A method for preparing nanometer titanium-silicon molecular sieve TS-1

Abstract

The application discloses a method for preparing nano titanium-silicon molecular sieve TS-1, which comprises the following steps: (1) adding a silicon source into a template agent aqueous solution, uniformly mixing, and obtaining a full-silicon mixed solution; (2) performing rotary evaporation and heating on the full-silicon mixed solution to prepare a full-silicon precursor solution; (3) adding a titanium source into an aqueous solution containing a complexing agent, stirring and hydrolyzing to obtain a titanium source hydrolysis solution; (4) adding the titanium source hydrolysis solution into the full-silicon precursor solution prepared in the step (2) to obtain a silicon-titanium mixed solution, sealing a reactor, hydrothermally crystallizing, and obtaining a crystallization product; and through filtering, washing, drying and calcining, the nano titanium-silicon molecular sieve TS-1 is obtained. The method is simple in operation, short in synthesis period, and can effectively avoid the formation of non-framework Ti caused by the mismatching of silicon-titanium species hydrolysis and crystallization rate, and the prepared TS-1 is a microcrystalline aggregate with a size of 100-200 nm.

Description

Technical Field

[0001] This invention belongs to the field of heteroatom zeolite molecular sieve synthesis technology, and particularly relates to a method for preparing nano-titanium silicon molecular sieve TS-1. Background Technology

[0002] In 1983, Taramasso et al. developed TS-1, a titanium-silicon molecular sieve with an MFI topology. The isolated four-coordinate Ti on its framework has unsaturated lone pairs of electrons and exhibits Lewis acidity. When combined with the oxidant H2O2, it can achieve a mild, green, and efficient selective catalytic oxidation process.

[0003] In the classical synthesis of TS-1, on the one hand, the hydrolysis rate of Ti source is much faster than that of Si source, leading to easy polymerization of Ti precursors and affecting the crystallization of Ti species into the molecular sieve framework. On the other hand, due to the atomic size difference between Si and Ti, Ti species gradually crystallize into the molecular sieve framework during crystal growth. This mismatch between the hydrolysis and crystallization rates of silicon and titanium species leads to the formation of a large amount of non-framework Ti, thus affecting the catalytic performance of the product TS-1. Existing methods mainly match the hydrolysis and crystallization rates of silicon and titanium species by adding crystallization aids (such as Tween-20, Tween-40, ammonium carbonate, etc.), controlling the feeding rate, or ultraviolet radiation, thereby increasing the framework Ti content in the molecular sieve. However, these measures increase the synthesis cost, increase the complexity of the operation, and prolong the preparation process and time of TS-1, which is not conducive to industrial scale-up production.

[0004] Furthermore, since TS-1 is a microporous molecular sieve, the diffusion of substrate molecules within its micropores is easily restricted. Moreover, for reactions involving macromolecular substrates, the microporous structure of TS-1 often leads to rapid deactivation due to surface carbon buildup or coking, causing pore blockage and thus limiting its applications. Currently, this problem is mainly addressed by introducing mesopores or even macropores into the microporous TS-1. Specific methods include post-processing pore creation and in-situ synthesis with the addition of mesoporous templates. The post-processing method is relatively complex, requiring precise control of post-processing conditions; otherwise, it may damage the molecular sieve's framework structure to some extent, reducing its crystallinity. In-situ addition of mesoporous templates can affect the nucleation and growth of the molecular sieve crystals during crystallization, and mesoporous templates are typically expensive, increasing synthesis costs. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for preparing nano-titanium silicate molecular sieve TS-1. By precisely controlling the crystallization process of the TS-1 molecular sieve, a fully silicate precursor solution containing a large number of MFI structural units is first prepared. Then, a Ti-source hydrolysate is introduced into the solution to induce hydrothermal crystallization of Ti species into the molecular sieve framework. Without adding crystallization aids or mesoporous template agents and without post-treatment, nano-titanium silicate molecular sieve TS-1 rich in framework Ti species is synthesized. The method described in this invention is simple to operate, has a short synthesis cycle, and effectively avoids the formation of non-framework Ti caused by the mismatch between the hydrolysis and crystallization rates of silicon and titanium species. The prepared nano-titanium silicate molecular sieve TS-1 is a microcrystalline aggregate with a size of 100~200 nm.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing nano-titanium silicate molecular sieve TS-1 includes the following steps:

