A catalyst for synthesizing 2,6-dimethylphenol and a preparation method thereof

Abstract

The application provides a catalyst for synthesizing 2,6-dimethylphenol and a preparation method thereof. The catalyst comprises iron oxide, vanadium oxide, silicon oxide and alkali metal oxide, and the molar ratio of the components is iron:vanadium:silicon:alkali metal = 1:1:(0.005-0.02):(0.001-0.005). The preparation method of the catalyst comprises the following steps: dissolving metavanadate and oxalic acid in water, adjusting the pH value to 1.5-3.0, adding a solution of soluble iron salt under stirring, and dropwise adding a solution containing a silicon compound and ammonia water, so that the pH value of the reaction system is kept at 1.5-3.0; after the feeding is completed, the stirring is continued for a period of time, and then the system is left to stand for a period of time; after solid-liquid separation, the obtained solid is impregnated with an alkali metal salt solution; after the impregnated solid is dried and calcined, the catalyst is obtained. The catalyst has the advantages of high catalytic activity and high 2,6-DMP selectivity.

Description

Technical Field

[0001] This invention relates to a catalyst for the synthesis of 2,6-dimethylphenol and its preparation method, belonging to the field of 2,6-dimethylphenol synthesis technology. Background Technology

[0002] 2,6-Dimethylphenol (2,6-DMP) is an important chemical raw material and fine chemical intermediate, used in the synthesis of engineering plastics polyphenylene oxide (PPO) and modified polyphenylene oxide (MPPO). PPO is an excellent thermoplastic, resistant to high temperatures and acids and alkalis, and is one of the five major engineering plastics. 2,6-DMP is also a raw material and intermediate for some pharmaceuticals, and can be used to synthesize 2,6-dimethylaniline and the antioxidant Lowinox 44M 26.

[0003] Currently, the vapor-phase alkylation of phenol with methanol is the main method for synthesizing 2,6-DMP. Catalysts for the synthesis of 2,6-DMP are mainly divided into two categories: Mg-based catalysts and Fe-based catalysts. Mg-based catalysts operate at high temperatures (above 400℃) and have high activity, but they are prone to deactivation at high temperatures and have low selectivity. Fe-based catalysts operate at low temperatures (350℃) and have high selectivity, but their catalyst composition is very complex, leading to low stability of the catalytic reaction.

[0004] CN104415761A discloses a phenol ortho-methylation catalyst, its preparation method, and a method for synthesizing o-cresol and 2,6-xylenol. This phenol ortho-methylation catalyst is a composite oxide containing iron, magnesium, cerium, vanadium, and potassium / sodium, with a molar ratio of Fe:Mg:Ce:V:K / Na = 100:(1-50):(0.5-5):(0.5-5):(0.1-0.5). The specific surface area of ​​this catalyst is 180-220 μm. 2 The catalyst exhibits a pore volume of 0.60–0.80 mL / g and a bulk density of 1.0–1.20 g / mL. It demonstrates high activity and good stability at low temperatures, capable of stable single-pass operation for over 2000 hours. However, its activity is not high, and its selectivity for 2,6-DMP is also low. When used for the co-production of 2,6-xylenol from o-cresol, after 2000 hours of operation, the average phenol conversion was 91.27%, and the 2,6-xylenol selectivity was only 49.03%.

[0005] It can be seen that the Fe-V catalyst disclosed in the above literature has low phenol conversion and low selectivity for 2,6-DMP, and is not suitable for processes that mainly produce 2,6-DMP.

[0006] Iron-vanadium catalysts are commonly used in fluidized bed reaction systems. Current techniques typically involve first preparing solutions of iron salt and metavanadate separately, then mixing and precipitating the two solutions to obtain the iron-vanadium catalyst. During the mixing process, iron ions partially precipitate to form Fe(OH)3, causing the Fe / V molar ratio in the precipitate to deviate from 1, reducing the formation of the highly active FeVO4 crystal phase and affecting the catalyst's reaction performance. Furthermore, to improve the wear resistance and mechanical strength of fluidized bed catalysts, current techniques often add more than 30% silica gel to the co-precipitation system, leading to difficulties in subsequent filtration and separation. Evaporating the precipitate system to dryness using a boiling water bath causes unprecipitated vanadium ions to precipitate and deposit on the precipitate crystal particles, disrupting the crystal integrity. Therefore, existing Fe-V catalyst systems exhibit low alkylation catalytic activity and 2,6-DMP selectivity, and have short catalyst lifetimes.

