A macroporous microporous fractionated molecular sieve and a preparation method thereof

Abstract

The present application belongs to the technical field of molecular sieve, and provides a preparation method of hierarchical microporous molecular sieve. The method comprises the following steps: mixing cetyltrimethylammonium bromide, tetraethyl orthosilicate, ammonia water and a solvent to obtain a white turbid solution, separating and calcining to obtain mesoporous silica beads; mixing the mesoporous silica beads, tetrapropylammonium hydroxide, tetraethylammonium hydroxide, an aluminum source, sodium hydroxide and ethanol, and aging to obtain a xerogel; grinding the xerogel and performing crystallization, and then calcining to obtain the hierarchical microporous molecular sieve. The hierarchical microporous molecular sieve has a size of 2-4 μm, is assembled from nanometer molecular sieve grains from bottom to top, has a hierarchical pore structure, and the hierarchical pore is a macropore and a micropore, wherein the size of the macropore is 100-700 nm, and the size of the micropore is 0.5-0.6 nm. The method for preparing the hierarchical microporous molecular sieve has the advantages of easy availability of raw materials, simple and easy-to-operate synthesis process, green and environmentally-friendly technology, no generation of organic waste liquid, high yield, regular crystal structure of the obtained hierarchical microporous molecular sieve, adjustable and controllable macropore channel, stable structure, a small amount of mesopore, and fast material transmission performance.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve technology, specifically relating to a macroporous and microporous hierarchical molecular sieve and its preparation method. Background Technology

[0002] Hierarchical structures are ubiquitous in natural systems (such as human blood vessels and alveoli; tree roots, stems, branches, and leaves), maximizing mass transport and reaction rates, thus leading to optimal utilization efficiency. In addition to micropores (<2 nm), hierarchical molecular sieves also possess secondary channels, such as mesopores (2–50 nm) and / or macropores (>50 nm), which facilitate rapid molecular diffusion and accessibility to catalytically active sites, making them a very promising catalytic and adsorption material.

[0003] Traditional methods for preparing hierarchical molecular sieve catalysts generally employ two strategies: top-down, involving dealumination and / or desilication; and bottom-up, using soft and hard templates. In these methods, the formation of hierarchical pores is primarily due to the removal or loss of the mesoscopic / macroscopic phases of the molecular sieve, including nano-gas / liquid / solid phases (e.g., bubbles, emulsions, and surfactants). Therefore, the traditional methods for preparing hierarchical molecular sieves inevitably involve cost issues: top-down methods result in material loss, while bottom-up methods require expensive templates or lead to low yields. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a macroporous / microporous hierarchical molecular sieve and its preparation method. The preparation method of this invention is simple, low-cost, and the conditions are easy to control, resulting in high product yield and molecular sieves with regular shapes and controllable pore structures.

[0005] To achieve the above objectives, the technical solution adopted in this method is as follows:

[0006] This invention provides a method for preparing macroporous and microporous hierarchical molecular sieves, comprising the following steps:

[0007] (1) Mix hexadecyltrimethylammonium bromide, ammonia, solvent, and tetraethyl orthosilicate until homogeneous to obtain a white turbid solution. Then separate the solid product and calcine it to obtain mesoporous silica microspheres.

[0008] (2) Mix and stir mesoporous silica microspheres, tetrapropylammonium hydroxide, tetraethylammonium hydroxide, aluminum source, sodium hydroxide and ethanol evenly, and then age to obtain dry gel;

[0009] (3) Grind the dry gel and crystallize it, then calcine it to obtain the macroporous and microporous graded molecular sieve.

[0010] As a preferred embodiment of the above technical solution, the size of the mesoporous silica microspheres in step (1) is 100-700 nm, and the mesopore size is 2-3 nm.

[0011] As a preferred embodiment of the above technical solution, in step (1), the mass ratio of hexadecyltrimethylammonium bromide to tetraethyl orthosilicate is (0.25-0.5):1, the concentration of hexadecyltrimethylammonium bromide in the solvent is 0.1wt%-2wt%, and the mass ratio of ammonia to solvent is (0.02-0.05):1.

