Preparation method of ultrathin oriented molecular sieve membrane
By coating nanocrystal seeds on the surface of molecular sieve membranes and performing multiple solid-phase transformations, the problems of excessive thickness and poor repeatability of DDR membranes were solved, achieving high permeability and high selectivity of ultrathin oriented molecular sieve membranes, which are suitable for industrial gas separation.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NANJING TECH UNIV
- Filing Date
- 2024-05-20
- Publication Date
- 2026-08-04
AI Technical Summary
In existing technologies, DDR membranes prepared by hydrothermal synthesis are typically 3-10 micrometers thick, which increases gas mass transfer resistance, reduces membrane permeability, and is not conducive to industrial applications; while DDR membranes prepared by gel-free methods have poor repeatability and are difficult to achieve high separation performance.
An optimized solid-phase conversion method was adopted to prepare an ultrathin oriented molecular sieve membrane by coating nanocrystal seeds on the surface defects of the molecular sieve membrane and performing secondary synthesis and calcination. The nanocrystal seeds consist of the supernatant after wet ball milling and induced seed crystals, and are synthesized multiple times in combination with the solid-phase conversion method to fill the defects.
Effective control of membrane thickness within the range of 0.02-2 micrometers improves membrane permeability and separation selectivity, reduces synthesis costs and pollution emissions, and enhances membrane repeatability and industrial application potential.
Smart Images
Figure CN118846821B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of molecular sieve membrane synthesis, and more specifically to a method for preparing an ultrathin oriented molecular sieve membrane. Background Technology
[0002] Molecular sieve membranes are aluminosilicate materials prepared by growing molecular sieves into continuous thin films on a carrier. Due to their regular microporous structure, molecular sieve membranes exhibit excellent gas separation performance, along with high mechanical strength and good hydrothermal stability, making them suitable for gas separation under industrial conditions. For example, DDR-type molecular sieve membranes have a pore size of 0.36 × 0.44 nanometers and are suitable for carbon dioxide-methane separation, hydrogen-methane separation, and carbon dioxide-nitrogen separation.
[0003] Currently, there are several methods for preparing molecular sieve membranes on supports, including hydrothermal synthesis, dry gelation, and gel-free methods. Hydrothermal synthesis involves placing a support or a support pre-coated with seed crystals in a synthetic gel to crystallize and form a membrane. However, the synthetic gel requires mixing the raw materials and undergoing different aging processes, often resulting in significant waste and increased costs for waste treatment. The gel-free method involves pre-coating the silicon source onto the support. The synthesis solution requires only a small amount of template agent and mineralizer, eliminating the need for gel preparation and thus avoiding gel aging and recovery steps. However, this method often results in poor membrane reproducibility and low separation performance in a single synthesis.
[0004] Chinese invention patent application CN106745026A discloses a highly selective DDR molecular sieve membrane, which is prepared by coating Sigma-1 seeds onto a four-channel hollow alumina fiber support and then performing hydrothermal synthesis to produce a highly selective randomly oriented DDR molecular sieve membrane. Chinese invention patent application CN115569534A discloses a method for preparing a DDR molecular sieve membrane for selectively separating carbon dioxide and methane, using different mineralizers to reduce membrane thickness and improve permeation performance.
[0005] In summary, existing hydrothermal synthesis methods typically produce DDR membranes with thicknesses ranging from 3 to 10 micrometers, increasing gas mass transfer resistance and reducing membrane permeability, which is detrimental to industrial applications. The inventors attempted to synthesize oriented molecular sieve membranes using a solid-phase conversion method to better control membrane thickness and thus improve permeability. However, they found that DDR membranes prepared using this method exhibited poor reproducibility and were prone to defects, hindering their application in separation. Therefore, optimizing the solid-phase conversion method for preparing DDR membranes to obtain DDR membranes with high separation performance is of great significance for the application of DDR membranes. Summary of the Invention
[0006] To address the aforementioned problems in the prior art, this invention employs an optimized solid-phase conversion method to prepare an ultrathin oriented molecular sieve membrane, which exhibits excellent permeation performance.
