Preparation and Application of Manganese Dioxide Catalyst for the Liquid-Phase Selective Oxidation of Benzyl Alcohol to Benzaldehyde

Abstract

The present invention discloses the preparation and application of a manganese dioxide catalyst for the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde. The preparation method of the manganese dioxide catalyst comprises the following steps: a potassium permanganate solution is slowly added dropwise to a manganese nitrate solution, redox precipitation is carried out at a low temperature, and then drying and calcination are performed. The method of the present invention has the advantages of low cost, simple process flow, mild and broad preparation conditions, etc.; the catalyst prepared by this method has a very high specific surface area, and in the selective oxidation reaction of benzyl alcohol with oxygen as the oxidant under mild conditions, it shows high benzyl alcohol conversion rate and high benzaldehyde selectivity, realizing the efficient production of benzaldehyde.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst preparation, and particularly relates to the preparation of a manganese dioxide catalyst and its application in the liquid-phase selective oxidation of benzyl alcohol to prepare benzaldehyde. Background Art

[0002] Benzaldehyde is a colorless or light yellow viscous liquid with a bitter almond odor. It is the simplest aromatic aldehyde and also the most widely used aromatic aldehyde in industry: in the field of organic chemical industry, it is used to prepare lauric aldehyde, lauric acid, phenylacetaldehyde, benzyl benzoate, etc.; in the pharmaceutical field, it is an intermediate and additive for medicines such as amoxicillin, esmolol, and ampicillin cephalosporin; in the pesticide field, it is a key intermediate for bromobenzonitrile, fenitrothion, and herbicides; in the textile industry, it is used for synthesizing common dyes and as a solvent; in addition, it is also a special top note fragrance and is widely used in fields such as food and daily chemicals.

[0003] Currently, the main preparation methods of benzaldehyde are toluene chlorination hydrolysis method, toluene direct oxidation method, aromatic ester / acid catalytic hydrogenation reduction method, and benzyl alcohol oxidation method, etc. In the toluene chlorination hydrolysis method, toluene undergoes side-chain chlorination under certain conditions to generate a mixture of benzyl chloride, dichlorobenzyl, and trichlorobenzyl. After removing benzyl chloride by distillation, dichlorobenzyl is hydrolyzed to obtain the main product benzaldehyde, and at the same time, trichlorobenzyl is hydrolyzed to generate the by-product benzoic acid. The advantages of this method are mature process and low equipment requirements, while the disadvantages are environmental unfriendliness, low product purity, and since the product contains chlorine, it cannot be used in applications in industries such as food and medicine. The toluene direct oxidation method has the characteristics of relatively simple reaction process and environmental friendliness, but the disadvantages are many by-products and harsh production conditions. The advantages of the aromatic ester / acid catalytic hydrogenation reduction method are high product yield, but it has no market competitiveness due to high raw material cost and production cost. The raw material benzyl alcohol in the benzyl alcohol oxidation method is cheap and easily available, and green oxidants such as oxygen and hydrogen peroxide are used, with low process cost and simple reaction, which is easy to industrialize. The gas-phase oxidation of benzyl alcohol uses a fixed bed as the reactor, with the advantages of high production efficiency and easy separation of products, but the disadvantages are high energy consumption and high equipment requirements, thus limiting its industrialization. Only the liquid-phase oxidation of benzyl alcohol has the characteristics of simple process, environmental friendliness, mild reaction conditions, and few by-products, and has good industrial prospects.

[0004] The key to the selective catalytic oxidation of benzyl alcohol lies in the selection of catalysts. At present, various catalysts for the oxidation of benzyl alcohol have been developed, mainly including noble metal catalysts and transition metal oxide catalysts, etc. Previous studies have mainly focused on noble metal catalysts based on Au and Pd: Patent CN104069857A discloses a preparation method and application of a nano Au / MgO catalyst. Under the condition of 110 °C and reacting for 3 hours, the conversion rate of benzyl alcohol reaches over 95%, and the yield of benzaldehyde can reach 90%; Patent CN114425332A discloses a preparation method and application of an ultrathin nanosheet constructed Au-Pd microflower. Under the condition of 50 °C, the complete reaction only takes 3.5 hours, and the yield of benzaldehyde reaches 100%. Although noble metal catalysts have high activity in the catalytic oxidation of benzyl alcohol, noble metal resources are scarce and expensive, which is not conducive to industrial applications. In recent years, transition metals have attracted much attention due to their rich resources, low price, and relatively excellent catalytic performance. Among many transition metal oxides, manganese oxides have become one of the catalysts widely studied because of their various oxidation states, diverse crystal structures, and excellent redox performance.

