Method for preparing anhydrous magnesium carbonate based on Mg-MOF-74

Abstract

The invention discloses a method for preparing anhydrous magnesium carbonate based on Mg-MOF-74, and belongs to the technical field of inorganic material preparation, in the method, a metal organic framework with adjustable components, structure and morphology is used as a precursor, and superfine anhydrous magnesium carbonate is designed and prepared based on the concept of structural inheritance by utilizing a simple hydrothermal synthesis technology. The metal organic framework is used as a magnesium source to construct a micro-nano cross-scale self-hybridization structure, so that the prepared anhydrous magnesium carbonate has the advantages of small size, good uniformity and high purity; meanwhile, the introduction of ethanol in the hydrothermal synthesis process not only regulates the polarity and solvation effect of the solution, but also contributes to taking away water in the later stage of the reaction and promoting the dehydration process of magnesium carbonate, thereby further ensuring the anhydrous characteristic of the product.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic material preparation, and specifically relates to a method for preparing anhydrous magnesium carbonate based on Mg-MOF-74. Background Art

[0002] Anhydrous magnesium carbonate is an important inorganic chemical product with a wide range of applications, including as a green and environmentally friendly flame retardant and filler for rubber products. Anhydrous magnesium carbonate is an inorganic compound with the chemical formula MgCO₃. It is the anhydrous form of magnesium carbonate and typically appears as a white powdery solid. It is insoluble in water, slightly soluble in ethanol, and soluble in acidic solutions. Its excellent thermal stability and chemical inertness make it widely used in various fields, including medicine, food, rubber, plastics, ceramics, and environmental protection. In the pharmaceutical industry, anhydrous magnesium carbonate is commonly used as an antacid, filter aid, and pharmaceutical excipient; in the rubber and plastics industries, it can be used as a filler and flame retardant to improve the mechanical properties and thermal stability of materials; in the ceramic industry, it is used to prepare refractory materials and glazes; and it can also be used as a desulfurizer, desiccant, and neutralizer. The production methods of anhydrous magnesium carbonate mainly include dehydration of magnesium carbonate hydrate, precipitation, or thermal decomposition. Due to its low toxicity and environmental friendliness, anhydrous magnesium carbonate is considered an important green chemical raw material with broad application prospects and development potential.

[0003] However, traditional methods for preparing anhydrous magnesium carbonate suffer from complex processes, large product sizes, and low product purity. Magnesium metal-organic frameworks (MgMOFs) have attracted widespread attention in materials science due to their unique structure and properties. Currently, research on the preparation of anhydrous magnesium carbonate using MgMOFs is limited. Developing a simpler method that can control the morphology of anhydrous magnesium carbonate is of great significance. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 in order to solve the above-mentioned problems.

[0005] The technical solution adopted by the present invention is as follows: a method for preparing anhydrous magnesium carbonate based on Mg-MOF-74, characterized in that it comprises the following steps:

[0006] S1: Prepare Mg-MOF-74 precursor, dissolve magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and 2,5-dihydroxyterephthalic acid in a mixed solution of DMF, C2H5OH and H2O at room temperature, stir for 10 minutes, and ultrasonicate for 30 minutes to completely dissolve it. Add the mixed solution to a polytetrafluoroethylene-lined reactor, set the oven temperature to 180°C, and the reaction time to 4 hours. After the reaction is completed, centrifuge at 3000 rpm for 5 minutes, wash three times with DMF, and vacuum dry at 80°C for 12 hours to obtain flower-shaped Mg-MOF-74;

[0007] S2: Mg-MOF-74 and NaHCO3 were dissolved in a mixed solution of 40 ml H2O and 40 ml C2H5OH at room temperature. The mixture was magnetically stirred for 10 min and then ultrasonicated for 30 min. The suspended solution was added to a polytetrafluoroethylene-lined reactor. The mixture was reacted at 160°C for 6 h, then centrifuged with deionized water, washed, and dried to obtain anhydrous magnesium carbonate.

