A g-c3n4@ptfe composite membrane for membrane distillation, a preparation method and application thereof

Abstract

The application provides a g-C3N4@PTFE composite membrane for membrane distillation, a preparation method and application, relates to the field of membrane separation technology, and the g-C3N4@PTFE composite membrane comprises two layers, wherein the base layer is a PTFE membrane, and the top layer is g-C3N4 modified by FAS; the g-C3N4 modified by FAS forms a hydrophobic layer on the surface of the PTFE membrane. The composite membrane combines g-C3N4 nanosheets with a traditional hydrophobic membrane, can not only improve the surface hydrophobicity of the PTFE membrane, but also maintain the complete nanochannel of ion sieving brought by the g-C3N4 nanosheet, and can greatly improve the surface hydrophobicity and MD performance of the membrane.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of membrane separation technology, and particularly relates to a g-C3N4@PTFE composite membrane for membrane distillation, a preparation method and application. BACKGROUND

[0002] Water scarcity is a growing global threat. Currently, only 1% of the world's water supply is available for human consumption, with the remaining 98% being seawater or brackish water. As a viable solution, seawater desalination has been considered by many researchers around the world. Seawater desalination is usually achieved using thermal technologies such as multi-stage flash (MSF) or multiple effect distillation (MED). However, these processes are very energy-intensive. Membrane-based separation is an effective method to reduce costs and simplify the process.

[0003] In water treatment, membrane separation can evolve into membrane distillation (MD) by combining single-stage distillation and membrane processes. The MD process is driven by a vapor pressure gradient established across a hydrophobic porous membrane using low-grade heat (30-90 °C). Under the driving force of this vapor pressure difference, water vapor passes through the hydrophobic membrane and condenses on the other side to obtain pure water. Compared with other conventional separation operations, MD has many advantages. It requires lower transmembrane pressure, can be used at low temperatures, and can handle high feed salinity. However, MD also has some disadvantages, such as low permeate flux during long-term operation and membrane pore wetting. In order to overcome these shortcomings, it is urgent to develop a high-performance MD separation membrane with high hydrophobicity, high porosity, appropriate pore size, and narrow pore size distribution.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] To solve the above technical problems, the present application provides a g-C3N4@PTFE composite membrane for membrane distillation, a preparation method and application. The composite membrane combines g-C3N4 nanosheets with traditional hydrophobic membranes, which not only improves the surface hydrophobicity of the PTFE membrane, but also maintains the complete nanochannel of the ion sieve brought by the g-C3N4 nanosheet, and greatly improves the surface hydrophobicity and MD performance of the membrane.

[0006] In order to achieve the purpose of the present application, the following technical solutions are adopted:

[0007] The present application provides a g-C3N4@PTFE composite membrane for membrane distillation, which comprises two layers, wherein the base layer is a PTFE membrane, and the top layer is g-C3N4 modified by FAS.

[0008] The g-C3N4 modified by FAS forms a hydrophobic layer on the surface of the PTFE membrane.

[0009] Further, the water contact angle of the g-C3N4@PTFE composite film is 130°-145°.

[0010] Further, the loading amount of the FAS-modified g-C3N4 on the PTFE film is 0.056 mg / cm 2 .

[0011] The application further provides a preparation method of the g-C3N4@PTFE composite film for membrane distillation.

[0012] S1, FAS and deionized water are added to an organic solvent for stirring to obtain hydroxylated FAS;

[0013] S2, melamine is calcined to obtain white powder, and the white powder is ultrasonically dispersed in an organic solvent, centrifuged to obtain a g-C3N4 nanosheet solution;

[0014] S3, the hydroxylated FAS is slowly added to the g-C3N4 nanosheet solution under magnetic stirring, and refluxed, then washed with an organic detergent, and dried to obtain FAS-modified g-C3N4;

[0015] S4, the FAS-modified g-C3N4 is ultrasonically dispersed in a PTFE emulsion to obtain a dispersion liquid, and the dispersion liquid is slowly cast drop by drop on a PTFE film, dried, and sintered to obtain a g-C3N4@PTFE composite film.

