A lithium type x molecular sieve adsorbent and a method for preparing the same

Abstract

The application discloses a lithium type X molecular sieve adsorbent and a preparation method thereof, and belongs to the technical field of adsorption separation. The preparation method of the lithium type X molecular sieve adsorbent comprises two steps of preparing a low-silicon aluminum X molecular sieve and ion exchange. The lithium type X molecular sieve adsorbent prepared by the method has a lithium ion exchange degree of 97.9-99.1%, a molar ratio of SiO2 and Al2O3 of 2.01-2.10, a nitrogen adsorption amount of 32.62-34.94 mL / g, an oxygen adsorption amount of 2.40-2.75 mL / g, and a nitrogen-oxygen separation coefficient of 11.86-14.56.

Description

Technical Field

[0001] This invention relates to a lithium-type X molecular sieve adsorbent and its preparation method, belonging to the field of adsorption and separation technology. Background Technology

[0002] Oxygen has extremely wide applications and plays a crucial role in the entire industrial system, making the air separation oxygen production industry a very broad market. The cryogenic distillation method (also known as the deep cryogenic method), which utilizes the differences in properties between nitrogen and oxygen, was the earliest industrialized air separation oxygen production method. However, this method requires large investments, consumes a lot of energy, and is complex to operate, limiting its application to large-scale oxygen production. In contrast, the pressure swing adsorption (PSA) method, developed in the 1960s, has rapidly gained popularity due to its advantages such as simple process, convenient operation, high product purity, low energy consumption, and low investment. The core technology of PSA is the adsorbent, and its performance directly affects the gas separation effect. The selection of adsorbents has undergone a relatively long process. In the 1950s, X molecular sieve (FAU structure, silica-to-alumina ratio 1.0~1.5) was first synthesized by Union Carbide, ushering in the era of artificially synthesized zeolite molecular sieves. In the 1960s and 1970s, research revealed that the adsorption performance of molecular sieves is closely related to their framework structure, silica-to-alumina ratio, and the types of exchangeable cations. Sodium-type X-zeolites (NaX) were used for gas separation, but their adsorption selectivity for nitrogen was low. In the 1980s, low-silica-alumina ratio X-zeolites (LSX) were synthesized: by reducing the silica-alumina ratio to near 1.0 (e.g., 1.0~1.1), their framework charge density was higher, their cation exchange capacity was stronger, and their adsorption performance for polar molecules (such as nitrogen) was significantly improved. In the early 1990s, French scientists first reported the superior performance of Li⁺-exchange LSX zeolites (low-silica-alumina ratio lithium-type X-zeolites). Lithium ions occupy specific sites in the zeolite framework (such as the SIII site), significantly increasing the adsorption capacity for nitrogen (more than 30% higher than NaX), making it a key material for efficient pressure swing adsorption (PSA) oxygen production. In the mid-1990s, low-silica-alumina ratio lithium-type X-zeolites were commercially produced by companies such as UOP in the United States, driving down energy consumption and improving efficiency in PSA oxygen production technology, gradually replacing the traditional cryogenic method in small- and medium-scale oxygen production.

[0003] Currently, to maximize the nitrogen and oxygen separation performance of lithium-type X-type molecular sieve adsorbents, the research direction in the molecular sieve adsorbent industry mainly focuses on two aspects: first, how to minimize the silicon-to-aluminum ratio and increase the number of metal cations inside the molecular sieve; and second, since the primary cation in X-type molecular sieves is Na... + or K + (or Na) + and K + Two types of ions), Li +The introduction of lithium must be achieved through ion exchange. Therefore, under the premise of ensuring that the X molecular sieve lattice is not destroyed, how to efficiently carry out ion exchange to maximize the lithium content in the X molecular sieve becomes the key to significantly improving the air separation efficiency of lithium-type X molecular sieve adsorbents.