[0008] (1) Add the silicon source to the template agent aqueous solution and mix well to obtain a total silicon mixed solution;

[0009] (2) The all-silicon mixed solution was rotary evaporated and heated to obtain an all-silicon precursor solution;

[0010] (3) Add the titanium source to an aqueous solution containing a complexing agent and hydrolyze to obtain a titanium source hydrolysate;

[0011] (4) Add the titanium source hydrolysate to the all-silicon precursor solution prepared in step (2) to obtain a silicon-titanium mixed solution. Seal the solution in a reactor and perform hydrothermal crystallization to obtain the crystallized product. After filtration, washing, drying and calcination, nano-titanium-silicon molecular sieve TS-1 is obtained.

[0012] In this invention, the template agent in the aqueous solution of the template agent in step (1) has a mass fraction of 10-18%, and the template agent is preferably tetraalkylammonium hydroxide, more preferably tetrapropylammonium hydroxide (TPAOH). The silicon source is an organosilicon source, preferably one or a mixture of tetraethyl orthosilicate, tetrapropyl orthosilicate, and tetrabutyl orthosilicate; the mass fraction of SiO2 in the prepared all-silicon mixed solution is 10-25%.

[0013] In this invention, the rotary evaporation operation temperature in step (2) is 20~50℃ and the rotary evaporation time is 20~120min; the heating temperature is 80~180℃ and the heating time is 5~60min. In particular, when the heating temperature is higher than 100℃, it needs to be sealed.

[0014] In this invention, the complexing agent in the aqueous solution containing the complexing agent in step (3) has a mass fraction of 2-10%; the complexing agent is one or a mixture of isopropanol, ethanol, tert-butanol, hydrogen peroxide, and tert-butanol peroxide; the titanium source is an organic titanium source, preferably one or a mixture of tetraethyl titanate, tetrapropyl titanate, and tetrabutyl titanate; the molar ratio of titanium source to complexing agent in the titanium source hydrolysate is 1:1-5; the titanium source hydrolysate is prepared and used immediately; if one or a mixture of hydrogen peroxide and tert-butanol peroxide is used as the complexing agent, the titanium source hydrolysis temperature is 0-10℃, preferably 0-5℃; if one or a mixture of isopropanol, ethanol, and tert-butanol is used as the complexing agent, the titanium source hydrolysis temperature is 10-30℃, preferably 20-30℃; the hydrolysis method is mechanical stirring assisted, the stirring time is 10-60 min, and the rotation speed is 50-350 rpm.

[0015] In this invention, the hydrothermal crystallization in step (4) is either static crystallization or dynamic crystallization. The rotation speed of dynamic crystallization is 5~30 rpm; the crystallization temperature is 160~200℃; and the crystallization time is 2~20h.

[0016] In this invention, the all-silicon precursor solution prepared in step (2) is a colorless and transparent solution. The solid residue after evaporation is detected by X-ray diffraction (XRD) as an amorphous structure, so as to ensure that step (4) can crystallize the Ti source into the TS-1 framework of the titanium-silicon molecular sieve.

[0017] In this invention, the silicon-titanium (Si:Ti) molar ratio in the silicon-titanium mixed solution in step (4) during hydrothermal crystallization is 1:0.02~0.05, and the skeleton Ti content in the prepared nano-titanium silicon molecular sieve TS-1 is 1.5~3.5wt.%.

