Method for rapidly preparing nano Beta molecular sieve by using microwave synthesis method

Abstract

The invention relates to the technical field of preparation of Beta molecular sieves, and discloses a method for rapidly preparing a nano Beta molecular sieve by using a microwave synthesis method, which comprises the following steps: selecting proper silicon sources (such as silicon dioxide, white carbon black and the like) and aluminum sources (such as sodium metaaluminate, aluminum sulfate and the like), adding a template agent (such as tetraethylammonium hydroxide), an alkali source (such as sodium hydroxide) and deionized water, and carrying out microwave synthesis to obtain the nano Beta molecular sieve. Uniformly mixing and stirring according to a specific molar ratio to obtain initial gel; and crystallizing the gel, wherein a conventional hydrothermal method or a microwave heating method can be adopted. And after crystallization is completed, carrying out post-treatment such as filtration, washing, drying and high-temperature calcination on the product to finally obtain the Beta molecular sieve product. According to the method for rapidly preparing the nano Beta molecular sieve by utilizing the microwave synthesis method, provided by the invention, the synthesis steps are simplified, the crystallization time is remarkably shortened, the production efficiency is improved, the particle size of the nano Beta molecular sieve is effectively controlled, and a nano-scale product with smaller particle size and more uniform distribution is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of Beta molecular sieve preparation, and in particular to a method for rapidly preparing nano Beta molecular sieve by using a microwave synthesis method. Background Art

[0002] Beta molecular sieve is a large-pore molecular sieve with a three-dimensional twelve-membered ring pore structure. It is widely used in petrochemicals, fine chemicals, environmental protection and other fields due to its unique pore structure, high specific surface area, excellent thermal stability and acidity. In industrial applications, Beta molecular sieve is mainly used as a catalyst or catalyst carrier, and exhibits excellent catalytic performance in reactions such as alkylation, isomerization, and cracking. Traditionally, Beta molecular sieve is mainly prepared by hydrothermal synthesis. This method usually needs to be carried out under high temperature and high pressure conditions, and the synthesis conditions (such as temperature, time, pH value, type of template agent, etc.) have a significant effect on the crystal phase, particle size, morphology and other properties of the final product.

[0003] However, the traditional hydrothermal synthesis method has some obvious disadvantages. First, the method has high energy consumption and low preparation efficiency. The crystallization time usually takes 2 to 3 days, which not only increases the production cost, but also limits the output of the product. Secondly, long-term high-temperature reaction will produce a large amount of chemical waste, which is not in line with the current development concept of green chemistry. In addition, the traditional heating method heats the reaction system through heat conduction and convection, with low energy transfer efficiency, and it is difficult to achieve uniform heating, which easily leads to unstable product quality. In contrast, microwave heating technology provides new possibilities for the synthesis of molecular sieves. Microwave heating directly acts on polar molecules in the reaction system, with extremely high energy transfer efficiency, and can cause high-frequency vibration of molecules, greatly increase the reaction rate, and significantly shorten the synthesis time. This not only improves production efficiency, but also reduces energy consumption. On the other hand, the performance of molecular sieves is closely related to their particle size, pore structure, etc. Studies have shown that nanoscale molecular sieves often exhibit better catalytic performance due to their larger specific surface area and shorter diffusion path. By using surfactants, the crystallization process of molecular sieves can be regulated to prepare nanomolecular sieves with smaller particle sizes, thereby further improving the catalytic performance of molecular sieves. Therefore, developing a fast, efficient and controllable method for synthesizing nano-Beta molecular sieves not only has important scientific significance, but also has great application value.

[0004] To this end, the patent application document with patent number CN110065953A introduces a method for rapidly synthesizing nano-Beta molecular sieves using microwave heating combined with a seed crystal method. However, this method requires the addition of Beta molecular sieve seeds and requires two stages of pre-crystallization and high-temperature crystallization. The synthesis steps are relatively complicated and time-consuming.

[0005] The patent application document with patent number CN110127715A introduces a method for accelerating the synthesis of Beta molecular sieve using a promoter. However, this method requires additional addition of a promoter and precise control of the ratio of the promoter to the silicon source. Dynamic crystallization is carried out at 95 to 145°C for 20 to 90 hours, which is time-consuming and the particle size of the product is not controlled.

[0006] The patent application document with patent number CN112939008A introduces a method for rapidly synthesizing Beta molecular sieves of different particle sizes using a crystal size regulator, but this method requires 30 to 80 hours of crystallization, which is time-consuming.

