A method for preparing hierarchical pore MFI molecular sieve nanosheets by semi-solid phase silicon precursor conversion

Abstract

The application discloses a method for preparing hierarchical MFI molecular sieve nanosheets by semi-solid silicon precursor conversion. The method comprises the following steps: grinding and mixing solid silica gel, alkali metal hydroxide and a water solution of tetrapropylammonium hydroxide, and then performing closed heating treatment to obtain a semi-solid silicon precursor with a large number of primary structure units; and mixing the silicon precursor with an amino acid water solution and then directly performing hydrothermal crystallization, so that the silicon precursor is orderly and rapidly oriented and assembled under the assistance of the amino acid, and finally, the MFI molecular sieve nanosheets with uniform particle size, good monodispersity, high yield and micro-mesopore hierarchical structure are obtained. The MFI molecular sieve nanosheets prepared by the method have excellent micro-mesopore hierarchical structure, and the mesopore structure can be adjusted. The method has the advantages of rapid reaction, low cost, simple operation, environmental friendliness and suitability for large-scale industrial production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of molecular sieve synthesis, and particularly relates to a method for preparing a hierarchical pore MFI molecular sieve nanosheet by converting a semi-solid phase silicon precursor. BACKGROUND

[0002] Molecular sieve materials have excellent hydrothermal stability, easily controllable acid-base properties, and unique ordered pore structures, and are a kind of porous material with excellent high-temperature resistance and performance; they have a wide and important application in the field of heterogeneous catalysis, adsorption and membrane separation. Among them, the MFI topological structure molecular sieve has three-dimensional ten-membered ring cross-pore and full-range adjustable silicon aluminum ratio (Si / Al), and plays an irreplaceable role in the field of catalysis such as methanol to gasoline, methanol to olefin (MTO), alkane dehydrogenation to olefin, aromatization, toluene disproportionation, and in the field of membrane separation such as organic solvent dehydration, alcohol permeation and isomer separation. However, in industrial applications, the traditional molecular sieve is greatly limited by the single and narrow micropore channel of the guest molecule in the channel, resulting in low catalytic efficiency and low membrane layer flux.

[0003] The method for improving the internal diffusion efficiency of the MFI molecular sieve includes two types, one is to construct a molecular sieve with a hierarchical pore structure, and the other is to prepare a nanometer-sized molecular sieve to reduce the diffusion resistance. For example, the MFI molecular sieve nanosheet with crystal face selectivity, the relatively thin crystal thickness in the b-axis direction corresponds to the relatively short straight pore, which ensures the selectivity and helps to shorten the diffusion path of the gas therein. It can be seen that the new hierarchical pore MFI molecular sieve nanosheet can integrate the intrinsic characteristics of the molecular sieve nanosheet and the excellent diffusion performance brought by the mesoporous structure, which shows great potential in the preparation of high-efficiency catalyst materials and oriented molecular sieve thin films.

[0004] In the current research, the preparation method of the hierarchical pore molecular sieve can be divided into two categories: one is the top-down post-processing method, and the other is the bottom-up hydrothermal synthesis method. Among them, the former is to etch part of the framework of the traditional molecular sieve by using acid or alkali to produce mesoporous structure; the method of the latter mostly depends on the template or the crystallization regulator to regulate the growth kinetics path of the molecular sieve crystallization, and then a hierarchical pore molecular sieve is prepared. Similarly, the preparation method of the molecular sieve nanosheet is usually divided into the "top-down" stripping-purification method and the "bottom-up" direct synthesis method. The top-down strategy usually needs to synthesize a layered zeolite precursor in advance, and then further stripping is carried out by ultrasonic post-processing method, and finally the ultra-thin two-dimensional zeolite nanosheet is obtained. In addition, researchers use the "bottom-up" strategy, that is, by designing a special structure of organic template as a structure directing agent for the crystallization of the molecular sieve nanosheet or using fluorine-containing inorganic acid / salt as a mineralizer to prepare a molecular sieve nanosheet with adjustable thickness.