[0007] The objective of this invention can be achieved through the following technical solutions:
[0008] A method for preparing an ultrathin oriented molecular sieve membrane, wherein the method involves coating nanocrystal seeds onto the surface defects of a molecular sieve membrane prepared by a secondary growth method, followed by secondary synthesis and calcination to obtain an ultrathin oriented molecular sieve membrane. The nanocrystal seeds are obtained by centrifuging the supernatant of a molecular sieve prepared by a seed crystal method after wet ball milling.
[0009] Furthermore, the specific steps of the preparation method are as follows:
[0010] (1) Preparation of nanocrystal seeds: The template agent, mineralizer, deionized water and silicon source are mixed to obtain a synthesis solution. After hydrothermal synthesis, a molecular sieve is obtained. The molecular sieve is wet ball-milled and centrifuged. The supernatant is used as a nanocrystal seed for later use. The centrifuged precipitate is added as an inducing seed to the synthesis solution of the controlled formula and hydrothermally synthesized again to obtain molecular sieve seed.
[0011] (2) The molecular sieve seed crystals are uniformly coated onto the carrier;
[0012] (3) Preparation of molecular sieve membrane precursor synthesis solution: Mix template agent and mineralizing agent, add water and stir to obtain casting solution;
[0013] (4) The carrier obtained in step (2) and the synthesis solution obtained in step (3) are subjected to solid-phase conversion to synthesize a molecular sieve membrane;
[0014] (5) Coat the nanocrystal seeds obtained in step (1) onto the defects on the film surface, and then repeat steps (3) and (4) to perform solid-phase conversion synthesis again;
[0015] (6) The re-synthesized molecular sieve membrane is calcined to obtain an ultrathin oriented molecular sieve membrane.
[0016] Further, in step (1), the template agent is adamantane, the mineralizing agent is ethylenediamine, sodium hydroxide, potassium hydroxide or potassium fluoride, the silicon source is silica sol, the molar ratio of template agent, mineralizing agent and H2O in the obtained in-situ hydrothermal synthesis solution is (1-12):(20-200):(1000-5000), the molar ratio of template agent, mineralizing agent and H2O in the obtained seed crystal hydrothermal synthesis solution is (1-12):(20-200):(1000-20000), and the amount of induced seed crystals added to the synthesis solution after formula adjustment is 0.01-5 wt.%.
[0017] Furthermore, in step (1), the in-situ hydrothermal synthesis temperature is 100-220℃ and the synthesis time is 24-240 hours, while the seed hydrothermal synthesis temperature is 100-220℃ and the synthesis time is 24-144 hours.
[0018] Further, the carrier described in step (2) is in the shape of a single-channel tubular, multi-channel tubular, flat, or hollow fiber tubular shape, and the material includes metal, alumina, zirconium dioxide, silicon dioxide, silicon carbide, or silicon nitride, with a pore size of 2-2000 nanometers; the coating method is wiping, dipping, spin coating, or spraying.
[0019] Furthermore, when using dip coating, the intact molecular sieve seed crystals are dispersed in water to form a seed crystal dispersion with a concentration of 0.1-5 wt.%, and 1-3 wt.% of binder is added. The binder includes hydroxypropyl cellulose, methyl cellulose, polyvinyl alcohol, polyethylene glycol or polymethyl methacrylate, etc., and 1-3 wt.% of silica sol is added. The seed crystal dispersion is uniformly dispersed by ultrasonication and stirring, and then dip coating is performed.
[0020] Further, the template agent in step (3) is adamantane, the mineralizing agent is ethylenediamine, and the molar ratio of template agent, mineralizing agent and water in the casting solution is (1-12):(20-200):(1000-20000).
[0021] Furthermore, the synthesis time in step (4) is 12-96 hours and the temperature is 100-220℃.
[0022] Furthermore, the synthesis time in step (5) is 12-96 hours and the temperature is 100-220℃.
