Method for preparing crack-free ceramic composite ultrafiltration membrane based on freeze drying technology and sol-gel method

Abstract

The invention discloses a method for preparing a crack-free ceramic composite ultrafiltration membrane based on a freeze drying technology, and belongs to the technical field of inorganic ceramic membranes. The method comprises the following steps: firstly, preparing stable sol through hydrolytic polycondensation of a ceramic precursor, and then coating the stable sol on a porous carrier to form a wet film; the core of the invention lies in that the obtained wet film is frozen to solidify the solvent in the pores, and then the solid solvent is directly sublimated by using a freeze-drying technology, so that the problems of gel network collapse and cracking caused by capillary force in the traditional drying process are avoided; and finally performing heat treatment to obtain a final product. The technical problem that a ceramic membrane prepared by a sol-gel method is easy to crack is solved, and the prepared ceramic composite ultrafiltration membrane is complete in structure, free of cracks, nanoscale in pore size, excellent in separation performance and suitable for ultrafiltration application in a severe environment.

Description

Technical Field

[0001] This invention relates to a method for preparing a ceramic separation membrane, belonging to the field of inorganic membranes. Background Technology

[0002] In the process of preparing ceramic ultrafiltration membranes using the sol-gel method, the aging method is a key process step that has a decisive influence on the formation of the membrane microstructure and the final separation performance.

[0003] Traditional aging processes typically involve placing a porous ceramic support coated with sol in a constant temperature and humidity drying oven. Precise control of the ambient temperature and humidity parameters suppresses rapid solvent evaporation, thus preventing cracks caused by surface stress concentration. While this method addresses surface cracking to some extent, it still relies on liquid phase evaporation. During this process, the vaporization of the solvent in the gel generates capillary forces. Under the continuous action of these forces, the gel network structure inevitably undergoes significant shrinkage and collapse, leading to closure of gel channels, decreased porosity, and ultimately resulting in a denser and thicker separation layer in the sintered ceramic film than expected, severely impacting its permeation flux. Summary of the Invention

[0004] To overcome the defects caused by the capillary effect, freeze-drying, as an aging technique that removes solvent by inhibiting the liquid phase, has attracted widespread attention. Its core principle is to rapidly freeze the solvent (usually water) in the wet gel system into solid ice crystals at low temperature, and then remove the ice crystals by direct sublimation into water vapor under high vacuum. This preserves the gel network structure intact, resulting in a film with a complete and crack-free surface. Furthermore, it is proposed to introduce ice crystal inhibitors such as trehalose into the sol, which work synergistically with the freeze-drying step to control the size and morphology of ice crystals from the source, further improving the structural stability of the gel network and the overall performance of the prepared ceramic composite ultrafiltration membrane during freeze-drying.

[0005] A method for preparing crack-free ceramic composite ultrafiltration membranes based on freeze-drying technology includes the following steps: (1) Select a porous ceramic support and clean and dry it to use it as the substrate; (2) The ceramic precursor, solvent, water and catalyst are mixed, and after hydrolysis and condensation reaction, and aging treatment is carried out, a stable ceramic sol is obtained. (3) The ceramic sol obtained in step (2) is coated on the surface of the porous ceramic support to form a wet film layer; (4) The porous ceramic support loaded with wet film layer is subjected to programmed cooling and freezing treatment to cause the liquid solvent in the pores of the wet film layer to undergo phase change and solidify. (5) Under vacuum, the solvent solidified in step (4) is removed by sublimation, while the gel skeleton structure is retained, and a dry film is obtained. (6) The dried sample is subjected to high-temperature heat treatment to obtain the crack-free ceramic composite ultrafiltration membrane.

[0006] The porous ceramic support in step (1) is made of one or more of alumina, zirconium oxide, kaolin or mullite; the average pore size of the porous ceramic support is 50-500 nm.

[0007] In step (2), an ice crystal inhibitor is also added to the ceramic sol; the ice crystal inhibitor is selected from at least one of sugar compounds, sugar alcohols or low molecular weight polyhydroxy polymers.