[0008] In a preferred embodiment, in step S1, the amount of magnesium nitrate hexahydrate used is 0.655 g, and the amount of 2,5-dihydroxyterephthalic acid used is 0.165 g.

[0009] In a preferred embodiment, in step S1, the amount of the mixed solution is 69.7 ml.

[0010] In a preferred embodiment, in step S1, the volume ratio of DMF:C2H5OH:H2O in the mixed solution is 15:1:1.

[0011] In a preferred embodiment, in step S1, the stirring time is at least 10 minutes, the ultrasonication time is at least 30 minutes, and the reaction is carried out at 180° C. for 4 hours.

[0012] In a preferred embodiment, in step S1, the centrifugal speed is at least 3000 rpm for 5 minutes, washing is performed 3 times, and the drying condition is vacuum drying at 80° C. for at least 12 hours.

[0013] In a preferred embodiment, in step S2, the amount of flower-shaped Mg-MOF-74 used is 0.4368 g, the amount of NaHCO3 used is 0.336 gg, and the volume ratio of anhydrous ethanol to aqueous solution is 1:1.

[0014] In a preferred embodiment, in step S2, the magnetic stirring time before the reaction is at least 10 minutes, the ultrasonic time is not less than 30 minutes, the reaction temperature is 160° C., and the reaction time is 6 hours.

[0015] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0016] 1. This invention uses a metal-organic framework (MOF) with adjustable composition, structure, and morphology as a precursor, employing a simple hydrothermal synthesis technique and based on the concept of "structural genetics" to design and prepare an ultrafine anhydrous magnesium carbonate. This MOF, using a magnesium source, creates a self-hybrid structure with micro-nanoscale crossover, resulting in an anhydrous magnesium carbonate with the advantages of small size, good uniformity, and high purity.

[0017] 2. In the present invention, the purity and crystal integrity of anhydrous magnesium carbonate are significantly improved. X-ray diffraction analysis shows that the main diffraction peaks of the product at diffraction angles of 32.5°, 42.9° and 54.5° correspond to the (104), (113) and (116) crystal planes of anhydrous magnesium carbonate, respectively. The peaks are sharp and the fluctuation of the miscellaneous peaks is small, indicating that the crystal structure is complete and the impurity content is low. This high-purity characteristic is mainly due to the high purity of the metal-organic framework precursor and the controllable chemical environment during the reaction process, which avoids the impurity generation caused by uneven mixing of raw materials or severe reaction conditions in traditional preparation methods. At the same time, the introduction of ethanol in the hydrothermal synthesis process not only regulates the polarity and solvation effect of the solution, but also helps to take away water in the later stage of the reaction, promotes the dehydration process of magnesium carbonate, and further ensures the anhydrous characteristics of the product.

[0018] 3. In the present invention, the preparation process has the advantages of simple operation and mild conditions. Compared with the traditional method that requires harsh conditions of extreme high temperature and high pressure, the present invention can complete the preparation at a reaction temperature of 160°C and a reaction time of 6 hours by optimizing the reaction parameters, which greatly reduces energy consumption and equipment requirements. During the reaction process, Mg-MOF-74 and NaHCO3 can undergo hydrothermal reaction in a water-ethanol mixed solution after magnetic stirring and ultrasonic treatment. Only simple centrifugation, washing and drying steps are required subsequently. The entire process is simple and efficient, and it is easy to scale up industrial production. This process improvement not only improves production efficiency, but also helps to reduce production costs, creating favorable conditions for the large-scale application of anhydrous magnesium carbonate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is the X-ray diffraction pattern of the petal-shaped Mg-MOF-74 described in Example 1;

[0020] Figure 2 This is a scanning electron microscope photograph of the petal-shaped Mg-MOF-74 described in Example 1;

[0021] Figure 3 is the X-ray diffraction pattern of the ultrafine anhydrous magnesium carbonate described in Example 1;