[0005] In summary, most of the reported strategies for the synthesis of hierarchical porous molecular sieves and nanosheet-like molecular sieves require the use of expensive organic templates or environmentally harmful additives, or involve complex exfoliation and purification processes, resulting in complex and tedious preparation processes, long time consumption, low yield, structure collapse and other problems, which seriously limit their practical industrial application. Further, it is more challenging to develop a new synthesis technology for hierarchical porous MFI molecular sieve nanosheets that can face industrialization. However, it is very urgent to design a simple and effective method for directly synthesizing hierarchical porous MFI molecular sieve nanosheets in one step for the development of high-efficiency catalyst materials and oriented molecular sieve thin films and other functional materials for the industrial application of molecular sieves. SUMMARY

[0006] The present application aims to provide a kind of hierarchical porous MFI molecular sieve nanosheet and its preparation method, the method provided by the present application is simple in operation, short in period, high in yield, environmentally friendly, low in cost.

[0007] In order to achieve the above-mentioned application purposes, the present application provides the following technical solutions:

[0008] The present application provides a kind of hierarchical porous sheet MFI molecular sieve preparation method, comprising the following steps:

[0009] (1) solid silica gel, alkali hydroxide and 20-40wt% tetrapropylammonium hydroxide solution are ground and mixed, then closed heating treatment is carried out, to obtain semi-solid silicon precursor;The amount of substance of the solid silica gel is calculated as SiO2, and the components in the mixture satisfy: the molar ratio of solid silica gel, alkali hydroxide, tetrapropylammonium hydroxide and water is 1:(0.05-0.2):(0.1-0.2):(3.0-6.0);The temperature of closed heating treatment is 30-60 DEG C;

[0010] (2) the silicon precursor is mixed with an aqueous solution of amino acid, and then hydrothermal crystallization is carried out, and after solid-liquid separation, drying and calcination, hierarchical porous MFI molecular sieve nanosheet is obtained.

[0011] Preferably, in step (1), the solid silica gel is a mixture of one or both of nanoscale fumed silica and micron-sized silica powder, and the alkali hydroxide is one or more of sodium hydroxide and potassium hydroxide.

[0012] Preferably, the amount of substance of the solid silica gel is calculated as SiO2, and the components in the mixture satisfy: the molar ratio of solid silica gel, alkali hydroxide, tetrapropylammonium hydroxide and water is 1:(0.10-0.15):(0.12-0.15):(4.0-5.0);The temperature of closed heating treatment is 40-50 DEG C.

[0013] Preferably, in step (1), the grinding time is 0.1-10 min; and the time for the closed heating treatment is 6-48 hours.

[0014] Preferably, in step (2), the amino acid comprises one or more of glycine, alanine, valine, leucine, isoleucine, phenylalanine, proline, tryptophan, tyrosine, cysteine, methionine, glutamic acid, threonine, arginine, histidine, aspartic acid, lysine and serine.

[0015] Preferably, in step (2), the amount of substance of the silicon precursor is calculated based on SiO2, and the components in the mixed solution satisfy the molar ratio of the silicon precursor, the amino acid and water being 1:(0.3-2):(15-300). More preferably, the components in the mixed solution satisfy the molar ratio of the silicon precursor, the amino acid and water being 1:(0.5-1):(50-200).

[0016] Preferably, in step (2), the hydrothermal crystallization is performed at a temperature of 90-180℃ for 2-96 hours. More preferably, in step (2), the hydrothermal crystallization is performed at a temperature of 140-160℃ for 24-72 hours.

[0017] The present application also provides a hierarchical MFI molecular sieve nanosheet prepared by the above method, which has a length of 0.3-3.2 μm, a width of 0.15-0.9 μm and a thickness of 30-200 nm.

[0018] Compared with the prior art, the present application has the following advantages:

[0019] 1. The present application adopts a low-temperature semi-solid phase system to prepare a silicon precursor, which can efficiently promote the dissolution of the silicon source and rapidly rearrange the silicon precursor to have a large number of primary structural units, thereby reducing energy consumption, accelerating the reaction, reducing production cost and being environmentally friendly.

