A method for preparing super-hydrophobic microporous separation membrane

Abstract

The invention discloses a method for preparing a super-hydrophobic microporous separation membrane, wherein nanoparticles are introduced into a casting liquid system to form a multi-component casting liquid system composed of polymer / solvent / additive / nanoparticles, and a separation membrane is prepared through a non-solvent-induced phase separation process, wherein the nanoparticles are microparticles hydrophobically modified using perfluoro and polyfluoroalkyl compounds, and the nanoparticles themselves have a through-channel structure and are rich in hydroxyl groups on the surface. The hydrophobic long fluorine-containing chains modified on the surface of the hydrophobic / hydrophilic channel particles significantly enhance their compatibility and dispersibility in the PVDF casting liquid, and the more abundant hydrophilic hydroxyl groups on the evenly distributed particle surface promote the exchange of the aqueous phase and the polymer-rich phase during phase separation, and the through-channel structure can also serve as a mass transfer channel for water coagulation bath during phase inversion and water vapor during membrane distillation, and through this interaction, the membrane pore size is increased to improve the porosity while inducing polymer crystallization to form a rough spherical structure, thereby constructing a super-hydrophobic surface.

Description

Technical Field

[0001] The invention relates to the technical field of membrane distillation, and relates to a method for preparing a microporous separation membrane for direct contact membrane distillation, in particular to a method for preparing a super-hydrophobic microporous separation membrane. Background Art

[0002] Membrane distillation (MD) is a membrane separation technology driven by the vapor pressure difference on both sides of a porous hydrophobic membrane, through which only water vapor molecules can pass. At present, the application of MD in many fields such as drinking water treatment, food processing, pharmaceutical biology, chemical industry and environmental protection has been studied. MD has a simple process device and the ability to utilize low-grade heat sources (such as solar energy, geothermal energy and waste heat, etc.). It can operate at low pressure (<100 kPa) and high temperature (60-90℃), and can achieve almost 100% rejection rate in treating high-salinity water. Blending inorganic nanoparticles into the casting solution is the most applicable method for manufacturing microporous separation membranes. However, it is necessary to solve the agglomeration, compatibility and interaction of inorganic nanoparticles in the casting solution during membrane preparation, otherwise the agglomeration of inorganic nanoparticles will lead to its uneven dispersion in the polymer matrix and impair the membrane performance. The compatibility and interaction between inorganic nanoparticles and the polymer matrix will determine the stability of the polymer matrix and the membrane performance. Therefore, the preparation of a stable microporous separation hydrophobic membrane is one of the research focuses.

[0003] Microporous separation membranes are prepared by introducing inorganic nanoparticles into an organic matrix. They combine the excellent properties of organic and inorganic components and have high thermal stability, mechanical strength and special chemical properties. In particular, nanoparticles as small as nanometers also have special nano effects, such as surface effects, small size effects and quantization effects. The methods for preparing microporous separation membranes include:

[0004] (1) Solution blending method: two or more matrices are dissolved in an appropriate solvent, then mixed evenly, and a microporous separation membrane is formed by solvent evaporation or other methods. (2) Microporous separation deposition method: solutions of different matrices are deposited on a substrate in sequence to form a microporous separation membrane. This can be achieved by spin coating, sputtering, spraying, etc. (3) Gel method: solutions of two or more matrices are mixed and gelled, and then a microporous separation membrane is formed by drying or other methods. (4) Microporous separation co-precipitation method: after the precursor solutions of two or more matrices are mixed, they are precipitated together by chemical reaction to form a microporous separation membrane. (5) Self-assembly method: the interaction between two or more matrix molecules, such as electrostatic interaction, hydrophobic interaction, etc., is utilized to make them self-assemble into a microporous separation membrane. (6) Microporous separation nanoparticle assembly method: after the nanoparticles of two or more matrices are mixed, a microporous separation membrane is formed by self-assembly or other methods. These methods can be adjusted and improved according to specific needs to obtain a microporous separation membrane with the required performance and structure.

