Molecular sieve membrane with performance improved through ion exchange as well as preparation method and application of molecular sieve membrane

Abstract

The invention discloses a molecular sieve membrane capable of improving performance through ion exchange and a preparation method and application thereof, and the preparation method comprises the following steps: S1, dissolving a metal salt in a solvent to prepare a metal ion solution; s2, contacting the molecular sieve membrane with a metal ion solution, and carrying out ion exchange under a microwave-assisted solvothermal condition; and S3, washing and drying the molecular sieve membrane after ion exchange to obtain the molecular sieve membrane with improved performance after metal ion exchange. According to the molecular sieve membrane improved by the method, the permeation flux and / or selectivity are obviously improved, and the molecular sieve membrane not only can be used for dehydration separation of organic solvents, but also can be used for gas separation (such as natural gas decarburization, carbon capture before combustion and collection of ammonia gas in synthetic ammonia) and the like; the method is simple to operate, short in time consumption, low in cost and suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention relates to the field of membrane separation technology, and in particular to a molecular sieve membrane with improved performance through ion exchange, and a preparation method and application thereof. Background Art

[0002] Anhydrous ethanol is an important organic solvent and chemical raw material. It is not only widely used in the national defense industry, organic synthesis, food industry, and medical and health care, but can also be used as a biofuel or gasoline additive for internal combustion engines. The separation of ethanol and water is an important step in the preparation of anhydrous ethanol, but because the two form an azeotrope, it is difficult to meet the separation requirements using traditional distillation. Azeotropic distillation, molecular sieve adsorption, pressure swing adsorption and other methods can be used for further dehydration of ethanol, but the equipment investment is large, the process is complex and the energy consumption is high.

[0003] As a new separation technology with high efficiency, low energy consumption and simple process, membrane separation plays an increasingly important role in production and life. Membrane separation technology uses materials with selective separation function and the selective separation of membranes to achieve the separation of different components of the feed liquid. Among them, pervaporation (PV) is a type of membrane separation technology, which is particularly suitable for the separation of azeotropes. PV uses the chemical potential difference of a certain component upstream and downstream of the feed liquid membrane as the driving force to achieve mass transfer, and uses the difference in membrane affinity and mass transfer resistance for different components in the feed liquid to achieve selectivity. It is widely used in the following areas: recovery of organic matter in water, separation of organic matter and mixed organic matter systems, and dehydration of organic solvents.

[0004] Membrane materials are the core of membrane separation. Molecular sieve membrane is a new type of membrane material that can achieve molecular sieving. It has a pore size that is comparable to the molecular size and is uniform, ion exchange performance, high temperature thermal stability, excellent shape-selective catalytic performance, and is easy to be modified. It has a variety of different types and structures to choose from. It is an ideal membrane separation and membrane catalysis material. There are many types of molecular sieves, which can be classified according to different characteristics: according to the structural type, molecular sieves can be divided into LTA type, FAU type, etc.; according to the different silicon-aluminum ratios, molecular sieves can generally be divided into low-silicon zeolite molecular sieves (Si / Al<10, such as type A, type X, type Y, etc.), high-silicon zeolite molecular sieves (10<Si / Al<100, such as MOR type, FER type, ZSM-5, etc.), all-silicon zeolite molecular sieves (Si / Al>100, such as MFI type, DDR type, etc.); according to the type of metal cations, molecular sieves can be divided into potassium type, sodium type, calcium type, etc.; according to the composition of skeleton elements, molecules can be divided into silicon-aluminum molecular sieves, phosphorus-aluminum molecular sieves, titanium-silicon molecular sieves, etc.; according to the different carrier shapes, molecular sieve membranes are also divided into single-channel molecular sieve membranes, multi-channel molecular sieve membranes, flat molecular sieve membranes, hollow fiber molecular sieve membranes, etc.

