MWF molecular sieve as well as preparation method and application thereof

Abstract

The invention provides an MWF molecular sieve as well as a preparation method and application thereof, and belongs to the technical field of molecular sieve synthesis. The preparation method comprises the following steps: mixing an aluminum source, an inorganic metal alkali source, water, a silicon source and a structure-directing agent to obtain gel; and carrying out hydrothermal crystallization on the gel to obtain the MWF molecular sieve. According to the present invention, the piperazine derivative or the piperidine derivative is adopted as the structure-directing agent, the sodium hydroxide and the potassium hydroxide are adopted as the inorganic alkali source, and the aluminosilicate MWF molecular sieve can be synthesized by using the one-step hydrothermal crystallization method, such that the cost is low, the yield is high, and the prepared MWF molecular sieve has characteristics of high crystallinity and good stability compared with the existing structure-directing agent-based MWF molecular sieve synthesis method; the MWF molecular sieve prepared by the method has a wide SiO2 / Al2O3 molar ratio window (SiO2 / Al2O3 = 5-20), the structure does not collapse after the structure-directing agent is removed by high-temperature roasting, the thermal stability is good, and the application of the MWF molecular sieve in the fields of catalysis and adsorption is expanded.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve synthesis technology, specifically to an MWF molecular sieve, its preparation method, and its application. Background Technology

[0002] As a member of the RHO family, MWF molecular sieves inherit the cage-like topology of RHO molecular sieves, but their pore arrangement and inter-cage connection methods differ (Nature 524, 74–78 (2015)). It can be observed that members of the RHO family share similar structural features, but their structural complexity increases with each generation. The first generation of RHO molecular sieves had only two building blocks, namely... lta The cage is formed by 8MR connection. d8r unit( a =14.77 Å), PAU molecular sieve has 7 constituent building blocks, and its structure is more complex than that of RHO, with cell parameters... a =34.34 Å. MWF molecular sieves have the same composite building blocks as PAU molecular sieves, but their stacking is more complex and they have larger cell parameters ( a =44.22 Å).

[0003] The complexity and instability of the RHO family of molecular sieves have hindered extensive research by scientists; currently, MWF molecular sieves can only be synthesized via tetraethylammonium hydroxide. Molecular sieves possess high specific surface area, ordered pore structure, controllable framework composition, and good hydrothermal stability, making them widely used in chemical processes such as catalysis, separation, ion exchange, and adsorption purification.

[0004] Hong et al. studied the adsorption of CO2 by five molecular sieves in the RHO family (RHO, PST-9, ECR18, ZSM-25, and PST-20). The results showed that Cs-TEA-ZSM-25 (MWF) and Cs-TEA-PST-20 had an adsorption capacity of 1.3 mmol·g⁻¹ for CO2 under conditions of 293 K–343 K. -1 It outperforms Na-A, which is commonly used in industry, and exhibits significant selective adsorption of CO2 in N2-CH4-CO2. In summary, MFW molecular sieves have great potential for selective CO2 adsorption, and the synthesis of thermally stable MWF molecular sieves shows broad prospects. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a MWF molecular sieve, its preparation method and application. The present invention can directionally prepare MWF molecular sieves, and the obtained MWF molecular sieves have a wide SiO2 / Al2O3 molar ratio window (SiO2 / Al2O3=5~20), good thermal stability and good CO2 adsorption performance.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing MWF molecular sieves, comprising the following steps: An aluminum source, an inorganic metal alkali source, water, a silicon source, and a structure-directing agent are mixed to obtain a gel. The gel was subjected to hydrothermal crystallization to obtain MWF molecular sieve; The inorganic metal alkali source is sodium hydroxide and potassium hydroxide; The silica-alumina molar ratio of the MWF molecular sieve is 5~20; The structure-directing agent has the structure shown in Formula I or Formula II: Formula I, Formula II; In Formula I, R1, R2, R3 and R4 are independently methyl, ethyl or isopropyl; In Formula II, R5, R6, R7, R8, R9, R 10 and R 11 It is independently hydrogen, methyl, ethyl or isopropyl, and R5 and R6 are not hydrogen.

[0007] Preferably, the structure-directing agent has one of the following structures: .

[0008] Preferably, in the inorganic metal alkali source, the molar ratio of sodium hydroxide to potassium hydroxide is 2~6.5:1.

[0009] Preferably, the aluminum source includes one or more of aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum hydroxide, sodium aluminate, boehmite, and aluminum isopropoxide.

[0010] Preferably, the silicon source includes one or more of sodium silicate, silica gel, silica powder, fumed silica, and tetraethyl orthosilicate.

[0011] Preferably, the aluminum source is calculated as Al2O3, the inorganic metal alkali source is calculated as alkali metal oxide, the silicon source is calculated as SiO2, and the molar ratio of the silicon source, aluminum source, inorganic metal alkali source, structure directing agent and water is (5~40):1:(0.5~2.5):(0~2):(40~150), and the molar ratio of the structure directing agent is not 0.