[0012] As a preferred embodiment of the above technical solution, the solvent mentioned in step (1) includes one or more of water, N,N-dimethylformamide, and ethanol, and the size of the mesoporous silica microspheres can be controlled within the range of 100-700 nm by adjusting the solvent. Specifically, when the solvent used is water and N,N-dimethylformamide in a mass ratio of 1:(0.95-1), the size of the mesoporous silica microspheres is approximately 100 nm; when the solvent used is water, the size of the mesoporous silica microspheres is approximately 300 nm; and when the solvent used is water and ethanol in a mass ratio of (0.7-0.75):1, the size of the mesoporous silica microspheres is approximately 700 nm.

[0013] As a preferred embodiment of the above technical solution, in step (1), when preparing mesoporous silica microspheres with sizes of approximately 100 nm and 700 nm, the stirring time is 3–4 h and the stirring temperature is 20–30 °C; when preparing mesoporous silica microspheres with a size of approximately 300 nm, the stirring is first carried out at 50–55 °C for 1–2 h, and then at 20–30 °C for 2–3 h. During mixing, hexadecyltrimethylammonium bromide, ammonia, and solvent are first mixed evenly, and then tetraethyl orthosilicate is added and the mixture is stirred evenly.

[0014] As a preferred embodiment of the above technical solution, the molar ratio of mesoporous silica microspheres, tetrapropylammonium hydroxide, tetraethylammonium hydroxide, and sodium hydroxide in step (2) is 1:(0.08~0.12):(0.08~0.12):(0.06~0.1).

[0015] As a preferred embodiment of the above technical solution, the aluminum source in step (2) includes one or more of aluminum isopropoxide, aluminum nitrate or aluminum chloride; the molar ratio of the mesoporous silica microspheres to the aluminum source is (40-120):1, wherein the molar ratio of the aluminum source is based on the Al2O3 that can be theoretically generated.

[0016] As a preferred embodiment of the above technical solution, in step (2), the molar ratio of ethanol to mesoporous silica microspheres is (3-4):1.

[0017] Preferably, in step (2), the mixing temperature is 10-30°C and the mixing time is 4-6 hours.

[0018] Preferably, in step (2), the aging temperature is 10-30°C and the aging time is 20-24 hours.

[0019] As a preferred embodiment of the above technical solution, the crystallization method in step (3) is steam-assisted crystallization, and the molar ratio of water to mesoporous silica microspheres is (20-200):1.

[0020] As a preferred embodiment of the above technical solution, the crystallization temperature in step (3) is 160-180°C and the crystallization time is 2-4 days.

[0021] As a preferred embodiment of the above technical solution, the calcination temperature in steps (1) and (3) is 500-600℃, the calcination time is 6-8h, the calcination atmosphere is air, and the heating rate is 2-4℃ / min.

[0022] The macroporous and microporous hierarchical molecular sieve obtained by the above preparation method of the present invention is assembled from nano-molecular sieve crystals from bottom to top, with a size of 2 to 4 μm and a hierarchical pore structure. The hierarchical pores mainly include macropores and micropores, and also have a small amount of mesopores, wherein the macropore size is 100 to 700 nm and the micropore size is 0.5 to 0.6 nm.

[0023] The technical concept of this invention is as follows: The core of this invention is the introduction of dual-structure directing agents tetraethylammonium hydroxide and tetrapropylammonium hydroxide, along with mesoporous silica microspheres. The dual-structure directing agents have the following functions: 1) During the molecular sieve nucleation stage: Tetraethylammonium hydroxide accelerates the dissolution and condensation of silica microspheres, promoting molecular sieve nucleation; 2) During the molecular sieve crystallization growth stage: Tetrapropylammonium hydroxide guides the formation of the molecular sieve framework structure, while tetraethylammonium hydroxide stabilizes the initial nano-molecular sieve crystals and inhibits the fusion between crystal grains. The mesoporous silica microspheres have a dual function: firstly, they act as a silicon source; secondly, they act as a hard template to introduce macropores into the molecular sieve crystals. Because tetraethylammonium hydroxide inhibits the fusion between crystal grains, the nano-molecular sieve crystals assemble from bottom to top to form a hierarchical molecular sieve. As the reaction proceeds, the silicon source, i.e., the mesoporous silica microspheres occupying the sites, is gradually consumed in the crystals, forming macropores, ultimately forming a macroporous-microporous hierarchical molecular sieve.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The method for preparing macroporous and microporous hierarchical molecular sieves provided by this invention uses readily available raw materials and has a simple synthesis method. Compared with traditional hydrothermal synthesis methods, the process is green and environmentally friendly, does not generate organic waste liquid, has a high yield, and consumes almost all of the silicon source to form molecular sieve crystals. Furthermore, the prepared macroporous and microporous hierarchical molecular sieve crystals have a regular structure, adjustable and controllable macropores, stable structure, and a small number of mesopores, resulting in faster mass transport performance. Attached Figure Description