[0005] Among them, zeolite molecular sieve is a kind of inorganic microporous material with regular nanopores and cage structure, which is constructed by connecting TO4 (T = Si, Al, P, etc.) tetrahedrons as basic units through bridging oxygen common vertices. Its pore size is generally less than 2nm. The typical LTA molecular sieve membrane has a three-dimensional pore system with a pore size at the nanoscale (0.3-0.5nm), between water molecules (0.29nm) and most organic molecules (>0.45nm). It has high chemical stability and thermal stability, and its regular pore structure enables it to achieve accurate screening of molecular size. In addition, due to its low silicon-aluminum ratio in the skeleton (only 1), the LTA molecular sieve membrane shows excellent hydrophilicity. Based on the adsorption-screening mechanism, the zeolite molecular sieve membrane with a low silicon-aluminum ratio shows excellent separation performance in the dehydration of organic solvents, and has been the first to achieve industrial application. In order to further enhance the pervaporation separation efficiency of the molecular sieve membrane and broaden the industrial application field of the molecular sieve membrane, it is necessary to further optimize the molecular sieve membrane materials and membrane processes.

[0006] Molecular sieve membranes are usually grown on the surface of porous supports (common tubular molecular sieve membranes such as Figure 4 As shown). A typical molecular sieve membrane tube is usually installed in a tubular membrane assembly device. The inside of the membrane tube is evacuated (the inside of the membrane tube is the vacuum side, i.e., the permeation side). When the raw material liquid flows through the outside of the membrane tube, the easily permeable components in the raw material (such as water) are preferentially adsorbed on the surface of the molecular sieve membrane. Driven by the partial pressure difference between the inside and outside of the membrane tube, the components permeate through the membrane layer and diffuse to the permeation side inside the membrane tube. The permeate passes through the outlet connected to the permeation side of the membrane tube and enters the cold trap tube to be condensed and collected; while the components that cannot permeate through the molecular sieve membrane are retained on the outside of the membrane tube and continue to circulate on the raw material side until the separation requirements are met.

[0007] Ion exchange is an important way to optimize the catalytic performance of molecular sieves or the separation performance of molecular sieve membranes. Unlike molecular sieve powders, the following situations may exist in the ion exchange process of molecular sieve membranes: 1. The solid-liquid interface contact area is prioritized, the ion exchange rate is slow, and the processing time is long; 2. Solvent water may destroy the microstructure of the low silicon-aluminum ratio molecular sieve membrane, reduce the crystallinity and stability of the molecular sieve membrane, and may cause the separation performance to decrease or even lose the separation performance; 3. Due to the limited ion diffusion, the ion exchange of the molecular sieve membrane may be limited to the surface layer, resulting in insufficient exchange depth and limited performance improvement. In response to the above problems, researchers have explored a variety of methods in recent years to improve the ion exchange efficiency and improve the comprehensive performance of the membrane. Zhu Meihua et al. immersed NaY molecular sieve membranes in different metal ion solutions for ion exchange. The study showed that different metal cations significantly affected the pervaporation performance of the molecular sieve membrane. Zhu Meihua et al. [Chinese Journal of Chemical Engineering, 2023, 59: 176-181.] immersed NaY molecular sieves in different metal ions (Ag +, K + , Ca 2+ , Zn 2+ ,Co 2+ ,Mg 2+ ) aqueous solution at 50℃ for 2h, and the pervaporation performance before and after ion exchange was compared. The results show that different metal cations have an important influence on the separation performance of NaY molecular sieve membranes. The selectivity of Y-type molecular sieve membranes after Zn(NO3)2 exchange in ethanol dehydration increased by 230%, but the total flux decreased by 45%, indicating that zinc ion exchange can significantly improve selectivity, but it may be accompanied by a decrease in flux.

[0008] On the other hand, NaA type molecular sieves have poor hydrothermal stability due to their low silicon-aluminum ratio (1:1), and are more prone to structural damage during ion exchange processes with water as the medium. Therefore, how to shorten the ion exchange time and optimize the ion exchange conditions to ensure the microstructural integrity and crystallinity of the molecular sieve membrane, while taking into account the improvement of permeation flux and selectivity, has become a hot topic in academia and industry. This not only involves basic research in the field of materials science, but also has important significance for the practical application of molecular sieve membranes in industrial dehydration and separation. Unlike the ion exchange of molecular sieve powders, the ion exchange of zeolite molecular sieve membranes is limited by the small contact area of ​​the solid-liquid interface, so the membrane processing time is longer.

