Magnesium aluminate spinel catalyst for efficiently depolymerizing lignin as well as preparation method and application of magnesium aluminate spinel catalyst

Abstract

The invention discloses a magnesium aluminate spinel catalyst for efficiently depolymerizing lignin as well as a preparation method and application of the magnesium aluminate spinel catalyst. The catalyst is prepared by the following steps: preparing a magnesium aluminate spinel carrier by a coprecipitation-calcination method, then slowly adding an active metal precursor salt solution into a uniformly dispersed carrier suspension, and then adding a reducing agent to carry out an active metal reduction reaction. According to the catalyst, active metal elements are tightly embedded by adjusting the multistage nanopore structure and acid-base active sites on the surface of the carrier, the effective contact area of catalytic reaction is greatly increased, and the catalytic hydrogenation activity is effectively improved. When the catalyst is applied to catalytic depolymerization of lignin under a hydrogen-free condition to prepare phenolic chemicals, the catalytic hydrogenation reaction activity can be remarkably improved, and the technical problems of harsh reaction conditions and low product yield and selectivity in the existing hydrogen-free catalytic reduction depolymerization technology are effectively solved.

Description

Technical Field

[0001] The invention relates to a magnesium aluminum spinel catalyst, in particular to a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin and a preparation method and application thereof, belonging to the technical field of energy chemical industry. Background Art

[0002] Lignocellulosic biomass is a potential alternative to fossil resources in the energy and chemical industries. It is mainly composed of cellulose, hemicellulose and lignin. Agricultural and forestry wastes (corn stalks, tea nut shells, poplar and pine, etc.) are the most abundant renewable resources in nature. However, these wastes are often used for incineration or composting, resulting in environmental pollution and waste of resources. Efficient use of these agricultural and forestry wastes can not only alleviate environmental problems, but also create considerable economic benefits.

[0003] Lignin is one of the three major components of wood fiber biomass. It is a three-dimensional network polymer formed by three basic structural units, namely syringylpropane (S), guaiacylpropane (G) and p-hydroxyphenylpropane (H), connected by CC bonds and COC bonds. By breaking the chemical bonds in the lignin molecules, small molecular depolymerization products containing methoxy, phenol (alcohol) hydroxyl, carbonyl, carboxyl and other functional groups can be obtained. The H / C content ratio of these small molecular compounds is similar to that of petroleum, and they have great application potential in the preparation of high-quality liquid fuels and high-value-added chemicals.

[0004] In 2015, the Sels and Abu-Omar research team proposed a catalytic reduction separation (RCF) strategy based on "lignin first". In this strategy, the catalyst works with hydrogen or hydrogen donor solvents (such as methanol, ethanol, isopropanol and water / alcohol co-solvents) to extract native lignin from wood fiber biomass and depolymerize it. Under relatively mild conditions (about 200°C–250°C), this method achieves a high yield of phenolic monomers. Among them, a catalyst with efficient catalytic hydrogenolysis is the key to the effective depolymerization of lignin under mild conditions. Metal oxide-supported noble metal catalysts are highly favored in the RCF process because they provide abundant active sites. However, the preparation process of such catalysts usually requires the introduction of hydrogen under high temperature conditions for activation. For example, the catalyst preparation scheme disclosed in the Chinese patent (CN11368860A) requires reduction at 350°C for 8 hours in a flowing 5% H2 atmosphere. This high-temperature activation process not only increases equipment and safety requirements, but also significantly increases energy consumption and time costs. Although the Chinese patent (CN116603515A) discloses a catalyst preparation scheme that does not require hydrogen activation, the application of this catalyst is mainly aimed at high-temperature pyrolysis of woody biomass to prepare aldehyde chemicals, which has high energy consumption, and the conversion products are different from the target products of this scheme, and it is difficult to predict its effect in the catalytic reduction depolymerization of lignin. (Applied Catalysis A, 2024, 670, 119555) involves the use of NaBH4 to reduce Au and prepare Au nanoparticles with a minimum particle size of 2.2±0.4nm. The reported scheme requires the use of PVP as a stabilizer, but this scheme is not applied to lignin depolymerization, and PVP is also introduced, and a complex process is required to remove PVP later. Therefore, it is still worth paying attention to the reduction of precious metals through a simple, safe and efficient process while ensuring the generation of smaller metal nanoparticles to improve the catalytic performance of the catalyst.