[0018] Using the method provided by this invention, the nano-titanium silicon molecular sieve TS-1 is a microcrystalline aggregate with a size of 100~200nm, wherein the microcrystalline size is 10~50nm, and the aggregate contains a mesoporous structure.

[0019] This invention introduces a titanium source hydrolysate into a solution of an all-silicon precursor containing some molecular sieve structural units, followed by hydrothermal crystallization. This process matches the hydrolysis and crystallization rates of the silicon and titanium sources, promoting the crystallization of Ti species into the molecular sieve framework and avoiding the formation of non-framework Ti. It also enables the crystallized TS-1 nanocrystals to self-assemble, forming microcrystalline aggregates with a certain mesoporous structure.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. No crystallization aids or mesoporous templates are required, making it green, environmentally friendly, and low-cost;

[0022] 2. The synthesis method is simple, has good reproducibility, and is easy to scale up for industrial production;

[0023] 3. Short synthesis cycle and high efficiency. Traditional methods require strict control of the titanium source droplet acceleration rate, generally taking at least 6 hours, and the crystallization process typically takes 2-4 days, resulting in a long synthesis cycle. This invention can obtain the crystallized product of TS-1 molecular sieve within 24 hours;

[0024] 4. It can effectively avoid the occurrence of non-framework Ti in TS-1, and the obtained nano TS-1 has a certain stacked mesoporous structure, which has good prospects for catalytic applications. Attached Figure Description

[0025] Figure 1 X-ray diffraction (XRD) spectra of the TS-1 samples prepared for comparative examples and embodiments;

[0026] Figure 2 The UV-Vis absorption (UV-Vis) spectra of the TS-1 samples prepared for comparative examples and embodiments;

[0027] Figure 3 Scanning electron microscope (SEM) images of TS-1 samples prepared for comparative examples and embodiments;

[0028] Figure 4 Transmission electron microscopy (TEM) image of the TS-1 sample prepared for the example. Detailed Implementation

[0029] The following detailed embodiments illustrate the implementation process and beneficial effects of the present invention, aiming to help better understand the essence and characteristics of the invention, and are not intended to limit the scope of implementation of this invention. All embodiments are performed according to the operating steps of the above technical solution.

[0030] XRD patterns were obtained using a Rigaku D / max-2400 X-ray diffractometer (Japan), employing CuKα rays, with a tube voltage of 40 kV, a current of 100 mA, and a scanning range of 2. θ The scanning angle is 5° to 50°, the scanning step is 0.02°, and the scanning speed is 8° / min.

[0031] UV-Vis spectra were characterized using a UV-2700 UV-Vis spectrophotometer manufactured by Shimadzu Corporation of Japan. BaSO4 powder was used as a reference sample. The wavelength measurement range was 190~500 nm, and the measurements were performed in diffuse reflectance high-speed mode.

[0032] SEM images were taken using a NOVA NanoSEM 450 field emission scanning electron microscope from FEI Corporation, USA, with a voltage of 230kV, a frequency of 60Hz, a current of 8A, and a magnification of 50.0kx. The sample was dispersed in anhydrous ethanol, dropped onto a silicon wafer using a capillary tube, and then fixed onto conductive adhesive for testing.

[0033] TEM images were taken using a G2F30 high-resolution transmission electron microscope manufactured by FEI Corporation, USA. The test voltage was 300 kV. The sample needed to be ground first, then ultrasonically dispersed in anhydrous ethanol for a period of time, and then dropped onto a Cu mesh for testing.

[0034] The Ti content was determined using an IRIS Intrepid II XSP atomic emission spectrometer manufactured by Thermo Elemental, USA. Before measurement, the sample required thorough digestion with a strong acid solution and dilution with deionized water to 10–500 ppm.