[0005] At present, PVDF membranes with a microscopic structure of microspheres have been successfully prepared by a non-solvent-induced phase separation method using alcohol (methanol, ethanol, etc.) as a coagulation bath. When alcohol is used as a coagulation bath, the phase separation process is delayed, and an interconnected structure without a finger-like bottom layer is obtained. In addition, a slow phase separation rate will also cause more crystallization in the membrane body. The surface of the resulting membrane structure has a microsphere structure, which increases the roughness of the membrane surface, making the membrane surface hydrophobic. When alcohol is used as a coagulation bath, the exchange rate between the solvent and the non-solvent is reduced, which is conducive to delaying the phase separation, and obtaining a membrane surface without a dense cortex and with particles. However, this method requires the use of alcohol as a coagulation bath, which will increase the cost of preparing the membrane and is not suitable for large-scale industrial production.

[0006] Common methods for preparing super-hydrophobic membranes: (1) Coating a layer of low-surface-energy material on the membrane surface, which can not only increase the roughness of the membrane surface but also reduce the surface energy of the membrane. However, the coating has poor bonding strength with the membrane matrix and is easy to fall off during long-term operation. In addition, the coating will increase the water vapor permeation resistance and reduce the flux. (2) Grafting low-surface-energy material on the membrane surface. By grafting low-surface-energy material, the surface energy of the membrane is reduced, thereby improving the hydrophobicity of the membrane. However, the equipment and process required are complex, which limits the large-scale industrial development of hydrophobic materials. (3) Improving the hydrophobicity of the membrane by blending. Blending generally involves mixing two hydrophobic organic polymers in a certain proportion and then making a membrane or mixing organic matter and inorganic nanoparticles together to make a membrane. By blending organic matter and inorganic nanoparticles to make a membrane, not only can the hydrophobicity of the membrane be effectively increased, but also the corrosion resistance and strength of the membrane can be increased. Moreover, this method is simple and easy to implement and does not require tedious post-processing.

[0007] At present, adding inorganic nanoparticles to the casting solution to prepare hydrophobic microporous separation membranes is an effective method that is simple, easy to operate and low-cost, for example:

[0008] Hou (Effects of calcium carbonate nano-particles on the properties of PVDF / nonwoven fabric flat-sheet composite membranes for direct contact membrane distillation, Desalination, Volume 347, 15 August 2014, Pages 25-33) added hydrophobic CaCO3 to the PVDF casting solution and found that the addition of hydrophobic nanoparticles can narrow the pore size distribution, increase the membrane porosity, and to a certain extent improve the membrane surface roughness and contact angle. The addition of nanoparticles also improves the crystallinity and thermal stability of the prepared membrane. Compared with the membrane without the addition of nanoparticles, the composite membrane has better mechanical properties.

[0009] Efome (Effects of superhydrophobic SiO2、nanoparticles on theperformance of PVDF flat sheet membranes for vacuum membrane distillation,Desalination, Volume 373, 1 October 2015, Pages 47-57) added hydrophobic SiO2 to the PVDF casting solution to prepare a membrane with an asymmetric structure, which has a thin top surface layer, a finger-like macroporous region and a lower sponge-like layer, and the hydrophobicity of the membrane is improved.

[0010] Baghbanzadeh (Effects of hydrophilic CuO nanoparticles on properties and performance of PVDF VMD membranes, Desalination, Volume 369, 3 August 2015, Pages 75-84) added hydrophilic CuO nanoparticles to the PVDF casting solution and prepared PVDF membranes using the immersion phase inversion method. It was found that the pore size, porosity, hydrophobicity and membrane distillation flux of the membrane could be effectively improved.

