A method for seed-induced synthesis of short-b-axis zsm-5 zeolite molecular sieve under a sodium-free system

Abstract

The application discloses a method for synthesizing short-b-axis ZSM-5 zeolite molecular sieves in a sodium-free system, wherein the b-axis thickness of the zeolite molecular sieves is 40-200 nm, and the Lc / Lb is controllable and adjustable within 5-30. The synthesis process is as follows: firstly, synthesizing amorphous silicon-aluminum nanoparticle seeds; then, dispersing the seeds into a silicon source solution; adding tetrapropylammonium hydroxide to control the pH of the system to be 7-11; and finally, realizing controllable adjustment of the b-axis thickness under hydrothermal conditions. The method has the advantages of simple preparation process, environmental friendliness and suitability for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve synthesis technology, specifically relating to a method for preparing short b-axis ZSM-5 zeolite molecular sieves by seed induction in a nano-free system. Background Technology

[0002] Zeolite molecular sieve materials are widely used in petroleum refining, energy and chemical industries, and environmental protection, and are key materials for solving energy and environmental problems. ZSM-5 zeolite, as one of the most important catalytic materials, possesses two intersecting ten-membered ring pore structures: a set of sinusoidal channels parallel to the a-axis and c-axis, with dimensions of approximately 0.51–0.55 nm; and a set of straight channels parallel to the b-axis, with dimensions of approximately 0.53–0.56 nm. Reaction / product molecules exhibit diffusion anisotropy along different intracrystalline channel directions of ZSM-5 molecular sieves. Because the diameter of the straight channels is larger than that of the sinusoidal channels, the diffusion rate of guest molecules along the b-axis is much faster than that along the a-axis and c-axis. Therefore, shortening the length of the straight channels along the b-axis helps overcome the limitations imposed by the molecular sieve channels on molecular mass transfer and diffusion, thereby improving the reactivity of the molecular sieve and even product selectivity.

[0003] To date, the main synthetic strategies for short b-axis ZSM-5 have been template-induced synthesis or additive-assisted synthesis. These two methods achieve precise control over crystal face growth by promoting or inhibiting the crystal growth rate. The template method involves introducing a surfactant composed of a long-chain alkyl group and two short-chain alkyl groups separated by quaternary ammonium groups during the molecular sieve synthesis process. During crystallization, the bisquaternary ammonium groups act as structure-directing agents for the synthesis of MFI, and the strong hydrophobic effect of the long alkyl chain restricts the excessive growth of the molecular sieve along straight channels, ultimately forming a sheet-like molecular sieve. For example, in patent document CN 111233001 A, C... 22-6-6 Using quaternary ammonium surfactants as templates, nano-thin ZSM-5 molecular sieves were prepared. Additive-assisted synthesis is another common method for synthesizing sheet-like ZSM-5 molecular sieves. For example, in patent document CN 112607746 A, a polymer selected from various polysubstituted guanidine compounds such as tetramethylguanidine, various monosubstituted guanidine compounds such as dodecyl guanidine hydrochloride, and various guanidine compounds such as polyhexamethylene biguanide hydrochloride was used as an additive to achieve the synthesis of thin-layer MFI molecular sieves. Another example is patent document CN 115321556 A, where methanol, ethanol, isopropanol, and n-butanol were added as crystal plane inhibitors, which facilitated the synthesis of sheet-like molecular sieves. However, the above synthesis strategies require the use of expensive templates or additives, and these substances are highly toxic and easily cause secondary pollution, limiting practical industrial production.

[0004] Chinese patent CN 115028175 A discloses a method for preparing sheet-like ZSM-5 molecular sieves by directionally controlling the growth orientation of the crystal growth solution through a seed-induced method. This method first obtains an acidic crystal growth solution under acidic conditions with a pH of 1-6, then adds seed crystals, adjusts the pH of the solution to 7.0-8.5, and synthesizes ZSM-5 molecular sieves in a low-alkalinity hydrothermal solution. In this patented method, the prepared seed crystals are generally larger than 90 nm and possess an MFI framework structure rather than being amorphous. Furthermore, the preparation process requires two pH adjustments of the crystal growth solution, making it cumbersome.

[0005] In summary, it is more meaningful to develop a green synthetic route for short b-axis ZSM-5 zeolite molecular sieves that is simple to prepare and can effectively control the growth orientation of the molecular sieve. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects of the prior art and provide a method for preparing short b-axis ZSM-5 molecular sieves in a sodium-free system. The b-axis thickness can be controlled and adjusted by seed induction, thereby obtaining a short b-axis ZSM-5 molecular sieve with a b-axis thickness of 40~200nm and an Lc / Lb ratio that can be controlled and adjusted between 5 and 30.

