Catalyst, preparation method thereof and application of catalyst in synthesis of imine and N-heterocyclic compound

Abstract

The invention belongs to the technical field of catalysts, and particularly relates to a catalyst, a preparation method thereof and application of the catalyst in synthesis of imine and N-heterocyclic compounds. The preparation method of the catalyst comprises the following steps: firstly, dissolving urea, Mg (NO3) 2, chitosan and glacial acetic acid in a solvent, uniformly mixing to obtain sol-gel containing Mg < 2 + >, then drying the obtained sol-gel, and pyrolyzing in an N2 atmosphere, so that Mg < 2 + > is dispersed on a nitrogen-doped carbon material in the form of MgO nanoclusters rich in defects. The preparation method of the catalyst is simple, the small-size metal nanoparticles with catalytic activity in the catalyst still have good stability after being recycled for 6 times, and the catalyst has high catalytic activity for synthesizing imine and N-heterocyclic compounds (constructing C = N) through reductive coupling of nitro compounds and alcohol compounds.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a catalyst and a preparation method thereof, and an application thereof in the synthesis of imines and N-heterocyclic compounds. Background Art

[0002] The selective construction of unsaturated C=N bonds is very important in both chemistry and biology because organic nitrogen compounds such as imines and N-heterocycles with C=N bonds are widely present in many fields such as fine chemicals, pharmaceuticals, and molecular motors. The reductive coupling of nitro compounds with alcohols is a sustainable route for constructing C=N bonds in imines and N-heterocycles, but this route is challenging due to the inertness of the α-C sp3 -H bond in alcohols and the vulnerability of C=N bonds to hydrogenation.

[0003] The borrowing hydrogen reduction reaction refers to the use of a metal catalyst to dehydrogenate a relatively inert organic molecule to form a metal hydride, while activating the organic molecule and participating in subsequent reactions, and then the metal hydride reduces the intermediate generated during the reaction process to generate a new coupling product. In recent years, new synthetic amine methods developed based on the borrowing hydrogen reduction strategy have attracted extensive attention from chemists. In this strategy, hydrogen sources such as formic acid, isopropyl alcohol, borane ammonia, or benzyl alcohol that can replace hydrogen molecules are used to achieve the reduction reaction to generate amine compounds. The by-product of the reaction is only water, so it is a very green method with high atom economy. The reaction does not require the use of high-pressure equipment and is simple and safe to operate.

[0004] There are many reports on the preparation of new amine compounds by directly using amines as substrates for borrowing hydrogen reduction reactions, but previous reports often used precious metals such as Pt, Pd, Ru, Ir, Au, etc., and the substrate amines used were often obtained by nitro reduction. Using nitroarenes as nitrogen sources for one-pot tandem synthesis of amines is relatively more challenging. There are currently few reports, and they also use precious metals as catalysts, and the selectivity of the reaction is not very ideal. Using cheap and abundant non-precious metals to replace rare precious metals as catalysts to achieve efficient conversion of important energy and chemical processes is a hot topic in current catalytic science and chemical engineering research. Precious metals are expensive and have limited reserves on Earth, which severely restricts their wide application in actual production, especially large-scale industrial production. Considering the market price and raw material sources, developing a cheaper and more readily available non-precious metal catalytic system to construct C=N bonds from nitro compounds and alcohols is more economically valuable and practically significant. Summary of the Invention

[0005] Aiming at the deficiencies in the prior art, the purpose of the present invention is to provide a catalyst (MgO / NC-T) and a preparation method thereof, and an application thereof in the synthesis of imines and N-heterocyclic compounds.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a catalyst MgO / NC-T, which is a nitrogen-doped carbon catalyst loaded with defective MgO nanoclusters, with a nitrogen-doped carbon material as the framework and active component MgO nanoclusters loaded on the framework.

