CN116474819A - Modified ZSM-5 zeolite supported perovskite catalyst and synthesis method thereof - Google Patents

Abstract

The invention belongs to the technical field of atmospheric pollution control and particularly discloses a modified ZSM-5 zeolite supported perovskite catalyst and a synthesis method thereof. The synthesis method has the characteristics of simple process, simple operation and low cost, realizes the coupling of adsorption and catalysis, improves the pore structure and specific surface area of the catalyst, improves the oxygen exchange capacity and catalytic activity of the catalyst, prevents migration, coalescence and sintering of the catalyst in the process of using the catalyst for catalysis, and effectively improves the catalytic performance and efficiency of the catalyst. The catalyst synthesized by the invention has good catalytic performance, high purity and long service life.

Description

A kind of modified ZSM-5 zeolite supported perovskite catalyst and its synthesis method

technical field

The invention belongs to the technical field of air pollution control, and in particular relates to a modified ZSM-5 zeolite-loaded perovskite catalyst and a synthesis method thereof.

Background technique

VOCs (Volatile Organic Compounds) are various organic compounds with a boiling point of 50°C to 260°C at room temperature. According to their chemical structure, they can be divided into alkanes, alkenes, alkynes, benzene series, alcohols, aldehydes, ethers, ketones, acids, esters, halogenated hydrocarbons and others. VOCs participate in the formation of ozone and secondary aerosols in the atmospheric environment, and have an important impact on regional atmospheric ozone pollution and PM2.5 pollution. In order to fundamentally solve PM2.5, ozone and other pollution problems and effectively improve the quality of the atmospheric environment, the governance of VOCs is particularly important.

Reduction and elimination of volatile organic compounds (VOCs) present in the air remains one of the challenges in the field of environmental catalysis. A variety of catalytic systems have been designed to address this issue, including mixed metal oxide systems and supported noble or non-noble metal systems. Perovskite catalysts are usually the preferred catalysts, which are composite metal oxides with a specific structure. Due to better oxygen mobility, they have higher catalytic activity than single oxides in certain reactions. The general formula of the perovskite catalyst is ABO ₃ , the A site is usually a rare earth or alkaline earth metal element with a large ionic radius, and the B site is usually a transition metal element with a small ionic radius. Generally, A has a tetrahedral structure, and B has an octahedral structure. The A site and the B site can form an alternating structure, and both are easily replaced by other elements to produce lattice defects, thereby changing the performance of the catalyst. Perovskite-type catalysts have high thermal stability and strong catalytic activity in the oxidation of many organic molecules, so they are often used as catalysts for the purification of VOCs.

However, the ABO ₃ perovskite catalyst has obvious disadvantages, that is, the specific surface area is small, usually lower than 10m ² /g, which limits its application.这个问题可以通过将钙钛矿催化剂支撑在具有发达表面积的多孔载体上来解决，但是目前的负载型钙钛矿催化剂工艺大多比较复杂，如专利CN107930626B公开的一种VOCs废气处理催化剂及其制备方法，所述催化剂以多孔金属作为载体，载体上涂覆有涂料；所述涂料包括催化活性组分、第二载体、第二载体改性剂，所述催化活性组分为Pt复配LaMnO ₃钙钛矿型复合氧化物，所述第二载体为γ-Al ₂ O ₃粉末，所述第二载体改性剂为以过渡金属元素中的一种或多种制成的固溶体氧化物；该催化剂的制备需要经历多步煅烧，还涉及切割、涂敷、改性等操作，工艺十分复杂，操作不便，成本高。

Contents of the invention

One of the purposes of the present invention is to provide a method for synthesizing a modified ZSM-5 zeolite-supported perovskite catalyst, which has the characteristics of simple process and easy operation, and the synthesized catalyst has good catalytic performance and high purity.

