A method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets

Abstract

The present invention relates to a method for synthesizing highly dispersed ZSM-5 nanosheets by in-situ assembly. Specifically, tetraethyl silicate (or sodium silicate), aluminum sulfate octahydrate, N,N,N,N-tetramethyl-1,6-hexanediamine, C6-C22-bromoalkanes (bromo-n-hexane, bromo-n-octane, bromo-n-decane, bromo-dodecane, bromo-tetradecane, bromo-hexadecane, bromo-octadecane, bromo-dodecane), sulfuric acid, sodium hydroxide and water are used as raw materials, and nanosheet ZSM-5 molecular sieves are dynamically synthesized using an in-situ assembly method. The in-situ assembly method dynamic synthesis method for preparing highly dispersed ZSM-5 nanosheets provided by the present invention has the advantages of simple and easy synthesis process and low cost. The ZSM-5 nanosheets obtained are multi-cell stacking structures, and the single-cell nanosheets obtained by a special template method have higher self-support and stability, and have a higher external specific surface area and a meso-micro composite pore structure.

Description

Technical Field

[0001] The present invention relates to a method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets. Specifically, the method uses tetraethyl silicate (or sodium silicate), aluminum sulfate octahydrate, N,N,N,N-tetramethyl-1,6-hexanediamine, C6-C24-bromoalkane, sulfuric acid, sodium hydroxide, and water as raw materials to dynamically synthesize nanosheet ZSM-5 molecular sieves using an in-situ assembly method. Background Art

[0002] Nanosheet molecular sieves are considered to be highly efficient catalysts or catalyst supports for macromolecular conversions due to their high accessibility of active sites and low diffusion restrictions of reactants. In recent years, researchers have made many attempts in the synthesis of nanosheet molecular sieves and have made great progress. Among them, the special template method is to use special organic molecules designed and synthesized in advance as directing agents to induce the synthesis of sheet molecular sieves. Ryoo et al. reported a specially designed bifunctional C 22 H 45 -N + (CH3)2-C6H 12 -N + -(CH3)2C6H 13 (Br - )2 template, named C 22-6-6 , in which the diammonium group directs the formation of the microporous structure of the MFI molecular sieve, and the long hydrophobic chain inhibits the growth of the MFI crystal along the b-axis, and an MFI crystal with a thickness of only 2 nm is synthesized. Roman-leshkov et al. reported a bifunctional structure-directing agent for the synthesis of the precursor MCM-22, and the hydrophobic segment is used to expand and separate the layers, resulting in MIT-1 nanosheets with good crystallinity. Xiao et al. successfully prepared FER nanosheet molecular sieves with a thickness of about 6 to 8 nm (3 to 5 unit cells) using N,N-diethyl-2,6-dimethylpyridine (DMP), and theoretical calculation results show that the structure of the nanosheet is more stable when it has 3 to 5 layers.

[0003] The use of specialized templates to synthesize nanosheet molecular sieves can yield nanosheets with a single-unit cell thickness, but the synthesis process is cumbersome, expensive, and challenging to scale up. Selecting low-cost templates and simplifying the synthesis process are key to achieving industrial production of nanosheet molecular sieves and are currently a focus of research.

[0004] Given this research background, this study used inexpensive and readily available raw materials to prepare nanosheet molecular sieves using an in situ assembly method. This method avoids the synthesis of special templates and allows the template raw materials to assemble in situ during the formation of the molecular sieve, guiding the formation of nanosheet molecular sieves. Summary of the Invention

[0005] The invention discloses a method for synthesizing highly dispersed ZSM-5 nanosheets through in-situ assembly. The synthesis method is simple and convenient, and the prepared ZSM-5 nanosheets have higher self-supporting property and stability.

[0006] The present invention discloses a novel method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets. The method uses tetraethyl orthosilicate (or sodium silicate) as a silicon source, aluminum sulfate octahydrate as an aluminum source, tetramethylhexanediamine, and C6-C24 bromoalkanes (n-hexane bromide, n-octane bromide, n-decane bromide, dodecane bromide, tetradecane bromide, hexadecane bromide, octadecane bromide, and docosane bromide) as template raw materials. The method utilizes a template-based in-situ assembly method to dynamically synthesize ZSM-5 nanosheet molecular sieves. The method has the advantages of a simple and easy synthesis process and low cost. The resulting ZSM-5 nanosheets have a multi-cell stacking structure, exhibiting higher self-support and stability than single-cell nanosheets produced using a special template method, and possess a higher external specific surface area and a meso-micro composite pore structure.

