Preparation method of nanosheet-shaped ts-1 molecular sieve and application thereof

Abstract

The application discloses a preparation method of nanosheet TS-1 molecular sieve and application thereof, relates to the technical field of catalysts, and synthesizes the TS-1 molecular sieve through a double quaternary ammonium salt structure directing agent containing a benzene ring; the obtained TS-1 molecular sieve has rich surface active sites and contains almost no impurities such as non-framework titanium and anatase TiO2; meanwhile, the nanosheet morphology significantly improves the diffusion and mass transfer performance of macromolecular reactants in a catalytic process, effectively enhances the accessibility of active sites; in a cyclohexene epoxidation reaction, the molecular sieve exhibits excellent catalytic performance and shows good application prospect; and the problems of significant channel diffusion limitation and low accessibility of active sites of the TS-1 molecular sieve in a catalytic process are solved.

Description

Technical Field

[0001] This invention belongs to the field of materials science and technology, and relates to a molecular sieve, specifically a method for preparing nanosheet-like TS-1 molecular sieve and its application. Background Technology

[0002] Titanium silicate molecular sieve (TS-1) is a heteroatom microporous molecular sieve with an MFI topology. In its lattice, some silicon atoms are introduced by titanium atoms in a isomorphous substitution form, forming Lewis acid sites. These framework titanium sites can effectively activate hydrogen peroxide, thereby catalyzing the selective oxidation reactions of various organic compounds such as alkanes, alkenes, ketones, and phenols (Nature, 1990, 345, 240-242; J. Chem. Soc., Chem. Commun., 1990, 6, 476-477; Nature, 1994, 368, 321-323). Since Enichem achieved commercial production of TS-1 in 1983 (US4410501), TS-1 has been widely used in industrial catalytic processes such as phenol hydroxylation, ketone ammonium oximeation, and olefin epoxidation (J. Catal., 1996, 161, 570-576; Chem. Eng. J., 2007, 132, 61-66; Chem. Eng. J., 2016, 295, 370-375; Appl. Catal. B, 2022, 319, 121837-121849), and has gradually become a key catalytic material in green oxidation processes.

[0003] However, the microporous structure of TS-1 limits its application in the catalytic conversion of macromolecular reactants, and the micropores are prone to carbon buildup or blockage, leading to a decrease in activity. To address this issue, patent CN116282060B proposes an improved synthesis strategy: a titanium-silicon precursor is prepared using organosilicon sources, organotitanium sources, and tetrapropylammonium hydroxide as raw materials. After alcohol evaporation treatment, a silanizing agent is introduced, followed by hydrothermal crystallization at 170 °C for 48 hours, successfully preparing a TS-1 molecular sieve with a mesoporous structure. This material exhibits excellent cyclohexene conversion and cyclohexane oxide selectivity in the cyclohexene epoxidation reaction. Further research indicates that reducing crystal size or introducing a mesoporous structure to enhance the accessibility of active sites is an effective way to improve the catalytic performance and lifespan of TS-1 (Adv. Mater. Interfaces 2021, 8, 2001095; J. Catal., 2012, 288, 16-23). However, TS-1 molecular sieves prepared using existing technologies still suffer from significant pore diffusion restriction and low accessibility of active sites during catalysis. Summary of the Invention

[0004] This invention overcomes the shortcomings of the prior art and proposes a method for preparing nanosheet-like TS-1 molecular sieves and their applications; it solves the problems of significant pore diffusion restriction and low accessibility of active sites in the catalytic process of TS-1 molecular sieves.

[0005] This invention is achieved through the following technical solution: A method for preparing nanosheet-like TS-1 molecular sieves involves synthesizing the TS-1 molecular sieve using a bis-quaternary ammonium salt structure directing agent containing a benzene ring.

[0006] Preferably, the preparation method of the bis-quaternary ammonium salt structure directing agent includes the following steps: Step 1: Dissolve N,N,N',N'-tetramethyldiamine in an organic solvent, add a monosubstituted haloalkane, and react in an oil bath. Step 2: Remove the solvent after the oil bath reaction, and then separate, wash, and dry to obtain the intermediate product; Step 3: After dissolving the intermediate product, add a reagent containing benzene ring-terminated molecules, and perform an oil bath reaction again. Separate, wash, and dry the product to obtain a bisquaternary ammonium salt structure directing agent.

[0007] Preferably, in step one, the organic solvent is ethanol; the oil bath reaction temperature in step one is 40-100℃, and the reaction time is 8-24 h.

