A preparation method and application of a NaX molecular sieve catalyst synthesized by in-situ growth

Abstract

This invention discloses a method for preparing and applying NaX molecular sieve catalysts using an in-situ growth method. KOH, deionized water, and a Si source are mixed and stirred. A metallic Al source solution is slowly added dropwise, and the mixture is heated in a water bath to form a homogeneous precursor solution. A carbon electrode support is immersed in this solution for a hydrothermal reaction, during which the NaX molecular sieve catalyst is grown in situ on the support. The catalyst is then washed and dried. This invention uses an in-situ growth method to directly generate the molecular sieve catalyst on the electrode surface. Compared to conventional catalysts and molecular sieve catalysts produced by coating, this molecular sieve catalyst exhibits better ion conversion efficiency and higher stability, and can reduce nitrate to ammonia in a system containing nitrate solution.

Description

Technical Field

[0001] This invention discloses a method for preparing NaX molecular sieve catalysts using in-situ growth and its application. Background Technology

[0002] Nitrate pollution, one of the most widespread pollutants in water bodies, can induce carcinogenic nitrite conversion when it accumulates excessively. Meanwhile, ammonia, a core raw material in modern industry, is traditionally synthesized using the energy-intensive and high-emission Haber-Bosch process. This process requires approximately 400-500°C and 150-300 atmospheres of pressure, emitting over 2 tons of carbon dioxide per ton of ammonia synthesized. To address the water pollution caused by nitrate and the high energy consumption and pollution issues of the HB ammonia synthesis method, various catalysts have been developed in recent years for the electrocatalytic reduction of nitrate to ammonia.

[0003] Photocatalytic reduction of nitrate to ammonia is a green technology that uses light energy to drive semiconductor materials (such as titanium dioxide or tungsten-based compounds) to convert nitrate into ammonia. However, the efficiency of photocatalytic reactions is generally low, the stability of photocatalysts is insufficient, semiconductor materials are prone to photocorrosion, and active sites gradually deactivate under long-term light exposure.

[0004] Surface-coated molecular sieve catalysts are applied by coating molecular sieve catalysts onto a support. The molecular sieve coating provides abundant active sites, enhancing the efficiency of the catalytic reaction. However, controlling the uniformity of the coating is difficult, potentially leading to uneven distribution of the molecular sieves and affecting catalytic performance. Mechanical strength is a particularly prominent issue; coatings that are too thick or too thin are prone to cracking or peeling. In contrast, molecular sieve electrocatalysts, with their high efficiency, reusability, and high stability, warrant further investigation.

[0005] The 8-electron transfer process of the electrocatalytic reduction of nitrate to ammonia is relatively complex and easily produces byproducts such as NO, N2, NH2OH, and N2O. NO3 -The potential for reduction to ammonia is typically below the hydrogen evolution reaction potential, resulting in the production of hydrogen gas, excessive electron consumption, and ultimately low Faradaic efficiency. Patent CN202410661850.X discloses a method for preparing Cu-doped Co3O4 nanosheet catalysts. The introduction of Cu enhances the conductivity of Co3O4, reduces charge transfer resistance, and improves electrocatalytic efficiency. However, its synthesis process lacks precise control, and the large range of calcination temperatures may affect the uniformity of material synthesis. Patent CN202311403822.X discloses a transition metal-doped molecular sieve catalyst and its electrocatalytic application. The H-MFI-FP catalyst, modified with Fe and P, reduces the coupling of adjacent N atoms in the reaction, reducing side reactions and improving the selectivity, ammonia yield, and Faradaic efficiency of electrocatalytic reduction. However, its molecular sieve catalyst is coated on the electrode surface, resulting in an unstable mechanical structure that easily detaches under molecular catalysis. Therefore, there is an urgent need for a stable molecular sieve catalyst capable of efficient electrocatalytic reduction to ammonia in nitrate solution. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing NaX molecular sieve catalysts using in-situ growth and its application. The NaX molecular sieve catalysts synthesized by the in-situ growth method of this invention can be used to reduce nitrate ions and can efficiently reduce nitrate ions in solution to ammonia.

[0007] The technical solution adopted in this invention is as follows: A method for synthesizing NaX molecular sieve catalysts using an in-situ growth method includes the following steps: S1 Preparation of precursor solution: Mix structure directing agent KOH, deionized water and Si source and stir, slowly add metal Al source solution, and heat in water bath to form a homogeneous precursor solution; S2 Support treatment and in-situ growth: The carbon electrode support is immersed in the precursor solution of step S1, the mixture is transferred to the reaction vessel, the reaction vessel is sealed and transferred to the oven, and a hydrothermal reaction is carried out under heating to grow NaX molecular sieve catalyst in situ on the support. S3 Product Processing: Cool the sample, wash with water, and then dry it to complete the preparation.

[0008] Furthermore, in step S1, the mass ratio of KOH to deionized water is 1:4-8.

[0009] Furthermore, the molar ratio of KOH to Al source is 0.15-0.4, preferably 0.3-0.35; the molar ratio of Si source to Al source is 0.95-1.05, preferably 1.