[0007] Therefore, the present invention first provides a method for preparing amorphous silicon-aluminum seed crystals, namely: preparing a tetrapropylammonium hydroxide solution, then adding tetraethyl orthosilicate and aluminum isopropoxide, stirring the resulting mixture thoroughly at 25~35 ℃ for 2~5 h, then heating to 80 ℃ for 5~8 h to obtain silicon-aluminum gel, then transferring it to a hydrothermal reactor, hydrothermally treating it at 150~190 ℃ for 2~5 days, and then centrifuging, washing and drying to obtain amorphous silicon-aluminum seed crystals;

[0008] The molar ratio of each raw material is (40~60)SiO2: Al2O3:(8~12)TPAOH:(1800~3000)H2O.

[0009] Based on this, the method for seed-induced synthesis of short b-axis ZSM-5 zeolite molecular sieve in a sodium-free system of the present invention includes the following steps: (1) using the above-mentioned amorphous silica-alumina seed crystals as seed crystals, dispersing the seed crystals in a silicon source solution, adjusting the pH of the system to 7~11 using tetrapropylammonium hydroxide, and crystallizing at 80~250 ℃ for 1~150 h after thorough stirring; (2) washing and drying the crystallized sample, and calcining at 500~650 ℃ for 4~8 h to obtain short b-axis ZSM-5 molecular sieve.

[0010] Preferably, in step (1), the silicon source is SiO2, and the molar ratio of silicon source to water in the silicon source solution is (1~100):(1~2000).

[0011] Preferably, the silicon source in step (1) is selected from at least one of fumed silica, type C silica gel, silica sol and silica fume.

[0012] Preferably, in step (1), the silicon source is SiO2, and the molar ratio of SiO2 in the seed crystal to SiO2 in the pure silicon solution is 0.05~2:1.

[0013] The method of this invention produces short b-axis ZSM-5 molecular sieves with a b-axis thickness of 40-200 nm and an Lc / Lb ratio that can be controllably adjusted between 5 and 30. Furthermore, the c-axis is significantly larger than the b-axis. In other words, the length of the straight channels along the b-axis in the molecular sieve can be greatly shortened according to actual needs. Therefore, when used as a catalyst, catalyst support, or gas adsorbent, the mass transfer and diffusion rate of molecules within the molecular sieve channels is increased, thereby enhancing the reactivity and product selectivity of the molecular sieve.

[0014] Compared with existing methods for synthesizing short b-axis ZSM-5 zeolite molecular sieves, the synthesis process of the short b-axis ZSM-5 zeolite molecular sieve of this invention first involves dispersing aluminum isopropoxide and tetraethyl orthosilicate in an aqueous solution of tetrapropylammonium hydroxide. During this process, aluminum isopropoxide and tetraethyl orthosilicate depolymerize to form isopropanol, ethanol, and negatively charged aluminate and silicate ions. The presence of isopropanol and ethanol reduces the rate of depolymerization and condensation of the silicon and aluminum sources, regulating crystal nucleation and growth. This allows silicate and aluminate ions to adsorb around the positively charged tetrapropylammonium hydroxide ions through electrostatic interactions, resulting in amorphous silicon-aluminum seed crystals. Nitrogen physisorption experiments show that the silicon-aluminum seed crystals prepared by the synthesis method of this invention have a microporous structure. By adding these seed crystals to a pure silicon growth solution and further controlling the amount of seed crystals added, the pH of the system, and the crystallization conditions, the depolymerization and condensation rates of the silicon source and seed crystals can be controlled, achieving controllable adjustment of Lc / Lb.

[0015] In summary, the short b-axis ZSM-5 molecular sieve of this invention is synthesized under sodium-free conditions, overcoming the problems of traditional zeolite molecular sieve synthesis methods that require the introduction of sodium hydroxide mineralizing agent. The introduction of sodium ions inevitably requires processes such as ion exchange and calcination to obtain molecular sieve catalysts with acidic sites, resulting in complex processes and significant pollution. Compared with existing technologies such as CN 111233001 A, CN 112607746 A, CN 115321556 A, and CN 115028175 A, this invention avoids the use of expensive special template agents and additives, eliminates the ion exchange process, and simplifies the operation. It can be used as a catalyst, catalyst carrier, and gas adsorbent in petroleum refining, energy chemical industry, and environmental protection, demonstrating high industrial application value. Attached Figure Description

[0016] Figure 1 This is a transmission electron microscope image of the seed crystals prepared in Example 1 of the present invention.