In order to achieve the above object, the concrete technical scheme that the present invention adopts is as follows:

A synthetic method of modified ZSM-5 zeolite-loaded perovskite catalyst, comprising the following steps:

S1. Weigh ZSM-5 zeolite and calcinate at 250-350°C for 3.5-4.5 hours;

S2. adding sodium hydroxide solution to the calcined zeolite and stirring;

S3. Centrifuge, wash the precipitate with deionized water, and then dry overnight at 55-65°C to obtain alkali-modified ZSM-5 zeolite;

S4. Weigh catalyst raw materials lanthanum nitrate, strontium nitrate and cobalt nitrate, first dissolve with deionized water, then add alkali-modified ZSM-5 zeolite, stir and mix;

S5. Add ammonia water dropwise to adjust the pH of the mixed solution to alkaline, then stop stirring, and age at a constant temperature;

S6. Wash the aged precipitate with deionized water, and then dry it overnight at 55-65° C. to obtain an alkali-modified ZSM-5 zeolite-supported perovskite catalyst.

First of all, in view of the shortcoming of the current perovskite-type catalyst with a small specific surface area, the present invention selects ZSM-5 zeolite as the carrier: on the one hand, it can effectively increase the specific surface area of the catalyst, effectively disperse the functional atoms of the catalyst and reduce its particle agglomeration, and provide more active sites for the catalytic reaction; on the other hand, it can enhance the catalytic reaction efficiency of the catalyst, which can play a catalytic role. After the catalyst is loaded, it can effectively promote the migration and conversion of active oxygen in the catalyst, and improve the ability of the catalyst to catalyze the oxidation of VOCs. Moreover, ZSM-5 zeolite has obvious advantages compared with other supports (such as activated carbon, graphene, carbon nanotubes, carbonized nitrogen, co-melt, perovskite, silica, etc.): ZSM-5 has easily accessible active sites, showing significant activity in the process of catalytic degradation of VOCs; at the same time, ZSM-5 has abundant acid centers and high surface acidity have also been proved to be beneficial to promote the catalytic performance of the catalyst.

Then, on the basis of selecting ZSM-5 zeolite as the carrier, the present invention further modifies it, because the traditional ZSM-5 zeolite has the following defects: on the one hand, it has a single microporous structure, which will hinder the diffusion of VOCs molecules and affect the catalytic efficiency; In the present invention, after alkali modification of ZSM-5 zeolite, a multi-level structure zeolite is formed. The multi-level structure zeolite retains the crystalline framework of the microporous zeolite, has good selectivity, thermal and hydrothermal stability, and the mesopore/macropore channel penetrating through the crystalline framework can provide a large specific surface area, expose more active centers, shorten the diffusion path of reactants and product molecules, and effectively improve the catalytic efficiency. Therefore, it is of great value to modify ZSM-5 zeolite and use it as a carrier of VOCs catalyst.

Finally, the present invention selects lanthanum nitrate, strontium nitrate, and cobalt nitrate as the main raw materials of the catalyst, and finally successfully loads the La _0.5 Sr _0.5 CoO ₃ perovskite catalyst on the modified ZSM-5 zeolite, realizing the coupling of adsorption and catalysis, effectively improving the pore structure and specific surface area of the perovskite catalyst while improving the oxygen exchange capacity and catalytic activity of the catalyst, and preventing the migration, coalescence and sintering of the catalyst in the process of catalysis, further improving the catalytic performance and efficiency of the catalyst. Significantly inventive. In addition, the synthesis of the modified ZSM-5 zeolite-supported perovskite catalyst of the present invention is basically a simple operation, without the need for multi-step calcination and complicated temperature change control. Compared with the existing supported perovskite catalyst, it is not only simpler in structure, but also significantly simpler in process, which can effectively reduce the cost of the catalyst.

Preferably, in step S2, the concentration of the sodium hydroxide solution is 0.5-2M. When the concentration exceeds 2M, the zeolite will dissolve or even gel.

Preferably, in step S2, the stirring method is magnetic stirring, the stirring time is 20-28 hours, and the rotation speed is 100-150 r/min.

Preferably, in step S4, the mass ratio of lanthanum acid, strontium nitrate and cobalt nitrate is 1:(0.8-1.2):(1.8-2.2).

Preferably, in step S4, the mass ratio of the catalyst raw material to the alkali-modified ZSM-5 zeolite is 1:(3-6).

Preferably, in step S4, the stirring method is magnetic stirring, and the stirring time is 20-40 minutes; in step S5, the heating temperature is controlled to be 35-45° C. during the stirring.