[0035] Example 1

[0036] (1) Add 100g of tetraethyl orthosilicate to 105g of a 17.1% TPAOH aqueous solution and stir until homogeneous to obtain a whole silicon mixed solution; (2) Place the whole silicon mixed solution at 50℃ for 40min and then heat at 90℃ for 120min to obtain a whole silicon precursor solution; (3) Add 5.4g of tetrabutyl titanate to 20g of a 5% H2O2 aqueous solution and place in a 0℃ ice-water bath for 50min with mechanical stirring at 150rpm to obtain a titanium source hydrolysate; (4) Add the titanium source hydrolysate to the whole silicon precursor mixed solution and stir at room temperature until homogeneous. Seal the mixture and place it in a reactor. Static crystallize at 190℃ for 6h. After crystallization, the product is filtered, washed, dried, and calcined to obtain nano TS-1 molecular sieve, denoted as TS-1-1#. The silicon-titanium (Si:Ti) molar ratio is 1:0.033, and the Ti content in the product TS-1-1# is 2.5%.

[0037] Example 2

[0038] (1) Add 100g of tetrabutyl orthosilicate to 115g of a 16.5% TPAOH aqueous solution and stir until homogeneous to obtain a silicon-based mixed solution; (2) Place the silicon-based mixed solution at 40℃ for 80min by rotary evaporation, then seal and heat at 140℃ for 60min to obtain a silicon-based precursor solution; (3) Add 1.8g of tetraethyl titanate to 14.7g of a 9% H2O2 aqueous solution and mechanically stir in a 5℃ ice-water bath for 30min at a stirring speed of 300rpm to obtain a titanium source hydrolysate; (4) Add the titanium source hydrolysate to the silicon-based precursor mixed solution and stir at room temperature until homogeneous. Seal and pack into a reactor and place at 160℃ for static crystallization for 20h. After crystallization, the product is filtered, washed, dried, and calcined to obtain nano-TS-1 molecular sieve, denoted as TS-1-2#. The silicon-titanium (Si:Ti) molar ratio is 1:0.025, and the Ti content in the product TS-1-2# is 1.9%.

[0039] Example 3

[0040] (1) Add 100g of tetraethyl orthosilicate to 160g of a 12% TPAOH aqueous solution and stir until homogeneous to obtain a whole silicon mixed solution; (2) Place the whole silicon mixed solution at 25℃ and rotary evaporate for 120min, then seal and heat at 170℃ for 30min to obtain a whole silicon precursor solution; (3) Add 3.3g of tetrabutyl titanate to 39.4g of a 3% isopropanol aqueous solution and mechanically stir in a 25℃ water bath for 15min at a stirring speed of 200rpm to obtain a titanium source hydrolysate; (4) Add the titanium source hydrolysate to the whole silicon precursor mixed solution and stir at room temperature until homogeneous. Seal and pack into a reactor and dynamically crystallize at 170℃ for 12h at a speed of 15rpm. After crystallization, the product is filtered, washed, dried, and calcined to obtain nano TS-1 molecular sieve, denoted as TS-1-3#. The silicon-titanium (Si:Ti) molar ratio is 1:0.02, and the Ti content in the product TS-1-3# is 1.56%.

[0041] Comparative Example 1

[0042] The comparative synthesis method follows the classical method (refer to patent: US4410501A), with the following specific steps: 43.9g TPAOH (20wt.%) was mixed with 38.8g H2O to form solution A; 50.0g tetraethyl orthosilicate and 2.7g tetrabutyl titanate were mixed to form solution B; solution B was slowly added dropwise to solution A and stirred for 30min; the homogeneous solution was hydrolyzed at 50℃ and subjected to alcohol removal until clear; the gel was transferred to a hydrothermal reactor and dynamically crystallized at 170℃ for 48h at 10rpm. After crystallization, the crystallized product was filtered, washed, dried, and calcined to obtain TS-1 molecular sieve, denoted as TS-1-R1#. The silicon-titanium (Si:Ti) molar ratio was 1:0.033, and the Ti content in product TS-1-R1# was 1.9%.

