CN116161674B - Hydrothermal preparation method of nano Ti-Beta molecular sieve - Google Patents

Abstract

The present invention discloses a hydrothermal preparation method of nano Ti-Beta molecular sieve. The method of the present invention is a one-step hydrothermal direct and rapid synthesis of Ti-Beta molecular sieves with excellent performance by preparing a precursor. Compared with the traditional method of directly synthesizing Ti-Beta molecular sieves, the method has a small amount of template agent, a short synthesis time, and no need to add fluoride, which can effectively reduce energy consumption and environmental pollution. In addition, the obtained Ti-Beta molecular sieve has a good titanium distribution. The synthesis method provided by the present invention is fast and simple, has strong repeatability, and has significant technical advancement compared with existing methods.

Description

A hydrothermal preparation method of nano Ti-Beta molecular sieve

Technical Field

The invention relates to a new method for preparing Ti-Beta molecular sieve, and specifically to a method for directly and rapidly synthesizing Ti-Beta molecular sieve with excellent performance in one step by preparing a precursor through hydrothermal method.

Background Art

Ti-Beta is a molecular sieve with a twelve-membered ring straight channel structure with a pore diameter of 0.77x0.67nm. Due to its wide range of silicon-aluminum ratios, large pores, abundant acid sites, and good stability, it has broad application prospects in the fields of epoxidation, alcohol selective oxidation, and oxidative desulfurization. So far, the preparation methods of Ti-Beta molecular sieves mainly include hydrothermal synthesis, gel synthesis, and post-synthesis.

Blasco T et al. hydrothermally synthesized Ti-Beta molecular sieve under the conditions of seeding (Chem. Commun, 1996, 20, 2367-368.) and seeding-free (J. Phys. Chem. B, 1998, 102 (1): 75-88): silicon source, titanium source, deionized water and template agent were added to the crystallization system, and the final product was obtained by crystallization at a certain temperature and for a certain period of time. In the hydrothermal synthesis method of Ti-Beta, the crystallization system can be roughly divided into a fluorine-containing system and a hydroxyl system (high pH). Crystallization in a neutral environment using fluorine-containing compounds as mineralizers does not require seeding guidance or the addition of aluminum-containing substances. However, the synthesized molecular sieve has large grains and the toxicity of the mineralizer will cause a great burden on the environment. In the hydrothermal synthesis under the strong alkaline conditions of the hydroxyl system, at least one of the seed and the aluminum-containing substance is selected, and aluminum will introduce B acid sites, affecting the catalytic performance. In addition, the traditional hydrothermal method has disadvantages such as large amount of template used, large amount of crystallization waste liquid discharged, and long crystallization time, which need further improvement.

Camblor MA et al. (catalysis. A, General, 1995, 133 (2): L185-L189) used "wet" gel to prepare Ti-Beta molecular sieve: first, TiO ₂ -SiO ₂ gel was prepared, then impregnated with a template directing agent, and finally crystallized to synthesize Ti-Beta molecular sieve. This method also requires the addition of aluminum or aluminum-containing seed crystals during the crystallization process, and still discharges a large amount of waste liquid. The crystallization time requires more than one week. Jappar N et al. (Journal of catalysis, 1998, 180 (2): 132-141.) used the dry gel method to prepare Ti-Beta molecular sieve for the first time. In this method, the gel powder is not in direct contact with water, but is transferred to a special crystallization kettle, and the Ti-Beta molecular sieve is synthesized using the auxiliary effect of water vapor. This dry gel method requires not only the addition of aluminum, but also a small amount of sodium, and the sodium content will affect the crystallinity of the molecular sieve and the amount of Ti incorporated. Although the amount of template used and the amount of waste liquid produced by the dry glue method are less than those of the hydrothermal method, its synthesized grains are larger (micrometer level) and its repeatability is poor, so it has not yet been applied in industry.