[0005] Most traditional manganese dioxide catalysts are synthesized by the hydrothermal method: For example, Patent CN114797853A discloses a preparation method of an α-crystalline manganese dioxide catalyst, which is prepared by the hydrothermal reaction of potassium permanganate and divalent manganese salt at 180 - 280 °C and has high catalytic activity for the oxidation of VOCs; Another example is Patent CN116273079A, which discloses a high-performance manganese dioxide for zinc-manganese batteries and its preparation method. It is prepared by dissolving potassium persulfate and manganese sulfate in a gelatin solution and then transferring it to a hydrothermal autoclave for high-temperature hydrothermal reaction and other steps, and the process is simple. However, the hydrothermal method generally has technical problems such as too high reaction temperature, low safety, and difficult reaction control, making it difficult to produce in large quantities, and the specific surface area of the catalyst is not high either (generally 20 - 70 m 2 / g). In addition to the hydrothermal method for preparing manganese dioxide, some other better preparation methods have emerged one after another. Patent CN106517341B discloses a method for preparing a manganese dioxide nanocatalyst and its application. This method obtains a nanowire-like manganese dioxide catalyst through pH adjustment and microwave heating and has high degradation performance for p-nitrophenol, but the disadvantage is that the pH value needs to be adjusted and stabilized at a certain value, and the reaction conditions are relatively harsh. Patent CN106902863A discloses a preparation method of a mesoporous manganese dioxide catalyst for the catalytic oxidation of ethanol to acetaldehyde: The template agent of the mesoporous silicon material is impregnated with a manganese salt, calcined to obtain a precursor, and then the template agent is removed. Its advantage is that it can be prepared under mild conditions and the product has a very high selectivity for acetaldehyde, but the specific surface area is 40 - 100 m 2 / g, the ethanol conversion rate is only 60%, and both the specific surface area and catalytic activity need to be improved. Patent CN108579729A discloses a preparation method of a catalyst for ozone decomposition. Mix potassium permanganate and reducing agent ammonium salt, and obtain manganese dioxide material through low-temperature redox reaction. After being treated with ammonium salt, this material has good ozone removal performance, but organic amines are not easily available and the cost is high. Patent CN113797925A provides a nano-manganese dioxide catalyst for removing formaldehyde and its preparation method. Using an organic reducing agent directly as a pore-forming agent can improve the specific surface area of manganese dioxide to a certain extent, but its preparation process is too complicated. Patent CN116903452A discloses a preparation method of a catalyst for the oxidation of benzyl alcohol to benzaldehyde. The manganese oxide catalyst obtained by this method is tested at 95°C. Although the benzaldehyde selectivity is as high as 100%, the conversion rate of benzyl alcohol is only 63%.

[0006] In summary, there are technical problems such as complex preparation process, too high reaction temperature, and harsh reaction conditions in current manganese dioxide catalysts. Therefore, the improvement of the catalyst preparation method is particularly important for developing an efficient catalyst for the catalytic oxidation of benzyl alcohol to benzaldehyde. The present invention adopts a forward-dropping low-temperature redox precipitation method to prepare a manganese dioxide catalyst. The prepared catalyst has a high specific surface area and high catalytic activity. The raw materials for preparation are common manganese salts, with low cost, mild and wide preparation conditions, and simple process. Summary of the Invention

[0007] The purpose of the present invention is to provide a preparation method of a manganese dioxide catalyst for the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde, so as to solve the technical problems such as complex preparation process, too high reaction temperature, and harsh reaction conditions of existing catalysts.