[0010] Furthermore, the Si source includes one or a mixture of two of silica sol, silicon dioxide, and silicates, and the Al source includes one or a mixture of two of aluminum salts, aluminum oxide, and aluminum hydroxide.

[0011] Furthermore, in step S2, the oven temperature is 150-300℃, preferably 200-250℃, and the hydrothermal reaction time is 2-4h, preferably 3h.

[0012] Furthermore, in step S2, the carbon electrode carrier is made of graphite.

[0013] The present invention also discloses the application of the NaX molecular sieve catalyst in the electrocatalytic reduction of nitrate to ammonia.

[0014] The twelve-membered ring molecular sieve mentioned in this invention possesses a FAU topology. Its three-dimensional, interconnected supercage structure and abundant microporous network provide efficient migration pathways for cations, enabling them to rapidly diffuse to internal active sites. Furthermore, the hydroxyl groups (Si-OH-Al) in its pore structure form acidic sites, capable of specifically adsorbing and activating cations. It is precisely for these reasons that the FAU molecular sieve exhibits excellent catalytic potential.

[0015] Furthermore, based on the structural characteristics of FAU-type molecular sieves, we found that higher specific surface areas in FAU-type molecular sieves result in better electrocatalytic performance. NaX molecular sieves achieved high levels in both Faradaic efficiency and ammonia yield. This is attributed to the synergistic effect of the twelve-membered ring structure and ultra-high specific surface area, which gives NaX molecular sieves excellent electrocatalytic performance. Moreover, its supercage effect facilitates better ion exchange, and the high density of negatively charged sites resulting from the low silica-alumina ratio (Si / Al=1.0) provides ample active centers for ion exchange.

[0016] The NaX molecular sieve catalyst described in this invention can be used for the catalytic reduction of nitrate to ammonia.

[0017] Compared with the prior art, the beneficial effects achieved by the present invention are: 1. This invention discovers that NaX molecular sieves with a low silicon-to-aluminum ratio (Si / Al=1.0) exhibit a supercage effect, high density of negatively charged sites, and a 770 m... 2 ·g -1 Its high specific surface area results in significantly better ion exchange efficiency than USY molecular sieves, with an ammonia yield of 0.87 mmol·h⁻¹. -1 ·cm -2 The Faraday efficiency reached 75.60%, and it was applied to the reaction of nitrate catalytic reduction to ammonia.

[0018] 2. The special microporous structure of the molecular sieve of the present invention enables molecular-level sieving, the robust silicon framework ensures structural stability during catalysis, and the hydroxyl groups in the pores provide specific active centers for the reaction, exhibiting good substrate compatibility and catalytic efficiency in the cathode reaction.

[0019] 3. This invention provides an in-situ growth method for forming NaX molecular sieves for the catalytic reduction of nitrate to ammonia. Compared with traditional coated molecular sieves that have weak physical adsorption binding force and insufficient thermal stability, this invention has strong binding force, excellent high stability, significantly increased mechanical strength, and can obtain a uniform and controllable film thickness. Attached Figure Description

[0020] Figure 1 These are electrocatalytic performance graphs of molecular sieves with different pore structures in Example 1; Figure 2 This is a graph showing the electrocatalytic performance of molecular sieves with different silicon-to-aluminum ratios in Example 2; Detailed Implementation

[0021] The present invention is described in detail below through embodiments, but the present invention is not limited to these embodiments.

[0022] In this embodiment of the invention, commercially available ZSM-5 molecular sieve, commercially available ZSM-35 molecular sieve, and commercially available USY molecular sieve are produced by Tianjin Nanhua Catalyst Co., Ltd. Commercially available SSZ-13 molecular sieve is produced by Dalian Zeer Catalytic Materials Co., Ltd. Commercially available NaX molecular sieve is produced by Shanghai Hengye Microcrystalline Materials Technology Co., Ltd.

[0023] Example 1: Comparison of the catalytic performance of different types of commercially available molecular sieves Different commercially available molecular sieves were used to make electrodes, and the electrodes were electroreduced to nitrate to obtain ammonia. The electrode making method was as follows: after grinding the molecular sieve, 5 mg of molecular sieve sample was dispersed in a mixed solution of 445 μL ultrapure water, 445 μL ethanol and 10 μL Nafion (5 wt%), sonicated for 1 h, and 100 μL was taken with a pipette and spread on 1 cm × 1 cm carbon paper. The sample was dried under an infrared lamp, and the process was repeated twice on the same side of the carbon paper.

[0024] Catalytic application experiment: An H-type electrolytic cell was used, which was divided into two chambers by a proton exchange membrane. The catholyte in the cathode chamber was a mixed aqueous solution of 1M KOH and 0.1M potassium nitrate, and the anolyte in the anode chamber was a mixed aqueous solution of 1M KOH. The electrode prepared above was inserted into the catholyte as the cathode, and a platinum sheet electrode was inserted into the anolyte as the counter electrode. A saturated calomel electrode was also set in the cathode chamber as a reference electrode.

[0025] Following the above catalytic application experimental procedure, the electrocatalytic performance of the prepared catalyst and the ammonia yield were tested. Test conditions: room temperature, initial voltage -1.6V, electrolysis time 2h, initial NO... 3- The concentration is 0.1 mol·L -1 .