[0012] Preferably, the hydrothermal crystallization temperature is 120~200 ℃ and the time is 1~30 days.

[0013] Preferably, when the structure-directing agent has the structure shown in Formula I, the method for preparing the structure-directing agent includes the following steps: A piperazine compound, a haloalkane, and an organic solvent are mixed and subjected to a first substitution reaction to obtain the structure-directing agent; the piperazine compound has the structure shown in Formula III, the haloalkane includes haloalkane 1 and haloalkane 2, and haloalkane 1 and haloalkane 2 have the structures shown in Formula IV and Formula V respectively, where X1 and X2 in Formula IV and Formula V are halogens. Formula III, Formula IV, Formula V; When the structure-directing agent has the structure shown in Formula II, the preparation method of the structure-directing agent includes the following steps: A piperidine compound, a haloalkane, and an organic solvent are mixed and subjected to a second substitution reaction to obtain the structure-directing agent; the piperidine compound has the structure shown in Formula VI, the haloalkane includes haloalkane 3 and haloalkane 4, and haloalkane 3 and haloalkane 4 have the structures shown in Formula VII and Formula VIII respectively, where X3 and X4 in Formula VII and Formula VIII are halogens. Formula VI, Equation VII, Formula VIII.

[0014] Preferably, the molar ratio of the piperazine compound to the haloalkane is 1:1 to 3; the temperature of the first substitution reaction is 40 to 60°C, and the time is 10 to 40 h. The molar ratio of the piperidine compound to the haloalkane is 1:1 to 3; the temperature of the second substitution reaction is 40 to 90°C, and the time is 24 to 72 h.

[0015] This invention provides MWF molecular sieves prepared by the above-described preparation method.

[0016] This invention provides the application of the above-mentioned MWF molecular sieve in the selective adsorption of CO2.

[0017] This invention provides a method for preparing MWF molecular sieves, comprising the following steps: mixing an aluminum source, an inorganic metal alkali source, water, a silicon source, and a structure directing agent to obtain a gel; subjecting the gel to hydrothermal crystallization to obtain MWF molecular sieves; wherein the inorganic metal alkali source is sodium hydroxide and potassium hydroxide; the silicon-to-aluminum molar ratio of the MWF molecular sieve is 5-20; and the structure directing agent has a structure as shown in Formula I or Formula II. Many factors influence the synthesis of molecular sieves (such as the type of inorganic precursor (Al, Si, P, etc.), gel composition (SiO2 / Al2O3 ratio, etc.), organic structure directing agent, type and concentration of inorganic cations, and mineralizing agent (OH-). - F - In the synthesis of molecular sieves, structure-directing agents often play a crucial role in the structure and composition of the synthesized products, taking into account factors such as mother liquor pH, crystallization temperature, and time. Located within the channels or cages of the molecular sieve, these agents guide the formation of molecular sieves with specific channel structures. This invention uses piperazine or piperidine derivatives as structure-directing agents and sodium hydroxide and potassium hydroxide as inorganic base sources to synthesize aluminosilicate MWF molecular sieves via a one-step hydrothermal crystallization method. Specifically, this invention uses sodium hydroxide and potassium hydroxide as base sources, providing the alkalinity required for molecular sieve synthesis, and... + K + The negative charge of the molecular sieve framework can be balanced because the [AlO4] tetrahedra in the molecular sieve are negatively charged and require cations for balance. On the other hand, different alkali metal cations have different basicities and cation radii. This invention uses sodium hydroxide and potassium hydroxide in combination to control the pore size of the molecular sieve, obtaining a MWF molecular sieve with an 8-membered ring channel structure and a pore size of 0.16 nm × 0.42 nm. Compared with previous methods for synthesizing MWF molecular sieves using structure-directing agents, the preparation method of this invention is low-cost, high-yield, and produces MWF molecular sieves with high crystallinity, a wide SiO2 / Al2O3 molar ratio window (SiO2 / Al2O3 = 5~20), and good thermal stability after high-temperature calcination to remove the structure-directing agent, which is beneficial for expanding the application of MWF molecular sieves in catalysis and adsorption. Attached Figure Description

