Preparation method of small crystal DD3R molecular sieve

Abstract

The present invention relates to a method for preparing small-crystalline DD3R molecular sieve, which adds Sigma-1 nanoseeds to a synthesis precursor and forms small-crystalline DD3R molecular sieve through hydrothermal crystallization. The nanoseeds are obtained by wet ball-milling the original molecular sieve to form a ball-milled seed solution, then centrifuging the ball-milled seed solution at high speed to obtain a supernatant and a lower solid precipitate, and finally drying the supernatant to form the nanoseeds. This method has a simple preparation process, a short reaction time, and the obtained DD3R molecular sieve has the characteristics of small particle size and uniform size.

Description

Technical Field

[0001] The present invention belongs to the field of molecular sieves, and particularly provides a method for preparing small-crystallite DD3R molecular sieves. Background Art

[0002] Molecular sieves are inorganic crystalline materials with regular pore structures, having a high specific surface area, excellent thermal stability, mechanical stability, and chemical stability, and are widely used as catalysts and adsorbents in various industries. The crystal morphology and size of molecular sieves are of great significance for their performance and applications. For example, reducing the particle size of molecular sieves can reduce the internal diffusion resistance within the molecular sieves to increase the activity and service life of molecular sieve catalysts.

[0003] DD3R molecular sieve is an eight-membered ring molecular sieve with an effective pore size of 0.36 nm × 0.44 nm, having certain advantages in the field of separating small molecule gases, especially in terms of CO2. Initially, ethylenediamine was used as a mineralizing agent in the synthesis of DD3R molecular sieves. However, ethylenediamine is volatile and causes pollution, so strict sealing is required. The article "Fine control of crystal morphologies of all-silica DD3R in ethylenediamine-free gel with inorganic base as mineralizing agent" successfully prepared DD3R molecular sieves with a hexagonal morphology by using KOH to replace ethylenediamine as the mineralizing agent. However, it requires microwave as a heating source, and on the premise that microwave can accelerate crystallization and reduce the particle size, the particle size of its DD3R molecular sieves is still as high as 1 μm, seriously hindering the application of DD3R molecular sieves.

[0004] During the synthesis of DD3R, hetero-crystals have always been an unavoidable problem. In previous work of this project, ethylenediamine was used as a mineralizing agent. During the preparation of DD3R using Sigma-1, it was found that using ball-milled seed crystals could reduce hetero-crystals. Therefore, we tried to use alkali as a mineralizing agent and ball-milled Sigma-1 molecular sieves as seed crystals, and it was found that the synthesized molecular sieves contained a large number of hetero-crystals.

[0005] Therefore, there is an urgent need to improve the existing DD3R molecular sieve synthesis technology, so as to develop a synthesis method for small-particle-size DD3R molecular sieves with high purity, uniform size, and controllable morphology. Summary of the Invention

[0006] The present invention provides a method for preparing small-crystallite DD3R molecular sieves, which is used to solve the defects in the prior art such as difficult control of the particle size of DD3R molecular sieves, easy generation of hetero-crystals, and long synthesis time.

[0007] A preparation method of small crystal DD3R molecular sieve. In this preparation method, nanocrystalline seeds are added as seeds to a synthesis precursor, and through hydrothermal crystallization, small crystal DD3R molecular sieve is formed. The nanocrystalline seeds are obtained by wet ball-milling the initial molecular sieve to form a ball-milled seed solution, then centrifuging the ball-milled seed solution at high speed into a supernatant and a lower solid precipitate, and finally drying the supernatant to form the nanocrystalline seeds.

[0008] Preferably, the initial molecular sieve is Sigma-1 molecular sieve or DD3R molecular sieve, with a particle size of 1-2 μm, and the particle size of the nanocrystalline seeds is 50-100 nm.

[0009] Preferably, the ratio of the original molecular sieve to the ball-milling solvent in wet ball-milling is 1:1-1:99, and the ball-milling solvent is water or ethanol.

[0010] Preferably, the rotation speed of the ball mill is set at 200-600 rpm, the ball-milling time is 2-4 h, the centrifugation speed is set at 5000-20000 rpm, and the time is 20-60 min.

[0011] Preferably, the preparation method specifically includes the following steps:

[0012] Step 1: Mix adamantylamine, mineralizer, deionized water and silica sol in a certain molar ratio to obtain a synthesis precursor;

[0013] Step 2: Add nanocrystalline seed molecular sieve as seeds to the above synthesis precursor for hydrothermal synthesis, and obtain small crystal DD3R molecular sieve through washing, centrifugation and drying.