[0026] Figure 1 The image shows the XRD characterization of the macroporous and microporous hierarchical molecular sieve in Example 1.

[0027] Figure 2 The image shows the SEM characterization of the macroporous and microporous hierarchical molecular sieve in Example 1.

[0028] Figure 3 The image shows the BET characterization of the macroporous and microporous hierarchical molecular sieve in Example 1.

[0029] Figure 4 This is a characterization diagram of the pore size distribution of the macroporous and microporous graded molecular sieve in Example 1.

[0030] Figure 5 The image shows the XRD characterization of the macroporous and microporous graded molecular sieve in Example 2.

[0031] Figure 6 This is a SEM characterization image of the macroporous and microporous hierarchical molecular sieve in Example 2.

[0032] Figure 7 The image shows the BET characterization of the macroporous and microporous graded molecular sieve in Example 2.

[0033] Figure 8 This is a characterization diagram of the pore size distribution of the macroporous and microporous graded molecular sieve in Example 2.

[0034] Figure 9 The image shows the XRD characterization of the macroporous and microporous graded molecular sieve in Example 3.

[0035] Figure 10 This is a SEM characterization image of the macroporous and microporous hierarchical molecular sieve in Example 3.

[0036] Figure 11 The image shows the BET characterization of the macroporous and microporous hierarchical molecular sieve in Example 3.

[0037] Figure 12 This is a characterization diagram of the pore size distribution of the macroporous and microporous graded molecular sieve in Example 3.

[0038] Figure 13 The fluorescence spectrum of the macroporous and microporous hierarchical molecular sieve in Application Example 1 is shown.

[0039] Figure 14 The fluorescence spectrum of the commercial ZSM-5 molecular sieve in Application Example 1 is shown. Detailed Implementation

[0040] To better understand the present invention, the following detailed description of the invention is provided in conjunction with specific embodiments and accompanying drawings. However, the scope of the present invention is not limited to the embodiments described below.

[0041] In the following examples, unless otherwise specified, all reagents used are commercially available chemical reagents, and the concentration of ammonia water is 25% to 28%.

[0042] Example 1

[0043] A macroporous and microporous hierarchical molecular sieve is prepared by the following steps:

[0044] 1) Synthesis of 100nm mesoporous silica microspheres: 2.4g of hexadecyltrimethylammonium bromide, 57g of water, 57g of N,N-dimethylformamide, and 3g of ammonia were added sequentially and stirred until homogeneous. After the hexadecyltrimethylammonium bromide was completely dissolved, 5.58g of tetraethyl orthosilicate was added, and stirring continued for 3 hours to obtain a white turbid solution. The resulting product was then filtered, washed, dried, and calcined at 550℃ for 6 hours to obtain mesoporous silica microspheres with a size of approximately 100nm.

[0045] 2) Synthesis of macroporous and microporous hierarchical molecular sieves: 1 g (0.017 mol) of 100 nm mesoporous silica microspheres, 0.85 g of tetrapropylammonium hydroxide aqueous solution (concentration 40%, effective amount 0.0017 mol), 0.95 g of tetraethylammonium hydroxide aqueous solution (concentration 25%, effective amount 0.0016 mol), 0.05 g (0.00125 mol) of sodium hydroxide, 0.07 g (0.00034 mol) of aluminum isopropoxide and 2.5 g (0.054 mol) of ethanol were mixed and stirred at room temperature for 4 h, and then aged at room temperature for 20 h to obtain a dry gel.

