A method for preparing a three-layer ceramic composite membrane

Abstract

The application relates to a ceramic membrane method, in particular to a preparation method of a three-layer ceramic composite membrane. The Al2O3 film layer is prepared on a macroporous carrier in advance by using a boehmite sol film preparation liquid, and the TiO2 film layer is prepared on the Al2O3 film layer, so that the cracking of the TiO2 film layer can be avoided. After the TiO2 sol film preparation liquid is coated and vacuum freeze drying is adopted, calcination is carried out, so that the prepared three-layer ceramic composite membrane has the advantages of large flux and cracking avoidance.

Description

Technical Field

[0001] This invention relates to a method for preparing a ceramic membrane, and more particularly to a method for preparing a three-layer ceramic composite membrane. Background Technology

[0002] Compared to organic membranes, inorganic membranes possess superior thermal stability, mechanical strength, and high-temperature resistance. Ceramic membranes, a typical inorganic membrane material, are widely used in demanding production environments such as biomedicine and energy chemicals.

[0003] Among them, TiO2 material is an ideal material for preparing ceramic membranes due to its advantages such as good chemical stability, low cost, and high hydrophilicity, and the research on the preparation of TiO2 membranes has also received widespread attention. The sol-gel method is an important process for preparing ceramic membrane materials. This process can prepare the separation layer of high-performance ceramic membranes with uniform pore size, narrow pore size distribution, and reliable quality. Moreover, the sol-gel method for preparing TiO2 ceramic membranes has advantages such as low production cost, simple process, and easy adjustment of composition. Among its many methods, it is considered the preferred method for industrial-scale preparation of TiO2 ceramic membranes.

[0004] In the preparation of TiO2 ceramic membranes, large-pore supports are often used to ensure flux. However, the large pore size of the support causes uneven stress in the TiO2 membrane layer, leading to cracking during heat treatment and further affecting the membrane's integrity and performance. To address this issue, the inventors attempted to pre-prepare an Al2O3 membrane layer on the support surface before preparing the TiO2 membrane, and then continue preparing a TiO2 membrane layer on the Al2O3 membrane layer, thus forming a three-layer ceramic composite membrane. This method effectively solves the membrane cracking problem, but the three-layer ceramic membrane results in a decrease in pure water flux, making it difficult to meet the needs of large-scale applications. Therefore, how to improve the pure water flux of ceramic membranes while solving the membrane cracking problem during heat treatment is a pressing technical challenge in this field. Summary of the Invention

[0005] To address the aforementioned problems, this invention pre-prepares an Al2O3 film layer on a macroporous carrier using a boehmite sol film-forming solution, and then prepares a TiO2 film layer on top of the Al2O3 film layer, thus avoiding cracking of the TiO2 film layer. Furthermore, after coating with the TiO2 sol film-forming solution, the film is freeze-dried using a vacuum freeze dryer and then calcined, resulting in a three-layer ceramic composite membrane that not only avoids cracking but also boasts high flux.

[0006] The present invention provides a three-layer ceramic composite membrane, comprising a ceramic carrier, an Al2O3 membrane layer and a TiO2 membrane layer connected in sequence, wherein the ceramic carrier has an average pore size of 100-500 nm and the three-layer ceramic composite membrane has an average pore size of 2-5 nm.

[0007] The present invention also provides a method for preparing the above-mentioned three-layer ceramic composite film, which includes the following steps:

[0008] Step 1: Prepare film-forming solutions for boehmite sol and TiO2 sol respectively;

[0009] Step 2: Apply boehmite sol to the surface of the carrier, dry it, and then calcine it to form an Al2O3 film.

[0010] Step 3: Continue to coat the surface of the Al2O3 film with TiO2 sol film-forming solution, and place it in a vacuum freeze dryer. After freezing at a temperature below -50°C for 2-5 hours and vacuum drying for 5-20 hours, it is calcined to form the three-layer ceramic composite film.