[0022] Figure 4 This is a scanning electron microscope photograph of the ultrafine anhydrous magnesium carbonate described in Example 1;

[0023] Figure 5 The thermogravimetric and DSC images of the ultrafine anhydrous magnesium carbonate described in Example 1;

[0024] Figure 6 is the X-ray diffraction pattern of the anhydrous magnesium carbonate described in Comparative Example 1;

[0025] Figure 7 This is a scanning electron microscope photograph of the anhydrous magnesium carbonate described in Comparative Example 1. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0027] Reference Figure 1-7 ,

[0028] Example 1:

[0029] A method for preparing anhydrous magnesium carbonate based on magnesium MOF comprises the following steps:

[0030] Step 1: Prepare Mg-MOF-74 precursor. Dissolve 0.655g of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and 0.165g of 2,5-dihydroxyterephthalic acid in 69.7ml of a mixed solution of DMF:C2H5OH:H2O=15:1:1 at room temperature, stir for 10min, and ultrasonicate for 30min to completely dissolve it. Add the mixed solution to a polytetrafluoroethylene-lined reactor, set the oven temperature to 180°C, and the reaction time to 4h. After the reaction is completed, centrifuge at 3000rpm for 5min, wash three times with DMF, and vacuum dry at 80°C for 12h to obtain flower-shaped Mg-MOF-74.

[0031] Step 2: Take 0.4368g of Mg-MOF-74 and 0.336g of NaHCO3 at room temperature, dissolve them in a mixed solution of 40mlH2O and 40mlC2H5OH, stir magnetically for 10min, and then ultrasonicate for 30min. Add the suspended solution to a polytetrafluoroethylene-lined reactor, react at 160°C for 2h, then centrifuge with deionized water, wash, and dry to obtain a powdery product.

[0032] Example 2:

[0033] A method for preparing anhydrous magnesium carbonate based on magnesium MOF comprises the following steps:

[0034] Step 1: Prepare Mg-MOF-74 precursor. Dissolve 0.655g of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and 0.165g of 2,5-dihydroxyterephthalic acid in 69.7ml of a mixed solution of DMF:C2H5OH:H2O=15:1:1 at room temperature, stir for 10min, and ultrasonicate for 30min to completely dissolve it. Add the mixed solution to a polytetrafluoroethylene-lined reactor, set the oven temperature to 180°C, and the reaction time to 4h. After the reaction is completed, centrifuge at 3000rpm for 5min, wash three times with DMF, and vacuum dry at 80°C for 12h to obtain flower-shaped Mg-MOF-74.

[0035] Step 2: Take 0.4368g of Mg-MOF-74 and 0.336g of NaHCO3 at room temperature, dissolve them in a mixed solution of 40mlH2O and 40mlC2H5OH, stir magnetically for 10min, and then ultrasonicate for 30min. Add the suspended solution to a polytetrafluoroethylene-lined reactor, react at 160°C for 4h, then centrifuge with deionized water, wash, and dry to obtain a powdery product.

[0036] Example 3:

[0037] A method for preparing anhydrous magnesium carbonate based on magnesium MOF comprises the following steps:

[0038] Step 1: Prepare Mg-MOF-74 precursor. Dissolve 0.655g of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and 0.165g of 2,5-dihydroxyterephthalic acid in 69.7ml of a mixed solution of DMF:C2H5OH:H2O=15:1:1 at room temperature, stir for 10min, and ultrasonicate for 30min to completely dissolve it. Add the mixed solution to a polytetrafluoroethylene-lined reactor, set the oven temperature to 180°C, and the reaction time to 4h. After the reaction is completed, centrifuge at 3000rpm for 5min, wash three times with DMF, and vacuum dry at 80°C for 12h to obtain flower-shaped Mg-MOF-74.

