All-silicon MFI molecular sieve with mesoporous-microporous layered structure and preparation method of all-silicon MFI molecular sieve

Abstract

The invention belongs to the technical field of molecular sieve materials, and particularly relates to an all-silicon MFI molecular sieve with a mesoporous-microporous layered structure and a preparation method of the all-silicon MFI molecular sieve. The preparation method comprises the following steps: S1, mixing a silicon source, a template agent and water, and stirring to form an initial gel mixture; s2, adding a structure-directing agent into the initial gel mixture, and uniformly stirring to obtain synthetic gel; the structure-directing agent is selected from at least one of (4-sulfophenyl) porphyrin tetrasodium dodecahydrate, tetra (4-carboxyphenyl) porphyrin and tetra (4-trimethoxysilyl phenyl) porphyrin; s3, the synthetic gel is subjected to a hydrothermal crystallization reaction, the reaction temperature ranges from 130 DEG C to 160 DEG C, and the reaction time ranges from 24 hours to 96 hours; and S4, carrying out solid-liquid separation, washing, drying and roasting on the crystallized product to obtain the target product. The target product has the characteristics of stable structure, good mesoporous connectivity, high specific surface area and the like.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve materials technology, and particularly relates to an all-silica MFI molecular sieve with a mesoporous-microporous layered structure and its preparation method. Background Technology

[0002] All-silica MFI molecular sieves (Silicalite-1, or S-1 for short) have significant applications in catalysis and adsorption separation due to their regular pore structure and excellent chemical and thermal stability, showing broad market prospects. This material exhibits excellent catalytic performance in reactions such as methanol-to-propylene and the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, attracting attention from domestic and international industries. For example, Sinopec and Sumitomo Chemical of Japan have conducted related industrial demonstration studies. Furthermore, the shape-selective catalytic properties of S-1 are expected to expand into more niche application areas in fine chemical processes such as the shape-selective catalytic transformation of small molecules.

[0003] However, S-1 prepared by traditional methods is usually plate-shaped or ellipsoidal, and its specific surface area is mostly limited to 300-400 m². 2 Within the range of / g, its relatively small specific surface area and long diffusion path limit its catalytic efficiency and industrial application. Therefore, research on layered S-1 has gradually emerged. Currently, the preparation methods of layered S-1 mainly include two categories: one is the post-treatment exfoliation method, which involves secondary processing of dense S-1 through alkali treatment or organic acid exfoliation; the other is the use of a single long-chain template agent (such as C...). 16 H 33 Long-chain quaternary ammonium salts such as N(CH3)3OH can be directly synthesized. However, the layered S-1 structure prepared by long-chain template agents has poor stability and is prone to collapse, and the resulting mesopores are isolated and have poor connectivity; while the post-processing method is cumbersome and requires secondary processing, which increases the complexity and cost of the process.

[0004] Therefore, there is an urgent need to develop a structurally stable, well-connected, mesoporous layered MFI-type all-silica molecular sieve to overcome the limitations of existing materials and promote its wider application. Summary of the Invention

[0005] The first aspect of this invention aims to overcome the shortcomings of the prior art by providing an all-silica MFI molecular sieve with a stable structure, good mesoporous connectivity, and high specific surface area, and a meso-microporous layered structure.

[0006] A second aspect of the present invention provides a method for preparing the above-mentioned molecular sieve, which is simple in process and requires no post-processing steps.

[0007] To solve the above-mentioned technical problems, the present invention is implemented as follows: In a first aspect, the present invention provides an all-silica MFI molecular sieve with a mesoporous-microporous layered structure, having an MFI topology, a lamellar morphology, an average lamellar thickness of 15–20 nm, and a BET specific surface area of ​​450–650 m². 2 / g, total pore volume is 0.30~0.45cm³ 3 / g.

[0008] Secondly, the present invention provides a method for preparing the above-mentioned all-silica MFI molecular sieve having a mesoporous layered structure, comprising the following steps: S1. Mix the silicon source, template agent and water, and stir to form an initial gel mixture; S2. Add a structure-directing agent to the initial gel mixture described in step S1, stir until homogeneous, and obtain a synthetic gel; the structure-directing agent is selected from at least one of (4-sulfophenyl)porphyrin tetrasodium salt dodecahydrate, tetra(4-carboxyphenyl)porphyrin, and tetra(4-trimethoxysilylphenyl)porphyrin. S3. The synthesized gel described in step S2 is subjected to a hydrothermal crystallization reaction at a temperature of 130–160°C for 24–96 hours. S4. After crystallization in step S3, the product is subjected to solid-liquid separation, washing, drying and calcination to obtain the target product, an all-silica MFI molecular sieve with a mesoporous layered structure.