[0017] Figure 2 The image shows the XRD pattern of the seed crystals prepared in Example 1 of this invention.

[0018] Figure 3 This is the nitrogen physical adsorption-desorption isotherm of the seed crystals prepared in Example 1 of this invention.

[0019] Figure 4 This is a transmission electron microscope image of the short b-axis ZSM-5 molecular sieve prepared in Example 2 of the present invention.

[0020] Figure 5 The image shows the XRD pattern of the short b-axis ZSM-5 molecular sieve prepared in Example 2 of this invention. Detailed Implementation

[0021] The specific embodiments of the present invention will be described in further detail below.

[0022]

Example 1

[0023] (1) A tetrapropylammonium hydroxide solution was prepared according to the molar ratio of SiO2:Al2O3:TPAOH:H2O=40:1:9:1800. Then, tetraethyl orthosilicate and aluminum isopropoxide were added. The resulting mixture was stirred thoroughly at 25 °C for 5 h, and then heated to 80 °C for 8 h of aging. The aged silica-alumina gel was transferred to a hydrothermal reactor and hydrothermally treated at 150 °C for 5 days. After centrifugation, washing, and drying, amorphous silica-alumina seed crystals were obtained. Figure 1 The image shown is an electron microscope (EM) image of the amorphous aluminum silicate seed sample from Example 1. It can be seen that the sample exhibits a worm-like structure under the electron microscope. X-ray diffraction analysis revealed its structure (see...). Figure 2As can be seen, the sample has an amorphous structure. Further analysis of the micropore structure of the seed crystal using nitrogen physical adsorption-desorption experiments revealed a significant increase in the low-pressure region (P / P0 < 0.05), indicating the presence of abundant microporous structures in the sample (see...). Figure 3 ).

[0024] (2) Disperse the seed crystals prepared in step (1) into a silica sol solution with a molar ratio of SiO2:200H2O. The mass ratio of the seed crystals to SiO2 in the pure silicon solution is 30%. The pH of the system is adjusted to 8 using tetrapropylammonium hydroxide. After thorough stirring, crystallize at 100°C for 120 h.

[0025] (3) The crystallized sample was washed and dried, and then calcined at 550 °C for 6 h to obtain short b-axis ZSM-5 molecular sieve. The short b-axis ZSM-5 molecular sieve sample prepared in this embodiment is in the form of a typical thin sheet, with an average b-axis thickness (Lb) of about 130 nm and a c-axis length (Lc) of 1600 nm. The aspect ratio (Lc / Lb) is 12.

[0026]

Example 2

[0027] (1) A tetrapropylammonium hydroxide solution was prepared according to a molar ratio of SiO2:Al2O3:TPAOH:H2O=60:1:12:3000. Then, tetraethyl orthosilicate and aluminum isopropoxide were added. The resulting mixture was stirred thoroughly at 35 °C for 2 h, and then aged at 80 °C for 5 h. The aged silica-alumina gel was transferred to a hydrothermal reactor and hydrothermally treated at 190 °C for 2 days. After centrifugation, washing, and drying, amorphous silica-alumina seed crystals were obtained. The electron microscope image, XRD pattern, and microstructure of the amorphous silica-alumina seed crystal sample prepared in this example are basically the same as those in Example 1, and therefore will not be listed here again.

[0028] (2) The seed crystals prepared in step (1) are dispersed in a silica sol solution with a molar ratio of 10SiO2:2000H2O. The mass ratio of the seed crystals to SiO2 in the pure silicon solution is 30%. The pH of the system is adjusted to 9 with tetrapropylammonium hydroxide. After stirring thoroughly, the solution is crystallized at 100 °C for 120 h.

[0029] (3) The crystallized sample was washed and dried, and then calcined at 650℃ for 4 hours to obtain short b-axis ZSM-5 zeolite molecular sieve. Figure 4 The image shown is an electron microscope (EM) image of the short b-axis ZSM-5 zeolite molecular sieve sample synthesized in Example 2. It can be seen that the sample is a typical sheet-like structure with an average b-axis thickness (Lb) of approximately 140 nm and a c-axis length (Lc) of 1700 nm. The aspect ratio (Lc / Lb = 12) was used to measure the morphological variation of the sample. Further X-ray diffraction analysis revealed its structure (see [link to X-ray diffraction analysis]). Figure 5The XRD pattern of the sample showed obvious diffraction peaks at 2θ = 8.0°, 8.9°, 23.2°, 24.0° and 24.5°, indicating that the prepared sample has an MFI topology.