[0007] In summary, developing a novel catalyst for the synthesis of 2,6-dimethylphenol and its preparation method remains one of the urgent problems to be solved in this field. Summary of the Invention

[0008] To address the aforementioned technical problems, the present invention aims to provide a catalyst for the synthesis of 2,6-dimethylphenol and its preparation method. This catalyst possesses advantages such as high catalytic activity and high selectivity for 2,6-DMP.

[0009] To achieve the above objectives, a first aspect of the present invention provides a catalyst for the synthesis of 2,6-dimethylphenol, the catalyst comprising the following components: iron oxide, vanadium oxide, silicon oxide, and alkali metal oxide, wherein the molar ratio of each component is iron:vanadium:silicon:alkali metal = 1:1:(0.005-0.02):(0.001-0.005).

[0010] According to a specific embodiment of the present invention, preferably, the catalyst for synthesizing 2,6-dimethylphenol is prepared by the following steps:

[0011] (1) Dissolve metavanadate and oxalic acid in water to obtain a first solution, and adjust the pH of the first solution to 1.5-3.0;

[0012] (2) Under stirring conditions, a solution of soluble iron salt is added to the first solution, while a solution of silicon-containing compound is added dropwise, and ammonia is added dropwise to maintain the pH value of the reaction system at 1.5-3.0. After the solution of soluble iron salt and the solution of silicon-containing compound are added, stirring is continued for a period of time, and then the mixture is allowed to stand for a period of time to obtain the reaction mixture. The reaction mixture is then subjected to solid-liquid separation to obtain liquid and solid.

[0013] (3) After washing the solid, it is impregnated with an alkali metal salt solution. After drying and calcining, the impregnated solid is obtained as the catalyst for synthesizing 2,6-dimethylphenol.

[0014] In some specific embodiments of the present invention, the soluble iron salt includes one or a combination of several of ferric nitrate, ferric chloride, and ferric sulfate. Preferably, the soluble iron salt is ferric nitrate.

[0015] In some specific embodiments of the present invention, the metavanadate includes ammonium metavanadate and / or sodium metavanadate, etc. More preferably, the metavanadate is ammonium metavanadate.

[0016] In some specific embodiments of the present invention, the silicon-containing compound includes sodium silicate.

[0017] In some specific embodiments of the present invention, the alkali metal salt includes one or a combination of several of the following: alkali metal carbonates, nitrates, sulfates, and chlorides.

[0018] In some specific embodiments of the present invention, in step (1), metavanadate and oxalic acid are dissolved in water at 60-80°C.

[0019] In some specific embodiments of the present invention, step (2) is carried out at room temperature (generally 20-30°C).

[0020] In some specific embodiments of the present invention, in step (2), the stirring time after the addition of materials is completed is 0.5-4.0h.

[0021] In some specific embodiments of the present invention, in step (2), the settling time is 2-12 hours.

[0022] In some specific embodiments of the present invention, in step (3), the time for immersing and washing the solid with an alkali metal salt solution is 8-18 hours.

[0023] In some specific embodiments of the present invention, in step (3), the drying temperature is 120-180°C and the time is 6-12 hours.

[0024] In some specific embodiments of the present invention, in step (3), the calcination temperature is 400-600℃ and the time is 6-12h.

[0025] In some specific embodiments of the present invention, the preparation steps of the catalyst further include step (4): the liquid obtained by solid-liquid separation of the mixed system after the reaction in step (2) is reused to prepare the first solution in step (1).

[0026] A second aspect of the present invention provides a method for preparing the above-described catalyst for the synthesis of 2,6-dimethylphenol, comprising the following steps:

[0027] (1) Dissolve metavanadate and oxalic acid in water to obtain a first solution, and adjust the pH of the first solution to 1.5-3.0;

[0028] (2) Under stirring conditions, a solution of soluble iron salt is added to the first solution, while a solution of silicon-containing compound is added dropwise, and ammonia is added dropwise to maintain the pH value of the reaction system at 1.5-3.0. After the solution of soluble iron salt and the solution of silicon-containing compound are added, stirring is continued for a period of time, and then the mixture is allowed to stand for a period of time to obtain the reaction mixture. The reaction mixture is then subjected to solid-liquid separation to obtain liquid and solid.

