A method of synthesizing a microscale ts-2 molecular sieve having a hierarchical pore structure

Abstract

The application belongs to the field of molecular sieve material preparation, and relates to a method for synthesizing micron TS-2 molecular sieve with hierarchical pores, comprising the following steps: firstly, a silicon source, a titanium source, hydrochloric acid and deionized water are mixed according to a specific proportion, a high-dispersion SiO2-TiO2 precursor powder is obtained through a spray drying device, then the precursor powder is added into a TBAOH solution to form an initial gel, micron TS-2 molecular sieve is obtained by controlling hydrothermal crystallization conditions; then the micron TS-2 molecular sieve is fully mixed with an organic alkali solution and deionized water, and stirring is performed to obtain a mixed slurry; the mixed slurry is transferred into a crystallization kettle with a stirring device, and is subjected to a high-temperature crystallization process, and then is subjected to washing, separation, drying and calcination to obtain a micron TS-2 molecular sieve product with a hierarchical pore structure. The organic alkali post-treatment strategy is adopted, mesoporous structures are successfully introduced while the micron size is maintained, and the mass transfer efficiency and active site accessibility of the molecular sieve are significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve material preparation and relates to a method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores. Background Technology

[0002] TS-2 molecular sieves possess a unique MEL topology, consisting of two mutually perpendicular through-channels (approximately 0.53 nm × 0.54 nm in diameter), exhibiting excellent catalytic performance in selective oxidation reactions such as phenol hydroxylation, olefin epoxidation, and cyclohexanone ammonium oximeation. Compared to TS-1, the through-channel structure of TS-2 is beneficial for reducing diffusion resistance and improving the selectivity of hydroquinone. However, TS-2 molecular sieves synthesized by conventional hydrothermal methods only have a microporous structure with narrow pore sizes, resulting in significant mass transfer resistance in reactions involving macromolecular substrates, severely limiting the accessibility of active sites. Furthermore, although nanoscale TS-2 crystals have a high external surface area, separation and recovery in liquid-phase reaction systems are difficult, easily leading to catalyst loss and limiting industrial applications.

[0003] Aerosol-assisted hydrothermal synthesis has been applied to the synthesis of a series of molecular sieves, such as TS-1, ZSM, and Beta. This invention attempts to extend this method to the synthesis of micron-sized TS-2 molecular sieves. Based on the aerosol-assisted hydrothermal synthesis strategy, in-situ directional growth of the molecular sieve is achieved by systematically controlling the crystallization conditions, thereby completely preserving the inherent micron-sized particle morphology of the precursor. The core advantage of this technical route is that it transforms an amorphous precursor with specific micron-scale and morphological characteristics into a highly crystalline molecular sieve material, thus effectively avoiding the problems of morphology loss of control and decreased separation performance caused by excessive grain growth or aggregation in traditional synthesis methods. It can also improve the recyclability and stability of the catalyst in the liquid-phase reaction system.

[0004] Organic alkali post-treatment is a modification method that involves treating synthesized molecular sieves in an organic alkali solution to controllably introduce mesoporous structures into their framework via a dissolution-recrystallization mechanism. Compared with traditional inorganic alkali treatment (such as NaOH), organic alkali post-treatment has significant advantages: the organic alkali serves as both an alkali source and a structure-directing agent in molecular sieve synthesis. During desilication, it can simultaneously induce dissolved silicon species to recrystallize in situ at framework defects, achieving a dynamic balance between desilication and recrystallization. This avoids excessive framework damage and a significant decrease in crystallinity caused by inorganic alkali treatment. For titanium-silicon molecular sieves, organic alkali treatment can selectively remove framework silicon rather than active titanium species, effectively inhibiting the formation of non-framework titanium and protecting catalytic active centers. Existing research mainly focuses on the modification of nanoscale titanium-silicon molecular sieves, with limited research on the hierarchical pore construction of micron-scale titanium-silicon molecular sieves, making it difficult to balance mass transfer efficiency and separation and recovery performance.

