Taking the IPN ionic liquid gel as an example, the preparation steps of the composite membrane are as follows: C1. Treat the surface of a pre-washed and dried glass plate with air plasma for 5 minutes; C2. Subsequently, use a spin coater to coat an aqueous solution of a water-soluble polymer (such as sodium poly(4-styrenesulfonate) (PSS-Na)) with a mass concentration of 30 wt% on the glass plate under the conditions of a rotation speed of 3000 rpm and a duration of 1 minute; C3. Dry the glass plate coated with the PSS-Na layer on a hot plate at 120 °C for 5 minutes, and then treat its surface with air plasma for 10 seconds to increase the wettability of the IPN ionic liquid gel precursor solution on its surface; C4. Then, use a spin coater to coat the IPN ionic liquid gel precursor solution on the surface of the PSS-Na layer under the conditions of a certain rotation speed (3000 - 7000 rpm) and a duration of 1 minute, and heat the obtained coating in an oven at 60 °C for 24 hours to form an IPN ionic liquid gel layer; C5. Paste a hydrophobic polytetrafluoroethylene (PTFE) porous membrane on the IPN ionic liquid gel layer; C6. Subsequently, immerse the glass plate coated with the PSS-Na layer and the gel layer in an aqueous solution saturated with the dissolution of [EMIM][TFSI].

6. The process for preparing the ionic liquid gel thin film according to claim 5, characterized in that: After completing step C6, that is, when the sacrificial layer dissolves in water, the PTFE porous substrate and the ionic liquid gel layer will automatically peel off from the glass plate and remain adhered to each other, that is, a composite membrane composed of a porous PTFE base membrane and a PVDF-HFP / PEA IPN ionic liquid gel layer is obtained. Heat the obtained composite membrane in an oven at 100 °C for 24 hours to finally obtain a dry ionic liquid gel composite membrane without an intermediate layer.

7. The process for preparing the ionic liquid gel material into a thin film according to claim 5 and claim 6, characterized in that: D1. As the rotation speed of the spin coater increases during the spin coating of the gel layer, the thickness of the gel layer of the obtained composite membrane decreases significantly. When the spin coating speed increases from 3000 rpm to 7000 rpm, the thickness of the gel layer decreases from 15.9 μm to 9.3 μm. Therefore, the thickness of the gel layer can be significantly regulated by adjusting the spin coating speed. D2. As the concentration of the gel precursor solution decreases, the thickness of the gel layer of the obtained composite membrane decreases significantly. When the spin coating speed is fixed at 1000 rpm and the concentration of the precursor solution ((polymer + crosslinking agent + ionic liquid) / total mass of the solution × 100%) decreases from 62.5 wt% to 4.17 wt%, the thickness of the gel layer decreases from 19.5 μm to 559 nm. Therefore, the thickness of the gel layer can be significantly regulated by adjusting the concentration of the gel precursor solution.

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