[0029] (3) After washing the solid, it is impregnated with an alkali metal salt solution. After drying and calcining, the impregnated solid is obtained as the catalyst for synthesizing 2,6-dimethylphenol.

[0030] In the catalyst preparation method of this invention, the mixing method of the soluble iron salt solution and the metavanadate-containing solution is optimized. First, oxalic acid is used to adjust the pH of the metavanadate-containing solution to 1.5-3.0. Then, the soluble iron salt solution is slowly added to the metavanadate-containing solution, while ammonia is added dropwise to maintain the pH of the co-precipitation reaction system at 1.5-3.0. The mixture is thoroughly stirred and homogenized, promoting the reaction of iron ions with vanadium oxide to form iron vanadate precipitate (V / Fe molar ratio of 1), thus forming a highly active iron vanadate (i.e., FeVO4) crystal phase. The inventors of this invention have discovered that when the pH of the reaction system is less than 1.5, vanadium oxide precipitates from the metavanadate in the solution, and when the pH of the reaction system exceeds 3, iron ions precipitate to form iron hydroxide. Both of these conditions are unfavorable for the formation of the iron vanadate phase.

[0031] It should be noted that the components included in the catalyst of the present invention, namely iron oxide, vanadium oxide, silicon oxide and alkali metal oxide, only indicate that the catalyst of the present invention contains these substances, but do not describe the existence form of each substance in the catalyst of the present invention.

[0032] It should also be noted that the concentrations of metavanadate in the first solution, as well as the concentrations of the soluble iron salt solution, the silicon-containing compound solution, and the alkali metal salt solution, and their dosage relationships, can be adjusted by those skilled in the art according to the actual situation, as long as the proportions of each component in the catalyst as defined in this invention are satisfied.

[0033] In addition, in step (1), the amount of oxalic acid can also be adjusted by those skilled in the art according to the actual situation, as long as the pH value of the first solution as defined in this invention is met.

[0034] In the above preparation method, preferably, the soluble iron salt includes one or a combination of several of ferric nitrate, ferric chloride, and ferric sulfate. More preferably, the soluble iron salt is ferric nitrate.

[0035] In the above preparation method, preferably, the metavanadate includes ammonium metavanadate and / or sodium metavanadate, etc. More preferably, the metavanadate is ammonium metavanadate.

[0036] In the above preparation method, preferably, the silicon-containing compound includes sodium silicate.

[0037] In the above preparation method, preferably, the alkali metal salt includes one or a combination of several of alkali metal carbonates, nitrates, sulfates and chlorides.

[0038] In the above preparation method, preferably, in step (1), metavanadate and oxalic acid are dissolved in water at 60-80°C.

[0039] In the above preparation method, preferably, step (2) is carried out at room temperature (generally 20-30℃).

[0040] In the above preparation method, preferably, in step (2), the stirring time after the addition of materials is completed is 0.5-4.0h.

[0041] In the above preparation method, preferably, in step (2), the standing time is 2-12 hours.

[0042] In the above preparation method, preferably, in step (3), the solid is immersed in an alkali metal salt solution for 8-18 hours.

[0043] In the above preparation method, preferably, in step (3), the drying temperature is 120-180℃ and the time is 6-12h.

[0044] In the above preparation method, preferably, in step (3), the calcination temperature is 400-600℃ and the time is 6-12h.

[0045] According to a specific embodiment of the present invention, preferably, the above preparation method further includes step (4): the liquid obtained by solid-liquid separation of the mixed system after the reaction in step (2) is reused to prepare the first solution in step (1). The clear liquid after solid-liquid separation in step (2) contains vanadium ions. The present invention reuses it to prepare the first solution containing metavanadate, which not only avoids the precipitation and deposition of vanadium in the metavanadate in the iron vanadate crystal, affecting the performance of the catalyst, but also realizes the recycling of vanadium, reduces the energy consumption of the drying process, and avoids the environmental pollution caused by the discharge of vanadium in the solution.

