A method for synthesizing high-silica ZSM-5 molecular sieves

Abstract

This invention discloses a method for synthesizing high-silica ZSM-5 molecular sieves. The method uses amino-alkali metal salts as additives, natural minerals and supplementary silicon sources as raw materials, and a synthetic gel is prepared via hydrothermal synthesis with the assistance of ethanol. The high-silica ZSM-5 molecular sieve is then obtained through crystallization, calcination, and other processes. In this invention, the amino-alkali metal salts react with water to generate NH3 and hydroxides, which can be used as structure-directing agents and alkali sources, respectively, in the synthesis of high-silica ZSM-5 molecular sieves. This avoids the use of expensive organic amine templates, reducing synthesis costs. Furthermore, using ethanol as an auxiliary solvent effectively slows down the hydrolysis rate of the amino salts, which is beneficial for molecular sieve crystallization, thus obtaining a highly crystalline molecular sieve product.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve preparation, specifically relating to a method for synthesizing high-silica ZSM-5 molecular sieve. Background Technology

[0002] ZSM-5 molecular sieves, possessing an MFI-type topology, are crystalline aluminosilicate materials first synthesized by Landolt et al. in 1972. Due to their unique pore structure and excellent catalytic performance, ZSM-5 molecular sieves have been widely used in industrial catalysis. Generally, low-silica-alumina ratio ZSM-5 molecular sieves have large grain sizes and strong cracking performance, while high-silica-alumina ratio ZSM-5 molecular sieves exhibit more pronounced isomerization capabilities. However, low-silica-alumina ratio ZSM-5 molecular sieves typically have poor framework structure and hydrothermal stability, making them prone to framework dealuminization and pore structure collapse during prolonged use. In contrast, high-silica molecular sieves exhibit higher stability and stronger oleophilic and hydrophobic properties, thus the synthesis and research of high-silica ZSM-5 molecular sieves has attracted attention in the field of petrochemical catalysis.

[0003] The traditional hydrothermal synthesis method is currently the most common method for preparing ZSM-5 molecular sieves. It involves adding silicon, aluminum, alkali, a template agent, and water to a hydrothermal reactor, where the reactants are crystallized by increasing temperature and pressure to form the molecular sieve product. The template agents used in the hydrothermal preparation of ZSM-5 molecular sieves are typically quaternary ammonium salts and organic amines. According to current industrial production processes, the traditional hydrothermal method for synthesizing high-silicon-to-aluminum ratio ZSM-5 molecular sieves (referred to as high-silicon ZSM-5 molecular sieves) requires a large amount of template agent. This not only increases production costs but also generates a large amount of ammonia-nitrogen-containing waste liquid, thus increasing post-treatment costs or causing environmental pollution.

[0004] Chinese patent application CN115818663A discloses a method for preparing high-silicon ZSM-5 molecular sieves in an amine-free system. The method involves mixing a hydrothermal reactant of a first silicon source, an aluminum source, a first alkali source, water, and a first seed crystal with a second silicon source, a second alkali source, water, and a second seed crystal, followed by a hydrothermal reaction to obtain a ZSM-5 molecular sieve with a silicon-to-aluminum ratio (n(SiO2) / n(Al2O3)) of 60. This method requires two hydrothermal crystallization processes, resulting in a long preparation time, cumbersome operation, and a low silicon-to-aluminum ratio in the product.

[0005] Chinese patent application CN116265396A discloses a method for synthesizing high-silicon ZSM-5 molecular sieves without organic amines. The method involves uniformly mixing raw materials containing silicon, aluminum, and seed crystals with alkali, water, and ethanol, followed by hydrothermal crystallization to obtain a high-silicon ZSM-5 molecular sieve with a silicon-to-aluminum ratio of 310. This method requires a high amount of seed crystals, leading to increased costs and a long crystallization time.

