Composite membranes with superlyophobic layer and dense sieving layer and methods of making and using the same

Abstract

The application relates to the technical field of membrane distillation, and discloses a composite membrane with a super-hydrophobic layer and a dense screening layer and a preparation method and application thereof, wherein the composite membrane comprises a PVDF hydrophobic microporous membrane and a double-sided loaded polysiloxane nanofilament super-hydrophobic layer and a polyamide dense screening layer located on both sides of the PVDF hydrophobic microporous membrane; the composite membrane is applied to a membrane distillation process in the fields of high-salinity wastewater treatment and seawater desalination, and can effectively improve the comprehensive performance of a distillation membrane. The composite membrane has good anti-fouling and anti-wetting performances.

Description

Technical Field

[0001] This invention relates to the field of membrane distillation technology, and in particular to a composite membrane having a superhydrophobic layer and a dense sieve layer, its preparation method, and its application. Background Technology

[0002] High-salinity wastewater is characterized by its wide range of sources, high salinity, complex and diverse composition, significant influence of pollution source type on water quality characteristics, poor biochemical treatment conditions, and broad environmental impact, making it a key focus and challenge in current water treatment technologies. In recent years, membrane distillation (MD) technology has developed rapidly, showing promising prospects in high-salinity wastewater treatment. MD is a non-isothermal membrane process driven by the vapor pressure difference caused by the temperature difference across a hydrophobic microporous membrane. Water vapor in the hot feed solution passes through the hydrophobic membrane pores and condenses on the permeate side, while salts and other non-volatile components are trapped on the hot feed side, thus achieving substance separation. The main advantages of MD are: no external pressure required; no need to heat the feed solution to its boiling point; theoretically, it has a 100% rejection rate for non-volatile substances in the feed solution; it is less affected by salt content; and it can utilize low-grade heat sources such as industrial waste heat to heat the feed solution.

[0003] However, a key issue hindering the sustainable treatment of high-salinity wastewater using membrane fouling (MD) technology lies in membrane scaling and wetting. High-salinity wastewater contains a large amount of mineral salt ions (Ca... 2+ Mg 2+ Na + SiO3 2- Mineral salts undergo homogeneous and heterogeneous nucleation in the feed solution and on the membrane surface, respectively, forming "crystal clusters" and eventually developing into mineral scale. Mineral scale clogs membrane pores, increasing membrane mass transfer resistance and affecting permeate rate and quality. Furthermore, membrane wetting poses a particularly serious threat to MD processes based on hydrophobic separation membranes. Low surface tension liquids (surfactants, ethanol, and oils, etc.) reduce the surface tension of the feed solution, thus increasing the transmembrane pressure gradient (…). The pressure can easily exceed the liquid-to-gas pressure (LEP) entering the membrane pores. After membrane wetting occurs, the liquid-gas interface moves longitudinally along the membrane's cross-section, thus exacerbating thermal polarization and reducing the membrane's mass transfer driving force. Once the wetting channel is fully formed, contaminants in the feed solution will indiscriminately enter the permeate side through the "water channel," causing the MD process to fail.

[0004] Although the current development of super-hydrophobic membranes, full-hydrophobic membranes and Janus membranes exhibit advantages compared with conventional hydrophobic membranes in delaying membrane fouling and membrane wetting, in the long-term operation and continuous concentration process of MD, the salt concentration and low surface tension liquid concentration in the feed liquid gradually increase, eventually reaching saturation or even supersaturation, which inevitably still leads to the occurrence of membrane fouling and membrane wetting. Therefore, the design and development of composite membranes with excellent hydrophobic properties and dense screening structure can build a "double barrier" against membrane fouling and membrane wetting, thereby maximizing the improvement of membrane fouling and wetting process and promoting the application of MD technology. SUMMARY

[0005] The purpose of the present application is to solve the technical problems of membrane fouling and membrane wetting in the prior art hydrophobic microporous membranes for high-salinity wastewater treatment, and to provide a composite membrane with a super-hydrophobic layer and a dense screening layer and a preparation method thereof. The composite membrane is a multilayer "sandwich" structure, the middle layer is a hydrophobic polyvinylidene fluoride microporous membrane, and the upper and lower layers are a super-hydrophobic layer and a dense screening layer, respectively.

[0006] Another purpose of the present application is to provide the application of the composite membrane in high-salinity wastewater treatment.