[0039] Step 2: Take 0.4368g of Mg-MOF-74 and 0.336g of NaHCO3 at room temperature, dissolve them in a mixed solution of 40mlH2O and 40mlC2H5OH, and continue stirring to add sodium hydroxide dropwise until the pH of the solution rises to 9.6. After magnetic stirring for 10min, ultrasonicate for 30min. Add the suspended solution to a polytetrafluoroethylene-lined reactor, react at 160°C for 2h, centrifuge with deionized water, wash, and dry to obtain anhydrous magnesium carbonate.

[0040] Example 4:

[0041] Step 1: Prepare Mg-MOF-74 precursor. Dissolve 0.655g of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and 0.165g of 2,5-dihydroxyterephthalic acid in 69.7ml of a mixed solution of DMF:C2H5OH:H2O=15:1:1 at room temperature, stir for 10min, and ultrasonicate for 30min to completely dissolve it. Add the mixed solution to a polytetrafluoroethylene-lined reactor, set the oven temperature to 180°C, and the reaction time to 4h. After the reaction is completed, centrifuge at 3000rpm for 5min, wash three times with DMF, and vacuum dry at 80°C for 12h to obtain flower-shaped Mg-MOF-74.

[0042] Step 2: Take 0.4368g of Mg-MOF-74 and 0.336g of NaHCO3 at room temperature, dissolve them in a mixed solution of 40mlH2O and 40mlC2H5OH, continue stirring and add hydrochloric acid dropwise to raise the pH of the solution to 7.6, stir magnetically for 10min and then ultrasonicate for 30min, add the suspended solution to a polytetrafluoroethylene-lined reactor, react at 160°C for 2h, then centrifuge with deionized water, wash, and dry to obtain a powdery product.

[0043] Comparative Example 1

[0044] Take 50.8g of magnesium chloride hexahydrate and 30.06g of urea in a mortar, add 2.19g of ethylenediaminetetraacetic acid and grind for 10min to form a deep eutectic solution, preheat it to a transparent solution at 40°C, take 30ml of the deep eutectic solution and add it to 50ml of polytetrafluoroethylene lining and then put it into a hydrothermal reactor at 180°C for 1h, then centrifuge at 3000rpm for 5min, wash it with deionized water 3 times, and vacuum dry it at 80°C for 12h to obtain anhydrous magnesium carbonate.

[0045] The results of the above examples and comparative examples are shown in Figures 1 to 7 , result analysis:

[0046] Figure 1 The X-ray diffraction pattern of Mg-MOF-74 shows that the main diffraction peaks at diffraction angles of 6.9° and 11.9° correspond to the (110) and (030) crystal planes, respectively. The diffraction peak intensity is large, and the sample has a high degree of crystallinity. Figure 2 The Mg-MOF-74 in the sample is a petal-shaped sample with a smooth surface area, a particle size of 0.5 μm, a length of about 2-4 μm, and a uniform distribution. It has high crystallinity and regularity. In the X-ray diffraction pattern of the ultrafine anhydrous magnesium carbonate prepared using it, the diffraction angle is 32.5°, and the main diffraction peaks at 42.9° and 54.5° correspond to the (104), (113) and (116) crystal planes, respectively. These are the classic structural crystal planes of anhydrous magnesium carbonate, which are different from those in the control example. Figure 6The peak shape is sharper and the fluctuation of impurity peaks is smaller, which proves that the purity and crystal integrity of anhydrous magnesium carbonate are higher. Figure 4 The anhydrous magnesium carbonate in the sample showed a short rod-like morphology with a diameter of 0.5–1 μm and a length of about 2 μm. Figure 7 Compared with the anhydrous magnesium carbonate in Mg-MOF-74, the particles are evenly dispersed without obvious agglomeration. The above analysis shows that the preparation of anhydrous magnesium carbonate using Mg-MOF-74 as the magnesium source can play a certain role in morphology control, and the prepared anhydrous magnesium carbonate is small in size, well dispersed, and has uniform particle size.

