Preparation method of metal-modified zinc oxide catalyst and application of metal-modified zinc oxide catalyst in light-concentrating catalytic conversion of methane into formaldehyde

Abstract

The application relates to the technical field of catalyst preparation, in particular to a preparation method of a metal-modified zinc oxide catalyst and application of the metal-modified zinc oxide catalyst in concentrated catalytic methane-to-formaldehyde. The preparation method comprises the following steps: S1, adding nanocellulose whiskers, a zinc salt and 2-methyl imidazole into a methanol aqueous solution, vacuum drying after hydrothermal reaction; S2, calcining the vacuum-dried product in step S1; S3, placing the calcined product obtained in step S2 in deionized water, and adding a metal salt solution dropwise to react; S4, adding a sodium hypophosphite solution dropwise to the reaction liquid obtained in step S3 to reduce, filtering, cleaning and drying after the reaction is completed to obtain the catalyst. In the application, nanocellulose whiskers are used as a matrix, ZIF-8 can be combined on the matrix, and the matrix is combined with calcination, so that the morphology of the prepared zinc oxide is effectively adjusted, and the performance of the prepared catalyst is finally improved.

Description

Technical Field

[0001] This invention relates to the field of catalyst preparation technology, specifically a method for preparing a metal-modified zinc oxide catalyst and its application in photocatalytic methane-to-formaldehyde conversion. Background Technology

[0002] Formaldehyde is a key raw material in the chemical industry, with huge global annual demand. Currently, industrial production mainly uses the "methanol oxidation method," which is a long process (requiring the conversion of syngas into methanol before oxidation to formaldehyde), energy-intensive, and has high carbon emissions. In contrast, directly converting methane into formaldehyde in a single step offers significant atom economy and environmental value.

[0003] However, methane molecules have a highly symmetrical tetrahedral structure and extremely high CH bond energy (439 kJ / mol), making them very difficult to activate. Existing methane conversion technologies face the following main challenges: (1) Traditional thermocatalysis often requires high temperatures (>600℃) to activate methane, but the generated formaldehyde is more reactive than methane and is easily oxidized to CO2 at high temperatures, resulting in extremely low selectivity for the target product. (2) Conventional photocatalysis has mild reaction conditions and can suppress peroxidation, but it is limited by the high recombination rate of photogenerated carriers and the narrow light absorption range (mostly limited to the ultraviolet region) of semiconductor materials, resulting in a usually low methane conversion rate, which is difficult to meet industrial needs. (3) Many laboratory studies are still conducted in batch reactors, and the products are difficult to be stably produced in large-scale continuous flow.

[0004] In recent years, concentrated photothermal catalysis has become a research hotspot. It utilizes concentrated sunlight or simulated light sources to generate high light intensity, producing a synergistic "light + heat" effect on the catalyst surface. To achieve efficient conversion, it is urgent to develop a catalyst that can withstand concentrated light conditions, effectively separate photogenerated charges, and possess specific active sites.

[0005] ZnO, a classic wide-bandgap semiconductor, is inexpensive and responds well to ultraviolet light, but pure ZnO has low activity. Some noble metals possess excellent electron transport properties. Loading noble metals onto the surface of ZnO holds promise for solving carrier recombination problems through metal-semiconductor interactions, and utilizing the localized surface plasmon resonance effect or photothermal conversion capability of noble metals to achieve efficient methane conversion in continuous flow systems.

[0006] Existing technologies generally involve adsorbing metal ions onto the surface of zinc oxide and then reducing the metal ions on the zinc oxide surface. The nano zinc oxide used is mostly prepared by ordinary methods, and there is a lack of methods to further optimize the surface structure of zinc oxide. As a result, the efficiency of the prepared catalysts needs to be improved. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a metal-modified zinc oxide catalyst and its application in photocatalytic methane-to-formaldehyde conversion, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A method for preparing a metal-modified zinc oxide catalyst includes the following steps:

[0010] S1. Add nanocellulose whiskers, zinc salt and 2-methylimidazole to a methanol aqueous solution and react continuously at 50-80℃ under nitrogen protection for 8-12 hours. Then filter, wash the filtered product with sufficient deionized water and dry it under vacuum at 40-60℃ for 5-8 hours.

[0011] S2. Place the vacuum-dried product from step S1 into a muffle furnace and calcine it at 500-700℃ for 2-4 hours to obtain the calcined product.

[0012] S3. Place the calcined product obtained in step S2 in deionized water, then add metal salt solution dropwise, and stir continuously for 1-2 hours to obtain the reaction solution.

[0013] S4. Add sodium hypophosphite solution dropwise to the reaction solution obtained in step S3, stir the reaction at 80-90℃ for 1-2 hours, then filter, wash the filtered product with sufficient deionized water, and dry it under vacuum at 60-70℃ for 10-12 hours to obtain the catalyst.

