A delaminated mww sheet-like molecular sieve and method for making same

Abstract

The application discloses a kind of exfoliated MWW sheet molecular sieves and its preparation method.The method is fully dispersed in carboxylic acid aqueous solution MCM-22P, in turn, stirring, centrifugal washing, drying and calcination after, namely obtain;Carboxylic acid is calculated with carboxyl in the carboxylic acid aqueous solution, and its mole ratio with deionized water is 0.1~+∞;The mass-volume ratio of MCM-22P and formic acid aqueous solution is 0.5~1.5g:100ml.The method does not need additional swelling and ultrasonic operation steps, and is completed at room temperature, with the advantages of simple operation, mild conditions, etc., and the required material is easy to obtain and low in price, has potential for large-scale industrial production of related layered molecular sieve materials.The exfoliated molecular sieve prepared based on the method is mainly single-layer and few-layer structure with layer number less than or equal to 4, without using strong acid and ultrasonic process in the exfoliation process, and the obtained molecular sieve retains rich acid sites on the surface, which can effectively improve the catalytic activity of molecular sieve.

Description

Technical Field

[0001] This invention designs and synthesizes a layered molecular sieve material, specifically involving a peelable MWW-type layered molecular sieve and its preparation method, belonging to the technical fields of porous material synthesis and catalysis. Background Technology

[0002] Zeolite molecular sieves are a class of microporous aluminosilicates. Due to their excellent properties such as high thermal (hydrothermal) stability, diverse framework structures, and tunable acid-base properties, they are widely used as adsorbents and heterogeneous catalysts. Compared with traditional three-dimensional zeolites, two-dimensional zeolites have higher pore diffusion and transport characteristics and a larger external specific surface area, exposing more accessible acidic sites. Therefore, research on two-dimensional molecular sieves has attracted increasing attention.

[0003] MWW-type molecular sieves are among the most widely used two-dimensional zeolite molecular sieves in scientific research and practical applications. The pore structure of MWW-type molecular sieves is characterized by a cage-like structure (12-membered ring, 0.71 nm × 0.71 nm × 1.82 nm) with sinusoidal 10-membered ring channels connected between interlayer layers, as well as intralayer 10-membered ring channels. MCM-22P is a layered zeolite precursor of MWW, consisting of an ordered arrangement of two-dimensional layered zeolite formed by the sequential stacking of MWW-type nanomolecular sieve monolayers (2.5 nm) through hydrogen bonds formed by interlayer silanol groups.

[0004] Currently, methods for preparing two-dimensional zeolite molecular sieves can be mainly divided into in-situ hydrothermal synthesis of single-layer or multi-layer molecular sieves and exfoliation of layered zeolite precursors. To obtain two-dimensional layered MCM-22 molecular sieves, MCM-22P is swollen with an exfoliating agent solution (such as a surfactant like hexadecyltrimethylammonium hydroxide), followed by ultrasonic exfoliation under suitable pH conditions to prepare ITQ-2. This is the earliest reported exfoliated MWW molecular sieve. After calcination, this material exhibits a high external surface area (~700 m²). 2 / g). Compared to three-dimensional MCM-22 zeolite, ITQ-2 exhibits enhanced catalytic activity in various reactions, including catalytic cracking. However, conventional methods for preparing ITQ-2 require very harsh conditions (such as high pH and heating), which can damage the material and lead to amorphization. From an economic and practical application perspective, it is essential to develop a one-step synthesis method under mild conditions to produce zeolite MWW nanosheets with large external surfaces. Furthermore, strategies for achieving exfoliated MWW-type materials with similar properties to ITQ-2 reported in the literature are still scarce. Although several methods have been reported for preparing monolayer MWW-type materials, these methods require multi-step synthesis, use of surfactants for swelling or oxidation with Piranha solutions, heating, and ultrasonic treatment; these preparation routes are complex, polluting, or highly toxic. Therefore, designing a one-step synthesis method for exfoliating MWW-type zeolite is one of the urgent problems to be solved in this field. Summary of the Invention

[0005] To address the problems existing in the prior art, the first objective of this invention is to provide a stripped MWW sheet molecular sieve. This MWW sheet molecular sieve exposes abundant acidic sites on its outer surface. The main body of the molecular sieve is a single-layer or few-layer structure (≤4 layers). Since strong acid-base and ultrasonic sample synthesis methods are not used in the separation process, the resulting molecular sieve surface retains abundant acidic sites, which can effectively improve the catalytic activity of the molecular sieve.

