Method for preparing pure-phase CHA molecular sieve by using out-of-phase induction of KFI molecular sieve seed crystal

Abstract

The invention relates to the field of preparation of CHA molecular sieves, in particular to a method for preparing a CHA molecular sieve with controllable morphology through heterogeneous induction by using a KFI molecular sieve as a seed crystal in a system without using an organic template agent, a fluorine-containing medium and heavy metal ions. According to the method, the KFI molecular sieve is used as the seed crystal, the CHA molecular sieve is prepared through heterogeneous induction, the use of an organic template agent, a fluorine-containing medium and heavy metal ions is effectively avoided, the preparation cost and the synthesis waste liquid treatment cost are reduced, the synthesis time of the CHA molecular sieve can be shortened, the yield can be increased, and the method is suitable for large-scale preparation of the CHA molecular sieve.

Description

Technical Field

[0001] This invention relates to the field of CHA molecular sieve preparation, and in particular to a method for preparing morphology-controllable CHA molecular sieves by using KFI molecular sieves as seed crystals in a heterogeneous induction system without the use of organic template agents, fluorine-containing media and heavy metal ion systems. Background Technology

[0002] CHA-type molecular sieves (chabazite, abbreviated as CHA) belong to the rhombohedral crystal system and have a pore size of 0.38 nm, which is comparable to the kinetic diameter of many gas molecules. Its silicon-to-aluminum ratio is 2-∞, and the wide adjustable range of silicon-to-aluminum ratio makes CHA molecular sieves exhibit excellent performance in adsorption separation, membrane separation and catalysis.

[0003] The preparation of CHA molecular sieves mainly falls into two categories: one is synthesis using amorphous aluminosilicates as raw materials, and the other is synthesis via a crystal transformation method. In the synthesis method using amorphous aluminosilicates as raw materials, there are two main approaches. One is synthesis guided by organic structure-directing agents. Although the use of organic templates can effectively increase the silica-to-alumina ratio of CHA molecular sieves, its synthesis cost is relatively high. For example, invention patent CN 202010593655.X successfully synthesized pure silicon and high-silicon CHA molecular sieves using organic templates; invention patent CN 202010361283.8 uses N,N,N-trimethyl-1-adamantyl ammonium hydroxide (TMAdaOH) as a template to rapidly synthesize pure silicon and high-silicon CHA molecular sieves in dilute solutions. The other approach is to synthesize CHA molecular sieves without adding any organic templates, but this method requires the use of heavy metal ions (such as Sr...). 2+ Cs + (etc.) or fluorine-containing systems. Although the above methods can all achieve the synthesis of CHA molecular sieves, their preparation costs are relatively high, and the post-treatment of synthesis waste liquid is difficult and easily pollutes the environment.

[0004] To address the aforementioned issues, the inventors attempted hydrothermal synthesis of CHA molecular sieves using CHA seed crystals in a synthesis solution consisting only of an alkali source, an aluminum source, a silicon source, and water. Through a series of optimization attempts with different synthesis solution ratios, they were able to synthesize CHA molecular sieves. However, the synthesis time required was at least 10 hours, and a yield of over 50% could only be achieved after at least 16 hours. This limited the wider application of this method.

[0005] Therefore, there is an urgent need to explore a method for the rapid and high-yield synthesis of CHA zeolite without the need for fluorides, organic structure directing agents, or heavy metals. Summary of the Invention

[0006] To address the aforementioned issues, this invention uses KFI molecular sieve as a seed crystal to induce the preparation of CHA molecular sieves through heterogeneous induction. This effectively avoids the use of organic template agents, fluorine-containing media, and heavy metal ions, reducing preparation costs and wastewater treatment costs. Furthermore, it can shorten the synthesis time of CHA molecular sieves and increase the yield, making it suitable for large-scale preparation of CHA molecular sieves.

[0007] Therefore, the present invention provides the following technical solution: A method for preparing pure-phase CHA molecular sieves using KFI molecular sieve seed crystal heterogeneous induction involves adding KFI molecular sieve as a seed crystal to a synthesis solution, followed by high-temperature hydrothermal synthesis to obtain the CHA molecular sieve. The synthesis solution consists of an alkali source, an aluminum source, a silicon source, and water.

[0008] Preferably, the alkali source in the synthesis solution is potassium hydroxide, and the molar ratio of K2O / H2O in the synthesis solution is 3-5:450-2100, more preferably 4:1000-1400.

