A method for synthesizing zsm-5 molecular sieve with ultra-low silicon / aluminum ratio, the synthesized molecular sieve and application thereof

Abstract

The application discloses a method for synthesizing ZSM-5 molecular sieve with ultra-low silicon-aluminum ratio, the synthesized molecular sieve and application thereof. The method comprises the following steps: mixing inorganic alkali, a silicon source, nano Silicalite-1 seeds and water, activating, adding an aluminum source, crystallizing in a closed container, and obtaining the ZSM-5 molecular sieve with ultra-low silicon-aluminum ratio; the size of the nano Silicalite-1 seeds is 100-500 nm. When the Silicalite-1 with a crystal size less than 500 nm is used as the seeds, the lower limit of the Si / Al of the ZSM-5 molecular sieve can be reduced to 7.5, the sample has higher specific surface area and relative crystallinity, and the solid yield of the product is greater than 98%.

Description

Technical Field

[0001] This application relates to a method for synthesizing ZSM-5 molecular sieves with ultra-low silica-to-alumina ratio, as well as the synthesized molecular sieves and their applications, belonging to the field of molecular sieve synthesis. Background Technology

[0002] ZSM-5 molecular sieves exhibit excellent catalytic performance in important reactions such as cracking, aromatization, alkylation, isomerization, and dehydration due to their high hydrothermal / thermal stability and tunable acidity. In these reactions, framework aluminum sites are the main active sites; therefore, synthesizing ZSM-5 molecular sieves with a low silica-to-alumina ratio is crucial for improving catalyst activity. Taking the cracking reaction of 1,3,5-triisopropylbenzene as an example, its large molecular size prevents it from diffusing into the micropores of ZSM-5 molecular sieves for reaction. Therefore, aluminum sites on the outer surface and at the pore openings are the main active sites. However, the high silica-to-alumina ratio of ZSM-5 molecular sieves results in a limited content of aluminum sites on the outer surface, leading to low activity in the cracking reaction of 1,3,5-triisopropylbenzene. Reducing the silica-to-alumina ratio and synthesizing ZSM-5 molecular sieves with ultra-low silica-to-alumina ratios is key to improving its cracking reaction activity. Furthermore, the products obtained by reducing the silica-to-alumina ratio of molecular sieves often contain impurity phases such as MOR and FER. Currently, synthesizing pure-phase ZSM-5 molecular sieves with ultra-low silica-alumina ratios still faces significant challenges.

[0003] Louis et al. [Chem.Sci.,2018,9:6532-6539] reduced the silica-alumina ratio of ZSM-5 to 8.5 using a biomass-assisted synthesis method; further reduction of the silica-alumina ratio resulted in a significant decrease in the specific surface area of ​​the ZSM-5 molecular sieve. This patent requires pretreatment of the biomass and the addition of an organic template agent during synthesis, resulting in high preparation costs. Patent CN110885089A describes a hydrothermal crystallization method using a temperature environment provided by a preset temperature curve to obtain ZSM-5 molecular sieves with a silica-alumina ratio of around 10, with the addition of an organic template agent during the process. Patent CN114394605A reports a method for synthesizing a low silica-alumina ratio hierarchical porous ZSM-5 molecular sieve, with the obtained sample exhibiting low micropore volume and BET specific surface area. Rimer et al. [Adv.Mater.2021,33,2100897] also reported the synthesis of low silica-alumina ratio MFI or MEL molecular sieves. Jiao et al. [Angew. Chem. Int. Ed. 2020, 59: 19478-19486] obtained hollow MFI zeolite with a silica-to-alumina ratio of 16 via a post-processing method. However, this post-processing method is complex, and the silica-to-alumina ratio remains relatively high. In summary, a highly efficient method for synthesizing ZSM-5 molecular sieves with ultra-low silica-to-alumina ratios is still lacking. Summary of the Invention

[0004] This application employs a template-free synthesis method guided by nano-Silicalite-1 crystals. By adding a crystal activation step, the controllable synthesis of ZSM-5 molecular sieves with ultra-low silicon-to-aluminum ratio can be achieved, with the lowest silicon-to-aluminum ratio reduced to 7.5.

[0005] This application relates to a method for synthesizing ZSM-5 molecular sieves with ultra-low silicon-to-aluminum ratio, which mainly solves the problems of poor adjustable silicon-to-aluminum ratio and low yield of solid products in existing ZSM-5 molecular sieve materials.

[0006] According to one aspect of this application, a method for synthesizing ZSM-5 molecular sieves with ultra-low silica-to-alumina ratio is provided, characterized in that...

[0007] Includes the following steps:

[0008] Inorganic alkali, silicon source, nano-Silicalite-1 seed crystals and water are mixed and activated, aluminum source is added, and crystallization is carried out in a sealed container to obtain the ultra-low silicon-aluminum ratio ZSM-5 molecular sieve.

[0009] The size of the nano-Silicalite-1 seed crystals is 100–500 nm.