[0016] Further, in S1, the concentration of FAS is 1wt%-4wt%;

[0017] The organic solvent used is N,N-dimethylformamide;

[0018] The volume ratio of the deionized water to FAS is (1-3):1;

[0019] The stirring speed is 300r / min-800r / min, and the reaction time is 3h-12h.

[0020] Further, in S2, the calcination temperature of melamine is 520℃-550℃;

[0021] The calcination time is 6h-7h;

[0022] The heating or cooling rate during calcination is 3℃ / min-5℃ / min.

[0023] Further, in S2, the ultrasonic dispersion time is 1h-3h;

[0024] The centrifugation time is 15-30 min, and the centrifugation speed is 5000-8000 rpm.

[0025] Further, in the S4, the concentration of the PTFE emulsion is 3-6 wt%;

[0026] The ultrasonic treatment time is 3-6 h.

[0027] The drying temperature is 60-80℃.

[0028] The drying time is 12 h.

[0029] Further, in the S4, the sintering temperature is 250-400℃, and the sintering time is 10-30 min.

[0030] Further, in the S3, the speed of adding the hydroxylated FAS is 0.1-0.5 mL / min, the reaction time is 8-16 h, and the magnetic stirring speed is 400-900 r / min.

[0031] Further, in the S2, the organic solvent used is N,N-dimethylformamide.

[0032] Further, in the S3, the organic washing agent used is N,N-dimethylformamide.

[0033] The application also provides the use of the g-C3N4@PTFE composite membrane for membrane distillation in water treatment.

[0034] The application has the following technical effects:

[0035] (1) The obtained g-C3N4@PTFE composite membrane exhibits higher surface hydrophobicity, higher flux and higher rejection rate.

[0036] (2) The obtained g-C3N4@PTFE composite membrane has less performance decay in long-term operation, exhibits better wet resistance, and has broad application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0038] Figure 1 : SEM image of the g-C3N4 nanosheet prepared in Example 1;

[0039] Figure 2 : SEM images of the surface of the PTFE membrane before and after modification in Example 1, wherein (a) is before modification of the PTFE membrane, (b) is after modification of the PTFE membrane;

[0040] Figure 3 : Water contact angles of the PTFE membrane before and after modification in Example 1, wherein (a) is before modification of the PTFE membrane, (b) is after modification of the PTFE membrane;

[0041] Figure 4 : Stability diagram of the g-C3N4@PTFE composite membrane prepared in Example 1 during 24 h membrane distillation, wherein (a) is before modification of the PTFE membrane, (b) is after modification of the PTFE membrane. DETAILED DESCRIPTION

[0042] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0043] In a first aspect, the present application provides a g-C3N4@PTFE composite membrane for membrane distillation, which comprises two layers, wherein the base layer is a PTFE membrane and the top layer is g-C3N4 modified by FAS.

[0044] The g-C3N4 modified by FAS forms a hydrophobic layer on the surface of the PTFE membrane.

[0045] The base layer is a PTFE membrane and the top layer is g-C3N4 modified by FAS. The PTFE membrane itself has hydrophobicity and porous structure, but by adding g-C3N4 nanosheets modified by FAS, a dense hydrophobic layer is formed on the surface of the PTFE membrane. FAS is a perfluoroalkylsilane, which can react with the functional groups on the surface of g-C3N4 nanosheets after hydroxylation, introducing hydrophobic groups (such as -CF3), thereby enhancing the surface hydrophobicity. At the same time, g-C3N4 nanosheets have regular nanochannels (about 0.3-0.5 nm), which can provide ion sieving function, allowing water vapor to pass through while blocking salt ions. This structure combines the mechanical strength of PTFE and the nanochannel advantage of g-C3N4, realizing the synergistic improvement of hydrophobicity and sieving ability.

