Preparation method and application of amorphous zirconium-doped manganese oxide catalyst

Abstract

The invention provides a preparation method and application of an amorphous zirconium-doped manganese oxide catalyst, and belongs to the field of organic catalytic synthesis. The catalyst is Zr-MnO2-x, and is rich in lattice oxygen with high activity and high stability; x is the molar ratio of Zr to the sum of Mn and Zr, and x is larger than or equal to 0.2 and smaller than or equal to 0.6; zirconium mainly exists in the form of zirconium hydroxide. The preparation method comprises the following steps: slowly dropwise adding a mixed salt solution of a zirconium source and a manganese source into a precipitant under a continuous stirring condition, and carrying out a co-precipitation reaction; after the reaction is completed, standing and aging, carrying out solid-liquid separation and collecting a precipitate; and washing and drying to obtain the amorphous zirconium-doped manganese oxide catalyst which is used for catalyzing aniline and derivatives thereof to synthesize azobenzene compounds. According to the invention, the characteristic of'difunctional lattice oxygen 'with high activity and high stability is introduced through zirconium doping, and the catalyst has the advantages of high activity, high selectivity, wide substrate applicability, excellent stability and reusability and the like when used for catalytic synthesis of azobenzene compounds.

Description

Technical Field

[0001] This invention belongs to the field of organic catalytic synthesis technology, and specifically relates to a method for preparing an amorphous zirconium-doped manganese oxide catalyst and its application. Background Technology

[0002] Azobenzene compounds (containing –N=N– bonds) possess unique chemical structures and photoresponsive properties, making them key intermediates in dyes, pigments, pharmaceuticals, and functional materials. Traditional synthetic methods primarily include reductive coupling via nitro compounds and diazo coupling. Diazo coupling uses highly toxic nitrosating reagents such as sodium nitrite to react with aromatic primary amines to generate diazonium salts, which then undergo electrophilic coupling reactions with electron-rich aromatic compounds (such as phenols and amines). Existing traditional methods suffer from drawbacks such as cumbersome procedures, poor atom economy, the use of highly toxic reagents, and the generation of large amounts of waste, which are inconsistent with the development direction of green chemistry.

[0003] To circumvent the aforementioned problems, the route of directly catalytically oxidizing aniline derivatives using air or oxygen as an oxygen source has become a research hotspot. Common catalytic systems mainly include: 1. Noble metal catalysts, such as supported gold catalysts (Au / TiO2), platinum / palladium nanowires, etc., which have high activity but are expensive and often require the addition of auxiliary agents, which limits their industrial application (see Grirrane et al., Science, 2008, 322, 1661); 2. Non-noble metal catalysts, such as manganese-based oxide catalysts, which show great potential due to their low price, environmental friendliness and rich redox chemical properties (see Biswanath Dutta et al., Angew. Chem. Int. Ed., 2016, 55, 2171–2175); Zirconium-based catalysts (Zr(OH)4) can be used as highly efficient heterogeneous catalysts, which can use O2 as an oxidant to achieve the selective oxidation of aniline to synthesize azobenzene compounds (see Long Yu et al., Angew. Chem. Int. Ed., 61 (2022) e202112907).

[0004] However, existing non-precious metal manganese-based catalysts still have significant drawbacks: First, their catalytic performance is limited by the contradiction between the activity and stability of lattice oxygen—high activity leads to easy deactivation of the structure, while a stable structure results in insufficient activity; second, they exhibit low catalytic activity for specific substrates such as nitrogen-containing heterocycles, and their substrate universality is insufficient.

[0005] Therefore, there is an urgent need to develop catalysts that combine the cost advantages of non-precious metals, high catalytic activity under additive-free conditions, excellent stability, and broad substrate universality, so as to achieve efficient and green synthesis of azobenzene compounds under mild conditions. Summary of the Invention

[0006] Therefore, the present invention aims to provide a method for preparing an amorphous zirconium-doped manganese oxide catalyst and its application, in order to solve at least one technical problem in the background art.