[0008] Another purpose of the present invention is to provide the application of a manganese dioxide catalyst obtained by the above preparation method in the reaction of liquid-phase selective oxidation of benzyl alcohol to benzaldehyde. This catalyst has good catalytic activity, high conversion rate of benzyl alcohol, and high selectivity of benzaldehyde, and has the characteristics of efficient production when used for the oxidation of benzyl alcohol.

[0009] The technical solutions that the present invention can provide are as follows:

[0010] In the first aspect, the present invention provides a preparation method of a manganese dioxide catalyst for the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde, including the following steps:

[0011] 1) Dissolve a certain amount of manganese nitrate in deionized water to form solution A, and dissolve slightly excessive potassium permanganate in deionized water to form solution B;

[0012] 2) Conduct low-temperature redox reaction: Keep solution A stirred at 30 - 60 °C, slowly add solution B dropwise. After the addition is complete, control the pH value of the reaction system between 0 - 4, and continue to stir and age at 30 - 60 °C for 3 - 10 h;

[0013] 3) Filter and separate the reaction mixture obtained in step 2), and wash the precipitate;

[0014] 4) Dry the precipitate obtained in step 3) and then calcine it at 200 - 400 °C to obtain the manganese dioxide catalyst described.

[0015] In step 1) of the present invention, the so-called "slightly excessive" means that the addition amount of potassium permanganate does not exceed 20% of the theoretical addition amount of potassium permanganate.

[0016] Preferably, in step 1), the concentration of potassium permanganate is 10 - 40 g / L, and the concentration of manganese nitrate is 15 - 62 g / L.

[0017] In step 2) of the present invention, the dropping order is to add solution B to solution A (forward dropping), and the dropping rate is 1 - 2 drops per second.

[0018] Preferably, in step 2), the temperature of the redox reaction is 30 °C. After the dropping is complete, keep stirring and aging at 30 °C for 8 h.

[0019] In step 2) of the present invention, the control of the pH value of the reaction system generally makes the pH value of the reaction system naturally fall within the above pH range by controlling the concentrations and dosages of solution A and solution B; or the pH value can be adjusted by additionally adding acids or bases. The acid added can be nitric acid, and the bases added can be ammonia water or potassium hydroxide. Preferably, the pH value of the reaction system is controlled between 0 - 1.

[0020] Preferably, in step 4), the drying temperature is 60 - 100 °C, and the drying time is 8 - 12 h.

[0021] Preferably, in step 4), the calcination is carried out in a muffle furnace, the calcination temperature is 200 - 400 °C, the calcination time is 5 - 10 h, and the heating rate is 2 - 10 °C / min. As a further preference, the calcination temperature is 200 - 300 °C. Most preferably, the calcination temperature is 300 °C, the calcination time is 6 h, and the heating rate is 10 °C / min.

[0022] In the second aspect, the present invention provides an application of the manganese dioxide catalyst obtained by the preparation method according to the first aspect in the reaction of liquid-phase selective oxidation of benzyl alcohol to benzaldehyde, and the reaction of liquid-phase selective oxidation of benzyl alcohol to benzaldehyde uses oxygen as the oxidant.

[0023] The application is specifically as follows: Add benzyl alcohol, catalyst and solvent into a high-pressure reactor, and introduce oxygen to make the initial pressure of oxygen be 1-5 MPa, and stir and react at 60-100 °C to generate benzaldehyde.

[0024] Preferably, the solvent is toluene or acetone.

[0025] Preferably, the reaction time is 1-2 h.

[0026] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0027] (1) The preparation method of the present invention adopts a low-temperature redox precipitation method, and slowly drops potassium permanganate solution into manganese nitrate solution, so that the specific surface area of manganese dioxide is greatly increased, overcoming problems such as low specific surface area, high reaction temperature, and low safety in the preparation of manganese oxides by the hydrothermal method.

[0028] (2) The preparation method of the present invention has a simple process, does not require pH adjustment, has mild preparation conditions, and is easy to operate.