[0047] It can be inferred that the mechanism process of MOF preparing anhydrous magnesium carbonate is divided into the following four steps.

[0048] 1. Precursor dissolution and dissociation: Mg-MOF-74 in a mixed solution of water and ethanol, the Mg 2+ Will gradually dissociate and enter the solution. At the same time, sodium bicarbonate will also dissociate in the solution to produce Na + and HCO3 - . And HCO3 - It can further dissociate into H + and CO3 2- , thereby providing a certain amount of CO3 in the solution 2- ;

[0049] 2. Complexation and stabilization: The organic ligands of Mg-MOF-74 may react with Mg 2+ and HCO3 - There is a certain interaction between them, forming a complex or a stable intermediate product, which affects Mg 2+ and CO3 2- reaction pathways and rates;

[0050] 3. Nucleation and growth: Mg 2+ and CO3 2- When they meet in the solution and reach a certain supersaturation, crystal nuclei of magnesium carbonate begin to form. In the environment of a mixed solvent of water and ethanol, the presence of ethanol may change the polarity and solvation effect of the solution, affecting the crystallization process of magnesium carbonate. The porous structure and large specific surface area of ​​Mg-MOF-74 may provide a favorable place and template for the nucleation and growth of magnesium carbonate, enabling magnesium carbonate crystals to grow orderly on its surface or in its pores.

[0051] 4. Dehydration process: During the preparation process, as the reaction proceeds and the conditions are controlled, the magnesium carbonate crystals will gradually lose their crystal water and eventually convert into anhydrous magnesium carbonate. The volatility of ethanol may help remove water during the reaction and promote the dehydration process of magnesium carbonate.

[0052] From the above we can know:

[0053] This invention innovatively uses a metal-organic framework (MOF) with adjustable composition, structure, and morphology as a precursor, combined with a simple hydrothermal synthesis technique, to successfully prepare ultrafine anhydrous magnesium carbonate based on the concept of "structural genetics." Its core advantages are reflected in the following aspects. First, the characteristics of the metal-organic framework precursor provide a flexible means for regulating product properties. Because Mg-MOF-74 itself has a regular porous structure and a large specific surface area, it can serve as an ideal structural template during the reaction process. Through the "structural genetics" effect, it guides the orderly growth of magnesium carbonate crystals on its surface or within its pores, effectively limiting excessive crystal aggregation and disordered expansion, thereby obtaining a small and uniformly distributed product. Experimental results show that the prepared anhydrous magnesium carbonate exhibits a short rod-like morphology with a diameter of 0.5–1μm and a length of approximately 2μm. Compared with traditional methods, the particle dispersion is significantly improved, with no obvious agglomeration, which lays the foundation for its uniform dispersion in composite materials.

[0054] Secondly, this method significantly improves the purity and crystal integrity of anhydrous magnesium carbonate. X-ray diffraction analysis shows that the main diffraction peaks of the product at diffraction angles of 32.5°, 42.9° and 54.5° correspond to the (104), (113) and (116) crystal planes of anhydrous magnesium carbonate, respectively. The peaks are sharp and the fluctuation of the miscellaneous peaks is small, indicating that the crystal structure is complete and the impurity content is low. This high-purity characteristic is mainly due to the high purity of the metal-organic framework precursor and the controllable chemical environment during the reaction process, which avoids the impurity generation caused by uneven mixing of raw materials or severe reaction conditions in traditional preparation methods. At the same time, the introduction of ethanol during the hydrothermal synthesis process not only regulates the polarity and solvation effect of the solution, but also helps to remove water in the later stage of the reaction, promotes the dehydration process of magnesium carbonate, and further ensures the anhydrous characteristics of the product.