[0006] The second objective of this invention is to provide a method for preparing exfoliated MWW layered molecular sieves. This method uses carboxylic acid as the exfoliating liquid, and the exfoliated MWW layered molecular sieves can be synthesized in one step by stirring and centrifuging at room temperature. This solves the problems of harsh synthesis conditions, highly destructive synthesis process and environmental unfriendliness of traditional exfoliated molecular sieves, and provides a new, simple and efficient synthesis method. This method is suitable for the industrial production of exfoliated MWW molecular sieves.

[0007] To achieve the above technical objectives, this invention provides a method for preparing a peelable MWW layered molecular sieve, wherein MCM-22P is fully dispersed in an aqueous carboxylic acid solution, and then subjected to stirring, centrifugation, washing and separation, drying and calcination in sequence to obtain the product;

[0008] In the aqueous solution of the carboxylic acid, the carboxylic acid is calculated as a carboxyl group, and its molar ratio to deionized water is 0.1 to +∞.

[0009] The mass-to-volume ratio of MCM-22P to formic acid aqueous solution is 0.5–1.5 g: 100 ml.

[0010] The concentration of carboxylic acid or the molar ratio of carboxylic acid to water directly affects the stripping effect of MCM-22P. The ideal stripping effect cannot be achieved if there is no carboxylic acid or the carboxylic acid content is too high.

[0011] The preparation process of MCM-22P used in this invention is as follows: raw materials including colloidal silica (30 wt.%), sodium hydroxide, sodium aluminate, and hexamethyleneimine are thoroughly mixed into a gel, and hydrothermal crystallization is carried out under stirring. The resulting crystalline product is then calcined to obtain the final product. The MCM-22P raw material used in this invention is self-prepared. If the synthesis parameters change during the preparation process, it may affect the yield of the exfoliated layered molecular sieve of this invention.

[0012] As a preferred embodiment, the mass ratio of colloidal silica, sodium hydroxide, sodium aluminate, hexamethyleneimine, and deionized water is 110–120:5–6:1.5–2:15–20:180–220. More preferably, the mass of the various raw materials required for the synthesis of MCM-22P is 114.89 g of colloidal silica (30 wt.%), 5.62 g of sodium hydroxide, 1.86 g of sodium aluminate, 18.92 g of hexamethyleneimine, and 200 g of deionized water.

[0013] As a preferred embodiment, the hydrothermal crystallization conditions are: a temperature of 130–150°C and a time of 90–100 hours. More preferably, the hydrothermal crystallization conditions are: a temperature of 143°C and a time of 96 hours.

[0014] As a preferred embodiment, the calcination conditions for the crystalline product are: calcination for 6-10 hours using a hot air stream at 500-600°C as the medium and a heating rate of 1-3°C / min. More preferably, the calcination conditions for the crystalline product are: calcination for 6 hours using a hot air stream at 550°C as the medium and a heating rate of 2°C / min.

[0015] As a preferred embodiment, the MCM-22P is fully dispersed in the carboxylic acid aqueous solution by stirring, under the following conditions: at room temperature, stirring at a speed of 100-600 rpm for 5 minutes to 24 hours. More preferably, the stirring conditions are: at room temperature, stirring at a speed of 500 rpm for 5 minutes to 24 hours.

[0016] As a preferred embodiment, the centrifugation conditions are: centrifugation at 4000-10000 rpm for 5-30 minutes; the washing method is centrifugal washing, the washing agent is deionized water, and the centrifugal washing process is repeated 1-3 times.

[0017] As a preferred embodiment, the drying method is vacuum drying, freeze drying, and oven drying; when the drying method is oven drying, the conditions are: drying at 20-120°C for 12-48 hours.

[0018] As a preferred embodiment, the calcination conditions are as follows: the product obtained after washing is heated to 500-600°C in a muffle furnace at a heating rate of 1-5°C / min, and calcined at a constant temperature for 6-10 hours.

[0019] The present invention also provides a peelable MWW layered molecular sieve, obtained by any of the preparation methods described above; the molecular sieve has 1 to 4 layers and a thickness of 2.5 to 10 nm.

[0020] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0021] 1) The MWW layered molecular sieve provided by the present invention has a rich exposed surface area. The main body of the molecular sieve is a single-layer or few-layer structure. Since strong acid and alkali and ultrasonic experimental procedures are not used in the separation process, the surface of the obtained molecular sieve retains a rich number of acidic sites, which can effectively improve the catalytic activity of the molecular sieve.