[0029] (3) The catalyst prepared by the present invention has high catalytic activity in the reaction of catalytic oxidation of benzyl alcohol to prepare benzaldehyde, and both the conversion rate of benzyl alcohol and the selectivity of benzaldehyde are very high, overcoming problems such as low activity of manganese oxides and high cost of precious metals at the present stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of a batch reaction device for the selective oxidation of benzyl alcohol to benzaldehyde: 1-oxygen cylinder, 2-pressure reducing valve, 3-gas flowmeter, 4-pressure gauge, 5-exhaust valve & gas sampling valve, 6-fixer, 7-thermocouple for temperature measurement, 8-heating module, 9-stainless steel reactor body, 10-liquid sampling tube, 11-thermal insulation sleeve, 12-exhaust valve & liquid sampling valve, 13-explosion-proof valve, 14-magnetic coupling stirrer. DETAILED DESCRIPTION OF THE INVENTION

[0031] The following further illustrates the present invention through specific examples, but the protection scope of the present invention is not limited thereto.

[0032] Example 1

[0033] Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A under magnetic stirring at 30 °C and add solution B dropwise to solution A (at a rate of about 1 drop per second). Measure that the pH value gradually drops from 4 to 0.8 during this process. After the dropwise addition is completed, keep stirring and aging at 30 °C for 8 h, filter out the precipitate by suction filtration, wash it with deionized water more than 3 times, place it in a drying oven and dry it at 70 °C for 12 h. Finally, heat it in a muffle furnace at a rate of 10 °C / min to 300 °C and hold for 6 h to obtain catalyst C1.

[0034] Example 2

[0035] Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A under magnetic stirring at 30 °C and add solution B dropwise to solution A (at a rate of about 1 drop per second). After the dropwise addition is completed, keep stirring and aging at 30 °C for 8 h, filter out the precipitate by suction filtration, wash it with deionized water more than 3 times, place it in a drying oven and dry it at 70 °C for 12 h. Finally, heat it in a muffle furnace at a rate of 10 °C / min to 200 °C and hold for 6 h to obtain catalyst C2.

[0036] Example 3

[0037] Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A under magnetic stirring at 30 °C and add solution B dropwise to solution A (at a rate of about 1 drop per second). After the dropwise addition is completed, keep stirring and aging at 30 °C for 8 h, filter out the precipitate by suction filtration, wash it with deionized water more than 3 times, place it in a drying oven and dry it at 70 °C for 12 h. Finally, heat it in a muffle furnace at a rate of 10 °C / min to 400 °C and hold for 6 h to obtain catalyst C3.

[0038] Example 4

[0039] Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A under magnetic stirring at 30 °C, add solution B dropwise to solution A (at a rate of about 1 drop per second), and at the same time add concentrated HNO3 solution to adjust the pH value to be stable at 0. After the dropping is completed, keep stirring and aging at 30 °C for 8 h, filter by suction to separate the precipitate, wash it with deionized water more than 3 times, place it in a drying oven and dry at 70 °C for 12 h. Finally, heat it in a muffle furnace at a rate of 10 °C / min to 300 °C and hold for 6 h to obtain catalyst C4.

[0040] Example 5

[0041] Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A under magnetic stirring at 30 °C, add solution B dropwise to solution A (at a rate of about 1 drop per second), and at the same time add KOH solution to adjust the pH value to be stable at 4. After the dropping is completed, keep stirring and aging at 30 °C for 8 h, filter by suction to separate the precipitate, wash it with deionized water more than 3 times, place it in a drying oven and dry at 70 °C for 12 h. Finally, heat it in a muffle furnace at a rate of 10 °C / min to 300 °C and hold for 6 h to obtain catalyst C5.

[0042] Example 6

[0043] Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A under magnetic stirring at 60 °C, add solution B dropwise to solution A (at a rate of about 1 drop per second), without adjusting the pH value. After the dropping is completed, keep stirring and aging at 60 °C for 8 h, filter by suction to separate the precipitate, wash it with deionized water more than 3 times, place it in a drying oven and dry at 70 °C for 12 h. Finally, heat it in a muffle furnace at a rate of 10 °C / min to 300 °C and hold for 6 h to obtain catalyst C6. [[ID=?]]