[0020] 2. The silicon precursor is orderly and directionally assembled with the aid of the amino acid, and the nanosheet-shaped MFI molecular sieve with uniform particle size, high yield (~100%) and micro-mesopore hierarchical pore structure can be obtained in a short time, and the molecular sieve nanosheet has adjustable mesopore structure.

[0021] 3、The straight pore direction of the nanosheet molecular sieve prepared by the method is perpendicular to the largest crystal face, and the internal part has abundant mesopores, so that the diffusion path of the guest molecules in the pore is shorter and the diffusion resistance is smaller; and the length in the b-axis direction can be regulated between 30-200nm, so that the molecular sieve can catalyze the reactions of various molecules of different sizes, and the oriented molecular sieve film with high flux can be prepared. Therefore, the MFI molecular sieve nanosheet prepared by the method provided by the application can be used as a high-performance functional material in the field of catalysis and separation, and has a wide application prospect.

[0022] 4、The preparation method provided by the application uses cheap and easily available solid silica gel and amino acid as raw materials, does not need to use expensive tetraethyl orthosilicate as a silicon source, and does not need to add additional expensive organic template agents, so that the raw material cost is reduced; and no fluorine-containing raw materials are needed, and the environment is friendly.

[0023] In summary, the MFI molecular sieve nanosheet prepared by the method provided by the application has excellent micro-mesoporous hierarchical pore structure, and can be used as a high-efficiency catalyst material and an oriented molecular sieve film and other functional materials; the method provided by the application is low in cost, simple in operation, environmentally friendly, and suitable for large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The XRD spectrum of the hierarchical-pore MFI molecular sieve nanosheet obtained in Example 1 is shown in the figure;

[0025] Figure 2 The high-magnification SEM image of the hierarchical-pore MFI molecular sieve nanosheet obtained in Example 1 is shown in the figure;

[0026] Figure 3 The high-magnification TEM image of the hierarchical-pore nanosheet-shaped MFI molecular sieve prepared in Example 1 is shown in the figure;

[0027] Figure 4 The N2 adsorption / desorption isotherm graph of the hierarchical-pore nanosheet-shaped MFI molecular sieve prepared in Example 1 is shown in the figure;

[0028] Figure 5 The SEM image of the MFI molecular sieve prepared in Comparative Example 1 is shown in the figure;

[0029] Figure 6 The SEM image of the MFI molecular sieve prepared in Comparative Example 2 is shown in the figure;

[0030] Figure 7 The SEM image of the MFI molecular sieve prepared in Comparative Example 3 is shown in the figure;

[0031] Figure 8 The XRD spectrum of the sample prepared in Comparative Example 4 is shown in the figure;

[0032] Figure 9 The SEM image of the MFI molecular sieve prepared in Comparative Example 5 is shown in the figure;

[0033] Figure 10 This is the XRD spectrum of the MFI molecular sieve prepared in Comparative Example 6;

[0034] Figure 11 This is the SEM image of the MFI molecular sieve prepared in Comparative Example 6;

[0035] Figure 12 This is the XRD spectrum of the MFI molecular sieve prepared in Comparative Example 7;

[0036] Figure 13 This is the SEM image of the MFI molecular sieve prepared in Comparative Example 7. DETAILED DESCRIPTION

[0037] To better illustrate the present invention's method for preparing hierarchical MFI molecular sieve nanosheets by converting a semi-solid-phase silicon precursor, several examples of precursor preparation and synthesis of hierarchical MFI molecular sieve nanosheets are provided below. Obviously, the examples described are only a subset of the present invention, and are not exhaustive. All other examples derived by persons of ordinary skill in the art based on the examples herein without inventive effort are considered within the scope of protection of the present invention.

