A shaped lta structure aluminophosphate molecular sieve membrane and a preparation method and application thereof

Abstract

The present application relates to the technical field of shaped molecular sieve membrane, in particular to a kind of shaped LTA structure aluminum phosphate molecular sieve membrane and its preparation method and application;Its technical scheme uses the shaped aluminum phosphate dry gel containing fluorine as precursor, and directly heats solid phase synthesis shaped LTA structure aluminum phosphate molecular sieve membrane in open system;The shaped LTA structure aluminum phosphate molecular sieve membrane prepared by the present application has high mechanical strength, high density, good gas permeation separation performance, and potential application value in the field of gas separation and purification.

Description

Technical Field

[0001] This invention relates to the field of molded molecular sieve membrane technology, specifically to a molded LTA structure aluminum phosphate molecular sieve membrane, its preparation method, and its application. Background Technology

[0002] Molecular sieve-based materials (molecular sieve membranes) have important applications in catalysis, adsorption, and separation due to their regular pore structure, good hydrothermal stability, high surface area, and other characteristics. LTA-type aluminum phosphate molecular sieves (ALPO4-42) belong to the cubic crystal system and have a neutral framework structure composed of aluminum-oxygen tetrahedra and phosphorus-oxygen tetrahedra. The framework contains SOD and LTA cages, as well as three-dimensional intersecting channels composed of double four-membered ring structural units. The eight-membered ring pore size of LTA-type aluminum phosphate molecular sieves is 0.4 nm, which is close to or smaller than the kinetic diameter of low-carbon hydrocarbon molecules. Therefore, the separation of H2, N2, and low-carbon hydrocarbons using LTA-type aluminum phosphate molecular sieves holds promise for achieving good separation results through type-selective diffusion or molecular sieving.

[0003] Traditional methods for preparing and synthesizing molecular sieve membranes mainly include:

[0004] (1) In-situ growth method: The substrate is directly immersed in the synthesis solution and crystallized to obtain a molecular sieve membrane, such as the method disclosed in Prog. Chem. 2001, 13, 392-397 and Chinese patent CN103318916A; the in-situ synthesis method has strict requirements on the substrate and synthesis conditions.

[0005] (2) The secondary growth method, which is also the most commonly used film-forming strategy, involves first attaching a layer of pre-synthesized nanoscale molecular sieve crystals as seed crystals to the substrate, and then immersing it in a synthesis solution for further crystallization to form a continuous molecular sieve membrane with good gas permeability, as disclosed in the literature J. Am. Chem. Soc. 2010, 132, 2140-2141. This method inevitably requires the pre-preparation of nanoscale molecular sieve crystals as seed crystals, making the coating process cumbersome and complex.

[0006] Furthermore, since both methods rely on synthesis techniques using liquid solvents, the system self-generated pressure during synthesis can pose safety hazards when the liquid phase is water or a volatile solvent. When the liquid phase is an ionic liquid, the system self-generated pressure is almost zero due to the non-volatile nature of ionic liquids; however, the high cost of ionic liquids increases the synthesis cost. Additionally, the large amounts of acid and alkali waste liquid generated during the in-situ growth and secondary growth methods can also cause environmental pollution.

[0007] High-temperature solid-state synthesis is an effective method for preparing thermally stable materials. Inorganic solids are produced by mixing, grinding, and calcining solid reactants, and this process can be carried out under open system conditions, such as with ceramic oxides, silicates, and metal alloys (Acc. Chem. Res. 2018, 51, 2918-2925). In the field of zeolite materials, more than a dozen silica-based zeolites can be prepared by topological condensation of corresponding two-dimensional precursors (Catal. Today 2020, 354, 133-140).

[0008] Therefore, compared with conventional hydrothermal and solvothermal synthesis methods, open-system solid-phase synthesis can be carried out under normal pressure, thus eliminating the safety hazards caused by the high pressure generated by the solvent during the reaction. Furthermore, open-system solid-phase synthesis is simple, easy to operate, produces no wastewater, and is environmentally friendly.