[0007] This invention is implemented as follows: The first aspect of the present invention provides an amorphous zirconium-doped manganese oxide catalyst, wherein the catalyst has the molecular formula Zr-MnO2-x; wherein x represents the molar ratio of Zr to the sum of Mn and Zr, and 0.2≤x≤0.6; wherein Zr mainly exists in the form of zirconium hydroxide Zr(OH)4; and the catalyst is rich in highly active lattice oxygen and highly stable lattice oxygen.

[0008] Furthermore, x = 0.4.

[0009] A second aspect of this invention provides a method for preparing an amorphous zirconium-doped manganese oxide catalyst, the method comprising the following steps: Prepare a mixed salt solution of zirconium and manganese sources; The mixed salt solution was slowly added dropwise to the precipitant under continuous stirring, and stirring was continued for 2 to 6 hours to carry out the co-precipitation reaction. After the coprecipitation reaction is completed, allow the mixture to stand for 10-14 hours to age, then separate the solid and liquid phases and collect the precipitate. The precipitate was washed and dried to obtain an amorphous zirconium-doped manganese oxide catalyst Zr-MnO2-x; where x represents the molar ratio of Zr to the sum of Mn and Zr, and 0.2≤x≤0.6.

[0010] Furthermore, x = 0.4.

[0011] Furthermore, the zirconium source is selected from at least one of zirconium oxychloride, zirconium oxynitrate, and zirconium sulfate; the manganese source is selected from at least one of potassium permanganate, potassium manganate, manganese nitrate, manganese sulfate, manganese chloride, and manganese acetate.

[0012] Furthermore, the precipitant is an alkaline solution.

[0013] Furthermore, the alkaline solution is selected from at least one of ammonia, NaOH, KOH, and Na2CO3; The amount of precipitant used is 2.5 to 5.0 times the total mass of the zirconium source and manganese source.

[0014] A third aspect of the present invention provides the application of the above-mentioned amorphous zirconium-doped manganese oxide catalyst, characterized in that the catalyst is used to catalyze the reaction of aniline and its derivatives with an oxidant to synthesize azobenzene compounds.

[0015] Furthermore, the reaction steps for synthesizing azobenzene compounds include: using aniline and its derivatives as substrates, air or oxygen as oxidant, in a catalyst and aprotic solvent, setting the oxygen pressure to 0.6 MPa to 1 MPa, and heating to 90°C to 130°C for 1 h to 24 h.

[0016] Furthermore, the amount of the catalyst used is 5% to 150% of the substrate mass.

[0017] Furthermore, the aprotic solvent is selected from at least one of toluene, xylene, trimethylbenzene, chlorobenzene, N,N-dimethylformamide, and 1,4-dioxane.

[0018] Furthermore, the amount of the aprotic solvent used is 0.05 mL / mg to 0.2 mL / mg of substrate.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. The zirconium-doped manganese oxide prepared by the method of the present invention has an amorphous structure and a "bifunctional lattice oxygen" characteristic of highly active lattice oxygen + highly stable lattice oxygen introduced by zirconium doping; it has the advantages of high activity and high selectivity, wide substrate applicability, excellent stability and reusability when catalyzing the synthesis of azobenzene compounds.

[0020] 2. The Zr-MnO2-x catalyst prepared in this invention exhibits a conversion rate and selectivity exceeding 99% when catalyzing homogeneous coupling of anilines. It demonstrates excellent catalytic performance for various substituted anilines (such as p-chloroaniline, p-toluidine, and p-methoxyaniline), with high yields of azobenzene products.

[0021] 3. The catalyst prepared by the invention does not contain any precious metals (Au, Pt, Pd, etc.), and no alkali or other additives need to be added during the catalytic oxidative coupling reaction, which is low in cost and environmentally friendly.

[0022] 4. The catalytic oxidative coupling reaction of the present invention is under mild conditions, using air as the oxygen source, and can react efficiently at normal pressure and moderate temperature. The operation is safe and simple.