[0004] To minimize the silica-to-alumina ratio of X-type molecular sieves, current techniques mainly include: a two-step synthesis method of low-temperature aging followed by high-temperature crystallization; a one-step synthesis method of low-temperature crystallization; synthesis using a directing agent; and microwave synthesis. The two-step method is a classic approach for synthesizing low-silica-to-alumina ratio X-type molecular sieves. Its biggest problem is its high sensitivity to changes in water content; too much or too little water will produce impurities. The one-step method for synthesizing low-silica-to-alumina ratio X-type molecular sieves is very simple and low-cost, but the resulting low-silica-to-alumina ratio X-type molecular sieves usually contain impurities such as P-type molecular sieves and sodalite, resulting in poor quality and making it unsuitable for use in air separation oxygen production. For example, Chinese patent CN113880104A discloses a method for preparing low-silica-to-alumina ratio NaX molecular sieves using low-temperature crystallization. The preparation method includes: preparing low-silicon-to-aluminum ratio (SiA) NaX molecular sieve seed crystals using silicon and aluminum sources in a specific ratio; adding a small amount of these seed crystals during the preparation of low-SiA NaX allows for one-step low-temperature crystallization to produce high-crystallinity NaX raw powder with a SiA ratio between 2.0 and 2.2. This patent, through low-temperature crystallization, synthesizes X molecular sieves in one step, but the high SiA ratio makes it difficult to meet the requirements for use in air separation oxygen production. Directing agent technology offers significant advantages, not only shortening crystallization time but also improving the crystallinity of molecular sieves and inhibiting the formation of impurities. However, the composition of the directing agent needs precise control, the preparation process is relatively complex, and the requirements for process operation precision are high, making industrial application very difficult. Microwave heating synthesis has characteristics such as penetration, low thermal inertia, and selectivity. Compared with traditional heating methods, microwave heating has advantages such as high thermal effect, uniform heating, small temperature gradient, environmental friendliness, energy saving, and convenient operation. In recent years, microwave heating has been applied to molecular sieve synthesis. However, microwave synthesis is still in the laboratory stage, and microwave reactors commonly used are household microwave ovens. The reaction is discontinuous, and industrial applications face problems such as difficulty in controlling microwave power and the risk of rapid explosions. Based on the above comparison, the two-step synthesis method of low-temperature aging and high-temperature crystallization remains the most effective method, but the problem of impurity crystal growth caused by slight changes in water volume must be addressed.

[0005] Numerous studies have shown that increasing the lithium content in X-type molecular sieves through ion exchange only significantly improves nitrogen adsorption capacity when the lithium-ion exchange degree exceeds 85%. However, lithium ions have small radii and strong polarity, forming multiple layers of hydrated lithium ions in aqueous solutions, resulting in slow migration and low exchange rates with sodium ions in the molecular sieve. Increasing the exchange degree requires breaking the hydrated lithium-ion bonds. Because of the high bond energy of hydrated lithium ions, conventional methods such as heating and stirring are ineffective in breaking them, leading to slow lithium-sodium ion exchange rates and making it difficult to obtain high-exchange-degree, low-silicon-aluminum ratio lithium-type X-type molecular sieves. A large excess of lithium salt is required, typically necessitating multiple / stage exchanges, which are time-consuming and inefficient. For example, Chinese patent CN101125664A discloses an ion exchange method for preparing lithium-type low-silicon-aluminum X-type zeolite molecular sieves, which involves first calcining the Na-X molecular sieve and then passing it through a Li... + Aqueous solution exchange was performed, and finally, LiNa-X molecular sieves with a certain degree of exchange were mixed with solid lithium salt powder for solid-phase exchange to obtain Li-modified X molecular sieves with an exchange degree of over 96%. However, this method has the drawback of Li... + Uneven exchange and excessive Na + Problems exist. Chinese patent CN101766987A describes a lithium-modified low-silicon aluminum X-type molecular sieve adsorbent and its preparation method. It involves modifying commercially available low-silicon molecular sieve powder through a single-cross-calcination, double-cross-calcination, and triple-cross-calcination process to obtain Li-LSX molecular sieve powder with a Li exchange degree greater than 95%. This method requires calcination twice at 400-450℃ for 2-3 hours each time, which can lead to a decrease in the crystallinity of the X molecular sieve due to dealuminization. Besides ion exchange in an aqueous environment, there is also the molten salt exchange method, which has higher ion exchange efficiency. This method uses molten salts with high ionization properties, such as alkali metal halides, sulfates, or nitrates, as the molten salt solution for cation exchange. However, the temperature at which the molten salt solution is formed must be lower than the destruction temperature of the zeolite structure. Furthermore, in addition to cation exchange reactions, some salts are trapped within the zeolite cages in the molten salt solution (the degree of trapping depends on the size of the anion and the exchange temperature), potentially forming zeolites with special properties.