[0009] As can be seen from the above introduction to the synthesis of molecular sieve membranes and open systems, if molecular sieve membranes can be prepared using open systems, it is expected to develop a simple, environmentally friendly, and rapid method for preparing molecular sieve membranes. To date, the synthesis of molecular sieve membranes in open containers remains a significant challenge. Summary of the Invention

[0010] To address the problems existing in the prior art, the purpose of this invention is to provide a method for preparing a molded LTA structure aluminum phosphate molecular sieve membrane. The molded LTA structure aluminum phosphate molecular sieve membrane prepared by the technical solution provided by this invention has high mechanical strength, no cracks, and good gas permeation and separation performance.

[0011] To achieve the above objectives, this invention uses fluorine-containing molded aluminum phosphate gel as a precursor and heats it under open conditions to directly prepare molded LTA structured aluminum phosphate molecular sieve membranes.

[0012] Specifically, the technical solution of the present invention is as follows:

[0013] In a first aspect of the present invention, a method for preparing an LTA-structured aluminum phosphate molecular sieve membrane involves preparing a hydrogel from reactants, including an aluminum source, a phosphorus source, an organic amine, and an ionic liquid, in the presence of hydrofluoric acid. The resulting hydrogel is then dried and shaped, and the LTA-structured aluminum phosphate molecular sieve membrane is formed by direct heating in an open system.

[0014] (1) An aluminum source, a phosphorus source, hydrofluoric acid, and an organic amine were added sequentially to deionized water. After stirring evenly at room temperature, an ionic liquid was added dropwise to prepare a hydrogel.

[0015] (2) The gel is aged at room temperature for 3-6 hours, and then treated at 80℃-100℃ for 5-10 hours to remove some of the water from the gel, resulting in a malleable solid. This solid is then molded to obtain a molded dry gel.

[0016] (3) Place the molded dry adhesive in an open system and heat it to crystallize in air without adding liquid. The crystallization reaction should be carried out at 90-300℃ for ≥10 minutes.

[0017] (4) After crystallization, a solid product is obtained and dried at room temperature to 120°C for 2 to 12 hours; then it is calcined at 400 to 600°C in air for 7 to 13 hours to obtain a shaped LTA structure aluminum phosphate molecular sieve membrane.

[0018] In a more specific embodiment, the molar ratio of Al2O3∶P2O5∶HF∶organic amine∶H2O∶ionic liquid in the hydrogel of step (1) is 1∶0.5-2∶0.1-5∶0.1-10∶10-150∶0.5-3.

[0019] In a more specific embodiment, the molar ratio of Al2O3∶P2O5∶HF∶organic amine∶H2O∶ionic liquid in the molded aluminum phosphate dry adhesive of step (2) is 1∶0.5-2∶0.1-5∶0.1-10∶1-10∶0.5-3.

[0020] Preferably, the molar amounts of aluminum and phosphorus sources are calculated in their oxide form.

[0021] The aluminum source is one or more of boehmite, activated alumina, aluminum hydroxide, aluminum isopropoxide, aluminum sulfate, or aluminum nitrate.

[0022] Preferably, the phosphorus source is phosphoric acid, and the phosphoric acid concentration is 85 wt%.

[0023] Preferably, the fluorine source is hydrofluoric acid, and the concentration of the hydrofluoric acid is 40 wt%.

[0024] Preferably, the organic amine is tetramethylammonium hydroxide, and the concentration of tetramethylammonium hydroxide is 25%.

[0025] Preferably, the ionic liquid is an imidazole ionic liquid; more preferably, the ionic liquid is a 1-butyl-3-methylimidazolium bromide ionic liquid.

[0026] In one embodiment of the present invention, a molded LTA structure aluminum phosphate molecular sieve membrane prepared by the above method is provided.

[0027] The molded LTA structure aluminum phosphate molecular sieve membrane described in this embodiment has the characteristics of high mechanical strength and no cracks, thus having good gas permeation and separation performance, and is used for gas separation.

[0028] In a second aspect of the invention, a molded LTA-structured aluminum phosphate molecular sieve membrane prepared by the method described in the first aspect is provided.

[0029] In a third aspect of the invention, an application of the molded LTA structure aluminum phosphate molecular sieve membrane described in the first aspect in gas separation is provided.

[0030] The specific embodiments of the present invention have the following beneficial effects:

[0031] 1. The technical solution provided by the present invention uses fluorine-containing molded aluminum phosphate gel as a precursor, processes it into the desired shape, and makes molded aluminum phosphate dry gel. It is heated and crystallized under open system conditions without the need for solvents. It has the advantages of simple operation steps, low cost, environmental friendliness and easy industrial application.