[0046] In some embodiments, the water contact angle of the g-C3N4@PTFE composite membrane is 130°-145°.

[0047] The high water contact angle can make the membrane surface less likely to be wetted by water, reducing the risk of membrane hole blockage and pollution, thereby prolonging the service life of the membrane.

[0048] In some embodiments, the loading amount of the FAS-modified g-C3N4 on the PTFE membrane is 0.056 mg / cm2. 2 .

[0049] The loading amount is achieved by optimizing the preparation parameters (such as ultrasonic time, centrifugation conditions and casting speed), ensuring that the g-C3N4 nanosheets are uniformly distributed on the PTFE surface to form a continuous and non-blocking hydrophobic layer. Too low loading amount will result in incomplete coverage, and too high loading amount may block the PTFE membrane holes, affecting the air permeability. This loading amount balances the hydrophobicity and air permeability, so that the composite membrane has both high water contact angle and high water vapor flux during MD process.

[0050] In a second aspect, the present application also provides a preparation method of a g-C3N4@PTFE composite membrane for membrane distillation, characterized in that it comprises the following steps:

[0051] S1, adding FAS and deionized water into an organic solvent and stirring to obtain hydroxylated FAS;

[0052] S2, calcining melamine to obtain white powder, ultrasonic treatment and dispersion of the white powder in an organic solvent, and centrifugation to obtain a g-C3N4 nanosheet solution;

[0053] S3, slowly adding the hydroxylated FAS to the g-C3N4 nanosheet solution under magnetic stirring, refluxing, then washing with an organic detergent, and drying to obtain FAS-modified g-C3N4;

[0054] S4, ultrasonic treatment of the FAS-modified g-C3N4 in a PTFE emulsion to obtain a dispersion, slowly dropping the dispersion onto a PTFE membrane, drying, and sintering to obtain a g-C3N4@PTFE composite membrane.

[0055] In some embodiments, the organic solvent used in S1 is N,N-dimethylformamide;

[0056] The concentration of the FAS is 1wt%-4wt%;

[0057] The volume ratio of the deionized water to FAS is (1-3):1;

[0058] The stirring speed is 300r / min-800r / min, and the reaction time is 3h-12h.

[0059] In some embodiments, the calcination temperature of the melamine in S2 is 520-550℃.

[0060] The calcination time is 6-7h.

[0061] The heating or cooling rate during the calcination is 3-5℃ / min.

[0062] In some embodiments, the ultrasonic dispersion time in S2 is 1-3h.

[0063] The centrifugation time is 15-30min, and the centrifugation speed is 5000-8000rpm.

[0064] In some embodiments, the concentration of the PTFE emulsion in S4 is 3-6wt%.

[0065] The ultrasonic treatment time is 3-6h.

[0066] The drying temperature is 60-80℃.

[0067] The drying time is 12h.

[0068] In some embodiments, the sintering temperature in S4 is 250-400℃, and the sintering time is 10-30min.

[0069] In some embodiments, the speed of adding the hydroxylated FAS in S3 is 0.1-0.5mL / min, the reaction time is 8-16h, and the magnetic stirring speed is 400-900r / min.

[0070] In some embodiments, the organic solvent used in S2 is N,N-dimethylformamide.

[0071] In some embodiments, the organic solvent used in S3 is N,N-dimethylformamide.

[0072] In the step S1 of the present application, the FAS reacts with deionized water in an organic solvent (such as N,N-dimethylformamide) to generate hydroxylated FAS, so as to make it have reactivity.

[0073] In the step S2 of the present application, the melamine is calcined at 520-550℃ to form g-C3N4 nanosheets, which are then purified by ultrasonic dispersion and centrifugation to obtain a single-layer or few-layer nanosheet solution.

