Non-metal element-doped carbon catalysts, preparation and synthesis of imines

Abstract

The application discloses a preparation method of a non-metal element doped carbon catalyst, the catalyst contains two or more than two of N, P, B and O elements in addition to C element, the catalyst is prepared through pretreatment by alkali metal hydroxide and high-temperature calcination, is a non-metal element doped carbon catalyst with high specific surface area, the content of non-metal elements in the catalyst in addition to C element is 5-20wt%, and the specific surface area of the catalyst can reach 1500-3700m 2 ·g ‑1 The preparation method is simple and convenient to operate, has excellent catalytic activity for the reaction of cross-coupling of monohydric alcohol with primary amine to produce corresponding imine, the catalyst is low in price and green and environment-friendly, and some main disadvantages of metal catalysts are avoided.

Description

Technical fields:

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to a method for preparing a high specific surface area non-metallic element-doped carbon catalyst and its application. Background technology:

[0002] In current industrial production processes, heterogeneous catalytic reactions, compared to homogeneous reactions, can significantly reduce catalyst costs and improve economic efficiency. Currently used heterogeneous catalysts are predominantly metal catalysts, including both noble and non-noble metal catalysts. Given the current global shortage of metal resources, developing non-metallic heterogeneous catalysts to replace metal catalysts is crucial for conserving non-renewable metal resources. Carbon catalysts doped with non-metallic elements are an ideal example of non-metallic heterogeneous catalysts. By using different doping elements, the electronic properties of carbon catalysts can be tuned. Furthermore, the specific surface area of ​​carbon catalysts can be increased through physical means, thereby improving their mass transfer efficiency and making it possible for their catalytic performance to approach or even surpass that of traditional metal catalysts.

[0003] Imines are an important class of nitrogen-containing organic intermediates, widely used in the synthesis of imines in biological, agricultural, and pharmaceutical fields, including reduction, addition, cyclization, and aziridine reactions. Traditional synthetic methods often involve unstable aldehydes and acid catalysts, which have significant drawbacks from both economic and environmental perspectives. In recent years, a series of green and efficient synthetic methods have been developed based on this approach. Among them, the one-step synthesis of imines via the condensation of alcohols and amines is a highly promising green process route. This route simplifies the reaction steps by directly oxidizing alcohols to aldehydes and then coupling them with amines to form imines. Furthermore, the process produces only water as the sole byproduct, making it environmentally friendly. Currently, catalysts used in this reaction include Au, Pd, or Ru supported catalysts, as well as MnO. x Catalysts based on metal oxides such as CeO2 are available, but the presence of metals increases the preparation cost. Moreover, especially for the pharmaceutical industry, there is a potential for metal pollution, which limits their widespread application. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for preparing a non-metallic element-doped carbon catalyst and its application in imine synthesis. The catalyst preparation method is simple and convenient to operate, and it has excellent catalytic activity for the cross-coupling of primary amines with monohydric alcohols containing α-H to produce corresponding imines. Moreover, the catalyst is inexpensive and environmentally friendly.

[0005] This invention is achieved through the following technical solution:

[0006] This invention provides a non-metallic element-doped carbon catalyst, wherein the active centers of the catalyst include non-metallic element active sites other than carbon; the total content of non-metallic elements excluding carbon in the catalyst is approximately 5-20 wt%. The specific surface area of ​​the catalyst is 1500-3700 m². 2 ·g -1 .

[0007] Based on the above technical solutions, preferably, the non-metallic element is two or more of N, P, B, and O.

[0008] Based on the above technical solution, preferably, the mass fraction of non-metallic elements other than carbon is in the range of 5-20 wt%, and the specific surface area of ​​the catalyst is 1500-3700 m². 2 ·g -1 .

[0009] Another aspect of the present invention provides a method for preparing the above-mentioned non-metallic element-doped carbon catalyst, characterized in that: the aromatic fused-ring organic precursor is treated by mixing and grinding with metal hydroxide, an appropriate amount of water is added to the obtained homogeneous solid powder, and the mixture is stirred at 70-90℃ for 4-6 hours. Then, the temperature is raised to 100-160℃ to evaporate the water, resulting in a dry brown-black powdery solid mixture. The mixture is calcined at 600-800℃ under N2 for 4-6 hours to obtain a fluffy black solid. After pulverizing, the solid is filtered and washed with a large amount of deionized water until the filtrate is neutral, and then dried to obtain the catalyst.