[0046] This invention provides a catalyst for the synthesis of 2,6-dimethylphenol and its preparation method. The catalyst preparation method of this invention promotes the formation of highly active iron vanadate precipitate by adjusting the pH of the metavanadate-containing solution to 1.5-3.0 and maintaining the pH value of the system stable at 1.5-3.0 during the precipitation process. Furthermore, this invention introduces silicon and alkali metal doping into the catalyst components, which can improve the conversion rate of phenol and the selectivity of 2,6-DMP. In addition, the preparation method of this invention washes the precipitate after solid-liquid separation to prevent unprecipitated vanadium from depositing on the precipitate particles, ensuring the uniformity of the catalyst's crystal phase and improving its activity and stability. Simultaneously, the preparation method of this invention reuses the liquid obtained from solid-liquid separation, i.e., the vanadium-containing filtrate, which not only avoids residual vanadium deposition on the active phase and affecting the catalyst performance, but also achieves efficient utilization of vanadium, eliminates wastewater discharge during catalyst preparation, effectively reduces energy and water consumption, and improves the overall economic efficiency of the process.

[0047] The technical solution of the present invention has at least the following beneficial effects:

[0048] (1) The catalyst preparation method of the present invention first adjusts the pH of the solution containing metavanadate to 1.5-3.0, then adds the solution of soluble iron salt to the solution containing metavanadate, and keeps the pH of the system stable at 1.5-3.0 during the precipitation process, so as to promote the reaction of iron ions with vanadium oxide to generate iron vanadate precipitate (V / Fe molar ratio is 1), forming a highly active iron vanadate (i.e., FeVO4) crystal phase;

[0049] (2) The catalyst preparation method of the present invention uses solid-liquid separation to separate the unprecipitated vanadium element, preventing vanadium from precipitating and depositing on the precipitated crystal particles, which would damage the integrity of the crystal and thus affect the performance of the catalyst. At the same time, the vanadium-containing filtrate is reused to prepare the first solution containing metavanadate, realizing the recycling of vanadium, reducing energy and water consumption, and avoiding environmental pollution caused by the discharge of vanadium.

[0050] (3) The catalyst of the present invention has high catalytic activity and 2,6-DMP selectivity. After 800 hours of single-pass operation, the phenol conversion rate is above 95% and the 2,6-DMP selectivity is above 88%. Furthermore, the catalyst has high stability and long lifespan. After 1500 hours of single-pass operation, the catalytic activity only decreases slightly. Detailed Implementation

[0051] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0052] Example 1

[0053] This embodiment provides a catalyst for the synthesis of 2,6-dimethylphenol.

[0054] The preparation method of this catalyst includes the following steps:

[0055] (1) Weigh 28.96g NH4VO3 and 2.9g oxalic acid, dissolve them in 1000mL of deionized water at 70℃ to obtain the first solution, and the pH value of the first solution is 1.5;

[0056] (2) Weigh 100g Fe(NO3)3·9H2O and 0.7g Na2SiO3·9H2O and dissolve them in 200mL and 20mL of deionized water respectively to obtain ferric nitrate solution and sodium silicate solution respectively. Slowly add ferric nitrate solution to the first solution, while adding sodium silicate solution dropwise and adding ammonia water dropwise to ensure that the pH value of the reaction system is 1.5. Stir simultaneously. After the ferric nitrate solution and sodium silicate solution are added, continue stirring for 2 hours, and then let stand for 4 hours. After solid-liquid separation, liquid and solid (i.e., filter cake) are obtained.

[0057] (3) After washing the obtained filter cake, it was soaked in an aqueous solution of potassium carbonate (concentration of 0.05%) containing 0.02g of potassium carbonate for 12h, then dried at 150℃ for 8h, and then calcined at 500℃ for 6h to obtain a catalyst for the synthesis of 2,6-dimethylphenol.

[0058] (4) The liquid obtained from solid-liquid separation in step (2) is reused to prepare the first solution in step (1).

[0059] Example 2

[0060] This embodiment provides a catalyst for the synthesis of 2,6-dimethylphenol.

[0061] The preparation method of this catalyst includes the following steps:

[0062] (1) Weigh 28.96g NH4VO3 and 1.8g oxalic acid, dissolve them in 1000mL of deionized water at 80℃ to obtain the first solution, and the pH value of the first solution is 3.0;

[0063] (2) Weigh 40.15g FeCl3 and 1.05g Na2SiO3·9H2O and dissolve them in 200mL and 20mL of deionized water respectively to obtain ferric chloride solution and sodium silicate solution respectively. Slowly add ferric chloride solution to the first solution, while adding sodium silicate solution dropwise and adding ammonia water dropwise to ensure that the pH value of the reaction system is 3.0. Stir simultaneously. After the ferric chloride solution and sodium silicate solution are added, continue stirring for 2 hours, and then let stand for 4 hours. After solid-liquid separation, liquid and solid (i.e., filter cake) are obtained.