Preferably, in step S5, ammonia water is added dropwise to adjust the pH to 11-12. If the pH is less than 11, it is difficult to precipitate and cannot form a catalyst; if the pH is greater than 12, it will dissolve.

Preferably, in step S5, the aging time is 24-36 hours, and the aging temperature is normal temperature. If the aging time is less than 24h, there will be more impurity phases in the synthesized sample, which cannot be clearly identified as a supported perovskite catalyst.

The second object of the present invention is to provide the above-mentioned modified ZSM-5 zeolite-supported perovskite catalyst prepared by the above-mentioned synthesis method.

The present invention has the following beneficial effects:

1. The method for synthesizing the modified ZSM-5 zeolite-supported perovskite catalyst provided by the present invention realizes the coupling of adsorption and catalysis, improves the pore structure and specific surface area of the catalyst while improving the oxygen exchange capacity and catalytic activity of the catalyst, and prevents the migration, agglomeration and sintering of the catalyst in the process of its use in catalysis, and improves the catalytic performance and efficiency of the catalyst. Compared with the synthesis of pure perovskite catalysts, it is creative.

2. The synthesis method of the present invention has simple process and convenient operation, can effectively reduce the catalyst cost, and has obvious advantages compared with the existing supported perovskite catalysts.

3. The present invention uses alkali-modified ZSM-5 zeolite as the catalyst matrix, and its multi-level pore structure shortens the diffusion path of reactants and product molecules, thereby effectively improving the catalytic efficiency, reducing the contact between reactants and acid sites, effectively inhibiting the progress of side reactions, and finally improving the purity and service life of the catalyst.

Description of drawings

Fig. 1: SEM figure of unmodified zeolite ZSM-5 (a, b) and alkali-modified ZSM-5 zeolite (c, d) in embodiment 1;

Fig. 2: the XRD spectrum of the modified ZSM-5 zeolite supported perovskite catalyst synthesized in Example 1;

Fig. 3: the SEM figure of the modified ZSM-5 zeolite supported perovskite catalyst synthesized in embodiment 1;

Figure 4: BET diagram of the modified ZSM-5 zeolite-supported perovskite catalyst synthesized in Example 1.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

The test materials used are as follows: ZSM-5 zeolite (Sinopharm), lanthanum nitrate, strontium nitrate, cobalt nitrate (Sinopharm), flake sodium hydroxide (McLean), ammonia water (McLean), deionized water. The instruments used are as follows: muffle furnace, magnetic stirrer, vacuum oven.

Example 1

A synthetic method of modified ZSM-5 zeolite-loaded perovskite catalyst, comprising the following steps:

S1. Weigh 5g of ZSM-5 zeolite, place it in a crucible, and calcinate it in a muffle furnace at 300°C for 4h, with a heating rate of 5°C/min;

S2. Add a sodium hydroxide solution with a concentration of 1M to the calcined zeolite, and perform magnetic stirring, the stirring time is 24h, and the rotation speed is 120r/min;

S3. Centrifuge, wash the precipitate with deionized water for 3 times, and then dry at 60°C overnight to obtain alkali-modified ZSM-5 zeolite;

S4. Weigh the catalyst raw materials lanthanum nitrate, strontium nitrate and cobalt nitrate in a beaker according to the mass ratio of 1:1:2. The total mass of the three is 1g, dissolve it with deionized water first, then add 3g of alkali-modified ZSM-5 zeolite, place the beaker on a magnetic stirrer, and stir magnetically for 30 minutes at room temperature to mix;

S5. Set the heating temperature of the magnetic stirrer to 40°C, add ammonia water dropwise to adjust the pH of the mixed solution to 11.5, then stop stirring, and age at room temperature for 30 hours;

S6. After aging, filter the precipitate, wash it with deionized water three times, and then dry it overnight at 60° C. to obtain an alkali-modified ZSM-5 zeolite-supported perovskite catalyst.