[0005] On the other hand, the current RCF process usually relies on external hydrogen to increase the yield of phenolic monomers, but the use of hydrogen significantly increases the process cost and poses safety hazards due to its flammable and explosive properties. These factors, coupled with strict environmental and equipment requirements, limit the pilot-scale application of the RCF process (ChemSusChem, 2023, 16(13), e202300103). For example, the MgAl2O4-loaded precious metal Ru catalyst mentioned in the Chemical Engineering Journal, 2024, 495, 153612 document discloses the depolymerization of the lignin model diphenyl ether under the synergistic effect of hydrogen. Although a good depolymerization effect was obtained, the document only discloses the application of simple model compounds derived from lignin, and there is no report on the application effect under more complex real lignin and without external hydrogen. Therefore, it is of great industrial application value to develop an RCF production process without the need for external hydrogen to reduce costs and improve safety. Summary of the invention

[0006] In view of the problems existing in the prior art, the first purpose of the present invention is to provide a magnesium-aluminum-spinel catalyst for efficiently depolymerizing lignin. The catalyst is based on the synergistic effect between a magnesium-aluminum-spinel carrier and active metal elements. By adjusting the multi-level nanopore structure and acid-base active sites on the carrier surface, the active metal elements are firmly embedded while greatly increasing the effective contact area of ​​the catalytic reaction. This ensures that the catalyst has excellent stability while greatly improving its activity in catalyzing lignin depolymerization reactions, especially the catalytic activity of RCF reactions in the absence of external hydrogen.

[0007] The second object of the present invention is to provide a method for preparing a magnesium-aluminum spinel catalyst for efficiently depolymerizing lignin. The method first prepares a magnesium-aluminum spinel carrier with a high specific surface area and a multi-level nanopore structure by a co-precipitation-calcination method, and then reduces the active metal ions loaded on the carrier to a zero-valent state by a solid-liquid method at low temperature. The entire process does not require hydrogen reduction or activation, and due to the synergy between the carrier and the zero-valent active metal, not only the molecular mass transfer capacity of the catalyst is effectively improved, but also its hydrogen supply efficiency is greatly improved, thereby improving the catalytic performance of the RCF reaction under the condition of no external hydrogen addition.

[0008] The third object of the present invention is to provide an application of a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, which is used for catalytic depolymerization of lignin in the absence of hydrogen to prepare phenolic chemicals. Based on the excellent catalytic performance of the above catalyst, it is applied to the catalytic depolymerization of lignin in a hydrogen-free environment to prepare phenolic chemicals, which can significantly improve the yield and selectivity of the reaction; after testing, the yield of phenolic monomers was as high as 39% (poplar wood) when the catalyst provided by the present invention was used to catalyze the depolymerization reaction of native lignin in the absence of hydrogen, and the selectivity of propanol-substituted monomers in the monomer product was as high as 91% (pine wood), which effectively solved the technical problems of harsh reaction conditions, low product yield and selectivity in the existing hydrogen-free catalytic reduction depolymerization technology.

[0009] In order to achieve the above technical objectives, the present invention provides a method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, comprising:

[0010] Step S1, dissolving a carrier raw material including hydrated magnesium nitrate and hydrated aluminum nitrate, and adding the dissolved raw material to a mixed alkali solution under stirring, and sequentially performing coprecipitation reaction, filtering, washing and drying to obtain a carrier precursor;

[0011] Step S2, calcining the carrier precursor by a programmed temperature method until it is completely crystallized into a spinel crystal form, thereby obtaining a multi-level nanoporous structure carrier;

[0012] Step S3, dispersing the carrier obtained in step S2 into a suspension, then slowly adding the active metal precursor salt solution into the uniformly dispersed carrier suspension, and then adding a reducing agent to carry out an active metal reduction reaction, thereby obtaining;

[0013] The atomic ratio of magnesium to aluminum in the hydrated magnesium nitrate and hydrated aluminum nitrate is 1:2-4; the loading amount of active metal in the catalyst is 3-5wt%.