[0009] Preferably, the silicon source is silica sol, sodium silicate, or fumed silica; the aluminum source is sodium aluminate, aluminum hydroxide, aluminum isopropoxide, or aluminum sulfate; and the molar ratio of SiO2 / Al2O3 in the synthesis solution is 8-10:0.6-2.0, preferably 6-12:1.

[0010] Preferably, the molar ratio of Al2O3 / H2O in the synthesis solution is 0.6-2:450-2100, and more preferably 1:810-1200.

[0011] Preferably, the hydrothermal synthesis temperature is 140-170℃; the synthesis time is 6-48 h, preferably 8-12 h.

[0012] Preferably, the KFI molecular sieve is added to the synthesis solution after aging, and the aging temperature is 5-40 ℃ and the aging time is 0-5 h.

[0013] Preferably, the particle size of the KFI molecular sieve is 200-800 nm, and the mass of the KFI molecular sieve added to the synthesis solution accounts for 5-15% of the SiO2 source.

[0014] The present invention also provides a pure-phase CHA molecular sieve prepared by heterogeneous induction using KFI molecular sieve seeds. The CHA molecular sieve is a highly crystalline plate-like or walnut-like aggregate composed of small cubic crystals of 50-100 nanometers, with a Si / Al ratio of 3-3.5.

[0015] The application of the pure-phase CHA molecular sieve prepared by heterogeneous induction using KFI molecular sieve seeds provided by the present invention includes one application in the fields of gas adsorption separation and drying.

[0016] Compared with the prior art, the present invention has the following beneficial effects: First, this invention, by using KFI molecular sieves as seed crystals for heterogeneous induction, successfully avoids the use of organic template agents, fluorine-containing media, and heavy metal ions in the synthesis of CHA molecular sieves, thereby significantly reducing preparation costs and subsequent wastewater treatment costs. This innovation not only meets environmental protection requirements and reduces the risk of environmental pollution, but also makes the preparation process of CHA molecular sieves more economical and efficient, which is conducive to its application and promotion in large-scale industrial production.

[0017] Secondly, by optimizing the synthesis solution ratio and introducing KFI molecular sieve seeds, this invention significantly shortens the synthesis time of CHA molecular sieves and improves the synthesis yield. Compared to existing technologies that require at least 10 hours of synthesis time and 16 hours to achieve a yield of over 50%, the method of this invention can achieve high-yield synthesis of CHA molecular sieves in a shorter time, greatly improving production efficiency and providing strong support for the industrial production of CHA molecular sieves. Attached Figure Description

[0018] Figure 1 This is a schematic diagram illustrating the mechanism of synthesizing CHA molecular sieves according to the present invention; Figure 2 The XRD pattern (a) and SEM image (b) of the KFI molecular sieve synthesized in Example 1 are shown. Figure 3 XRD patterns of CHA zeolite synthesized at different times; Figure 4 SEM images of CHA zeolite synthesized at different times; Figure 5 To compare the effects of samples synthesized at 298 K and 0-1 atm with different water contents on CO2 ( Figure 5 a) and CH4 ( Figure 5 b) Single-component adsorption isotherms and CO2 / CH4 selectivity curves ( Figure 5 c). Detailed Implementation

[0019] To better illustrate the advantages of the method for preparing CHA molecular sieves using KFI molecular sieve heterogeneous induction provided by the present invention, specific embodiments are given below, but the scope of protection of this patent is not limited to these embodiments.

[0020] Figure 1This diagram illustrates the mechanism of heterogeneous induction preparation of CHA zeolite using KFI zeolite in this invention. As shown, small aluminum and silicon species in the synthesis solution first aggregate into larger gel particles through polymerization during hydrothermal treatment. Then, the added KFI zeolite rapidly dissolves into composite building blocks during hydrothermal treatment, particularly the d6r structure, which is crucial for CHA zeolite crystallization. The polymerized gel particles form disc-shaped and walnut-shaped CHA zeolite aggregates on the d6r structure. With prolonged synthesis time, the disc-shaped crystals further aggregate into walnut-shaped CHA zeolite crystals.

[0021] Example 1 The steps for preparing CHA molecular sieves in Example 1 are as follows: (1) Synthesis of KFI molecular sieve seeds The gel composition of the KFI zeolite synthesis was: 0.236 K₂O: 0.125 Al₂O₃: 0.06 Cs₂O: 1 SiO₂: 8 H₂O: 0.2004 HF. Al(OH)₃, CsOH·H₂O, and KOH were added to deionized water in a specific ratio and boiled in a sealed PTFE reagent bottle until Al(OH)₃ was completely dissolved. After cooling to room temperature, Ludox AS-40 was added dropwise under stirring. After stirring for 2 hours, HF was added, and the precursor was aged at room temperature for 2 hours. Subsequently, hydrothermal synthesis was performed at 100°C for 190 hours. The resulting solid was washed with deionized water and dried overnight at 75°C.