[0023] Furthermore, the coating method described in step (5) includes vacuum suction, wiping, dipping, or spraying.
[0024] Furthermore, when using vacuum suction, the nanocrystal seeds are dispersed in water to form a nanocrystal seed dispersion with a concentration of 0.1-10 wt.%. The seed dispersion is then uniformly dispersed by ultrasonication and stirring, and then vacuum suction is performed for 1-10 minutes.
[0025] Furthermore, the roasting atmosphere in step (6) is ozone or air, the roasting time is 24-144 hours, the temperature is 150-400℃, and the heating and cooling rates are 1℃ / min.
[0026] The molecular sieve membranes prepared by the preparation method of the present invention include DDR molecular sieve membranes, MFI molecular sieve membranes, CHA molecular sieve membranes, LTA molecular sieve membranes, or T-type molecular sieve membranes.
[0027] Furthermore, the thickness of the molecular sieve membrane is 0.02-2 micrometers.
[0028] Compared with the prior art, the present invention has the following advantages:
[0029] (1) This invention employs a solid-phase conversion method to synthesize oriented molecular sieve membranes. Compared to the traditional hydrothermal method, the solid-phase conversion method synthesizes membranes by converting a limited silicon source within the seed layer or on the surface of the carrier. The crystallization process is an epitaxial growth of the seed crystal, effectively controlling the orientation. Furthermore, as the limited silicon source is consumed, crystal growth gradually stops, thereby achieving control over the membrane thickness. It also avoids the use of large quantities of synthetic raw materials, reducing pollution emissions and lowering synthesis costs. The use of intact molecular sieve seed crystals reduces porosity and seepage.
[0030] (2) In view of the problem of poor reproducibility in the preparation of molecular sieve membranes by solid-phase conversion method, the present invention further fills the membrane layer with nanocrystal seeds, and then uses solid-phase conversion method to synthesize again. The seeds are used as part of the silicon source to continue to grow and fill the defects in the membrane, thereby improving the selectivity while maintaining the membrane permeability. Attached Figure Description
[0031] Figure 1 SEM image of the DDR molecular sieve membrane prepared in Example 1;
[0032] Figure 2 SEM image of the DDR molecular sieve membrane prepared in Example 2;
[0033] Figure 3 The XRD pattern of the DDR molecular sieve membrane prepared in Example 2; Detailed Implementation
[0034] The present invention will now be described in detail with reference to specific embodiments. These embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.
[0035] Example 1
[0036] This embodiment uses a single solid-phase conversion to synthesize an ultrathin oriented DDR molecular sieve membrane. The specific steps are as follows:
[0037] Step 1: Mix adamantane and ethylenediamine, then add deionized water and stir for 2 hours. Add silica sol (30 wt.% SiO2) and continue stirring for 2 hours to obtain a seed crystal synthesis solution. The molar ratio of the synthesis solution is ADA (adamantane): EDA (ethylenediamine): SiO2: H2O = 9:150:100:4000. Perform hydrothermal synthesis at 160℃ for 48 hours to obtain an all-silicon DDR molecular sieve. Ball mill at 350 rpm for 4 hours. Centrifuge the ball-milled suspension at a maximum speed of 11000 rpm for 30 minutes. Evaporate and concentrate the supernatant containing nanocrystals for later use. Dry the centrifuged product for later use. ADA, EDA, H2O and silica sol were mixed sequentially, and then the obtained ball-milled seed crystals were added and stirred for 12 hours to obtain a molecular sieve seed crystal synthesis solution. The solution was hydrothermally synthesized at 160℃ for 72 hours to obtain oriented DDR molecular sieve seed crystals. The molar ratio of the synthesis solution was ADA:EDA:SiO2:H2O = 3:50:100:4000.