[0014] Furthermore, in step S1, the zinc salt is zinc nitrate hexahydrate, and the mass ratio between nanocellulose whiskers, zinc salt, 2-methylimidazole, methanol and deionized water is (3-5):1:(1-2):(70-80):(5-10).

[0015] Furthermore, the calcination condition in step S2 is an air atmosphere.

[0016] Furthermore, in step S3, the mass ratio between the calcined product and deionized water is 1:(400-500).

[0017] Furthermore, the metal salt solution in step S3 is one of ruthenium chloride solution, chloroplatinic acid solution, palladium chloride solution, iridium chloride solution, chloroauric acid solution, and silver nitrate solution; the concentration of the metal salt solution is 1 wt%, and the mass ratio between the metal salt solution and the calcined product is (5-10):1.

[0018] Furthermore, in step S4, the mass ratio between the sodium hypophosphite solution and the metal salt solution is (1-2):1, and the concentration of the sodium hypophosphite solution is 10wt%.

[0019] Furthermore, the nanocellulose whiskers in step S1 need to undergo pretreatment, which specifically includes the following steps:

[0020] Cellulose nanofibers were placed in an aqueous ethanol solution containing aminopropyltriethoxysilane and reacted at 70-85°C for 2-4 hours. The mixture was then filtered, and the filtered product was washed with sufficient deionized water and then vacuum dried at 50-60°C for 4-6 hours.

[0021] Furthermore, the mass ratio of the nanocellulose whiskers, aminopropyltriethoxysilane, ethanol and deionized water is (1-2):1:(30-40):(5-10).

[0022] The application of the catalyst prepared by the above-mentioned method for preparing metal-modified zinc oxide catalyst in the photocatalytic production of formaldehyde from methane.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] This invention synthesizes ZIF-8 on nanocellulose whiskers, and then performs joint calcination on the material. After calcination, ZIF-8 produces nano-zinc oxide particles. The calcined zinc oxide particles of ZIF-8 have a porous structure, which facilitates the subsequent loading and bonding of metal ions. The nanocellulose whiskers act as a framework template, preventing thermal aggregation between nano-zinc oxide particles during sintering, improving the morphology and structure of the prepared nano-zinc oxide, and providing better conditions for subsequent loading of metal ions. Attached Figure Description

[0025] Figure 1 This is a process flow diagram for preparing the catalyst of the present invention. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Please see Figure 1 The present invention provides:

[0028] Example 1

[0029] A method for preparing a metal-modified zinc oxide catalyst includes the following steps:

[0030] S1. 8g of nanocellulose whiskers, 2g of zinc salt, and 3g of 2-methylimidazole were added to a methanol aqueous solution. The amounts of methanol and deionized water were 150g and 16g, respectively. The zinc salt was zinc nitrate hexahydrate. The reaction was carried out continuously at 65°C under nitrogen protection for 10h. After filtration, the filtered product was washed with sufficient deionized water and then vacuum dried at 55°C for 7h. In step S1, a composite material of nanocellulose whiskers and ZIF-8 was prepared by hydrothermal reaction of nanocellulose whiskers, zinc salt, and 2-methylimidazole.

[0031] S2. Place the vacuum-dried product from step S1 in a muffle furnace and calcine it at 650°C in an air atmosphere for 3 hours to obtain the calcined product. In step S2, the nanocellulose whiskers and organic ligands are removed by air calcination, thereby leaving a multi-level porous structure and uniformly dispersed nano zinc oxide.

[0032] S3. Place 0.4g of the calcined product obtained in step S2 into 180g of deionized water, and then add 3.2g of a 1wt% metal salt solution, which is a chloroauric acid solution. Stir continuously for 1.5h to obtain a reaction solution. In step S3, the metal salt solution is added to the zinc oxide dispersion to deload the zinc oxide with metal ions.

[0033] S4. Add 4.8 g of 10 wt% sodium hypophosphite solution dropwise to the reaction solution obtained in step S3, stir the reaction at 85 °C for 1.2 h, then filter, wash the filtered product with sufficient deionized water and dry it under vacuum at 65 °C for 11 h to obtain the catalyst; in step S4, the metal ions loaded with zinc oxide in step S3 are reduced by sodium hypophosphite to obtain zinc oxide with metal particle structure, thus obtaining a complete catalyst structure;

[0034] The nanocellulose whiskers in step S1 above need to undergo pretreatment, which specifically includes the following steps:

[0035] 10g of nanocellulose whiskers were placed in an ethanol-water solution containing aminopropyltriethoxysilane, with the amounts of aminopropyltriethoxysilane, ethanol, and deionized water being 8g, 260g, and 55g, respectively. The mixture was reacted at 75°C for 3 hours, then filtered. The filtered product was washed with sufficient deionized water and then vacuum dried at 55°C for 5 hours.