[0022] 2) In this invention, the carboxylic acid solution is not completely ionized, resulting in a weakly acidic environment. In a solution with a molar ratio of carboxylic acid to deionized water of 0.1 to +∞, the pH value ranges from -1.75 to 1.33, and the reaction temperature is room temperature, making the reaction conditions relatively mild.

[0023] 3) The preparation method provided by this invention uses carboxylic acid as the stripping liquid, and the stripped MWW layered molecular sieve can be synthesized in one step by stirring and centrifuging at room temperature. This helps to solve the problems of harsh synthesis conditions, strong destruction of molecular sieve structure, environmental unfriendliness and high cost in the traditional synthesis of layered molecular sieves. It provides a new, simple and efficient synthesis method that can be applied to the industrial production of stripped layered molecular sieves. Attached Figure Description

[0024] Figure 1 XRD patterns of exfoliated layered molecular sieves and the original MCM-22P molecular sieve obtained by treating carboxylic acid solutions of different concentrations prepared in Example 1 of the present invention.

[0025] Figure 2 The above is the nitrogen adsorption-desorption isotherm of the exfoliated sheet molecular sieve prepared in Example 1 of this invention.

[0026] Figure 3 TEM images of the exfoliated layered molecular sieves prepared in Examples 1 and 2;

[0027] Figure 4 The image shows the XRD pattern of the sample prepared in Example 3. Detailed Implementation

[0028] The specific implementation schemes of the present invention will be described in detail and clearly below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] The preparation process of MCM-22P used in this embodiment of the invention is as follows: 114.89g of colloidal silica (30wt.%), 5.62g of sodium hydroxide, 1.86g of sodium aluminate, 18.92g of hexamethyleneimine, and 200g of deionized water are mixed to form a uniform gel, which is then transferred to an autoclave and hydrothermally crystallized at 143°C with mechanical stirring for 96 hours. The resulting crystallized product is washed with deionized water and dried overnight in an oven at 60°C. The dried sample is MCM-22P.

[0030] Example 1

[0031] 1. Compare the effect of carboxylic acid concentration on the peeling effect of MCM-22P. Seven molar ratios of formic acid to deionized water were set up: 0:6, 0.5:5.5, 1:5, 2:4, 3:3, 4:2, and 6:0, labeled as experiments 1 to 7. In each experiment, 0.1g of MCM-22P was weighed, and the mass-to-volume ratio of MCM-22P to the formic acid aqueous solution was 1g:100ml.

[0032] 2. Stir the 7 samples at room temperature for 6 hours at a speed of 500 minutes / revolution.

[0033] 3. The mixture obtained in step 2 was centrifuged and washed at a speed of 5000 minutes / revolution. After three centrifugation washes, the solid precipitate was dried at 60°C for 36 hours.

[0034] 4. Place the dried solid product obtained in step 3 in a muffle furnace and heat it to 550°C at a heating rate of 2°C / min. Calcinate at this temperature for 6 hours to obtain the exfoliated MCM-22 molecular sieve.

[0035] The molecular sieves MCM-22P and the exfoliated molecular sieves were characterized by XRD and nitrogen adsorption, respectively, and the results are as follows: Figure 1 and Figure 2 As shown. From Figure 1 The XRD characterization results show that in this embodiment, the (002) diffraction peak disappears, and the characteristic diffraction peaks at (101) and (102) weaken. Meanwhile, the characteristic peaks of the MCM-22P molecular sieve at (101) and (102) show obvious peak splitting, indicating that the synthesized product in this example is indeed an exfoliated MWW sheet molecular sieve. Furthermore, in solutions of different proportions, except for sample 1 (without formic acid), which failed to exfoliate successfully, samples 2 to 7 were exfoliated to varying degrees, indicating that formic acid concentration is a key factor in sample exfoliation during the exfoliation process. Figure 2 The image shows the N2- adsorption-desorption isotherm for sample No. 5, with a specific surface area of ​​479.7 m². 2 / g.

[0036] Example 2

[0037] Take a sample with a formic acid:water ratio of 3:3 and sonicate it for 3 hours.