[0005] Therefore, this invention develops a method for controllably constructing hierarchical porous structures in micron-sized TS-2 molecular sieves, which significantly improves mass transfer efficiency and catalytic performance while maintaining its micron-sized dimensions (easy to separate and recover), and is an efficient hierarchical porous molecular sieve construction strategy. Summary of the Invention

[0006] This invention utilizes a combination of spray drying, hydrothermal crystallization, and post-treatment to prepare hierarchical porous micron-sized TS-2 molecular sieves. The process includes the following steps: First, highly dispersed SiO2-TiO2 precursor powder is obtained through spray drying; then, micron-sized TS-2 molecular sieves are obtained through hydrothermal crystallization. Next, the micron-sized TS-2 molecular sieves are mixed with an organic alkali solution and deionized water, and treated under hydrothermal conditions to obtain micron-sized TS-2 molecular sieves with a hierarchical porous structure. The product exhibits high crystallinity, abundant mesoporous structure, and micron-sized morphology. The molecular sieve synthesized using this method demonstrates excellent catalytic performance in the hydroxylation reaction of phenol.

[0007] The technical solution of this invention: A method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores, comprising the following steps: S1. Mix silicon source, titanium source, hydrochloric acid and deionized water, and stir at room temperature until complete hydrolysis to form an initial gel; use a spray drying device to treat the initial gel at a temperature of 220°C to obtain a white powder, and then dry it at 110°C overnight to obtain a precursor powder. S2. Add organic template agent A1 and deionized water to the precursor powder obtained in step S1 in sequence, stir thoroughly to ensure uniform mixing at room temperature, and transfer to a hydrothermal crystallization kettle with PTFE lining for crystallization for a period of time. S3. After crystallization, cool to room temperature, wash the solid powder until neutral, dry it overnight in a constant temperature oven, and obtain micron-sized TS-2 molecular sieve after high-temperature calcination. S4. Add organic template agent A2 and deionized water to the micron-sized TS-2 molecular sieve obtained in step S3 in sequence, stir thoroughly to ensure uniform mixing at room temperature, and transfer to a hydrothermal crystallization kettle with PTFE lining for crystallization for a period of time. After crystallization, S5 is filtered, washed, dried and calcined to obtain micron-sized TS-2 molecular sieve with multi-level pores.

[0008] In step S1, the silicon source is any one of tetraethyl orthosilicate, silica sol, fumed silica, or silica gel.

[0009] In step S1, the molar ratio of each component in the initial gel is 1SiO2:0.0167TiO2:0.067HCl:20H2O; wherein the number of moles of silicon source is calculated based on the SiO2 contained in the silicon source; and the number of moles of titanium source is calculated based on TiO2.

[0010] In step S2, the organic template agent A1 is any one of tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, and tetraethylammonium hydroxide.

[0011] In step S2, the molar ratio of the organic template agent A1 to the silicon source is 0.1, and the molar ratio of deionized water to the silicon source is 7.

[0012] In step S2, the crystallization conditions are as follows: temperature 170℃, time 72h.

[0013] In step S3, the drying temperature is 110℃, the calcination temperature is 550℃, and the calcination time is 6h.

[0014] In step S4, the organic template agent A2 is any one of tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, and tetraethylammonium hydroxide.

[0015] In step S4, the molar ratio of the organic template agent A2 to the silicon source is 0.1-0.3.

[0016] In step S4, the crystallization conditions are as follows: temperature 170℃, time 24-72h.

[0017] In step S5, the drying temperature is 110℃; the calcination temperature is 550℃; and the calcination time is 6h.

[0018] The beneficial effects of this invention are as follows: This invention utilizes spray drying technology combined with hydrothermal synthesis and organic base post-treatment to prepare micron-sized TS-2 molecular sieves with abundant mesoporous structures. This method involves hydrolysis under acidic conditions, resulting in uniform dispersion of Ti species within amorphous silica. Hydrothermal crystallization is then performed, and by adjusting the crystallization conditions, the molecular sieve is crystallized in situ, maintaining the micron-sized morphology of the amorphous precursor and yielding micron-sized molecular sieves with high crystallinity. Furthermore, the TPAOH post-treatment introduces abundant mesoporous structures, which is beneficial for the contact between reactants and active sites during catalysis and for reducing separation and recovery costs, exhibiting excellent catalytic performance in the hydroxylation of phenol. Attached Figure Description

[0019] Figure 1 The images are SEM images of samples obtained at different crystallization times in Comparative Example 1; where (a) is 12h, (b) is 24h, (c) is 48h, (d) is 72h, and (e) is 96h.

[0020] Figure 2This is the XRD pattern of the sample obtained in Example 1.

[0021] Figure 3 This is the UV-Vis image of the sample obtained in Example 1.

[0022] Figure 4 This is a SEM image of the sample obtained in Example 1.

[0023] Figure 5 This is a TEM image of the sample obtained in Example 1. Detailed Implementation

[0024] The specific embodiments of the present invention are described below in conjunction with the technical solution and accompanying drawings.