[0007] The technical scheme adopted to achieve the purpose of the present application is:

[0008] A preparation method of a composite membrane with a super-hydrophobic layer and a dense screening layer, comprising the following steps:

[0009] Step 1: Alkali treatment of a PVDF hydrophobic microporous membrane to produce hydroxyl groups on both sides, drying to obtain a hydroxyl membrane, and then hydrolysis-polymerization reaction of trichloromethylsilane on both sides of the hydroxyl membrane to obtain a super-hydrophobic membrane with a super-hydrophobic layer of polysiloxane nanofilaments on both sides;

[0010] Step 2: Etching of the super-hydrophobic membrane obtained in step 1 in hydrogen peroxide aqueous solution to oxidize Si-CH3 in the super-hydrophobic layer of polysiloxane nanofilaments on one side of the super-hydrophobic membrane to Si-OH (thereby changing the super-hydrophobicity to hydrophilicity, facilitating subsequent interfacial polymerization of trimesoyl chloride and m-phenylenediamine) to form an oxidation side, removing the intermediate membrane, and then polymerizing and heat-crosslinking trimesoyl chloride and m-phenylenediamine on the oxidation side to form a polyamide dense screening layer, thereby obtaining a composite membrane with a super-hydrophobic layer on one side and a dense screening layer on the other side.

[0011] In the above technical solution, in step 1, the specific steps of the hydrolysis-polymerization reaction are:

[0012] The hydroxyl film is immersed in a toluene / n-heptane mixed solution containing acetic acid and water, cooled in an ice bath, then trichloromethylsilane is added to the mixed solution to induce hydrolysis-polymerization of the trichloromethylsilane, after the reaction is completed, the product is washed with n-hexane and dried under N2 flow, thus obtaining a polysiloxane nanofilament layer loaded on both sides of the super-oleophobic film.

[0013] In the above technical solution, in step 2, the specific steps of polymerization and thermal crosslinking are as follows:

[0014] The intermediate film is fixed in a frame mold, and under preset environmental temperature and humidity conditions, n-hexane solution containing trimesoyl chloride and aqueous solution containing m-phenylenediamine are sequentially added to induce interfacial polymerization and then thermal crosslinking to form a polyamide dense screening layer.

[0015] In the above technical solution, in step 1, the PVDF hydrophobic microporous film is immersed in an NaOH aqueous solution for alkaline treatment, the concentration of the NaOH aqueous solution is 3.5-7.5 M, the volume ratio of toluene to n-heptane in the toluene / n-heptane mixed solution is 1:1-2:3, the water content in each kilogram of the toluene / n-heptane mixed solution is 100-300 mg, the concentration of trichloromethylsilane in the toluene / n-heptane mixed solution is 0.013-0.023 M, the pH of the toluene / n-heptane mixed solution is 4-5, the ice bath cooling temperature is 0-5℃, and the hydrolysis-polymerization reaction time is 6-12 h.

[0016] In the above technical solution, in step 2, the preset environmental temperature is 18-22℃, the preset environmental humidity is 50-60%, the concentration of the hydrogen peroxide aqueous solution is 30-60wt%, the concentration of the n-hexane solution containing trimesoyl chloride is 0.15-0.25wt%, the concentration of the aqueous solution containing m-phenylenediamine is 1.5-2.5wt%, the interfacial polymerization time is 5-10 min, and the thermal crosslinking temperature is 60-90℃.

[0017] Another aspect of the present application also includes a composite film with a super-oleophobic layer and a dense screening layer obtained by the preparation method.

[0018] In the above technical solution, the thickness of the composite film is 120-130μm, the thickness of the PVDF hydrophobic microporous film is 117-127μm, the thickness of the super-oleophobic layer is 1.5-3.0μm, the diameter of the polysiloxane nanofilament in the super-oleophobic layer is 50-90nm, and the thickness of the dense screening layer is 80-100nm.

[0019] The pore size of the PVDF hydrophobic microporous film is 0.35-0.45μm, the pore size of the super-oleophobic layer is 0.06-0.10μm, and the pore size of the dense screening layer is 0.50-0.90nm.

[0020] The liquid entry pressure LEP of the composite membrane is > 20 bar, the water contact angle of the super-hydrophobic layer is > 150°, and the underwater oil contact angle of the dense screening layer is > 150°.

[0021] Another aspect of the present application also includes the application of the composite membrane with a super-hydrophobic layer and a dense screening layer.

[0022] In the above technical solution, the composite membrane with a super-hydrophobic layer and a dense screening layer is used as a distillation membrane in wastewater treatment.