[0008] The present invention also provides a preparation method of the above catalyst, and the preparation method successively includes the following steps:

[0009] (1) Dissolve urea, Mg(NO 3 ) 2 ·6H 2 O, chitosan and glacial acetic acid in solvent A; stir evenly to obtain a sol-gel precursor containing Mg 2+ ;

[0010] (2) After drying the sol-gel precursor containing Mg 2+ , heat it from room temperature (25 °C, the same below) to the target temperature of 400 - 600 °C at a heating rate of (1 - 4) °C / min (preferably 2 °C / min) in a nitrogen atmosphere, and then keep it at the target temperature for 1 - 4 h (preferably 2 h), and cool it to room temperature to obtain the MgO / NC-T catalyst, and the mass percentage content of Mg element in the catalyst is 4.6 - 7%.

[0011] Further, the solvent A in step (1) is distilled water.

[0012] Further, the molar ratio of urea, Mg(NO 3 ) 2 ·6H 2 O and glacial acetic acid in step (1) is (100 - 400):(1 - 4):(90 - 360), preferably 200:2:175; the mass ratio of urea and chitosan is (6 - 24):(0.5 - 2), preferably 12:1.

[0013] Further, the drying condition in step (2) is oven drying at 30 - 80 °C for 8 - 12 h, preferably drying at 70 °C for 10 h.

[0014] A brief flow chart of the above preparation method of the catalyst is as shown in the attached Figure 5 of the specification.

[0015] The present invention also provides an application of the above catalyst in the synthesis of imines or N-heterocyclic compounds.

[0016] Further, the steps of the application are as follows: adding the catalyst described in claim 1 or the catalyst obtained by the preparation method described in any one of claims 2-5, an alcohol compound, and a nitro compound in a ratio of (15-60) mg: (90-360) mmol: (0.25-1) mmol (preferably 30 mg: 171 mmol: 0.5 mmol) into a reaction vessel, adding solvent B, sealing and filling with an inert gas, and reacting under heating conditions to obtain the product imine or N-heterocyclic compound.

[0017] Further, in the application, the structural formula of the alcohol compound is one of the following:

[0018]

[0019] and / or the structural formula of the nitro compound is one of the following:

[0020]

[0021] Further, in the application, the pressure of the inert gas filled is (0.5-2) MPa, preferably 1 MPa; the reaction conditions are reaction at (170-220 °C) for (6-36) h, preferably reaction at 200 °C for 6 h.

[0022] Further, the solvent B is one or more of the alcohol compound, tetrahydrofuran, toluene, water, acetonitrile, and n-hexane, preferably n-hexane. Specifically, when the substrate is an aliphatic alcohol compound, the aliphatic alcohol compound serves as both a solvent and a reaction substrate; when the substrate is an aromatic alcohol compound, one or more of tetrahydrofuran, toluene, acetonitrile, and n-hexane are selected as solvents.

[0023] Further, in the application, the solvent B contains trace amounts of water, and the water content ratio to the total volume of the solvent B is 1: (200-50).

[0024] Compared with the prior art, the advantages and beneficial effects of the catalyst and its application of the present invention are as follows:

[0025] The catalyst of the present invention uses a nitrogen-doped carbon material as a skeleton, and uniformly loaded MgO nanoclusters with reducibility and rich defects on the skeleton as active components. The local environment of oxygen vacancies (represented by O v ), and magnesium (Mg-O) significantly reduces the energy barrier for the transfer of hydrogen atoms from α-Csp 3 -H in the alcohol compound to the nitro group in the nitro compound during the adsorption and activation of nitroso compounds. Taking ethanol as an example of the alcohol compound, when ethanol dissociates into CH 3 CH 2 O on the surface of MgO nanoclusters and Ov-MgO nanoclusters- and H + After that, nitrobenzene can also be adsorbed on the surfaces of MgO nanoclusters and Ov-MgO nanoclusters, and the adsorption energies are 29.5 and 35.0 kcal / mol, respectively (see Figures 3 and 3' in the attached drawings of the specification). After that, CH Figure 3 a). Then, the H atom in the α-C 3 CH 2 O - -H of CH sp3 CH Figure 3 is transferred to the O atom of the adsorbed nitrobenzene, and the energy barrier for this step obtained by DFT calculation is 59.0 kcal / mol (see Figure b in the attached drawings of the specification). At the same time, the calculation shows that with the help of H 2 O molecules, the energy barrier for the H atom transfer step of the α-C 3 CH 2 O - -H in CH sp3 CH Figure 3 is greatly reduced to 29.4 kcal / mol (see TS3-4 in Figure a of the attached drawings of the specification). Therefore, this catalyst has high activity and can efficiently catalyze the reductive coupling reaction of alcohol compounds and nitro compounds with the help of trace water.