[0008] The ice crystal inhibitor is selected from one or more of trehalose, sucrose, glucose, maltose, mannitol, sorbitol or xylitol; the amount of ice crystal inhibitor added is such that its mass ratio with water in the ceramic sol is 1wt% - 10wt%.

[0009] The ceramic precursor mentioned in step (2) is selected from alkoxides or inorganic salts of titanium, aluminum, zirconium, silicon or yttrium, specifically including tetrabutyl titanate, aluminum isopropoxide, aluminum sec-butoxide, aluminum nitrate, n-propyl zirconate, tetraethyl orthosilicate or yttrium nitrate; the preparation method of the sol includes: dissolving the ceramic precursor in an alcohol solvent, carrying out a hydrolysis-condensation reaction at 25-80°C under the action of an acidic or alkaline catalyst, adding a polymer binder, and then aging for 5-100 hours to obtain a stable sol.

[0010] The environmental conditions for coating in step (3) are: temperature 20-40℃, relative humidity 40%-90%.

[0011] The cooling rate of the freezing process in step (4) is 0.5-5℃ / min, the freezing endpoint temperature is -80℃ to -20℃, and the temperature is kept at the endpoint temperature for a certain period of time to ensure that the solvent is completely frozen.

[0012] The freeze-drying conditions described in step (5) are: cold trap temperature < -50℃, vacuum degree < 10Pa, and drying time of 2-50h.

[0013] The heating rate of the sintering in step (6) is 0.5-5℃ / min, the final sintering temperature is 300-1000℃, and the holding time is 0.5-5h.

[0014] A ceramic composite ultrafiltration membrane prepared by any of the methods described herein, characterized in that the separation layer of the ceramic composite ultrafiltration membrane is free of cracks, has an average pore size of 1-10 nm, and has a retention rate of ≥90% for substances with a molecular weight >1 kDa. Beneficial effects

[0015] 1. Crack-free structure: By using freeze-drying technology, the solvent in the wet film is removed by sublimation, avoiding the formation of the gas-liquid interface, thereby eliminating the capillary forces that cause gel cracking, and successfully preparing a crack-free ceramic separation layer.

[0016] 2. Excellent separation performance: The ceramic composite ultrafiltration membrane prepared by this method has a precise pore size of nanoscale (1-10 nm), exhibiting high water flux and high molecular weight substance rejection rate.

[0017] 3. High process reliability: This method has good process repeatability and high yield, and is particularly suitable for preparing large-size and complex-configured ceramic membrane modules, with promising prospects for industrial application.

[0018] 4. Controllable structure: By adjusting the sol composition and freezing rate, precise control can be achieved over key parameters such as membrane pore size and thickness.

[0019] 5. Controllable Ice Crystals and Further Optimized Pore Structure: Introducing an appropriate amount of trehalose into the sol system as an ice crystal inhibitor (cryoprotectant) utilizes the property of its numerous hydroxyl groups forming multiple hydrogen bonds with water molecules, lowering the freezing point and significantly increasing the low-temperature viscosity. This inhibits the rapid nucleation and growth of large ice crystals during freezing, resulting in the formation of smaller, more uniformly distributed ice crystals, and even localized glassy structures, within the wet gel. During subsequent freeze-drying, these fine ice crystals sublimate directly, leaving a uniform and delicate pore network within the separation layer. This effectively avoids stress concentration and microcrack formation caused by excessively large ice crystals and uneven local volume expansion during traditional freeze-drying. Compared with the case of using only freeze-drying process, the present invention achieves further fine control of ice crystal growth behavior by introducing trehalose into the sol. This not only further reduces the probability of microcracks and hidden cracks in the membrane layer and improves the structural integrity and mechanical stability of the membrane layer, but also obtains higher porosity and narrower pore size distribution while maintaining the same pore size class and retention performance. This significantly improves the pure water flux, pressure resistance, and thermal shock resistance of the ceramic composite ultrafiltration membrane. Attached Figure Description

[0020] Figure 1 SEM images of the surface of the ultrafiltration membrane prepared in Example 1; Figure 2 SEM images of the surface of the ultrafiltration membrane prepared in Comparative Example 1; Figure 3 SEM images of the surface of the ultrafiltration membrane prepared in Comparative Example 2; Figure 4 SEM images of the surface of the ultrafiltration membrane prepared in Example 3; Detailed Implementation Example 1: Preparation of TiO2 ceramic ultrafiltration membrane (1) Preparation of TiO2 sol: Using tetrabutyl titanate as a precursor, ethanol as a solvent, and nitric acid and deionized water as catalysts, a light blue TiO2 sol was obtained by mixing Ti(OC4H9)4: HNO3: C2H5OH: H2O in a molar ratio of 1: 0.5: 3.6: 5.2. The obtained TiO2 sol was added to 0.5wt% HPC solution and stirred evenly to obtain a stable TiO2 sol casting solution.

