Synthesis method of hierarchical pore molecular sieve nanosheet

Abstract

The invention discloses a synthesis method of a hierarchical pore molecular sieve nanosheet, and belongs to the technical field of molecular sieve synthesis. Comprising the following steps: mixing a tetrapropylammonium hydroxide solution, tetraethyl orthosilicate and water, stirring for pre-hydrolysis, standing for aging, then adding a nanocellulose aqueous dispersion, carrying out ultrasonic treatment, then adding urea, water and a metal source, stirring, and finally crystallizing at 150-200 DEG C for 12-30 hours, after crystallization is finished, carrying out solid-liquid separation on a product, and washing, drying and roasting a solid product to obtain the hierarchical pore molecular sieve nanosheet. According to the invention, nano-crystalline cellulose and urea are compounded and introduced into a molecular sieve synthesis system for the first time, and controllable growth of flaky morphology and integrated construction of intracrystalline mesopores are realized in situ under a fluoride-free condition. The method is green and environment-friendly, raw materials are cheap, the process is simple, and the method has extremely high industrial application value.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve synthesis technology, and in particular to a method for synthesizing hierarchical porous molecular sieve nanosheets. Background Technology

[0002] ZSM-5 molecular sieves are widely used in petroleum refining and fine chemical industries due to their unique MFI-type cross-channel structure, excellent hydrothermal stability, tunable surface acidity, and good shape-selective catalytic performance, such as catalytic cracking, isomerization, alkylation, and methanol-to-hydrocarbon processes. However, the channels of traditional ZSM-5 molecular sieves are only micropores (pore size <0.6 nm), and the long diffusion path severely limits the mass transfer efficiency of reactant and product molecules, easily leading to carbon deposition and deactivation, and significantly shortening the catalyst's lifespan. To solve the above diffusion limitation problem, domestic and foreign scholars have mainly carried out research in two directions: one is to introduce mesopores into microporous crystals through post-treatment or template methods to construct "hierarchical" molecular sieves; the other is to synthesize nanosheet-like molecular sieves with reduced thickness along the b-axis to shorten the diffusion distance. Patent CN117142484A discloses a method for synthesizing hierarchical porous ZSM-5 molecular sieve nanosheets with controllable b-axis thickness. This method uses tetrabutylammonium hydroxide as the main template agent and ammonium fluoride as the mineralizing agent, preparing hierarchical porous nanosheets by crystallization at 170℃ for 60 hours. However, ammonium fluoride is highly corrosive and biotoxic, requiring high equipment corrosion resistance, and the treatment cost of fluorine-containing wastewater is enormous, failing to meet the requirements of green chemical development. Patent CN112624145A discloses a method for synthesizing MFI molecular sieve nanosheets. This method adjusts the pH of the system to 7.0-9.5 by adding acidic reagents such as sulfuric acid and hydrochloric acid, achieving the synthesis of sheet-like MFI molecular sieves without adding fluorine-containing reagents. However, this method mainly relies on pH to control crystal orientation, limiting its ability to construct mesoporous structures. The resulting product is mainly composed of pure micropores, lacking effective intracrystalline mesoporous mass transfer channels, and its diffusion advantage in macromolecular catalytic reactions is not fully utilized.

[0003] Therefore, how to simultaneously achieve the controllable growth of ZSM-5 molecular sieves with plate-like morphology and efficient construction of intracrystalline mesopores without using fluorine-containing reagents, while maintaining a simple synthesis process and low raw material costs, remains a technical bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0004] The purpose of this invention is to solve the above-mentioned problems in the prior art and provide a method for synthesizing multi-level porous molecular sieve nanosheets, using nanocellulose as a mesoporous template agent and urea as a morphology control agent, to synthesize ZSM-5 molecular sieve nanosheets with intracrystalline mesoporous structure in situ in a fluorine-free hydrothermal system.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for synthesizing hierarchical porous molecular sieve nanosheets includes the following steps:

[0007] 1) Mix tetrapropylammonium hydroxide solution, tetraethyl orthosilicate and water, stir to pre-hydrolyze, and then let stand to age to obtain mixed solution A;

[0008] 2) Add nanocellulose aqueous dispersion to the mixed solution A, mix, and then sonicate to obtain mixed solution B;

[0009] 3) Add urea and water, or add urea, metal source and water to the mixed solution B, stir, and obtain the initial gel mixture C;

[0010] 4) The initial gel mixture C is placed in a hydrothermal crystallization vessel and crystallized at 150~200℃ for 12~30h;

[0011] 5) After crystallization, the product is separated into solid and liquid phases. The solid product is washed, dried and calcined to obtain the multi-level porous molecular sieve nanosheets.