[0011] Preferably, the preparation steps of boehmite sol in step 1 are as follows: aluminum isopropoxide is dissolved in water and mixed evenly, then nitric acid solution with a certain dilution ratio is added, and finally polyvinyl alcohol solution and anhydrous ethanol are added to obtain boehmite sol.

[0012] Preferably, the mass ratio of aluminum isopropoxide, nitric acid, polyvinyl alcohol, water and anhydrous ethanol in the boehmite sol is 1:0.05-5:0.1-0.5:10-50:10-50.

[0013] Preferably, in step 1, the preparation steps of the TiO2 sol-film forming solution are as follows:

[0014] Tetrabutyl titanate and concentrated nitric acid were added to anhydrous ethanol solution, mixed, and then slowly added dropwise to a solution containing deionized water, concentrated nitric acid, and ethanol. After continuous stirring, an aqueous solution of hydroxypropyl cellulose was added to obtain TiO2 sol film-forming solution.

[0015] Preferably, the molar ratio of Ti(OC4H9)4:HNO3:C2H5OH:H2O in the TiO2 sol film-forming solution is 1:0.25-1:2-5:3-10, and the mass fraction of hydroxypropyl cellulose in the TiO2 film-forming solution is 0.1-0.5%.

[0016] Preferably, the carrier in step 2 is a ceramic carrier with a pore size of 100-500 nm.

[0017] Preferably, the drying in step 2 is done at room temperature for 5-20 hours, the calcination temperature is 600-1200℃, and the calcination time is 1-6 hours.

[0018] Preferably, the coating of boehmite sol in step 2 and the coating of TiO2 sol film-forming solution in step 3 are both applied by dip-coating method, with a dip-coating rate of 2-10 mm / s and a dip time of 5-120 s.

[0019] Preferably, the freezing temperature in step 3 is (-80) - (-55°C).

[0020] Preferably, the calcination temperature in step 3 is 200-400℃, the calcination time is 2-5h, and the heating and cooling rate is 0.5-2℃ / min.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] First, this invention innovatively prepares an Al2O3 film layer between the macroporous support and the TiO2 film layer in advance, thereby ensuring the stress uniformity of the TiO2 film layer and solving the problem that the TiO2 film layer prepared by the sol-gel method on the macroporous support is prone to cracking.

[0023] Secondly, while addressing the issue of easy cracking in TiO2 films prepared on macroporous carriers, this invention employs a vacuum freeze dryer followed by calcination after coating the TiO2 sol film-forming solution. This reduces changes in the film structure caused by high temperatures during the drying process, thereby producing an ultrathin TiO2 film structure. Compared with existing technologies, the film thickness in this invention does not exceed 5 μm, and the average pore size of the three-layer ceramic composite film is 2-5 nm, significantly improving the water flux of the ceramic film. Attached Figure Description

[0024] Figure 1 The surface / cross-section SEM images of the coatings prepared by coating TiO2 sol film-forming solutions with three different acid-titanium molar ratios onto the transition layer are shown, where the acid-titanium ratios of a, b, and c are 1, 0.75, and 0.5, respectively.

[0025] Figure 2 The particle size distribution of TiO2 sol is shown in the four acid-to-titanium ratios.

[0026] Figure 3 This diagram shows the main location distribution of aluminum and titanium elements in the composite film.

[0027] Figure 4 The graph shows the retention data of coatings prepared on two carriers with different average pore sizes under different aging methods, where L represents vacuum freeze-drying and H represents constant temperature and humidity. Detailed Implementation

[0028] Example 1

[0029] Example 1: The three-layer ceramic composite membrane was prepared using the following steps:

[0030] Aluminum isopropoxide was added to pure water heated to 85°C and stirred until homogeneous. Nitric acid solution was then slowly added, and stirring continued until the solution became a clear, pale blue color. This clear solution was then mixed with polyvinyl alcohol solution and anhydrous ethanol to prepare a white boehmite sol. The ratio of aluminum isopropoxide:nitric acid:polyvinyl alcohol:deionized water:anhydrous ethanol in the solution was 1:0.1:0.23:26.1:23.8, where the density of the nitric acid solution was 0.01 g / ml and the mass fraction of the polyvinyl alcohol solution was 6.5%. A carrier with an average pore size of 400 nm was placed in this film-forming solution using an dip-coating machine at a rate of 5 mm / s for coating. The immersion time was 60 s, and one coating was sufficient. The coated carrier was placed at room temperature for 12 h and then calcined at 900°C for 3 h to obtain an alumina film. The heating and cooling rates during calcination were both 1 min / °C.