[0018] Figure 1 The structure-directing agent 1,1,4,4-tetramethylpiperazine in Example 1 13 C NMR spectrum; Figure 2 The structure-directing agent 1,1,4,4-tetramethylpiperazine in Example 1 1 H NMR spectrum; Figure 3 The mass spectra of the structure-directing agent 1,1,4,4-tetramethylpiperazine in Example 1; Figure 4The structure-directing agent 1-ethyl-1,3,5-trimethylpiperidine in Example 2 1 H NMR spectrum; Figure 5 The structure-directing agent 1-ethyl-1,3,5-trimethylpiperidine in Example 2 13 C NMR spectrum; Figure 6 This is the X-ray diffraction pattern of the MWF molecular sieve obtained by hydrothermal synthesis in the Na-K system using 1,1,4,4-tetramethylpiperazine as a structure directing agent in Example 3 after calcination. Figure 7 The images are scanning electron microscope (SEM) images at different scales of the MWF molecular sieve obtained by hydrothermal synthesis in the Na-K system using 1,1,4,4-tetramethylpiperazine as a structure directing agent in Example 3 after calcination. Figure 8 This is the X-ray diffraction pattern of the MWF molecular sieve obtained by hydrothermal synthesis in the Na-K system using 1,1,4,4-tetramethylpiperazine as a structure directing agent in Example 4 after calcination. Figure 9 The images are scanning electron microscope (SEM) images at different scales of the MWF molecular sieve obtained by hydrothermal synthesis in the Na-K system using 1,1,4,4-tetramethylpiperazine as a structure directing agent in Example 4 after calcination. Figure 10 The X-ray diffraction pattern of the solid obtained by hydrothermal synthesis in the Na-K system using 1-ethyl-1,3,5-trimethylpiperidine as a structure directing agent in Example 5 is shown. Figure 11 The image shows a scanning electron microscope (SEM) image of the solid obtained by hydrothermal synthesis in a Na-K system using 1-ethyl-1,3,5-trimethylpiperidine as a structure directing agent in Example 5. Figure 12 X-ray diffraction pattern of the solid obtained by hydrothermal synthesis in Comparative Example 1 with 1-ethyl-1,3,5-trimethylpiperidine as an organic structure directing agent and reducing the molar ratio of SiO2 to Al2O3 in the composition of the synthetic gel. Figure 13 X-ray diffraction pattern of the solid obtained by hydrothermal synthesis in Comparative Example 2, where 1,1,4,4-tetramethylpiperazine was used as an organic structure directing agent to increase the amount of alkali metal added to the composition of the synthetic gel. Figure 14 The CO2 adsorption isotherms are for the samples in Examples 3, 4, 5 and Comparative Example 2. Detailed Implementation

[0019] This invention provides a method for preparing MWF molecular sieves, comprising the following steps: An aluminum source, an inorganic metal alkali source, water, a silicon source, and a structure-directing agent are mixed to obtain a gel. The gel was subjected to hydrothermal crystallization to obtain MWF molecular sieve; The inorganic metal alkali source is sodium hydroxide and potassium hydroxide; The silica-alumina molar ratio of the MWF molecular sieve is 5~20; The structure-directing agent has the structure shown in Formula I or Formula II: Formula I, Formula II; In Formula I, R1, R2, R3 and R4 are independently methyl, ethyl or isopropyl; In Formula II, R5, R6, R7, R8, R9, R 10 and R 11 It is independently hydrogen, methyl, ethyl or isopropyl, and R5 and R6 are not hydrogen.

[0020] Unless otherwise specified, the raw materials used in this invention are commercially available.

[0021] This invention involves mixing an aluminum source, an inorganic metal alkali source, water, a silicon source, and a structure-directing agent to obtain a gel. In this invention, the structure-directing agent preferably has one of the following structures: .

[0022] In this invention, when the structure-directing agent has the structure shown in Formula I, the preparation method of the structure-directing agent preferably includes the following steps: A piperazine compound, a haloalkane, and an organic solvent are mixed and subjected to a first substitution reaction to obtain the structure-directing agent; the piperazine compound has the structure shown in Formula III, the haloalkane includes haloalkane 1 and haloalkane 2, and haloalkane 1 and haloalkane 2 have the structures shown in Formula IV and Formula V respectively, where X1 and X2 in Formula IV and Formula V are halogens. Formula III, Formula IV, Formula V; In this invention, R1 and R3 in Formula III are identical to R1 and R3 in Formula I, and R2 and R4 in Formulas IV and V are identical to R2 and R4 in Formula I, respectively. X1 and X2 are halogens, which can be chlorine, bromine, or iodine. In this invention, the molar ratio of the piperazine compound to the haloalkane is preferably 1:1 to 3, and the molar ratio of haloalkane 1 and haloalkane 2 is preferably the same. In this invention, the organic solvent is preferably ethyl acetate.

[0023] In this invention, the preferred method for mixing the piperazine compound, the haloalkane, and the organic solvent is to add the piperazine compound to the organic solvent, stir until homogeneous, and then slowly add the haloalkane to the resulting solution.

[0024] In this invention, the temperature of the first substitution reaction is preferably 40~60℃, more preferably 50℃; the time is preferably 10~40 h, more preferably 20~30 h, and the time of the first substitution reaction is calculated from the time when the haloalkane is completely added.

[0025] After the first substitution reaction is completed, the present invention preferably filters the obtained reaction product, and washes the obtained solid phase in acetone (washing the solid phase in acetone), filters and vacuum dries it in sequence to obtain a structure-directing agent having the structure shown in Formula I.