[0038] The samples were characterized using transmission electron microscopy (FEI Talos) and formic acid solution exfoliation, and the results are as follows: Figure 3 As shown. Figure 3 The results showed that the thickness of the molecular sieve after peeling with a formic acid:water 3:3 solution was 2.5-10 nm, which was not significantly different from the sample without ultrasonic treatment in Example 1. Therefore, it can be concluded that ultrasound has no significant effect on the delamination of MCM-22P.

[0039] Example 3

[0040] 1. Experiments were conducted to investigate the peeling effect of different solutions by replacing formic acid with acetic acid, methanol, ethanol, and acetaldehyde. The molar ratio of X:water = 1:1, where X represents acetic acid, methanol, ethanol, and acetaldehyde, was used. 0.1g of MCM-22P was weighed, and the mass-to-volume ratio of MCM-22P to the formic acid aqueous solution was 1g:100ml.

[0041] 2. Stir the four samples in a centrifuge tube at room temperature for 6 hours at a speed of 500 minutes per revolution.

[0042] 3. The mixture obtained in step 2 was centrifuged and washed at a speed of 5000 minutes / revolution. After three centrifugation washes, the solid precipitate was dried at 60°C for 36 hours.

[0043] 4. The dried solid product obtained in step 3 is placed in a muffle furnace and heated to 550°C at a heating rate of 2°C / min, and calcined at this temperature for 6 hours to obtain the exfoliated MCM-22 molecular sieve.

[0044] The molecular sieves MCM-22P and the exfoliated sieve were characterized by XRD, and the results are as follows: Figure 4 As shown in the figure, formic acid and acetic acid are effective in stripping MCM-22P, while methanol, ethanol and acetaldehyde do not play a stripping role, indicating that protons and hydrogen bond acceptors may work together in the stripping process.

Claims

1. A method for preparing a peelable MWW layered molecular sieve, characterized in that: First, MCM-22P is fully dispersed in an aqueous carboxylic acid solution, and then subjected to stirring, centrifugation, washing, drying, and calcination in sequence to obtain the product; In the aqueous solution of the carboxylic acid, the carboxylic acid is calculated as a carboxyl group, and its molar ratio to deionized water is 0.1 to +∞. The mass-to-volume ratio of MCM-22P to formic acid aqueous solution is 0.5~1.5g: 100ml.

2. The method for preparing a stripped MWW layered molecular sieve according to claim 1, characterized in that: The preparation process of MCM-22P is as follows: raw materials including colloidal silica, sodium hydroxide, sodium aluminate and hexamethyleneimine are fully mixed and hydrothermally crystallized under stirring. The resulting crystallized product is then calcined to obtain the final product.

3. The method for preparing a stripped MWW layered molecular sieve according to claim 2, characterized in that: The mass ratio of colloidal silica, sodium hydroxide, sodium aluminate, hexamethyleneimine, and deionized water is 110~120:5~6:1.5~2:15~20:180~220; the hydrothermal crystallization conditions are: temperature 130~150℃ and time 90~100h.

4. The method for preparing a stripped MWW layered molecular sieve according to claim 2, characterized in that: The calcination conditions for the crystalline product are as follows: using hot air at 500~600℃ as the medium, calcining at a heating rate of 1~3℃ / min for 6~10h.

5. The method for preparing a stripped MWW layered molecular sieve according to claim 1, characterized in that: The MCM-22P is fully dispersed in the carboxylic acid aqueous solution by stirring, under the following conditions: at room temperature, stirring at a speed of 100~600 rpm for 5 minutes to 24 hours.

6. The method for preparing a stripped MWW layered molecular sieve according to claim 1, characterized in that: The washing method is centrifugal washing, the detergent is deionized water, and the centrifugal washing process is repeated 1 to 3 times. The conditions are: centrifugation at 4000 to 10000 rpm for 5 to 30 minutes.

7. The method for preparing a stripped MWW layered molecular sieve according to claim 1, characterized in that: The drying method is vacuum drying, freeze drying, or oven drying; when the drying method is oven drying, the conditions are: drying at 20~120℃ for 12~48 hours.

8. The method for preparing a stripped MWW layered molecular sieve according to claim 1, characterized in that: The calcination conditions are as follows: the product obtained after washing is heated to 500-600℃ in a muffle furnace at a heating rate of 1-5℃ / min, and calcined at a constant temperature for 6-10 hours.

9. A peelable MWW layered molecular sieve, characterized in that: The molecular sieve is prepared by any one of claims 1 to 8; the molecular sieve has 1 to 4 layers and the sample crystal thickness is 2.5 to 10 nm.

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