[0038] Step 2: Select four-channel hollow fiber alumina with a pore size of 150 nm as the carrier. Seal both ends of the carrier with PTFE raw material tape. Dip silica sol (20 wt.% SiO2) onto the outer surface of the carrier. Disperse the intact molecular sieve seed crystals in water, add 2 wt.% hydroxypropyl cellulose and silica sol, and treat with ultrasound and stirring to make the seed crystals disperse evenly. Then, immerse the modified carrier in the seed crystal dispersion for 15 seconds by dip coating and then lift it out and dry it.
[0039] Step 3: Mix ADA and EDA, add H2O and stir for 1 hour to obtain casting solution. The molar ratio of each component in the casting solution is ADA:EDA:H2O = 3:50:4000.
[0040] Step 4: Place the carrier in the reactor, add 1 gram of casting solution and provide a steam environment, synthesize in an oven at 160°C for 24 hours, and remove the carrier after cooling the reactor;
[0041] Step 5: The oriented DDR molecular sieve membrane obtained in Step 4 is calcined at 220°C for 96 hours in an ozone atmosphere to remove the template agent in the pores. The heating and cooling rates are both 1°C / min. After cooling, an ultrathin oriented DDR molecular sieve membrane is obtained.
[0042] The surface and cross-section of the obtained oriented DDR molecular sieve membrane are as follows: Figure 1 As shown, (a) is a SEM image of the film surface; (b) is a SEM image of the film cross section. It can be seen from the images that the DDR crystal morphology on the film surface is a typical diamond shape, which conforms to the h0h orientation morphology. The crystals grow well together, the film thickness is uniform, and the total thickness of the film layer and the silicon source is 2.3 micrometers, while the actual film thickness is about 0.8 micrometers.
[0043] CO2 / CH4 gas separation was tested on a support with a DDR molecular sieve membrane on its surface. The test conditions were: temperature 25℃, atmospheric pressure 102 kPa, feed gas flow rate 100 mL / min, and molar composition CO2 / CH4 = 50 / 50. The gas flow rate on the permeate side was measured using a soap bubble flow meter; the gas composition on the permeate side was analyzed using a gas chromatograph (Aglient-7820A).
[0044] The formula for calculating gas permeability is: P = V / (ΔP × S). Where V is the flow rate of the permeating gas (CO2 or CH4), in mol / s, and S is the membrane area, in m². 2 ΔP is the pressure difference between the feed side and the permeate side of the membrane tube, in Pa.
[0045] Separation selectivity calculation formula: α=PCO2 / PCH4, that is, the ratio of the permeability of CO2 to CH4.
[0046] The CO2 / CH4 gas separation test results for this membrane showed that, at 0.1 MPa, its average CO2 permeability was 2 × 10⁻⁶. - 7 mol·m -2 ·s -1 ·Pa -1 The average selectivity for separating CO2 / CH4 was 30.
[0047] Therefore, compared with the traditional hydrothermal synthesis of randomly oriented molecular sieve membranes, the solid-phase conversion method can reduce the membrane thickness and improve the orientation. The membrane thickness is reduced from about 2-10 micrometers in the traditional hydrothermal synthesis to 0.8 micrometers. The CO2 permeability is increased by nearly 5 times. However, the CO2 / CH4 separation selectivity is significantly reduced because the reproducibility of the single synthesis is poor and intergranular defects are easily generated, which leads to the decrease in separation selectivity.
[0048] Example 2
[0049] This embodiment uses nanocrystal seeds to fill membrane defects and synthesize a high-performance DDR molecular sieve membrane. The specific steps are as follows:
[0050] Step 1: Mix adamantane and ethylenediamine, then add deionized water and stir for 2 hours. Add silica sol (30 wt.% SiO2) and continue stirring for another 2 hours to obtain a seed crystal synthesis solution. The molar ratio of the synthesis solution is ADA (adamantane):EDA (ethylenediamine):SiO2:H2O = 9:150:100:4000. Perform hydrothermal synthesis at 160℃ for 48 hours to obtain all-silicon DDR molecular sieve seed crystals. After ball milling and centrifugation, evaporate and concentrate the supernatant containing the nano-seed crystals for later use. Dry the centrifuged product for later use. Alternatively, mix ADA, EDA, H2O, and silica sol sequentially, then add the obtained ball-milled seed crystals and stir for 12 hours to obtain a molecular sieve seed crystal synthesis solution. Perform hydrothermal synthesis at 160℃ for 72 hours to obtain oriented DDR molecular sieve seed crystals. The molar ratio of the synthesis solution is ADA:EDA:SiO2:H2O = 3:50:100:4000.