[0007] The typical synthesis process is as follows (Si / Al=50, crystallization temperature 150°C, crystallization time 120h as an example):

[0008] Accurately weigh 1.44g of sodium hydroxide, 2.00g of bromododecane, 0.99g of n-hexane bromide, and 1.03g of tetramethylhexanediamine and dissolve them in 30g of deionized water (Solution a). Accurately weigh 0.40g of aluminum sulfate octahydrate and dissolve it in 13.2g of deionized water. Then, add 1.06g of concentrated sulfuric acid dropwise to adjust the pH (Solution b). Slowly add Solution b dropwise to Solution a and stir to mix thoroughly. After cooling to room temperature, slowly add 9.5g of tetraethyl orthosilicate dropwise and continue stirring for 3 hours to obtain an initial gel. The gel was then transferred to a Teflon-lined stainless steel reactor and subjected to rotational crystallization at 150°C (40 rpm) for 120 hours. After the crystallization is completed, wait for it to cool naturally to room temperature, wash the precipitate until it is close to neutral, dry it at 100°C for 10 hours, and then place the obtained product in a muffle furnace, set the heating rate to 2°C / min, first heat it to 300°C, keep it warm for 2 hours to allow the template to fully decompose, then heat it to 570°C and calcine for 6 hours to completely remove the template to obtain ZSM-5 nanosheet molecular sieve.

[0009] Compared with the previous preparation method, the present invention has the following advantages:

[0010] 1) The present invention adopts a one-pot synthesis method, which is simple and easy to operate;

[0011] 2) The ZSM-5 nanosheet molecular sieve obtained by the present invention has a multi-cell stacking structure and has higher self-supporting property and stability; BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1is the X-ray powder diffraction pattern of the catalyst obtained in Example 1;

[0013] Figure 2 is a scanning electron microscope image of the catalyst obtained in Example 1;

[0014] Figure 3 TEM image of the catalyst obtained in Example 1;

[0015] Figure 4 is the X-ray powder diffraction pattern of the catalyst obtained in Example 2;

[0016] Figure 5 is a scanning electron microscope image of the catalyst obtained in Example 2;

[0017] Figure 6 is the X-ray powder diffraction pattern of the catalyst obtained in Example 3;

[0018] Figure 7 is a scanning electron microscope image of the catalyst obtained in Example 3; DETAILED DESCRIPTION

[0019] Example 1: 1.44g of sodium hydroxide, 2.00g of bromododecane, 0.99g of n-hexane bromide, and 1.03g of tetramethylhexanediamine were accurately weighed and dissolved in 30g of deionized water (Solution a). 0.40g of aluminum sulfate octahydrate was accurately weighed and dissolved in 13.2g of deionized water, followed by the dropwise addition of 1.06g of concentrated sulfuric acid to adjust the pH (Solution b). Solution b was slowly added dropwise to Solution a, stirred and mixed thoroughly, and then cooled to room temperature. 9.5g of tetraethyl orthosilicate was slowly added dropwise, followed by continued stirring for 3 hours to obtain an initial gel. The gel was then transferred to a polytetrafluoroethylene-lined stainless steel reactor, with a silicon-to-aluminum ratio of 50, and subjected to rotational crystallization at 180°C for 120 hours at 40 rpm. After crystallization, the product was cooled naturally to room temperature, washed until nearly neutral, and dried at 100°C for 10 hours. The resulting product was then placed in a muffle furnace and heated at a rate of 2°C / min to 300°C for 2 hours to fully decompose the template. The temperature was then raised to 570°C and calcined for 6 hours to completely remove the template. This yielded nanosheet ZSM-5 molecular sieve.