[0046] 3) Synthesis of macroporous and microporous hierarchical molecular sieves: The dry gel obtained in 2) was ground to obtain a white powder. 0.2g of the powder was placed in a crucible and transferred to a reaction vessel. 3ml of water was added to the bottom of a 100ml reaction vessel, ensuring that the water and the powder in the crucible did not come into contact. The reaction vessel was then sealed and placed in an oven at 180℃ for 3 days. After the reaction was completed, the powder was filtered, washed, and dried. The obtained powder was then placed in a muffle furnace and calcined at 550℃ for 6 hours to obtain macroporous and microporous hierarchical molecular sieves.

[0047] The macroporous and microporous hierarchical molecular sieve obtained in Example 1 was characterized by XRD, as follows: Figure 1 As shown, the diffraction peaks of a typical MFI-type molecular sieve can be observed, indicating that the microporous crystal material ZSM-5 molecular sieve has been successfully synthesized, with a micropore diameter in the range of 0.5–0.6 nm.

[0048] The SEM characterization image of the macroporous and microporous hierarchical molecular sieve in this embodiment is as follows: Figure 2As shown, the prepared macroporous and microporous hierarchical molecular sieves are assembled from nano-molecular sieve crystals from bottom to top to form a hierarchical structure. The nano-molecular sieve crystals are 20-50 nm in size, and the macroporous and microporous hierarchical molecular sieve particles are uniform in size, 2-4 μm in size, and have macropores of 80-250 nm on their surface.

[0049] The BET characterization diagram of the macroporous and microporous hierarchical molecular sieve in this embodiment is as follows: Figure 3 As shown, nitrogen adsorption in the low-pressure region is inherent to the microporous adsorption of MFI-type molecular sieves, while the curve in the high-pressure region exhibits a hysteresis loop, indicating that mesoporous structures also exist in the macroporous-microporous hierarchical molecular sieve. The pore size distribution characterization diagram of the macroporous-microporous hierarchical molecular sieve in this embodiment is shown below. Figure 4 As shown, there is a peak distribution in the range of 0–10 nm, which proves that there are micropores and mesopores in the macroporous-microporous hierarchical molecular sieve.

[0050] Example 2

[0051] A macroporous and microporous hierarchical molecular sieve is prepared by the following steps:

[0052] 1) Synthesis of 300nm mesoporous silica microspheres: 0.56g hexadecyltrimethylammonium bromide, 442g water, and 21.6g ammonia were added sequentially and stirred at 50℃ for 1h. After the hexadecyltrimethylammonium bromide was completely dissolved, 2.6g tetraethyl orthosilicate was added, and stirring continued at room temperature for 3h to obtain a white turbid solution. The resulting product was then filtered, washed, dried, and calcined at 550℃ for 6h to obtain mesoporous silica microspheres with a size of approximately 300nm.

[0053] 2) Synthesis of macroporous and microporous hierarchical molecular sieves: 1 g (0.017 mol) of 300 nm mesoporous silica microspheres, 0.85 g of tetrapropylammonium hydroxide aqueous solution (concentration 40%, effective amount 0.0017 mol), 0.95 g of tetraethylammonium hydroxide aqueous solution (concentration 25%, effective amount 0.0016 mol), 0.05 g (0.00125 mol) of sodium hydroxide, 0.07 g (0.00034 mol) of aluminum isopropoxide and 2.5 g (0.054 mol) of ethanol were mixed and stirred at room temperature for 4 h, and then aged at room temperature for 20 h to obtain a dry gel.

[0054] 3) Synthesis of macroporous and microporous hierarchical molecular sieves: The dry gel obtained in 2) was ground to obtain a white powder. 0.2g of the powder was placed in a crucible and transferred to a reaction vessel. 3ml of water was added to the bottom of a 100ml reaction vessel, ensuring that the water and the powder in the crucible did not come into contact. The reaction vessel was then sealed and placed in an oven at 180℃ for 3 days. After the reaction was completed, the powder was filtered, washed, and dried. The obtained powder was then placed in a muffle furnace and calcined at 550℃ for 6 hours to obtain macroporous and microporous hierarchical molecular sieves.