[0051] Step 2: Select four-channel hollow fiber alumina with a pore size of 150 nm as the carrier. Seal both ends of the carrier with PTFE raw material tape. Dip silica sol (20 wt.% SiO2) onto the outer surface of the carrier. Disperse the intact molecular sieve seed crystals in water, add 2 wt.% hydroxypropyl cellulose and silica sol, and treat with ultrasound and stirring to make the seed crystals disperse evenly. Then, immerse the modified carrier in the seed crystal dispersion for 15 seconds by dip coating and then lift it out and dry it.
[0052] Step 3: Mix ADA and EDA, add H2O and stir for 1 hour to obtain molecular sieve membrane synthesis solution. The molar ratio of each component in the synthesis solution is ADA:EDA:H2O = 3:50:4000.
[0053] Step 4: The synthesis solution prepared in Step 4 is coated onto the porous support with the oriented DDR molecular sieve seed layer prepared in Step 3 by dip coating. Then the support is placed in a reaction vessel, 1 gram of synthesis solution is added to provide a steam environment, and the synthesis is carried out in an oven at 160°C for 24 hours. After cooling the reaction vessel, the porous support is taken out.
[0054] Step 5: The nanocrystal seeds prepared in Step 1 are coated onto the surface of the membrane prepared in Step 4 using a vacuum suction method. The concentration of the nanocrystal seed dispersion is 0.1 wt.%, and the suction time is 2 min. Then, Step 4 is repeated to perform solid-phase transformation synthesis again.
[0055] Step 6: The oriented DDR molecular sieve membrane obtained in Step 5 is calcined in an ozone atmosphere at 220°C for 96 hours to remove the template agent in the pores. The heating and cooling rates are both 1°C / min. After cooling, an ultrathin oriented DDR molecular sieve membrane is obtained.
[0056] The surface and cross-section of the obtained oriented DDR molecular sieve membrane are as follows: Figure 2As shown, (a) is a SEM image of the film surface; (b) is a SEM image of the film cross-section. From the images, it can be seen that the DDR crystal morphology on the film surface is a typical diamond shape, conforming to the h0h orientation morphology. The crystals grow well together, and the film thickness is uniform. The total thickness of the film layer and the silicon source is 2.3 micrometers, while the actual film thickness is approximately 1.2 micrometers. Figure 3 The XRD pattern of the prepared DDR molecular sieve membrane is shown.
[0057] The CO2 / CH4 gas separation test results for this membrane showed that, at 0.1 MPa, its average CO2 permeability was 1.5 × 10⁻⁶. - 7 mol·m -2 ·s -1 ·Pa -1 The average CO2 / CH4 separation selectivity was 150. This demonstrates that the solid-phase conversion method for synthesizing oriented molecular sieve membranes can effectively control membrane thickness. After resynthesis using nanocrystal seeds to fill defects, the membrane thickness increased slightly, CO2 permeability decreased slightly, but CO2 / CH4 selectivity was significantly improved.
[0058] By filling membrane defects with nanocrystal seeds and then synthesizing oriented membranes using solid-phase conversion, effective control of membrane thickness was achieved. This improved permeability without affecting membrane separation selectivity, and significantly reduced membrane preparation costs by eliminating the need for synthesis solutions. It also avoided the use of high-temperature reactors, thus reducing equipment costs.
[0059] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.
[0060] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.