[0026] The catalyst prepared by the present invention is a non-precious metal catalyst with the characteristics of low cost, simple preparation process, high activity under alkali-free conditions, and wide substrate range.

[0027] This catalyst is applicable to systems using alcohol compounds, tetrahydrofuran, toluene, acetonitrile or n-hexane as solvents, and can achieve high conversion rates in all cases.

[0028] The catalyst of the present invention has a simple preparation process, good stability, and can be recycled more than 6 times. Description of the Drawings

[0029] Figure 1 In a and b of the attached drawings, the transmission electron microscope images of the MgO / NC-400 and MgO / NC-500 catalysts prepared in Example 1 and Example 2 are shown respectively;

[0030] Figure 2 are the TEM image and element distribution diagram of MgO / NC-500 prepared in Example 2: (a) is the TEM image of the MgO / NC-500 catalyst, (b) is the HR-TEM image, (c) is the AC-HAADF-STEM image (insert: particle size distribution of MgO nanoclusters), (d) is the high-angle annular dark field-scanning transmission electron microscope (HAADF-STEM), and (e-h) are the energy dispersive spectroscopy element diagrams (EDS). The bright spots in (c) are the MgO nanoclusters;

[0031] Figure 3(a) DFT calculation results for the transfer hydrogenation of nitrobenzene to nitrosobenzene in Example 5, (b) Process diagram of the direct transfer of the H atom in the α-C 3 CH 2 O - of CH sp3 -H to the O atom of nitrobenzene;

[0032] Figure 4 Results of nitrobenzene conversion and product selectivity when the catalyst was used 1 to 6 times in the cyclic experiment in Example 7;

[0033] Figure 5 Brief flow chart of the preparation method of the catalyst MgO / NC-T of the present invention;

[0034] Figure 6 Possible reaction mechanism for the reductive coupling of nitro compounds and alcohol compounds by the catalyst MgO / NC-T of the present invention. Detailed implementation manners

[0035] The following lists some specific examples to further illustrate the present invention, but do not limit the scope of protection claimed by the present invention.

[0036] Example 1:

[0037] A preparation method of a magnesium-doped carbon material supported Mg (MgO / NC-400) catalyst, comprising the following steps:

[0038] Dissolve urea (12.0 g) and Mg(NO 3 ) 2 ·6H 2 O (0.6 g) with a molar ratio of 200:2 in distilled water (10 mL) to form a solution, and then slowly add chitosan (1.0 g, deacetylation degree ≥ 95%, viscosity 100 - 200 mPa·s, the same below) to the above solution under vigorous stirring (450 rpm, the same below) at a slow rate (1 g / 5 min, the same below). Quickly add glacial acetic acid (1 mL) and continue vigorous stirring for 30 minutes to obtain a homogeneous translucent sol-gel containing Mg 2+ . Dry the obtained sol-gel overnight at 70 °C (10 h, the same below) to evaporate the water, and place it on an alumina boat in a quartz tube furnace. Under a nitrogen atmosphere, heat it from room temperature (25 °C, the same below) to the required temperature (400 °C) at a heating rate of 2 °C·min -1 , and then hold it at 400 °C for 2 hours. After cooling to room temperature with the kettle, obtain a black powdery MgO / NC-400 catalyst, and grind it for further use and characterization. The active component in the catalyst is MgO, and the mass percentage content of Mg element in the catalyst is 4.6%.