[0014] Preferably, in the first step, the mineralizer is sodium hydroxide, cesium hydroxide, potassium hydroxide, potassium fluoride, ethylenediamine and triethylenetetramine

[0015] Preferably, in the first step, the mineralizers used are sodium hydroxide, cesium hydroxide, potassium hydroxide, potassium fluoride, ethylenediamine and triethylenetetramine, and preferably sodium hydroxide and potassium hydroxide are used.

[0016] Preferably, in the first step, based on SiO2 in the silicon source, the molar ratio of SiO2, adamantylamine, mineralizer and water is 0.05-1:0.3:0.2:100-2000; in the second step, based on SiO2 in the silicon source, the addition amount of nanocrystalline seed molecular sieve added to the synthesis precursor is 16-64 wt.%.

[0017] Preferably, in the second step, the hydrothermal synthesis time is 1-24 h, and the hydrothermal synthesis temperature is 120-180 °C.

[0018] Technical effects

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1. The present invention optimizes the treatment method of molecular sieve. Aiming at the uneven crushing of molecular sieve during wet ball milling, it proposes to use the nanoscale nanocrystalline seeds in the supernatant obtained by ball milling and centrifugation as seeds, which well overcomes the serious unevenness of ball milling crushing.

[0021] 2. The present invention uses nanocrystalline seeds as seeds, and small particle size DD3R molecular sieves with a size of 200 - 600 nm can be obtained under traditional heating methods (non-microwave) such as an oven. Moreover, even when an alkali is used as a mineralizer, the DD3R molecular sieves prepared by the preparation method of the present invention have a high purity and basically do not contain impurity crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the particle size distribution diagram of Sigma-1 molecular sieve seeds.

[0023] Figure 2 It is the SEM diagram of DD3R molecular sieves prepared with seeds of different sizes.

[0024] Figure 3 It is the XRD diagram of DD3R molecular sieves prepared with seeds of different sizes.

[0025] Figure 4 It is the SEM diagram of small crystal grain DD3R molecular sieves prepared with different amounts of seed addition.

[0026] Figure 5 It is the XRD diagram of small crystal grain DD3R molecular sieves prepared with different amounts of seed addition.

[0027] Figure 6 It is the SEM diagram of small crystal grain DD3R molecular sieves prepared at different crystallization temperatures.

[0028] Figure 7 It is the XRD diagram of small crystal grain DD3R molecular sieves prepared at different crystallization temperatures.

[0029] Figure 8 It is the SEM diagram of small crystal grain DD3R molecular sieves prepared at different crystallization times.

[0030] Figure 9 It is the XRD diagram of small crystal grain DD3R molecular sieves prepared at different crystallization times.

[0031] Figure 10 It is the SEM diagram of small crystal grain DD3R molecular sieves prepared with different mineralizers.

[0032] Figure 11 It is the XRD diagram of small crystal grain DD3R molecular sieves prepared with different mineralizers. SPECIFIC EMBODIMENTS

[0033] The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings and technical solutions.

[0034] Preparation of seed crystals: Sigma-1 molecular sieve (particle size 1.5 μm) and water were mixed in a ratio of 1:20 and ball-milled and crushed. The rotation speed of the ball mill was set at 400 rpm, and the ball-milling time was 3.34 h. After ball-milling, it was washed with deionized water and centrifuged at a high speed. The centrifugation speed was set at 11,000 rpm for 30 min to obtain the supernatant and the lower solid sample. The small molecular sieve fragments that could not settle after centrifugation in the supernatant were labeled as Sigma-1 nanoseed crystals, and the lower solid sample was labeled as Sigma-1 ball-milled seed crystals.

[0035] Figure 1 is the particle size distribution diagram of Sigma-1 molecular sieve seed crystals. The average particle size of Sigma-1 nanoseed crystals is about 80 nm, and the average particle size of Sigma-1 ball-milled seed crystals is about 350 nm, and the particle size distribution of Sigma-1 nanoseed crystals is narrower.

[0036] Example 1:

[0037] In this example, the molar ratio is 1SiO2: 0.3ADA: 0.2NaOH: 100H2O. The added seed crystals are nanoseed crystals, and the content is 32 wt.% (based on the mass of SiO2). The crystallization temperature is 140 °C, and DD3R molecular sieve is prepared by reacting for 24 h.