[0009] Preferably, in step S1, the silicon source is at least one of tetraethyl orthosilicate, tetramethyl orthosilicate, tetrapropyl orthosilicate, and tetrabutyl orthosilicate.

[0010] Preferably, in step S1, the template agent is an aqueous solution of tetrapropylammonium hydroxide.

[0011] Preferably, in step S3, the temperature of the hydrothermal crystallization reaction is 140–150°C, and the time is 48–72 hours.

[0012] Preferably, in step S2, the amount of the structure-directing agent added is 0.001 to 0.008, based on the molar amount of SiO2 in the synthetic gel being 1.

[0013] Preferably, in step S1, the amount of the template agent is calculated based on a molar amount of SiO2 in the synthetic gel of 1, and the amount of TPA in the template agent is... + The molar amount is 0.25 to 0.4.

[0014] Preferably, in step S1, the total amount of water used is calculated based on the molar amount of SiO2 in the synthesized gel being 1, and the molar amount of H2O being 20 to 25.

[0015] Preferably, between step S1 and step S2, the process further includes: subjecting the initial gel mixture to alcohol removal treatment at 60–80°C, followed by replenishment of water.

[0016] Thirdly, the present invention provides the application of the above-mentioned all-silica MFI molecular sieve with mesoporous layered structure as a catalyst or adsorbent.

[0017] Compared with the prior art, the present invention has the following technical effects: 1. This invention introduces porphyrin derivatives as structure directing agents into the synthesis system. By utilizing their large π-conjugated structure and N-heterocyclic sites, they form hydrogen bonds and coordination with silicon species, thereby breaking the traditional three-dimensional MFI framework of S-1 and constructing a layered structure with uniform thickness, which significantly improves the specific surface area of ​​the molecular sieve.

[0018] 2. After calcination, the residual nitrogen atoms of porphyrin molecules can be anchored in the S-1 framework, exposing more surface active sites. At the same time, the thermal decomposition of porphyrin can generate well-connected mesoporous structures between layers, effectively alleviating the diffusion limitation caused by the single micropore in traditional S-1 and significantly improving the mass transfer efficiency of reactants and products.

[0019] 3. The mesopores formed by the pyrolysis of porphyrins significantly contribute to the total pore volume, endowing the material with superior adsorption properties and further expanding its application potential in the fields of catalysis and separation. Attached Figure Description

[0020] Figure 1 The XRD pattern of sample S-1-A; Figure 2 The XRD pattern of sample S-1-B; Figure 3 For the low angle (2-5) of the S-1-B sample o XRD pattern; Figure 4 Scanning electron microscope image of sample S-1-A; Figure 5 Scanning electron microscope image of sample S-1-B; Figure 6 This is a scanning electron microscope image of sample S-1-F. Detailed Implementation

[0021] The present invention will now be described in detail through specific embodiments. These embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art. As used throughout the specification and claims, the terms "comprising" or "including" are open-ended and are interpreted as "comprising but not limited to". The following description is a preferred embodiment for carrying out the invention; however, this description is intended to illustrate the general principles of the specification and is not intended to limit the scope of the invention. The scope of protection of the invention is determined by the appended claims. Unless otherwise specified, all reagents and materials used in the present invention are commercially available.

[0022] Comparative Example 1 46.2 g of tetraethyl orthosilicate was added to a beaker and stirred. Then, 44 g of 25 wt% TPAOH aqueous solution and 38 g of water were added. The mixture was hydrolyzed at 30 °C for 3 h to obtain a silicon source hydrolysate. The hydrolysate was then removed from the alcohol at 80 °C for 1.5 h, and 50.2 g of water was added and stirred for 30 min. The resulting transparent solution was placed in a crystallization vessel and crystallized at 170 °C for 48 h. The crystallized product was washed, dried, and then calcined at 550 °C for 6 h to obtain a traditional nanosphere-type S-1 sample, denoted as S-1-A.