[0006] In summary, the synthesis of high-silica ZSM-5 molecular sieves without the participation of organic amine template agents is typically complex and costly, hindering industrial production. Therefore, developing an efficient and easy-to-implement method for synthesizing high-silica ZSM-5 molecular sieves is crucial. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for synthesizing high-silica ZSM-5 molecular sieves, which can produce high-silica ZSM-5 molecular sieves with high crystallinity.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for synthesizing high-silica ZSM-5 molecular sieve, characterized by comprising the following steps:

[0010] (1) Add ethanol, sub-molten salt activated natural silica-alumina minerals and supplementary silicon source to deionized water, mix and stir evenly, then add amino alkali metal salt, and continue stirring evenly to obtain synthetic gel.

[0011] (2) The above-mentioned synthetic gel is transferred to a crystallization vessel with a polytetrafluoroethylene liner and placed in an oven for crystallization. After crystallization, the crystallized product is cooled, washed, and dried at 90-105℃ for 6-12h to obtain a solid product. The solid product is then calcined at 500-600℃ for 3.5-4.5h to obtain high-silicon ZSM-5 molecular sieve.

[0012] Further, the natural silica-alumina mineral mentioned in step (1) is one or more of rettoite, kaolinite, illite, and perlite; the supplementary silicon source is one or more of solid silica, silica sol, and silica; and the amino alkali metal salt is one or more of lithium amino, sodium amino, and potassium amino.

[0013] Furthermore, the mass ratio of ethanol to deionized water in step (1) is 1 to 3:1.

[0014] Further, the composition of the synthetic gel in step (1) is: n(SiO2) / n(Al2O3) = 200~1000, n(H2O) / n(SiO2) = 6.0~7.5, n(aminoalkali metal salt) / n(SiO2) = 0.7~1.0.

[0015] Furthermore, the crystallization temperature in step (2) is 150–180°C, and the crystallization time is 12–24 h.

[0016] Furthermore, the silicon-to-aluminum ratio of the obtained high-silicon ZSM-5 molecular sieve is 100–600.

[0017] This invention employs the above technical solution, using amino alkali metal salts as additives and natural minerals and supplementary silicon sources as raw materials, to prepare high-silica ZSM-5 molecular sieves via hydrothermal synthesis with the assistance of ethanol. Compared with existing synthesis methods, the high silica-to-alumina ratio ZSM-5 molecular sieves synthesized by this invention have the following advantages:

[0018] (1) Using ammonia and ethanol, which are generated by the reaction of low-cost amino alkali metal salts with water, as structure guiding agents in the preparation system, avoids the use of expensive organic amine template agents, reduces post-processing, and lowers production costs; using ethanol as an auxiliary solvent effectively slows down the hydrolysis rate of amino salts, which is conducive to molecular sieve crystallization, thereby obtaining molecular sieve products with high crystallinity.

[0019] (2) High-quality high-silica ZSM-5 molecular sieves were synthesized in a system without organic template agents and seed crystals, which greatly reduced the preparation cost.

[0020] (3) Using natural minerals activated by submolten salt as a source of silicon-aluminum alkali reduces the degree of environmental pollution.

[0021] (4) Compared with the existing technology for synthesizing high-silicon ZSM-5 without organic amines, the present invention can complete crystallization in a shorter time to obtain high-quality products with low production energy consumption.

[0022] (5) The ZSM-5 molecular sieve prepared has a high relative crystallinity and a silicon-to-aluminum ratio, with a relative crystallinity of 96-101% and a silicon-to-aluminum ratio of 100-600. Attached Figure Description

[0023] Figure 1 The X-ray diffraction (XRD) spectra of the ZSM-5 molecular sieves prepared in Examples 1-4 and the comparative examples of this invention are shown. Detailed Implementation

[0024] The present invention will be described in detail below through specific embodiments, but this does not limit the present invention.

[0025] Example 1

[0026] A method for synthesizing high-silica ZSM-5 molecular sieves

[0027] (1) Add 14g of ethanol, 0.4g of molten salt activated rettoiter and 8.8g of solid silica to 14g of deionized water, mix and stir evenly, then add 3.6g of sodium amide and continue stirring evenly to obtain a synthetic gel with a synthesis molar ratio of 1SiO2:0.005Al2O3:6H2O:0.7NaNH2.