[0023] 5. The Zr-MnO2-x catalyst prepared by this invention benefits from the structural stabilization effect introduced by Zr doping, and the amorphous structure of the catalyst can remain stable during the reaction. Experiments show that the catalyst can be reused at least 5 times without significant decrease in activity after simple recovery, washing, and drying.

[0024] 6. The Zr-MnO2-x catalyst prepared in this invention exhibits good tolerance to halogen-containing substrates (such as 4-chloroaniline), high yield of coupling products, and no obvious dehalogenation side reactions were observed, solving a common problem in such reactions and demonstrating broad substrate applicability. Through Zr doping, a highly active O2 catalyst was cleverly achieved. L "and "high stability O" L The separation and synergy of these elements simultaneously meet the requirements of high reactivity and high structural stability, providing a new strategy for resolving the inherent contradictions in transition metal oxide catalysis. Attached Figure Description

[0025] Figure 1 The XRD pattern of the amorphous zirconium-doped manganese oxide catalyst Cat-Zr0.4 prepared in Example 1 of this invention; Figure 2 The image shows the EDS spectrum of the amorphous zirconium-doped manganese oxide catalyst Cat-Zr0.4 prepared in Example 1 of this invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] An amorphous zirconium-doped manganese oxide catalyst, the catalyst having the molecular formula Zr-MnO2-x; where x represents the molar ratio of Zr to the sum of Mn and Zr, i.e., x=n zr / (n Zr +n Mn The catalyst is rich in highly active lattice oxygen and highly stable lattice oxygen, and 0.2≤x≤0.6 (preferably x=0.4), wherein Zr mainly exists in the form of zirconium hydroxide Zr(OH)4; the catalyst is rich in highly active lattice oxygen and highly stable lattice oxygen.

[0028] A method for preparing an amorphous zirconium-doped manganese oxide catalyst, the method comprising the following steps: S1, Solution preparation: Weigh out zirconium source and manganese source according to the Zr / (Mn+Zr) molar ratio of x (0.2≤x≤0.6), dissolve them together in deionized water, and stir to form a homogeneous mixed salt solution; The zirconium source is selected from at least one of zirconium oxychloride, zirconium oxynitrate, and zirconium sulfate; the following examples use ZrOCl2·8H2O as the zirconium source, but are not limited to the listed zirconium sources, and other unlisted zirconium sources within the above range are also applicable.

[0029] The manganese source is selected from at least one of potassium permanganate, manganese nitrate, and manganese acetate. The following examples use KMnO4 as the manganese source, but are not limited to the listed manganese sources. Other unlisted manganese sources within the above range are also applicable.

[0030] S2, Coprecipitation reaction: The mixed salt solution is slowly added dropwise to the precipitant under continuous stirring, and the coprecipitation reaction is carried out by stirring for 2 to 6 hours. The precipitant is an alkaline solution; the alkaline solution is selected from at least one of ammonia, NaOH, KOH, and Na2CO3, and the amount of precipitant is 2.5 to 5.0 times the total mass of the zirconium source and manganese source; the following examples use 28 wt% ammonia as the precipitant, but are not limited to the listed precipitants, and other unlisted precipitants within the above range are also applicable.

[0031] The stirring time for the coprecipitation reaction is 2h to 6h, for example, 2h, 4h, or 6h; the following example preferably uses 4h, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0032] S3, Aging and Separation: After the co-precipitation reaction is completed, allow the mixture to stand for aging for 10-14 hours, then separate the solid and liquid phases and collect the precipitate. The aging time is 10h to 14h, for example, it can be 10h, 12h, or 14h; the following embodiment preferably uses 12h, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] S4, Post-treatment: The precipitate is washed and dried to obtain the amorphous zirconium-doped manganese oxide catalyst Zr-MnO2-x; where x represents the molar ratio of Zr to the sum of Mn and Zr, and 0.2≤x≤0.6; During washing, the precipitate is repeatedly washed with deionized water until the filtrate is neutral to remove impurity ions. The drying step specifically involves freeze-drying or low-temperature drying (e.g., 60℃~80℃) of the washed product.

