A method for synthesizing a template-free ultra-low silicon-aluminum ratio multi-stage pore ZSM-5 molecular sieve

Abstract

The application discloses a kind of synthesis methods of template-free ultra-low silicon-aluminum ratio multi-stage pore ZSM-5 molecular sieve, steps include: sodium hydroxide, deionized water, aluminum source, seed and silicon source are placed in hydrothermal reaction kettle and stirred uniformly, and after aging 3-5 hours at room temperature, hydrothermal reaction kettle is sealed, and dynamic crystallization is carried out in two stages temperature;After hydrothermal crystallization product is washed, dried and calcined, pure phase and higher crystallinity ZSM-5 molecular sieve is obtained.By adjusting the gel ratio and seed, effectively avoid the system in ultra-low silicon-aluminum ratio system due to high aluminum content, system is more likely to form higher aluminum content, more stable mordenite heterocrystal phase;By controlling crystallization conditions, effectively avoid the phenomenon of low crystallinity caused by uneven mass transfer and heat transfer when crystallization mode is static;And N2-adsorption and desorption analysis is carried out to the synthesized ZSM-5 molecular sieve, and it is obvious that it is a multi-stage pore structure.This method does not use template, reduces the pollution to environment, reduces production cost, and the synthesis method is simple and efficient.

Description

Technical Field

[0001] This invention relates to the preparation of molecular sieve materials, specifically to a method for synthesizing a template-free, ultra-low silica-alumina ratio hierarchical porous ZSM-5 molecular sieve. Background Technology

[0002] ZSM-5 molecular sieve is a novel three-dimensional cross-channel zeolite molecular sieve developed by Mobile International Inc. in 1972. This molecular sieve exhibits good hydrophobicity, high thermal and hydrothermal stability, and its unique pore structure makes it one of the most important catalytic materials for shape-selective reactions in the petroleum industry. Conventional ZSM-5 typically has a silica-to-alumina ratio (SAR) above 30, even reaching 100% silica. ZSM-5 with a SAR < 30 is called ultra-low silica ZSM-5 zeolite, and studies have shown that it contains more Brønsted acid (B), Lewis acid (L), and total acid content, exhibiting higher low-temperature activity. Currently, synthesizing pure ZSM-5 zeolite molecular sieves with a SAR of 25 using liquid-phase hydrothermal synthesis is quite difficult, and the resulting products often exhibit low crystallinity and are prone to the formation of impurities.

[0003] In the conventional crystallization synthesis of ZSM-5 molecular sieves, organic amines are generally used as template agents (common template agents include tetrapropylammonium hydroxide, tetraethylammonium hydroxide, and n-butylamine). Patents CN 1715186 A and CN 1247457 C have prepared ZSM-5 molecular sieves using different template agents. Due to the presence of template agents, the preparation process suffers from drawbacks such as high synthesis costs, severe environmental pollution, and complex procedures.

[0004] To address the numerous challenges in the synthesis of ZSM-5 molecular sieves, the current industry standard employs an amine-free method and precise control of the gel ratio to suppress mordenite. Patent CN 102757068 A discloses an amine-free method for synthesizing ZSM-5 molecular sieves, using a mixture of solid silica gel and liquid water glass as the silicon source to achieve amine-free synthesis of ZSM-5 molecular sieves at high concentrations. This method is simple, environmentally friendly, and significantly improves synthesis efficiency. However, this method yields a high silica-to-alumina ratio. Currently, synthesizing ultra-low silica-to-alumina ratio ZSM-5 molecular sieves is quite difficult, resulting in products with low crystallinity and a tendency to contain impurities. Therefore, we conducted a static amine-free method for preparing ultra-low silica-to-alumina ratio ZSM-5 molecular sieves, finding that the conditions were quite demanding and the crystallinity was extremely low. The reason is that, under a low silicon-to-aluminum ratio, the high aluminum content in the hydrothermal synthesis system leads to premature hydrolysis of the precursor, resulting in an excessively high gel concentration and the presence of impurities or amorphous silica in the hydrothermal synthesis product; and when the silicon-to-aluminum ratio is below 30, the synthesized product tends to transform into mordenite. Summary of the Invention

[0005] The purpose of this invention is to provide a method for synthesizing template-free, ultra-low silica-alumina ratio hierarchical porous ZSM-5 molecular sieves, in order to solve the problems of low crystallinity and impurity phases in the synthesis of amine-free ultra-low silica-alumina ratio ZSM-5. By adjusting the gel ratio and controlling the crystallization conditions, pure phase and high crystallinity ZSM-5 molecular sieves can be obtained.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for synthesizing a template-free, ultra-low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve includes the following steps: sodium hydroxide, deionized water, aluminum source, seed crystals, and silicon source are placed in a hydrothermal reactor and stirred evenly. After aging at room temperature for 3-5 hours, the hydrothermal reactor is sealed, and dynamic crystallization is performed in two stages at different temperatures. The hydrothermal crystallization product is washed, dried, and then calcined to obtain a pure-phase ZSM-5 molecular sieve with high crystallinity.