[0011] However, the hydrophobic microporous separation membrane prepared by adding inorganic nanoparticles to the casting solution has the problems of low flux and insufficient hydrophobicity. Summary of the invention

[0012] The purpose of the present invention is to overcome the shortcomings of low flux in the membrane distillation process that cannot meet the requirements of practical applications, as well as the membrane wetting phenomenon caused by insufficient hydrophobicity, thereby providing a method for preparing a super-hydrophobic microporous separation membrane. The super-hydrophobic microporous separation membrane prepared by this method has greatly improved permeation flux, stable super-hydrophobic properties, and the preparation method is simple and easy.

[0013] The objective of the present invention is achieved by the following technical solutions:

[0014] The present invention provides a method for preparing a super-hydrophobic microporous separation membrane, wherein nanoparticles are introduced into a casting liquid system to form a multi-component casting liquid system consisting of a polymer / solvent / additive / nanoparticles, and a separation membrane is prepared through a non-solvent-induced phase separation process. The nanoparticles are hydrophobically modified nanoparticles using perfluoro and polyfluoroalkyl compounds, and the nanoparticles themselves have a through-channel structure and have abundant hydroxyl groups on the surface.

[0015] Hydrophobically modified nanoparticles with through-pore structures containing hydroxyl groups on the surface are added to a casting solution system composed of polymer / solvent / additive / inorganic nanoparticles. The modification effect is to enhance the dispersibility and compatibility of the pore particles in the PVDF casting solution by utilizing the fluorine-containing long chains modified on the surface of the hydrophobic / hydrophilic pore particles. The more abundant hydrophilic hydroxyl groups on the surface of the evenly distributed pore particles promote the exchange of the aqueous phase and the polymer-rich phase, and the through-pore structure of the particles can serve as a mass transfer channel for water during phase transformation. Through this interaction, the pore size is increased and the porosity of the membrane is improved. Through the synergistic effect of the fluorine-containing long chains and the surface hydroxyl groups, a super-hydrophobic microporous separation membrane with good hydrophobicity and high flux is obtained.

[0016] Furthermore, the perfluoro and polyfluoroalkyl compounds are one or more of perfluorohexanoic acid, perfluorooctanoic acid, and perfluorononanoic acid.

[0017] Furthermore, the method for hydrophobically modifying nanoparticles with perfluoroalkyl and polyfluoroalkyl compounds is as follows: blending a dispersion of nanoparticles with a solution of perfluoroalkyl and polyfluoroalkyl compounds, wherein the mass of the perfluoroalkyl and polyfluoroalkyl compounds is 1 to 3 times the mass of the nanoparticles, adding a water-carrying agent and a catalyst, performing a grafting reaction at 65-75°C for 6-7 hours, centrifuging and washing after the reaction, drying and grinding to obtain hydrophobically modified nanoparticles, wherein the catalyst is one or more of concentrated sulfuric acid, boric acid, phosphoric acid, formic acid, and acetic acid, and the water-carrying agent is cyclohexane, petroleum ether, or xylene. The mass of the catalyst is 1 to 3 times the mass of the nanoparticles, and the mass of the water-carrying agent is 10 to 20 times the mass of the nanoparticles.

[0018] Furthermore, the nanoparticles are one or more of mesoporous titanium dioxide, mesoporous zinc oxide, iron oxyhydroxide, and hydroxylated carbon nanotubes.

[0019] Furthermore, the polymer is polyvinylidene fluoride or polytetrafluoroethylene or polyvinyl chloride.

[0020] Furthermore, the solvent is one or more of dimethylacetamide, dimethylformamide, and N-methylpyrrolidone.

[0021] Furthermore, the additive is one or more of ethanol, propylene glycol, butanol, pentanediol and water.

[0022] Furthermore, the concentration of the hydrophobically modified nanoparticles of perfluoroalkyl and polyfluoroalkyl compounds is 0.01 wt% to 0.50 wt%, preferably 0.05 wt% to 0.20 wt%, relative to the total mass of the casting solution.

[0023] Furthermore, the temperature of the casting solution is controlled at 60°C to 90°C, preferably 70°C to 90°C, and more preferably 75°C to 85°C.