[0055] The XRD characterization pattern of the macroporous and microporous hierarchical molecular sieve obtained in Example 2 is shown below. Figure 5 As shown, the diffraction peaks of a typical MFI-type molecular sieve can be observed, indicating that the MFI-type molecular sieve has been successfully synthesized.

[0056] The SEM characterization image of the macroporous and microporous hierarchical molecular sieve in this embodiment is as follows: Figure 6 As shown, the prepared macroporous and microporous hierarchical molecular sieves are assembled from nano-molecular sieve crystals from bottom to top to form a hierarchical structure. The nano-molecular sieve crystals are 50-80 nm in size, and the macroporous and microporous hierarchical molecular sieve particles are uniform in size, 2-3 μm in size, and have macropores of 100-300 nm on their surface.

[0057] The BET characterization diagram of the macroporous and microporous hierarchical molecular sieve in this embodiment is as follows: Figure 7 As shown, the hysteresis loop of the nitrogen adsorption-desorption curve is smaller than that of Example 1.

[0058] The pore size distribution characterization diagram of the macroporous and microporous hierarchical molecular sieve in this embodiment is shown in the figure below. Figure 8 As shown, there is a peak distribution in the range of 0–10 nm, which proves that there are micropores and mesopores in the macroporous-microporous hierarchical molecular sieve.

[0059] Example 3

[0060] A macroporous and microporous hierarchical molecular sieve is prepared by the following steps:

[0061] 1) Synthesis of 700nm mesoporous silica microspheres: 1g hexadecyltrimethylammonium bromide, 276g water, 380g ethanol, and 24g ammonia were added sequentially and stirred until homogeneous. After the hexadecyltrimethylammonium bromide was completely dissolved, 2.8g tetraethyl orthosilicate was added, and stirring was continued for 3 hours to obtain a white turbid solution. The obtained product was then filtered, washed, dried, and calcined at 550℃ for 6 hours to obtain mesoporous silica microspheres with a size of approximately 700nm.

[0062] 2) Synthesis of macroporous and microporous hierarchical molecular sieves: 1 g (0.017 mol) of 700 nm mesoporous silica microspheres, 0.85 g of tetrapropylammonium hydroxide aqueous solution (concentration 40%, effective amount 0.0017 mol), 0.95 g of tetraethylammonium hydroxide aqueous solution (concentration 25%, effective amount 0.0017 mol), 0.05 g (0.00125 mol) of sodium hydroxide, 0.07 g (0.00034 mol) of aluminum isopropoxide and 2.5 g (0.054 mol) of ethanol were mixed and stirred at room temperature for 4 h, and then aged at room temperature for 20 h to obtain a dry gel.

[0063] 3) Synthesis of macroporous and microporous hierarchical molecular sieves: The dry gel obtained in 2) was ground to obtain a white powder. 0.2g of the powder was placed in a crucible and transferred to a reaction vessel. 3ml of water was added to the bottom of a 100ml reaction vessel, ensuring that the water and the powder in the crucible did not come into contact. The reaction vessel was then sealed and placed in an oven at 180℃ for 3 days. After the reaction was completed, the powder was filtered, washed, and dried. The obtained powder was then placed in a muffle furnace and calcined at 550℃ for 6 hours to obtain macroporous and microporous hierarchical molecular sieves.

[0064] The macroporous and microporous hierarchical molecular sieve obtained in Example 3 was characterized by XRD, such as... Figure 9 As shown, the diffraction peaks of a typical MFI-type molecular sieve can be observed, indicating that the MFI-type molecular sieve has been successfully synthesized.

[0065] The SEM characterization image of the macroporous and microporous hierarchical molecular sieve in this embodiment is as follows: Figure 10 As shown, the prepared macroporous and microporous hierarchical molecular sieves are assembled from nano-molecular sieve crystals from bottom to top to form a hierarchical structure. The nano-molecular sieve crystals are 20-30 nm in size, and the macroporous and microporous hierarchical molecular sieve particles are uniform with a size of 3-5 μm and macropores of 400-700 nm on the surface.