[0008] The molar ratio of silicon source, aluminum source, sodium hydroxide, and water is 17-23:1:1-2:400-600; wherein, silicon source, aluminum source, and sodium hydroxide are calculated as oxides SiO2:Al2O3:Na2O.

[0009] The seed crystal is a nano-grade ZSM-5 molecular sieve from Zibo Qichuang New Material Technology Co., Ltd., and the amount added is 16%-21% of the mass of silicon dioxide in the raw material silicon source.

[0010] As a preferred option:

[0011] The silicon source is silica sol or inorganic silica gel.

[0012] The aluminum source is sodium aluminate or boehmite.

[0013] The rotational speed for dynamic crystallization is 30–200 rpm.

[0014] The two-stage temperature dynamic crystallization is as follows: at a low temperature, the crystallization temperature is 80-100℃ and the crystallization time is 12-18h; at a high temperature, the crystallization temperature is 160-200℃ and the crystallization time is 18-86h.

[0015] The roasting temperature is 500-800℃, and the roasting time is 3-10h.

[0016] The washing process uses deionized water.

[0017] The sodium hydroxide can be replaced by potassium hydroxide.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1. The ultra-low silica-alumina ratio ZSM-5 molecular sieve synthesized in this invention has the characteristics of high microporous specific surface area (microporous specific surface area / total specific surface area is 72.45%), high acidity, and strong acidity compared with the lowest silica-alumina ratio of 30 used in commercially available ZSM-5 molecular sieves.

[0020] 2. This invention obtains ZSM-5 molecular sieves with 100% purity and crystallinity greater than 85% by adjusting the gel ratio and controlling crystallization conditions, effectively solving the problems of low crystallinity and impurity phases in the current industrial production of ZSM-5 molecular sieves with ultra-low silicon-aluminum ratio.

[0021] 3. The synthesis method of the amine-free ultra-low silica-alumina ratio hierarchical porous ZSM-5 molecular sieve under dynamic conditions of the present invention has a simple process and a short crystallization time.

[0022] 4. The method for synthesizing ultra-low silica-alumina ratio hierarchical porous ZSM-5 molecular sieve under dynamic conditions without amines, as described in this invention, has low cost, high yield, and broad prospects for industrial production and application. Attached Figure Description

[0023] Figure 1 This is the XRD pattern of the ZSM-5 molecular sieve prepared in Example 1.

[0024] Figure 2 This is the SEM image of the ZSM-5 molecular sieve prepared in Example 1.

[0025] Figure 3 This is the XRD pattern of the ZSM-5 molecular sieve prepared in Comparative Example 1.

[0026] Figure 4 This is the SEM image of the ZSM-5 molecular sieve prepared in Comparative Example 1.

[0027] Figure 5 This is the XRD pattern of the ZSM-5 molecular sieve prepared in Comparative Example 2.

[0028] Figure 6 This is the SEM image of the ZSM-5 molecular sieve prepared in Comparative Example 2. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to embodiments, in order to better understand the content of the present invention, rather than to limit the present invention.

[0030] Example 1:

[0031] 33.7g distilled water, 2.08g sodium aluminate, 0.74g sodium hydroxide, and 2g of nano-sized ZSM-5 seed crystals from Zibo Qichuang New Materials Technology Co., Ltd. were added to a 100ml hydrothermal reactor and stirred until homogeneous. While stirring, 38.52g of silica sol (30% by mass) was slowly added, then the reactor was sealed and placed at 30r / min. Dynamic hydrothermal crystallization was carried out at 80℃ for 12 hours, followed by dynamic hydrothermal crystallization at 200℃ for 86 hours. The hydrothermal crystallization product was washed, dried, and calcined at 500℃ for 10 hours to synthesize a pure-phase ultra-low silica-alumina ratio ZSM-5 molecular sieve with high crystallinity. The mass of the obtained molecular sieve was measured to be 8.15g. The purity of the obtained molecular sieve was determined to be 100%, and the crystallinity was as high as 85%. The XRD pattern, SEM image, and BET data of the sample are shown below. Figure 1 , Figure 2 As shown in Table 1.