[0007] It can be seen that the prior art still has the following problems in preparing Beta molecular sieves:

[0008] 1. Complex synthesis steps and long crystallization time: In the prior art, even if microwave heating, seed crystal method or promoter method are used, the synthesis of Beta molecular sieve still takes 4 to 20 hours, or even longer. At the same time, some methods require two complex stages of pre-crystallization and high-temperature crystallization, or require precise control of the ratio of promoter to silicon source, or the addition of seed crystals to promote the crystallization process, which increases the complexity of the operation.

[0009] 2. Limited particle size control, large particle size: In the prior art, the particle size control capability of nano-Beta molecular sieve is limited, the resulting product particle size is often large, or a crystal particle size regulator needs to be added to achieve particle size control. This limits the performance of molecular sieves in certain application areas.

[0010] Therefore, it is necessary to provide a method for rapidly preparing nano-Beta molecular sieves using microwave synthesis, aiming to simplify the synthesis steps, significantly shorten the crystallization time, improve production efficiency, and achieve effective control of the particle size of the nano-Beta molecular sieve to obtain nano-scale products with smaller particle size and more uniform distribution. Summary of the invention

[0011] The purpose of the present invention is to provide a method for rapidly preparing nano Beta molecular sieves by microwave synthesis, aiming to solve the problems of complex synthesis steps, long crystallization time and limited control of nanoparticle size in the prior art. The method achieves a significant reduction in synthesis time and precise control of nanoparticle size by optimizing the microwave heating process and reaction parameters and combining the use of surfactants. It does not require the addition of seed crystals or complex pretreatment processes, greatly simplifies the synthesis steps, and has the important significance of improving production efficiency, reducing energy consumption, and enhancing product performance.

[0012] The invention provides a method for rapidly preparing nano Beta molecular sieve by using microwave synthesis, which comprises the following steps: adding a silicon source and an aluminum source into an alkaline solution respectively, and self-assembling to form a crystal nucleus with an ordered structure under the action of a structure directing agent; subsequently adding a surfactant to regulate the growth process of the crystal nucleus, and evaporating excess water to form a gel system; rapidly crystallizing the crystal nucleus under the action of microwaves to generate a nano particle product with a Beta molecular sieve structure; and finally obtaining the nano Beta molecular sieve through post-treatments such as centrifugation, filtration, washing, drying, grinding, and calcination.

[0013] It should be noted that the term "Beta" in the Beta molecular sieve involved in the present invention can be translated into the Greek letter "β" or the Chinese word "beta", etc.

[0014] The method for rapidly preparing nano Beta molecular sieve by microwave synthesis provided by the present invention comprises the following steps:

[0015] S1. Synthesis of initial gel: Mix aluminum source, silicon source, alkali, structure directing agent, surfactant and deionized water. The silicon source is SiO 2 The aluminum source is Al 2 O 3 The template agent is TEA + The alkali metal source is A 2 O, surfactant is calculated as R, and its original molar ratio is SiO 2 :Al 2 O 3 :TEA + :A 2 O:R:H 2 O=40~50:1:15~30:2~5:2.5~8:200~500;

[0016] S2, preparing a concentrated gel system: heating and stirring the initial gel prepared in step S1 to evaporate water, controlling H 2 O / SiO 2 5 to 10;

[0017] S3, microwave heating crystallization: the concentrated glue is transferred into a microwave reactor and sealed, and heated to 150-180°C using microwaves and maintained for 3-6 hours for crystallization;

[0018] S4. The crystallized product is centrifuged, washed several times and then dried to obtain a white solid product, which is ground and calcined at 600-700° C. for 6-10 h to obtain a nano Beta molecular sieve product.

[0019] Preferably, the silicon source is one or more of fumed silica, tetraethyl orthosilicate, silica sol, white carbon black, water glass, chromatography silica gel or macroporous silica gel.

[0020] Preferably, the aluminum source is one or more of sodium aluminate, aluminum powder, pseudo-boehmite, aluminum sulfate, aluminum chloride, aluminum nitrate or aluminum acetate.

[0021] Preferably, the base is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate or potassium carbonate.

[0022] Preferably, the structure directing agent is one or more of tetraethylammonium hydroxide, tetraethylammonium bromide or tetraethylammonium chloride.