[0030]

Example 3

[0031] (1) A tetrapropylammonium hydroxide solution was prepared according to a molar ratio of SiO2:Al2O3:TPAOH:H2O=50:1:10:2000. Then, tetraethyl orthosilicate and aluminum isopropoxide were added. The resulting mixture was stirred thoroughly at 30 °C for 3 h, and then aged at 80 °C for 7 h. The aged silica-alumina gel was transferred to a hydrothermal reactor and hydrothermally treated at 170 °C for 3 days. After centrifugation, washing, and drying, amorphous silica-alumina seed crystals were obtained. The electron microscope image, XRD pattern, and microstructure of the amorphous silica-alumina seed crystal sample prepared in this example are basically the same as those in Example 1, and therefore will not be listed here again.

[0032] (2) Disperse the seed crystals prepared in step (1) into a fumed silica solution with a molar ratio of 100SiO2:2000H2O. The mass ratio of the seed crystals to SiO2 in the pure silicon solution is 30%. Adjust the pH of the system to 7.5 with tetrapropylammonium hydroxide. After stirring thoroughly, crystallize at 250 °C for 1 h.

[0033] (3) The crystallized sample was washed and dried, and then calcined at 500 °C for 8 h to obtain a short b-axis ZSM-5 molecular sieve with an average b-axis thickness of approximately 120 nm, an Lc of 2000 nm, and an Lc / Lb ratio of 17. The electron microscopy image and XRD pattern of the short b-axis ZSM-5 molecular sieve sample prepared in this embodiment are basically the same as those in Example 2, and will not be listed here again.

[0034]

Example 4

[0035] (1) A tetrapropylammonium hydroxide solution was prepared according to a molar ratio of SiO2:Al2O3:TPAOH:H2O = 60:1:12:2760. Then, tetraethyl orthosilicate and aluminum isopropoxide were added. The resulting mixture was stirred thoroughly at 35 °C for 3 h, and then aged at 80 °C for 5 h. The aged silica-alumina gel was transferred to a hydrothermal reactor and hydrothermally treated at 170 °C for 3 days. After centrifugation, washing, and drying, amorphous silica-alumina seed crystals were obtained. The electron microscope image, XRD pattern, and microstructure of the amorphous silica-alumina seed crystal sample prepared in this example are basically the same as those in Example 1, and therefore will not be listed here again.

[0036] (2) Disperse the seed crystals prepared in step (1) into a type C silica gel solution with a molar ratio of 1SiO2:2000H2O. The mass ratio of the seed crystals to SiO2 in the pure silica solution is 8%. The pH of the system is adjusted to 11 with tetrapropylammonium hydroxide. After stirring thoroughly, crystallize at 210 °C for 3 h.

[0037] (3) After washing and drying the crystallized sample, ZSM-5 molecular sieve with an average b-axis thickness of approximately 80 nm, an Lc of 800 nm, and an Lc / Lb ratio of 10 was obtained by calcining at 600 °C for 5 h. The electron microscopy image and XRD pattern of the short b-axis ZSM-5 molecular sieve sample prepared in this embodiment are basically the same as those in Example 2, and will not be listed here again.

[0038]

Example 5

[0039] (1) A tetrapropylammonium hydroxide solution was prepared according to a molar ratio of SiO2:Al2O3:TPAOH:H2O=55:1:9:2000. Then, tetraethyl orthosilicate and aluminum isopropoxide were added. The resulting mixture was stirred thoroughly at 28 °C for 4 h, and then aged at 80 °C for 5 h. The aged silica-alumina gel was transferred to a hydrothermal reactor and hydrothermally treated at 160 °C for 4 days. After centrifugation, washing, and drying, amorphous silica-alumina seed crystals were obtained. The electron microscope image, XRD pattern, and microstructure of the amorphous silica-alumina seed crystal sample prepared in this example are basically the same as those in Example 1, and therefore will not be listed here again.

[0040] (2) Disperse the seed crystals prepared in step (1) into a silica solution with a molar ratio of 1SiO2:1000H2O. The mass ratio of the seed crystals to SiO2 in the pure silicon solution is 200%. Adjust the pH of the system to 10 with tetrapropylammonium hydroxide. After stirring thoroughly, crystallize at 170 °C for 12 h.

[0041] (3) The crystallized sample was washed and dried, and then calcined at 500 °C for 8 h to obtain a short b-axis ZSM-5 molecular sieve with an average b-axis thickness of about 45 nm, an Lc of 690 nm, and an Lc / Lb ratio of 15. The electron microscopy image and XRD pattern of the short b-axis ZSM-5 molecular sieve sample prepared in this embodiment are basically the same as those in Example 2, and will not be listed here again.