[0009] Zhong et al. (Journal of Membrane Science, 2022, 643, 119998) used an aqueous solution of cationic chloride (1.0 mol L -1 ), and continuously stirred for 48 h in a 333K water bath to exchange the Na+ in the membrane for monovalent (H+, Li+, K+, Cs+) and divalent (Mg2+, Ca 2 +、Ba 2 +) cations. Among them, the H+ exchanged MOR membrane has the largest pore size and the highest water adsorption capacity. When a 90wt% acetic acid / water mixture is pervaporated at 348K, it shows a maximum permeation flux of 3.68kg m-2h-1 and a separation factor of 472. However, the high water content and long ion exchange process are not suitable for NaA molecular sieve membranes with lower silicon-aluminum ratios.

[0010] Huang Aisheng et al. (Chinese Chemical Letters, 2019, 30, 1204–1206) used 0.005 mol L -1 After ion exchange of copper acetate (Cu(Ac)2) methanol solution at 50℃ for 5h, Cu-LTA molecular sieve membrane was obtained. 2+The degree of ion exchange was low. Compared with the original Na-LTA membrane, the separation factor decreased slightly in 90 wt.% ethanol aqueous solution at 75°C (Cu-LTA was 3591, Na-LTA was 4082), and the permeate flux increased by 113% (Cu-LTA was 3.52 kg m-2h-1, Na-LTA was 1.65 kg m-2h-1). Further attempts to pass high concentrations of copper acetate (0.1 mol L -1 and 0.01, 0.005 mol L -1 )Methanol solution increases the exchange ratio, but the LTA zeolite membrane is easily damaged and cracked under this condition.

[0011] In summary, the current methods for modifying molecular sieve membranes have the following main disadvantages: 1. It takes a long time; 2. The current ion exchange technology such as the impregnation method often requires a large amount of solution, which is costly; 3. The current ion exchange technology often cannot take into account the flux and selectivity of the molecular sieve membrane when improving the performance of the molecular sieve membrane. Whether the time of ion exchange can be shortened, the degree of ion exchange can be improved, and the integrity and high crystallinity of the membrane microstructure can be ensured, while improving the permeation flux and selectivity of the molecular sieve membrane are the focus of academic and industrial attention. At present, there is no report on the application of microwave-assisted solvothermal method to molecular sieve membrane ion exchange. Summary of the invention

[0012] The object of the present invention is to provide a molecular sieve membrane with improved performance through ion exchange, and a preparation method and application thereof in view of the above problems.

[0013] In order to achieve its purpose, the present invention adopts the following technical solution:

[0014] A first aspect of the present invention provides a method for preparing a high-performance molecular sieve membrane by ion exchange, comprising the following steps:

[0015] S1: dissolving a metal salt into a solvent to prepare a metal ion solution;

[0016] S2: contacting the molecular sieve membrane with the metal ion solution prepared in step S1, and performing ion exchange under microwave-assisted solvothermal conditions;

[0017] S3: washing and drying the molecular sieve membrane after ion exchange in step S2 to obtain a molecular sieve membrane with improved performance after metal ion exchange.

[0018] In step S1, the metal salt is selected from one or more of chlorides, nitrates, sulfates, and carbonates of neodymium ions, lanthanum ions, silver ions, copper ions, potassium ions, calcium ions, cesium ions, or lithium ions;

[0019] The solvent in step S1 is one or a mixture of two or more of deionized water, methanol, ethanol, isopropanol, n-propanol, and ethylene glycol; the concentration of the metal ions in the metal ion solution is 0.001 to 0.5 mol L -1 .

[0020] Preferably, in step S1, the metal salt is selected from one or more of chlorides, nitrates, sulfates and carbonates of neodymium ions, lanthanum ions and lithium ions; the solvent is preferably selected from deionized water, methanol, ethanol and isopropanol; the concentration of the metal ions in the metal ion solution is 0.01 to 0.3 mol L -1 or 0.03~0.1mol L -1 or 0.04-0.07 mol L -1 .

[0021] The molecular sieve membrane comprises a carrier and a skeleton structure material loaded on the carrier; the carrier is in the form of a flat plate, a tube or a hollow fiber;

[0022] The carrier is selected from alumina carrier, titanium dioxide carrier, stainless steel mesh carrier, and the alumina carrier includes α-Al2O3 carrier, γ-Al2O3 carrier, and anodized aluminum oxide carrier; the pore size of the micropores on the carrier is 5nm-1μm;

[0023] The skeleton structure material is selected from LTA, CHA, FAU, MOR, MFI, and UFI molecular sieve membranes, preferably LTA or CHA molecular sieve membranes.