[0034] The method of this invention eliminates the need for a high-temperature calcination step, directly yielding a catalyst with high specific surface area and an amorphous structure. The simplified formula for amorphous zirconium-doped manganese oxide is Zr-MnO2-x, where x represents the molar ratio of Zr to the sum of Mn and Zr, and 0.2≤x≤0.6, for example 0.2, 0.4, 0.6, etc., with x=0.4 being the most preferred.

[0035] The catalyst is characterized by its amorphous structure and the "bifunctional lattice oxygen" property introduced by zirconium doping. (1) Highly active lattice oxygen: exists around the Zr-O-Mn interface, has a relatively low binding energy, is easy to participate in oxidation reactions, and is the main active site for aniline oxidative coupling. (2) Highly stable lattice oxygen: mainly composed of Zr 4+It is stable, has a high binding energy, and is difficult to reduce. Its main function is to maintain the amorphous framework structure of the catalyst, prevent it from undergoing phase transition in the reaction or reducing atmosphere, and ensure the stability of the catalyst.

[0036] The aforementioned amorphous zirconium-doped manganese oxide catalyst can be used to catalyze the reaction of aniline and its derivatives with oxidants to synthesize azobenzene compounds.

[0037] The reaction formula for synthesizing azobenzene compounds is as follows: ; In the formula, R is hydrogen, an electron-donating group (such as –CH3, –OCH3), or an electron-withdrawing group (such as –Cl).

[0038] The reaction steps for synthesizing azobenzene compounds include: using aniline and its derivatives as shown in Formula I as substrates, air or oxygen as oxidant, adding an amorphous zirconium-doped manganese oxide catalyst Zr-MnO2-x as a catalyst, and heating the reaction in an aprotic solvent to synthesize azobenzene compounds as shown in Formula II.

[0039] The amount of catalyst used is 5% to 150% of the substrate mass; the following examples preferably use about 107%, but are not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0040] The aprotic solvent is selected from at least one of toluene, xylene, chlorobenzene, and 1,4-dioxane; toluene is preferred as the aprotic solvent in the following examples, but it is not limited to the listed aprotic solvents, and other aprotic solvents not listed in the above range are also applicable. The amount of aprotic solvent used is 0.05 mL / mg to 0.2 mL / mg of substrate, and about 0.11 mL / mg of substrate is preferred in the following examples, but it is not limited to the listed values, and other values ​​not listed in the value range are also applicable.

[0041] The reaction temperature is 90℃~130℃, for example, it can be 90℃, 100℃, 110℃, or 130℃; the following examples preferably use 80℃ or 110℃, but are not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] The oxygen pressure is 0.6MPa to 1MPa, for example, it can be 0.6MPa, 0.8MPa, or 1MPa; but it is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0043] The reaction time is 1h to 24h; for example, it can be 1h, 2h, 6h, 8h, 12h, 16h, or 24h; preferably 2h to 6h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0044] Example 1 catalyst Zr 0.4 MnO x Preparation: Weigh 2.062 g ZrOCl2·8H2O (6.399 mmol) and 1.516 g KMnO4 (9.593 mmol) (n Zr / n (Mn+Zr) = 0.4), dissolved in 20 mL of deionized water, and stirred for 30 minutes to form a homogeneous solution. Under magnetic stirring, 40 mL of 28 wt% ammonia solution was rapidly added to the solution, and stirring was continued for 4 h, followed by aging by standing for 12 h. The brownish-black precipitate was collected by suction filtration and washed several times with deionized water until the filtrate was free of chloride ions when tested with AgNO3. The solid product was dried for 12 h to obtain an amorphous Zr-MnO2-0.4 catalyst, denoted as Cat-Zr0.4.

[0045] Adjust the amount of KMnO4 to make n Zr / n (Mn+Zr) = 0.2, 0.4, 0.6, 0.8, 1, with other conditions unchanged, amorphous Zr-MnO2-x catalysts were obtained, denoted as Cat-Zr0.2, Cat-Zr0.4, Cat-Zr0.6, Cat-Zr0.8, and Cat-Zr1.