[0006] As can be seen from the above, lithium-type X molecular sieve adsorbents still have two major problems: it is difficult to synthesize high-quality X molecular sieves with low silica-to-alumina ratio and it is difficult to obtain X molecular sieves with high lithium content with high efficiency. Therefore, it is urgent to develop new preparation methods to obtain high-quality X molecular sieves with high lithium content and low silica-to-alumina ratio. Summary of the Invention

[0007] To address the shortcomings of the existing technology, this invention provides a lithium-type X-type molecular sieve adsorbent and its preparation method, achieving the following objectives: to prepare a high-quality, high-lithium-content, low-silicon-aluminum-ratio X-type molecular sieve adsorbent, which has a high nitrogen adsorption capacity and exhibits excellent characteristics of high nitrogen-oxygen separation coefficient and high air separation efficiency when used for air separation oxygen production.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A lithium-type X molecular sieve adsorbent and its preparation method are disclosed. The lithium-type X molecular sieve adsorbent has a lithium-ion exchange capacity of 97.9-99.1%, a SiO2 to Al2O3 molar ratio of 2.01-2.10, a nitrogen adsorption capacity of 32.62-34.94 mL / g, an oxygen adsorption capacity of 2.40-2.75 mL / g, and a nitrogen-oxygen separation coefficient of 11.86-14.56. The preparation method of the lithium-type X molecular sieve adsorbent includes two steps: preparing a low-silica-alumina X molecular sieve and ion exchange. The following are further improvements to the above technical solution:

[0009] Step 1: Preparation of low-silica-alumina X molecular sieves Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. Then, a silicon source, polyaminopolyether methylenephosphonic acid, and magnesium aluminum silicate are added to the deionized water and stirred until homogeneous to obtain a silicon solution. Next, the alkali aluminum solution and the silicon solution are mixed according to a SiO2 to Al2O3 molar ratio of 2.1 to 2.3 and stirred rapidly to obtain a gel. After low-temperature aging and high-temperature crystallization, the gel is cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. The aluminum source is sodium aluminate; The alkaline source is one or a mixture of two of sodium hydroxide and potassium hydroxide in any mass ratio; The silicon source is one or a mixture of two of sodium metasilicate and potassium silicate in any mass ratio. In the alkaline aluminum solution, the mass fraction of the aluminum source is 6-16 wt%, and the mass fraction of the alkaline source is 15-33 wt%. In the silicon solution, the mass fraction of the silicon source is 25-48 wt%, the mass fraction of polyaminopolyether methylenephosphonic acid is 2-5 wt%, and the mass fraction of magnesium aluminum silicate is 1.5-6 wt%. The low-temperature aging process involves an aging temperature of 40-65°C and an aging time of 16-23 hours. The high-temperature crystallization is carried out at a temperature of 80-95°C for 9-15 hours. The washing process involves washing with deionized water until the pH of the washing solution reaches 7.5-8. The drying process involves a drying temperature of 85-95℃ and a drying time of 10-16 hours. The grinding process involves grinding the dried product into powder with a particle size of 200-600 mesh.

[0010] Step 2, Ion exchange After mixing low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt evenly, the mixture is placed in a muffle furnace, heated and kept at a constant temperature, and then ion exchange is carried out at the constant temperature. After the exchange is complete, the mixture is cooled to room temperature, and then washed and dried to obtain lithium-type X molecular sieve adsorbent. The organic lithium salt is one or a mixture of two or more of lithium stearate, lithium 12-hydroxystearate, dilithium sebate, and dilithium azelate in any mass ratio. The low-melting-point mixed molten salt is a mixture of KCl and ZnCl2 in a molar ratio of 23:27; The mass ratio of the low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt is 40~120:10~90:1~15; The heating and holding process involves a heating rate of 1~5℃ / min and a holding temperature of 235~300℃. After the exchange is complete, the ion exchange time is 3.5 to 5 hours at a constant temperature; The washing process involves washing with deionized water at a temperature of 80-95°C until the pH of the washing solution reaches 7.2-7.8. The drying process involves a drying temperature of 85-110℃ and a drying time of 10-20 hours.