[0055] In addition, the preparation process has the advantages of simple operation and mild conditions. Compared with the traditional method that requires extremely high temperature and high pressure conditions, the present invention can complete the preparation at a reaction temperature of 160°C and a reaction time of 6 hours by optimizing the reaction parameters, which greatly reduces energy consumption and equipment requirements. During the reaction process, Mg-MOF-74 and NaHCO3 can undergo hydrothermal reaction in a water-ethanol mixed solution after magnetic stirring and ultrasonic treatment. Only simple centrifugation, washing and drying steps are required. The entire process is simple and efficient, and it is easy to scale up industrial production. This process improvement not only improves production efficiency, but also helps to reduce production costs, creating favorable conditions for the large-scale application of anhydrous magnesium carbonate.

[0056] Finally, the product's ultrafine structure and excellent performance expand its application prospects. Anhydrous magnesium carbonate, as a new type of inorganic material, has environmentally friendly characteristics such as good heat absorption, release of CO2 to dilute oxygen during fire extinguishing, and generation of magnesium oxide to adsorb toxic substances. The ultrafine anhydrous magnesium carbonate prepared by the present invention has further improved flame retardant efficiency and adsorption performance due to its small size and large specific surface area. In the fields of rubber product fillers, small and uniform particles can also better combine with organic matrices to improve the mechanical properties and processing properties of the material. Therefore, the anhydrous magnesium carbonate prepared by this method shows a broader application potential in the fields of industrial flame retardancy, environmental protection, and polymer material modification.

[0057] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprises" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further limitations, an element defined by the sentence "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device that includes the element.

[0058] The above description is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing anhydrous magnesium carbonate based on Mg-MOF-74, characterized in that: The method comprises the following steps: S1: Prepare Mg-MOF-74 precursor, dissolve magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and 2,5-dihydroxyterephthalic acid in a mixed solution of DMF, C2H5OH and H2O at room temperature, stir for 10 minutes, and ultrasonicate for 30 minutes to completely dissolve it. Add the mixed solution to a polytetrafluoroethylene-lined reactor, set the oven temperature to 180°C, and the reaction time to 4 hours. After the reaction is completed, centrifuge at 3000 rpm for 5 minutes, wash three times with DMF, and vacuum dry at 80°C for 12 hours to obtain flower-shaped Mg-MOF-74; S2: Mg-MOF-74 and NaHCO3 were dissolved in a mixed solution of 40 ml H2O and 40 ml C2H5OH at room temperature. The mixture was magnetically stirred for 10 min and then ultrasonicated for 30 min. The suspended solution was added to a polytetrafluoroethylene-lined reactor. The mixture was reacted at 160°C for 6 h, then centrifuged with deionized water, washed, and dried to obtain anhydrous magnesium carbonate.

2. A method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, characterized in that: In step S1, the amount of magnesium nitrate hexahydrate used is 0.655 g, and the amount of 2,5-dihydroxyterephthalic acid used is 0.165 g.

3. A method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, characterized in that: In step S1, the amount of the mixed solution is 69.7 ml.

4. The method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, wherein: In step S1, the volume ratio of DMF:C2H5OH:H2O in the mixed solution is 15:1:

1.

5. The method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, wherein: In the step S1, the stirring time is at least 10 minutes, the ultrasonication time is at least 30 minutes, and the reaction is carried out at 180° C. for 4 hours.

6. A method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, characterized in that: In the step S1, the centrifugal speed is at least 3000 rpm for 5 minutes, washing is performed 3 times, and the drying condition is vacuum drying at 80° C. for at least 12 hours.

7. The method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, wherein: In the step S2, the amount of flower-shaped Mg-MOF-74 used is 0.4368 g, the amount of NaHCO3 used is 0.336 gg, and the volume ratio of anhydrous ethanol to aqueous solution is 1:

1.

8. The method for preparing anhydrous magnesium carbonate based on Mg-MOF-74 according to claim 1, wherein: In step S2, the magnetic stirring time before the reaction is at least 10 minutes, the ultrasonic time is not less than 30 minutes, the reaction temperature is 160° C., and the reaction time is 6 hours.

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