[0032] 2. The molded LTA structure aluminum phosphate molecular sieve membrane prepared by the technical solution provided by the present invention has high mechanical strength, no cracks, and good gas permeation separation performance. Attached Figure Description

[0033] Figure 1 The image shows the XRD pattern of the molded LTA structure aluminum phosphate molecular sieve membrane prepared in Example 1 of this invention.

[0034] Figure 2 SEM image of the molded LTA structure aluminum phosphate molecular sieve membrane prepared in Example 1 of the present invention. Detailed Implementation

[0035] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0036] The following embodiments further illustrate the present invention, but the specific implementation is not limited to the embodiments.

[0037] The open system described in this application is carried out in an open reactor, thereby eliminating the safety hazards caused by the high pressure generated by the solvent during the reaction process.

[0038] Example 1

[0039] 13.02 g of boehmite (78.3 wt%), 11.53 g of 85% phosphoric acid, 8 g of 40% hydrofluoric acid, and 29.17 g of 25% tetramethylammonium hydroxide were sequentially added to 162 g of deionized water. After stirring thoroughly at room temperature, 21.9 g of 1-butyl-3-methylimidazolium bromide ionic liquid was added dropwise, and stirring continued for 30 min to obtain a hydrogel. The hydrogel was aged at room temperature for 6 h, then baked at 80 °C for 10 h to remove some of the water, resulting in a malleable solid. This solid was then pressed into discs to obtain a molded dry gel. The original molded dry gel was placed in an open system and crystallized at 160 °C for 12 h in air without adding liquid. The product was washed three times with deionized water, dried in an oven at 120 °C for 6 h, and then calcined in air at 550 °C for 6 h to obtain a molded LTA-structured aluminum phosphate molecular sieve membrane.

[0040] Depend on Figure 1 It can be seen that the prepared molecular sieve membrane is a pure phase, free of impurities. Figure 2 It can be seen that the prepared molecular sieve membrane is dense, intact, and continuous.

[0041] Gas separation performance test of LTA type molecular sieve membrane:

[0042] The prepared disc-shaped LTA molecular sieve membrane was sealed in a membrane permeation cell using fluorosilicone rubber O-rings. Equal volumes of H2 and binary gases of CO2, O2, N2, and CH4 were introduced into one side of the membrane. A pressure difference was generated across the membrane using purge gas to provide the driving force for gas permeation, thus evaluating the gas separation performance of the prepared LTA molecular sieve membrane. The gas separation performance test results of the molecular sieve prepared in Example 1 are shown in Table 1. The test results show that at a temperature of 293 K and a pressure difference of 1 bar, the separation coefficients of the LTA molecular sieve membrane in Example 1 for H2 / CO2, H2 / O2, H2 / N2, and H2 / CH4 mixed gases were 6.8, 5.7, 6.2, and 4.4, respectively, all higher than the corresponding Knudsen diffusion coefficients, indicating excellent separation selectivity.

[0043] In Table 1, the Knudsen diffusion coefficient for binary gases is the reciprocal of the square root of the molecular weights of the two gases; the ideal gas separation coefficient is the ratio of the permeation amounts of the single-component gases; and the mixed gas separation coefficient is the ratio of the permeation amounts of the mixed gases.

[0044] Table 1. Two-component gas separation data of LTA membrane

[0045]

[0046] Example 2

[0047] 13.02 g of boehmite (78.3 wt%), 5.77 g of 85% phosphoric acid, 100 g of 40% hydrofluoric acid, and 18.23 g of 25% tetramethylammonium hydroxide were sequentially added to 180 g of deionized water. After stirring thoroughly at room temperature, 11.29 g of 1-butyl-3-methylimidazolium bromide ionic liquid was added dropwise, and stirring continued for 30 min to obtain a hydrogel. The hydrogel was aged at room temperature for 6 h, then baked at 100 °C for 10 h to remove some of the water, resulting in a malleable solid. This solid was then pressed into discs to obtain a molded dry gel. The original molded dry gel was placed in an open system under air atmosphere without the addition of liquid and crystallized at 220 °C for 20 min. The product was washed three times with deionized water, then dried in an oven at 120 °C for 12 h, and finally calcined in air atmosphere at 550 °C for 10 h to obtain a molded LTA-structured aluminum phosphate molecular sieve membrane.