[0021] (2) Sol coating: The pretreated alumina support with an average length of 10 cm (average pore size of 160 nm) was immersed in the above sol at a rate of 2 cm / min for 30 s. After being pulled out, it was dried at room temperature for 40 min and then coated again. It was allowed to stand at room temperature for 2 h to complete gelation.

[0022] (3) Freezing treatment: Place the sample in a programmed cooling freezer and cool it from 25 °C to -40 °C at a rate of 1 °C / min, and keep it warm for 1 hour.

[0023] (4) Freeze-drying: Quickly transfer the frozen sample to a freeze dryer pre-cooled to -60 °C. Turn on the vacuum pump to reduce the vacuum to below 5 Pa and continue drying for 48 h.

[0024] (5) Sintering: The dried sample is placed in a muffle furnace and heated to 400°C at a rate of 1°C / min. The temperature is held for 3 hours and then cooled naturally to obtain the TiO2 ceramic composite ultrafiltration membrane.

[0025] The surface morphology and properties of the prepared TiO2 ceramic ultrafiltration membrane were characterized, such as... Figure 1 As shown, the membrane surface is continuous and intact, without any visible cracks. Under a pressure of 0.2 MPa, its retention rate of dextran with a molecular weight of 5000 Da reached over 95%, with an average pore size of 4.1 nm. Comparative Example 1 The steps are exactly the same as in Example 1, except that steps (3) and (4) (freezing and freeze drying) are replaced with drying in a constant temperature and humidity chamber: keep at 50°C and 75% relative humidity for 12 hours; then slowly reduce the humidity to 50% at a rate of 5% RH per hour, and dry to constant weight under these conditions.

[0026] Result: As Figure 2 As shown, there are a large number of cracks on the surface of the membrane, which cannot form a complete separation layer and has no retention capacity in the performance test. Example 2: Preparation of Al2O3 ceramic ultrafiltration membrane (1) Preparation of boehmite sol: Aluminum isopropoxide was used as a precursor and mixed with deionized water. The mixture was heated and stirred at 80°C for 3 hours to allow for complete hydrolysis. Then, an appropriate amount of nitric acid was slowly added dropwise, and the mixture was heated and stirred for another 10 hours until the reaction was complete to obtain boehmite sol. The obtained boehmite sol was added to polyvinyl alcohol and anhydrous ethanol solution and mixed. After stirring evenly, a boehmite casting solution was obtained. The molar ratios of the components were as follows: aluminum isopropoxide: ethanol: water: nitric acid: polyvinyl alcohol = 1:50:102:0.5:7.

[0027] (2) Coating: The pretreated alumina support with an average length of 10 cm (average pore size of 200 nm) is immersed in the above sol at a rate of 2 cm / min for 15 s. After being pulled out, it is dried at room temperature for 40 min. The above steps are repeated to coat twice. The gelation is completed by standing at room temperature for 2 h.

[0028] (3) Freezing treatment: Place the sample in a programmed cooling freezer and cool it from 25 °C to -40 °C at a rate of 2 °C / min, and keep it warm for 1 hour.

[0029] (4) Freeze-drying: Quickly transfer the frozen sample to a freeze dryer pre-cooled to -60 °C. Turn on the vacuum pump to reduce the vacuum to below 5 Pa and continue drying for 48 h.

[0030] (5) Sintering: The dried sample is placed in a muffle furnace and heated to 900°C at a rate of 1°C / min. The temperature is held for 3 hours and then cooled naturally to obtain the Al2O3 ceramic ultrafiltration membrane.