[0064] (3) After washing the obtained filter cake, it was soaked in an aqueous solution of potassium carbonate (concentration of 0.125%) containing 0.05g of potassium carbonate for 12h, then dried at 150℃ for 8h, and then calcined at 500℃ for 6h to obtain a catalyst for the synthesis of 2,6-dimethylphenol.

[0065] (4) The liquid obtained from solid-liquid separation in step (2) is reused to prepare the first solution in step (1).

[0066] Comparative Example 1

[0067] This comparative example provides a catalyst for the synthesis of 2,6-dimethylphenol.

[0068] The preparation method of this catalyst includes the following steps:

[0069] (1) Weigh 28.96g NH4VO3 and dissolve it in 1000mL of deionized water at 70℃ to obtain ammonium metavanadate solution; weigh 100g Fe(NO3)3·9H2O and 0.7g Na2SiO3·9H2O and dissolve them in 200mL and 20mL of deionized water respectively to obtain ferric nitrate solution and sodium silicate solution respectively.

[0070] (2) Slowly add ferric nitrate solution to ammonium metavanadate solution, while simultaneously adding sodium silicate solution dropwise, and stir simultaneously. After the ferric nitrate solution and sodium silicate solution are added, continue stirring for 2 hours, and then let stand for 4 hours. After solid-liquid separation, liquid and solid (i.e., filter cake) are obtained.

[0071] (3) After washing the obtained filter cake, it was soaked in an aqueous solution of potassium carbonate (concentration of 0.05%) containing 0.02g of potassium carbonate for 12h, then dried at 150℃ for 8h, and then calcined at 500℃ for 6h to obtain a catalyst for the synthesis of 2,6-dimethylphenol.

[0072] Comparative Example 2

[0073] This comparative example provides a catalyst for the synthesis of 2,6-dimethylphenol.

[0074] The preparation method of this catalyst includes the following steps:

[0075] (1) Weigh 28.96g NH4VO3 and 1.8g oxalic acid, dissolve them in 1000mL of deionized water at 70℃ to obtain the first solution, and the pH value of the first solution is 3.0;

[0076] (2) Weigh 40.15g FeCl3 and 5.47g InCl3 and dissolve them in 200mL of deionized water. Weigh 0.7g Na2SiO3·9H2O and dissolve it in 20mL of deionized water to obtain solutions containing ferric chloride and indium trichloride and sodium silicate solution, respectively. Slowly add the solutions containing ferric chloride and indium trichloride to the first solution, while adding sodium silicate solution dropwise and adding ammonia water dropwise to ensure that the pH of the reaction system is 3.0. Stir simultaneously. After the solutions containing ferric chloride and indium trichloride and sodium silicate solution are added, continue stirring for 2 hours, and then let stand for 4 hours. After solid-liquid separation, liquid and solid (i.e., filter cake) are obtained.

[0077] (3) After washing the obtained filter cake, it was soaked in an aqueous solution of potassium carbonate (concentration of 0.05%) containing 0.02g of potassium carbonate for 12h, then dried at 150℃ for 8h, and then calcined at 500℃ for 6h to obtain a catalyst for the synthesis of 2,6-dimethylphenol.

[0078] The catalysts prepared in Examples 1-2 and Comparative Examples 1-2 were crushed and used for catalyst performance evaluation. The catalyst performance was evaluated using a small-scale phenol alkylation evaluation system. The reaction raw materials were a mixed solution of phenol, methanol, and water, with a molar ratio of phenol:methanol:water of 1:5:3. A fixed-bed reactor was used, with a catalyst loading of 10 mL and a liquid hourly space velocity (LISH) of 0.8 h⁻¹. -1 The carrier gas nitrogen flow rate was 10 mL / min, the reaction temperature was 350 °C, and the reaction pressure was 0.1 MPa. The reaction products were analyzed using gas chromatography with internal standard method to obtain phenol conversion, o-cresol selectivity, and 2,6-DMP selectivity. The catalyst performance evaluation results of Examples 1-2 and Comparative Examples 1-2 are shown in Table 1.