[0046] As a control, without adding a zirconium source, 1.516 g of KMnO4 (9.593 mmol) was treated in the same way as described above to obtain amorphous MnO. x Catalyst, denoted as Cat-Mn.

[0047] The XRD and EDS spectra of the amorphous zirconium-doped manganese oxide catalyst Cat-Zr0.4 prepared in Example 1 are shown below. Figure 1 and Figure 2 As shown. Figure 1 The XRD patterns show that Cat-Zr0.4 retains amorphous features. Figure 2 The EDS spectrum shows that Cat-Zr0.4 is a zirconium-doped manganese oxide.

[0048] Example 2 Catalytic oxidation of aniline to azobenzene: In a 50 mL high-pressure reactor, aniline (186.26 mg, 2.0 mmol), catalyst (200 mg), and toluene (20 mL) were added sequentially. The high-pressure reactor was heated to 110 °C and reacted for 2 hours under an oxygen pressure of 0.6 MPa. After the reaction was completed, the mixture was cooled to room temperature, and the catalyst was separated by centrifugation.

[0049] Catalyst selection: Amorphous Zr-MnO2-x catalyst prepared in Example 1 and amorphous MnO xCatalysts (i.e., Cat-Zr0.2, Cat-Zr0.4, Cat-Zr0.6, Cat-Zr0.8, Cat-Zr1 and the control sample Cat-Mn), as well as commercial C-Mn2O3 (Aladdin, 99%), were used. The reaction conditions were adjusted, and after the reaction was completed, the reaction solution was analyzed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) to test the aniline conversion rate and the selectivity of azobenzene. The results are shown in Table 1 below.

[0050] Table 1

[0051] Note: Cat-Zr0.4-550 refers to the product obtained by calcining Cat-Zr0.4 in a muffle furnace at 550℃ for 5 hours.

[0052] As shown in Table 1, with a reaction time of only 1 hour, the selectivity of azobenzene obtained by the Zr-MnO2-x catalyst prepared in this invention is >72%, and the x value is preferably 0.2~0.6. Comparison of data from groups 1 to 8 shows that 0.4 is the most preferred value. Among them, when Cat-Zr0.4 is used as a catalyst, the aniline conversion rate is higher than 95% and the selectivity of azobenzene is higher than 99% in just 1 hour, which is superior to amorphous MnO with other doping ratios. x Catalyst, undoped amorphous MnO x Catalysts and commercially available nonporous Mn2O3 were used. GC analysis showed that when commercially available nonporous Mn2O3 was used as a catalyst, the aniline conversion rate was only ~21.3% within 1 hour, indicating that the amorphous structure and zirconium doping are crucial for high activity. In group 8, the catalytic activity of Cat-Zr0.4 decreased significantly after high-temperature calcination. Comparisons of data from groups 1 to 3 and groups 9 to 11 showed that the aniline conversion rate of Cat-Zr0.2, Cat-Zr0.4, and Cat-Zr0.6 increased with increasing reaction time; meanwhile, comparisons of data from groups 10 and 12 to 15 showed that decreasing oxygen pressure and temperature reduced the aniline conversion rate. In group 16, nitrogen was replaced with oxygen in the reaction process. Comparison with group 10 showed that the catalytic effect of group 16 decreased significantly, indicating that the oxidation was not due to the lattice oxygen of the catalyst itself, but rather that the catalyst catalyzed the oxidation of oxygen, which in turn oxidized the aniline.

[0053] The recovered catalyst Cat-Zr0.4 was washed with toluene and acetonitrile and dried at 110 °C for 12 h for use in the next cycle experiment. The above reaction was repeated, and the catalyst was used three times consecutively. The aniline conversion and azobenzene selectivity were tested, and the results are shown in Table 2 below.