[0024] Furthermore, deionized water, ethanol or a mixture of the two is used as the coagulation bath.

[0025] The present invention uses the synergistic effect of the fluorine-containing long chains carried by the hydrophobically modified pore structure nanoparticles and the hydroxyl groups themselves to affect the hydrophobic membrane structure, forming a sponge layer structure of a spherical structure in the film forming process, and the surface of the microsphere structure is wrinkled, so as to increase the roughness and surface hydrophobicity of the membrane, thereby preparing a super-hydrophobic microporous separation membrane.

[0026] The advantages and positive effects of the present invention are:

[0027] 1. The present invention adopts hydrophobic modification of pore structure nanoparticles with hydroxyl groups, and utilizes the fluorine-containing long chain modified on the surface of hydrophobic / hydrophilic pore particles to enhance the dispersibility and compatibility of the pore particles in the PVDF casting solution. The more abundant hydrophilic hydroxyl groups on the surface of the evenly distributed through-pore particles promote the exchange of the aqueous phase and the polymer-rich phase, and the through-pore structure of the particles can serve as a mass transfer channel for water during phase transformation. Through this interaction, the pore size is increased to improve the porosity of the membrane and induce crystallization to construct a super-hydrophobic surface, and finally a high-performance super-hydrophobic microporous separation membrane is prepared.

[0028] 2. The present invention uses one or more of ethanol, propylene glycol, butanol, pentanediol and water as non-solvent additives. Alcohol as a non-solvent additive can inhibit the mass transfer rate of the solvent and non-solvent during the film formation process, which is beneficial to the crystallization of the polymer. Therefore, by adding hydrophobic / hydrophilic pore particles as cores to induce crystallization and bring hydrophobicity, a rough surface can be constructed, and the film has super-hydrophobic properties.

[0029] 3. The present invention adds hydrophobic pore structure nanoparticles to induce crystallization and guide the formation of super-hydrophobic microstructures during the liquid phase separation process of the casting membrane, and improves the pore size and porosity of the membrane body to obtain a super-hydrophobic microporous separation membrane. The method is easy to operate, and the membrane microstructure and hydrophobic properties are stable.

[0030] 4. Through the induced phase separation of the method of the present invention, the sponge layer structure of the membrane body is changed from a network pore structure to a fiber-connected spherical structure. The surface of the spherical structure is wrinkled, and the roughness of the membrane surface is greatly increased, so that the membrane has superhydrophobic properties.

[0031] 5. The super-hydrophobic microporous separation membrane prepared by the method of the present invention realizes uniform distribution of super-hydrophobic microstructures on the membrane surface and in the membrane body, and the super-hydrophobic property of the membrane is stable, which is conducive to the stable operation of the membrane, and the preparation method is simple and easy. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a schematic diagram of a direct contact membrane distillation test apparatus;

[0033] Figure 2 is a scanning electron microscope (SEM) image of the lower surface of the pure polymer film prepared in Example 1;

[0034] Figure 3 is a cross-sectional SEM image of the pure polymer film prepared in Example 1;

[0035] Figure 4 is a SEM image of the lower surface of the modified polymer film prepared in Comparative Example 1;

[0036] Figure 5 is a cross-sectional SEM image of the modified polymer film prepared in Comparative Example 1. DETAILED DESCRIPTION

[0037] In order to further understand the present invention, specific examples are given below to describe the present invention. These descriptions are only to further illustrate the features and advantages of the present invention rather than to limit the patent requirements of the present invention.

[0038] Example 1

[0039] 0.5g of oxyhydroxide iron nanoparticles were dispersed in 100ml of anhydrous ethanol, ultrasonically dispersed for 45min, and then 1g of perfluorooctanoic acid dispersed in 20ml of anhydrous ethanol was added to the solution at room temperature. The two were then mixed in a three-necked flask connected to a condensation reflux device and stirred for 6h at an oil bath heating temperature of 70°C for grafting reaction. During this period, 10g of cyclohexane and 1g of concentrated sulfuric acid were added to promote the forward reaction. After the reaction, the hydrophobically modified oxyhydroxide iron nanoparticles were obtained by centrifugation washing, drying and grinding.