[0023] The XRD diffraction results of the sample are as follows Figure 2 As shown, the characteristic diffraction peaks of MFI zeolite (2θ=7.8) o 8.8 o 23.1 o 23.9 o The presence of both peaks indicates a complete structure without impurities. The peak-to-height ratio (I7.8 / I8.8) shows that the two peaks are of equal intensity, indicating no obvious growth bias in the crystals. This sample is a conventional nanoscale S-1.

[0024] The results of scanning electron microscopy are from Figure 4 As can be seen, the particles are elliptical spheres, uniform in size, and approximately 200 nm in size. Nitrogen physisorption characterization analysis showed that the BET specific surface area of ​​sample S-1-A was 360.7 m². 2 / g, pore volume 0.28m 3 / g, of which the micropore volume is 0.15m 3 / g.

[0025] Example 1 46.2 g of tetraethyl orthosilicate was added to a beaker and stirred. Then, 44 g of 25 wt% TPAOH aqueous solution and 23 g of water were added. The mixture was hydrolyzed at 30 °C for 3 h to obtain a silicon source hydrolysate. 15 g of water and 1.50 g of (4-sulfophenyl)porphyrin tetrasodium salt dodecahydrate were stirred until homogeneous to obtain a porphyrin aqueous solution. The porphyrin aqueous solution was added dropwise to the silicon source hydrolysate and stirred for 30 min. The mixture was then subjected to alcohol removal at 80 °C for 1.5 h, followed by the addition of 50.2 g of water and stirring for 30 min. The resulting transparent solution was placed in a crystallization vessel and crystallized at 140 °C for 48 h. The crystallized product was separated, washed, dried, and then calcined at 550 °C for 6 h to obtain S-1-B.

[0026] The XRD diffraction results of the sample are as follows Figure 2 As shown, the characteristic diffraction peaks of MFI zeolite (2θ=7.8) o 8.8 o 23.1 o 23.9 o The presence of all peaks indicates a complete structure without impurities. The peak height ratio (I7.8 / I8.8) shows that the (020) crystal plane peak is significantly stronger than the (101) crystal plane peak, indicating a strong growth orientation. This sample is an S-1 with a thin-plate structure. Under the strong induction of porphyrin, silicon species stack along the plate-like direction, forming a layered stacked morphology. The XRD results of the sample at low angles of 2–5° are as follows: Figure 3 As shown, a sharp, single peak is visible with a stable baseline and no obvious extraneous peaks, indicating a high degree of order in the layered structure of the product. The results of the scanning electron microscope are derived from... Figure 5 As can be seen, the sample has a layered structure with a layer thickness of 15–20 nm. Nitrogen physisorption characterization analysis showed that the BET specific surface area of ​​sample S-1-B was 597.4 m². 2 / g, pore volume 0.43m 3 / g, of which the micropore volume is 0.15m 3 / g, mesoporous pore volume is 0.28m 3 / g.

[0027] Example 2 Repeat Example 1, replacing (4-sulfophenyl)porphyrin tetrasodium salt dodecahydrate with the same amount of tetra(4-carboxyphenyl)porphyrin or tetra(4-trimethoxysilylphenyl)porphyrin, and placing the resulting transparent liquid into a crystallization vessel for crystallization at 140°C for 48 h. After separation, washing, and drying, the crystallized products were calcined at 550°C for 6 h to obtain S-1-C and S-1-D, respectively.

[0028] The XRD diffraction results and scanning electron microscopy results of the sample are consistent with Figure 2 and Figure 5 same.

[0029] Example 3 Repeat Example 1, crystallize at 150°C for 48 h, and after separation, washing and drying, calcine at 550°C for 6 h to obtain S-1-E.

[0030] The XRD diffraction results and scanning electron microscopy results of the sample are consistent with Figure 2 and Figure 5 same.

[0031] Example 4 Repeat Example 1, but change the crystallization time to 72h. After separation, washing and drying, the crystallized product is calcined at 550℃ for 6h to obtain S-1-E.

[0032] The XRD diffraction results and scanning electron microscopy results of the sample are consistent with Figure 2 and Figure 5 same.

[0033] Example 5 Repeat Example 1, crystallize at 170°C for 48 hours, and then calcine the crystallized product at 550°C for 6 hours after separation, washing and drying to obtain S-1-F.

[0034] The SEM images of the sample show a plate-like morphology. It can be seen that when the synthesis temperature is increased to 170℃, the hydrolysis and condensation rate of silicon species is significantly accelerated. At high temperature, the thermal motion of porphyrin is intensified, the hydrogen bonding / coordination with silicon species is weakened, the guiding ability for layered growth is reduced, and the three-dimensional skeleton formation cannot be suppressed, so the morphology of the product is biased towards the traditional S-1 morphology.