[0039] Example 2:

[0040] A preparation method of a magnesium-doped carbon material supported Mg (MgO / NC-500) catalyst, comprising the following steps:

[0041] Dissolve urea (12.0 g) and Mg(NO 3 ) 2 ·6H 2 O (0.6 g) in distilled water (10 mL) to form a solution, and then slowly add chitosan (1.0 g) to the above solution under vigorous stirring. Quickly add glacial acetic acid (1 mL) and continue vigorous stirring for 30 minutes to obtain a homogeneous translucent sol-gel. Dry the obtained sol-gel overnight at 70 °C to evaporate the water, and place it on an alumina boat in a quartz tube furnace. Under a nitrogen atmosphere, heat it from room temperature to the desired temperature (500 °C) at a heating rate of 2 °C·min -1 . Then hold at 500 °C for 2 hours. After cooling to room temperature with the kettle, obtain the MgO / NC-500 catalyst in the form of a black powder, and grind it for further use and characterization. The active component in the catalyst is MgO, and the mass percentage content of Mg element in the catalyst is 6.0%.

[0042] Figure 1 In a and b are the transmission electron micrographs of the MgO / NC-400 and MgO / NC-500 catalysts prepared in Example 1 and Example 2, respectively. Figure 1 and Figure 2 Both together confirm the presence and state of the MgO particles.

[0043] Example 3: Activity of the MgO / NC-500 catalyst at different reaction temperatures

[0044] Add nitrobenzene (0.5 mmol), the MgO / NC-500 catalyst (30 mg) prepared in Example 2, and ethanol (10 mL, analytical pure) into a 40 mL autoclave. Purge the air in the autoclave 5 times through N 2 , and after sealing, fill the autoclave with N at 1 MPa 2Six groups of reactions were set up in the same way and then reacted at different temperatures (170 - 220 °C) for 6 hours. After cooling to room temperature with the autoclave, the catalyst was collected by centrifugation, and the remaining solution was analyzed by gas chromatography (GC) and gas chromatography - mass spectrometry (GC - MS). After detection, the conversion rate of nitrobenzene and the product yields are shown in Table 1 below. It can be seen from Table 1 that the reaction temperature has a more obvious influence in this reaction. As the reaction temperature increases, the conversion rate of nitrobenzene and the yield of imine gradually increase. After raising the temperature to 210 °C, the yield of N - phenylethylideneamine product (Product 2) in this process increases with the increase of the conversion rate, but the selectivity does not increase significantly, and a small amount of other by - products other than Products 1 and 2 are detected. Therefore, 200 °C was selected as the reaction temperature for practicality testing.

[0045] Table 1

[0046]

[0047]

[0048] Note: a in the table indicates that the reaction time is 12 h.

[0049] Example 4: Activity of the MgO / NC - 500 catalyst prepared in Example 2 under different solvents

[0050] Nitrobenzene (0.5 mmol, 1.0 equiv.), the MgO / NC - 500 catalyst prepared in Example 2 (30 mg), benzyl alcohol (5.0 equiv.), ultrapure water, and different types of analytical - grade solvents (10 mL) were added to a 40 - mL autoclave respectively. After purging the air in the autoclave 5 times with N 2 and sealing, 1 MPa of N 2 was filled into the autoclave. Then the reaction was carried out at 200 °C for 6 hours. After cooling to room temperature with the autoclave, the catalyst was collected by centrifugation, and the remaining solution was analyzed by gas chromatography (GC) and gas chromatography - mass spectrometry (GC - MS). After detection, the conversion rate of nitrobenzene and the product yields are shown in Table 2 below. Water can promote the conversion of nitrobenzene to a certain extent. When other analytical - grade solvents are selected, the catalyst also has a certain activity. However, the yield of imine (Product 2) in this model reaction is the highest in n - hexane. When the time was extended with n - hexane as the solvent, the yield of imine could reach 96.6%. Therefore, when exploring other alcohol compounds as substrates except ethanol, the preferred solvent is n - hexane (analytical grade).

[0051] Table 2

[0052]

[0053]

[0054] Note: In the table, a indicates that the reaction time is 32 h.