[0014] In the prior art, the depolymerization catalyst of lignin has low efficiency in the catalytic depolymerization process due to the low number of acid and base sites and their uneven distribution, especially in the RCF reaction without external hydrogenation. The technical solution provided by the present invention strictly controls the ratio between the components, improves the synergistic effect between the carrier and the active metal, and improves the catalyst molecular mass transfer efficiency while also improving the catalyst hydrogen supply efficiency, especially maintaining a high yield and selectivity under hydrogen-free conditions.

[0015] As a preferred solution, the mixed alkali solution is a mixed solution of soda ash and caustic soda, and the molar ratio of the two is 1:1-3. The carbonate ions provided by sodium carbonate can form carbonate or bicarbonate precipitation with magnesium ions and aluminum ions. The precipitation is relatively loose, which is conducive to the subsequent calcination to form a porous structure; and the use of a double alkali mixing method can control the reaction rate of the precipitation on the one hand, avoid the agglomeration of particles caused by too fast reaction, and on the other hand, ensure the formation of a uniformly mixed magnesium-aluminum composite precipitate, which is conducive to the subsequent production of magnesium-aluminum spinel.

[0016] As a preferred solution, the coprecipitation reaction process is: adjust the system pH to 9-11, continue stirring for 20-50 minutes, and age at room temperature for 8-12 hours. The aging process helps the growth of crystals and the rearrangement of the precipitation structure, making the precursor more regular and uniform, thereby generating appropriate pore sizes during subsequent calcination.

[0017] As a preferred solution, the washing process uses ultrapure water to wash until it is neutral.

[0018] As a preferred solution, the conditions of the drying process are: drying temperature is 70-100°C and time is 10-14h.

[0019] As a preferred solution, the calcination process is: placing the carrier precursor in a muffle furnace, heating from room temperature to 800-900°C at 3-8°C / min, keeping the temperature for 3-5h, and then cooling to room temperature with the furnace.

[0020] The present invention needs to calcine the carrier precursor strictly in accordance with the above requirements. During the calcination process, temperature and heating rate are key factors for regulating the pore structure of the carrier. Only when the temperature reaches 800°C can the complete crystallization of spinel be guaranteed, and the heating rate can effectively avoid the drastic shrinkage of particles due to excessive temperature change, thereby achieving effective regulation of the carrier pore size.

[0021] As a preferred solution, the catalyst active metal is at least one of Pd, Pt and Ru.

[0022] As a preferred solution, the process of adding the salt solution of the catalyst active metal to the suspension is: slowly adding dropwise under stirring at 250-350 rpm, and after the addition is completed, stirring is continued for 5-6 hours and allowed to stand overnight.

[0023] As a preferred solution, the reducing agent is sodium borohydride and / or potassium borohydride, and the molar ratio of sodium borohydride and potassium borohydride to the catalyst active metal is 15-25:1.

[0024] As a preferred solution, the reduction reaction process is: dissolving the reducing agent in water, dripping into the system at 3-5 ml / min under ice bath state and stirring rate of 300-500 rpm, and then continuously stirring at 200-300 rpm for 2-5 hours after the dropwise addition is completed.

[0025] Compared with the prior art which uses hydrogen for gas-solid phase reduction, the present invention uses a solid-liquid method to reduce active metals, and the reaction conditions are warmer and the reduction is more complete. In addition, the alkaline environment is also conducive to the stability of the reducing agent, avoiding the decomposition and failure of the reducing agent before adding materials, while the ice bath state can slow down the reaction rate, promote the uniformity of reduction, and reduce particle agglomeration.

[0026] The present invention also provides a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, which is obtained by any one of the preparation methods described above; the active metal particle size of the catalyst is 1.2-2.0 nm; the specific surface area of ​​the carrier of the catalyst is 180-200 m 2 / g, pore size is 10~20nm.

[0027] The present invention also provides an application of a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, which is used for catalytically depolymerizing lignin to prepare phenolic chemicals in a hydrogen-free environment.

[0028] As a preferred solution, the raw material of the lignin is agricultural and forestry waste containing lignin.