[0011] Compared with the prior art, the present invention achieves the following beneficial effects: 1. This invention uses a two-step synthesis method of low-temperature aging followed by high-temperature crystallization to synthesize X-type molecular sieves with low silicon-to-aluminum ratio. Then, lithium-ion exchange is performed on the low silicon-to-aluminum ratio X-type molecular sieves using a highly efficient molten salt exchange method. As a result, lithium-type X-type molecular sieves with low silicon-to-aluminum ratio, high lithium content, large nitrogen adsorption capacity, small oxygen adsorption capacity, and excellent nitrogen-oxygen separation coefficient are obtained. 2. In order to obtain X molecular sieve with a low silicon-to-aluminum ratio, this invention adds two substances, polyamino-polyether methylenephosphonic acid and magnesium aluminum silicate, to the silicon solution during the preparation of low silicon-to-aluminum X molecular sieve. Among them, polyamino-polyether methylenephosphonic acid has a strong electrostatic attraction effect on silicate anions, which can slow down the gelation rate of silicate and aluminum ions and regulate and reduce their ratio. Similarly, magnesium aluminum silicate dispersed in the reaction system will form a colloidal form with good thickening properties, which will also reduce the rate of silicate and aluminum ions colliding and reacting to form gels. The synergistic effect of these two substances will better control the structure of the prepolymer of the crystal clusters formed by silicate and aluminum ions in the low-temperature aging reaction stage and reduce its size, laying a good foundation for the fine crystallization in the subsequent high-temperature crystallization process. Therefore, the addition of these two substances can effectively reduce the silicon-to-aluminum ratio of X molecular sieve and refine the crystal structure of the final molecular sieve, promoting the overall performance improvement of X molecular sieve. 3. This invention uses organic lithium salts such as lithium stearate, lithium 12-hydroxystearate, dilithium sebate, and dilithium azelate, combined with a low-melting-point mixed molten salt, to perform ion exchange on low-silicon-aluminum X molecular sieves. The organic lithium salts have low melting points, which can lower the operating temperature of lithium ion exchange and reduce the negative impact of high-temperature ion exchange on the molecular sieve framework. However, the viscosity of the organic lithium salts after melting is high, and the penetration of the molten liquid into the molecular sieve is slow, which slows down the ion exchange rate. In order to reduce the viscosity of the organic lithium salt molten liquid and increase the ion exchange rate, this invention adds a low-melting-point mixed molten salt composed of KCl and ZnCl2 in a molar ratio of 23:27. This mixed molten salt has a very low viscosity. After being incorporated into the organic lithium salt molten liquid, it can significantly reduce the viscosity of the organic lithium salt molten liquid, thereby increasing the rate at which the organic lithium salt molten liquid penetrates into the molecular sieve, and ultimately significantly increasing the lithium ion exchange rate. 4. The lithium-type X molecular sieve adsorbent prepared by this invention has a lithium-ion exchange degree of 97.9~99.1%, a molar ratio of SiO2 to Al2O3 of 2.01~2.10, a nitrogen adsorption capacity of 32.62~34.94 mL / g, an oxygen adsorption capacity of 2.40~2.75 mL / g, and a nitrogen-oxygen separation coefficient of 11.86~14.56. Attached Figure Description

[0012] Figure 1 The X-ray spectra of the low-silica aluminum X-zeolite (before ion exchange) and lithium-type X-zeolite adsorbent (after ion exchange) obtained in Example 1 are shown. Figure 2 The image shows a scanning electron microscope image of the surface of the lithium-type X molecular sieve adsorbent obtained in Example 1, magnified 5000 times. Figure 3 The image shown is a scanning electron microscope image of the surface of the lithium-type X molecular sieve adsorbent obtained in Example 2, magnified 5000 times. Detailed Implementation

[0013] The preferred embodiments of the present invention are described below. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0014] Example 1: A method for preparing a lithium-type X-type molecular sieve adsorbent Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. Then, a silicon source, polyamino-polyether methylene phosphonic acid, and magnesium aluminum silicate are added to the deionized water and stirred until homogeneous to obtain a silicon solution. Next, the alkali aluminum solution and the silicon solution are mixed according to a SiO2 to Al2O3 molar ratio of 2.2 and stirred rapidly to obtain a gel. After low-temperature aging and high-temperature crystallization, the gel is cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. The aluminum source is sodium aluminate; The alkali source is sodium hydroxide; The silicon source is sodium metasilicate; In the alkaline aluminum solution, the mass fraction of the aluminum source is 11 wt%, and the mass fraction of the alkali source is 25 wt%. In the silicon solution, the mass fraction of the silicon source is 35 wt%, the mass fraction of polyaminopolyether methylenephosphonic acid is 4 wt%, and the mass fraction of magnesium aluminum silicate is 4 wt%. The low-temperature aging process involves an aging temperature of 50°C and an aging time of 20 hours. The high-temperature crystallization is carried out at a crystallization temperature of 85°C for 13 hours. The washing process involves washing with deionized water until the pH of the washing solution reaches 7.8. The drying process is carried out at a temperature of 90°C for 13 hours. The grinding process involves grinding the dried product into powder with a particle size of 500 mesh.