[0031] The surface morphology and properties of the prepared Al2O3 ceramic ultrafiltration membrane were characterized, and no cracks were found on the surface of the membrane. At a pressure of 0.2 MPa, its pure water flux was measured to be 65 L / (m²·h·bar), and its retention efficiency for PEG-5000 was found to be over 90%. Comparative Example 2: The sol preparation, coating, and sintering steps were the same as in Example 2, and the aging method was the same as in Comparative Example 1. The surface morphology of the obtained Al2O3 ceramic ultrafiltration membrane was characterized by SEM images (…). Figure 3 The surface film layer shows detachment and there is no separation performance. Example 3: Preparation of TiO2 ceramic composite ultrafiltration membrane containing trehalose This embodiment is basically the same as Embodiment 1, except that trehalose is introduced into the casting solution as an ice crystal inhibitor.

[0032] Preparation of trehalose-containing TiO2 sol casting solution: TiO2 sol was prepared according to the method in Example 1. The prepared TiO2 sol was added to 0.5 wt% HPC solution and stirred evenly. Then, trehalose was added to make the mass ratio of trehalose to water in the sol 5 wt%. The mixture was magnetically stirred at room temperature for 2 h until it was completely dissolved to obtain TiO2 sol casting solution containing trehalose.

[0033] (2) Coating and gelation: The coating steps are the same as in Example 1: The pretreated alumina support is immersed in the casting solution at a rate of 2 cm / min, and after immersion for 30 s, it is taken out and dried at room temperature for 40 min. The coating is repeated once, and then the gelation is completed by standing at room temperature for 2 h.

[0034] (3) Freezing and freeze drying: The freezing steps are the same as in Example 1: the temperature is reduced from 25°C to -40°C at a rate of 1°C / min and kept at that temperature for 1 h; then the sample is transferred to a freeze dryer pre-cooled to -60°C and dried for 48 h under a vacuum of less than 5 Pa.

[0035] (4) Sintering: The sintering procedure is the same as in Example 1: the temperature is increased to 400 ℃ at 1 ℃ / min, held for 3 h, and then cooled naturally to obtain a TiO2 ceramic composite ultrafiltration membrane containing trehalose.

[0036] like Figure 4 As shown, SEM characterization indicates that the film surface is continuous and dense, with no through cracks, and the surface micropores are more uniformly distributed, with the ice crystal pore size being significantly smaller than that in Example 1.

[0037] The pure water flux was tested at 0.2 MPa, and the obtained flux was approximately 80 L / (m²·h·bar). Using a dextran solution with a molecular weight of 5000 Da as a model solution, the retention rate remained above 95%, and the average pore size was approximately 4.0 nm. Compared with Example 1 without trehalose, the pure water flux increased by approximately 15% while maintaining essentially the same retention performance, the pore size distribution was more concentrated, and the surface microcrack density was significantly reduced. Example 4: Preparation of Al2O3 ceramic composite ultrafiltration membrane containing trehalose This embodiment is based on Example 2, but introduces trehalose into the boehmite sol casting solution.

[0038] Preparation of Boehmite Casting Solution Containing Trehalose: Boehmite sol was prepared according to the method in Example 2, and then mixed with polyvinyl alcohol and anhydrous ethanol solution and stirred until homogeneous. Trehalose was then added to make the mass ratio of trehalose to water in the sol 3 wt%, and stirred at room temperature for 2 h until completely dissolved to obtain Boehmite Casting Solution containing trehalose. The molar ratios of other components remained the same as in Example 2.

[0039] Coating and gelation: The coating steps are the same as in Example 2: The pretreated alumina support is immersed in the above-mentioned trehalose casting solution at a rate of 2 cm / min, and after immersion for 15 s, it is taken out and dried at room temperature for 40 min. The coating is repeated twice, and the gelation is completed by standing at room temperature for 2 h.

[0040] Freezing and freeze-drying: The freezing and freeze-drying conditions were the same as in Example 2: the temperature was lowered from 25 ℃ to -40 ℃ at a cooling rate of 2 ℃ / min and held for 1 h; then freeze-dried at -60 ℃ and a vacuum of less than 5 Pa for 48 h.