[0029] As a preferred solution, the catalytic depolymerization process of lignin is: subjecting the lignin raw material to a hydrothermal reaction with a catalyst under protective atmosphere conditions.

[0030] As a preferred solution, the conditions of the hydrothermal reaction are: stirring rate of 800-900 rpm, temperature of 200-250° C., and time of 2-4 h.

[0031] As a preferred solution, the protective atmosphere is nitrogen and / or argon.

[0032] Compared with the prior art, the beneficial technical effects of the technical solution of the present invention are:

[0033] 1) The catalyst provided by the present invention is based on the synergistic effect between the magnesium aluminum spinel carrier and the active metal elements. By adjusting the multi-level nanopore structure and acid-base active sites on the carrier surface, the active metal elements are firmly embedded while greatly increasing the effective contact area of ​​the catalytic reaction. This ensures that the catalyst has excellent stability while greatly improving its activity in catalyzing lignin depolymerization reactions, especially the catalytic activity in RCF reactions without external hydrogenation.

[0034] 2) The preparation method provided by the present invention first prepares a magnesium-aluminum spinel carrier with a high specific surface area and a multi-level nanopore structure through a co-precipitation-calcination method, and then reduces the active metal ions loaded on the carrier to a zero-valent state through a solid-liquid method at low temperature. The entire process does not require hydrogen reduction or activation. In addition, due to the synergy between the carrier and the zero-valent active metal, it not only effectively improves the molecular mass transfer capacity of the catalyst, but also greatly improves its hydrogen supply efficiency, thereby improving the catalytic performance of the RCF reaction in the absence of external hydrogen.

[0035] 3) In the technical solution provided by the present invention, based on the excellent catalytic performance of the above-mentioned catalyst, it is applied to the catalytic depolymerization of lignin to produce phenolic chemicals in a hydrogen-free environment, which can significantly improve the yield and selectivity of the reaction; after testing, the yield of phenolic monomers can reach 39% and the selectivity of propanol-substituted monomers in the monomer product can reach 91% by using the catalyst provided by the present invention to carry out hydrogen-free catalytic depolymerization reaction of native lignin, which effectively solves the technical problems of harsh reaction conditions and low product yield and selectivity in the existing hydrogen-free catalytic reduction depolymerization technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a nitrogen adsorption-desorption curve of the carrier provided in Example 1 of the present invention

[0037] in, Figure 1 (a) Nitrogen adsorption-desorption isotherms of the carrier, Figure 1 (b) Pore size distribution curve of the carrier.

[0038] Figure 2 X-ray diffraction patterns of the catalysts provided in Examples 1 and 2 of the present invention;

[0039] Figure 3 TEM image of the catalyst provided in Example 1 of the present invention and a particle size distribution diagram of Pd nanoparticles;

[0040] in, Figure 3 (a) is the TEM image of the catalyst, with a scale of 50 nm. Figure 3 (b) is the TEM image of the catalyst, with a scale of 20 nm. Figure 3 (c) is the particle size distribution diagram of Pd nanoparticles in the catalyst;

[0041] Figure 4 NH3-TPD image of the catalyst provided in Example 1 of the present invention;

[0042] Figure 5 Gas chromatogram of liquid product after lignin depolymerization by the catalyst provided in Example 1 of the present invention;

[0043] in, Figure 5(a) is the gas chromatogram of the liquid product after lignin depolymerization under hydrogenation conditions. Figure 5 (b) is the gas chromatogram of the liquid product after lignin depolymerization without hydrogenation. DETAILED DESCRIPTION

[0044] In order to facilitate the understanding of the present invention, the present invention will be described in more comprehensive and detailed manner in conjunction with the accompanying drawings and preferred embodiments of the specification. It should be noted that the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] Example 1

[0046] This embodiment provides a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, and the preparation process is as follows:

[0047] 1. Preparation of multi-level nanoporous structure carrier:

[0048] 0.02 mol of Mg(NO3)2·6H2O and 0.04 mol of Al(NO3)3·9H2O were dissolved in 100 mL of ultrapure water, and added to the mixed alkali solution under stirring, and the pH was adjusted to 10, and then the mixed solution was stirred for 35 min and aged at room temperature for 10 hours. After the aging, the precipitate was separated, washed with ultrapure water to a pH of 7, and then dried at 80°C for 12 hours to obtain a carrier precursor;

[0049] The carrier precursor was placed in a muffle furnace, heated from room temperature to 800°C at a heating rate of 5°C / min, and calcined for 4 hours to obtain a hierarchical nanoporous structure carrier.