[0024] Preferably, in step S2, the microwave-assisted solvothermal method has an operating temperature of 30 to 150°C and a processing time of 5 to 60 minutes.

[0025] Further preferably, the working temperature of the microwave-assisted solvothermal method is 50-130° C. or 50-120° C. or 90-105° C., and the processing time is 15-35 min or 20-30 min.

[0026] In step S3, the washing reagent used for washing the molecular sieve membrane is deionized water, methanol, ethanol, isopropanol or n-propanol, and the washing is performed until the eluent is close to neutral;

[0027] The drying conditions are: drying at a temperature of 50 to 200° C. and a vacuum degree of 0 to 0.1 MPa for 0.5 to 24 hours; preferably drying at a temperature of 50 to 80° C. and a vacuum degree of 0 to 0.1 MPa for 8 to 16 hours.

[0028] Preferably, in the above method and technical solution, the molecular sieve membrane is composited on the outer surface of the tubular porous carrier, and the object to be treated is a molecular sieve membrane tube;

[0029] In step S2, one end of the molecular sieve membrane tube is plugged with a stopper, a metal ion solution is added into the molecular sieve membrane tube (preferably, the metal ion solution is added in a volume of 30% to 100% of the volume of the molecular sieve membrane tube), and then the other end of the molecular sieve membrane tube is plugged with a stopper with a pinhole, and the molecular sieve membrane tube filled with the ion exchange solution is placed in a microwave reactor for microwave-assisted solvothermal reaction;

[0030] In step S3, after the microwave-assisted solvent thermal reaction is completed, the molecular sieve membrane tube is taken out after the reaction system is cooled down (preferably to below 50° C.), the plug is removed, and the molecular sieve membrane tube is washed until the eluent is close to neutral; then the molecular sieve membrane tube is dried to obtain the product.

[0031] The second aspect of the present invention provides a high-performance molecular sieve membrane, which is prepared by any of the methods described above.

[0032] The third aspect of the present invention provides the use of the above molecular sieve membrane in pervaporation, steam permeation or gas separation; preferably in deep dehydration of organic solvents or dehydration of azeotropic or near-boiling systems of organic solvents / water.

[0033] The beneficial effects of the present invention are:

[0034] 1. Previous studies have shown that when water is used as a solvent for the ion solution, if the ion solution is in direct contact with the molecular sieve membrane, since the silicon-aluminum ratio of the NaA molecular sieve membrane (NaA is a common LTA type molecular sieve) is only 1, the hydrothermal stability is poor, and direct contact with an ion exchange solution with a high water content (water is a highly polar solvent) may destroy the structure of the molecular sieve membrane under heating conditions, and long-term high temperature conditions may cause the molecular sieve membrane skeleton to collapse and lose separation performance. The present invention has found through research that by treating the tubular LTA molecular sieve membrane, adding an ion exchange solution to the carrier side (i.e., the inside of the membrane tube) instead of directly contacting the ion exchange solution with the membrane side outside the membrane tube, the possible destructive effect of the solvent on the membrane under heating conditions can be reduced.

[0035] 2. The present invention adopts a microwave-assisted solvent thermal method to carry out ion exchange of the molecular sieve membrane. The treatment process can carry out rapid and uniform ion exchange under mild reaction conditions. The ion exchange rate is fast and the operation process is simple, which can effectively shorten the treatment time. The heating uniformity is good, which helps to improve the uniformity of ion exchange. Microwave heating can also accelerate the migration of ions, which helps to increase the depth of ion exchange. By introducing an ion exchange solution into the molecular sieve membrane tube for treatment, the amount of ion solution required can be greatly reduced, thereby reducing costs. The method of the present invention can achieve controllable batch preparation and be applied on a large scale in industry.

[0036] 3. Compared with the untreated blank molecular sieve membrane, the molecular sieve membrane improved by the method of the present invention has significantly improved permeation flux and / or selectivity. By changing the operating parameters (temperature and concentration of raw liquid) in the pervaporation test, the molecular sieve membrane has excellent dehydration performance and is suitable for the application of dehydration of different solvents, especially for the deep dehydration of organic solvents or the dehydration of azeotropic or near-boiling systems of organic solvents / water. This lays the foundation for the further extensive application of molecular sieve membranes in pervaporation separation.