[0023] In the above technical solution, the composite membrane with a super-hydrophobic layer and a dense screening layer is used as a distillation membrane in seawater desalination.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The composite membrane prepared by the present application is in a "sandwich" shape, the super-hydrophobic nanofilament layer has a rough reentrant structure and low surface energy, so that the liquid exists in a Cassie-Baxter state (suspended state), and a large number of "air pockets" reduce the actual contact area between the liquid and the solid membrane surface, thereby slowing down the occurrence of membrane fouling and membrane wetting.

[0026] 2. The polyamide dense screening layer of the composite membrane of the present application has a sub-nanometer pore size, which can effectively prevent ion penetration into the product water side while increasing the LEP value, thereby further enhancing the anti-fouling and anti-wetting properties of the composite membrane. The super-hydrophobic layer and the dense screening layer constitute a "double barrier" to inhibit membrane fouling and membrane wetting. At the same time, the modified PVDF-NA membrane flux is not significantly reduced, and it exhibits long-term stable selective permeation performance.

[0027] 3. The composite membrane of the present application can effectively improve the comprehensive performance of the distillation membrane when applied to the membrane distillation process in the fields of high-salinity wastewater treatment and seawater desalination. The composite membrane has good anti-fouling and anti-wetting properties, making the composite membrane durable and effective in the membrane distillation process. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a schematic diagram of the preparation process of the composite membrane of the present application.

[0029] Figure 2 is a scanning electron microscope image of a PVDF hydrophobic microporous membrane (#PVDF), a polyamide membrane (#PVDF-PA), a super-hydrophobic membrane (#PVDF-PN), and the composite membrane of the present application (#PVDF-NA).

[0030] Figure 3are the water contact angle and the underwater oil contact angle plots of the PVDF, PVDF-PA, PVDF-PN membrane surfaces and the top (#PVDF-NA-T) and bottom (#PVDF-NA-B) surfaces of PVDF-NA.

[0031] Figure 4 are DCMD anti-fouling performance tests.

[0032] Figure 5 are DCMD anti-wetting performance tests.

[0033] Figure 6 are DCMD seawater desalination performance tests. DETAILED DESCRIPTION

[0034] The application will be further described below in conjunction with specific examples. It should be understood that the specific examples described herein are intended to be illustrative only and are not intended to limit the scope of the present application.

[0035] The PVDF hydrophobic microporous membrane in the following examples or comparative examples was purchased from Merck Milipore (IPVH00010).

[0036] Example 1

[0037] A composite membrane with super-hydrophobic layer and dense sieving layer was prepared by the following method:

[0038] Step 1, the PVDF hydrophobic microporous membrane (#PVDF) was immersed in 7.5M NaOH solution to generate a large number of hydroxyl groups on the membrane surface, and then washed with a large amount of deionized water under ultrasonic to neutral (pH≈7.0) and dried at 45℃ for 15min;

[0039] Then the dried membrane was immersed in a toluene:n-heptane (1:1=v / v) mixed solution containing 150ppm water for 6h (acetic acid was used to adjust the pH=4.5, ice bath temperature control T=2℃), and the mixed solution contained 0.017M trichloromethylsilane; then, the sample membrane was cleaned with n-hexane solution and dried under N2 flow to obtain a super-hydrophobic membrane with a double-sided polysiloxane nanowire super-hydrophobic layer, denoted as #PVDF-PN.

[0040] Step 2, the #PVDF-PN prepared in step 1 was contacted with 40% hydrogen peroxide solution for 10 min to oxidize Si-CH3 in the polysiloxane nanowire super-hydrophobic layer to Si-OH to form an oxidized side, and then was fixed on a frame mold. The stable ambient temperature and humidity were 20℃ and 55%, respectively. 0.2wt% trimesoyl chloride in n-hexane was poured on the oxidized side of the #PVDF-PN, and the residual liquid was poured out after 2 min of immersion. Then, 2.0wt% m-phenylenediamine aqueous solution was poured, and the interfacial polymerization reaction was carried out for 5 min. Then, the sample was placed in a vacuum oven at 85℃ for thermal crosslinking for 5 min to form a dense screening layer (PA layer). Finally, a composite membrane with a "sandwich" structure was obtained, which comprises a PVDF hydrophobic microporous membrane and a super-hydrophobic layer and a dense screening layer on both sides of the membrane, respectively, and is denoted as #PVDF-NA.