[0031] According to the molar ratio of Ti(OC4H9)4:HNO3:C2H5OH:H2O of 1:0.5:3.7:5, tetrabutyl titanate and concentrated nitric acid were added to anhydrous ethanol solution. After mixing, the mixture was slowly added dropwise to a solution containing deionized water, concentrated nitric acid, and ethanol, and stirred continuously for 2 hours to prepare a light blue, clear TiO2 sol. 38.75 g of 0.3% hydroxypropyl cellulose aqueous solution was added as an organic additive to form a stable and uniform TiO2 sol film-forming solution. The solution was coated onto the surface of an alumina film using an dip-coating machine at a rate of 5 mm / s, and coated four times at room temperature, with each dip lasting 15 seconds.

[0032] After coating, the material was placed in a cold trap at -70°C in a vacuum freeze dryer for 3 hours, then dried in a vacuum environment for 12 hours. Finally, it was placed in a muffle furnace and calcined in an air atmosphere. The temperature was increased to 350°C at a rate of 1°C / min and held for 3 hours. Then, it was cooled to room temperature at a rate of 1°C / min to obtain a uniform and complete three-layer ceramic composite film.

[0033] Example 2

[0034] The difference from Example 1 is that in Example 2, the TiO2 sol film-forming solution was applied to the Al2O3 film surface twice.

[0035] Example 3

[0036] The difference from Example 1 is that a carrier with an average pore size of 200 nm is selected.

[0037] Comparative Example 1

[0038] The difference from Example 1 is that the TiO2 sol film coating solution was aged in a constant temperature and humidity chamber after coating, with a temperature of 60°C, a relative humidity of 70%, and a time of 12 hours.

[0039] Comparative Example 2

[0040] The difference from Comparative Example 1 is that the molar ratio of tetrabutyl titanate: concentrated nitric acid: anhydrous ethanol: deionized water in the TiO2 sol film-forming solution is 1:1:3.7:5.

[0041] Comparative Example 3

[0042] The difference from Comparative Example 1 is that the molar ratio of tetrabutyl titanate: concentrated nitric acid: anhydrous ethanol: deionized water in the TiO2 sol film-forming solution is 1:0.75:3.7:5.

[0043] Comparative Example 4

[0044] The difference from implementation 3 is that the TiO2 sol film-forming solution is coated twice.

[0045] Characterization

[0046] The average pore size and pure water flux of the samples from the above embodiments and comparative examples were measured. The pressures for the retention test and pure water flux test were 0.2 MPa and 0.1 MPa, respectively. The results are as follows:

[0047] Table 1 Performance results of ceramic composite films prepared in different embodiments and comparative examples

[0048]