[0037] 4. The improved molecular sieve membrane of the present invention can be used not only for dehydration and separation of organic solvents, but also for gas separation (such as natural gas decarbonization, pre-combustion carbon capture, and collection of ammonia in synthetic ammonia). BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a temperature gradient diagram of pervaporation of a blank control LTA membrane tube at different temperatures after microwave treatment of a neodymium ion solution in the LTA membrane tube (90°C, 20 min).

[0039] Figure 2 The temperature gradient diagram of pervaporation of neodymium ion solution and lanthanum ion solution in LTA membrane tube under microwave treatment (120℃, 20min) at different temperatures.

[0040] Figure 3 This is a concentration gradient diagram of pervaporation of a blank control LTA membrane tube at different concentrations after microwave treatment of a neodymium ion solution in the LTA membrane tube (90°C, 20 min).

[0041] Figure 4 It is a conventional molecular sieve membrane tube.

[0042] Figure 5 It is a molecular sieve membrane tube performance testing device and process schematic diagram. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with embodiments, but the present invention is not limited thereto.

[0044] The experimental methods in the following examples are conventional methods unless otherwise specified.

[0045] Explanation of terms

[0046] Tubular molecular sieve membrane:

[0047] The tubular molecular sieve membrane (or molecular sieve membrane tube) used in each embodiment and comparative example of the present invention is a commonly used membrane tube (such as a molecular sieve membrane tube) with an Al2O3 porous ceramic substrate as a carrier (or support body) and a membrane layer as a molecular sieve membrane. Figure 4As shown), the molecular sieve membrane is a thin film. In the molecular sieve membrane tube, the molecular sieve membrane is composited on the outer surface of a tubular porous carrier. During the experiment, it is installed into a tubular membrane module device for experiment.

[0048] The molecular sieve membrane is grown on one side of a support, which is usually divided into a support side (eg, the inner side of a tubular support of a tubular molecular sieve membrane) and a membrane side (eg, the side of the membrane attached to the outer surface of the tubular support).

[0049] The tubular molecular sieve membrane used in the embodiment of the present invention is of LTA type (expressed using the international structural code), specifically NaA type, which is commercially available; the length of the molecular sieve membrane tube is 50 mm and the inner diameter is 8 mm.

[0050] Example

[0051] The present invention provides a method for preparing a molecular sieve membrane with improved performance through ion exchange, which is operated according to the following steps:

[0052] S1: Take 0.6265g of neodymium chloride (NdCl3) powder and add solvent (deionized water or alcohol) to dissolve to 50mL, ultrasonicate for 20min, and obtain a concentration of 0.05mol L -1 of neodymium chloride solution.

[0053] S2: Use a polytetrafluoroethylene plug to plug the bottom of the tubular LTA molecular sieve membrane (hereinafter referred to as LTA molecular sieve membrane), take 3.5mL of the solution prepared in S1 and add it into the tube of the LTA molecular sieve membrane until the tube is almost filled with the metal ion solution, and then use a polytetrafluoroethylene plug with a pinhole to plug the top of the LTA molecular sieve membrane. Transfer the LTA molecular sieve membrane to a microwave reactor, set the microwave parameters, heat it to 90°C within 6 minutes, keep it at 90°C for 20 minutes, and then cool it down naturally.

[0054] Since the microwave heating temperature is close to the boiling point of alcohol solvents, the vapor pressure is relatively high under closed conditions. The molecular sieve membrane tube is blocked with a stopper to prevent the metal ion solution from flowing out of the two ends of the membrane tube in large quantities, ensuring that there is always a small amount of metal ion solution in the membrane tube. The metal ion solution passes through the micropores on the carrier from the inside of the membrane tube to the molecular sieve membrane loaded on the outer surface of the membrane tube for ion exchange. The pinhole on the stopper at the top of the membrane tube is to prevent the stopper from being flushed out and falling off due to excessive vapor pressure inside the membrane tube.

[0055] In our previous experiments, we tried to use metal ion solution in direct contact with the molecular sieve membrane for microwave heating-assisted ion exchange. The microstructure of the molecular sieve membrane did not change significantly, but the repeatability of the membrane performance was poor. The method of using metal ion solution in the membrane tube and ion exchange on the molecular sieve membrane on the outer surface of the membrane tube under the assistance of microwave heating can achieve a sufficient degree of ion exchange while avoiding a large amount of contact between the solution and the membrane surface, preventing possible damage to the membrane structure.