[0074] In the step S3 of the present application, the g-C3N4 nanosheets undergo grafting reaction with the hydroxylated FAS, and the hydrophobic groups of the FAS are covalently bonded to the surface of the g-C3N4.

[0075] In the step of S4 of the present application, the modified g-C3N4 is dispersed in the PTFE emulsion, cast on the PTFE film, dried and sintered (250-400 ℃) to form a firmly combined composite film. The sintering process makes the PTFE particles melt and form a physical interweaving with the g-C3N4 layer, enhancing the interfacial bonding force.

[0076] The FAS modification makes the g-C3N4 change from hydrophilic to hydrophobic while retaining its nanochannel, and the sintering step makes the PTFE form a stable interface with the g-C3N4 layer, preventing interlayer peeling and improving the mechanical strength and long-term stability of the film.

[0077] In a third aspect, the present application also provides the use of the above-mentioned g-C3N4@PTFE composite film for membrane distillation in water treatment.

[0078] The following will be described in conjunction with specific examples:

[0079] Example 1

[0080] (1) FAS and deionized water (FAS: deionized water v:v = 1:1) were added to N,N-dimethylformamide to obtain a mixed solution with a FAS concentration of 2 wt%, and then the mixed solution was fully stirred to complete the hydroxylation of FAS. The stirring speed was 500 r / min, and the reaction time was 8 h.

[0081] (2) 10 g of melamine was placed in a muffle furnace and calcined, with a calcination temperature of 520 ℃ and a holding time of 6.5 h, and a temperature rise and fall rate of 3 ℃ / min. The white powder collected was g-C3N4 nanosheets.

[0082] (3) The g-C3N4 nanosheets (12 mg) obtained in step (2) were dispersed in 50 mL of N,N-dimethylformamide by ultrasonic treatment for 2 h, and then the dispersion was centrifuged at 8000 rpm for 15 min to remove unpeeled g-C3N4 fragments, obtaining a g-C3N4 nanosheet solution.

[0083] (4) 30 mL of the above g-C3N4 nanosheet solution was heated to 100 ℃, and then the hydroxylated FAS solution obtained in step (1) was slowly added to the g-C3N4 dispersion at a speed of 0.1 mL / min under magnetic stirring, and refluxed for 8 h. The product was washed with N,N-dimethylformamide to remove unreacted FAS, and dried in an oven at 80 ℃ for 10 h to obtain FAS-g-C3N4.

[0084] (5) 5 mg of FAS-g-C3N4 powder was re-dispersed in 5 mL of PTFE slurry with a concentration of 6 wt% by ultrasonic treatment for 6 h. The dispersant was then slowly cast drop by drop on the PTFE film to ensure uniform distribution on the entire surface. The g-C3N4@PTFE composite film was obtained by placing it in a vacuum oven at 70 °C for 12 h.

[0085] (6) The g-C3N4@PTFE composite film obtained in step (5) was sintered in a muffle furnace at 360 °C for 10 min to obtain a g-C3N4@PTFE composite film with a tightly bonded interface.

[0086] Example 2

[0087] (1) FAS and deionized water (FAS: deionized water v:v = 1:2) were added to N,N- dimethylformamide to obtain a mixed solution with a FAS concentration of 3 wt%, and then the mixed solution was stirred to complete the hydroxylation of FAS. The stirring speed was 800 r / min, and the reaction time was 12 h.

[0088] (2) 11 g of melamine was calcined in a muffle furnace, with a calcination temperature of 528 °C and a holding time of 6.5 h, and a temperature rise and fall rate of 5 °C / min. The white powder obtained was collected, which was g-C3N4 nanosheets.

[0089] (3) The g-C3N4 nanosheets (10 mg) obtained in step (2) were dispersed in 50 mL of N,N-dimethylformamide by ultrasonic treatment for 3 h, and then the dispersion was centrifuged at 8000 rpm for 15 min to remove unpeeled g-C3N4 fragments, obtaining a g-C3N4 nanosheet solution.