[0026] When the structure-directing agent has the structure shown in Formula II, the preparation method of the structure-directing agent includes the following steps: A piperidine compound, a haloalkane, and an organic solvent are mixed and subjected to a second substitution reaction to obtain the structure-directing agent; the piperidine compound has the structure shown in Formula VI, the haloalkane includes haloalkane 3 and haloalkane 4, and haloalkane 3 and haloalkane 4 have the structures shown in Formula VII and Formula VIII respectively, where X3 and X4 in Formula VII and Formula VIII are halogens. Formula VI, Equation VII, Formula VIII.

[0027] In this invention, R7, R8, R9, and R of formula VI... 10 and R 11 Respectively related to R7, R8, R9, and R in Equation II 10 and R 11 To maintain consistency, R5 in Formula VII and R6 in Formula VIII are consistent with R5 and R6 in Formula II, respectively. In this invention, the molar ratio of the piperidine compound to the haloalkane is preferably 1:1 to 3, and the molar ratio of haloalkane 3 and haloalkane 4 is preferably the same. In this invention, the organic solvent is preferably ethanol; this invention does not have particular requirements on the amount of the organic solvent used, as long as it is sufficient to fully dissolve the raw materials.

[0028] In this invention, the preferred method for mixing the piperidine compound, the haloalkane, and the organic solvent is to add the piperidine compound to the organic solvent, stir until homogeneous, and then slowly add the haloalkane dropwise to the resulting solution.

[0029] In this invention, the temperature of the second substitution reaction is preferably 40~90℃, more preferably 50~60℃, and the time is preferably 24~72 h, more preferably 50~70 h. The time of the second substitution reaction is calculated from the time when the haloalkane is completely added.

[0030] After the second substitution reaction is completed, the present invention preferably filters the obtained reaction product, and washes the obtained solid in acetone (washing the solid in acetone), filters and vacuum dries it in sequence to obtain a structure-directing agent having the structure shown in Formula II.

[0031] The method for preparing structure-directing agents provided by this invention uses inexpensive and readily available raw materials, and the preparation process is simple and easy to scale up.

[0032] This invention mixes an aluminum source, an inorganic metal alkali source, water, a silicon source, and a structure-directing agent to obtain a gel. In this invention, the inorganic metal alkali source is preferably sodium hydroxide and potassium hydroxide; the molar ratio of sodium hydroxide to potassium hydroxide is preferably 2~6.5:1, more preferably 2.5~5.8:1, and even more preferably 3~5:1. The alkali source in this invention is the simultaneous presence of sodium hydroxide and potassium hydroxide; a suitable ratio is beneficial for controlling the pore size of the molecular sieve, thereby improving the adsorption selectivity for CO2.

[0033] In this invention, the aluminum source preferably includes one or more of aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum hydroxide, sodium aluminate, boehmite, and aluminum isopropoxide; the silicon source preferably includes one or more of sodium silicate, silica gel, silica gel powder, silica fume, and tetraethyl orthosilicate. In embodiments of this invention, the solid content of the silica gel is preferably 40 wt%; the water is preferably deionized water.

[0034] In this invention, the aluminum source is calculated as Al2O3, the inorganic metal alkali source is calculated as alkali metal oxide M2O (M represents Na and K), the silicon source is calculated as SiO2, and the molar ratio of the silicon source, aluminum source, inorganic metal alkali source, structure directing agent and water is preferably (5~40):1:(0.5~2.5):(0~2):(40~150), more preferably (5~30):1:(1~2):(0.1~1):(50~120), and even more preferably (8~20):1:(1.2~2.0):(0.2~0.5):(80~110), and the structure directing agent is not 0.

[0035] In this invention, the preferred method for mixing the aluminum source, inorganic metal alkali source, water, structure directing agent, and silicon source is as follows: The aluminum source, inorganic metal alkali source and water are mixed sequentially, stirred to dissolve and clarify to obtain a first alkaline aluminum solution, and then cooled to room temperature; A structure-directing agent is added to the first alkaline aluminum solution for a second mixing, followed by mechanical stirring to obtain a second organic aluminum-containing solution. A silicon source is added to the second organic aluminum-containing solution for a third mixing to obtain the silicon-aluminum gel.

[0036] In this invention, the first mixing is preferably carried out at room temperature, and is preferably stirred until homogeneous. After homogeneous stirring, the mixture is placed in a high-temperature, high-pressure hydrothermal reactor for heating and dissolution. The heating and dissolution temperature is preferably 120°C, and the heating and dissolution time is preferably 12 hours. The heating and dissolution is preferably carried out under dynamic conditions. This invention, through heating and dissolution, ensures the complete dissolution of the aluminum source, resulting in a clear solution. After adding the silicon source, a uniform gel is obtained, avoiding the formation of impurity phases. In this invention, both the second and third mixing are preferably stirred until a uniform gel is formed.