[0022] (2) Preparation of CHA zeolite using KFI seed crystals The pure-phase CHA zeolite gel was prepared by dissolving Al(OH)3 in KOH solution, followed by boiling in a sealed PTFE reagent bottle until the Al(OH)3 was completely dissolved. After cooling to room temperature, the remaining water was added, and colloidal silica was added to the solution. The mixture was stirred at room temperature for 1 hour to form a synthesis solution. The composition of the synthesis solution was 9 SiO2: 1 Al2O3: 4 K2O: 1200 H2O. KFI zeolite seed crystals (10% of the SiO2 source mass) were then added and stirred for 1 minute. The synthesis gel was then transferred to an autoclave and subjected to hydrothermal synthesis at 150 °C for 4 to 48 hours. The resulting CHA zeolite was washed with deionized water and dried overnight at 100 °C.

[0023] Example 2 The steps for synthesizing the CHA molecular sieve in this embodiment are basically the same as those in Example 1, except that the composition of the synthesis solution is adjusted (see Table 1 for specific adjustments).

[0024] Example 3 The steps for synthesizing the CHA molecular sieve in this embodiment are basically the same as those in Example 1, except that the hydrothermal synthesis time is adjusted.

[0025] Comparative Example 1 The steps for synthesizing the CHA molecular sieve in this comparative example are basically the same as those in Example 1, except that it uses CHA molecular sieve as seed crystal (see Table 2 for specific synthesis conditions).

[0026] Characterization results Figure 2 Figures a and 2b show the XRD patterns and scanning electron microscope (SEM) images of the synthesized KFI molecular sieve, respectively, indicating that the synthesized KFI zeolite is a pure phase with a cubic morphology. The average crystal size of the synthesized KFI seeds is approximately 500 nm. Energy-dispersive X-ray spectroscopy (EDS) results show that the Si / Al and K+ / Cs+ ratios in the synthesized KFI seeds are approximately 3.1 and 1.36, respectively.

[0027] Figure 3 The XRD patterns of CHA zeolite synthesized at 150 °C using a standard gel composition (9 SiO2: 1 Al2O3: 4 K2O: 1200 H2O) for different times (samples 1-8) are shown. As can be seen from the figures, an amorphous phase was observed after 4-7 hours of crystallization. After 8 hours of crystallization, the peak intensity of CHA zeolite increased, while the peak of KFI zeolite disappeared and remained stable until 48 hours. This indicates that CHA zeolite completely crystallized within 8 hours. Figure 4 SEM images of CHA zeolite crystallized for 4–48 hours are shown. An amorphous phase was obtained after 4 hours of crystallization. Figure 4 a) indicates that the KFI seed crystals completely dissolved into composite structural units (CBUs) or an amorphous phase within 4 hours, which is consistent with... Figure 4 The XRD patterns shown are consistent with the observations. Crystallization occurred 7 hours later ( Figure 4 (b) Low-crystallinity walnut-shaped crystals are produced, with a diameter of approximately 2-4 micrometers. After 8 hours of crystallization, highly crystalline plate-like and walnut-shaped aggregates composed of small cubic crystals of 50-100 nanometers are produced. Figure 4 c). As the crystallization time was further extended to 48 hours, the morphology and crystal size of the platy and walnut-like aggregates did not change significantly. Figure 4 The time (d-4h) indicates that CHA zeolite was fully crystallized within 8 hours, consistent with the XRD results. The product yield was calculated to be approximately 0.63-0.72 during the crystallization period of 8-48 hours. The Si / Al and K / Al ratios of the samples synthesized within 8-48 hours were approximately 3.2 and 1.0, respectively. However, the percentage of walnut-shaped aggregates increased slightly with increasing crystallization time, indicating that platy aggregates gradually aggregated into walnut-shaped aggregates during synthesis. Therefore, a crystallization time of 8-12 hours is sufficient to produce highly crystalline CHA zeolite aggregates.