[0038] Example 1

[0039] (1) Preparation of semi-solid silicon precursor

[0040] Tetrapropylammonium hydroxide was dissolved in water to prepare a 25 wt% tetrapropylammonium hydroxide aqueous solution (TPAOH aqueous solution). 1.0 g of micronized silica powder, 0.087 g of sodium hydroxide, and 1.898 g of TPAOH aqueous solution were mixed and ground for 5 minutes. The mixture was then transferred to a 20 mL hydrothermal reactor and reacted at 50°C for 12 hours. The mixture maintained a molar ratio of SiO₂:NaOH:TPAOH:H₂O of 1:0.13:0.14:4.75. After the reaction, the hydrothermal reactor was removed from the reactor and immersed in cold water to cool to room temperature to produce a silicon precursor.

[0041] (2) Preparation of multi-level pore MFI molecular sieve nanosheets

[0042] 1.22g of lysine was dispersed in 13.58g of water and stirred for 5 minutes. Next, 3g of a silicon precursor was dispersed in the amino acid aqueous solution and stirred thoroughly to obtain a mixed solution. The mixture was transferred to a reactor and crystallized at 160°C for 72 hours. The crystallized product was washed by centrifugation, dried, and calcined at 550°C for 6 hours to obtain hierarchical MFI molecular sieve nanosheets.

[0043] Figure 1 The XRD spectrum of the multi-level porous nano-sheet MFI molecular sieve prepared in Example 1 is shown in FIG. Figure 1 It can be seen that the product prepared in Example 1 is a pure phase MFI molecular sieve.

[0044] Figure 2 This is the SEM image of the multi-level porous nano-sheet MFI molecular sieve prepared in Example 1, with a scale of 1 μm. Figure 2 It can be seen that the multi-level porous nano-platelet MFI molecular sieve crystals prepared in Example 1 have a length × width × thickness = 1.0 μm × 0.3 μm × 70 nm, and are nano-platelet MFI molecular sieve crystals in the short b-axis direction with high crystal face selectivity. The nanosheets have good uniformity and are highly monodisperse.

[0045] Figure 3 This is a high-magnification TEM image of the multi-level porous nano-sheet MFI molecular sieve prepared in Example 1, with a scale of 50 nm. Figure 3 It can be seen that the multi-level porous nano-sheet MFI molecular sieve prepared in Example 1 has abundant mesoporous channels inside.

[0046] Figure 4 The N2 adsorption / desorption isotherm of the multi-level porous nano-sheet MFI molecular sieve prepared in Example 1 is shown in FIG. Figure 4 It can be seen that the multi-level porous nano-sheet MFI molecular sieve prepared in Example 1 has a very high micropore surface area and a very rich mesopore volume, and the BET specific surface area is 432m 2 g -1 , the micropore volume is 0.17 cm 3 g -1 , the mesopore volume is 0.17 cm 3 g -1 It can be found that, unlike the stacked mesopores produced by molecular sieve nanoparticles, the mesopores of the nanosheets prepared by the present invention are embedded inside the nanosheets. However, the mesoporous structure does not cause the molecular sieve structure to collapse, and the nanosheets still maintain high crystallinity.

[0047] Example 2

[0048] A multi-level porous nano-platelet MFI molecular sieve was prepared according to the method of Example 1, except that 2.44 g of lysine was added in step (2). The multi-level porous nano-platelet MFI molecular sieve crystals prepared in Example 2 had a length × width × thickness = 0.9 μm × 0.3 μm × 60-70 nm.

[0049] Example 3

[0050] The multi-level porous nano-sheet MFI molecular sieve was prepared according to the method of Example 1, the only difference from Example 1 being that glycine was used instead of lysine in step (2), and the amount of glycine added was 1.251 g.

[0051] The length x width x thickness of the hierarchical nanoplatelet MFI molecular sieve crystals prepared in Example 3 is 2.5 pm x 0.4 pm x 100 nm.

[0052] Example 4

[0053] The hierarchical nanoplatelet MFI molecular sieve was prepared according to the method of Example 1, with the only difference from Example 1 being that L-glutamine was used instead of lysine in step (2), and the amount of L-glutamine added was 1.218 g.