[0055] Example 5: Comparison of the catalytic activities of the MgO / NC-400 catalyst prepared in Example 1 and the MgO / NC-500 catalyst prepared in Example 2 for the model reaction and exploration of the reaction mechanism

[0056] Add nitrobenzene, aniline or nitroso-benzene (0.5 mmol), the MgO / NC-500 catalyst prepared in Example 2 or the MgO / NC-400 catalyst prepared in Example 1 (30 mg), and ethanol (10 mL, analytical grade) into a 40 mL autoclave. Purge the air in the autoclave 5 times with N 2 and then fill the autoclave with N at 1 MPa after sealing. 2 Then react at 200 °C for 6 h respectively. After cooling the autoclave to room temperature, collect the catalyst by centrifugation and analyze the remaining solution by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). After detection, the conversion rate of nitrobenzene and the product yield are shown in Table 3 below. The present invention focuses on the subsequent dissociation of H in α-C 3 CH 2 O - and the transfer hydrogenation of nitrobenzene on the surface of MgO nanoclusters and Ov-MgO nanoclusters. Since the transfer hydrogenation of nitrobenzene to nitroso-benzene is the rate-determining step in the whole reaction process (as can be seen from the comparison between Entry 2 and Entry 3 in Table 3), detailed DFT calculation studies were carried out on this key step. After ethanol dissociates into CH sp3 CH v O v and H 3 CH 2 O - on the surface of MgO nanoclusters and Ov-MgO nanoclusters, nitrobenzene can also be adsorbed on the surface of MgO nanoclusters and Ov-MgO nanoclusters, and the adsorption energies are 29.5 and 35.0 kcal / mol respectively (see + 3 and 3' in Figure 3 a). The co-adsorption of nitrobenzene causes the H atom to directly transfer from the α-C 3 CH 2 O - in CH sp3 to the O atom on the catalyst surface, and the energy barrier of this step obtained by DFT calculation is 59.0 kcal / mol (see Figure 3 b). DFT calculations show that with the help of H 2 O molecules, the H atom transfer step from CH 3 CH 2 O -The α-C in sp3 -H is greatly reduced to 29.4 kcal / mol (see Figure 3 TS3-4 in a). The possible reaction mechanism is speculated as Figure 6 shown below.

[0057] Table 3

[0058]

[0059]

[0060] Note: In the table, a indicates that the substrate is nitrobenzene (0.5 mmol) and ethanol, and the reaction time is 6 h; b indicates that the substrate is aniline (0.5 mmol) and ethanol, and the reaction time is 6 h.

[0061] Example 6: Effect of water content on the activity of the MgO / NC-500 catalyst prepared in Example 2

[0062] Add nitrobenzene (0.5 mmol), the MgO / NC-500 catalyst prepared in Example 2, and ultradry ethanol (10 mL) or ethanol (10 mL, analytical grade) containing different volumes of water (0 - 200 μL) into a 40 mL autoclave. Purge the air in the autoclave 5 times through N 2 and then charge 1 MPa of N 2 into the autoclave after sealing. Then react at 200 °C for 6 hours respectively. After cooling the autoclave to room temperature, collect the catalyst by centrifugation and analyze the remaining solution by gas chromatography (GC) and gas chromatography - mass spectrometry (GC-MS). After detection, the conversion rate of nitrobenzene and the product yield are shown in Table 4 below. The experimental data show that adding a trace amount of water to ultradry ethanol will accelerate the reduction of nitrobenzene. Similarly, adding a trace amount of water to ethanol (analytical grade) can also accelerate the reduction of nitrobenzene. However, as the water content increases, the MgO nanoclusters on the catalyst surface will hydrolyze to form Mg(OH) 2 resulting in catalyst deactivation and a decrease in substrate conversion rate. Therefore, analytical grade solvents are used in all experiments except for the experiments exploring the water content.

[0063] Table 4

[0064]

[0065]

[0066] Note: In the table, a indicates that the solvent is ethanol (analytical grade).