[0090] (4) 30 mL of the above g-C3N4 nanosheet solution was heated to 60 °C, and then the hydroxylated FAS solution obtained in step (1) was slowly added to the g-C3N4 dispersion at a speed of 0.5 mL / min under magnetic stirring, and refluxed for 16 h. Then, the product was washed with N,N-dimethylformamide to remove unreacted FAS, and dried in an oven at 80 °C for 10 h to obtain FAS-g-C3N4.

[0091] (5) 5 mg of FAS-g-C3N4 powder was re-dispersed in 5 mL of PTFE slurry with a concentration of 5 wt% by ultrasonic treatment for 5 h. The dispersant was then slowly cast drop by drop on the PTFE film to ensure uniform distribution on the entire surface. The g-C3N4@PTFE composite film was obtained by placing it in a vacuum oven at 80 °C for 12 h.

[0092] (6) The g-C3N4@PTFE composite film obtained in step (5) is sintered in a muffle furnace at 360 °C for 20 min to obtain a g-C3N4@PTFE composite film with a tightly bonded interface.

[0093] Example 3

[0094] (1) FAS and deionized water (FAS: deionized water v:v = 1:1) were added to N,N- dimethylformamide to obtain a mixed solution with a FAS concentration of 1 wt%, and then the mixed solution was fully stirred to complete the hydroxylation of FAS. The stirring speed was 300 r / min, and the reaction time was 3 h.

[0095] (2) 12 g of melamine was placed in a muffle furnace for calcination, wherein the calcination temperature was 550 °C, the holding time was 6.5 h, and the temperature rising and falling rate was 3 °C / min. The white powder obtained was collected, which was g-C3N4 nanosheets.

[0096] (3) The g-C3N4 nanosheets (8 mg) obtained in step (2) were dispersed in 50 mL of N,N- dimethylformamide by ultrasonic treatment for 2 h, and then the dispersion was centrifuged at 5000 rpm for 30 min to remove unexfoliated g-C3N4 fragments to obtain a g-C3N4 nanosheet solution.

[0097] (4) 30 mL of the above g-C3N4 nanosheet solution was heated to 80 °C, and then the hydroxylated FAS solution obtained in step (1) was slowly added to the g-C3N4 dispersion at a speed of 0.3 mL / min under magnetic stirring, and refluxed for 12 h. Then, the product was washed with N,N-dimethylformamide to remove unreacted FAS, and dried in an oven at 80 °C for 10 h to obtain FAS-g-C3N4.

[0098] (5) 5 mg of FAS-g-C3N4 powder was re-dispersed in 5 mL of PTFE slurry with a concentration of 3 wt% by ultrasonic treatment for 3 h. Then the dispersant was slowly cast drop by drop on the PTFE film to ensure uniform distribution of the whole surface. The FAS-g-C3N4 layer modified g-C3N4@PTFE composite film was obtained by standing in a vacuum oven at 60 °C for 12 h.

[0099] (6) The g-C3N4@PTFE composite film obtained in step (5) was sintered in a muffle furnace at 300 °C for 30 min to obtain a g-C3N4@PTFE composite film with a tightly bonded interface.

[0100] The g-C3N4 nanosheets prepared in step (3) of Example 1 were subjected to SEM testing, and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that the g-C3N4 nanosheets are very thin.

[0101] SEM tests were performed on the unmodified PTFE membrane and the g-C3N4@PTFE composite membrane prepared in step (6) of Example 1. The results are as follows: Figure 2 As shown. By Figure 2 It can be seen that the pore size of the membrane surface before modification is larger, while no obvious pinholes or cracks are seen on the surface of the membrane after modification.

[0102] The water contact angle of the unmodified PTFE membrane and the g-C3N4@PTFE composite membrane prepared in step (6) of Example 1 was tested, and the results are as follows: Figure 3 As shown. By Figure 3 It can be seen that the modified membrane surface is more hydrophobic, and the water contact angle increases from 122.8° of the original PTFE membrane to 131.5° of the g-C3N4@PTFE composite membrane.