[0012] In step 1), the concentration of the tetrapropylammonium hydroxide solution is 25wt%~40wt%; in the mixed solution A, the molar ratio of tetraethyl orthosilicate, tetrapropylammonium hydroxide and water is 1:0.4~0.8:20~30.

[0013] In step 1), the pre-hydrolysis time is 10-20 hours, the temperature is 40-90°C, and the settling and aging time is 2-4 hours.

[0014] In step 2), the concentration of the nanocellulose aqueous dispersion is 5~20wt%, and the mass ratio of the amount of nanocellulose added to the mass of tetraethyl orthosilicate in step 1) is 0.02~0.04:1 on a dry basis.

[0015] In step 3), the mass ratio of the amount of urea added to the mass of tetraethyl orthosilicate in step 1) is 0.2~0.5:1.

[0016] In step 3), the metal source is selected from at least one of aluminum source and titanium source; the aluminum source is one or more of aluminum isopropoxide, aluminum sulfate, sodium aluminate or aluminum nitrate nonahydrate, and the titanium source is one or more of tetrabutyl titanate or titanium oxysulfate.

[0017] When only an aluminum source is added in step 3), the product is ZSM-5 molecular sieve nanosheets; when only a titanium source is added in step 3), the product is TS-1 molecular sieve nanosheets; when no metal source is added in step 3), the product is all-silica Silicalite-1 molecular sieve nanosheets.

[0018] In step 4), the crystallization temperature is 170~180℃ and the crystallization time is 12~24h.

[0019] In step 5), the roasting temperature is 500~600℃ and the roasting time is 2~6h.

[0020] A hierarchical porous molecular sieve nanosheet was prepared using the method described above.

[0021] Compared with the prior art, the beneficial effects achieved by the technical solution of this invention are:

[0022] 1. This invention proposes for the first time a dual-functional adjuvant system of "nanocellulose-urea" to achieve integrated in-situ synthesis of plate-like morphology and intracrystalline mesopores under fluorine-free conditions. This invention creatively combines nanocellulose as a mesoporous hard template and urea as a crystal morphology guiding agent in the ZSM-5 hydrothermal synthesis system. Nanocellulose, rich in hydroxyl groups, forms hydrogen bonds with silicon and aluminum species, leaving uniformly distributed intracrystalline mesopores in situ after calcination. Urea slowly decomposes under alkaline hydrothermal conditions, releasing ammonia gas, adjusting local supersaturation, and inducing restricted crystal growth along the b-axis, forming a plate-like morphology. Through the synergistic effect of nanocellulose and urea, this invention, for the first time, simultaneously solves the two major challenges of plate-like oriented growth and mesopore formation under fluoride-free conditions.

[0023] 2. Green and environmentally friendly, completely eliminating fluoride pollution. Compared with fluorine-containing synthesis processes represented by CN117142484A, this invention does not use toxic mineralizing agents such as ammonium fluoride at all. The reaction system is mild and non-corrosive, and the waste liquid does not require special defluorination treatment, significantly reducing environmental protection investment.

[0024] 3. The raw materials are inexpensive and readily available, and the process is extremely simple, making it suitable for industrial scale-up. This invention eliminates the need for synthesizing expensive quaternary ammonium salt surfactants, pre-preparing seed crystals, two-step temperature-controlled crystallization, and precise pH control. Commercially available industrial-grade urea, commercially available nanocellulose, and conventional TPAOH can be used for synthesis. The process flow is highly compatible with conventional ZSM-5 production, demonstrating excellent prospects for industrial scale-up.

[0025] 4. The product structure is precisely controllable, and the catalytic performance is excellent. This invention can achieve a b-axis thickness of approximately 50 nm and a mesopore diameter of around 13 nm by simply adjusting the amount of nanocellulose, urea, and crystallization temperature. The resulting hierarchical porous nanosheets possess both short-axis diffusion paths and abundant mesoporous mass transfer channels, significantly improving the accessibility of acid centers. Attached Figure Description

[0026] Figure 1 The X-ray diffraction (XRD) pattern of the hierarchical porous molecular sieve nanosheets prepared in Example 1;

[0027] Figure 2Scanning electron microscope (SEM) image of the hierarchical porous molecular sieve nanosheets prepared in Example 1;

[0028] Figure 3 The nitrogen adsorption-desorption isotherm of the hierarchical porous molecular sieve nanosheets prepared in Example 1;

[0029] Figure 4 The pore size distribution of the hierarchical porous molecular sieve nanosheets prepared in Example 1 (BJH method).