[0025] Comparative Example 1 According to the molar ratio of 1SiO2:0.0167TiO2:0.067HCl:20H2O, 30g of tetraethyl orthosilicate, 0.8351g of tetrabutyl titanate, 0.9518g of hydrochloric acid, and 60.95g of deionized water were first mixed and stirred until the solution was clear. The precursor solution was then spray-dried to obtain precursor powder. According to the molar ratio of 1SiO2:0.0167TiO2:7H2O:0.1TBAOH, the above precursor powder, TBAOH solution, and deionized water were mixed and stirred at room temperature for 30 minutes to form a gel. Finally, the gel was transferred to a stainless steel high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 170℃ in a constant temperature oven for a period of time. The crystallized suspension was filtered, washed, and dried, and then calcined in air at 550℃ for 6 hours to obtain micron-sized TS-2 molecular sieves.

[0026] Example 1 According to the molar ratio of 1SiO2:0.0167TiO2:0.067HCl:20H2O, 30g of tetraethyl orthosilicate, 0.8351g of tetrabutyl titanate, 0.9518g of hydrochloric acid, and 60.95g of deionized water were first mixed and stirred until the solution was clear. The precursor solution was then spray-dried to obtain precursor powder. According to the molar ratio of 1SiO2:0.0167TiO2:7H2O:0.1TBAOH, the above precursor powder, TBAOH solution, and deionized water were mixed and stirred at room temperature for 30 min to form a gel. Finally, the gel was transferred to a stainless steel high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 170℃ for 72 h in a constant temperature oven. The crystallized suspension was filtered, washed, and dried, and then calcined in air at 550℃ for 6 h to obtain micron-sized TS-2 molecular sieves. The micron-sized TS-2 molecular sieve, TPAOH solution, and deionized water were mixed according to the molar ratio of 1SiO2:0.0167TiO2:10H2O:0.2TPAOH and stirred at room temperature for 30 min to form a gel. The gel was then transferred to a stainless steel high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 170℃ for 48 h in a constant-temperature oven. The crystallized suspension was filtered, washed, and dried, and then calcined in air at 550℃ for 6 h to obtain a micron-sized TS-2 molecular sieve with hierarchical pores.

[0027] Example 2 The difference between this embodiment and Example 1 is that the molar ratio of organic base added is TPAOH / SiO2=0.1, while the rest is the same as in Example 1.

[0028] Example 3 The difference between this embodiment and Example 1 is that the molar ratio of organic base added is TPAOH / SiO2=0.3, while the rest is the same as in Example 1.

[0029] Example 4 The difference between this embodiment and Example 1 is that the molar ratio of organic base added is TBAOH / SiO2 = 0.1, while the rest is the same as in Example 1.

[0030] Example 5 The difference between this embodiment and Example 1 is that the molar ratio of organic base added is TBAOH / SiO2 = 0.2, while the rest is the same as in Example 1.

[0031] Example 6 The difference between this embodiment and Example 1 is that the molar ratio of organic base added is TBAOH / SiO2 = 0.3, while the rest is the same as in Example 1.

[0032] Table 1. Relative crystallinity of samples synthesized with different molar ratios of organic base addition.

[0033] As shown in Table 1, post-treatment with organic bases significantly affected the relative crystallinity of TS-2 molecular sieves. Compared to micron-sized TS-2 molecular sieves, the relative crystallinity of the post-treated samples increased significantly. The reason for this phenomenon is that the two organic bases can provide an alkaline environment and also act as structure-directing agents in molecular sieve synthesis. During the desilication process, they can simultaneously induce dissolved silicon species to recrystallize at framework defects, further increasing the crystallinity.

[0034] Example 7 The difference between this embodiment and Embodiment 1 is that the TPAOH treatment time is 24 hours, while the rest is the same as in Embodiment 1.

[0035] Example 8 The difference between this embodiment and Embodiment 1 is that the TPAOH treatment time is 72 hours, while the rest is the same as in Embodiment 1.

[0036] Table 2. Relative crystallinity of synthesized samples at different treatment times

[0037] Table 2 shows the relative crystallinity of the synthesized samples after treatment with TPAOH for different times. The relative crystallinity did not change significantly with increasing treatment time. After 48 hours of treatment, the crystallinity reached 100%, indicating that the molecular sieve was completely crystallized.

[0038] Example 9 The samples prepared by crystallization for 72 hours in Comparative Example 1 and the samples prepared in Example 1 were used to catalyze the hydroxylation reaction of phenol with hydrogen peroxide (30% wt). The reaction conditions were 10 g methanol phenol solution, 1 g H2O2 and 0.5 g molecular sieve, and the reaction was carried out at 333 K for 2 hours.