The post-synthesis method is to synthesize Ti-Beta molecular sieve by dealuminizing Beta zeolite and doping titanium in two steps. At present, it is roughly divided into gas-solid, solid-solid, and liquid-solid isomorphous substitution methods. The post-synthesis method generally has a short synthesis time, and Ti-Beta molecular sieve can be prepared in 1-2 days.

It is much more difficult to incorporate Ti into the framework of Beta zeolite than TS-1. Therefore, the crystallization of Ti-Beta and the amount of titanium incorporated into the framework are two key issues faced by industrial production. The crystallization time of the hydrothermal method and the gel method is 7-14 days, and their framework titanium incorporation is limited. The advantages of the post-synthesis method are its short time and high titanium incorporation, but no matter which post-synthesis method is used, it is difficult to control the coordination state of titanium, and there is a large amount of non-framework titanium and anatase; in addition, the defect sites cannot be completely occupied, resulting in the formation of B acid, which affects the catalytic performance.

Therefore, it is of great practical significance to study new Ti-Beta preparation methods in view of the shortcomings of existing processes.

Summary of the invention

The purpose of the present invention is to provide a preparation method for hydrothermally synthesizing nano Ti-Beta molecular sieves with simple steps, short time consumption, no need for additional addition of fluoride and aluminum source, and good distribution of titanium species.

A hydrothermal preparation method of nano Ti-Beta molecular sieve, the method comprising the following steps:

(1) The silicon source is calculated as SiO ₂ , and the titanium source is calculated as TiO ₂ , and the molar ratio of SiO ₂ :TiO ₂ :H ₂ O:HCl=1:0.01-0.05:20-80:0.033-0.135 is fully mixed, and stirred at room temperature for 20 minutes; then the solution is spray-dried at 220° C. using an aerosol generator to obtain a titanium silicon precursor powder;

(2) calcining the titanium silicon precursor powder at 550°C for 3-6 hours to obtain a solid powder; adding a template agent, tetraethylammonium hydroxide, and then adding Beta molecular sieve seeds equivalent to 2%-5% of the mass of the silicon source, and distilling and concentrating the mixed liquid in a water bath at 60-80°C to achieve a molar ratio of _SiO2 : _TiO2 : _H2O :TEAOH=1:0.01-0.05:3-10:0.2-0.55;

(3) The obtained turbid liquid is transferred to an autoclave and crystallized at 130-150° C. for 3-7 days. After washing with deionized water, drying and calcining, a Ti-Beta molecular sieve is obtained.

In step 1, the silicon source used is tetraethyl orthosilicate or silica sol, and the titanium source used is tetra-n-butyl titanate.

In step 3, the drying temperature is 110° C., the calcination temperature is 550° C., and the calcination time is 6 h.

Beneficial effects of the present invention:

Compared with the existing Ti-Beta molecular sieve preparation technology, this method has the following significant advantages:

a. The special precursor preparation method of the present invention: hydrolyze the silicon source and the titanium source under acidic conditions, and prepare the titanium silicon microsphere precursor by combining aerosol spray drying with high temperature calcination. The coordination state of the titanium species is effectively regulated before crystallization, solving the primary problem of Ti-Beta molecular sieve synthesis; the titanium silicon microsphere precursor is dissolved with a template agent, and the appropriate ratio is the key to whether it can be synthesized. In addition, see step 2, the concentration of tetraethylammonium hydroxide available on the market is 25% to 35%, which cannot meet the ratio requirements of this method, so additional water evaporation is required; and industrialization can use a template agent with a concentration of more than 90%. The method can also find a reasonable ratio, and the water distillation and concentration step is omitted, and the synthesis path can be further simplified. The aerosol precursor is treated with high temperature calcination to achieve the purpose of reducing the amount of synthesis water, eliminating the influence of the acid content in the precursor, and optimizing the titanium distribution.

b. The hydrothermal synthesis of Ti-Beta molecular sieve by this method does not require additional treatment of the titanium source and silicon source. The synthesis process is simple and convenient, and there are no stringent requirements for conditions in each link, and the repeatability is very strong. In addition, the crystallization time of this method is significantly shorter than that of the traditional hydrothermal method.

c. This synthesis method does not add additional fluorine source, aluminum source and alkali metal (Na), which minimizes resource waste and environmental pollution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 3 are XRD characterization results of the Ti-Beta molecular sieve synthesized in the present invention in the examples.