[0042]

Example 6

[0043] (1) A tetrapropylammonium hydroxide solution was prepared according to a molar ratio of SiO2:Al2O3:TPAOH:H2O=60:1:12:3000. Then, tetraethyl orthosilicate and aluminum isopropoxide were added. The resulting mixture was stirred thoroughly at 35 °C for 3 h, and then aged at 80 °C for 5 h. The aged silica-alumina gel was transferred to a hydrothermal reactor and hydrothermally treated at 170 °C for 3 days. After centrifugation, washing, and drying, amorphous silica-alumina seed crystals were obtained. The electron microscope image, XRD pattern, and microstructure of the amorphous silica-alumina seed crystal sample prepared in this example are basically the same as those in Example 1, and therefore will not be listed here again.

[0044] (2) The seed crystals prepared in step (1) are dispersed in a silica sol solution with a molar ratio of 10SiO2:2000H2O. The mass ratio of the seed crystals to SiO2 in the pure silicon solution is 5%. The pH of the system is adjusted to 9 with tetrapropylammonium hydroxide. After stirring thoroughly, the solution is crystallized at 130 °C for 24 h.

[0045] (3) The crystallized sample was washed and dried, and then calcined at 550 °C for 6 h to obtain a short b-axis ZSM-5 molecular sieve with an average b-axis thickness of approximately 75 nm, an Lc of 2000 nm, and an Lc / Lb ratio of 27. The electron microscopy image and XRD pattern of the short b-axis ZSM-5 molecular sieve sample prepared in this embodiment are basically the same as those in Example 2, and will not be listed here again.

[0046] It should be noted that the above examples are only preferred embodiments of the present invention. Any modifications and refinements made in accordance with the present invention without departing from its spirit and essence are within the scope of protection of the present invention.

Claims

1. A method for seed-induced synthesis of short b-axis ZSM-5 zeolite molecular sieves in a sodium-free system, comprising the following steps: (1) Prepare a tetrapropylammonium hydroxide solution, then add tetraethyl orthosilicate and aluminum isopropoxide. Stir the mixture thoroughly at 25-35 °C for 2-5 h, then heat it to 80 °C and age it for 5-8 h to obtain aluminosilicate gel. Then transfer it to a hydrothermal reactor and hydrothermally treat it at 150-190 °C for 2-5 days. After centrifugation, washing and drying, obtain amorphous aluminosilicate seed crystals. The molar ratio of each raw material is (40-60)SiO2:Al2O3:(8-12)TPAOH:(1800-3000)H2O. (2) Using the amorphous silicon-aluminum seed crystals described in step (1) as seed crystals, disperse them in the silicon source solution, add an appropriate amount of tetrapropylammonium hydroxide to adjust the pH of the system to 7 to 11, stir thoroughly, and crystallize at 80-250 ℃ for 1 h to 150 h; wash and dry the crystallized sample, and calcine at 500-650 ℃ for 4 to 8 h to obtain short b-axis ZSM-5 molecular sieve with a b-axis thickness of 40 to 200 nm and an Lc / Lb ratio between 5 and 30.

2. The method for seed-induced synthesis of short b-axis ZSM-5 zeolite molecular sieves in a sodium-free system according to claim 1, characterized in that, In step (2), the silicon source is SiO2, and the molar ratio of silicon source to water in the silicon source solution is (1 ~ 100):(1 ~ 2000).

3. The method for seed-induced synthesis of short b-axis ZSM-5 zeolite molecular sieves in a sodium-free system according to claim 1 or 2, characterized in that, The silicon source in step (2) is selected from at least one of fumed silica, type C silica gel, and silica sol.

4. The method for seed-induced synthesis of short b-axis ZSM-5 zeolite molecular sieves in a sodium-free system according to claim 3, characterized in that, The molar ratio of SiO2 in the seed crystal to SiO2 in the silicon source solution is 0.05 ~ 2:

1.

5. A short b-axis ZSM-5 zeolite molecular sieve obtained by any one of claims 1 to 3, characterized in that, The b-axis thickness is 40 ~ 200 nm, and the Lc / Lb ratio is between 5 and 30.

6. The application of the short b-axis ZSM-5 zeolite molecular sieve as described in claim 5 as a catalyst, catalyst support, and gas adsorbent in the fields of petroleum refining, energy chemical industry, and environmental protection.

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