Claims
1. A method for preparing an ultrathin oriented molecular sieve membrane, characterized in that The method involves coating nanocrystal seeds onto surface defects of a molecular sieve membrane prepared by a secondary growth solid-phase conversion method, followed by secondary synthesis and calcination to obtain an ultrathin oriented molecular sieve membrane. The nanocrystal seeds are obtained by ball milling and centrifuging of a molecular sieve prepared by a seed crystal method, and the supernatant is collected. The centrifuged precipitate is added as an inducing seed crystal to the synthesis solution of the controlled formula. Molecular sieve seeds are obtained by hydrothermal synthesis using the seed crystal method, and the molecular sieve seeds are uniformly coated onto a carrier and synthesized by solid-phase conversion with the casting solution to form the molecular sieve membrane. The specific steps of the preparation method are as follows: (1) Preparation of nano-seeds: The template agent, mineralizer, deionized water and silicon source are mixed to obtain a synthesis liquid. After in-situ hydrothermal synthesis, molecular sieves are obtained. The molecular sieves are wet ball-milled and centrifuged. The supernatant is taken as nano-seeds for later use. The centrifuged precipitate is added to the synthesis liquid of the control formula as an inducing seed. Molecular sieve seeds are obtained by hydrothermal synthesis using the seed method. (2) The molecular sieve seed crystals are uniformly coated onto the carrier; (3) Preparation of molecular sieve membrane precursor synthesis solution: Mix template agent and mineralizing agent, add water and stir to obtain casting solution; (4) The carrier obtained in step (2) and the synthesis solution obtained in step (3) are subjected to solid-phase conversion to synthesize a molecular sieve membrane; (5) Coat the nanocrystal seeds obtained in step (1) onto the defects on the film surface, and then repeat steps (3) and (4) to perform solid-phase conversion synthesis again; (6) The re-synthesized molecular sieve membrane is calcined to obtain an ultrathin oriented molecular sieve membrane. The template agent mentioned in step (1) is adamantane, the mineralizing agent is ethylenediamine, sodium hydroxide, potassium hydroxide or potassium fluoride, the silicon source is silica sol, the molar ratio of template agent, mineralizing agent and H2O in the obtained in-situ hydrothermal synthesis solution is (1-12):(20-200):(1000-5000), the molar ratio of template agent, mineralizing agent and H2O in the obtained seed crystal hydrothermal synthesis solution is (1-12):(20-200):(1000-20000), and the amount of induced seed crystals added in the synthesis solution after formula adjustment is 0.01-5 wt.%. In step (1), the in-situ hydrothermal synthesis temperature is 100-220℃ and the synthesis time is 24-240 hours; the seed hydrothermal synthesis temperature is 100-220℃ and the synthesis time is 24-144 hours. The ball milling jar material used in the wet ball milling in step (1) includes zirconium oxide, polytetrafluoroethylene or corundum, the ball milling beads material includes zirconium oxide, quartz or silicon carbide, the diameter is 0.5-5 mm, the ball milling speed is 100-600 rpm, the ball milling time is 1-12 hours, and the nano-seed particle size is 5-500 nanometers. The template agent in step (3) is adamantane, and the mineralizing agent is ethylenediamine; the molar ratio of template agent, mineralizing agent and water in the casting solution is (1-12):(20-200):(1000-20000).
2. The production method according to claim 1, characterized by, The carrier in step (2) is in the shape of a single-channel tubular, multi-channel tubular, flat, or hollow fiber tubular material, including metal, alumina, zirconium dioxide, silicon dioxide, silicon carbide, or silicon nitride, with a pore size of 2-2000 nanometers; the coating method is wiping, dipping, spin coating, or spraying.
3. The preparation method according to claim 1, characterized in that, The synthesis time in steps (4) and (5) is 12-96 hours and the temperature is 100-220℃, respectively.
4. The method of claim 1, wherein, The coating method described in step (5) includes vacuum suction, wiping, dipping or spraying.
5. The preparation method according to claim 1, characterized in that, The roasting atmosphere in step (6) is ozone or air, the roasting time is 24-144 hours, the temperature is 150-400℃, and the heating and cooling rate is 1℃ / min.