FIG. 2 is a UV-vis graph showing the control comparison of titanium species of the Ti-Beta molecular sieve synthesized in the present invention.

FIG. 4 is a SEM image of the Ti-Beta molecular sieve synthesized in the present invention.

DETAILED DESCRIPTION

The present invention is described in detail below by way of examples, but the present invention is not limited to these examples.

The crystallization process in the following embodiments is a crystallization process known in the art, and the crystallization process is a static crystallization process.

The washing, drying and roasting methods described in the following examples are well known to those skilled in the art. For example, washing with deionized water until the pH is neutral, and drying at 110°C for 3 to 10 hours.

In the following examples, the molar mass of the silicon source is calculated as _SiO2 , the molar mass of the titanium source is calculated as _TiO2 , the molar mass of the template is calculated as TEA ⁺ , and the mass of the silicon source is calculated as the contained _SiO2 .

Example 1

This example is used to illustrate a method for preparing nano-Ti-Beta molecular sieve.

The titanium source, silicon source (tetraethyl orthosilicate), deionized water and hydrochloric acid were fully mixed in a molar ratio of SiO2:TiO2:H2O:HCl=1:0.016:20:0.135, and stirred at room temperature for 20 minutes; the solution was then spray-dried at 220°C using an aerosol generator to obtain a titanium silicon precursor powder; the precursor powder was calcined at 550°C for 6 hours to obtain a solid powder, 11.61 g of tetraethylammonium hydroxide was added, and then Beta molecular sieve seeds equivalent to 5% of the mass of the silicon source were added, and the mixed liquid was concentrated by distillation in a 70°C water bath to reach a molar ratio of SiO2:TiO2:H2O:TEAOH=1:0.016:3.4:0.55; finally, the obtained turbid solution was transferred to an autoclave, crystallized at 140°C for 5 days, and Ti-Beta molecular sieve was obtained after washing with deionized water, drying and calcining. The product was recorded as "Sample 1". Its XRD spectrum is shown in FIG1 ; its UV-vis spectrum is shown in FIG2 “after regulation”, that is, the prepared aerosol powder is calcined at high temperature; and its SEM is shown in FIG3 .

Example 2

This example is compared with Example 1: used to further illustrate the regulation of the coordination state of titanium species by the present invention

The titanium source, silicon source, deionized water and hydrochloric acid were fully mixed in a molar ratio of SiO2:TiO2:H2O:HCl=1:0.016:20:0.135, and stirred at room temperature for 20 minutes; the solution was then spray-dried at 220°C using an aerosol generator to obtain a titanium silicon precursor powder; the obtained precursor powder was directly added to 11.61g of tetraethylammonium hydroxide without treatment, and then Beta molecular sieve seeds equivalent to 5% of the mass of the silicon source were added, and the mixed liquid was concentrated by distillation in a 70°C water bath to reach a molar ratio of SiO2:TiO2:H2O:TEAOH=1:0.016:3.4:0.55; finally, the obtained turbid solution was transferred to an autoclave, crystallized at 140°C for 5 days, and the Ti-Beta molecular sieve was obtained after washing with deionized water, drying and calcining. Its UV-vis spectrum is shown in “before regulation” in FIG2 , that is, the prepared aerosol powder is not subjected to calcination treatment and is directly fed into the feed for crystallization.