[0008] Preferably, the drying temperature in step two is 40-80°C.

[0009] Preferably, the solvent for dissolving the intermediate product in step three is toluene and / or acetonitrile.

[0010] Preferably, the reagent containing benzyl ring-terminated molecules is a benzyl halide containing a benzyl ring, wherein the benzyl halide containing a benzyl ring is one of benzyl bromide, x-methylbenzyl chloride / bromine, x-vinylbenzyl chloride / bromine, and x-tert-butylbenzyl chloride / bromine.

[0011] Preferably, the synthesis steps are as follows: under stirring, titanium source, silicon source, structure directing agent and water are mixed evenly, and the mixture is transferred to a crystallization reactor. Crystallization is carried out under dynamic crystallization conditions of 120-200 °C for 4-14 days. The obtained product is centrifuged, dried at 80-150 °C, and calcined in air at 500-600 °C to obtain TS-1 molecular sieve. The rotation speed of the dynamic crystallization is 20-120 r / min.

[0012] Preferably, the molar ratio of titanium source, silicon source, structure directing agent, and water component is SiO2 / TiO2=20-200; SiO2 / SDA=5-100; H2O / SiO2=20-200.

[0013] Preferably, the silicon source is an organosilicon source or an inorganic silicon source, wherein the organosilicon source is tetraethyl orthosilicate, and the inorganic silicon source is fumed silica or silica sol; the titanium source is an organotitanium source or an inorganic titanium source, wherein the organotitanium source is one of tetrabutyl titanate, tetraethyl titanate, and tetraisopropyl titanate, and the inorganic titanium source is titanium tetrachloride or titanium oxysulfate.

[0014] The nanosheet-like TS-1 molecular sieve prepared by the aforementioned method is applied to the cyclohexene epoxidation reaction.

[0015] The beneficial effects of this invention compared to the prior art are as follows: This invention addresses the problems of significant pore diffusion restriction and low accessibility of active sites in existing TS-1 molecular sieves during catalysis by optimizing their structure. By constructing a nanosheet morphology, the prepared molecular sieve has a larger specific surface area, more abundant surface acidic sites, and a shorter pore structure, thereby significantly improving the diffusion efficiency of reactants in the pores and the utilization rate of active sites, and exhibiting higher catalytic activity in the epoxidation of cyclohexene.

[0016] This invention achieves the preparation of ultrathin nanosheet TS-1 molecular sieves by designing a bis-quaternary ammonium salt structure directing agent containing a benzene ring. Each nanosheet is about 6 nm thick and has a larger specific surface area and surface active sites, exhibiting higher catalytic activity than traditional nano TS-1 molecular sieves in the cyclohexene epoxidation reaction. Attached Figure Description

[0017] Figure 1 The bis-quaternary ammonium salt structure directing agent prepared in Example 1 of this invention 1 H NMR spectrum; Figure 2 The XRD pattern of the TS-1 molecular sieve prepared in Example 2 of this invention; Figure 3 The images shown are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the TS-1 molecular sieve prepared in Example 2 of this invention. Figure 4 The nitrogen adsorption-desorption curve of the TS-1 molecular sieve prepared in Example 2 of this invention; Figure 5 The UV-Vis spectrum of the TS-1 molecular sieve prepared in Example 2 of this invention; Figure 6 The infrared spectrum of the TS-1 molecular sieve prepared in Example 2 of this invention; Figure 7 The XRD pattern of the TS-1 molecular sieve prepared in Example 3 of this invention; Figure 8These are scanning electron microscope and transmission electron microscope images of the TS-1 molecular sieve prepared in Example 3 of the present invention; Figure 9 The XRD pattern of the TS-1 molecular sieve prepared in Example 4 of this invention; Figure 10 These are scanning electron microscope and transmission electron microscope images of the TS-1 molecular sieve prepared in Example 4 of this invention; Detailed Implementation

[0018] To make the technical problems to be solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail with reference to the embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solutions of this invention are described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto. Example 1