[0056] S3: When the temperature of the reaction system drops below 50°C, take out the LTA molecular sieve membrane, remove the upper and lower polytetrafluoroethylene stoppers, soak the entire membrane tube in deionized water and wash it three times at room temperature (the eluent after washing is close to neutral at this time), transfer it to a 60°C oven and bake it for 12 hours to obtain the LTA molecular sieve membrane after ion exchange optimization under microwave-assisted solvothermal conditions.

[0057] The molecular sieve membranes of Examples 1-11 and Comparative Examples 1-2 were prepared according to the above method, except that the type and concentration of the metal salt, the solvent used to dissolve the metal salt, the heating method (the comparative example did not use microwave heating, but oven heating) and the operating parameters were different. The specific experimental conditions are shown in Table 1:

[0058] Table 1 Molecular sieve membranes of Examples and Comparative Examples

[0059]

[0060]

[0061] Comparative Example

[0062] Different comparative examples were set to compare the effects of different metal salts, solvents and heating treatment methods on the improvement of membrane performance, as shown in Table 1.

[0063] Performance Testing

[0064] The performance of the tubular molecular sieve membrane treated with metal salt solution prepared in the above examples and comparative examples was tested: the molecular sieve membrane tube treated with metal salt solution was installed in a tubular membrane assembly device for experiment (the detection schematic diagram is shown in FIG. Figure 5As shown). The tubular membrane assembly device is a common assembly device in the current technical field. One end of the assembly device is closed and the other end is connected to the pervaporation device. The assembled membrane tube and membrane assembly are placed in a raw material tank filled with raw material liquid (90wt.% ethanol / water system). The inside of the membrane tube is evacuated (the inside of the membrane tube is the vacuum side, i.e., the permeation side), and the raw material liquid flows through the outside of the membrane tube. The easily permeable components (such as water) in the raw material liquid are preferentially adsorbed on the surface of the molecular sieve membrane, and driven by the partial pressure difference between the inside and outside of the membrane tube, they penetrate through the molecular sieve membrane on the outer surface of the membrane tube and enter the permeation side of the membrane tube, and are collected by a cold trap tube connected to the permeation side of the membrane tube (i.e., permeate, which refers to the component in the raw material liquid that can pass through the molecular sieve membrane. In the case of this raw material liquid, most of the permeate is water); and the components that cannot penetrate through the molecular sieve membrane remain in the raw material liquid.

[0065] The pervaporation test was carried out according to the above method to detect the pervaporation performance of the molecular sieve membrane.

[0066] In the experiment, the flux of all molecular sieve membrane products was characterized. The definition and detection method of permeation flux refer to the industry standard HG / T 5540-2019:

[0067] Membrane permeation flux (J, kg m -2 h -1 ) and selective α 水 / 乙醇 Calculated by formula (1) and (2) respectively:

[0068] J=M / (t×A)=(m1-m2)) / (t×π×D×l) (1)

[0069] α (水 / 乙醇) =(y 水 / y 乙醇 ) / (x 水 / x 乙醇 )=f×[(A 水，渗透液 / A 乙醇，渗透液 ) / (A 水，原料液 / A 乙醇，原料液 )] (2)

[0070] Where M is the mass of permeate (kg); D is the outer diameter of the membrane tube (m); l is the effective membrane tube length (m); t is the test time (h); x 水 and x 乙醇 are the mass fractions of water and ethanol in the raw material solution, y 水 and 乙醇 are the mass fractions of water and ethanol in the permeate, f is the relative correction factor, A 水，原料液 and A 乙醇，原料液 are the percentage of chromatographic peak area corresponding to water and ethanol in the raw material solution, A 水，渗透液 and A 乙醇，渗透液are the percentages of the chromatographic peak areas corresponding to water and ethanol in the permeate, respectively.