[0054] The length x width x thickness of the hierarchical nanoplatelet MFI molecular sieve crystals prepared in Example 4 is 2.6 pm x 0.4 pm x 100 nm.

[0055] Example 5

[0056] The hierarchical nanoplatelet MFI molecular sieve was prepared according to the method of Example 1, with the only difference from Example 1 being that the reaction conditions in step (1) were 30°C for 48 hours.

[0057] The length x width x thickness of the hierarchical nanoplatelet MFI molecular sieve crystals prepared in Example 5 is 2.1 pm x 0.6 pm x 100 nm.

[0058] Comparative Example 1

[0059] The MFI molecular sieve was prepared according to the method of Example 1, with the only difference from Example 1 being that 3.08 g of H2O was additionally added in step (1), so that the mixture solution met the molar ratio of SiO2:NaOH:TPAOH:H2O = 1:0.13:0.14:15.

[0060] Figure 5 The SEM image of the MFI molecular sieve prepared in Comparative Example 1, with a scale of 50 pm; it can be seen from Figure 5 that the MFI molecular sieve prepared in Comparative Example 1 is a disordered large block-shaped molecular sieve crystal, and does not have a nanoplatelet hierarchical pore structure. The semi-solid precursor is crucial for the preparation of a nanoplatelet, and the water content in step (1) needs to be strictly controlled. According to the conventional hydrothermal system formula, if the water content is too high, then the semi-solid precursor cannot be obtained, and the molecular sieve nanoplatelet cannot be prepared.

[0061] Comparative Example 2

[0062] The MFI molecular sieve was prepared according to the method of Example 1, with the only difference from Example 1 being that no sodium hydroxide was added in step (1).

[0063] Figure 6 The SEM image of the MFI molecular sieve prepared in Comparative Example 2, with a scale of 20 pm; it can be seen from Figure 6It can be seen that the MFI molecular sieve prepared in Comparative Example 2 is a disordered large block molecular sieve crystal, and does not have a nanosheet-like hierarchical pore structure. Without adding alkali metal hydroxide in step (1), the solid silicon source cannot be fully dissolved, so that the molecular sieve nanosheet cannot be prepared.

[0064] Comparative Example 3

[0065] The MFI molecular sieve was prepared according to the method of Example 1, and the only difference from Example 1 was that 0.087 g of sodium hydroxide was additionally added in step (2) without adding sodium hydroxide in step (1).

[0066] Figure 7 The SEM image of the MFI molecular sieve prepared in Comparative Example 3 is shown, wherein the scale is 20 μm; it can be seen from Figure 7 It can be seen that the MFI molecular sieve prepared in Comparative Example 3 is an uneven plate brick-shaped molecular sieve crystal, and does not have a nanosheet-like hierarchical pore structure. The order of adding alkali metal hydroxide is crucial. Without adding alkali metal hydroxide in step (1), the solid silicon source cannot be fully dissolved, so that the molecular sieve nanosheet cannot be prepared.

[0067] Comparative Example 4

[0068] The MFI molecular sieve was prepared according to the method of Example 1, and the only difference from Example 1 was that tetraethyl orthosilicate was used as the silicon source in step (1).

[0069] Figure 8 The XRD spectrum of the sample prepared in Comparative Example 4 is shown, wherein Figure 8 It can be seen that the product prepared in Comparative Example 4 is amorphous material, and does not have an MFI molecular sieve structure. Tetraethyl orthosilicate commonly used for preparing nanosheets is not suitable for the method for preparing hierarchical pore molecular sieve nanosheets from a semi-solid precursor according to the application.

[0070] Comparative Example 5

[0071] The MFI molecular sieve was prepared according to the method of Example 1, and the only difference from Example 1 was that the closed heating treatment temperature in step (1) was 70°C.