[0035] The results above show that at crystallization temperatures below 150℃, porphyrins have strong structure-directing ability and can form a uniform layered structure, resulting in an all-silica MFI-type layered S-1 molecular sieve.

[0036] Example 6 The adsorption performance of the obtained molecular sieve samples was evaluated under the following conditions.

[0037] 1.0 g of 20-40 mesh adsorbent was loaded into the fixed-bed constant-temperature zone, followed by the addition of 800 cm⁻¹ adsorbent. 3 / m 3 Acetone feed gas (with air as the balance gas) is introduced into the fixed-bed reactor at a rate of 100 ml / min. The adsorption temperature is 35°C. o C, Mass airspeed (WHSV) is The adsorption capacity, breakthrough capacity, and breakthrough time of acetone are shown in the table below.

[0038] Adsorbent Saturated adsorption capacity (mg / g) Penetration capacity (mg / g) Penetration time min S-1-A 143 132 293 S-1-B 270 256 553 S-1-C 268 253 550 S-1-D 271 255 552 S-1-E 269 254 551 S-1-F 268 251 552 The data above show that the traditional S-1 molecular sieve has a much lower adsorption capacity, breakthrough capacity, and breakthrough time than the S-1 molecular sieve obtained by adding a structure-directing agent. This is because the addition of porphyrin-based structure-directing agents greatly increases the specific surface area and pore volume of S-1.

[0039] It is understood that the above specific description of the present invention is only for illustrating the present invention and is not limited to the technical solutions described in the embodiments of the present invention. Those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention to achieve the same technical effect; as long as the use needs are met, they are all within the protection scope of the present invention.

Claims

1. A fully silica MFI molecular sieve with a mesoporous-microporous layered structure, characterized in that, It has an MFI topology with a lamellar morphology, an average lamellar thickness of 15–20 nm, and a BET specific surface area of ​​450–650 m². 2 / g, total pore volume is 0.30~0.45cm³ 3 / g.

2. A method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure as described in claim 1, characterized in that, Includes the following steps: S1. Mix the silicon source, template agent and water, and stir to form an initial gel mixture; S2. Add a structure-directing agent to the initial gel mixture described in step S1, stir until homogeneous, and obtain a synthetic gel; the structure-directing agent is selected from at least one of (4-sulfophenyl)porphyrin tetrasodium salt dodecahydrate, tetra(4-carboxyphenyl)porphyrin, and tetra(4-trimethoxysilylphenyl)porphyrin. S3. The synthesized gel described in step S2 is subjected to a hydrothermal crystallization reaction at a temperature of 130–160°C for 24–96 hours. S4. After crystallization in step S3, the product is subjected to solid-liquid separation, washing, drying and calcination to obtain the target product, an all-silica MFI molecular sieve with a mesoporous layered structure.

3. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to claim 2, characterized in that, In step S1, the silicon source is at least one of tetraethyl orthosilicate, tetramethyl orthosilicate, tetrapropyl orthosilicate, and tetrabutyl orthosilicate.

4. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to claim 2, characterized in that, In step S1, the template agent is an aqueous solution of tetrapropylammonium hydroxide.

5. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to claim 2, characterized in that, In step S3, the temperature of the hydrothermal crystallization reaction is 140–150°C, and the time is 48–72 hours.

6. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to claim 2, characterized in that, In step S2, the amount of the structure-directing agent added is 0.001 to 0.008, based on the molar amount of SiO2 in the synthetic gel being 1.

7. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to any one of claims 2 to 6, characterized in that, In step S1, the amount of the template agent is calculated based on a molar amount of SiO2 in the synthesized gel of 1, and the amount of TPA in the template agent is... + The molar amount is 0.25 to 0.

4.

8. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to claim 7, characterized in that, In step S1, the total amount of water used is calculated based on the molar amount of SiO2 in the synthesized gel being 1, and the molar amount of H2O being 20 to 25.

9. The method for preparing the all-silica MFI molecular sieve with a mesoporous layered structure according to claim 2, characterized in that, Between step S1 and step S2, there is an additional step: the initial gel mixture is subjected to alcohol removal treatment at 60-80°C, and then water is added.

10. A catalyst or adsorbent, characterized in that, It includes the layered all-silica MFI molecular sieve as described in claim 1.

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