[0040] 186g of dimethylacetamide, 48g of polyvinylidene fluoride, 66g of propylene glycol, 30g of anhydrous ethanol, and 0.3g of hydrophobically modified iron oxyhydroxide nanoparticles were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath.

[0041] A laboratory-made direct contact membrane distillation test device was used, as shown in the schematic diagram. Figure 1 The MD performance of the prepared membrane was tested. The schematic diagram of the device is shown in Figure 1 In the test, 10.0 wt% NaCl solution was used as the feed solution to simulate the high-salinity seawater environment, and the conductivity was about 10.0 μS·cm −1 Deionized water was used as the permeate. The temperatures of the feed solution and the permeate were set at 70 °C and 20 °C, respectively, and the flow rates were 0.4 L·min −1 and 0.2 L·min −1 , both flow rates are monitored by glass rotor flowmeter. The salt retention rate is monitored by conductivity meter. The water production w is calculated by the change of balance indication, and connected to the computer for real-time monitoring. The conductivity of the produced water is tested by the conductivity meter. After stable operation for 5 minutes, the water production and water conductivity are recorded every 10 minutes. The test is repeated 3 times and the average value is taken. The MD flux is calculated according to formula (1):

[0042]

[0043] Where J is the membrane distillation flux, kg / (m 2 ·h); ΔW is the water production, A is the effective filtration area of ​​the flat membrane, m 2 ;Δt is the test time, h.

[0044] The interception rate R is calculated according to formula (2):

[0045]

[0046] Among them, C f and C pare the conductivity of the feed solution and the produced water, μs / cm, respectively. As the test device continues to operate, the stable operation time before the MD flux and retention rate show a downward trend due to membrane wetting is the MD duration.

[0047] The water contact angle of the obtained membrane is 151°, and the membrane distillation flux is 35.6 Kg / m 2 ·h, MD time is 120 h, pore size is 238.0 nm, and porosity is 80.75%.

[0048] When alcohol is used as a non-solvent additive, the mass transfer rate of the solvent and non-solvent during the film formation process can be inhibited, which is beneficial to the crystallization of the polymer. The hydrophobic inorganic nanoparticles can play a nucleation role, thereby inducing the crystal morphology of the polymer, constructing a rough surface, and making the film have super-hydrophobic properties. Figure 2 and Figure 3 .

[0049] Comparative Example 1

[0050] 186g of dimethylacetamide, 48g of polyvinylidene fluoride, 66g of propylene glycol and 30g of anhydrous ethanol were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath.

[0051] The water contact angle of the obtained membrane is 100° and the membrane distillation flux is 20.8 Kg / m 2 ·h, MD time is 15 h, pore size is 134.6 nm, and porosity is 73.14%.

[0052] When alcohol is used as the only non-solvent additive, it has no inducing effect on the polymer crystallization process. The polymers are entangled with each other, the structure is very dense, and the roughness of the membrane surface is low, so that the membrane does not have superhydrophobic properties. Figure 4 and Figure 5 .

[0053] Example 2

[0054] The mesoporous zinc oxide was hydrophobically modified using 2 times the mass fraction of perfluorooctanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified mesoporous zinc oxide nanoparticles were evenly mixed, the casting liquid temperature was 80 ° C, the film was cast on a glass plate, and the phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0055] Example 3

[0056] The mesoporous titanium dioxide was hydrophobically modified using 2 times the mass fraction of perfluorooctanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified mesoporous titanium dioxide nanoparticles were evenly mixed, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0057] Example 4

[0058] The hydroxylated carbon nanotubes were hydrophobically modified using 2 times the mass fraction of perfluorooctanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified hydroxylated carbon nanotubes were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0059] Example 5