[0079] Table 1

[0080]

[0081] The data above show that this invention effectively improves the catalytic activity and 2,6-DMP selectivity of the catalyst. After 800 hours of single-pass operation, the phenol conversion rate is above 95%, and the 2,6-DMP selectivity is above 88%. Furthermore, the catalyst activity decays slowly with increasing operating time. Comparative Example 1 did not use the precipitation method of this invention, and Comparative Example 2 introduced doped In metal. However, the experimental results above show that the catalysts obtained in both comparative examples have relatively low activity, and their activity decays significantly with increasing time. Therefore, this invention improves the stability and lifespan of the catalyst; the catalytic activity only slightly decreases after 1500 hours of single-pass operation. Moreover, this invention also improves the overall economic efficiency of the preparation process.

Claims

1. A catalyst for the synthesis of 2,6-dimethylphenol, the catalyst comprising: iron oxide, vanadium oxide, silicon oxide and alkali metal oxide, wherein the molar ratio of each component is iron:vanadium:silicon:alkali metal = 1:1:(0.005-0.02):(0.001-0.005); The catalyst is prepared by the following steps: (1) Dissolve metavanadate and oxalic acid in water to obtain a first solution, and adjust the pH of the first solution to 1.5-3.0; (2) Under stirring conditions, a solution of soluble iron salt is added to the first solution, while a solution of silicon-containing compound is added dropwise, and ammonia is added dropwise to keep the pH value of the reaction system between 1.5 and 3.

0. After the solution of soluble iron salt and the solution of silicon-containing compound are added, stirring is continued for a period of time, and then the mixture is allowed to stand for a period of time to obtain the mixed system after the reaction. The mixed system after the reaction is subjected to solid-liquid separation to obtain liquid and solid. (3) After washing the solid, it is impregnated with an alkali metal salt solution. After drying and calcining the impregnated solid, the catalyst for synthesizing 2,6-dimethylphenol is obtained.

2. The method for preparing the catalyst for synthesizing 2,6-dimethylphenol according to claim 1, comprising the following steps: (1) Dissolve metavanadate and oxalic acid in water to obtain a first solution, and adjust the pH of the first solution to 1.5-3.0; (2) Under stirring conditions, a solution of soluble iron salt is added to the first solution, while a solution of silicon-containing compound is added dropwise, and ammonia is added dropwise to keep the pH value of the reaction system between 1.5 and 3.

0. After the solution of soluble iron salt and the solution of silicon-containing compound are added, stirring is continued for a period of time, and then the mixture is allowed to stand for a period of time to obtain the mixed system after the reaction. The mixed system after the reaction is subjected to solid-liquid separation to obtain liquid and solid. (3) After washing the solid, it is impregnated with an alkali metal salt solution. After drying and calcining the impregnated solid, the catalyst for synthesizing 2,6-dimethylphenol is obtained.

3. The preparation method according to claim 2, wherein, The soluble iron salt includes one or a combination of ferric nitrate, ferric chloride, and ferric sulfate.

4. The preparation method according to claim 2, wherein, The metavanadate includes ammonium metavanadate and / or sodium metavanadate.

5. The preparation method according to claim 2, wherein, The silicon-containing compound includes sodium silicate.

6. The preparation method according to claim 2, wherein, The alkali metal salts include one or a combination of several of the following: carbonates, nitrates, sulfates, and chlorides of alkali metals.

7. The preparation method according to claim 2, wherein, In step (1), metavanadate and oxalic acid are dissolved in water at 60-80°C.

8. The preparation method according to claim 2, wherein, Step (2) is carried out under normal temperature conditions.

9. The preparation method according to claim 2, wherein, In step (2), the stirring time after the addition of materials is completed is 0.5-4.0 h.

10. The preparation method according to claim 2, wherein, In step (2), the settling time is 2-12 h.

11. The preparation method according to claim 2, wherein, In step (3), the solid is immersed in an alkali metal salt solution for 8-18 hours.

12. The preparation method according to claim 2, wherein, In step (3), the roasting temperature is 400-600℃ and the time is 6-12 h.

13. The preparation method according to claim 2, wherein, The preparation method further includes step (4): the liquid obtained by solid-liquid separation of the mixed system after the reaction in step (2) is reused to prepare the first solution in step (1).

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