[0037] After obtaining the gel, the present invention performs hydrothermal crystallization on the gel to obtain a crystallized product. In the present invention, the temperature of the hydrothermal crystallization is preferably 120~200℃, more preferably 140~160℃, and the time is preferably 1~30 days, more preferably 3~21 days, and even more preferably 3~5 days; the hydrothermal crystallization is preferably dynamic hydrothermal crystallization, which is preferably carried out under rotational conditions, and the rotational speed is preferably 15~50 rpm. The present invention preferably transfers the gel to a hydrothermal reactor for hydrothermal crystallization.

[0038] After the hydrothermal crystallization is completed, the present invention preferably cools the obtained crystallization reaction solution and then sequentially performs solid-liquid separation, solid phase washing, and drying to obtain the MWF molecular sieve. In the present invention, the solid-liquid separation method is preferably centrifugation.

[0039] In this invention, the silicon-aluminum molar ratio (SiO2 / Al2O3) of the MWF molecular sieve is preferably 5~20, more preferably 8.5~20, even more preferably 10~18, and even more preferably 12~16.

[0040] The structure-directing agent used in this invention is inexpensive and easy to synthesize, enabling the direct one-step hydrothermal synthesis of MWF molecular sieves. The synthesized MWF molecular sieves have high yield, high crystallinity, short crystallization time, and a wide SiO2 / Al2O3 ratio window (SiO2 / Al2O3=5~20).

[0041] In use, the MWF molecular sieve is calcined to remove the structure-directing agent; the calcination temperature is preferably 450~600℃, more preferably 500~550℃, and the calcination time is preferably 5~10 h, more preferably 6~8 h. The MWF molecular sieve prepared by this invention has good thermal stability, and its structure does not collapse after the structure-directing agent is removed by high-temperature calcination.

[0042] This invention provides a MWF molecular sieve prepared by the above-described method. In this invention, the MWF molecular sieve has an 8-membered ring channel structure with a pore size of 0.16 nm × 0.42 nm.

[0043] This invention provides the application of the above-mentioned MWF molecular sieve in the selective adsorption of CO2.

[0044] The following detailed description of the MWF molecular sieve, its preparation method, and its application provided by the present invention, with reference to specific embodiments, should not be construed as limiting the scope of protection of the present invention.

[0045] Example 1 1,1,4,4-Tetramethylpiperazine was prepared using 1,4-dimethylpiperazine and iodomethane as raw materials, and ethyl acetate and methanol as solvents. Its structural formula is as follows: .

[0046] The preparation method is as follows: 0.3 mol of 1,4-dimethylpiperazine was added to 50 mL of N,N-dimethylformamide and stirred at room temperature for 10 min. Then, 0.3 mol of iodomethane was slowly added dropwise to the above solution and reacted at room temperature for 36 h. The resulting reaction product was then filtered, and the filtered solid was washed with methanol, filtered, and vacuum dried for 24 h to obtain a pale yellow solid product, 1,1,4,4-tetramethylpiperazine.

[0047] 1,1,4,4-Tetramethylpiperazine 13 C 1 H NMR and mass spectrometry detection, results as follows Figures 1-3 As shown in the figure. NMR and mass spectrometry results confirm the successful preparation of 1,1,4,4-tetramethylpiperazine.

[0048] Example 2 1-Ethyl-1,3,5-trimethylpiperidine was prepared using 1,3,5-trimethylpiperidine and iodoethane as raw materials and ethanol as solvent. The structural formula is as follows: .

[0049] 0.3 mol of 1,3,5-dimethylpiperidine was added to 50 mL of ethanol and stirred at room temperature for 10 min. Then, 0.3 mol of iodoethane was slowly added dropwise to the solution and the reaction was carried out at 60 °C for 48 h. The resulting reaction product was then filtered, and the filtered solid was washed with diethyl ether, filtered, and vacuum dried for 24 h to obtain a white solid product, 1-ethyl-1,3,5-trimethylpiperidine.

[0050] 1-Ethyl-1,3,5-trimethylpiperidine 1 H, 13 C NMR detection, results are as follows Figure 4 and Figure 5 As shown. NMR results confirm the successful preparation of 1-ethyl-1,3,5-trimethylpiperidine.

[0051] Example 3 MWF molecular sieves were prepared using 1,1,4,4-tetramethylpiperazine as an organic structure directing agent, potassium hydroxide and sodium hydroxide as alkali sources, and aluminum hydroxide as an aluminum source. The preparation method is as follows: (1) Weigh 2.90 g sodium hydroxide (96 wt%), 1.82 g potassium hydroxide (85 wt%), 4.00 g aluminum hydroxide (99.9 wt%), and 6.75 g deionized water, stir evenly at room temperature, and let it stand statically for 12 h at 120℃. (2) Cool the solution obtained in step (1) to room temperature, add 3.30 g of the 1,1,4,4-tetramethylpiperazine aqueous solution (25 wt%) prepared in Example 1, and stir at room temperature for 1 h; (3) Add 69.16 g of silica gel (JN-40) to the solution obtained in step (2), stir at room temperature for 24 h to form a uniform gel; transfer the obtained gel to a 100 mL hydrothermal reactor and perform dynamic hydrothermal crystallization (60 rpm rotation) at 120 ℃ for 7 days. (4) Centrifuge, wash and dry the solid obtained in step (3) to obtain MWF molecular sieve.