[0072] Figure 9 The SEM image of the MFI molecular sieve prepared in Comparative Example 5 is shown, wherein the scale is 1 μm; it can be seen from Figure 9 It can be seen that the MFI molecular sieve prepared in Comparative Example 5 is a nanosheet, and does not have a nanosheet-like hierarchical pore structure. The closed heating treatment temperature also affects the preparation of the molecular sieve nanosheet.

[0073] Comparative Example 6

[0074] The MFI molecular sieve was prepared according to the method of Example 1, and the only difference from Example 1 was that the raw material was directly subjected to step (2) without closed heating treatment after grinding in step (1).

[0075] Figure 10 The XRD spectrum of the MFI molecular sieve prepared in Comparative Example 6 is shown in FIG. Figure 10 It can be seen that the product prepared in Comparative Example 6 is a pure phase MFI molecular sieve.

[0076] Figure 11 This is the SEM image of the MFI molecular sieve prepared in Comparative Example 6, where the scale is 50 μm; Figure 11 It can be seen that the MFI molecular sieve prepared in Comparative Example 6 is a disordered large block molecular sieve crystal, and does not have a nano-sheet multi-level pore structure.

[0077] Comparative Example 7

[0078] The MFI molecular sieve was prepared according to the method of Example 1, the only difference from Example 1 being that no amino acid was added in step (2).

[0079] Figure 12 The XRD spectrum of the MFI molecular sieve prepared in Comparative Example 7 is shown in FIG. Figure 12 It can be seen that the product prepared in Comparative Example 7 is a pure phase MFI molecular sieve.

[0080] Figure 13 The SEM image of the MFI molecular sieve prepared in Comparative Example 7, where the scale is 100 μm; Figure 13 It can be seen that the MFI molecular sieve prepared in Comparative Example 7 is a disordered block-like molecular sieve crystal and does not have a nano-sheet-like multi-level pore structure.

[0081] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for preparing multi-level pore MFI molecular sieve nanosheets by converting a semi-solid phase silicon precursor, characterized in that: The following steps are involved: (1) After grinding and mixing solid silica gel, alkali metal hydroxide and 20-40 wt% tetrapropylammonium hydroxide aqueous solution, a closed heating treatment is performed to obtain a semi-solid silicon precursor; the solid silica gel is one or two of nano-scale fumed silica and micron-scale silica powder; the amount of the solid silica gel is calculated as SiO2, and the components in the mixture meet the following requirements: the molar ratio of solid silica gel, alkali metal hydroxide, tetrapropylammonium hydroxide and water is 1: (0.05-0.2): (0.1-0.2): (3.0-6.0); the temperature of the closed heating treatment is 30-60°C; (2) The silicon precursor is mixed with an amino acid aqueous solution and then hydrothermally crystallized. After solid-liquid separation, the mixture is dried and calcined to obtain hierarchical pore MFI molecular sieve nanosheets; the amount of the silicon precursor is calculated as SiO2, and the molar ratio of the silicon precursor, amino acid and water is 1: (0.3-2): (15-300).

2. The method according to claim 1, wherein In step (1), the alkali metal hydroxide is one or more of sodium hydroxide and potassium hydroxide.

3. The method according to claim 1, wherein In step (1), the grinding time is 0.1 to 10 min.

4. The method according to claim 1, wherein In step (1), the closed heat treatment time is 6 to 48 hours.

5. The method according to claim 1, wherein In step (2), the amino acids include one or more of glycine, alanine, valine, leucine, isoleucine, phenylalanine, proline, tryptophan, tyrosine, cysteine, methionine, glutamic acid, threonine, arginine, histidine, aspartic acid, lysine and serine.

6. The method according to claim 1, wherein In step (2), the hydrothermal crystallization temperature is 90 to 180° C., and the time is 2 to 96 hours.

7. A multi-level pore MFI molecular sieve nanosheet prepared by the method according to any one of claims 1 to 6, having a length of 0.3 to 3.2 μm, a width of 0.15 to 0.9 μm, and a thickness of 30 to 200 nm.

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