[0071] Test Results

[0072] The performance of the membrane tube before (i.e., blank membrane tube) and after treatment with the metal ion solution was tested. The raw material solution was a 90wt.% ethanol / water system, and the pervaporation temperature was 60°C. The test results are shown in Table 2:

[0073] Table 2 Performance test results

[0074]

[0075] *Note: An embodiment may have multiple repetitions. Even for molecular sieve membrane tubes of the same specifications, the initial performance of different blank membrane tubes may be different. Embodiment 1-Repetition 1, 2, 3 in Table 2 refers to one of the membrane tubes among multiple LTA molecular sieve membrane tubes of the same specifications improved by the same treatment method. The performance of each membrane tube before and after treatment was tested.

[0076] The membrane tube of Comparative Example 1 was pervaporated in a 90wt.% EtOH / H2O system at 60°C, and the permeate flux increased from 1.46 kgh -1 m -2 Reduced to 0.47 kg h -1 m -2 , decreased by 67.81%; the average water content of the permeate decreased from 99.62% to 95.98%, decreased by 3.66%; the selectivity decreased from 1936.48 to 180.39, decreased by 90.68%. The flux of the membrane tube of comparative example 2 decreased from 1.62 kg h -1 m -2 Reduced to 1.45 kg / h -1 m -2 , the water content decreased from 99.78% to 99.48%, and the selectivity decreased from 3861.42 to 1686.21.

[0077] Figure 1 The temperature gradient diagram of the neodymium ion solution after microwave treatment (90°C, 20 min) in the LTA membrane tube and the blank control LTA membrane tube pervaporation at different temperatures (Example 1-Repeat 2), the raw material liquid is 90wt.% ethanol / water system, Figure 1 It can be seen that after the neodymium ion solution was microwave-treated in the LTA membrane tube, the water flux and selectivity of the blank membrane tube were improved in the tests at different temperatures. In particular, at 60°C, the water content in the permeate increased from 91.18% to 99.53%, and the corresponding selectivity (86.17→1746.84) increased by 1927.20%, with a significant increase in selectivity and mild reaction conditions.

[0078] Figure 2 The temperature gradient diagram of the pervaporation of the neodymium ion solution (Example 3) and the lanthanum ion solution (Example 4) (the solvent is water) in the LTA membrane tube under microwave treatment (120°C, 20min) at different temperatures, the raw material liquid is a 90wt.% ethanol / water system, Figure 2 It can be seen that at different temperatures, compared with neodymium ion exchange, the selectivity of the membrane tube after lanthanum ion exchange is better, and its selectivity performance is maintained at a high level.

[0079] Figure 3 The concentration gradient diagram of (Example 1-Repeat 3) neodymium ion solution after microwave treatment in LTA membrane tube (90°C, 20min) and blank control LTA membrane tube pervaporation at 60°C under different ethanol / water system raw liquid concentrations (10, 15, 20, 25, 30wt% water concentration, corresponding to 90, 85, 80, 75, 70wt% ethanol concentration). The membrane tube after neodymium ion exchange can maintain high selectivity in different raw liquid concentrations. The test results show that for raw liquids of different concentrations, the Nd-LTA molecular sieve membrane after microwave-assisted ion exchange can maintain high separation performance and has a wide range of applications. It can be seen from the results of Examples 1-11 and Comparative Examples 1-2 that when the heating method adopts oven heating, the flux and selectivity of the membrane tube are reduced, and the performance of the membrane tube cannot be improved but decreases. When microwave heating is adopted, the flux and / or selectivity of the membrane tube can be improved. Compared with the case where water was used as the solvent (Examples 1-4), the flux was reduced and the selectivity was improved. When alcohols were used as the solvent (Examples 5-11), the repeatability of the membrane performance improvement was better, and both the flux and the selectivity were improved. (Example 9) After the lanthanum ion solution was microwave-treated in the LTA membrane tube (50°C, 20 min, the solvent was isopropanol), the flux was 1.96 kg h when the raw material liquid was a 90 wt.% ethanol / water system and the pervaporation temperature was 60°C. -1 m -2 , the selectivity is 2719.61 (data in Table 2); when the test system is changed to 90wt.% methanol / water, the flux is 0.72kg h -1 m -2 , the selectivity was 46.78; when the system was changed to 90 wt.% isopropanol / water, the flux increased to 1.82 kg h -1 m -2 , the selectivity increased to 222.64. The results show that the membrane optimized by ion exchange has a universal enhancement effect in the dehydration of many solvents.