[0032] Table 1. BET data of ZSM-5 molecular sieve prepared in Example 1

[0033]

[0034] Example 2:

[0035] 82g distilled water, 2.08g sodium aluminate, 0.35g sodium hydroxide, and 2.2g of nano-sized ZSM-5 seed crystals from Zibo Qichuang New Materials Technology Co., Ltd. were added to a 100ml hydrothermal reactor and stirred until homogeneous. While stirring, 13.1g of inorganic silica gel (88.1% by mass) was slowly added. The hydrothermal reactor was sealed, and dynamic hydrothermal crystallization was carried out at 30r / min at a low temperature of 80℃ for 16 hours, followed by dynamic hydrothermal crystallization at a high temperature of 170℃ for 48 hours. The hydrothermal crystallization product was washed, dried, and calcined at 600℃ for 6 hours to obtain a pure-phase ultra-low silica-alumina ratio ZSM-5 molecular sieve with high crystallinity. The mass of the obtained molecular sieve was measured to be 7g.

[0036] Example 3:

[0037] 39g distilled water, 1.16g boehmite, 0.78g sodium hydroxide, and 2.5g of nano-sized ZSM-5 seed crystals from Zibo Qichuang New Materials Technology Co., Ltd. were added to a 100ml hydrothermal reactor and stirred until homogeneous. While stirring, 38.52g of silica sol (30% by mass) was slowly added. The hydrothermal reactor was sealed, and dynamic hydrothermal crystallization was carried out at 100℃ for 12 hours at a low temperature of 30r / min, followed by dynamic hydrothermal crystallization at 160℃ for 18 hours. The hydrothermal crystallization product was washed, dried, and calcined at 800℃ for 3 hours to synthesize a pure-phase ultra-low silica-alumina ratio ZSM-5 molecular sieve with high crystallinity. The mass of the obtained molecular sieve was measured to be 7.5g.

[0038] Comparative Example 1:

[0039] Using the exact same conditions and procedures as in Example 1, the difference was that when the amount of sodium hydroxide added was 0.9 g, the resulting ultra-low silica-alumina ratio ZSM-5 molecular sieve had a crystallinity of less than 20% and contained impurities, with a purity of less than 40%. The XRD pattern and SEM image of the sample are shown below. Figure 3 , Figure 4 As shown.

[0040] Comparative Example 2:

[0041] Using the exact same conditions and procedures as in Example 1, except for the amount of deionized water added (18g), the resulting ultra-low silica-alumina ratio ZSM-5 molecular sieve exhibited crystallinity below 70% and contained impurities, with a purity below 80%. The XRD pattern and SEM image of the sample are shown below. Figure 5 , Figure 6 As shown.

[0042] Comparative Example 3:

[0043] The same amount and order of feeding were used as in Example 1. The difference was that the crystallization method was changed from low temperature and high temperature to static crystallization. Finally, the hydrothermal crystallization product was washed, dried and calcined at 500°C for 10 hours. The resulting ultra-low silica-alumina ratio ZSM-5 molecular sieve had a crystallinity of less than 65% and a purity of less than 60%.

[0044] This invention has been described through specific embodiments, but those skilled in the art should understand that this invention is not limited to the embodiments. Without departing from the principles and essence of this invention, this invention can be simplified, improved, etc., and these are all included within the scope of protection of this invention.

Claims

1. A method for synthesizing template-free, ultra-low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve, characterized in that, The process includes the following steps: Sodium hydroxide, deionized water, aluminum source, seed crystal and silicon source are placed in a hydrothermal reactor and stirred evenly. After aging at room temperature for 3 to 5 hours, the hydrothermal reactor is sealed and dynamic crystallization is carried out in two stages at different temperatures. The hydrothermal crystallization product is washed, dried and then calcined to obtain pure phase and high crystallinity ZSM-5 molecular sieve. The molar ratio of silicon source, aluminum source, sodium hydroxide, and water is 17-23:1:1-2:400-600; wherein, silicon source, aluminum source, and sodium hydroxide are calculated as oxides SiO2:Al2O3:Na2O. The seed crystal is a nano-grade ZSM-5 molecular sieve from Zibo Qichuang New Material Technology Co., Ltd., and the amount added is 16% to 21% of the mass of silicon dioxide in the raw material silicon source. The two-stage temperature dynamic crystallization is as follows: at a low temperature, the crystallization temperature is 80–100℃ and the crystallization time is 12–18h; at a high temperature, the crystallization temperature is 160–200℃ and the crystallization time is 18–86h. The silicon source is silica sol or inorganic silica gel; The aluminum source is sodium aluminate or boehmite.

2. The method for synthesizing template-free, ultra-low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve as described in claim 1, characterized in that, The sodium hydroxide was replaced with potassium hydroxide.

3. The method for synthesizing template-free, ultra-low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve as described in claim 1, characterized in that, The rotational speed for dynamic crystallization is 30–200 rpm.

4. The method for synthesizing template-free, ultra-low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve as described in claim 1, characterized in that, The roasting temperature is 500–800℃, and the roasting time is 3–10 hours.

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