[0048] Example 3

[0049] 13.02 g of boehmite (78.3 wt%), 23.06 g of 85% phosphoric acid, 2 g of 40% hydrofluoric acid, and 364.6 g of 25% tetramethylammonium hydroxide were sequentially added to 270 g of deionized water. After stirring thoroughly at room temperature, 65.7 g of 1-butyl-3-methylimidazolium bromide ionic liquid was added dropwise, and stirring continued for 30 min to obtain a hydrogel. The hydrogel was aged at room temperature for 6 h, then baked at 100 °C for 10 h to remove some of the water, resulting in a malleable solid. This solid was then pressed into discs to obtain a molded dry gel. The original molded dry gel was placed in an open system under air atmosphere without the addition of liquid and crystallized at 170 °C for 12 h. The product was washed three times with deionized water, dried in an oven at 100 °C for 10 h, and then calcined in air atmosphere at 450 °C for 13 h to obtain a molded LTA-structured aluminum phosphate molecular sieve membrane.

[0050] Example 4

[0051] 13.02 g of boehmite (78.3 wt%), 11.53 g of 85% phosphoric acid, 16 g of 40% hydrofluoric acid, and 3.65 g of 25% tetramethylammonium hydroxide were sequentially added to 162 g of deionized water. After stirring thoroughly at room temperature, 21.9 g of 1-butyl-3-methylimidazolium bromide ionic liquid was added dropwise, and stirring continued for 30 min to obtain a hydrogel. The hydrogel was aged at room temperature for 6 h, then baked at 80 °C for 10 h to remove some of the water, resulting in a malleable solid. This solid was then pressed into discs to obtain a molded dry gel. The original molded dry gel was placed in an open system under air atmosphere without the addition of liquid and crystallized at 160 °C for 26 h. The product was washed three times with deionized water, dried in an oven at 120 °C for 6 h, and then calcined in air atmosphere at 550 °C for 6 h to obtain a molded LTA-structured aluminum phosphate molecular sieve membrane.

[0052] Example 5

[0053] 13.02 g of boehmite (78.3 wt%), 11.53 g of 85% phosphoric acid, 20 g of 40% hydrofluoric acid, and 36.46 g of 25% tetramethylammonium hydroxide were sequentially added to 162 g of deionized water. After stirring thoroughly at room temperature, 10.95 g of 1-butyl-3-methylimidazolium bromide ionic liquid was added dropwise, and stirring continued for 30 min to obtain a hydrogel. The hydrogel was aged at room temperature for 6 h, then baked at 80 °C for 10 h to remove some of the water, resulting in a malleable solid. This solid was then pressed into discs to obtain a molded dry gel. The original molded dry gel was placed in an open system under air atmosphere without the addition of liquid and crystallized at 160 °C for 18 h. The product was washed three times with deionized water, dried in an oven at 120 °C for 6 h, and then calcined in air atmosphere at 550 °C for 8 h to obtain a molded LTA-structured aluminum phosphate molecular sieve membrane.

[0054] Comparative Example 1: Conventional hydrothermal preparation of LTA-structured molecular sieve membranes

[0055] (1) Preparation of a substrate with functionalized surface:

[0056] A porous alumina ceramic matrix was placed in a toluene solution containing CPTMS (solution concentration of 0.2 mol / L) and reacted at 383 K for 1 hour to introduce chloropropyl silane linkers onto the surface of the porous alumina ceramic matrix.

[0057] (2) Preparation of LTA-type molecular sieve membrane synthesis solution:

[0058] Weigh 22.22g of NaOH and dissolve it in 47.5g of distilled water. After clarification, add 0.30g of aluminum foil and stir to form an aluminum solution. Measure 4.17g of silica sol and add it to 50g of preheated distilled water, stirring thoroughly. Slowly add the preheated aluminum solution to the silica sol and stir thoroughly. Then, continue stirring the solution at room temperature for 24 hours. The final solution contains Na2O, Al2O3, SiO2, and H2O (equivalent to a silicon-aluminum precursor containing aqueous complexes of NaAlO2 and Na2SiO3) in a molar ratio of 50:1:5:1000.