[0028] Table 1 shows the results of synthesizing CHA zeolite using KFI seeds under different conditions. In addition to the synthesis time, the effects of synthesis temperature, basicity, water content, and the SiO2 / Al2O3 ratio were also investigated. The optimal conditions for synthesizing highly crystalline CHA zeolite aggregates were determined to be: SiO2: 0.833~1.667, Al2O3: 4, K2O: 810-2100, H2O, at 150℃ for 8 to 48 hours. Table 2 shows the results of synthesizing CHA molecular sieves using CHA seeds under different conditions. Combining Tables 1 and 2, it can be seen that using KFI seeds can shorten the crystallization time to 8 hours, achieving a Si / Al ratio greater than 3 and a yield of 0.63-0.72. However, using CHA seeds requires at least 10 hours of crystallization, with a Si / Al range of 2.24 to 2.66 and a yield less than 0.6, and a yield of over 0.5 is only achieved after at least 16 hours.

[0029] Table 1. Results of CHA molecular sieve synthesis under different conditions using KFI as seed crystals Table 2. Results of CHA molecular sieves synthesized under different synthesis conditions using CHA as seed crystals. Figure 5 The effects of samples synthesized at 298 K and 0-1 atm with different water contents on CO2 ( Figure 5 a) and CH4 ( Figure 5 b) Single-component adsorption isotherms and CO2 / CH4 selectivity curves ( Figure 5 c). The CO2 adsorption isotherm showed a sharp increase at low pressure in all samples, indicating the presence of significant micropores ( Figure 5 a). Interestingly, CHA zeolite with a water content of 1200 exhibited the highest CO2 and CH4 adsorption capacity ( Figure 5 (a and 5b), while the CHA zeolite with a water content of 1800 exhibited the lowest CO2 and CH4 adsorption throughout the test. The ideal selectivity of different components calculated from the single-component adsorption was used to evaluate the separation performance. With increasing relative pressure, the CO2 / CH4 selectivity of all CHA zeolite samples decreased (a and 5b). Figure 5(c) This is due to the strong interaction between CO2 and certain specific adsorption sites. Although CHA zeolite with a water content of 1200 exhibits the highest CO2 adsorption capacity, CHA zeolite with a water content of 1500-1800 shows the highest ideal selectivity for CO2 and CH4. The ideal selectivity for CO2 and CH4 is 7.3 at P / P0 = 0.5 and 4.3 at P / P0 = 1.0. Therefore, CHA zeolite with a water content of 1200-1500 is more suitable for adsorbing CO2 from CH4 and capturing carbon, indicating its potential in pressure swing adsorption. The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing pure-phase CHA molecular sieves using KFI molecular sieve seed crystals induction, characterized in that, KFI molecular sieve is added as a seed crystal to the synthesis solution, and the CHA molecular sieve is obtained by high-temperature hydrothermal synthesis. The synthesis solution consists of an alkali source, an aluminum source, a silicon source, and water.

2. The method according to claim 1, characterized in that, The alkali source in the synthesis solution is potassium hydroxide, and the molar ratio of K2O / H2O in the synthesis solution is 3-5:450-2100, preferably 4:1000-1400.

3. The method according to claim 1, characterized in that, The silicon source is silica sol, sodium silicate, or fumed silica; the aluminum source is sodium aluminate, aluminum hydroxide, aluminum isopropoxide, or aluminum sulfate; the molar ratio of SiO2 / Al2O3 in the synthesis solution is 8-10:0.6-2.0, preferably 6-12:

1.

4. The method according to claim 1, characterized in that, The molar ratio of Al2O3 / H2O in the synthesis solution is 0.6-2:450-2100, preferably 1:810-1200.

5. The method according to claim 1, characterized in that, The hydrothermal synthesis temperature is 140-170℃; the synthesis time is 6-48 h, preferably 8-12 h.

6. The method according to claim 1, characterized in that, After aging, KFI molecular sieves are added to the synthesis solution. The aging temperature is 5-40 ℃ and the aging time is 0-5 h.

7. The method according to claim 1, characterized in that, The KFI molecular sieve has a particle size of 200-800 nm, and the mass of KFI molecular sieve added to the synthesis solution accounts for 5-15% of the SiO2 source.

8. A pure-phase CHA molecular sieve prepared by heterogeneous induction using KFI molecular sieve seeds, characterized in that, The CHA molecular sieve is a highly crystalline, plate-like or walnut-like aggregate composed of small cubic crystals of 50-100 nanometers, with a Si / Al ratio of 3-3.

5.

9. The application of the pure-phase CHA molecular sieve prepared by heterogeneous induction using KFI molecular sieve seeds according to claim 8, characterized in that, The application includes one of the fields of gas adsorption separation and drying.

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