[0060] The oxyhydroxide iron was hydrophobically modified using 2 times the mass fraction of perfluorohexanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified oxyhydroxide iron were evenly mixed, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0061] Example 6

[0062] The mesoporous zinc oxide was hydrophobically modified using 2 times the mass fraction of perfluorohexanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified mesoporous zinc oxide were evenly mixed, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0063] Example 7

[0064] The mesoporous titanium dioxide was hydrophobically modified using 2 times the mass fraction of perfluorononanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified mesoporous titanium dioxide were evenly mixed, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0065] Example 8

[0066] The hydroxylated carbon nanotubes were hydrophobically modified using 2 times the mass fraction of perfluorononanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophobically modified hydroxylated carbon nanotubes were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0067] Comparative Example 2

[0068] 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of unmodified hydrophilic ferric hydroxide were mixed evenly. The casting liquid temperature was 80°C. The film was cast on a glass plate and then scraped. Phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0069] It can be seen from the data in Table 1 that if hydrophilic ferric hydroxide is used as an additive to prepare the film, due to its own characteristics of easy agglomeration and poor compatibility, it is impossible to achieve a rich hydroxyl content to effectively induce crystallization, and the surface water contact angle is much lower than that of Examples 1-8, and the superhydrophobic effect cannot be achieved.

[0070] Comparative Example 3

[0071] 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophilic graphene oxide were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained film are shown in Table 1.

[0072] It can be seen from the data in Table 1 that if hydrophilic graphene oxide is used as an additive to prepare a membrane, due to its own characteristics of easy agglomeration and poor compatibility, it is impossible to achieve a rich hydroxyl content to effectively induce crystallization, and the surface water contact angle is much lower than that of Examples 1-8, and the superhydrophobic effect cannot be achieved. At the same time, due to the lack of a pore structure, the MD flux is also low. The test results of the obtained membrane are shown in Table 1.

[0073] Comparative Example 4

[0074] 186g of dimethylacetamide, 48g of polyvinylidene fluoride, 66g of propylene glycol, 30g of anhydrous ethanol and 0.3g of hydrophobic non-porous calcium carbonate were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0075] It can be seen from the data in Table 1 that if hydrophobic non-porous calcium carbonate is used as an additive to prepare a membrane, the MD flux is lower than that of Examples 1-8 because it does not have a pore structure.

[0076] Comparative Example 5

[0077] The hydrophilic non-porous silica was hydrophobically modified using 2 times the mass fraction of perfluorooctanoic acid (other hydrophobic modification conditions were the same as in Example 1), 186 g of dimethylacetamide, 48 g of polyvinylidene fluoride, 66 g of propylene glycol, 30 g of anhydrous ethanol, and 0.3 g of hydrophilic non-porous silica were evenly mixed, the casting liquid temperature was 80°C, the film was cast on a glass plate, and phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0078] It can be seen from the data in Table 1 that if hydrophobically modified non-porous silica is used as an additive to prepare a membrane, the MD flux is lower than that of Examples 1-8 because it does not have a pore structure.

[0079] Comparative Example 6

[0080] The hydrophilic non-porous tungsten trioxide was hydrophobically modified with 2 times the mass fraction of perfluorohexanoic acid (the other hydrophobic modification conditions were the same as those in Example 1), 186g of dimethylacetamide, 48g of polyvinylidene fluoride, 66g of propylene glycol, 30g of anhydrous ethanol, and 0.3g of hydrophilic non-porous tungsten trioxide were mixed evenly, the casting liquid temperature was 80°C, the film was cast on a glass plate, and the phase separation was completed in a deionized water coagulation bath. The test results of the obtained membrane are shown in Table 1.

[0081] It can be seen from the data in Table 1 that if hydrophobically modified non-porous tungsten trioxide is used as an additive to prepare a membrane, the MD flux is lower than that of Examples 1-8 because it does not have a pore structure.

[0082] Table 1 Performance statistics of the membranes prepared under the conditions of Examples 1-8 and Comparative Examples 1-6

[0083]

[0084] The contents not described in the embodiments of the present invention are prior art and will not be described in detail.