[0043] Comparative Example 2

[0044] The difference between Comparative Example 2 and Example 1 is that step (2) in Example 1 is removed, while the rest of the operation is the same as in Example 1: (1) 100g of tetraethyl orthosilicate is added to 105g of an aqueous solution with a mass fraction of 17.1% TPAOH and stirred until homogeneous to obtain a whole silicon mixed solution; (2) 5.4g of tetrabutyl titanate is added to 20g of an aqueous solution with a mass fraction of 5% H2O2 and mechanically stirred in an ice-water bath at 0℃ for 50min at a stirring speed of 150rpm to obtain a titanium source hydrolysate; (3) The titanium source hydrolysate is added to the whole silicon mixed solution and stirred at room temperature until homogeneous. The mixture is then sealed in a container and placed at 190℃ for static crystallization for 6h. After crystallization, the product is filtered, washed, dried, and calcined to obtain nano TS-1 molecular sieve, denoted as TS-1-R2#. The silicon-titanium (Si:Ti) molar ratio is 1:0.033, and the Ti content in the product TS-1-R2# is 1.8%.

[0045] Depend on Figure 1 It can be seen that samples TS-1-1#, TS-1-2#, TS-1-3#, TS-1-R1# and TS-1-R2# all have typical MFI topological structures, are highly crystalline and free of impurities.

[0046] Depend on Figure 2 It can be seen that samples TS-1-1#, TS-1-2#, and TS-1-3# only contain framework four-coordinated Ti species (210nm), while comparative examples TS-1-R1# and TS-1-R2# contain not only framework four-coordinated Ti species, but also a certain amount of non-framework six-coordinated Ti species (260nm) anatase (330nm).

[0047] Depend on Figure 3It can be seen that the crystal size of sample TS-1-R1# is 400~600nm, and the crystal size of samples TS-1-1#, TS-1-2#, TS-1-3# and TS-1-R2# is 100~200nm.

[0048] Depend on Figure 4 It can be seen that the crystals of samples TS-1-1#, TS-1-2#, and TS-1-3# are composed of nanocrystals with a size of 10~50nm stacked together, and there are stacked mesopores.

[0049] Using the same silicon-titanium feedstock, and synthesizing samples using the method of this invention (Example 1) and the classical method (Comparative Example 1), it was found that the Ti species in TS-1-1# were mainly framework four-coordinated Ti, while TS-1-R1# contained not only framework four-coordinated Ti, but also a large amount of non-framework six-coordinated Ti and anatase. Furthermore, the Ti content in sample TS-1-1# was higher than that in TS-1-R1#. In addition, the crystal size of sample TS-1-1# was significantly smaller than that of sample TS-1-R1#. These results indicate that the scheme provided by this invention can promote the crystallization of Ti in the raw materials into the molecular sieve framework, avoid the appearance of non-framework Ti and anatase, and prevent the loss of Ti species. Simultaneously, it can achieve the synthesis of nano-TS-1 with smaller crystal sizes by controlling the crystallization process without adding crystallization aids.

[0050] According to the characterization results of samples TS-1-1# and TS-1-R2#, both are well crystallized and free of impurities, and have similar crystal sizes. However, in addition to the framework four-coordinated Ti species, TS-1-R2# also contains a large amount of non-framework six-coordinated Ti and anatase. Moreover, the Ti content of TS-1-1# is significantly higher than that of TS-1-R2#, indicating that step (2) of the present invention is the key to ensuring that Ti is effectively crystallized and enters the molecular sieve framework.

[0051] Example of catalytic effect verification:

[0052] The nano-TS-1 molecular sieves synthesized in Examples 1-3 and those prepared in Comparative Examples 1-2 were compared using a 1-hexene epoxidation reaction. The 1-hexene epoxidation reaction conditions were as follows: catalyst, 50 mg; 1-hexene, 10 mmol; hydrogen peroxide (30 wt.%), 10 mmol; solvent methanol, 10 mL; reaction temperature, 60 °C; reaction time, 2 h.