[0050] 2. Preparation of catalyst

[0051] 1 g of MgAl2O4 spinel was evenly dispersed in 20 ml of deionized water, and 0.1286 g of Pd(NO3)2·2H2O active metal salt was dissolved in 10 ml of deionized water. The active metal salt solution was slowly added dropwise to the MgAl2O4 spinel mixed solution under stirring at 300 rmp / min. After stirring for 6 hours, the solution was allowed to stand overnight.

[0052] Weigh 0.374 g of NaBH4 and dissolve it in 25 ml of a mixed solvent of water / ethanol in a volume ratio of 4:6 to obtain a reducing agent with a pH of 11. Add it dropwise to the mixed solution that has been standing overnight at a rate of 5 ml / min under ice bath conditions (the temperature is 0°C under the condition of solid-liquid coexistence) and stir vigorously (400 rpm). Continue stirring at a stirring rate of 250 rpm for 3 hours, wash with a large amount of deionized water and ethanol solvent until the pH is 7, and dry the filtered solid in vacuum at room temperature to obtain the obtained product.

[0053] The present invention also performs a performance test of catalytic depolymerization of the catalyst obtained in this embodiment to prepare phenolic chemicals, and the process is as follows: 1g biomass raw material, 0.2g Pd / MgAl2O4 catalyst and 50 mL (ethanol / water=7:3) co-solvent are mixed in a 100ml autoclave reactor. Before the reaction starts, nitrogen is used to exhaust the autoclave reactor to remove air, and then N2 or H2 is pressurized into the autoclave reactor to reach a pressure of 2MPa, and the stirring rate is set to 800rpm, and the reactor is heated to 240°C, reacted for 3h, cooled to room temperature after the reaction ends, and the liquid part is rotary evaporated at 60°C after filtration to obtain lignin oil, and then extracted with dichloromethane and saturated sodium chloride solution for multiple times to obtain phenolic monomers. The liquid product is quantitatively analyzed by gas chromatography to determine the phenolic monomer yield and the selectivity of specific phenolic monomers, and the results are shown in Tables 1 and 2.

[0054]

[0055]

[0056] It can be seen from Tables 1 and 2 that the catalyst prepared by this scheme can achieve efficient catalytic depolymerization of native lignin under mild conditions, and still maintain a high phenolic monomer yield and high selectivity for specific substituted monomers even without the addition of hydrogen.

[0057] Example 2

[0058] This example is exactly the same as Example 1, except that the active metal salt is replaced with 0.13 g H2PtCl6·6H2O.

[0059] The present invention also carries out a performance test of catalytic depolymerization of the catalyst obtained in this embodiment to prepare phenolic chemicals, and the process is exactly the same as that in Example 1, and the results are as follows: under the condition of adding H2, the yield of phenolic monomers using oil-tea camellia shells as raw materials is 13.2 wt%, the selectivity of propanol-substituted monomers is 57.8%, and the selectivity of propyl-substituted monomers is 28.7%; under the condition of no adding H2 (i.e., an atmosphere of added N2), the yield of phenolic monomers using oil-tea camellia shells as raw materials is 10.2 wt%, the selectivity of propanol-substituted monomers is 31.9%, and the selectivity of propyl-substituted monomers can reach 50.5%.

[0060] Example 3

[0061] This embodiment is exactly the same as embodiment 1, except that the amount of Pd(NO3)2·2H2O added is 0.077 g.