[0085] The present invention ensures that the membrane surface has a super-hydrophobic structure while having a suitable pore size and high porosity, greatly improving the permeation flux and stable operation time in the membrane distillation process. Compared with the commonly used membrane for membrane distillation, the preparation cost is reduced and it is simple and easy.

[0086] The above description is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept, which all belong to the protection scope of the present invention.

Claims

1. A method for preparing a super-hydrophobic microporous separation membrane, wherein nanoparticles are introduced into a casting liquid system to form a multi-component casting liquid system consisting of polymer / solvent / additive / nanoparticles, and a separation membrane is prepared by a non-solvent-induced phase separation process, characterized in that: The nanoparticles are hydrophobically modified nanoparticles using perfluoro and polyfluoroalkyl compounds. The nanoparticles themselves have a through-pore structure and are rich in hydroxyl groups on the surface. The surface-modified fluorine-containing long chains are used to enhance the dispersibility and compatibility of the nanoparticles in the casting solution. The evenly distributed rich hydrophilic hydroxyl groups on the surface of the nanoparticles promote the exchange of the aqueous phase and the polymer-rich phase, and the through-pore structure of the nanoparticles can serve as a mass transfer channel for the aqueous phase during phase conversion. Through this interaction, the pore size is increased and the porosity of the membrane is improved. Through the synergistic effect of the fluorine-containing long chains and the surface hydroxyl groups, a super-hydrophobic microporous separation membrane with good hydrophobicity and high flux is obtained; The perfluoro and polyfluoroalkyl compounds are one or more of perfluorohexanoic acid, perfluorooctanoic acid and perfluorononanoic acid; The method for hydrophobically modifying nanoparticles with perfluoroalkyl and polyfluoroalkyl compounds is as follows: a dispersion of nanoparticles is mixed with a solution of perfluoroalkyl and polyfluoroalkyl compounds, wherein the mass of the perfluoroalkyl and polyfluoroalkyl compounds is 1 to 3 times the mass of the nanoparticles; a water-carrying agent and a catalyst are added, and a grafting reaction is performed at 65 to 75° C. for 6 to 7 hours; after the reaction is completed, the nanoparticles are centrifuged and washed, and dried and ground to obtain hydrophobically modified nanoparticles, wherein the catalyst is one or more of concentrated sulfuric acid, boric acid, phosphoric acid, formic acid, and acetic acid; the water-carrying agent is cyclohexane, petroleum ether, or xylene; the mass of the catalyst is 1 to 3 times the mass of the nanoparticles; and the mass of the water-carrying agent is 10 to 20 times the mass of the nanoparticles; The nanoparticles are one or more of mesoporous titanium dioxide, mesoporous zinc oxide, iron oxyhydroxide, and hydroxylated carbon nanotubes; The polymer is polyvinylidene fluoride or polytetrafluoroethylene or polyvinyl chloride.

2. The method for preparing a super-hydrophobic microporous separation membrane according to claim 1, wherein: The solvent is one or more of dimethylacetamide, dimethylformamide and N-methylpyrrolidone.

3. The method for preparing a super-hydrophobic microporous separation membrane according to claim 1, characterized in that: The additive is one or more of ethanol, propylene glycol, butanol, pentanediol and water.

4. The method for preparing a super-hydrophobic microporous separation membrane according to claim 1, wherein: The concentration of the hydrophobically modified nanoparticles of perfluoro and polyfluoroalkyl compounds is 0.01 wt % to 0.50 wt % relative to the total mass of the casting solution.

5. The method for preparing a super-hydrophobic microporous separation membrane according to claim 1, characterized in that: The temperature of the casting liquid is controlled at 60℃~90℃.

6. The method for preparing a super-hydrophobic microporous separation membrane according to claim 1, characterized in that: Deionized water, ethanol or a mixture of the two is used as the coagulation bath.

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