[0052] The gel composition formed after adding silica gel in step (3) is: SiO2:Al2O3:M2O:R:H2O=18:1:1.9:0.22:110, where R represents the structure directing agent, Na2O:Al2O3=1.368, K2O:Al2O3=0.54, NaOH:KOH=2.52.

[0053] The MWF molecular sieve obtained in Example 3 was tested by XRF and found that the SiO2 / Al2O3 molar ratio was 16.2, and the MWF molecular sieve yield was 90% (the yield was calculated as the ratio of the mass of the molecular sieve obtained after the reaction to the theoretically expected mass of the molecular sieve).

[0054] The MWF molecular sieve prepared in Example 3 was calcined at 550°C for 6 h. Figure 6 and Figure 7 The images shown are X-ray diffraction (XRD) and scanning electron microscopy (SEM) images of the MWF molecular sieve prepared in Example 3 after calcination. The XRD and SEM results confirm the successful synthesis of the MWF molecular sieve, and the XRD results show that the prepared MWF molecular sieve has high crystallinity and its structure did not collapse after the structure-directing agent was removed by high-temperature calcination.

[0055] Example 4 MWF molecular sieves were prepared using 1,1,4,4-tetramethylpiperazine as an organic structure directing agent, potassium hydroxide and sodium hydroxide as alkali sources, and aluminum hydroxide as an aluminum source. The preparation method is as follows: (1) Weigh 2.85 g sodium hydroxide (96 wt%), 0.8 g potassium hydroxide (85 wt%), 4.00 g aluminum hydroxide (99.9 wt%), and 15.96 g deionized water, stir evenly at room temperature, and let it stand statically for 12 h at 120℃. (2) Cool the solution obtained in step (1) to room temperature, add 4.42 g of 1,1,4,4-tetramethylpiperazine aqueous solution (25 wt%) (1,1,4,4-tetramethylpiperazine was prepared in Example 1), and stir at room temperature for 1 h; (3) Add 30.74 g of silica gel (JN-40) to the solution obtained in step (2) and stir at room temperature for 24 h to form a uniform gel; transfer the obtained gel to a 100 mL hydrothermal reactor and perform dynamic hydrothermal crystallization at 120 °C (60 rpm rotation) for 7 days. (4) Centrifuge, wash and dry the solid obtained in step (3) to obtain MWF molecular sieve.

[0056] The gel composition formed after adding silica powder in step (3) is: SiO2:Al2O3:M2O:R:H2O=8:1:1.57:0.3:80, where R represents the structure directing agent, Na2O:Al2O3=1.3354, K2O:Al2O3=0.2370, NaOH:KOH=5.63.

[0057] The Si2O / Al2O3 ratio of the MWF molecular sieve obtained in Example 4 was 7.12 as measured by XRF, and the yield of the MWF molecular sieve was 89% (the yield was calculated as the ratio of the mass of the molecular sieve obtained after the reaction to the theoretically expected mass of the molecular sieve).

[0058] The MWF molecular sieve prepared in Example 4 was calcined at 500℃ for 6 hours. Figure 8 and Figure 9 The images shown are X-ray diffraction (XRD) and scanning electron microscopy (SEM) images of the MWF molecular sieve prepared in Example 4 after calcination. The XRD and SEM results confirm the successful synthesis of the MWF molecular sieve, and the XRD results show that the prepared MWF molecular sieve has high crystallinity and its structure did not collapse after the structure-directing agent was removed by high-temperature calcination.

[0059] Example 5 MWF molecular sieves were prepared using 1-ethyl-1,3,5-trimethylpiperidine as an organic structure directing agent, potassium hydroxide and sodium hydroxide as alkali sources, and aluminum hydroxide as an aluminum source. The preparation method is as follows: (1) Weigh 2.90 g sodium hydroxide (96 wt%), 0.81 g potassium hydroxide (85 wt%), 4.00 g aluminum hydroxide (99.9 wt%), and 29.98 g deionized water, stir evenly at room temperature, and let it stand statically for 12 h at 120℃. (2) Cool the gel obtained in step (1) to room temperature, add 7.99 g of 1-ethyl-1,3,5-trimethylpiperidine aqueous solution (25 wt%) (1-ethyl-1,3,5-trimethylpiperidine was prepared in Example 2), and continue stirring for 1 h; (3) Add 18.12 g of silica gel powder (SiO2 content is 95%) to the solution obtained in step (2), stir at room temperature for 24 h to form a uniform gel; transfer the obtained gel to a 100 mL hydrothermal reactor and perform dynamic hydrothermal crystallization at 120 °C (60 rpm rotation) for 14 days. (4) Centrifuge, wash and dry the solid obtained in step (3) to obtain MWF molecular sieve.