[0026] Following the above experimental procedure, when commercially available ZSM-5, ZSM-35, β-zeolite, and SSZ-13 molecular sieves were selected as the molecular sieves, the comparison results of ammonia yield and Faradaic efficiency catalyzed by electrode materials with different pore sizes are summarized below. Figure 1 middle.

[0027] Following the above experimental procedure, when commercially available USY-5.5 molecular sieve, USY-15 molecular sieve, and NaX molecular sieve (silicon-to-aluminum ratio = 1.0) were selected as the molecular sieves, the comparison results of ammonia yield and Faraday efficiency catalyzed by electrode materials with different pore sizes are summarized in the following table. Figure 2 middle.

[0028] Example 2: A method for preparing NaX molecular sieve catalysts using in-situ growth. 1) Take the graphite electrode material and cut it into cylinders with an average diameter of 10mm and a height of 20mm as carriers.

[0029] 2) Mix potassium hydroxide (KOH) and water at a mass ratio of 1:4. Then, take 2.53g of the above KOH aqueous solution and dissolve it in 1.2g of water. Then, add 28.20mmol of sodium aluminate and 29.61mmol of sodium silicate in sequence. Heat at 100℃ for 3h to dissolve and obtain a homogeneous precursor solution.

[0030] 3) Immerse the graphite electrode support described in step 1) into the precursor solution described in step 2), transfer the mixture to a reaction vessel, seal the reaction vessel and transfer it to an oven, and carry out a hydrothermal reaction at 225°C for 3 hours. After the reaction is completed, cool to room temperature, wash with water and dry to obtain the NaX molecular sieve catalyst.

[0031] Example 3: The preparation steps of its NaX molecular sieve catalyst are the same as those in Example 2, except that the amount of sodium silicate used in "step 2) is 28.20 mmol", and the other conditions remain unchanged.

[0032] Example 4: The preparation steps of its NaX molecular sieve catalyst are the same as those in Example 2, except that the amount of sodium silicate used in "step 2) is 26.79 mmol", and the other conditions remain unchanged.

[0033] Catalytic application experiments: An H-type electrolytic cell was used, divided into two chambers by a proton exchange membrane. The catholyte in the cathode chamber was a mixed aqueous solution of 1M KOH and 0.1M potassium nitrate, while the anolyte in the anode chamber was a 1M KOH aqueous solution. The catalyst electrode prepared in Examples 2, 3, or 4 was inserted into the catholyte as the cathode, and a platinum sheet electrode was inserted into the anolyte as the counter electrode. A saturated calomel electrode was also provided in the cathode chamber as a reference electrode.

[0034] Following the above catalytic application experimental procedure, the electrocatalytic performance of the prepared catalyst and the ammonia yield were tested. Test conditions: room temperature, initial voltage -1.6V, electrolysis time 2h, initial NO... 3- The concentration is 0.1 mol·L -1 The results of comparing the catalytic ammonia yield and Faraday efficiency, following the experimental procedure described above, are summarized in Table 1.

[0035] Table 1 .

[0036] This invention uses an in-situ growth method to form NaX molecular sieves on a graphite support, which are then used as cathode working electrodes in the catalytic reduction of nitrate to ammonia reaction. Compared with the traditional coating method, this method is more stable and has a longer lifespan.

Claims

1. A method for preparing a NaX molecular sieve catalyst by in-situ growth synthesis, characterized in that, Includes the following steps: S1 Preparation of precursor solution: Mix structure directing agent KOH, deionized water and Si source and stir, slowly add metal Al source solution, and heat in water bath to form a homogeneous precursor solution; S2 Support treatment and in-situ growth: The carbon electrode support is immersed in the precursor solution of step S1, the mixture is transferred to the reaction vessel, the reaction vessel is sealed and transferred to the oven, and a hydrothermal reaction is carried out under heating to grow NaX molecular sieve catalyst in situ on the support. S3 Product Processing: Cool the sample, wash with water, and then dry it to complete the preparation.

2. The production method according to claim 1, wherein In step S1, the mass ratio of KOH to deionized water is 1:4-8.

3. The production method according to claim 1, wherein The molar ratio of KOH to Al source is 0.15-0.4, preferably 0.3-0.35; the molar ratio of Si source to Al source is 0.95-1.05, preferably 1.

4. The production method according to claim 1, wherein The Si source includes one or a mixture of two of silica sol, silicon dioxide, and silicates, while the Al source includes one or a mixture of two of aluminum salts, aluminum oxide, and aluminum hydroxide.

5. The production method according to claim 1, wherein In step S2, the oven temperature is 150-300℃, preferably 200-250℃, and the hydrothermal reaction time is 2-4h, preferably 3h.

6. The production method according to claim 1, wherein In step S2, the carbon electrode carrier is made of graphite.

7. The NaX molecular sieve catalyst prepared by any one of the methods described in claims 1-6.

8. The application of the NaX molecular sieve catalyst as described in claim 7 in the electrocatalytic reduction of nitrate to ammonia.

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