[0028] (2) The synthesized gel was transferred to a crystallization vessel lined with polytetrafluoroethylene and crystallized in an oven at 150°C for 24 hours. After crystallization, the crystallized product was cooled, washed, and dried at 100°C for 6 hours to obtain a solid product. Finally, it was calcined at 550°C for 6 hours to obtain a high-silica ZSM-5 molecular sieve, denoted as sample A. Its XRD pattern is shown below. Figure 1 As shown, the relative crystallinity is 99%.

[0029] Example 2

[0030] A method for synthesizing high-silica ZSM-5 molecular sieves

[0031] (1) Add 16g of ethanol, 0.16g of molten salt activated rettoiter and 8.80g of solid silica to 16g of deionized water, mix and stir evenly, then add 4.1g of sodium amide and continue stirring evenly to obtain a synthetic gel with a synthesis molar ratio of 1SiO2:0.002Al2O3:6.500H2O:0.800NaNH2;

[0032] (2) The synthesized gel was transferred to a crystallization vessel lined with polytetrafluoroethylene and placed in an oven at 160°C for 16 hours for crystallization. After crystallization, the crystallized product was cooled, washed, and dried at 100°C for 6 hours to obtain a solid product. Finally, it was calcined at 550°C for 6 hours to obtain a high-silica ZSM-5 molecular sieve, denoted as sample B. Its XRD pattern is shown below. Figure 1 As shown, the relative crystallinity is 101%.

[0033] Example 3

[0034] A method for synthesizing high-silica ZSM-5 molecular sieves

[0035] (1) Add 25g of ethanol, 0.11g of molten salt activated rettoiter and 8.80g of solid silica source to 17g of deionized water, mix and stir evenly, then add 4.6g of sodium amide and continue stirring evenly to obtain a synthetic gel with a synthesis molar ratio of 1SiO2:0.0014Al2O3:7H2O:0.9000NaNH2;

[0036] (2) The synthesized gel was transferred to a crystallization vessel lined with polytetrafluoroethylene and placed in an oven at 170°C for 14 hours for crystallization. After crystallization, the crystallized product was cooled, washed, and dried at 100°C for 6 hours to obtain a solid product. Finally, it was calcined at 550°C for 4 hours to obtain a high-silica ZSM-5 molecular sieve, denoted as sample C. Its XRD pattern is shown below. Figure 1 As shown, the relative crystallinity is 97%.

[0037] Example 4

[0038] A method for synthesizing high-silica ZSM-5 molecular sieves

[0039] (1) Add 27g ethanol, 0.08g submolten salt activated retto clay and 8.80g solid silica to 18g deionized water, mix and stir evenly, then add 5.10g sodium amide and continue stirring evenly to obtain a synthetic gel with a synthesis molar ratio of 1SiO2:0.001Al2O3:7.500H2O:1NaNH2;

[0040] (2) The synthesized gel was transferred to a crystallization vessel lined with polytetrafluoroethylene and placed in an oven at 180°C for 12 hours for crystallization. After crystallization, the crystallized product was cooled, washed, and dried at 100°C for 6 hours to obtain a solid product. Finally, it was calcined at 550°C for 4 hours to obtain a high-silica ZSM-5 molecular sieve, denoted as sample D. Its XRD pattern is shown below. Figure 1 As shown, the relative crystallinity is 99%.

[0041] Comparative Example

[0042] A method for synthesizing ZSM-5 molecular sieves

[0043] (1) Add 14g of ethanol, 0.4g of molten salt activated retto clay and 8.8g of solid silica to 14g of deionized water, mix and stir evenly to obtain a synthetic gel with a synthesis molar ratio of 1SiO2:0.005Al2O3:6H2O.