[0041] The preparation process of #PVDF-NA is shown in Figure 1 The morphology of #PVDF-NA is shown in Figure 2 The upper surface of #PVDF-NA has a large number of nanowires, which is not significantly different from #PVDF-PN. At the same time, the lower surface of #PVDF-NA (enlarged SEM image) is completely covered by the PA layer and is firmly combined, which is not significantly different from #PVDF-PA (polyamide membrane). Therefore, #PVDF-NA has the characteristics of super-hydrophobicity of #PVDF-PN and pore size screening of #PVDF-PA, forming a "double barrier" to inhibit membrane wetting, membrane fouling and membrane scaling during membrane distillation.

[0042] Comparative Example 1

[0043] A composite membrane with a super-hydrophobic layer was prepared by the following method:

[0044] Step 1, a PVDF hydrophobic microporous membrane (#PVDF) was immersed in a 7.5M NaOH solution to produce a large number of hydroxyl groups on the membrane surface, and then was washed with a large amount of deionized water under ultrasonic to neutralize (pH≈7.0), and was dried at 45℃ for 15 min;

[0045] Step 2, the dried membrane obtained in step 1 was immersed in a toluene:n-heptane (1:1=v / v) mixed solution containing 150ppm water for 6h (pH=4.5, T=2℃), and the mixed solution contained 0.017M trichloromethylsilane; then, the sample membrane was cleaned with n-hexane solution and dried under N2 flow to obtain a PVDF membrane with a super-hydrophobic layer on both sides, denoted as #PVDF-PN.

[0046] The SEM images of the surface and cross-section of #PVDF-PN are shown in Figure 2As shown in (B) and (F), a nanoporous layer formed by interwoven polysiloxane nanowires covers the surface of the microporous substrate membrane, exhibiting a hanging morphology and an inwardly curved reentrant structure. This surface morphology facilitates the formation of numerous "cavitation" structures, resulting in #PVDF-PN possessing extremely low surface energy and excellent hydrophobic properties. This effectively prevents the adhesion and adsorption of contaminants, inhibiting membrane wetting, membrane fouling, and membrane scaling.

[0047] Comparative Example 2

[0048] A composite membrane with a dense sieve layer is prepared by the following method:

[0049] PVDF hydrophobic microporous membrane (#PVDF) Figure 2 (A) and (E) are the surface and cross-sectional morphology of #PVDF. A 0.2 wt% solution of pyromellitic trimethylolpropionate chloride n-hexane was poured into one side of the surface and the solution was soaked for 2 min. The remaining liquid was then poured out. Immediately afterwards, a 2.0 wt% aqueous solution of m-phenylenediamine was poured in. After the interfacial polymerization reaction was carried out for 5 min, the surface was transferred to a vacuum oven and thermally crosslinked at 85°C for 5 min. A dense sieve layer was formed on one side of the #PVDF, which was denoted as #PVDF-PA.

[0050] pass Figure 2 The surface and cross-sectional morphology of the (C) and (G)#PVDF-PA show that the membrane pores are covered by a PA layer with a nodular fold structure. The coverage is relatively complete and the membrane is firmly bonded to the surface of the #PVDF base membrane. The #PVDF-PA membrane prepared in this way has a nanoscale pore structure, which can effectively increase the liquid inlet pressure (LEP) value of the membrane, has a size sieving effect, and the fold structure can increase the effective evaporation area of ​​the membrane.

[0051] Test Example 1

[0052] Surface wetting properties were tested on #PVDF-NA obtained in Example 1, #PVDF-PN of Comparative Example 1, #PVDF-PA of Comparative Example 2, and #PVDF. Figure 3As shown, #PVDF-NA exhibits excellent anti-wetting properties, with a water contact angle of 161.6±2.9° and an underwater oil contact angle of 119.2±7.4° on the superhydrophobic side (#PVDF-NA-T); and a water contact angle of 76.6±15.4° and an underwater oil contact angle of 143.7±9.6° on the dense sieve side (#PVDF-NA-B). #PVDF is a hydrophobic film, with a water contact angle of 140.6±6.8° and an underwater oil contact angle of 94.1±10.1°. #PVDF-PA exhibits enhanced hydrophilicity, with its water contact angle decreasing to 87.3±1.8° and its underwater oil contact angle increasing to 139.3±13.8°. Due to the hydrophobic polysiloxane nanonetwork of #PVDF-PN, its water contact angle increases to 162.7±2.1° and its underwater oil contact angle decreases to 57.7±3.3°. This demonstrates that #PVDF-NA was successfully constructed.