[0053] The results of the comparative experiments on catalytic effects are shown in the table below:

[0054] catalyst 1-Hexene conversion TS-1-R1# 18% TS-1-R2# 20% TS-1-1# 42% TS-1-2# 35% TS-1-3# 26%

[0055] Because the nano-TS-1 molecular sieve framework of the present invention has a high Ti content and is a nanocrystalline aggregate with a hierarchical porous structure, the samples of each embodiment exhibit better catalytic performance compared with the comparative sample.

Claims

1. A method for preparing nano-titanium silicate molecular sieve TS-1, characterized in that, Includes the following steps: (1) Add the silicon source to the template agent aqueous solution and mix well to obtain a total silicon mixed solution; (2) The all-silicon mixed solution was rotary evaporated and heated to obtain an all-silicon precursor solution; (3) Add the titanium source to an aqueous solution containing a complexing agent, stir and hydrolyze to obtain a titanium source hydrolysate; (4) Add the titanium source hydrolysate to the all-silicon precursor solution prepared in step (2) to obtain a silicon-titanium mixed solution. Seal the solution in a reactor and perform hydrothermal crystallization to obtain the crystallized product. After filtration, washing, drying and calcination, nano-titanium-silicon molecular sieve TS-1 is obtained. The prepared nano-titanium silicate molecular sieve TS-1 is a microcrystalline aggregate with a size of 100~200nm, wherein the microcrystalline size is 10~50nm, and the aggregate contains a mesoporous structure. In step (2), the rotary evaporation operation temperature is 20~50℃ and the rotary evaporation time is 20~120min; the heating temperature is 80~180℃ and the heating time is 5~60min.

2. The method according to claim 1, characterized in that, The template agent in the aqueous solution of step (1) has a mass fraction of 10-18%, and the template agent is tetraalkylammonium hydroxide.

3. The method according to claim 2, characterized in that, The template agent is tetrapropylammonium hydroxide (TPAOH).

4. The method according to claim 1, characterized in that, The silicon source mentioned in step (1) is an organosilicon source; the mass fraction of SiO2 in the prepared all-silicon mixed solution is 10~25%.

5. The method according to claim 4, characterized in that, The silicon source mentioned in step (1) is one or a mixture of tetraethyl orthosilicate, tetrapropyl orthosilicate, and tetrabutyl orthosilicate.

6. The method according to claim 1, characterized in that, The complexing agent in the aqueous solution containing the complexing agent in step (3) has a mass fraction of 2-10%; the complexing agent is one or a mixture of isopropanol, ethanol, tert-butanol, hydrogen peroxide, and tert-butanol peroxide.

7. The method according to claim 1, characterized in that, The titanium source mentioned in step (3) is an organic titanium source; the molar ratio of titanium source to complexing agent in the titanium source hydrolysate is 1:1~5.

8. The method according to claim 7, characterized in that, The titanium source mentioned in step (3) is one or a mixture of tetraethyl titanate, tetrapropyl titanate, and tetrabutyl titanate.

9. The method according to claim 1, characterized in that, The hydrothermal crystallization in step (4) is either static or dynamic crystallization; the crystallization temperature is 160~200℃ and the crystallization time is 2~20h.

10. The method according to any one of claims 1 to 9, characterized in that, The all-silicon precursor solution prepared in step (2) is a colorless and transparent solution. The solid residue after evaporation is detected by X-ray diffraction as an amorphous structure, which ensures that step (4) can crystallize the Ti source into the TS-1 framework of the titanium-silicon molecular sieve.

11. The method according to any one of claims 1 to 9, characterized in that, In step (4), the silicon-titanium mixed solution undergoing hydrothermal crystallization has a silicon-titanium molar ratio of 1:0.02~0.05, and the skeleton Ti content in the prepared nano-titanium silicon molecular sieve TS-1 is 1.5~3.5 wt.%.

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