[0010] The inorganic base is selected from at least one of NaOH, Na2CO3, NaHCO3, NaAlO2, NH3·H2O, KOH, K2CO3, and KHCO3;

[0011] The silicon source is selected from at least one of tetraethyl orthosilicate, silica sol, water glass, silica gel, fumed silica, and activated clay.

[0012] The aluminum source is selected from at least one of sodium aluminate, aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum isopropoxide, and aluminum oxide.

[0013] The molar ratio of the silicon source to the aluminum source is 12 to 28;

[0014] The molar ratio of the inorganic base to the silicon source is 0.05 to 0.20;

[0015] The molar ratio of water to silicon source is 5–80;

[0016] The mass of the Silicalite-1 seed crystal is 2 to 30 wt% of the mass of the silicon source;

[0017] Wherein, the molar amount of the silicon source is expressed as the molar amount of silicon dioxide;

[0018] The molar amount of the aluminum source is expressed as the molar amount of aluminum oxide;

[0019] The molar amount of the inorganic base is expressed as the molar amount of the alkali metal oxide corresponding to the basic substance therein.

[0020] The mass of the Silicalite-1 seed crystals is expressed in molar amounts of silicon dioxide.

[0021] The activation temperature is 45–90°C;

[0022] The activation time is 0.5 to 5 hours.

[0023] The crystallization temperature is 120–200°C;

[0024] The crystallization time is 12 to 96 hours.

[0025] According to another aspect of this application, an ultra-low silica-alumina ratio ZSM-5 molecular sieve prepared by the above-described preparation method is provided;

[0026] The Si / Al molar ratio of the ultra-low Si / Al ratio ZSM-5 molecular sieve is 6 to 14.

[0027] According to another aspect of this application, an application of the above-mentioned ultra-low silica-alumina ratio ZSM-5 molecular sieve is provided for catalytic cracking reactions, methanol aromatization, hydration reactions, and dehydration reactions.

[0028] The beneficial effects that this application can produce include:

[0029] This application relates to a method for synthesizing ZSM-5 molecular sieves with ultra-low silicon-to-aluminum ratios, mainly addressing the problems of poor tunability of the silicon-to-aluminum ratio and low solid product yield in existing ZSM-5 molecular sieve materials. When using Silicalite-1 crystals with a size less than 500 nm as seed crystals, the lower limit of Si / Al in ZSM-5 molecular sieves can be reduced to 7.5, and the sample exhibits high specific surface area and relative crystallinity, with a solid product yield greater than 98%. Attached Figure Description

[0030] Figure 1 The image shown is a scanning electron microscope (SEM) image of the seed crystal from Example 1, at a scale of 1 μm.

[0031] Figure 2 The image is a scanning electron microscope (SEM) image of the ultra-low silicon-to-aluminum ratio ZSM-5 sample prepared in Example 1, at a scale of 1 μm.

[0032] Figure 3 The image shows the XRD pattern of the sample from Example 1.

[0033] Figure 4 The nitrogen physical adsorption curve is shown for the sample in Example 1.

[0034] Figure 5 The image shown is a scanning electron microscope (SEM) image of the seed crystals in Example 2, with a scale of 1 μm.

[0035] Figure 6The image is a scanning electron microscope (SEM) image of the ultra-low silicon-to-aluminum ratio ZSM-5 sample prepared in Example 2, at a scale of 1 μm.

[0036] Figure 7 The image shows the XRD pattern of the sample from Example 2. Detailed Implementation

[0037] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0038] Unless otherwise specified, the raw materials and reagents used in the embodiments of this application were all purchased commercially.

[0039] Example 1:

[0040] Silicalite-1 seed crystals with a size smaller than 250 nm (10% of the silica sol mass) were mixed evenly with silica sol, deionized water, and sodium hydroxide. After activation at 45°C for 1 h, sodium aluminate was added, and the mixture was stirred evenly before being placed in a reactor for rotary crystallization at 180°C for 40 h. The molar ratio of each component was 1 SiO2:0.0714 Al2O3:30 H2O:0.135 Na2O.

[0041] XRF results showed that the Si / Al molar ratio of the obtained solid sample was 7.5;

[0042] Appendix Figure 2 Scanning electron microscopy results showed that the size of Silicalite-1 seed crystals was ~250 nm, the prepared sample was a rod-shaped stack, and no obvious impurity phases were observed.

[0043] Appendix Figure 3 XRD results showed that it was a ZSM-5 molecular sieve and that no impurity phases were formed.

[0044] Appendix Figure 4 Physical adsorption results show that it has a microporous structure with a specific surface area of ​​381 m². 2 / g, micropore volume is 0.136cm³ 3 / g.

[0045] Example 2:

[0046] Silicalite-1 seed crystals with a size smaller than 450 nm (10% of the silica sol mass) were mixed evenly with silica sol, deionized water, and sodium hydroxide. After activation at 60°C for 0.8 h, sodium aluminate was added, and the mixture was stirred evenly before being placed in a reactor for rotary crystallization at 180°C for 48 h. The molar ratio of each component was 1 SiO2:0.0714 Al2O3:30 H2O:0.135 Na2O.