Example 2

A synthetic method of modified ZSM-5 zeolite-loaded perovskite catalyst, comprising the following steps:

S1. Weigh 5g of ZSM-5 zeolite, place it in a crucible, and calcinate it in a muffle furnace at 250°C for 4.5h, with a heating rate of 5°C/min;

S2. Add a sodium hydroxide solution with a concentration of 0.5M to the calcined zeolite, and perform magnetic stirring, the stirring time is 28 hours, and the rotation speed is 150r/min;

S3. Centrifuge, wash the precipitate with deionized water for 3 times, and then dry at 55°C overnight to obtain alkali-modified ZSM-5 zeolite;

S4. Weigh the catalyst raw materials lanthanum nitrate, strontium nitrate and cobalt nitrate in a beaker according to the mass ratio of 1:1:2. The total mass of the three is 1g, dissolve it with deionized water first, then add 4g of alkali-modified ZSM-5 zeolite, place the beaker on a magnetic stirrer, and stir magnetically at room temperature for 20min to mix;

S5. Set the heating temperature of the magnetic stirrer to 35°C, add ammonia water dropwise to adjust the pH of the mixed solution to 12, then stop stirring, and age at room temperature for 24 hours;

S6. After aging, filter the precipitate, wash it with deionized water three times, and then dry it overnight at 55° C. to obtain an alkali-modified ZSM-5 zeolite-supported perovskite catalyst.

Example 3

A synthetic method of modified ZSM-5 zeolite-loaded perovskite catalyst, comprising the following steps:

S1. Weigh 5g of ZSM-5 zeolite, place it in a crucible, and calcinate it in a muffle furnace at 350°C for 3.5h, with a heating rate of 5°C/min;

S2. Add a sodium hydroxide solution with a concentration of 2M to the calcined zeolite, and perform magnetic stirring, the stirring time is 20h, and the rotation speed is 100r/min;

S3. Centrifuge, wash the precipitate with deionized water for 3 times, and then dry at 65°C overnight to obtain alkali-modified ZSM-5 zeolite;

S4. Weigh the catalyst raw materials lanthanum nitrate, strontium nitrate and cobalt nitrate according to the mass ratio of 1:1:2 and place them in a beaker. The total mass of the three is 1g. Dissolve them with deionized water first, then add 6g of alkali-modified ZSM-5 zeolite, place the beaker on a magnetic stirrer, and stir magnetically for 40 minutes at room temperature to mix evenly;

S5. Set the heating temperature of the magnetic stirrer to 45°C, add ammonia water dropwise to adjust the pH of the mixed solution to 11, then stop stirring, and age at room temperature for 36 hours;

S6. After aging, filter the precipitate, wash it with deionized water three times, and then dry it overnight at 65° C. to obtain an alkali-modified ZSM-5 zeolite-supported perovskite catalyst.

Comparative example 1-3

The alkali-modified zeolites in Examples 1-3 were respectively replaced with unmodified ZSM-5 zeolites, and the operations were performed according to steps S4-S6 to synthesize ZSM-5 zeolite-supported perovskite catalysts.

The ZSM-5 zeolite-supported perovskite catalyst synthesized in Examples 1-3 has multi-level pores, a large specific surface area, and the phase is a pure catalyst crystal phase, and the pore structure of the modified zeolite and the granular perovskite catalyst loaded on the surface of the zeolite are clearly observed under SEM, which proves that the synthesis of the supported perovskite catalyst is very successful. Specifically, the performance characterization is as follows:

(1) The SEM images of the unmodified ZSM-5 zeolite and the alkali-modified ZSM-5 zeolite in Example 1 are as shown in Figure 1, the surface of the unmodified ZSM-5 zeolite is smooth, with sharp edges and corners, and it is in the shape of a cube, a cuboid and a polyhedron; the modified ZSM-5 zeolite gradually disappears, small holes gradually appear, and the zeolite gradually decreases. The comparison shows that the pore structure of ZSM-5 zeolite is richer after alkali modification, which is beneficial to the loading of catalyst.

(2) The XRD test results of the modified ZSM-5 zeolite-supported perovskite catalyst synthesized in Example 1 are shown in Figure 2. Through XRD standard card comparison, the phase contains a ZSM-5 zeolite framework, and the synthesized product is La _0.5 Sr _0.5 CoO ₃ perovskite catalyst. The results prove that the alkali-modified ZSM-5 zeolite-supported perovskite catalyst was successfully synthesized.