[0053] Test Example 2

[0054] Membrane distillation tests were performed on the #PVDF-NA obtained in Example 1, the #PVDF-PN of Comparative Example 1, the #PVDF-PA of Comparative Example 2, and the #PVDF. Specifically, the feed solution consisted of 20 wt% NaCl, and under this feed solution condition, the feed side and the permeate side were stabilized at 60°C and 20°C, respectively, for 12 hours of continuous testing.

[0055] Depend on Figure 4 The membrane distillation test results in (A) show that #PVDF-NA exhibits superior anti-scaling performance compared to #PVDF, #PVDF-PA, and #PVDF-PN. During 12 hours of DCMD operation, #PVDF-NA demonstrated stable permeate flux and salt rejection rate. With high-concentration brine as feed, the average permeate flux of the #PVDF-NA membrane was 26.31 kg·m³. -2 ·h -1 The conductivity of the produced water is stable at approximately 3 μS·cm. -1 This indicates that no membrane scaling or wetting has occurred, and the produced water is pure.

[0056] like Figure 4 Images (B), (C), (D), and (E) show the scanning electron microscopy cross-sectional morphologies of #PVDF, #PVDF-PA, #PVDF-PN, and #PVDF-NA, respectively. These images indicate that after 12 hours of testing with 20wt% NaCl solution, the different membrane cross-sections exhibit significantly different sodium chloride crystal distribution characteristics. Specifically, #PVDF-NA... Figure 4Middle (E) showed the best anti-fouling performance: only a few sparse crystals were attached on the upper surface, and there was no crystal on the middle and lower surface, which indicated that the #PVDF-NA membrane effectively inhibited the vertical migration and horizontal diffusion of crystals, and had excellent anti-fouling performance, which could be applied to membrane distillation technology to treat high-salinity wastewater in practical production.

[0057] Test Example 3

[0058] The anti-wetting performance of the #PVDF-NA obtained in Example 1, the #PVDF-PN of Comparative Example 1, the #PVDF-PA of Comparative Example 2, and the #PVDF was tested. Specifically, a feed liquid composed of 3.5wt% NaCl and different concentration gradients of ethanol. Under the condition of the feed liquid, the feed side and the permeate side were stabilized at 60°C and 20°C respectively, and the continuous test was carried out for 12h.

[0059] From Figure 5 The membrane distillation test results of Middle (A) showed that when 13.3%(v / v) ethanol was added, the water production flux of #PVDF decreased to zero within 5h, and the water production conductivity increased to 23.01μS·cm -1 , indicating that serious pore wetting occurred. Due to the excellent hydrophobicity of #PVDF-PN, the conductivity began to rise when the ethanol addition reached 20.1%(v / v), indicating that water channels formed in the partially wetted membrane pores allowed salt ions to freely enter the permeate. Relatively speaking, due to the nano-scale pore size structure of #PVDF-PA, it has a higher LEP value, and the water production conductivity does not rise until the ethanol addition in the feed liquid reaches 26.8%(v / v). #PVDF-NA has the best anti-wetting performance, and its normalized water flux decreases most slowly. When the ethanol addition is 33.5%(v / v), its water production conductivity is only 3.18μS·cm⁻¹, indicating that it still has certain membrane distillation capacity and the quality of the produced water can be guaranteed. The digital photos of the samples of #PVDF, #PVDF-PN, #PVDF-PA and #PVDF-NA after testing are shown in Figure 5 Middle (B), (C), (D) (E) show that #PVDF-NA with a "double barrier" functional layer does not become transparent, and maintains the characteristics of a relatively hydrophobic channel. Due to the excellent anti-wetting performance of #PVDF-NA membrane, it can be applied to membrane distillation technology to treat various types of wastewater in practical production.

[0060] Example 2

[0061] The performance of #PVDF-NA obtained in Example 1 was tested by actual seawater desalination (sampled from the Bohai Bay Tianjin coast). As Figure 6As shown, the membrane distillation test results show that PVDF-NA has excellent seawater desalination performance. In the 48h long-term test, PVDF-NA shows stable water flux (31.57kg·m -2 ·h -1 ) and almost complete salt rejection (>99.99%), so the PVDF-NA provided by the application can be applied to seawater desalination.