[0015] Step 2, Ion exchange After mixing low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt evenly, the mixture is placed in a muffle furnace, heated and kept at a constant temperature, and then ion exchange is carried out at the constant temperature. After the exchange is complete, the mixture is cooled to room temperature, and then washed and dried to obtain lithium-type X molecular sieve adsorbent. The organic lithium salt is lithium stearate; The low-melting-point mixed molten salt is a mixture of KCl and ZnCl2 in a molar ratio of 23:27; The mass ratio of the low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt is 80:40:7; The heating and holding process involves a heating rate of 2℃ / min and a holding temperature of 285℃. After the exchange is complete, the ion exchange time is 4 hours at a constant temperature; The washing process involves washing with deionized water at 90°C until the pH of the washing solution reaches 7.4. The drying process is carried out at a temperature of 100°C for 17 hours.

[0016] Example 2: A method for preparing a lithium-type X-type molecular sieve adsorbent Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. Then, a silicon source, polyaminopolyether methylenephosphonic acid, and magnesium aluminum silicate are added to the deionized water and stirred until homogeneous to obtain a silicon solution. Next, the alkali aluminum solution and the silicon solution are mixed according to a SiO2 to Al2O3 molar ratio of 2.1 and stirred rapidly to obtain a gel. After low-temperature aging and high-temperature crystallization, the gel is cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. The aluminum source is sodium aluminate; The alkaline source is potassium hydroxide; The silicon source is potassium silicate; In the alkaline aluminum solution, the mass fraction of the aluminum source is 6 wt%, and the mass fraction of the alkali source is 15 wt%. In the silicon solution, the mass fraction of the silicon source is 25 wt%, the mass fraction of polyaminopolyether methylenephosphonic acid is 2 wt%, and the mass fraction of magnesium aluminum silicate is 1.5 wt%. The low-temperature aging process involves an aging temperature of 40°C and an aging time of 23 hours. The high-temperature crystallization is carried out at a crystallization temperature of 80°C for 15 hours. The washing process involves washing with deionized water until the pH of the washing solution reaches 7.5. The drying process is carried out at a temperature of 85°C for 16 hours. The grinding process involves grinding the dried product into powder with a particle size of 200 mesh.

[0017] Step 2, Ion exchange After mixing low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt evenly, the mixture is placed in a muffle furnace, heated and kept at a constant temperature, and then ion exchange is carried out at the constant temperature. After the exchange is complete, the mixture is cooled to room temperature, and then washed and dried to obtain lithium-type X molecular sieve adsorbent. The organic lithium salt is lithium 12-hydroxystearate; The low-melting-point mixed molten salt is a mixture of KCl and ZnCl2 in a molar ratio of 23:27; The mass ratio of the low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt is 40:10:1; The heating and holding process involves a heating rate of 1℃ / min and a holding temperature of 235℃. After the exchange is complete, the ion exchange time at a constant temperature is 3.5 hours; The washing process involves washing with deionized water at 80°C until the pH of the washing solution reaches 7.2. The drying process involves a drying temperature of 85°C and a drying time of 20 hours.

[0018] Example 3: A method for preparing a lithium-type X-type molecular sieve adsorbent Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. Then, a silicon source, polyamino-polyether methylenephosphonic acid, and magnesium aluminum silicate are added to the deionized water and stirred until homogeneous to obtain a silicon solution. Next, the alkali aluminum solution and the silicon solution are mixed according to a SiO2 to Al2O3 molar ratio of 2.3 and stirred rapidly to obtain a gel. After low-temperature aging and high-temperature crystallization, the gel is cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. The aluminum source is sodium aluminate; The alkali source is sodium hydroxide; The silicon source is sodium metasilicate; In the alkaline aluminum solution, the mass fraction of the aluminum source is 16 wt%, and the mass fraction of the alkali source is 33 wt%. In the silicon solution, the mass fraction of the silicon source is 48 wt%, the mass fraction of polyaminopolyether methylenephosphonic acid is 5 wt%, and the mass fraction of magnesium aluminum silicate is 6 wt%. The low-temperature aging process involves an aging temperature of 65°C and an aging time of 16 hours. The high-temperature crystallization is carried out at a crystallization temperature of 95°C for 9 hours. The washing process involves washing with deionized water until the pH of the washing solution reaches 8. The drying process is carried out at a temperature of 95°C for 10 hours. The grinding process involves grinding the dried product into powder with a particle size of 600 mesh.