[0020] Example 2: 2.88g of sodium hydroxide, 3.00g of octadecane bromide, 1.22g of n-hexane bromide, and 1.81g of tetramethylhexanediamine were accurately weighed and dissolved in 40g of deionized water (Solution a). 0.55g of aluminum sulfate octahydrate was accurately weighed and dissolved in 15.2g of deionized water, followed by the dropwise addition of 3.06g of concentrated sulfuric acid to adjust the pH (Solution b). Solution b was slowly added dropwise to Solution a and stirred to mix thoroughly. The mixture was then cooled to room temperature, and 7.5g of sodium silicate was slowly added dropwise. Stirring was continued for 3 hours to obtain an initial gel. The gel was then transferred to a polytetrafluoroethylene-lined stainless steel reactor, with a silicon-to-aluminum ratio of 75, and subjected to rotational crystallization at 150°C for 48 hours at 60 rpm. After crystallization, the product was naturally cooled to room temperature, washed until it was close to neutral, and dried at 100°C for 10 hours. The product was then placed in a muffle furnace, and the heating rate was set to 2°C / min. The temperature was first raised to 400°C and kept warm for 2 hours to allow the template to fully decompose. The temperature was then raised to 600°C and calcined for 4 hours to completely remove the template to obtain nanosheet ZSM-5 molecular sieve.

[0021] Example 3: 1.64g of sodium hydroxide, 1.99g of n-hexyl bromide, and 1.03g of tetramethylhexanediamine were accurately weighed and dissolved in 25g of deionized water (Solution a). 0.40g of aluminum sulfate octahydrate was accurately weighed and dissolved in 10.2g of deionized water. 1.56g of concentrated sulfuric acid was then added dropwise to adjust the pH (Solution b). Solution b was slowly added dropwise to Solution a and stirred to mix thoroughly. The mixture was then cooled to room temperature, and 12.5g of tetraethyl orthosilicate was slowly added dropwise. Stirring was continued for 3 hours to obtain an initial gel. The gel was then transferred to a stainless steel reactor lined with polytetrafluoroethylene (PTFE) and subjected to rotational crystallization at 160°C for 240 hours at a speed of 30 rpm, with a silicon-to-aluminum ratio of 100. After crystallization, the product was cooled naturally to room temperature, washed until nearly neutral, and dried at 100°C for 10 hours. The resulting product was then placed in a muffle furnace and heated at a rate of 2°C / min to 400°C for 2 hours to fully decompose the template. The temperature was then raised to 550°C and calcined for 8 hours to completely remove the template. This resulted in nanosheet ZSM-5 molecular sieves.

Claims

1. A method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets, characterized by: Nanosheet ZSM-5 was synthesized using tetraethyl orthosilicate or sodium silicate, aluminum sulfate 18-hydrate, N,N,N,N-tetramethyl-1,6-hexanediamine, C6-C24 bromoalkane, sulfuric acid, sodium hydroxide, and deionized water as raw materials. The synthesis steps are as follows: (1) Accurately weigh sodium hydroxide, N,N,N,N-tetramethyl-1,6-hexanediamine, and C6-C24 bromoalkane, dissolve them in deionized water, and stir at 70°C for 1-2 hours; (2) Accurately weigh aluminum sulfate 18hydrate and sulfuric acid, dissolve in an appropriate amount of deionized water, and stir at room temperature until a homogeneous solution is obtained; (3) Mix the solutions obtained in (1) and (2) above evenly, add tetraethyl orthosilicate or sodium silicate while stirring, and stir at room temperature for 3 hours to allow it to be fully hydrolyzed; (4) transferring the mixed solution obtained in step (3) into a polytetrafluoroethylene-lined reactor and performing rotational crystallization at a certain temperature for a certain period of time; (5) After the crystallization is completed, the kettle is cooled and opened, and the sample washed to neutrality is placed in an oven at 100°C and dried overnight to obtain a white powder sample; (6) The white powder sample obtained in the above step is placed in a muffle furnace, calcined at a certain temperature to remove the template, and cooled to room temperature to obtain nanosheet ZSM-5 molecular sieve.

2. The method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets according to claim 1, characterized in that: The C6-C24 brominated alkane used in step (1) includes one or two of brominated n-hexane, brominated n-octane, brominated n-decane, brominated dodecane, brominated tetradecane, brominated hexadecane, brominated octadecane and brominated docosane.

3. The method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets according to claim 1, characterized in that: The silicon-aluminum ratio adjustment range in steps (2) and (3) is 30-200.

4. The method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets according to claim 1, characterized in that: In step (4), the rotational crystallization speed is 30-60 r / min, the crystallization temperature is 120-160° C., and the crystallization time is 48-120 h.

5. The method for in-situ assembly synthesis of highly dispersed ZSM-5 nanosheets according to claim 1, characterized in that: The calcination step in step (6) is to first increase the temperature to 250-400°C at 2°C / min, maintain for 2 hours, then increase the temperature to 500-700°C at 5°C / min, and calcine for 4-8 hours.

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