[0049] in addition, Figure 1 SEM images of the surface / cross-section of coatings prepared by coating three groups of TiO2 sol-gel film-forming solutions with different acid-to-titanium molar ratios onto a transition layer. Figure 1 It can be seen that the TiO2 sol film-forming solutions under all three acid-to-titanium ratios can successfully adhere. Figure 2 The particle size of TiO2 sol under different acid-to-titanium ratios showed that as the acid-to-titanium ratio decreased, the sol particle size increased, resulting in better coating effect of the film-forming solution. EDX elemental characterization of the cross-section of the calcined composite ceramic film revealed (e.g.) Figure 3 Titanium is mainly concentrated on the outermost layer, while due to the small particle size of TiO2 sol, a very small amount of titanium is present in the inner alumina transition layer and ceramic carrier. Figure 4 It can be seen that in Example 2, when the acid-to-titanium ratio is 0.5 and the coating is applied twice, the film prepared by aging the TiO2 sol coated on the surface of a ceramic alumina carrier with an average pore size of 400 nm using a freeze-drying process is tested for retention. The dextran molecular weight corresponding to a retention rate of 90% is 12521 Da, which is obtained through the Stokes equation. Calculations show that the pore radius is d=2r, and the average pore size of the film is 5.06nm. This coating has the largest average pore size, resulting in the highest pure water flux of 25.3L / m³.2 ·h -1 ·bar -1 Based on the above characterization data, it can be seen that the TiO2 film prepared by aging the film-forming solution (TiO2 sol film-forming solution coated on the surface of Al2O3 film layer twice) on the surface of a carrier with an average pore size of 400 nm using a vacuum freeze-drying process has the highest pure water permeation flux.

Claims

1. A method for preparing a three-layer ceramic composite membrane for increasing pure water flux, characterized in that, The three-layer ceramic composite membrane comprises a ceramic carrier, an Al2O3 membrane layer, and a TiO2 membrane layer connected in sequence. The ceramic carrier has an average pore size of 100-500 nm, and the three-layer ceramic composite membrane has an average pore size of 2-5 nm. The method includes the following steps: Step 1: Prepare film-forming solutions for boehmite sol and TiO2 sol respectively; Step 2: Apply boehmite sol to the surface of the carrier, dry it, and then calcine it to form an Al2O3 film. Step 3: Continue to coat the surface of the Al2O3 film with TiO2 sol film-forming solution, and place it in a vacuum freeze dryer. After freezing at a temperature below -50°C for 2-5 hours and vacuum drying for 5-20 hours, it is calcined to form the three-layer ceramic composite film.

2. The preparation method according to claim 1, characterized in that, The preparation steps of boehmite sol in step 1 are as follows: Aluminum isopropoxide was dissolved in water and mixed evenly. Then, a nitric acid solution with a certain dilution ratio was added. Finally, polyvinyl alcohol solution and anhydrous ethanol were added to obtain boehmite sol.

3. The preparation method according to claim 2, characterized in that, The mass ratio of aluminum isopropoxide, nitric acid, polyvinyl alcohol, water and anhydrous ethanol in boehmite sol is 1:0.05-5:0.1-0.5:10-50:10-50.

4. The preparation method according to claim 1, characterized in that, In step 1, the preparation steps of the TiO2 sol-film forming solution are as follows: Tetrabutyl titanate and concentrated nitric acid were added to anhydrous ethanol solution, mixed, and then slowly added dropwise to a solution containing deionized water, concentrated nitric acid, and ethanol. After continuous stirring, an aqueous solution of hydroxypropyl cellulose was added to obtain TiO2 sol film-forming solution.

5. The preparation method according to claim 4, characterized in that, The molar ratio of Ti(OC4H9)4:HNO3:C2H5OH:H2O in the TiO2 sol film-forming solution is 1:0.25-1:2-5:3-10, and the mass fraction of hydroxypropyl cellulose in the TiO2 film-forming solution is 0.1-0.5%.

6. The preparation method according to claim 1, characterized in that, In step 2, the drying process involves drying at room temperature for 5-20 hours, and the calcination temperature is 600-1200℃ for 1-6 hours.

7. The preparation method according to claim 1, characterized in that, In step 2, the boehmite sol coating and in step 3, the TiO2 sol film-forming solution coating are both applied by dip-coating method, with a dip-coating rate of 2-10 mm / s and a dip time of 5-120 s.

8. The preparation method according to claim 1, characterized in that, The freezing temperature in step 3 is -80℃ to -55℃.

9. The preparation method according to claim 1, characterized in that, The calcination temperature in step 3 is 200-400℃, the calcination time is 2-5h, and the heating and cooling rate is 0.5-2℃ / min.

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