[0062] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for preparing a composite membrane having a superlyophobic layer and a dense sieving layer, characterized in that, The method comprises the following steps: Step 1: Alkali treatment is performed on a PVDF hydrophobic microporous membrane to generate hydroxyl groups on both sides of the membrane, and the membrane is dried to obtain a hydroxyl membrane. Then, hydrolysis-polymerization of trichloromethylsilane occurs on both sides of the hydroxyl membrane to obtain a super-oleophobic membrane with a super-oleophobic layer of polysiloxane nanofilaments on both sides; Step 2: The super-oleophobic membrane obtained in Step 1 is floated in a hydrogen peroxide aqueous solution for etching, so that Si-CH3 in the super-oleophobic layer of polysiloxane nanofilaments on one side of the super-oleophobic membrane is oxidized to Si-OH to form an oxidized side. The intermediate membrane is obtained by taking it out. Then, interfacial polymerization of trimesoyl chloride and m-phenylenediamine occurs on the oxidized side, and thermal crosslinking is performed to form a dense screening layer of polyamide, thereby obtaining a composite membrane with a super-oleophobic layer on one side and a dense screening layer on the other side.

2. The production method according to claim 1, wherein In Step 1, the specific steps of the hydrolysis-polymerization reaction are as follows: The hydroxyl membrane is immersed in a toluene / n-heptane mixed solution containing acetic acid and water, cooled in an ice bath, and then trichloromethylsilane is added to the mixed solution to induce hydrolysis-polymerization of trichloromethylsilane. After the reaction is completed, n-hexane is used for cleaning, and drying treatment is performed under N2 flow to obtain a super-oleophobic membrane with a polysiloxane nanofilament layer on both sides.

3. The production method according to claim 1, wherein In Step 2, the specific steps of the polymerization and thermal crosslinking are as follows: The intermediate membrane is fixed in a frame mold, and under preset environmental temperature and humidity conditions, n-hexane solution containing trimesoyl chloride and aqueous solution containing m-phenylenediamine are sequentially added to induce interfacial polymerization and then thermal crosslinking to form a dense screening layer of polyamide.

4. The production method according to claim 2, wherein In Step 1, the PVDF hydrophobic microporous membrane is immersed in an NaOH aqueous solution for alkali treatment. The concentration of the NaOH aqueous solution is 3.5-7.5 M. The volume ratio of toluene to n-heptane in the toluene / n-heptane mixed solution is 1:1-2:

3. The water content in each kilogram of the toluene / n-heptane mixed solution is 100-300 mg. The concentration of trichloromethylsilane in the toluene / n-heptane mixed solution is 0.013-0.023 M. The pH of the toluene / n-heptane mixed solution is 4-5. The ice bath cooling temperature is 0-5℃. The hydrolysis-polymerization reaction time is 6-12 h.

5. The production method according to claim 3, wherein In Step 2, the preset environmental temperature is 18-22℃, the preset environmental humidity is 50-60%, the concentration of the hydrogen peroxide aqueous solution is 30-60 wt %, the concentration of the n-hexane solution containing trimesoyl chloride is 0.15-0.25 wt %, the concentration of the aqueous solution containing m-phenylenediamine is 1.5-2.5 wt %, the interfacial polymerization time is 5-10 min, and the thermal crosslinking temperature is 60-90℃.

6. The composite membrane with a super-oleophobic layer and a dense screening layer obtained by the preparation method of claim 1.

7. The composite membrane having a superlyophobic layer and a dense sieving layer of claim 6, wherein, The thickness of the composite membrane is 120-130 μm, the thickness of the PVDF hydrophobic microporous membrane is 117-127 μm, the thickness of the super-oleophobic layer is 1.5-3.0 μm, the diameter of the polysiloxane nanofilaments in the super-oleophobic layer is 50-90 nm, and the thickness of the dense screening layer is 80-100 nm. The PVDF hydrophobic microporous membrane has a pore size of 0.35-0.45 μm, the super-hydrophobic layer has a pore size of 0.06-0.10 μm, and the dense sieving layer has a pore size of 0.50-0.90 nm; The composite membrane has a liquid entry pressure (LEP) of >20 bar, the super-hydrophobic layer has a water contact angle of >150°, and the dense sieving layer has an under-water oil contact angle of >150°.

8. Use of the composite membrane with a super-hydrophobic layer and a dense sieving layer according to claim 6.

9. Use according to claim 8, wherein the compound is ###0002### The composite membrane with a super-hydrophobic layer and a dense sieving layer is used as a distillation membrane in wastewater treatment.

10. The use according to claim 8, wherein the compound is ###00003### or a pharmaceutically acceptable salt thereof. The composite membrane with a super-hydrophobic layer and a dense sieving layer is used as a distillation membrane in seawater desalination.

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