[0023] Preferably, the surfactant is one or more of L-lysine or citric acid, and the molar ratio of the silicon source to the surfactant is 5-20.

[0024] Preferably, in the microwave heating crystallization step, the microwave heating temperature is 150-180° C., and the crystallization time is 3-6 hours.

[0025] Preferably, the particle size of the obtained nano Beta molecular sieve is 30-80 nm.

[0026] Preferably, by adjusting the raw material ratio and crystallization conditions, the rapid synthesis of nano Beta molecular sieves is achieved, and the hydrogen-type, ammonium-type or Beta molecular sieves loaded with other metal cations are obtained by ion exchange technology.

[0027] Preferably, the rotation speed of the centrifugation step is 12000-15000 rpm, and the centrifugation time is 10-30 min.

[0028] Compared with the related art, the method for rapidly preparing nano Beta molecular sieve by microwave synthesis provided by the present invention has the following beneficial effects:

[0029] The present invention uses microwave synthesis to rapidly prepare nano Beta molecular sieves. Microwaves can directly act on reactant molecules to promote molecular motion and collision. Compared with the traditional hydrothermal method, microwave heating can achieve rapid and uniform heating, accelerate the formation and growth of crystal nuclei, and significantly shorten the crystallization time, which not only improves the synthesis efficiency but also helps to form a more uniform crystal structure.

[0030] The present invention adopts fumed silica, tetraethyl orthosilicate or silica sol as silicon source, sodium aluminate or aluminum powder as aluminum source, tetraethylammonium hydroxide or tetraethylammonium bromide as structure directing agent, L-lysine or citric acid as surfactant, which can interact with crystal nuclei in the crystal nucleus formation stage, adsorb on the crystal surface, limit the further growth of the crystal, and thus realize the control of the particle size of the nano Beta molecular sieve.

[0031] The Beta molecular sieve synthesized by this method has a particle size of 30-80nm, a high specific surface area and a rich pore structure, and exhibits excellent catalytic performance and adsorption characteristics. Through the synergistic effect of microwave heating and surfactants, this method significantly shortens the synthesis time and achieves precise control of the crystal size, providing a new technical approach for the large-scale production of high-performance nano Beta molecular sieves. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Transmission electron microscope images of molecular sieve particles prepared in various embodiments and comparative examples of the present invention;

[0033] Figure 2 The XRD diffraction patterns of various embodiments and comparative examples of the present invention and the standard XRD data of the BEA (Beta molecular sieve structure code) structure;

[0034] Figure 3 This is a nitrogen adsorption-desorption curve diagram of the molecular sieve particles prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further described below in conjunction with the accompanying drawings and implementation modes.

[0036] 1. Embodiment 1

[0037] (1) At room temperature, add 10.42 g of ethyl orthosilicate (99%) to 16.2 g of tetraethylammonium hydroxide aqueous solution (25 wt%) and stir evenly. Add 0.22 g of sodium hydroxide (≥96%) and stir to dissolve, then add 5.4 g of deionized water. Heat the solution to 50°C and stir at constant temperature for 6 to 10 hours until the ethyl orthosilicate is completely hydrolyzed to obtain synthetic solution A.

[0038] (2) Dissolve 0.205 g of sodium aluminate (reagent grade) in 8 mL of deionized water to obtain synthetic solution B.

[0039] (3) 0.731 g of L-lysine (98%) was dissolved in 4.6 mL of deionized water to obtain synthetic solution C.

[0040] (4) At 50°C, slowly add synthesis solutions B and C into synthesis solution A and stir for 1 h to obtain the initial synthesis gel.

[0041] (5) The initial synthetic gel was placed under a heating lamp and slowly stirred to evaporate water until the mass was reduced by 24 g to obtain a synthetic concentrated gel.

[0042] (6) The synthesized concentrated gel was transferred to a microwave reactor and heated to 180°C by microwave. After reacting for 3 h, a white liquid product was obtained.

[0043] (7) The white liquid product was separated by centrifugation at 14000 rpm to obtain a solid product, which was then repeatedly washed by centrifugation with deionized water and ethanol until the washing liquid was nearly neutral.

[0044] (8) The product was placed in an oven at 90°C and dried overnight to obtain molecular sieve raw powder.

[0045] (9) The molecular sieve raw powder was ground and placed in a muffle furnace, heated to 650°C at a heating rate of 2°C / min, calcined for 6 hours and then naturally cooled to finally obtain a nano-Beta molecular sieve with a sample particle size of about 30 nm and a BET specific surface area of ​​767.1 m 2 / g, much higher than ordinary commercial Beta molecular sieve (about 500m 2 / g).