[0030] Figure 5 SEM image of the molecular sieve prepared for Comparative Example 1;

[0031] Figure 6 Nitrogen adsorption-desorption isotherm for the molecular sieve prepared in Comparative Example 1;

[0032] Figure 7 The pore size distribution of the molecular sieve prepared for Comparative Example 1 is shown.

[0033] Figure 8 SEM images of the molecular sieve prepared for Comparative Example 2;

[0034] Figure 9 Nitrogen adsorption-desorption isotherm for the molecular sieve prepared in Comparative Example 2;

[0035] Figure 10 The pore size distribution of the molecular sieve prepared in Comparative Example 2 is shown.

[0036] Figure 11 SEM image of the molecular sieve prepared for Comparative Example 3;

[0037] Figure 12 Nitrogen adsorption-desorption isotherm for the molecular sieve prepared in Comparative Example 3;

[0038] Figure 13 The pore size distribution of the molecular sieve prepared for Comparative Example 3 is shown. Detailed Implementation

[0039] To make the technical problems, technical solutions and beneficial effects of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0040] Example 1

[0041] This embodiment provides a method for synthesizing hierarchical porous molecular sieve nanosheets. The amounts of each raw material added are by mass fraction, and the specific steps are as follows:

[0042] (1) Pre-hydrolysis and aging: Weigh 1.5 parts of a 40wt% tetrapropylammonium hydroxide solution, add 1.875 parts of deionized water, and mix thoroughly by magnetic stirring. Slowly add 1.0 part of tetraethyl orthosilicate while stirring, and continue stirring at 40℃ for 10h to obtain a clear and transparent mixed solution A. Let mixed solution A stand at room temperature for 2h to age.

[0043] (2) Mesoporous template assembly: Add 0.3 parts of 10wt% nanocellulose aqueous dispersion (0.03 parts of nanocellulose dry basis) to the above aged mixed solution A, shake well by hand, and then place it in an ultrasonic cleaner for ultrasonic treatment for 10 min (power 200W, frequency 40kHz) to obtain a uniform milky white dispersion B.

[0044] (3) Morphology control and aluminum source introduction: 1.0 part of deionized water, 0.25 part of urea and 0.01 part of aluminum nitrate nonahydrate were added to the above mixed solution B in sequence. The mixture was magnetically stirred at room temperature for 1 h to fully dissolve the aluminum source and urea, and the initial gel mixture C was obtained.

[0045] (4) Hydrothermal crystallization: The above initial gel mixture C is transferred to a stainless steel hydrothermal crystallization vessel lined with polytetrafluoroethylene, sealed and placed in a homogeneous reactor at 170°C for static crystallization for 24 hours.

[0046] (5) Post-processing: After crystallization, remove the reactor and allow it to cool naturally to room temperature. Transfer the product to a centrifuge tube and centrifuge at 8000 rpm for 5 min. Discard the supernatant, add deionized water, sonicate and disperse, then centrifuge again. Repeat washing 3-5 times until the pH of the centrifuged liquid is 7.0. Place the obtained solid product in an 80℃ vacuum drying oven and dry overnight. Transfer the dried solid to a muffle furnace and heat to 550℃ at a heating rate of 2℃ / min. Calcinate at this temperature for 6 h to obtain the hierarchical porous molecular sieve nanosheets.

[0047] See Figure 1 In this example, ZSM-5 molecular sieve was successfully synthesized.

[0048] See Figure 2 The hierarchical porous molecular sieve nanosheets prepared in Example 1 exhibit a uniform ultrathin nanosheet morphology with a b-axis thickness of approximately 50 nm, which is much smaller than that of conventional ZSM-5 molecular sieves. The short diffusion path is beneficial for improving the efficiency of catalytic reactions.

[0049] Figures 3-4 The nitrogen adsorption-desorption isotherm and pore distribution diagram of the hierarchical porous molecular sieve nanosheets prepared in Example 1 are shown. The specific surface area of ​​the prepared sample is 263.42 m² / g, the pore volume is 0.58 cm³ / g, and the mesopore diameter is 13.94 nm.

[0050] Comparative Example 1: No urea, no nanocellulose

[0051] The specific preparation steps of this comparative example are the same as those in Example 1, except that: no nanocellulose aqueous dispersion is added in step (2), no urea is added in step (3), and the other conditions are exactly the same.