Example 3

This example is compared with Example 1: used to further verify the uniqueness of the invention

The titanium source, silicon source (tetraethyl orthosilicate), deionized water and hydrochloric acid were fully mixed in a molar ratio of SiO2:TiO2:H2O:HCl=1:0.016:20:0.135, and stirred at room temperature for 20 minutes; the solution was then spray-dried at 220°C using an aerosol generator to obtain a titanium silicon precursor powder; the precursor powder was calcined at 550°C for 6 hours to obtain a solid powder, 11.61 g of tetraethylammonium hydroxide was added, and then Beta molecular sieve seeds equivalent to 5% of the mass of the silicon source were added, and the mixed liquid was stirred evenly at room temperature; finally, the obtained turbid solution was transferred to an autoclave, crystallized at 140°C for 5 days, and Ti-Beta molecular sieve was obtained after washing with deionized water, drying and calcining, that is, the precursor template was not concentrated, and the product was recorded as "sample 2", and its XRD spectrum is shown in Figure 1. Through X-ray powder diffraction characterization, the crystallinity of sample 1 was used as a reference, and the relative crystallinity of sample 2 was only 46%. In addition, due to the addition of Beta seeds in the synthesis route, combined with the obtained yield, it was judged that only a very small amount or no Ti-Beta molecular sieve was synthesized.

As can be seen from the mutual support of Examples 1, 2, and 3, the preparation of Ti-Beta molecular sieves in the present invention has distinct characteristics: the distribution of titanium species will be deteriorated by simply using aerosol technology without calcination, and the failure to concentrate the template will directly lead to the inability to synthesize Ti-Beta molecular sieves. Each process of precursor preparation complements each other and is indispensable.

Example 4

This example is used to illustrate that this method can be used to synthesize Ti-Beta molecular sieves with different template dosages.

Example 1 was repeated, but in the final water distillation and concentration step, the molar ratio was SiO2: TiO2: H2O: TEAOH = 1: 0.016: 4.5: 0.2/0.3/0.4. The products obtained were recorded as "sample 3", "sample 4" and "sample 5", and their XRD spectra were shown in Figure 3. Taking the crystallinity of sample 1 as a reference, the relative crystallinity of samples 3-5 reached 70%, 90% and 115% respectively. It can be seen that the template dosage of the present invention can reach the best at 0.4, which is less than the template dosage of the traditional hydrothermal method (0.55).

Example 5

This example is used to illustrate that the present method can be used to synthesize Ti-Beta molecular sieves through different crystallization times.

Example 1 was repeated, but the final crystallization time was 140°C for 3d, 4d, 6d, and 7d. The products obtained were recorded as "Sample 6", "Sample 7", "Sample 8", and "Sample 9", and their XRD spectra are shown in Figure 3. Taking the crystallinity of sample 1 as a reference, the relative crystallinity of samples 6-9 reached 80%, 89%, 114%, and 102%, respectively. It can be seen that the present invention reaches the best crystallization state after 6 days of crystallization, but the relative crystallinity has reached 80% after 3 days of crystallization. The crystallization time required for synthesizing Ti-Beta molecular sieve using the present invention is much shorter than the traditional hydrothermal method for directly synthesizing Ti-Beta molecular sieve.

Example 5

This example is used to illustrate that the powdered precursor can be calcined for different lengths of time to synthesize Ti-Beta molecular sieve.

Example 1 was repeated, but the spray powder precursor was calcined at 550°C for 3h, 4h, and 5h. The sample characterization results obtained were not much different from those in Example 1, which shows that the calcination time has little effect on the preparation of Ti-Beta molecular sieve.

Example 6

This example is used to illustrate that Ti-Beta molecular sieve can be prepared with different Ti contents.