[0046] 2. Embodiment 2

[0047] In Example 1, the amount of L-lysine added in step (3) was changed to 1.462 g, and the other steps remained unchanged. A gel-like product was obtained after crystallization. After centrifugation, washing, drying, grinding and calcination, a nano-Beta molecular sieve was obtained. The sample particle size was about 80 nm.

[0048] 3. Comparative Example 1

[0049] In Example 1, L-lysine was not added, other conditions remained unchanged, and synthetic liquid C was not added. The synthetic concentrated gel was crystallized for 3 hours under microwave heating at 180°C to obtain a white liquid product. After centrifugation, washing, drying, grinding, and calcination, Beta molecular sieve was obtained, and the sample particle size was about 200 nm.

[0050] 4. Comparative Example 2

[0051] In Example 1, L-lysine was not added, microwave heating method was not used, other conditions remained unchanged, and synthetic liquid C was not added. The synthetic concentrated gel was transferred to a reactor, placed in a high-temperature oven and heated to 180°C and crystallized for 3 hours to obtain a gel product. XRD analysis showed that the synthetic liquid failed to crystallize successfully. The heating temperature was adjusted to 150°C and crystallized for 48 hours to obtain a white liquid product, which was centrifuged, washed, dried, ground, and calcined to obtain a Beta molecular sieve. The sample particle size of the obtained Beta molecular sieve was about 200nm.

[0052] 5. Embodiment 3

[0053] (1) 3 g of fumed silica was slowly added to 14.336 g of tetraethylammonium hydroxide aqueous solution (25 wt%), and the mixture was heated and stirred in a 75° C. water bath for 6 to 10 h until the fumed silica was completely reacted to obtain synthetic solution A.

[0054] (2) Add 0.179 g of sodium aluminate (reagent grade) and 0.13 g of sodium hydroxide (≥96%) to 8 g of deionized water, respectively, and stir to dissolve to obtain synthetic solution B.

[0055] (3) Dissolve 0.96 g of citric acid in 10 mL of deionized water to obtain synthetic solution C.

[0056] (4) In a 75°C water bath, slowly add synthesis solutions B and C into synthesis solution A and stir for 1 h to obtain the initial synthesis gel.

[0057] (5) Place the initial synthetic gel under a heating lamp and slowly stir and evaporate the water until the mass is reduced by 20 g to obtain a synthetic concentrated gel.

[0058] (6) The synthesized concentrated gel was transferred to a microwave reactor and heated to 160°C by microwave. After reacting for 6 hours, a white liquid product was obtained.

[0059] (7) The subsequent steps are the same as those in Example 1 to obtain a nano-Beta molecular sieve with a particle size of about 50 nm.

[0060] 6. Embodiment 4

[0061] (1) 10.24 g of tetraethylammonium bromide (98%) was added to 21.51 g of deionized water to dissolve, and then 3 g of fumed silica was slowly added, and the mixture was heated and stirred in a 75° C. water bath for 6 to 10 h until the fumed silica was completely reacted to obtain synthetic liquid A.

[0062] (2) Dissolve 0.066 g of sodium hydroxide (≥96%) in 8 mL of deionized water, add 0.059 g of aluminum powder, and stir at room temperature until the aluminum powder is fully reacted to obtain synthetic solution B.

[0063] (3) The subsequent steps are the same as those in Example 3 to obtain a nano-Beta molecular sieve with a particle size of about 80 nm.

[0064] 7. Comparative Example 3

[0065] In Example 4, the microwave heating method is not used, and other conditions remain unchanged. The synthetic concentrated gel is transferred to a reactor, placed in a high-temperature oven and heated to 150°C and crystallized for 48 hours to obtain a white liquid product. After centrifugation, washing, drying, grinding and calcination, a nano Beta molecular sieve is obtained, and the sample particle size is about 100 nm.