[0067] Example 7: Cyclic experiment to explore the stability of the MgO / NC-500 catalyst prepared in Example 2

[0068] Add nitrobenzene (0.5 mmol), the MgO / NC-500 catalyst prepared in Example 2 (30 mg), and ethanol (10 mL, analytical grade) into a 40 mL autoclave. Purge the air in the autoclave 5 times with N 2 to remove the air in the autoclave, and then fill the autoclave with N at 1 MPa after sealing 2 . React at 200 °C for 6 hours. After cooling the autoclave to room temperature, collect the catalyst by centrifugation, and wash it 3 times with water and ethanol respectively; after each repetition, use this method to recover the catalyst. After the recovery is completed, weigh it, and the recovery rate each time > 95%; the recovered catalyst is directly used for the next imine synthesis reaction. Repeat the above steps (since there is a loss of less than 5% of the catalyst recovered after each use, each time according to the ratio of the weight of the recovered catalyst to the weight of the catalyst at the first use (30 mg), reduce the usage amount of other reaction raw materials according to this ratio), and analyze the remaining solution after each reaction by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The conversion rate of nitrobenzene and the product selectivity of each reaction are as Figure 4 shown. The detection results show that the MgO / NC-500 catalyst prepared in Example 2 was recycled 6 times, and the catalytic activity did not decrease significantly, indicating that the catalyst has good stability.

[0069] Example 8: Explore the catalytic activity of the MgO / NC-500 catalyst prepared in Example 2 towards different substrates

[0070] As shown in Table 5, add different nitro compounds (0.5 mmol), the MgO / NC-500 catalyst prepared in Example 2 (30 mg), and different aromatic alcohol compounds (2.5 mmol), n-hexane (analytical grade solvent, 10 mL) into a 40 mL autoclave (when using aliphatic alcohols, they are also used as reaction solvents; 10 equiv. of 1,2-ethylene glycol was used as both substrate and solvent when synthesizing quinoline). Purge the air in the autoclave 5 times with N 2 to remove the air in the autoclave, and then fill the autoclave with N at 1 MPa after sealing 2 . Then react at 200 °C for 6 hours respectively. After cooling the autoclave to room temperature, collect the catalyst by centrifugation, and analyze the remaining solution by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). After detection, the conversion rate of nitrobenzene and the yield of imine products are shown in Table 5.

[0071] As can be seen from Table 5, in addition to ethanol, other types of aliphatic alcohols are also suitable for reductive coupling with nitrobenzene, and the isolated yields of the imines exceed 90% (see Entries 1-6 in Table 5). The activity of aliphatic alcohols decreases with the increase in the number of carbon atoms, which is due to the larger steric hindrance of aliphatic alcohols with more carbon atoms. The reductive coupling of nitrobenzene with aromatic / heteroaromatic alcohols also gives the corresponding imines with satisfactory yields (see Entries 7-16 in Table 5). The activity of aromatic alcohols with electron-withdrawing groups is slightly higher than that of aromatic alcohols with electron-donating groups (see Entries 10-12 in Table 5). Heterocyclic aromatic alcohols, such as biomass-derived furfuryl alcohol and 4-pyridylethanol, usually cause the deactivation of metal nanoparticle catalysts due to the strong affinity of heteroatoms for metal nanoparticles. However, the MgO / NC-500 catalyst prepared in Example 2 is also active for the reductive coupling of heteroatom-containing alcohols with nitrobenzene (see Entries 13-15 in Table 5). This activity towards heteroaromatic alcohols may be due to the strong interaction between the oxygen atom in the alcohol and the Ov sites in MgO / NC-500, resulting in the selective adsorption of the hydroxyl group and avoiding the poisoning of heteroatoms on the aromatic ring. In addition to primary alcohols, the bulky secondary alcohol 1-phenylethanol can also effectively couple with nitrobenzene to form the corresponding imine with a high yield, but a longer reaction time is required (see Entries 16 and 7 in Table 5). Similar steric effects can also be observed for p-nitrotoluene, m-nitrotoluene, and o-nitrotoluene (see Entries 17, 18, and 19 in Table 5).