Example 1 was repeated, but the titanium source, silicon source (tetraethyl orthosilicate), deionized water, and hydrochloric acid were fully mixed in a molar ratio of SiO2: TiO2: H2O: HCl = 1: 0.05/0.025/0.0125/0.01: 20: 0.135; similarly, the molar ratio reached in the final distillation and concentration step was SiO2: TiO2: H2O: TEAOH = 1: 0.05/0.025/0.0125/0.01: 4.5: 0.55, and Ti-Beta molecular sieves with different Ti contents were successfully prepared. It can be seen that the present invention can be used to prepare Ti-Beta molecular sieves with different titanium loadings as needed.

Example 7

This example is used to illustrate that Ti-Beta molecular sieve can be prepared by making some changes or modifications without departing from the technical solution of the present invention. In this example, Ti-Beta molecular sieve is prepared by replacing the silicon source and not adding seed crystals.

0.386g of tetrabutyl titanate, 2.5g of deionized water, and 1.0g of hydrogen peroxide (31wt%) were stirred at room temperature for 1h to make the solution uniform without precipitation, and then 7.1g of silica sol was added and stirred for 1.5h until the solution was uniform and clear; then the solution was spray-dried at 220°C using an aerosol generator to obtain titanium silicon precursor powder; the precursor powder was roasted at 550°C for 6h to obtain solid powder, added to 11.61g of tetraethylammonium hydroxide, and the mixed liquid was concentrated in a 70°C water bath by distillation to reach a molar ratio of SiO2:TiO2:H2O:TEAOH=1:0.016:3.4:0.55; finally, the obtained turbid solution was transferred to an autoclave, crystallized at 140°C for 5d, washed with deionized water, dried, and calcined to obtain Ti-Beta molecular sieve. The obtained product was recorded as "sample 10", and its XRD spectrum is shown in Figure 3.

From the above data, it can be seen that the special precursor hydrothermal direct synthesis Ti-Beta molecular sieve method provided by the present invention uses less crystallization time to obtain nano Ti-Beta molecular sieve with high crystallinity and good titanium species distribution state, and has strong repeatability. Based on the above analysis, the method of the present invention has the characteristics of realizing large-scale industrialization of Ti-Beta molecular sieve, greatly reducing the synthesis cost and environmental load, and greatly improving the application field of Ti-Beta molecular sieve.

Claims (4)

1. The hydrothermal preparation method of the nano Ti-Beta molecular sieve is characterized by comprising the following steps of:

(1) Silicon source is calculated as SiO ₂, titanium source is calculated as TiO ₂, and the silicon source is calculated as SiO ₂：TiO₂：H₂ O: hcl=1: 0.01 to 0.05: 20-80: mixing the mixture fully according to the molar ratio of 0.033-0.135, and stirring the mixture for 20min at room temperature; then using an aerosol generator to spray-dry the solution at 220 ℃ to obtain titanium-silicon precursor powder;

(2) Roasting the titanium silicon precursor powder at 550 ℃ for 3-6 hours to obtain solid powder; adding template tetraethylammonium hydroxide, then adding Beta molecular sieve seed crystal accounting for 2-5% of the mass of the silicon source, evaporating water and concentrating the mixed liquid in a water bath kettle at 60-80 ℃ to obtain a molar ratio of SiO ₂：TiO₂：H₂ O: teaoh=1: 0.01 to 0.05: 3-10: 0.2 to 0.55;

(3) Transferring the obtained turbid liquid into an autoclave, crystallizing for 3-7 d at 130-150 ℃, washing with deionized water, drying, and calcining to obtain the Ti-Beta molecular sieve.

2. The method for preparing nano Ti-Beta molecular sieve according to claim 1, wherein in the step 1, the silicon source is tetraethyl orthosilicate or silica sol.

3. The method for preparing nano Ti-Beta molecular sieve according to claim 1, wherein the titanium source is tetra-n-butyl titanate.

4. The method for preparing nano Ti-Beta molecular sieve according to claim 1, wherein in the step 3, the drying temperature is 110 ℃, the roasting temperature is 550 ℃, and the roasting time is 6 hours.