[0066] In combination with the above embodiments and comparative examples, and referring to Figure 1-3 , Figure 1 The transmission electron microscope images of the molecular sieve particles prepared in various embodiments and comparative examples of the present invention mainly reflect the difference in particle size between different products;

[0067] Figure 2The XRD diffraction patterns of various embodiments and comparative examples of the present invention and the standard XRD data of the BEA (Beta molecular sieve structure code) structure indicate that all prepared products have a Beta molecular sieve structure;

[0068] Figure 3 This is a nitrogen adsorption-desorption curve of the molecular sieve particles prepared in Example 1 of the present invention. The BET surface area calculated based on the curve data is 767.1 m 2 / g, which is much higher than the BET surface area data of common Beta molecular sieves in the literature (about 500m 2 / g), and the nitrogen adsorption-desorption curve of ordinary Beta molecular sieve can also be supplemented as a comparison.

[0069] Compared with the related art, the method for rapidly preparing nano Beta molecular sieve by microwave synthesis provided by the present invention has the following beneficial effects:

[0070] This method greatly shortens the synthesis time through microwave heating technology, solves the key problem of the long synthesis cycle of the traditional hydrothermal method, and significantly improves production efficiency. Secondly, microwave heating achieves rapid and uniform heating, effectively overcoming the problem of product quality fluctuations caused by uneven temperature distribution in traditional methods, and improving product uniformity and synthesis stability.

[0071] By precisely controlling the synthesis parameters, this method can regulate the crystal size and morphology of nano-Beta molecular sieves, obtain a larger specific surface area and optimized pore structure, and thus significantly improve the catalytic performance of the molecular sieve. This feature provides new possibilities for improving the efficiency of various catalytic reactions. In addition, this method conforms to the concept of green chemistry, greatly reduces the emission of chemical waste and energy consumption, and provides a new way for environmentally friendly production.

[0072] The method of the present invention is simple and easy to implement, has low cost and good scalability, and lays a foundation for the industrial large-scale production of nano Beta molecular sieves. This not only meets the demand for high-performance molecular sieves in traditional fields such as petrochemicals and fine chemicals, but also opens up broad prospects for applications in emerging fields such as new energy and environmental protection. In general, the present invention brings innovation to the preparation technology of nano Beta molecular sieves, and has important practical significance and long-term value for promoting the development of related industries.

[0073] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for rapidly preparing nano Beta molecular sieves by microwave synthesis, characterized in that: The following steps are involved: S1. Synthesis of initial gel: Mix aluminum source, silicon source, alkali, structure directing agent, surfactant and deionized water. The silicon source is calculated as SiO2, the aluminum source is calculated as Al2O3, and the template is calculated as TEA. + The alkali metal source is calculated as A2O, and the surfactant is calculated as R. The original molar ratio is SiO2:Al2O3:TEA + :A2O:R:H2O=40~50:1:15~30:2~5:2.5~8:200~500; S2, preparing a concentrated gel system: heating and stirring the initial gel prepared in step S1 to evaporate water, and controlling the H2O / SiO2 ratio to be 5-10; S3, microwave heating crystallization: the concentrated glue is transferred into a microwave reactor and sealed, and heated to 150-180°C using microwaves and maintained for 3-6 hours for crystallization; S4. The crystallized product is centrifuged, washed several times and then dried to obtain a white solid product, which is ground and calcined at 600-700° C. for 6-10 h to obtain a nano Beta molecular sieve product.

2. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The silicon source is one or more of fumed silica, tetraethyl orthosilicate, silica sol, white carbon black, water glass, chromatography silica gel or macroporous silica gel.

3. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The aluminum source is one or more of sodium aluminate, aluminum powder, pseudo-boehmite, aluminum sulfate, aluminum chloride, aluminum nitrate or aluminum acetate.

4. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The alkali is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate or potassium carbonate.

5. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The structure directing agent is one or more of tetraethylammonium hydroxide, tetraethylammonium bromide or tetraethylammonium chloride.

6. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The surfactant is one or more of L-lysine or citric acid, and the molar ratio of the silicon source to the surfactant is 5-20.

7. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: In the microwave heating crystallization step, the microwave heating temperature is 150-180° C., and the crystallization time is 3-6 hours.

8. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The particle size of the obtained nano Beta molecular sieve is 30-80 nm.

9. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: By adjusting the raw material ratio and crystallization conditions, the rapid synthesis of nano Beta molecular sieves can be achieved, and hydrogen-type, ammonium-type or Beta molecular sieves loaded with other metal cations can be obtained through ion exchange technology.

10. The method for rapidly preparing nano-Beta molecular sieve by microwave synthesis as claimed in claim 1, characterized in that: The rotation speed of the centrifugation step is 12000-15000 rpm, and the centrifugation time is 10-30 min.

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