[0036] Example 2

[0037] A method for preparing a metal-modified zinc oxide catalyst includes the following steps:

[0038] S1. Add 6g of nanocellulose whiskers, 2g of zinc salt and 2g of 2-methylimidazole to a methanol aqueous solution. The zinc salt is zinc nitrate hexahydrate. The amounts of methanol and deionized water are 140g and 10g, respectively. React continuously at 50℃ and under nitrogen protection for 8 hours. After filtration, the filtered product is washed with sufficient deionized water and then vacuum dried at 40℃ for 5 hours.

[0039] S2. Place the vacuum-dried product from step S1 in a muffle furnace and calcine it at 500°C in an air atmosphere for 2 hours to obtain the calcined product.

[0040] S3. Place 0.4g of the calcined product obtained in step S2 into 160g of deionized water, then add 2g of a 1wt% metal salt solution (chloroauric acid solution) dropwise, and stir continuously for 1h to obtain the reaction solution.

[0041] S4. Add 2g of 10wt% sodium hypophosphite solution to the reaction solution obtained in step S3, stir the reaction at 80℃ for 1h, then filter, wash the filtered product with sufficient deionized water and dry it under vacuum at 60℃ for 10h to obtain the catalyst.

[0042] The nanocellulose whiskers in step S1 above need to undergo pretreatment, which specifically includes the following steps:

[0043] 8g of nanocellulose whiskers were placed in an ethanol-water solution containing aminopropyltriethoxysilane. The amounts of aminopropyltriethoxysilane, ethanol, and deionized water were 8g, 240g, and 40g, respectively. The mixture was reacted at 70℃ for 2h, then filtered. The filtered product was washed with sufficient deionized water and then vacuum dried at 50-60℃ for 4h.

[0044] Example 3

[0045] A method for preparing a metal-modified zinc oxide catalyst includes the following steps:

[0046] S1. Add 10g of nanocellulose whiskers, 2g of zinc salt and 4g of 2-methylimidazole to a methanol aqueous solution. The zinc salt is zinc nitrate hexahydrate. The amounts of methanol and deionized water are 160g and 20g, respectively. React continuously at 80℃ and under nitrogen protection for 12h. After filtration, the filtered product is washed with sufficient deionized water and then vacuum dried at 60℃ for 8h.

[0047] S2. Place the vacuum-dried product from step S1 in a muffle furnace and calcine it at 700°C in an air atmosphere for 4 hours to obtain the calcined product.

[0048] S3. Place 0.4g of the calcined product obtained in step S2 into 200g of deionized water, and then add 4g of a 1wt% metal salt solution, which is a chloroauric acid solution. Stir continuously for 2 hours to obtain the reaction solution.

[0049] S4. Add 8g of 10wt% sodium hypophosphite solution to the reaction solution obtained in step S3, stir the reaction at 90℃ for 2h, then filter, wash the filtered product with sufficient deionized water and dry it under vacuum at 70℃ for 12h to obtain the catalyst.

[0050] The nanocellulose whiskers in step S1 above need to undergo pretreatment, which specifically includes the following steps:

[0051] 16g of nanocellulose whiskers were placed in an ethanol-water solution containing aminopropyltriethoxysilane, with the amounts of aminopropyltriethoxysilane, ethanol, and deionized water being 8g, 320g, and 80g, respectively. The mixture was reacted at 85°C for 4 hours, then filtered. The filtered product was washed with sufficient deionized water and then vacuum dried at 60°C for 6 hours.

[0052] In this invention, the metal salt solution in step S3 can also be one of ruthenium chloride solution, chloroplatinic acid solution, palladium chloride solution, iridium chloride solution, and silver nitrate solution. This invention uses chloroauric acid solution for verification.

[0053] Comparative Example 1

[0054] The difference between Comparative Example 1 and Example 1 is that the pretreatment step of nanocellulose whiskers was omitted, while the remaining steps are exactly the same as in Example 1.

[0055] Comparative Example 2

[0056] The difference between Comparative Example 2 and Example 1 is that ZIF-8 was calcined directly in an air atmosphere, and the calcined product was then used in step S3.

[0057] Comparative Example 3

[0058] The difference between Comparative Example 3 and Example 1 is that nano zinc oxide is directly applied to step S3, while the remaining steps are exactly the same as in Example 1. The nano zinc oxide used in Comparative Example 3 is nano zinc oxide prepared by conventional process (nano zinc oxide is prepared by precipitation method).