[0038] Adamantylamine, sodium hydroxide, deionized water, and silica sol were prepared into a synthetic liquid precursor. Sigma-1 nanoseed crystals and ball-milled seed crystals were added respectively, and stirred and mixed evenly. The oven temperature was set at 140 °C and the time was set at 24 h. After the reaction, the sample was washed, centrifuged, and dried to obtain DD3R molecular sieve.

[0039] Comparative Example 1: The difference from Example 1 is that the added Sigma-1 ball-milled seed crystals have the same content.

[0040] Figure 2 is the SEM image of the small crystal DD3R molecular sieve prepared in Example 1 and Comparative Example 1. It can be seen from the figure that under the same conditions, the particle size and morphology of the DD3R molecular sieve obtained from the ball-milled seed crystals are quite different. The DD3R molecular sieve prepared from the nanoseed crystals is more uniform, about 400 nm.

[0041] Figure 3 is the XRD pattern of the DD3R molecular sieve prepared in Example 1 and Comparative Example 1. It was found that the sample obtained using the ball-milled seed crystals contained a certain amount of miscellaneous crystals, while the sample prepared using the nanoseed crystals had less miscellaneous crystals.

[0042] Table 1 Results of samples obtained with different seed crystals

[0043]

[0044] Note: The number of * indicates the content of impurity crystals. / means there are no impurity crystals. The same below.

[0045] Example 2

[0046] In this example, the molar ratio is 1SiO2: 0.3ADA: 0.2NaOH: 100H2O, and the Sigma-1 nanocrystal seed contents are 3.2 wt.%, 16 wt.%, 32 wt.%, 48 wt.% and 64 wt.% (based on the mass of SiO2) respectively. The crystallization temperature is 140 °C, and small crystal DD3R zeolite is prepared by reacting for 24 h.

[0047] Figure 4 SEM image of the small crystal DD3R zeolite prepared by this method. It can be seen from the figure that the crystal grain size of the DD3R zeolite decreases with the increase of the seed crystal content, and the crystal morphology has no obvious change, all being hexagonal. When the added seed crystal amount is 32 wt.%, the reaction temperature is 140 °C, and the reaction time is 24 h, the particle size of the DD3R zeolite is about 400 nm.

[0048] Figure 5 XRD pattern of the small crystal DD3R zeolite prepared by this method, which is basically consistent with the standard pattern. However, there are a small amount of impurity crystals in the low-content nanocrystal seeds.

[0049] Table 2 Results of samples obtained with different seed crystal contents

[0050]

[0051]

[0052] Example 3

[0053] In this example, 32 wt.% nanocrystal seeds are added, and the reaction temperatures are 120 °C, 130 °C, 150 °C, 160 °C, 170 °C, 180 °C respectively, and small crystal DD3R zeolite is prepared by reacting for 24 h.

[0054] Figure 6 SEM images of the small crystal DD3R zeolite prepared at different crystallization temperatures. In the range of 120 - 180 °C of the crystallization temperature, small crystal DD3R zeolite can be prepared, and the particle size has no obvious change. As the temperature rises above 170 °C, the morphology of the DD3R zeolite tends to grow into polyhedrons.

[0055] Figure 7The XRD pattern of the small-crystallite DD3R zeolite prepared by this method is consistent with the standard pattern. Well-crystallized DD3R zeolite was obtained within the temperature range of 120 - 180 °C.

[0056] Example 4

[0057] In this example, 32 wt.% of nanoseeds was added, and the reaction temperatures were 140 °C respectively, and the reaction times were 1 h, 3 h, 4 h, 6 h, 12 h, and 18 h to prepare small-crystallite DD3R zeolite. The remaining steps were the same as those in Example 1.

[0058] Figure 8 SEM images of small-crystallite DD3R zeolite prepared with different crystallization times. DD3R zeolite with a size of about 200 nm started to appear at a reaction time of 3 h; when the time was extended to 4 h, the crystallinity of the crystals increased, and the size was about 400 nm. When the crystallization time was continuously extended to 24 h, compared with the reaction time of 4 h, there was no significant change in the particle size and morphology of the zeolite.