[0019] 52 mL of N,N,N',N'-tetramethyl-1,6-hexanediamine (0.25 mol) and 200 mL of ethanol were added to a round-bottom flask as solvents, followed by 5 mL of bromohexane (0.035 mol). The mixture was stirred and reacted in an oil bath at 70 °C for 12 h under reflux. The solvent was removed by a vacuum rotary evaporator. The product was allowed to stand and separate into layers. The lower layer was then dried in a vacuum drying oven at 40 °C to obtain product a: C6H 13 -N(CH3)2-C6H 13 -N(CH3)2; Subsequently, 15 mmol of product a and 10 mmol of 4-tert-butylbenzyl chloride were added to 50 mL of toluene and 50 mL of acetonitrile solution, and reacted at 70 °C for 12 h under reflux. Most of the solvent was removed by rotary evaporation, and the product was precipitated with a large amount of ethyl acetate. After filtration and separation, the product was dried in a vacuum drying oven at 40 °C to obtain the bisquaternary ammonium salt structure directing agent C. TBB . Example 2

[0020] Tetrabutyl titanate (TBOT) was added to a dilute H₂O₂ solution and stirred thoroughly to obtain a pale yellow, clear liquid. Tetramethylammonium hydroxide (TMAOH) was then added and stirred until clear. Finally, structure-directing agent C was added. TBBAfter stirring and dissolving, tetraethyl orthosilicate (TEOS) was added, and the mixture was sealed and stirred to ensure complete hydrolysis of TEOS. After complete hydrolysis, the ethanol produced by TEOS hydrolysis was removed by an oil bath at 80 °C, yielding a mother liquor with a molar ratio of 1.0 SiO2: 0.025 TiO2: 0.2 SDA: 0.4 TMAOH: 40 H2O. The mother liquor was transferred to a stainless steel reactor lined with polytetrafluoroethylene and crystallized for 7 days in a homogeneous reactor at 170 °C and 30 rpm. The sample was separated from the mixture by centrifugation and washed with deionized water until pH < 8. The sample was dried in an oven at 80 °C for 24 h and then calcined in a muffle furnace at 550 °C for 6 h. The resulting sample was labeled TS-1-0.2-0.025.

[0021] Its XRD phase diagram is as follows Figure 2 As shown, this is a pure-phase TS-1 molecular sieve; scanning electron microscopy and transmission electron microscopy tests are as follows. Figure 3 As shown, the sample has a uniform morphology, exhibiting a sheet-like structure with a sheet thickness of approximately 3.6 nm; the ultraviolet-visible spectrum is as follows. Figure 5 As shown in the spectrum, the sample only exhibits an absorption peak at 210 nm, indicating that only framework titanium is present in the sample, with almost no extra-framework titanium (260 nm) and anatase TiO2 (330 nm); similarly, in the infrared spectrum... Figure 6 It can be 960 cm -1 Characteristic peaks of skeletal titanium are present at this location. Example 3

[0022] The amount of titanium source was changed to: SiO2:TiO2=100, and the remaining steps were the same as in Example 2. The TS-1 molecular sieve sample prepared in Example 3 was labeled as TS-1-0.2-0.01.

[0023] The XRD phase diagram of the sample is as follows Figure 7 As shown, similar to Example 2, it is a pure phase TS-1 molecular sieve; Figure 8 SEM images and TEM tests showed that the sample also exhibited a sheet-like structure with no change in sheet thickness. Example 4

[0024] The amount of structure directing agent was changed to: SiO2 / SDA=10, and the remaining steps were the same as in Example 2. The TS-1 molecular sieve sample prepared in Example 4 was labeled as TS-1-0.1-0.025.

[0025] Example 4: Sample XRD phase diagram as follows Figure 9 As shown, similar to Examples 2 and 3, the sample is a pure phase TS-1 molecular sieve; Figure 10 SEM images and TEM tests showed that the sample exhibited a sheet-like structure with no change in sheet thickness.

[0026] Comparative Example 1 Tetrabutyl titanate (TBOT) was added to a dilute H₂O₂ solution and stirred thoroughly to obtain a pale yellow, clear liquid. Then, tetrapropylammonium hydroxide (TPAOH) was added as a template agent, followed by stirring. Tetraethyl silicate (TEOS) was then added, and the mixture was sealed and stirred until TEOS was completely hydrolyzed. After complete hydrolysis, the ethanol produced by TEOS hydrolysis was removed in an 80 °C oil bath. Polyacrylic acid (PAA) was added and stirred for 10 min to obtain a mother liquor with a molar ratio of 1.0 SiO₂: 0.025 TiO₂: 0.4 TPAOH: 30 H₂O: 0.25 PAA. The mother liquor was transferred to a polytetrafluoroethylene-lined stainless steel reactor and crystallized for 2 days in a homogeneous reactor at 170 °C and 30 rpm. The sample was separated from the mixture by centrifugation and washed with deionized water until pH < 8. It was then dried in an 80 °C oven for 24 h and calcined in a muffle furnace at 550 °C for 6 h. The resulting sample was labeled TS-1-ref. Example 5

[0027] Take 25 mg of the TS-1 sample synthesized in Examples 2-4 above, 5 mL of acetonitrile, 5 mmol of cyclohexene, and 5 mmol of hydrogen peroxide tert-butanol (decane solution), and react at 60 °C for 2 hours. Then, analyze the content of reactants and products using gas chromatography (see Table 1).