[0059] (3) Preparation of LTA-type molecular sieve membranes:

[0060] The surface-modified substrate was placed in a reactor containing the synthesis solution of LTA-type molecular sieve membrane (2). The reactor was heated to 333 K using an oven. After hydrothermal synthesis at this temperature for 24 hours, the reaction solution was removed, cooled to room temperature, washed with distilled water until neutral, and the product was dried at 383 K to obtain the LTA-type molecular sieve membrane. The test results showed that at a temperature of 293 K and a pressure difference of 1 bar, the separation coefficients of H2 / CO2, H2 / O2, H2 / N2, and H2 / CH4 mixed gases of the LTA molecular sieve membrane of Comparative Example 1 were 6.0, 5.4, 5.3, and 3.9, respectively.

[0061] Compared with the separation performance results of Example 1 of this application (the separation coefficients of the LTA molecular sieve membrane for H2 / CO2, H2 / O2, H2 / N2, and H2 / CH4 mixed gases in Example 1 were 6.8, 5.7, 6.2, and 4.4, respectively), it can be seen that the separation performance of the LTA molecular sieve membrane in Comparative Example 1 is lower than that of Example 1 of this application. A possible reason is that the comparative example used a hydrothermal in-situ synthesis method for the molecular sieve, resulting in incomplete conversion of the alumina base and a low LTA molecular sieve content. In contrast, this application first mixes the required raw materials evenly and prepares a molded dry gel, which is then crystallized under certain conditions to completely convert the dry gel into LTA molecular sieves. The molecular sieve membrane prepared in Example 1 of this application is entirely composed of LTA molecular sieves, resulting in superior gas separation performance.

Claims

1. A method for preparing a molded LTA structure aluminum phosphate molecular sieve membrane, characterized in that, The steps are as follows: Aluminum source, phosphoric acid, hydrofluoric acid and tetramethylammonium hydroxide were added sequentially to deionized water. After stirring evenly at room temperature, ionic liquid was added dropwise to prepare hydrogel. The hydrogel prepared in step 1) is aged at room temperature for 3-6 hours, and then baked at 80 ℃-100 ℃ for 5-10 hours to obtain a malleable solid. The solid is then pressed into a dry gel. The dry gel prepared in step 2) was placed in an open system and crystallized at 160 ℃-220 ℃ for 20 min-26 h in air without the addition of liquid. The crystallized product from step 3) is dried at 25℃~120℃ for 2~12h, and then calcined in air at 400~600℃ for 7~13h to obtain a molded LTA structure aluminum phosphate molecular sieve membrane. The molar ratio of the aluminum source, phosphoric acid, hydrofluoric acid, tetramethylammonium hydroxide, H2O, and ionic liquid is 1:0.5-2:0.1-5:0.1-10:10-150:0.5-3.

2. The method for preparing the molded LTA structure aluminum phosphate molecular sieve membrane according to claim 1, characterized in that, The aluminum source is one or more of boehmite, activated alumina, aluminum hydroxide, aluminum isopropoxide, aluminum sulfate, or aluminum nitrate.

3. The method for preparing the molded LTA structure aluminum phosphate molecular sieve membrane as described in claim 1, characterized in that, The phosphoric acid concentration is 65-99 wt%; the hydrofluoric acid concentration is 30-75 wt%; and the tetramethylammonium hydroxide concentration is 10 wt%-60 wt%.

4. The method for preparing the molded LTA structure aluminum phosphate molecular sieve membrane as described in claim 3, characterized in that, The phosphoric acid concentration is 85 wt%; the hydrofluoric acid concentration is 40 wt%; and the tetramethylammonium hydroxide concentration is 25 wt%.

5. The method for preparing the molded LTA structure aluminum phosphate molecular sieve membrane according to claim 1, characterized in that, The ionic liquid is an imidazole-based ionic liquid.

6. The method for preparing the molded LTA structure aluminum phosphate molecular sieve membrane according to claim 5, characterized in that, The imidazole-based ionic liquid is either 1-butyl-3-methylimidazolium bromide ionic liquid or 1-ethyl-3-methylimidazolium bromide ionic liquid.

7. A molded LTA-structured aluminum phosphate molecular sieve membrane, characterized in that, Prepared by the method according to any one of claims 1-6.

8. The application of the molded LTA structure aluminum phosphate molecular sieve membrane according to claim 7 in gas separation.

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