[0019] Step 2, Ion exchange After mixing low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt evenly, the mixture is placed in a muffle furnace, heated and kept at a constant temperature, and then ion exchange is carried out at the constant temperature. After the exchange is complete, the mixture is cooled to room temperature, and then washed and dried to obtain lithium-type X molecular sieve adsorbent. The organic lithium salt is lithium sebacate; The low-melting-point mixed molten salt is a mixture of KCl and ZnCl2 in a molar ratio of 23:27; The mass ratio of the low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt is 120:90:15; The heating and holding process involves a heating rate of 5°C / min and a holding temperature of 300°C. After the exchange is complete, the ion exchange time is 5 hours at a constant temperature; The washing process involves washing with deionized water at a temperature of 95°C until the pH of the washing solution reaches 7.8. The drying process is carried out at a temperature of 110°C for 10 hours.

[0020] Example 4: A method for preparing a lithium-type X-type molecular sieve adsorbent Step 1 is the same as in Example 1; Step 2, Ion exchange The organic lithium salt is dilithium azelate, and other operations are the same as in Example 1; Step 3 is the same as in Example 1.

[0021] Comparative Example 1: Based on Example 1, in step 1, the preparation of low-silica-alumina X molecular sieve, polyamino-polyether methylenephosphonic acid and magnesium aluminum silicate were not added. The specific operation is as follows: Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. A silicon source is then added to deionized water and stirred until homogeneous to obtain a silicon solution. The alkali aluminum solution and silicon solution are then mixed according to a SiO2 to Al2O3 molar ratio of 2.2 and stirred rapidly to obtain a gel. The gel is then subjected to low-temperature aging and high-temperature crystallization, cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. In the silicon solution, the mass fraction of the silicon source is 35 wt%, and other operations are the same as in Example 1; Step 2 is the same as in Example 1.

[0022] Comparative Example 2: Based on Example 1, in step 1, the preparation of low-silica-alumina X molecular sieve, polyamino-polyether methylenephosphonic acid was not added. The specific operation is as follows: Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. Then, a silicon source and magnesium aluminum silicate are added to the deionized water and stirred until homogeneous to obtain a silicon solution. Next, the alkali aluminum solution and the silicon solution are mixed according to a SiO2 to Al2O3 molar ratio of 2.2 and stirred rapidly to obtain a gel. After low-temperature aging and high-temperature crystallization, the gel is cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. In the silicon solution, the mass fraction of the silicon source is 35 wt%, the mass fraction of magnesium aluminum silicate is 4 wt%, and other operations are the same as in Example 1; Step 2 is the same as in Example 1.

[0023] Comparative Example 3: Based on Example 1, in step 1, the preparation of low-silica aluminum X molecular sieve, magnesium aluminum silicate was not added. The specific operation is as follows: Step 1: Preparation of low-silica-alumina X molecular sieves An aluminum source is dissolved in deionized water, and an alkali source is added to obtain an alkali aluminum solution. A silicon source and polyamino-polyether methylene phosphonic acid are then added to the deionized water and stirred until homogeneous to obtain a silicon solution. The alkali aluminum solution and silicon solution are then mixed according to a SiO2 to Al2O3 molar ratio of 2.2 and stirred rapidly to obtain a gel. The gel is then subjected to low-temperature aging and high-temperature crystallization, cooled to room temperature, and then filtered, washed, dried, and ground to obtain a low-silicon aluminum X molecular sieve. In the silicon solution, the mass fraction of the silicon source is 35 wt%, the mass fraction of the polyaminopolyether methylenephosphonic acid is 4 wt%, and other operations are the same as in Example 1; Step 2 is the same as in Example 1.

[0024] Comparative Example 4: Based on Example 1, in step 2, ion exchange, instead of adding a low-melting-point mixed molten salt, 7 parts of the low-melting-point mixed molten salt were replaced with 7 parts of an organic lithium salt. The specific operation is as follows: Step 1 is the same as in Example 1; Step 2, Ion exchange Replace 7 parts of low-melting-point mixed molten salt with 7 parts of organic lithium salt, and perform the other operations as in Example 1.