[0062] The present invention also carries out a performance test of catalytic depolymerization of the catalyst obtained in this embodiment to prepare phenolic chemicals, and the process is exactly the same as that in Example 1, and the results are as follows: under the condition of adding H2, the yield of phenolic monomers using oil-tea camellia shells as raw materials is 13.4 wt%, the selectivity of propanol-substituted monomers is 86.4%, and the selectivity of propyl-substituted monomers is 9.1%; under the condition of no adding H2 (i.e., an atmosphere of added N2), the yield of phenolic monomers using oil-tea camellia shells as raw materials is 9.8 wt%, the selectivity of propanol-substituted monomers is 44.0%, and the selectivity of propyl-substituted monomers can reach 41.1%.

Claims

1. A method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, characterized in that: include: Step S1, dissolving a carrier raw material including hydrated magnesium nitrate and hydrated aluminum nitrate, and adding the dissolved raw material to a mixed alkali solution under stirring, and sequentially performing coprecipitation reaction, filtering, washing and drying to obtain a carrier precursor; Step S2, calcining the carrier precursor by a programmed temperature method until it is completely crystallized into a spinel crystal form, thereby obtaining a multi-level nanoporous structure carrier; Step S3, dispersing the carrier obtained in step S2 into a suspension, then slowly adding the active metal precursor salt solution into the uniformly dispersed carrier suspension, and then adding a reducing agent to carry out an active metal reduction reaction, thereby obtaining; The atomic ratio of magnesium to aluminum in the hydrated magnesium nitrate and hydrated aluminum nitrate is 1:2-4; the loading amount of active metal in the catalyst is 3-5wt%.

2. The method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin according to claim 1, characterized in that: The mixed alkali solution is a mixed solution of soda ash and caustic soda, and the molar ratio of the two is 1:1-3; the process of the coprecipitation reaction is: adjusting the pH of the system to 9-11, continuing stirring for 20-50 minutes, and aging at room temperature for 8-12 hours.

3. The method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin according to claim 1, characterized in that: The washing process uses ultrapure water to wash until it is neutral; the conditions of the drying process are: the drying temperature is 70-100° C. and the time is 10-14 hours.

4. The method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin according to claim 1, characterized in that: The calcination process is as follows: placing the carrier precursor in a muffle furnace, heating from room temperature to 800-900° C. at 3-8° C. / min, keeping the temperature for 3-5 hours, and then cooling to room temperature along with the furnace.

5. The method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin according to claim 1, characterized in that: The catalyst active metal is at least one of Pd, Pt and Ru; the process of adding the salt solution of the catalyst active metal to the suspension is: slowly adding dropwise under stirring at 250-350 rpm, and continuously stirring for 5-6 hours after the addition is completed and standing overnight.

6. The method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin according to claim 1, characterized in that: The reducing agent is sodium borohydride and / or potassium borohydride, and the molar ratio of sodium borohydride to the catalyst active metal is 15-25:

1.

7. The method for preparing a magnesium aluminum spinel catalyst for efficiently depolymerizing lignin according to claim 1, characterized in that: The reduction reaction process is as follows: dissolving the reducing agent in a co-solvent of ethanol and water, dripping into the system at 3-5 ml / min under an ice bath state and a stirring rate of 300-500 rpm, and continuously stirring at 200-300 rpm for 2-5 hours after the dripping is completed.

8. A magnesium aluminum spinel catalyst for efficiently depolymerizing lignin, characterized in that: Obtained by the preparation method according to any one of claims 1 to 7; the active metal particle size of the catalyst is 1.2 to 2.0 nm; the specific surface area of ​​the catalyst carrier is 180 to 200 m 2 / g, pore size is 10~20nm.

9. The use of a magnesium aluminum spinel catalyst for efficient depolymerization of lignin according to claim 8, characterized in that: The method is used for catalytic depolymerization of lignin to prepare phenolic chemicals under hydrogen-free conditions; the raw material of the lignin is agricultural and forestry waste containing lignin.

10. The use of a magnesium aluminum spinel catalyst for efficient depolymerization of lignin according to claim 9, characterized in that: The catalytic depolymerization process of lignin is as follows: the lignin raw material and the catalyst are subjected to a hydrothermal reaction under protective atmosphere conditions; the conditions of the hydrothermal reaction are as follows: a stirring rate of 800-900 rpm, a temperature of 200-250° C., and a time of 2-4 hours; the protective atmosphere is nitrogen and / or argon.

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