[0103] Membrane distillation tests were conducted using a prepared 3.5 wt% NaCl salt solution, with a cold-side temperature of 10 ℃ and a hot-side salt solution temperature of 65 ℃. The g-C3N4@PTFE composite membrane obtained in step (6) was subjected to membrane distillation for 20 h. The test results of membrane flux and membrane effluent conductivity are as follows: Figure 4 As shown. By Figure 4 It can be seen that the g-C3N4 modified PTFE membrane exhibits a higher membrane flux compared to the original PTFE membrane. The effluent conductivity is also lower, indicating an improved membrane rejection rate. Furthermore, in long-term testing, the g-C3N4 modified PTFE membrane also demonstrated improved anti-wetting performance, with minimal performance degradation after 20 hours of operation, proving the necessity of adding g-C3N4 to improve anti-wetting performance.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A g-C3N4@PTFE composite membrane for membrane distillation, characterized in that, The g-C3N4@PTFE composite film comprises two layers, wherein the base layer is a PTFE film, and the top layer is g-C3N4 modified by FAS; The g-C3N4 modified by FAS forms a hydrophobic layer on the surface of the PTFE film; The preparation method of the g-C3N4@PTFE composite film for membrane distillation comprises the following steps: S1, stirring FAS and deionized water into an organic solvent to obtain hydroxylated FAS; S2, calcining melamine to obtain white powder, and dispersing the white powder in an organic solvent by ultrasonic treatment and centrifuging to obtain a g-C3N4 nanosheet solution; S3, slowly adding the hydroxylated FAS to the g-C3N4 nanosheet solution under magnetic stirring, refluxing, then washing with an organic detergent, and drying to obtain g-C3N4 modified by FAS; S4, dispersing the g-C3N4 modified by FAS in a PTFE emulsion by ultrasonic treatment to obtain a dispersion, slowly casting the dispersion drop by drop on a PTFE film, drying, and sintering to obtain a g-C3N4@PTFE composite film.

2. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 1, characterized in that, The water contact angle of the g-C3N4@PTFE composite film is 130°-145°.

3. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 1, characterized in that, The FAS-modified g-C3N4 has a loading amount of 0.056 mg / cm on the PTFE membrane 2 .

4. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 1, characterized in that, In S1, the concentration of FAS is 1 wt%-4 wt%; The organic solvent used is N,N-dimethylformamide; The volume ratio of deionized water to FAS is (1-3):1; The stirring speed is 300 r / min-800 r / min, and the reaction time is 3 h-12 h.

5. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 1, characterized in that, In the S2 step, the calcination temperature of melamine is 520 ℃-550 ℃; The calcination time is 6 h-7 h; The heating or cooling rate during calcination is 3 ℃ / min-5 ℃ / min.

6. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 1, characterized in that, The ultrasonic dispersion time in S2 is 1 h-3 h; The centrifugation time is 15 min-30 min, and the centrifugation speed is 5000 rpm-8000 rpm.

7. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 1, characterized in that, In S3, the speed of adding hydroxylated FAS is 0.1 mL / min-0.5 mL / min; The reaction time is 8 h-16 h, and the magnetic stirring speed is 400 r / min-900 r / min.

8. The g-C3N4@PTFE composite membrane for membrane distillation according to claim 4, characterized in that, In S4, the concentration of PTFE emulsion is 3 wt%-6 wt%; The ultrasonic treatment time is 3 h-6 h; The drying temperature is 60 ℃-80 ℃; The drying time is 12 h; The sintering temperature is 250 ℃-400 ℃, and the sintering time is 10 min-30 min.

9. The g-C3N4@PTFE composite film for membrane distillation according to any one of claims 1-8 for use in water treatment.

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