[0044] Comparative Example 1

[0045] Catalyst D1 was prepared by a hydrothermal method: 0.02 mol of potassium permanganate was dissolved in 60 ml of deionized water to form a potassium permanganate solution; 0.05 mol of ammonium oxalate was dissolved in 40 ml of deionized water to form an ammonium oxalate solution. The potassium permanganate solution was maintained at room temperature with magnetic stirring. The ammonium oxalate solution was then added dropwise to the potassium permanganate solution. After the addition was complete, the solution was transferred to a stainless steel high-pressure hydrothermal autoclave and reacted in a drying oven at 180°C for 24 hours. The precipitate was removed, filtered, washed with deionized water, and dried in a drying oven at 70°C for 12 hours. Finally, the temperature was raised in a muffle furnace to 300°C at a rate of 10°C / min for 6 hours, then to 400°C at a rate of 10°C / min for 1 hour.

[0046] Comparative Example 2

[0047] Catalyst D2 was prepared by a hydrothermal method: 2.0 g of potassium permanganate was dissolved in 50 ml of deionized water, and 1 ml of 16 mol / L concentrated nitric acid was added with thorough stirring. 7.16 g of 50% manganese nitrate solution was then added and mixed thoroughly. The mixture was transferred to a stainless steel high-pressure hydrothermal autoclave and reacted in a drying oven at 120°C for 2 h. The precipitate was separated by filtration, washed with deionized water, and dried in a drying oven at 70°C for 12 h. Finally, the temperature was raised to 300°C in a muffle furnace at a rate of 10°C / min and maintained for 6 h.

[0048] Comparative Example 3

[0049] Catalyst D3 was prepared using the same method as in Example 1, except that the order of addition was reversed: 6.17 g of 50% manganese nitrate solution was dissolved in 200 ml of deionized water, designated Solution A. 2.0 g of potassium permanganate (10% excess) was dissolved in 200 ml of deionized water, designated Solution B. Solution B was then maintained at 30°C with magnetic stirring, and Solution A was added dropwise to Solution B at a rate of approximately one drop per second. After the addition was complete, the mixture was stirred and aged at 30°C for 8 hours. The precipitate was then filtered, washed with deionized water three or more times, dried in a drying oven at 70°C for 12 hours, and finally heated to 300°C in a muffle furnace at a rate of 10°C / min and held for 6 hours.

[0050] Comparative Example 4

[0051] The preparation method of catalyst D4 is the same as that of Example 1, but the pH value is adjusted to 7: Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A stirred magnetically at 30 °C, add solution B dropwise to solution A (at a rate of about 1 drop per second), and at the same time add KOH solution to adjust the pH value to be stable at 7. After the dropping is completed, keep stirring and aging at 30 °C for 8 h, filter by suction to obtain the precipitate, wash it with deionized water more than 3 times, place it in a drying oven and dry at 70 °C for 12 h, and finally heat it in a muffle furnace to 300 °C at a rate of 10 °C / min and hold for 6 h.

[0052] Comparative Example 5

[0053] The preparation method of catalyst D5 is the same as that of Example 1, but the calcination temperature is raised to 500 °C: Weigh 6.17 g of manganese nitrate solution (mass fraction 50%) and dissolve it in 200 ml of deionized water, denoted as solution A. Weigh 2.0 g of potassium permanganate (10% in excess) and dissolve it in 200 ml of deionized water, denoted as solution B. Then keep solution A stirred magnetically at 30 °C, add solution B dropwise to solution A (at a rate of about 1 drop per second). After the dropping is completed, keep stirring and aging at 30 °C for 8 h, filter by suction to obtain the precipitate, wash it with deionized water more than 3 times, place it in a drying oven and dry at 70 °C for 12 h, and finally heat it in a muffle furnace to 500 °C at a rate of 10 °C / min and hold for 6 h.