[0080] In general, compared with the prior art which is time-consuming and costly and can only improve one performance, the method of the present invention is time-saving, simple to operate, requires a small amount of ion exchange solution, is low in cost, and can effectively improve the flux and / or selectivity performance of the membrane tube.

Claims

1. A method for preparing a high-performance molecular sieve membrane by ion exchange, characterized in that: The following steps are involved: S1: dissolving a metal salt into a solvent to prepare a metal ion solution; S2: contacting the molecular sieve membrane with the metal ion solution prepared in step S1, and performing ion exchange under microwave-assisted solvothermal conditions; S3: washing and drying the molecular sieve membrane after ion exchange in step S2 to obtain a molecular sieve membrane with improved performance after metal ion exchange.

2. The method according to claim 1, characterized in that: In step S1, the metal salt is selected from one or more of chlorides, nitrates, sulfates, and carbonates of neodymium ions, lanthanum ions, silver ions, copper ions, potassium ions, calcium ions, cesium ions, or lithium ions; The solvent in step S1 is one or a mixture of two or more of deionized water, methanol, ethanol, isopropanol, n-propanol, and ethylene glycol; the concentration of the metal ions in the metal ion solution is 0.001 to 0.5 mol L -1 .

3. The method according to claim 2, characterized in that: In the step S1, The metal salt is preferably selected from one or more of chlorides, nitrates, sulfates and carbonates of neodymium ions, lanthanum ions and lithium ions; the solvent is selected from deionized water, methanol, ethanol and isopropanol; the concentration of the metal ions in the metal ion solution is 0.01 to 0.3 mol L -1 or 0.03~0.1mol L -1 or 0.04-0.07 mol L -1 .

4. The method according to claim 1, characterized in that: The molecular sieve membrane comprises a carrier and a skeleton structure material loaded on the carrier; the carrier is in the form of a flat plate, a tube or a hollow fiber; The carrier is selected from alumina carrier, titanium dioxide carrier, stainless steel mesh carrier, and the alumina carrier includes α-Al2O3 carrier, γ-Al2O3 carrier, and anodized aluminum oxide carrier; the pore size of the micropores on the carrier is 5nm-1μm; The skeleton structure material is selected from LTA, CHA, FAU, MOR, MFI, and UFI molecular sieve membranes, preferably LTA or CHA molecular sieve membranes.

5. The method according to claim 1, characterized in that: In the step S2, the working temperature of the microwave-assisted solvothermal method is 30 to 150° C., and the processing time is 5 to 60 minutes.

6. The method according to claim 5, characterized in that: The working temperature of the microwave-assisted solvent thermal method is 50-130° C. or 50-120° C. or 90-105° C., and the processing time is 15-35 min or 20-30 min.

7. The method according to claim 1, characterized in that: In the step S3, The washing reagent used for washing the molecular sieve membrane is deionized water, methanol, ethanol, isopropanol or n-propanol, and the washing is performed until the eluent is close to neutral; The drying conditions are: drying at a temperature of 50 to 200° C. and a vacuum degree of 0 to 0.1 MPa for 0.5 to 24 hours; preferably drying at a temperature of 50 to 80° C. and a vacuum degree of 0 to 0.1 MPa for 8 to 16 hours.

8. The method according to claim 1, characterized in that: The molecular sieve membrane is composited on the outer surface of the tubular porous carrier, and the object to be treated is the molecular sieve membrane tube; In step S2, one end of the molecular sieve membrane tube is plugged with a stopper, a metal ion solution is added into the molecular sieve membrane tube, and then the other end of the molecular sieve membrane tube is plugged with a stopper, and the molecular sieve membrane tube is placed in a microwave reactor for microwave-assisted solvothermal reaction; In step S3, after the microwave-assisted solvothermal reaction is completed, the molecular sieve membrane tube is taken out after the reaction system is cooled down, the plug is removed, and the molecular sieve membrane tube is washed until the eluent is close to neutral; Then the molecular sieve membrane tube is dried to obtain the product.

9. A high performance molecular sieve membrane, characterized in that: It is prepared by the method described in any one of claims 1 to 8.

10. Use of the molecular sieve membrane according to claim 9 in pervaporation, steam permeation or gas separation; preferably in deep dehydration of organic solvents or dehydration of azeotropic or near-boiling systems of organic solvents / water.

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