[0028] As can be seen from the table, after 120 minutes of reaction, the cyclohexene conversion rate of the comparative sample nanoparticles TS-1-ref was only 2.2%, while the samples prepared by the present invention all had higher cyclohexene conversion rates. Among them, the samples of Examples 2 and 4 had cyclohexene conversion rates of around 20% due to their high titanium content. Even though the titanium content of Example 3 was low, its cyclohexene conversion rate (13.2%) was still higher than that of the comparative sample. This indicates that the samples prepared by the present invention have more abundant surface active centers.

[0029] ; In summary, this invention designs a bis-quaternary ammonium salt structured directing agent containing a benzene ring. This directing agent can effectively guide the formation of nanosheet-like TS-1 molecular sieves, significantly reducing diffusion restriction and significantly increasing surface active sites, thus exhibiting superior catalytic performance in the epoxidation reaction of macromolecular cyclohexene.

[0030] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0031] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this invention.

Claims

1. A method for preparing nanosheet-like TS-1 molecular sieves, characterized in that, The TS-1 molecular sieve was synthesized using a bis-quaternary ammonium salt directing agent containing a benzene ring. The preparation method of the aforementioned quaternary ammonium salt structure directing agent includes the following steps: Step 1: Dissolve N,N,N',N'-tetramethyldiamine in an organic solvent, add a monosubstituted haloalkane, and react in an oil bath. Step 2: Remove the solvent after the oil bath reaction, and then separate, wash, and dry to obtain the intermediate product; Step 3: After dissolving the intermediate product, add a reagent containing benzene ring-terminated molecules, and perform an oil bath reaction again. Separate, wash, and dry the product to obtain a bisquaternary ammonium salt structure directing agent. The reagent containing benzyl ring-terminated molecules is a benzyl halide containing a benzyl ring, wherein the benzyl halide containing a benzyl ring is one of benzyl bromide, x-methylbenzyl chloride / bromine, x-vinylbenzyl chloride / bromine, and x-tert-butylbenzyl chloride / bromine; The prepared nanosheet-like TS-1 molecular sieve was applied to the cyclohexene epoxidation reaction.

2. The method for preparing nanosheet-like TS-1 molecular sieve according to claim 1, characterized in that, In step one, the organic solvent is ethanol; the oil bath reaction temperature in step one is 40-100℃, and the reaction time is 8-24 h.

3. The method for preparing nanosheet-like TS-1 molecular sieve according to claim 1, characterized in that, The drying temperature described in step two is 40-80℃.

4. The method for preparing nanosheet-like TS-1 molecular sieve according to claim 1, characterized in that, The solvent used to dissolve the intermediate product in step three is toluene and / or acetonitrile.

5. The method for preparing nanosheet-like TS-1 molecular sieve according to claim 1, characterized in that, The synthesis steps are as follows: under stirring, titanium source, silicon source, structure directing agent and water are mixed evenly, and the mixture is transferred to a crystallization reactor. Crystallization is carried out at 120-200 ℃ for 4-14 days. The obtained product is centrifuged, dried at 80-150 ℃, and calcined in air at 500-600 ℃ to obtain TS-1 molecular sieve. The rotation speed of the dynamic crystallization is 20-120 r / min.

6. The method for preparing a nanosheet-like TS-1 molecular sieve according to claim 5, characterized in that, The molar ratios of titanium source, silicon source, structure directing agent, and water components are SiO2 / TiO2 = 20-200; SiO2 / SDA = 5-100; H2O / SiO2 = 20-200.

7. The method for preparing a nanosheet-like TS-1 molecular sieve according to claim 5, characterized in that, The silicon source is an organosilicon source or an inorganic silicon source. The organosilicon source is tetraethyl orthosilicate, and the inorganic silicon source is fumed silica or silica sol. The titanium source is an organotitanium source or an inorganic titanium source. The organotitanium source is one of tetrabutyl titanate, tetraethyl titanate, and tetraisopropyl titanate, and the inorganic titanium source is titanium tetrachloride or titanium oxysulfate.

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