[0072] Regarding the influence of the electronic properties of substituents on nitroarenes, it can be seen that the hydrogen transfer coupling of nitroarenes containing electron-donating groups with benzyl alcohol is more active than that of nitroarenes containing electron-withdrawing groups (see Entries 17-22 and 22-25 in Table 5). In addition to nitroarenes, more challenging cyclic and aliphatic nitro compounds can also smoothly couple with benzyl alcohol to produce the corresponding imines with yields of 90-98% (see Entries 27-30 in Table 5). In addition to having high catalytic activity, MgO / NC-500 also exhibits excellent selectivity and good tolerance towards various reducing groups such as halogens, nitriles, and hydroxyl groups, especially towards vinyl (C=C bond) (see Entries 20-26 in Table 5), which is one of the most prominent advantages of the MgO / NC-500 catalyst.

[0073] The excellent catalytic performance of MgO / NC-500 prepared in Example 2 for the synthesis of imines further expands the synthetic methods of N-heterocyclic compounds (see Entries 31-54 in Table 5), and this method also proceeds through the reductive coupling of alcohol compounds and nitro compounds. The reductive coupling reactions of aliphatic alcohols and aromatic alcohols with 2-nitroaniline proceed smoothly at 200 °C to obtain the corresponding 2-substituted benzimidazoles in high yields (see Entries 31-45 in Table 5). Using 2-nitrophenol as the raw material for the reductive coupling reaction, 2-alkylbenzoxazoles were successfully generated (see Entries 46-48 in Table 5). In addition to the reductive coupling of 2-nitroaniline with primary alcohols to form benzimidazoles, MgO / NC-500 is also effective for the reductive coupling of 2-nitroaniline with 1,2-diols to synthesize quinoline in high yields (see Entries 49-54 in Table 5).

[0074] Table 5 Effects of Different Kinds of Nitro Compounds and Alcohol Compounds on the Target Products of Hydrogen Transfer Reductive Coupling Synthesis Reactions

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

Claims

1. A catalyst, characterized in that: The catalyst is a nitrogen-doped carbon catalyst loaded with defect-rich MgO nanoclusters, with nitrogen-doped carbon material as a skeleton, and active component MgO nanoclusters loaded on the skeleton.

2. A method for preparing the catalyst according to claim 1, characterized in that: The preparation method comprises the following steps in sequence: (1) Dissolve urea, Mg(NO3)2, chitosan and glacial acetic acid in solvent A; stir evenly to obtain a Mg-containing 2+ Sol-gel precursors; (2) The Mg 2+ After the sol-gel precursor is dried, it is heated from room temperature to a target temperature of 400-600°C at a heating rate of 1-4°C / min under a nitrogen atmosphere, and then maintained at the target temperature for 1-4 hours. After cooling to room temperature, a MgO / NC-T catalyst is obtained, in which the mass percentage content of Mg element in the catalyst is 4.6-7%.

3. The preparation method according to claim 2, characterized in that: In the step (1), the solvent A is distilled water.

4. The preparation method according to claim 2, characterized in that: The molar ratio of urea, Mg(NO3)2 and glacial acetic acid in step (1) is (100-400):(1-4):(90-360); the mass ratio of urea and chitosan is (6-24):(0.5-2).

5. The preparation method according to claim 2, characterized in that: The drying condition in step (2) is oven drying at 30-80°C for 8-12 hours.

6. Use of the catalyst according to claim 1 or the catalyst obtained by the preparation method according to any one of claims 2 to 5 in synthesizing imines or N-heterocyclic compounds.

7. The use according to claim 6, characterized in that: The application steps are as follows: adding the catalyst described in claim 1 or the catalyst obtained by the preparation method described in any one of claims 2 to 5, an alcohol compound, and a nitro compound in a ratio of (15-60) mg: (90-360) mmol: (0.25-1) mmol to a reaction container, adding solvent B, sealing and filling with inert gas, reacting under heating conditions to obtain a product imine or N-heterocyclic compound.

8. The use according to claim 7, characterized in that: In the application, the alcohol compound has a structural formula of one of the following: And / or the nitro compound structural formula is one of the following:

9. The use according to claim 7, characterized in that: In the application, the pressure of the inert gas is (0.5-2) MPa; the reaction conditions are (170-220° C.) and (6-36) h.

10. The use according to claim 7, characterized in that: The solvent B is one or more of the alcohol compound, tetrahydrofuran, toluene, acetonitrile, and n-hexane.

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