[0041] Sintering: The sintering process is the same as in Example 2: the temperature is increased to 900 ℃ at 1 ℃ / min, held for 3 h, and then naturally cooled to obtain an Al2O3 ceramic composite ultrafiltration membrane containing trehalose.

[0042] The membrane surface has finer and more uniform pores, with almost no microcracks observed, and the interface between the separation layer and the support is smoother and tighter.

[0043] The pure water flux was tested at 0.2 MPa, and the obtained flux was approximately 82 L / (m²·h·bar). Using PEG-5000 as a model solute, the rejection rate was approximately 93%, and the average pore size was approximately 4.0 nm. Compared with Example 2 (pure water flux 65 L / (m²·h·bar), rejection rate 90%), the flux increased by approximately 26% while the rejection performance was slightly improved, and the pore size distribution was more concentrated.

Claims

1. A method for preparing crack-free ceramic composite ultrafiltration membranes based on freeze-drying technology, characterized in that, Includes the following steps: (1) Select a porous ceramic support and clean and dry it to use it as the substrate; (2) The ceramic precursor, solvent, water and catalyst are mixed, and after hydrolysis and condensation reaction, and aging treatment is carried out, a stable ceramic sol is obtained. (3) The ceramic sol obtained in step (2) is coated on the surface of the porous ceramic support to form a wet film layer; (4) The porous ceramic support loaded with wet film layer is subjected to programmed cooling and freezing treatment to cause the liquid solvent in the pores of the wet film layer to undergo phase change and solidify. (5) Under vacuum, the solvent solidified in step (4) is removed by sublimation, while the gel skeleton structure is retained, and a dry film is obtained. (6) The dried sample is subjected to high-temperature heat treatment to obtain the crack-free ceramic composite ultrafiltration membrane.

2. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, The porous ceramic support in step (1) is made of one or more of alumina, zirconium oxide, kaolin or mullite; the average pore size of the porous ceramic support is 50-500 nm.

3. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, In step (2), an ice crystal inhibitor is also added to the ceramic sol; the ice crystal inhibitor is selected from at least one of sugar compounds, sugar alcohols or low molecular weight polyhydroxy polymers.

4. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 3, characterized in that, The ice crystal inhibitor is selected from one or more of trehalose, sucrose, glucose, maltose, mannitol, sorbitol or xylitol; the amount of ice crystal inhibitor added is such that its mass ratio with water in the ceramic sol is 1wt%-10wt%.

5. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, The ceramic precursor mentioned in step (2) is selected from alkoxides or inorganic salts of titanium, aluminum, zirconium, silicon or yttrium, specifically including tetrabutyl titanate, aluminum isopropoxide, aluminum sec-butoxide, aluminum nitrate, n-propyl zirconate, tetraethyl orthosilicate or yttrium nitrate; the preparation method of the sol includes: dissolving the ceramic precursor in an alcohol solvent, carrying out a hydrolysis-condensation reaction at 25-80°C under the action of an acidic or alkaline catalyst, adding a polymer binder, and then aging for 5-100 hours to obtain a stable sol.

6. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, The environmental conditions for coating in step (3) are: temperature 20-40℃, relative humidity 40%-90%.

7. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, The cooling rate of the freezing process in step (4) is 0.5-5℃ / min, the freezing endpoint temperature is -80℃~-20℃, and the temperature is kept at the endpoint temperature for a certain period of time to ensure that the solvent is completely frozen.

8. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, The freeze-drying conditions described in step (5) are: cold trap temperature < -50℃, vacuum degree < 10Pa, and drying time of 2-50h.

9. The method for preparing a crack-free ceramic composite ultrafiltration membrane based on freeze-drying technology according to claim 1, characterized in that, The heating rate of the sintering in step (6) is 0.5-5℃ / min, the final sintering temperature is 300-1000℃, and the holding time is 0.5-5h.

10. A ceramic composite ultrafiltration membrane prepared by the method according to any one of claims 1 to 9, characterized in that, The separation layer of the ceramic composite ultrafiltration membrane is crack-free, with an average pore size of 1-10 nm and a rejection rate of ≥90% for substances with a molecular weight >1 kDa.

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