[0044] (2) The above gel was transferred to a crystallization vessel with a polytetrafluoroethylene liner and placed in an oven at 150°C for 24 hours to crystallize. After crystallization, the crystallized product was cooled, washed, and dried at 100°C for 6 hours to obtain a solid product. Finally, it was calcined at 550°C for 6 hours to obtain ZSM-5 molecular sieve, denoted as sample E. Its XRD pattern is shown below. Figure 1 As shown, the relative crystallinity is 99%.

[0045] Depend on Figure 1 The XRD patterns shown indicate that, without the addition of organic amine templates, pure-phase ZSM-5 molecular sieves can be successfully synthesized using natural mineral rethrite as the silica-alumina base source and ammonia water generated from the reaction of ethanol and sodium amide with water as the structure directing agent. The crystallinity of the molecular sieves is greater than 90%, meeting production requirements. The products synthesized in Examples 1-4 and the comparative examples, after acid exchange and calcination, yielded H-type ZSM-5 molecular sieves, the corresponding chemical compositions of which are shown in Table 1. It can be seen that Examples 1-4 can all successfully synthesize high-silica ZSM-5 molecular sieves with silica-alumina ratios between 100 and 600.

[0046] Table 1 shows the chemical composition of the H-type ZSM-5 molecular sieves obtained from the synthetic products of Examples 1-4 and the comparative examples.

[0047]

[0048] The difference between the comparative example and the technical solutions of Examples 1-4 is that no amino alkali metal salt was added, resulting in a low silicon-aluminum ratio in the molecular sieve and insufficient crystallization. The product contains a lot of amorphous silicon, which leads to very low crystallinity and fails to meet production requirements.

[0049] The ZSM-5 molecular sieve synthesized in this invention has a high silicon-to-aluminum ratio and high crystallinity, providing a new synthetic route for the green and efficient preparation of high-silicon ZSM-5 molecular sieves.

[0050] It should be noted that the present invention is not limited to the specific embodiments described above. Any suitable modifications made without contradiction shall be considered as part of the content disclosed in the present invention.

Claims

1. A method for synthesizing high-silica ZSM-5 molecular sieve, characterized in that, Includes the following steps: (1) Add ethanol, natural silica-alumina minerals activated by submolten salt and supplementary silicon source to water, mix and stir evenly, then add amino alkali metal salt, and continue to stir evenly to obtain synthetic gel; (2) The above-mentioned synthetic gel is transferred to a crystallization kettle with a polytetrafluoroethylene liner and placed in an oven for crystallization. After crystallization, the crystallized product is cooled, washed and dried to obtain a solid product. The solid product is then calcined to obtain a high-silicon ZSM-5 molecular sieve.

2. The method for synthesizing a high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The natural silica-alumina minerals mentioned in step (1) are one or more of the following: rettoite, kaolinite, illite, and perlite.

3. The method for synthesizing a high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The supplementary silicon source mentioned in step (1) is one or more of solid silicon dioxide, silica sol, and fumed silica.

4. The method for synthesizing a high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The amino alkali metal salt mentioned in step (1) is one or more of amino lithium, amino sodium, and amino potassium.

5. The method for synthesizing a high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The mass ratio of ethanol to water in step (1) is 1~3:

1.

6. The method for synthesizing a high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The composition of the synthetic gel in step (1) is: n(SiO2) / n(Al2O3)=200~1000, n(H2O) / n(SiO2)=6.0~7.5, n(aminoalkali metal salt) / n(SiO2)=0.7~1.

0.

7. A method for synthesizing high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The crystallization temperature in step (2) is 150~180℃ and the crystallization time is 12~24 h.

8. A method for synthesizing high-silica ZSM-5 molecular sieve according to claim 1, characterized in that: The roasting temperature in step (2) is 500-600 ℃ and the roasting time is 3.5-4.5 h.

9. The high-silica ZSM-5 molecular sieve obtained by the synthesis method according to any one of claims 1 to 8.

10. The high-silica ZSM-5 molecular sieve according to claim 9, characterized in that, The silicon-aluminum ratio of the ZSM-5 molecular sieve is 100~600.

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