[0060] The gel composition formed after adding silica powder in step (3) is: SiO2:Al2O3:M2O:R:H2O=11.2:1:2:0.5:80, where R represents the structure directing agent, Na2O:Al2O3=1.36, K2O:Al2O3=0.24, NaOH:KOH=5.67.

[0061] The MWF molecular sieve obtained in Example 5 was tested by XRF and found that the SiO2 / Al2O3 molar ratio was 10.19, and the MWF molecular sieve yield was 91%.

[0062] The MWF molecular sieve prepared in Example 5 was calcined at a temperature of 500°C for 6 hours. Figure 10 and Figure 11 The images shown are X-ray diffraction (XRD) and scanning electron microscopy (SEM) images of the MWF molecular sieve prepared in Example 4 after calcination. The XRD and SEM results confirm the successful synthesis of the MWF molecular sieve. The XRD results also show that the prepared MWF molecular sieve has high crystallinity and that the structure did not collapse after the structure-directing agent was removed by high-temperature calcination.

[0063] Comparative Example 1 Using 1-ethyl-1,3,5-trimethylpiperidine as an organic structure directing agent, compared with Example 5, the molar ratio of SiO2 to Al2O3 in the synthesized gel composition was reduced, and the effect on the synthesis of MWF molecular sieves was compared. The preparation method is as follows: (1) Weigh 2.90 g sodium hydroxide (96 wt%), 0.81 g potassium hydroxide (85 wt%), 4.00 g aluminum hydroxide (99.9 wt%), and 30.53 g deionized water, stir evenly at room temperature, and let it stand statically for 12 h at 120℃. (2) Cool the gel obtained in step (1) to room temperature, add 7.99 g of 1-ethyl-1,3,5-trimethylpiperidine aqueous solution (25 wt%) (1-ethyl-1,3,5-trimethylpiperidine was prepared in Example 2), and continue stirring for 1 h; (3) Add 7.28 g of silica gel powder (SiO2 content is 95%) to the solution obtained in step (2), stir at room temperature for 24 h to form a uniform gel; transfer the obtained gel to a 100 mL hydrothermal reactor and perform dynamic hydrothermal crystallization at 120 °C (60 rpm rotation) for 14 days. (4) Centrifuge, wash and dry the solid obtained in step (3), and perform XRD test on the resulting solid powder.

[0064] The gel composition formed after adding silica powder in step (3) is: SiO2:Al2O3:M2O:R:H2O=4.5:1:2:0.5:80, of which Na2O:Al2O3=1.36 and K2O:Al2O3=0.24.

[0065] The difference between Comparative Example 1 and Example 5 lies in the molar ratio of SiO2 to Al2O3 in the gel composition during the synthesis process. In Comparative Example 1, the molar ratio of SiO2 to Al2O3 in the gel composition during the synthesis process is 4.5, which is outside the scope of protection of this technology. The lower initial gel Si-Al molar ratio makes it impossible to obtain MWF molecular sieves, and the product obtained after crystallization is an amorphous phase. The results are shown in […]. Figure 12 .

[0066] Comparative Example 2 Using 1,1,4,4-tetramethylpiperazine as an organic structure directing agent, and comparing with Example 4, the effect of increasing the amount of alkali metal added to the synthetic gel composition on the synthesis of MWF molecular sieves was compared. The preparation method is as follows: (1) Weigh 5.46 g sodium hydroxide (96 wt%), 0.92 g potassium hydroxide (85 wt%), 4.00 g aluminum hydroxide (99.9 wt%), and 15.12 g deionized water, stir evenly at room temperature, and let it stand statically for 12 h at 120℃. (2) Cool the solution obtained in step (1) to room temperature, add 4.42 g of 1,1,4,4-tetramethylpiperazine aqueous solution (25 wt%) (1,1,4,4-tetramethylpiperazine was prepared in Example 1), and stir at room temperature for 1 h; (3) Add 30.74 g of silica gel (JN-40) to the solution obtained in step (2), stir at room temperature for 24 h to form a uniform gel; transfer the obtained gel to a 100 mL hydrothermal reactor and perform dynamic hydrothermal crystallization at 120 °C (60 rpm rotation) for 7 days. (4) Centrifuge, wash and dry the solid obtained in step (3), and perform XRD test on the obtained powder.

[0067] The gel composition formed after adding silica powder in step (3) is: SiO2:Al2O3:M2O:R:H2O=8:1:2.83:0.3:80, where Na2O:Al2O3=2.56 and K2O:Al2O3=0.273. Compared with Example 4, the molar ratio of Na2O to Al2O3 increased from 1.3354 to 2.5566, and the molar ratio of K2O to Al2O3 increased from 0.237 to 0.273. The final molar ratio of the alkali source in the gel increased to 2.83, and the molar ratio of Na2O to K2O increased to 9.36. Excessive alkalinity affects the crystallization process of MWF molecular sieves, resulting in a large number of impurity crystals in the final synthesized product. The results are shown in […]. Figure 13 .