[0025] Performance testing: 1. Determination of lithium-ion exchange capacity: Number of lithium ions per unit mass of adsorbent / Number of cation exchange atoms per unit mass of adsorbent × 100%. Calculation of cation exchange atoms: The Al2O3 content in the molecular sieve is determined by ICP, and the number of Al atoms is calculated from this. Each aluminum atom corresponds to one cation charge. 2. Silicon-to-aluminum ratio: The molar ratio of SiO2 to Al2O3 in lithium-type X-zeolite adsorbents is the silicon-to-aluminum ratio. For the lithium-type X-zeolite adsorbents obtained in Examples 1-4 and Comparative Examples 1-4, the relative contents of SiO2 and Al2O3 were analyzed using an X-ray fluorescence spectrometer of model S8 TICER from Bruker GmbH, Germany, and the molar ratio of SiO2 to Al2O3 was calculated. 3. Nitrogen and oxygen adsorption and separation performance: The adsorption capacity of lithium-type X molecular sieve adsorbent for nitrogen and oxygen at 25℃ and 1 standard atmosphere was tested using a Type II Mcbain vacuum adsorption instrument. Before the test, the vacuum was evacuated to 3 Pa and then cooled to 25℃. The adsorbed gas (nitrogen or oxygen) was then introduced into the test system. The adsorption capacity of the sample for nitrogen or oxygen at adsorption equilibrium was tested at 25℃ and 1 standard atmosphere. The nitrogen-oxygen separation coefficient was calculated according to the formula: nitrogen adsorption capacity / oxygen adsorption capacity.

[0026] Table 1 Lithium-ion exchange capacity (%) <![CDATA[Molar ratio of SiO2 and Al2O3]]> Nitrogen adsorption capacity (mL / g) Oxygen adsorption capacity (mL / g) Nitrogen-oxygen separation coefficient Example 1 98.4 2.04 33.68 2.52 13.37 Example 2 99.1 2.01 34.94 2.40 14.56 Example 3 97.9 2.10 32.62 2.75 11.86 Example 4 98.6 2.02 33.88 2.61 12.98 Comparative Example 1 90.3 2.19 27.95 3.15 8.87 Comparative Example 2 92.4 2.14 28.72 2.97 9.67 Comparative Example 3 91.8 2.15 28.11 3.06 9.19 Comparative Example 4 83.3 2.02 23.87 5.84 4.09 As shown in Table 1, the lithium-ion exchange ratios in Examples 1-4 were all above 97%, the molar ratios of SiO2 and Al2O3 were all below 2.1, the nitrogen adsorption capacity was above 32.5 mL / g, the oxygen adsorption capacity was less than 2.8 mL / g, and the nitrogen-oxygen separation coefficient was above 11.8. This demonstrates that the present invention yields a lithium-type X molecular sieve with a low silicon-to-aluminum ratio, high lithium content, large nitrogen adsorption capacity, small oxygen adsorption capacity, and an excellent nitrogen-oxygen separation coefficient. In Comparative Examples 1-3, either without polyaminopolyether-based methylenephosphonic acid or with only one of the two substances, the molar ratio of SiO2 to Al2O3 was above 2.14. The lithium-ion exchange ratio decreased to below 92.4%, the nitrogen adsorption capacity decreased, the oxygen adsorption capacity increased, and the nitrogen-oxygen separation coefficient decreased accordingly. This indicates that the polyaminopolyether-based methylenephosphonic acid and magnesium aluminum silicate, through... The silicon-to-aluminum ratio indirectly affects the lithium-ion exchange capacity, ultimately influencing the adsorption capacity of the lithium-type X molecular sieve adsorbent for nitrogen and oxygen, and reducing the nitrogen-oxygen separation coefficient. A detailed comparison of the test data from Comparative Examples 1-3 shows that Comparative Example 1, without the addition of polyamino-polyether methylenephosphonic acid and magnesium aluminum silicate, exhibits worse performance indicators than Comparative Examples 2 and 3. This indicates that polyamino-polyether methylenephosphonic acid and magnesium aluminum silicate have a synergistic effect in reducing the silicon-to-aluminum ratio of the X-type molecular sieve. In Comparative Example 4, without the addition of a low-melting-point mixed molten salt in step 2 and ion exchange, the lithium-ion exchange capacity drastically decreased to 83.3%, the nitrogen adsorption capacity significantly decreased, the oxygen adsorption capacity significantly increased, and the nitrogen-oxygen separation coefficient also decreased significantly to 4.09. This demonstrates that the low-melting-point mixed molten salt plays a crucial role in improving the lithium-ion exchange capacity.