(3) The SEM image of the modified ZSM-5 zeolite-supported perovskite catalyst synthesized in Example 1 is shown in Figure 3. In the SEM images of different sizes, the pore structure of the modified zeolite and the granular perovskite catalyst are clearly and uniformly loaded on the surface of the zeolite, further indicating that the supported perovskite catalyst was successfully synthesized.

(4) After BET testing, as shown in Figure 4, the specific surface area of the modified ZSM-5 zeolite-supported perovskite catalyst synthesized in Example 1-3 reaches 300-310m2/g, while the specific surface area of the ZSM-5 zeolite-supported perovskite catalyst synthesized in Comparative Example 1-3 is ^{280-290m2 /} g, and the specific surface area of the pure perovskite-type catalyst is usually < ^10m2 /g, and the present invention has significant advantages.

(5) Through XPS and thermogravimetric test, the loading capacity of the modified ZSM-5 zeolite-supported perovskite catalyst synthesized in Example 1-3 is 65-80%, while the loading capacity of the ZSM-5 zeolite-supported perovskite catalyst synthesized in Comparative Example 1-3 is 40-55%, and the present invention has significant advantages.

This specific embodiment is only an explanation of the present invention, and is not a limitation of the present invention. Any changes made by those skilled in the art after reading the description of the present invention, as long as they are within the scope of the claims of the present invention, will be protected by the patent law.

Claims (9)

1. A method for synthesizing a modified ZSM-5 zeolite supported perovskite catalyst is characterized in that: the method comprises the following steps:

s1, weighing ZSM-5 zeolite, and calcining at the temperature of 250-350 ℃ for 3.5-4.5 hours;

s2, adding sodium hydroxide solution into the calcined zeolite, and stirring;

s3, centrifuging, cleaning the precipitate by deionized water, and drying overnight at 55-65 ℃ to obtain alkali modified ZSM-5 zeolite;

s4, weighing the raw materials of lanthanum nitrate, strontium nitrate and cobalt nitrate of the catalyst, firstly dissolving the raw materials with deionized water, then adding alkali modified ZSM-5 zeolite, and stirring and uniformly mixing;

s5, dropwise adding ammonia water to adjust the pH of the mixed solution to be alkaline, stopping stirring, and aging at a constant temperature;

s6, cleaning the aged precipitate by deionized water, and drying the precipitate at 55-65 ℃ overnight to obtain the alkali modified ZSM-5 zeolite supported perovskite catalyst.

2. The method for synthesizing the modified ZSM-5 zeolite supported perovskite catalyst as claimed in claim 1, wherein: in the step S2, the concentration of the sodium hydroxide solution is 0.5-2M.

3. The method for synthesizing the modified ZSM-5 zeolite supported perovskite catalyst as claimed in claim 2, wherein: in the step S2, the stirring mode is magnetic stirring, the stirring time is 20-28 h, and the rotating speed is 100-150 r/min.

4. The method for synthesizing the modified ZSM-5 zeolite supported perovskite catalyst as claimed in claim 1, wherein: in the step S4, the mass ratio of the lanthanum acid, the strontium nitrate and the cobalt nitrate is 1 (0.8-1.2):

(1.8～2.2)。

5. the method for synthesizing the modified ZSM-5 zeolite-supported perovskite catalyst according to claim 4, wherein the method comprises the following steps: in the step S4, the mass ratio of the catalyst raw material to the alkali modified ZSM-5 zeolite is 1:

(3～6)。

6. the method for synthesizing the modified ZSM-5 zeolite supported perovskite catalyst as claimed in claim 1, wherein: in the step S4, the stirring mode is magnetic stirring, and the stirring time is 20-40 min; in step S5, the heating temperature is controlled to be 35-45 ℃ during stirring.

7. The method for synthesizing the modified ZSM-5 zeolite supported perovskite catalyst as claimed in claim 1, wherein: in step S5, ammonia water is added dropwise to adjust the pH to 11-12.

8. The method for synthesizing the modified ZSM-5 zeolite supported perovskite catalyst as claimed in claim 1, wherein: in the step S5, the aging time is 24-36 h, and the aging temperature is normal temperature.

9. The alkali-modified ZSM-5 zeolite supported perovskite catalyst produced by the synthesis process of any one of claims 1-8.