[0054] Table 2

[0055] As shown in Table 2, the Cat-Zr0.4 catalyst prepared in this invention was recycled and reused three times. After 2 hours of reaction, the aniline conversion rate remained above 31%, and the azobenzene selectivity was >93%. When the reaction time was further increased, the aniline conversion rate was >97%, and the azobenzene selectivity was >98%.

[0056] Example 3 Catalytic oxidation of aniline derivatives to synthesize azobenzene derivatives: Using the same reaction steps as in Example 2, only the reaction time was adjusted. The oxidative coupling of a series of aniline derivatives was tested using Cat-Zr0.4 as a catalyst. The results are shown in Table 3.

[0057] Table 3

[0058] As shown in Table 3, the Cat-Zr0.4 catalyst prepared in this invention exhibits excellent catalytic performance for the oxidative coupling reactions of various aniline derivatives (such as p-chloroaniline, p-toluidine, and p-methoxyaniline), with high yields of azobenzene products. Its catalytic performance is far superior to that of undoped zirconium amorphous MnO. x catalyst.

[0059] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. An amorphous zirconium-doped manganese oxide catalyst, characterized in that, The catalyst has the molecular formula Zr-MnO2-x; where x represents the molar ratio of Zr to the sum of Mn and Zr, and 0.2≤x≤0.6; where Zr mainly exists in the form of zirconium hydroxide Zr(OH)4; the catalyst is rich in highly active lattice oxygen and highly stable lattice oxygen.

2. The amorphous zirconium-doped manganese oxide catalyst according to claim 1, characterized in that, x=0.4。 3. A method for preparing an amorphous zirconium-doped manganese oxide catalyst according to claim 1 or 2, characterized in that, The preparation method includes the following steps: Prepare a mixed salt solution of zirconium and manganese sources; The mixed salt solution was slowly added dropwise to the precipitant under continuous stirring, and stirring was continued for 2 to 6 hours to carry out the co-precipitation reaction. After the coprecipitation reaction is completed, allow the mixture to stand for 10-14 hours to age, then separate the solid and liquid phases and collect the precipitate. The precipitate was washed and dried to obtain an amorphous zirconium-doped manganese oxide catalyst Zr-MnO2-x; where x represents the molar ratio of Zr to the sum of Mn and Zr, and 0.2≤x≤0.

6.

4. The method for preparing an amorphous zirconium-doped manganese oxide catalyst according to claim 3, characterized in that, The zirconium source is selected from at least one of zirconium oxychloride, zirconium oxynitrate, and zirconium sulfate; the manganese source is selected from at least one of potassium permanganate, potassium manganate, manganese nitrate, manganese sulfate, and manganese chloride.

5. The method for preparing an amorphous zirconium-doped manganese oxide catalyst according to claim 3, characterized in that, The precipitant is an alkaline solution; the alkaline solution is selected from at least one of ammonia, NaOH, KOH, and Na2CO3, and the amount of precipitant used is 2.5 to 5.0 times the total mass of the zirconium source and the manganese source.

6. The application of the amorphous zirconium-doped manganese oxide catalyst according to claim 1 or 2, characterized in that, The catalyst is used to catalyze the reaction of aniline and its derivatives with an oxidant to synthesize azobenzene compounds.

7. The application according to claim 6, characterized in that, The reaction steps for synthesizing azobenzene compounds include: using aniline and its derivatives as substrates, air or oxygen as oxidant, and in the presence of a catalyst and aprotic solvent, setting the oxygen pressure to 0.6 MPa to 1 MPa, and heating to 90°C to 130°C for 1 to 24 hours.

8. The application according to claim 7, characterized in that, The amount of catalyst used is 5% to 150% of the substrate mass.

9. The application according to claim 7, characterized in that, The aprotic solvent is selected from at least one of toluene, xylene, trimethylbenzene, chlorobenzene, N,N-dimethylformamide, and 1,4-dioxane.

10. The application according to claim 9, characterized in that, The amount of the aprotic solvent used is 0.05 mL / mg to 0.2 mL / mg of substrate.

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