[0027] Appendix Figure 1 The images show the X-ray spectra of the low-silica-alumina X-ray molecular sieve (before ion exchange) and the lithium-type X-ray molecular sieve adsorbent (after ion exchange) obtained in Example 1. Figure 1As can be seen, in Example 1, the intensity of almost all characteristic peaks decreased before and after the ion exchange step. This indicates that after lithium ions replaced sodium and potassium ions, they had a certain impact on the entire framework lattice of the molecular sieve. This also shows that a violent ion exchange process occurred in Example 1, leading to a violent change in the framework lattice of the molecular sieve. Appendix Figure 2 and attached Figure 3 The images shown are scanning electron microscope (SEM) images of the lithium-type X molecular sieve adsorbents obtained in Examples 1 and 2, magnified 5000 times. The adsorbents obtained in both examples are basically large particles with irregular shapes and relatively rough surfaces. Many micropores can be seen distributed on the surface, which provides a good channel for the adsorption of gas through the internal micropores.

[0028] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium-type X-type molecular sieve adsorbent, characterized in that: The preparation method of the lithium-type X molecular sieve adsorbent includes two steps: preparing a low-silica-alumina X molecular sieve and ion exchange. The preparation of low-silicon aluminum X molecular sieve involves dissolving an aluminum source in deionized water, adding an alkali source to obtain an alkali aluminum solution, then adding a silicon source, polyaminopolyether methylenephosphonic acid, and magnesium aluminum silicate to the deionized water and stirring until homogeneous to obtain a silicon solution. Next, the alkali aluminum solution and silicon solution are mixed according to a SiO2 to Al2O3 molar ratio of 2.1 to 2.3 and rapidly stirred to obtain a gel. After low-temperature aging and high-temperature crystallization, the gel is cooled to room temperature, and then filtered, washed, dried, and ground to obtain the low-silicon aluminum X molecular sieve. The ion exchange involves uniformly mixing low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt, placing the mixture in a muffle furnace, heating and maintaining the temperature, and then carrying out ion exchange at the constant temperature. After the exchange is complete, the mixture is cooled to room temperature, washed, and dried to obtain a lithium-type X molecular sieve adsorbent. The organic lithium salt is one or a mixture of two or more of lithium stearate, lithium 12-hydroxystearate, dilithium sebate, and dilithium azelate in any mass ratio. The low-melting-point mixed molten salt is a mixture of KCl and ZnCl2 in a molar ratio of 23:

27.

2. The method for preparing the lithium-type X molecular sieve adsorbent according to claim 1, characterized in that: The aluminum source is sodium aluminate; The alkaline source is one or a mixture of two of sodium hydroxide and potassium hydroxide in any mass ratio; The silicon source is one or a mixture of two of sodium metasilicate and potassium silicate in any mass ratio. In the alkaline aluminum solution, the mass fraction of the aluminum source is 6-16 wt%, and the mass fraction of the alkaline source is 15-33 wt%. In the silicon solution, the mass fraction of silicon source is 25-48 wt%, the mass fraction of polyaminopolyether methylenephosphonic acid is 2-5 wt%, and the mass fraction of magnesium aluminum silicate is 1.5-6 wt%.

3. The method for preparing the lithium-type X-type molecular sieve adsorbent according to claim 1, characterized in that: The mass ratio of the low-silicon aluminum X molecular sieve, organic lithium salt, and low-melting-point mixed molten salt is 40~120:10~90:1~15.

4. The method for preparing the lithium-type X molecular sieve adsorbent according to claim 1, characterized in that: The heating and holding process involves a heating rate of 1~5℃ / min and a holding temperature of 235~300℃. After the exchange is complete, the ion exchange time is 3.5 to 5 hours at a constant temperature.

5. The lithium-type X molecular sieve adsorbent prepared by any one of the preparation methods according to claims 1-4, characterized in that: The lithium-type X molecular sieve adsorbent has a lithium-ion exchange capacity of 97.9-99.1%, a SiO2 to Al2O3 molar ratio of 2.01-2.10, a nitrogen adsorption capacity of 32.62-34.94 mL / g, an oxygen adsorption capacity of 2.40-2.75 mL / g, and a nitrogen-oxygen separation coefficient of 11.86-14.56.

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