[0059] The catalysts prepared in Examples 1-3 and Comparative Examples 1-3 were tested for formaldehyde production. The test procedures are as follows:

[0060] 100 mg of catalyst prepared in Examples 1-3 and Comparative Examples 1-3 were weighed out respectively, spread evenly on a quartz reactor, connected to a gas line, and a mixture of methane and oxygen (volume ratio CH4:O2 = 8:1) was introduced, with the total gas flow rate controlled at 22.5 mL / min. Then, the light source was turned on, using a 300W xenon lamp equipped with an AM 1.5G filter as the light source. The light spot was focused through a lens, and the light intensity on the catalyst surface was measured to be 620 mW / cm² using a light intensity meter. 2 At this point, the thermocouple monitored the catalyst bed temperature rising to approximately 130°C under illumination. After the reaction stabilized for 30 minutes, the tail gas was analyzed using an online gas chromatograph (GC). The formaldehyde yield is shown in Table 1 below.

[0061] Table 1: Formaldehyde yield of catalysts prepared in Examples 1-3 and Comparative Examples 1-3

[0062]

[0063] As can be seen from the data in Table 1 above, the zinc oxide structure obtained by combining ZIF-8 and nanocellulose whiskers and then calcining them together in this invention is more excellent. After loading metal particles, the catalyst obtained has higher catalytic efficiency. In this invention, the coordination ability of zinc ions is improved by pretreating nanocellulose whiskers. After joint calcination, the overall structure of zinc oxide is further optimized, and the performance of the prepared catalyst is improved.

[0064] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for producing a metal-modified zinc oxide catalyst, characterized by, The method comprises the following steps: S1, adding nanocellulose whiskers, zinc salt and 2-methylimidazole into a methanol aqueous solution, continuously reacting at 50-80℃ under nitrogen protection for 8-12h, then filtering, cleaning the filtered product with sufficient deionized water, and vacuum drying the cleaned product at 40-60℃ for 5-8h; S2, placing the vacuum dried product in step S1 into a muffle furnace, calcining at 500-700℃ for 2-4h to obtain a calcined product; S3, placing the calcined product obtained in step S2 into deionized water, then adding a metal salt solution dropwise, continuously stirring for 1-2h to obtain a reaction solution; S4, adding a sodium hypophosphite solution dropwise into the reaction solution obtained in step S3, stirring and reacting at 80-90℃ for 1-2h, then filtering, cleaning the filtered product with sufficient deionized water, and vacuum drying the cleaned product at 60-70℃ for 10-12h to obtain a catalyst; The metal salt solution in step S3 is one of a ruthenium chloride solution, a chloroplatinic acid solution, a palladium chloride solution, an iridium chloride solution, a chloroauric acid solution and a silver nitrate solution.

2. The method for preparing a metal-modified zinc oxide catalyst according to claim 1, characterized by, The zinc salt in step S1 is zinc nitrate hexahydrate, and the mass ratio among the nanocellulose whiskers, the zinc salt, 2-methylimidazole, methanol and deionized water is (3-5):1:(1-2):(70-80):(5-10).

3. The method for preparing a metal-modified zinc oxide catalyst according to claim 1, characterized by, The calcination condition in step S2 is an air atmosphere.

4. The method for preparing a metal-modified zinc oxide catalyst according to claim 1, characterized by, The mass ratio between the calcined product and deionized water in step S3 is 1:(400-500).

5. The method of preparing a metal-modified zinc oxide catalyst according to claim 1, characterized by, The concentration of the metal salt solution is 1wt%, and the mass ratio between the metal salt solution and the calcined product is (5-10):

1.

6. The method of preparing a metal-modified zinc oxide catalyst according to claim 1, characterized by, The mass ratio between the sodium hypophosphite solution and the metal salt solution in step S4 is (1-2):1, and the concentration of the sodium hypophosphite solution is 10wt%.

7. The method of producing a metal-modified zinc oxide catalyst according to claim 1, characterized by, The nanocellulose whiskers in step S1 need to be pretreated, and the pretreatment specifically comprises the following steps: placing the nanocellulose whiskers into an ethanol aqueous solution containing aminopropyltriethoxysilane, reacting at 70-85℃ for 2-4h, then filtering, cleaning the filtered product with sufficient deionized water, and vacuum drying the cleaned product at 50-60℃ for 4-6h.

8. The method of producing a metal-modified zinc oxide catalyst according to claim 7, characterized by, The mass ratio among the nanocellulose whiskers, aminopropyltriethoxysilane, ethanol and deionized water is (1-2):1:(30-40):(5-10).

9. Application of the catalyst prepared by the preparation method of the metal-modified zinc oxide catalyst according to any one of claims 1-8 in concentrating catalytic methane to formaldehyde.

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