[0054] Table 1 Comparison of preparation methods and conditions of different catalysts

[0055]

[0056] Note: Forward dropping means adding potassium permanganate solution to manganese nitrate solution, and reverse dropping means adding manganese nitrate solution to potassium permanganate solution.

[0057] The catalysts C1 - C6 in the examples of the present invention and the catalysts D1 - D5 in the comparative examples were used to test the catalytic performance of the catalytic oxidation of benzyl alcohol to benzaldehyde. The reactor was Figure 1 the shown polytetrafluoroethylene high-pressure reactor. The test conditions were: the dosage of the catalyst was 500 mg, the dosage of the raw material benzyl alcohol was 2.0 ml, the solvent was toluene, the solvent dosage was 20 ml, the oxidant was O2, the initial pressure after charging oxygen was 2.0 MPa, and the mechanical stirring speed was 1000 r / min. The test results of reacting for 1 hour at 60 °C are shown in Table 2. With the test conditions unchanged, the catalyst C1 in Example 1 was reacted for 2 hours at different reaction temperatures, and the test results are shown in Table 3.

[0058] Table 2 Results of different catalysts reacting for 1 h at 60 °C

[0059]

[0060]

[0061] Table 3 Test results of catalyst C1 at different reaction temperatures

[0062]

Claims

1. A preparation method of a manganese dioxide catalyst for the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde, characterized in that: The application of the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde is specifically as follows: Add benzyl alcohol, a catalyst, and a solvent into a high-pressure reactor, and introduce oxygen so that the initial pressure of oxygen is 1 - 5 MPa. Stir and react at 60 - 100 °C to produce benzaldehyde. The preparation method of the manganese dioxide catalyst includes the following steps: 1) Dissolve a certain amount of manganese nitrate in deionized water to form solution A, and dissolve a slightly excessive amount of potassium permanganate in deionized water to form solution B; the so-called slightly excessive amount means that the excessive amount of potassium permanganate does not exceed 20% of the theoretically added amount of potassium permanganate. 2) Conduct a low-temperature redox reaction: Keep solution A stirred at 30 - 60 °C, slowly dropwise add solution B. After the dropping is complete, control the pH value of the reaction system between 0 - 4, and continue to stir and age at 30 - 60 °C for 3 - 10 h. 3) Filter and separate the reaction mixture obtained in step 2), and wash the precipitate. 4) Dry the precipitate obtained in step 3) and then conduct a calcination treatment at 200 - 400 °C to obtain the manganese dioxide catalyst.

2. The preparation method according to claim 1, characterized in that: In step 2), the dropping rate of solution B is 1 - 2 drops per second.

3. The preparation method according to claim 1, characterized in that: In step 2), the temperature of the redox reaction is 30 °C. After the dropping is complete, keep stirring and aging at 30 °C for 8 h.

4. The preparation method according to claim 1, characterized in that: In step 2), control the pH value of the reaction system between 0 - 1.

5. The preparation method according to claim 1, characterized in that: In step 4), the drying temperature is 60 - 100 °C, and the drying time is 8 - 12 h.

6. The preparation method according to claim 1, wherein: In step 4), the calcination is carried out in a muffle furnace. The calcination temperature is 200 - 400 °C, the calcination time is 5 - 10 h, and the heating rate is 2 - 10 °C / min.

7. The preparation method according to claim 6, characterized in that: In step 4), the calcination temperature is 200 - 300 °C.

8. The preparation method according to claim 7, characterized in that: In step 4), the calcination temperature is 300 °C, the calcination time is 6 h, and the heating rate is 10 °C / min.

9. The application of a manganese dioxide catalyst obtained by the preparation method according to any one of claims 1 - 8 in the reaction of the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde. In the reaction of the liquid-phase selective oxidation of benzyl alcohol to benzaldehyde, oxygen is used as the oxidant. The application is specifically as follows: Add benzyl alcohol, a catalyst, and a solvent into a high-pressure reactor, and introduce oxygen so that the initial pressure of oxygen is 1 - 5 MPa. Stir and react at 60 - 100 °C to produce benzaldehyde.

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