[0047] XRF results showed that the Si / Al molar ratio of the obtained solid sample was 7.6;

[0048] Appendix Figure 6 Scanning electron microscopy results showed that the size of Silicalite-1 seed crystals was ~450 nm, the prepared sample was a rod-shaped stack, and no obvious impurity phases were observed.

[0049] Appendix Figure 7 XRD results showed that it was a ZSM-5 molecular sieve and that no impurity phases were formed.

[0050] Example 3

[0051] Silicalite-1 seed crystals with a size smaller than 150 nm (5% of the silica sol mass) were mixed evenly with silica sol, deionized water, and sodium hydroxide. After activation at 50°C for 1 h, sodium aluminate was added, and the mixture was stirred evenly before being placed in a reactor for rotary crystallization at 180°C for 48 h. The molar ratio of each component was 1 SiO2:0.0714 Al2O3:30 H2O:0.125 Na2O.

[0052] Example 4

[0053] Silicalite-1 seed crystals with a size smaller than 100 nm (3% of the mass of silica) were mixed evenly with silica, deionized water, and sodium hydroxide. After activation at 50°C for 1 h, aluminum nitrate was added, and the mixture was stirred evenly before being placed in a reaction vessel for rotary crystallization at 160°C for 60 h. The molar ratio of each component was 1 SiO2:0.0714 Al2O3:30 H2O:0.125 Na2O.

[0054] Example 5

[0055] Silicalite-1 seed crystals with a size smaller than 80 nm (2% of the silica sol mass) were mixed evenly with fumed silica, deionized water, and sodium hydroxide. After activation at 45°C for 2 hours, sodium aluminate was added, and the mixture was stirred evenly before being placed in a reaction vessel for rotary crystallization at 160°C for 60 hours. The molar ratio of each component was 1 SiO2:0.0556 Al2O3:30 H2O:0.135 Na2O.

[0056] Example 6

[0057] Silicalite-1 seed crystals with a size smaller than 500 nm (15% of the silica sol mass) were mixed evenly with fumed silica, deionized water, and sodium hydroxide. After activation at 60°C for 2 hours, sodium aluminate was added, and the mixture was stirred evenly before being placed in a reaction vessel for rotary crystallization at 180°C for 72 hours. The molar ratio of each component was 1 SiO2:0.0556 Al2O3:30 H2O:0.135 Na2O.

[0058] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and all fall within the scope of the technical solution.

Claims

1. A method for synthesizing ZSM-5 molecular sieves with ultra-low silica-to-alumina ratio, characterized in that, Includes the following steps: Inorganic alkali, silicon source, nano-Silicalite-1 seed crystals and water are mixed and activated, aluminum source is added, and crystallization is carried out in a sealed container to obtain the ultra-low silicon-aluminum ratio ZSM-5 molecular sieve. The size of the nano-Silicalite-1 seed crystals is 100–500 nm.

2. The method according to claim 1, characterized in that, The inorganic base is selected from at least one of NaOH, Na2CO3, NaHCO3, NaAlO2, NH3·H2O, KOH, K2CO3, and KHCO3; The silicon source is selected from at least one of tetraethyl orthosilicate, silica sol, water glass, silica gel, fumed silica, and activated clay. The aluminum source is selected from at least one of sodium aluminate, aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum isopropoxide, and aluminum oxide.

3. The method according to claim 1, characterized in that, The molar ratio of the silicon source to the aluminum source is 12 to 28; The molar ratio of the inorganic base to the silicon source is 0.05 to 0.20; The molar ratio of water to silicon source is 5–80; The mass of the Silicalite-1 seed crystal is 2 to 30 wt% of the mass of the silicon source; Wherein, the molar amount of the silicon source is expressed as the molar amount of silicon dioxide; The molar amount of the aluminum source is expressed as the molar amount of aluminum oxide; The molar amount of the inorganic base is expressed as the molar amount of the alkali metal oxide corresponding to the basic substance therein. The mass of the Silicalite-1 seed crystals is expressed in molar amounts of silicon dioxide.

4. The method according to claim 1, characterized in that, The activation temperature is 45–90°C; The activation time is 0.5 to 5 hours.

5. The method according to claim 1, characterized in that, The crystallization temperature is 120–200°C; The crystallization time is 12 to 96 hours.

6. A ZSM-5 molecular sieve with an ultra-low silica-to-alumina ratio prepared by the preparation method according to any one of claims 1 to 5; The Si / Al molar ratio of the ultra-low Si / Al ratio ZSM-5 molecular sieve is 6 to 14.

7. An application of the ultra-low silica-to-alumina ratio ZSM-5 molecular sieve according to claim 6, characterized in that, It is used in catalytic cracking reactions, methanol aromatization, hydration reactions, and dehydration reactions.

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