[0039] Table 3 Catalytic performance of TS-2 molecular sieve in phenol hydroxylation reaction

[0040] Compared to micron-sized TS-2 molecular sieves, the catalytic performance of samples post-treated with TPAOH was significantly improved. The molecular sieves post-treated with TPAOH showed a significant increase in phenol conversion and hydroquinone selectivity. This phenomenon can be attributed to the following: the higher mesoporous volume indicates the formation of hierarchical pores within the crystal, meaning that the post-treated sample can provide more unobstructed mass transfer channels for reactant molecules, accelerating the diffusion and transfer of reactant phenol and product hydroquinone, thereby enhancing the conversion of phenol and the selectivity of hydroquinone.

[0041] Example 10 The samples prepared in Example 1 and Example 5 were used to catalyze the hydroxylation reaction of phenol with hydrogen peroxide (30% wt). The reaction conditions were 10 g methanol phenol solution, 1 g H2O2 and 0.5 g molecular sieve, and the reaction was carried out at 333 K for 2 h.

[0042] Table 4 Catalytic performance of TS-2 molecular sieve in phenol hydroxylation reaction

[0043] Table 4 shows the catalytic performance of micron-sized TS-2 molecular sieves with hierarchical pores synthesized by two organic base post-treatment processes in the hydroxylation reaction of phenol. It was found that the phenol conversion rates of the two molecular sieves were similar, but the hydroquinone selectivity of the TPAOH-treated sample was higher than that of the TBAOH-treated sample. This phenomenon may be because the TPAOH-treated sample, due to its higher crystallinity, better titanium distribution, and larger mesoporous pore volume, can more effectively promote the diffusion and mass transfer of hydroquinone, effectively shortening the diffusion path and improving catalytic efficiency.

Claims

1. A method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores, characterized in that, The steps are as follows: S1. Mix silicon source, titanium source, hydrochloric acid and deionized water, and stir at room temperature until complete hydrolysis to form an initial gel; use a spray drying device to treat the initial gel at a temperature of 220°C to obtain a white powder, and then dry it at 110°C overnight to obtain a precursor powder. S2. Add organic template agent A1 and deionized water to the precursor powder obtained in step S1 in sequence, stir thoroughly to ensure uniform mixing at room temperature, and transfer to a hydrothermal crystallization kettle with PTFE lining for crystallization for a period of time. S3. After crystallization, cool to room temperature, wash the solid powder until neutral, dry it overnight in a constant temperature oven, and obtain micron-sized TS-2 molecular sieve after high-temperature calcination. S4. Add organic template agent A2 and deionized water to the micron-sized TS-2 molecular sieve obtained in step S3 in sequence, stir thoroughly to ensure uniform mixing at room temperature, and transfer to a hydrothermal crystallization kettle with PTFE lining for crystallization for a period of time. After crystallization, S5 is filtered, washed, dried and calcined to obtain micron-sized TS-2 molecular sieve with multi-level pores.

2. The method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores according to claim 1, characterized in that, In step S1, The silicon source is any one of tetraethyl orthosilicate, silica sol, fumed silica, and silica gel. The molar ratio of each component in the initial gel is 1SiO2:0.0167TiO2:0.067HCl:20H2O; wherein the molar number of the silicon source is calculated based on the SiO2 contained in the silicon source; and the molar number of the titanium source is calculated based on TiO2.

3. The method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores according to claim 1, characterized in that, In step S2, The organic template agent A1 is any one of tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, and tetraethylammonium hydroxide; The molar ratio of the organic template agent A1 to the silicon source is 0.1, and the molar ratio of deionized water to the silicon source is 7. The crystallization conditions are as follows: temperature 170℃, time 72h.

4. The method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores according to claim 1, characterized in that, In step S3, The drying temperature is 110℃, the calcination temperature is 550℃, and the calcination time is 6 hours.

5. The method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores according to claim 1, characterized in that, In step S4, The organic template agent A2 is any one of tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, and tetraethylammonium hydroxide; The molar ratio of the organic template agent A2 to the silicon source is 0.1-0.3; The crystallization conditions are as follows: temperature 170℃, time 24-72h.

6. The method for synthesizing micron-sized TS-2 molecular sieves with hierarchical pores according to claim 1, characterized in that, In step S6, The drying temperature is 110℃; the calcination temperature is 550℃; and the calcination time is 6 hours.

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