[0052] from Figure 5 As can be seen from the SEM images, the sample prepared in Comparative Example 1 does not have an obvious sheet-like structure, but rather an aggregated mass. Figure 6 and Figure 7 Nitrogen adsorption analysis showed that the sample prepared in Comparative Example 1 only had a microporous structure, and the microporous distribution was inherent to the molecular sieve itself, without any mesoporous distribution. This comparative example demonstrates that a hierarchical porous nanosheet structure cannot be obtained without the two auxiliary agents.

[0053] Comparative Example 2: Urea only, without nanocellulose

[0054] The specific preparation steps of this comparative example are the same as those in Example 1, except that: no nanocellulose aqueous dispersion is added in step (2), and the other conditions are exactly the same.

[0055] from Figure 8 The SEM images show that the sample prepared in Comparative Example 2 exhibits a plate-like structure, indicating that urea can effectively induce crystal growth along the b-axis orientation; however, Figure 9 and Figure 10 Nitrogen adsorption analysis showed that the pore size distribution consisted only of micropores, with no obvious mesopore distribution. This comparative example demonstrates that while urea can control the morphology, it cannot construct intracrystalline mesopores.

[0056] Comparative Example 3: Nanocellulose only, no urea

[0057] The specific preparation steps of this comparative example are the same as those in Example 1, except that urea is not added in step (3), while the other conditions are exactly the same.

[0058] from Figure 11 As can be seen from the SEM images, the product exhibits an irregular blocky morphology with large crystal size and no plate-like structure was observed, indicating that crystal growth is not limited in the absence of urea. Figure 12 and Figure 13 Nitrogen adsorption analysis revealed the presence of a certain amount of mesoporous distribution, confirming the mesoporous pore-forming function of nanocellulose. However, due to the longer diffusion path caused by the non-sheet-like morphology, its mass transfer efficiency was not as high as that of Example 1.

[0059] Therefore, it is evident that nanocellulose and urea have a synergistic effect in this invention: nanocellulose acts as a hard template to introduce mesopores within the crystal, while urea acts as a morphology control agent to induce the crystal to grow along the b-axis to form nanosheets. Both are indispensable, jointly achieving the integrated construction of sheet-like morphology and intracrystalline mesopores.

Claims

1. A method for synthesizing hierarchical porous molecular sieve nanosheets, characterized in that, Includes the following steps: 1) Mix tetrapropylammonium hydroxide solution, tetraethyl orthosilicate and water, stir to pre-hydrolyze, and then let stand to age to obtain mixed solution A; 2) Add nanocellulose aqueous dispersion to the mixed solution A, mix, and then sonicate to obtain mixed solution B; 3) Add urea and water, or add urea, metal source and water to the mixed solution B, stir, and obtain the initial gel mixture C; 4) The initial gel mixture C is placed in a hydrothermal crystallization vessel and crystallized at 150~200℃ for 12~30h; 5) After crystallization, the product is separated into solid and liquid phases. The solid product is washed, dried and calcined to obtain the multi-level porous molecular sieve nanosheets.

2. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 1), the concentration of the tetrapropylammonium hydroxide solution is 25wt%~40wt%; in the mixed solution A, the molar ratio of tetraethyl orthosilicate, tetrapropylammonium hydroxide and water is 1:0.4~0.8:20~30.

3. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 1), the pre-hydrolysis time is 10-20 hours, the temperature is 40-90°C, and the settling and aging time is 2-4 hours.

4. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 2), the concentration of the nanocellulose aqueous dispersion is 5~20wt%, and the mass ratio of the amount of nanocellulose added to the mass of tetraethyl orthosilicate in step 1) is 0.02~0.04:1 on a dry basis.

5. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 3), the mass ratio of the amount of urea added to the mass of tetraethyl orthosilicate in step 1) is 0.2~0.5:

1.

6. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 3), the metal source is selected from at least one of aluminum source and titanium source; the aluminum source is one or more of aluminum isopropoxide, aluminum sulfate, sodium aluminate or aluminum nitrate nonahydrate, and the titanium source is one or more of tetrabutyl titanate or titanium oxysulfate.

7. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 6, characterized in that: When only an aluminum source is added in step 3), the product is ZSM-5 molecular sieve nanosheets; when only a titanium source is added in step 3), the product is TS-1 molecular sieve nanosheets; when no metal source is added in step 3), the product is all-silica Silicalite-1 molecular sieve nanosheets.

8. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 4), the crystallization temperature is 170~180℃ and the crystallization time is 12~24h.

9. The method for synthesizing hierarchical porous molecular sieve nanosheets as described in claim 1, characterized in that: In step 5), the roasting temperature is 500~600℃ and the roasting time is 2~6h.

10. A hierarchical porous molecular sieve nanosheet, characterized in that: It is prepared by any one of the preparation methods of claims 1 to 9.

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