[0059] Figure 9 XRD patterns of small-crystallite DD3R zeolite prepared with different crystallization times. Within 3 h of reaction, there were no obvious characteristic peaks of DD3R zeolite. The XRD patterns of small-crystallite DD3R zeolite prepared with a reaction time of 4 - 18 h were consistent with the standard pattern, and the crystallinity of the zeolite was good.

[0060] Table 2 Results of samples obtained with different crystallization times

[0061]

[0062]

[0063] Example 5

[0064] In this example, small-crystallite DD3R zeolite was prepared with a molar ratio of 1SiO2:0.3ADA:0.2MA:100H2O, a Sigma-1 nanoseed amount of 32 wt.%, and a crystallization temperature of 140 °C for 24 h. MA was potassium hydroxide, cesium hydroxide, potassium fluoride, ethylenediamine, and triethylenetetramine respectively.

[0065] Figure 10 SEM images of small-crystallite DD3R zeolite prepared with different mineralizing agents. The type of mineralizing agent has a significant effect on the morphology of the zeolite. When potassium hydroxide and cesium hydroxide are used as mineralizing agents, the DD3R zeolite crystals show a hexagonal morphology; while when potassium fluoride, ethylenediamine, and triethylenetetramine are used as mineralizing agents, the crystals show a polyhedral morphology.

[0066] Figure 11The XRD pattern of the small crystal DD3R zeolite prepared by this method is consistent with the standard pattern. Well-crystallized DD3R zeolite was synthesized using different mineralizing agents.

[0067] Table 3 Results of samples obtained with different mineralizing agents

[0068] Sample Serial Number Mineralizer Average Particle Size of Product Content of Miscellaneous Crystals 5-1 NaOH 400nm / 5-2 KOH 400nm / 5-3 CsOH 1.5μm / 5-4 KF 400nm * 5-5 EDA 400nm * 6 TETA 400nm *

Claims

1. A preparation method of small crystal DD3R molecular sieve, characterized in that The preparation method adds nanocrystalline seeds to a synthetic precursor, and through hydrothermal crystallization, small-crystalline DD3R zeolite is formed. The nanocrystalline seeds are obtained by wet ball-milling the original zeolite to form a ball-milled seed solution, then centrifuging the ball-milled seed solution at a high speed into a supernatant and a lower solid precipitate, and finally drying the supernatant to form the nanocrystalline seeds. The original zeolite is Sigma-1 zeolite; the particle size of the original zeolite is 1-2 μm, and the particle size of the nanocrystalline seeds is 50-100 nm; the centrifugation speed is set at 5000-20000 rpm, and the time is 20-60 min; the particle size of the DD3R zeolite is 200-600 nm.

2. The method according to claim 1, wherein The ratio of the original zeolite for wet ball-milling to the ball-milling solvent is 1:1-1:99, and the ball-milling solvent is water or ethanol.

3. The method according to claim 1, wherein The rotation speed of the ball mill is set at 200-600 rpm, and the ball-milling time is 2-4 h.

4. The method according to claim 1, wherein The preparation method specifically includes the following steps: Step 1, mixing adamantylamine, a mineralizing agent, deionized water, and silica sol in a certain molar ratio to obtain a synthetic precursor; Step 2, adding nanocrystalline seed zeolite as seeds to the above synthetic precursor for hydrothermal synthesis, and obtaining small-crystalline DD3R zeolite through washing, centrifugation, and drying.

5. The preparation method according to claim 4, characterized in that, The mineralizing agent used in Step 1 is sodium hydroxide, cesium hydroxide, potassium hydroxide, potassium fluoride, ethylenediamine, or triethylenetetramine.

6. The preparation method according to claim 5, characterized in that, The mineralizing agent used in Step 1 is sodium hydroxide or potassium hydroxide.

7. The preparation method according to claim 4, characterized in that, In Step 1, based on SiO2 in the silica sol, the molar ratio of SiO2, adamantylamine, the mineralizing agent, and water is 0.05-1:0.3:0.2:100-2000; in Step 2, based on SiO2 in the silica sol, the addition amount of nanocrystalline seed zeolite added to the synthetic precursor is 16-64 wt.%.

8. The preparation method according to claim 5, characterized in that, In Step 2, the hydrothermal synthesis time is 1-24 h, and the hydrothermal synthesis temperature is 120-180 °C.

9. A small crystal DD3R molecular sieve prepared by any of the preparation methods according to claims 1-8, characterized in that The particle size of the DD3R zeolite is 200-600 nm.

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