[0068] Performance testing Figure 14 The CO2 adsorption isotherms for samples in Examples 3, 4, 5, and Comparative Example 2 are shown. First, the samples were placed on a degassing station and pretreated at 390°C for 4 hours under vacuum. After cooling to room temperature, they were quickly transferred to a physical adsorption analyzer to test their CO2 adsorption performance. Specific test conditions were: temperature 25°C, adsorption pressure 2~760 mmHg; Figure 14 It can be seen that as the adsorption pressure increases, the CO2 adsorption capacity of the samples obtained in Examples 3-5 is better than that of Comparative Example 2. The saturated adsorption capacity of CO2 at 760 mmHg reaches 58 mL / g, while the saturated adsorption capacity of CO2 of the sample obtained in Comparative Example 2 at 760 mmHg is only 33 mL / g. This is related to the fact that the large amount of ANA heterocrystalline phase in the sample obtained in Comparative Example 2 does not have CO2 adsorption capacity. It also shows that the pure phase MWF molecular sieve has good CO2 capacity and has good application prospects in the field of CO2 capture.

[0069] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing MWF molecular sieves, characterized in that, Includes the following steps: An aluminum source, an inorganic metal alkali source, water, a silicon source, and a structure-directing agent are mixed to obtain a gel. The gel was subjected to hydrothermal crystallization to obtain MWF molecular sieve; The inorganic metal alkali source is sodium hydroxide and potassium hydroxide; The silica-alumina molar ratio of the MWF molecular sieve is 5~20; The structure-directing agent has the structure shown in Formula I or Formula II: Formula I, Formula II; In Formula I, R1, R2, R3 and R4 are independently methyl, ethyl or isopropyl; In Formula II, R5, R6, R7, R8, R9, R 10 and R 11 It is independently hydrogen, methyl, ethyl or isopropyl, and R5 and R6 are not hydrogen.

2. The preparation method according to claim 1, characterized in that, The structure-directing agent has one of the following structures: 。 3. The preparation method according to claim 1, characterized in that, In the inorganic metal alkali source, the molar ratio of sodium hydroxide to potassium hydroxide is 2~6.5:

1.

4. The preparation method according to claim 1, characterized in that, The aluminum source includes one or more of aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum hydroxide, sodium aluminate, boehmite, and aluminum isopropoxide.

5. The preparation method according to claim 1, characterized in that, The silicon source includes one or more of sodium silicate, silica gel, silica powder, silica fume, and tetraethyl orthosilicate.

6. The preparation method according to claim 1, characterized in that, The aluminum source is calculated as Al2O3, the inorganic metal alkali source is calculated as alkali metal oxide, the silicon source is calculated as SiO2, and the molar ratio of the silicon source, aluminum source, inorganic metal alkali source, structure directing agent and water is (5~40):1:(0.5~2.5):(0~2):(40~150), and the molar ratio of the structure directing agent is not 0; The hydrothermal crystallization temperature is 120~200 ℃, and the time is 1~30 days.

7. The preparation method according to claim 1, characterized in that, When the structure-directing agent has the structure shown in Formula I, the preparation method of the structure-directing agent includes the following steps: A piperazine compound, a haloalkane, and an organic solvent are mixed and subjected to a first substitution reaction to obtain the structure-directing agent; the piperazine compound has the structure shown in Formula III, the haloalkane includes haloalkane 1 and haloalkane 2, and haloalkane 1 and haloalkane 2 have the structures shown in Formula IV and Formula V respectively, where X1 and X2 in Formula IV and Formula V are halogens. Formula III, Formula IV, Formula V; When the structure-directing agent has the structure shown in Formula II, the preparation method of the structure-directing agent includes the following steps: A piperidine compound, a haloalkane, and an organic solvent are mixed and subjected to a second substitution reaction to obtain the structure-directing agent; the piperidine compound has the structure shown in Formula VI, the haloalkane includes haloalkane 3 and haloalkane 4, and haloalkane 3 and haloalkane 4 have the structures shown in Formula VII and Formula VIII respectively, where X3 and X4 in Formula VII and Formula VIII are halogens. Formula VI, Equation VII, Formula VIII.

8. The preparation method according to claim 7, characterized in that, The molar ratio of the piperazine compound to the haloalkane is 1:1~3; the temperature of the first substitution reaction is 40~60 °C and the time is 10~40 h; The molar ratio of the piperidine compound to the haloalkane is 1:1 to 3; the temperature of the second substitution reaction is 40 to 90°C, and the time is 24 to 72 h.

9. The MWF molecular sieve prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the MWF molecular sieve according to claim 9 in the selective adsorption of CO2.

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