[0066] The BET characterization diagram of the macroporous and microporous hierarchical molecular sieve in this embodiment is as follows: Figure 11 As shown, the nitrogen adsorption-desorption curve is similar to that of Example 2, exhibiting a hysteresis loop.

[0067] The pore size distribution characterization diagram of the macroporous and microporous hierarchical molecular sieve in this embodiment is shown in the figure below. Figure 12 As shown, there is a peak distribution in the range of 0–10 nm, which proves that there are micropores and mesopores in the macroporous-microporous hierarchical molecular sieve.

[0068] Application examples

[0069] The 0.5 g macroporous and microporous hierarchical molecular sieve prepared in Example 3 was stirred with 20 ml of 1 mol / L ammonium nitrate solution at 80 °C for 4 h, then filtered and dried, and then calcined at 550 °C for 6 h. This process was repeated 3 times to obtain the hydrogen-form macroporous and microporous hierarchical molecular sieve.

[0070] The hydrogen-type macroporous and microporous hierarchical molecular sieves prepared using this application example and the commercial ZSM-5 molecular sieve were used in the furfural condensation reaction at room temperature, and fluorescence micrographs were obtained using a fluorescence microscope. Figure 13 As shown, compared to commercial ZSM-5 molecular sieves ( Figure 14 The macroporous and microporous hierarchical molecular sieves exhibit strong fluorescence signals and extremely high reactivity, indicating that the hydrogen-type macroporous and microporous hierarchical molecular sieves prepared in this application example have good mass transport performance.

[0071] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for preparing a macroporous / microporous hierarchical molecular sieve, characterized in that, Includes the following steps: (1) Mix hexadecyltrimethylammonium bromide, tetraethyl orthosilicate, ammonia and solvent to obtain a white turbid solution. Then separate the solid product and calcine it to obtain mesoporous silica microspheres with a size of 100-700 nm and a mesopore size of 2-3 nm. (2) Mesoporous silica microspheres, tetrapropylammonium hydroxide, tetraethylammonium hydroxide, aluminum source, sodium hydroxide and ethanol are mixed and aged to obtain a dry gel; wherein the molar ratio between mesoporous silica microspheres, tetrapropylammonium hydroxide, tetraethylammonium hydroxide and sodium hydroxide is 1:(0.08~0.12):(0.08~0.12):(0.06~0.1); (3) The dry gel is ground and crystallized by steam-assisted crystallization, and then calcined to obtain the macroporous and microporous hierarchical molecular sieve. The macroporous and microporous hierarchical molecular sieve has a size of 2-5 μm and a hierarchical pore structure; the hierarchical pores include macropores and micropores, wherein the macropore size is 100-700 nm and the micropore size is 0.5-0.6 nm.

2. The preparation method according to claim 1, characterized in that, The solvent mentioned in step (1) is one or more of water, N,N-dimethylformamide and ethanol.

3. The preparation method according to claim 1, characterized in that, The aluminum source in step (2) is one or more of aluminum isopropoxide, aluminum nitrate or aluminum chloride; the molar ratio of the mesoporous silica microspheres to the aluminum source is (40-120):1, wherein the molar ratio of the aluminum source is based on the Al2O3 that can be theoretically generated.

4. The preparation method according to claim 1, characterized in that, The aging temperature in step (2) is 10-30℃ and the aging time is 16-24h.

5. The preparation method according to claim 1, characterized in that, When crystallizing in step (3) using steam-assisted crystallization, the ratio of water to dry gel powder is (5-60):

1.

6. The preparation method according to claim 1, characterized in that, The crystallization temperature in step (3) is 160-180℃, and the crystallization time is 2-4 days.

7. The preparation method according to claim 1, characterized in that, The calcination temperature in steps (1) and (3) is 500-600℃, the calcination time is 6-8h, the calcination atmosphere is air, and the heating rate is 2-4℃ / min.

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