[0010] Based on the above technical solutions, preferably, the non-metallic element in the aromatic fused-ring organic precursor is one or two of N, P, B, and O; the aromatic fused-ring organic precursor contains 10-40 carbon atoms and 1-12 non-metallic element atoms per molecule.

[0011] Based on the above technical solutions, preferably, the metal hydroxide is one of LiOH, NaOH, KOH, and CsOH.

[0012] Based on the above technical solution, the preferred mass ratio of aromatic fused-ring organic precursor: metal hydroxide: water in the preparation method is 0.2-1.8:0.5-8:2-20.

[0013] In another aspect, the present invention provides a method for producing a corresponding imine by cross-coupling a primary amine with a monohydric alcohol containing α-H, wherein the above-mentioned catalyst is used to synthesize the imine by one-pot oxidative catalysis using a primary amine and a monohydric alcohol containing α-H as raw materials.

[0014] Based on the above technical solution, the preferred method for synthesizing the imine is as follows: A primary amine, an α-H-containing monohydric alcohol, a base, a solvent, and a catalyst are added to a reactor; the reactor is sealed, air at 1-10 bar is supplied, stirring is started, and the temperature is raised to 80-130°C; in the reaction mixture, the mass ratio of the primary amine to the α-H-containing monohydric alcohol in the reaction system is 1:0.5-2; in the reaction mixture, the mass concentration of the solvent in the reaction system is 60%-90%; in the reaction mixture, the mass concentration of the base in the reaction system is 0.2%-1.5%; in the reaction mixture, the mass concentration of the catalyst in the reaction system is 2%-30%; after the reaction proceeds for 6-24 hours, the reaction indicators obtained are: conversion rate of the α-H-containing monohydric alcohol ≥95%, and selectivity of the imine ≥92%.

[0015] Beneficial effects:

[0016] In this invention, the addition of alkali metal hydroxides acts as a pore-forming agent, increasing the specific surface area of ​​the material and fully exposing the heteroatom active centers of non-C elements, thus enabling them to better exert their catalytic effect. Therefore, the non-metallic element-doped carbon catalyst prepared by this method exhibits outstanding performance in the catalytic reaction of primary amines with α-H monohydric alcohols to produce corresponding imines.

[0017] The beneficial effects of this invention are: In this invention, a carbon-doped catalyst with a high specific surface area is prepared by calcining a precursor after pretreatment. Its active centers include non-metallic carbon active sites and carbon defect active sites. Figure 2 Raman spectroscopy reveals that the catalyst has carbon defects; the catalyst prepared by this method has low requirements for precursors, a wide range of which can be selected, and the catalyst specific surface area is as high as 3606 m². 2 ·g -1 This greatly increases mass transfer efficiency, resulting in higher selectivity and conversion rate of the catalyst. As a non-metallic catalyst, it is green and environmentally friendly, with no metal pollution. When this catalyst is applied to the reaction of primary amine and monohydric alcohol containing α-H cross-coupling to produce the corresponding imine, the conversion rate of the monohydric alcohol containing α-H is ≥95% and the selectivity of imine is ≥90% after running for more than 6 hours. Moreover, it has excellent stability after more than 5 cycles.

[0018] The preparation method used in this invention is simple and easy to operate. It has excellent catalytic activity for the cross-coupling of primary amines with monohydric alcohols containing α-H to produce corresponding imines. Moreover, the catalyst is inexpensive and environmentally friendly, avoiding some of the major drawbacks of metal catalysts. Attached Figure Description

[0019] Appendix Figure 1 This is an electron microscope image of the catalyst prepared in Example 1.

[0020] Appendix Figure 2 These are the Raman spectra of Example 1 and Comparative Example 1.

[0021] Appendix Figure 3 This is the N2 adsorption-desorption isotherm diagram of Example 1. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments, so as to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention. The scope of protection of the present invention includes, but is not limited to, the following embodiments. Any modifications made to the details and form of the technical solution of the present invention without departing from the spirit and scope of this application shall fall within the scope of protection of the present invention.

[0023] Example 1

[0024] Weigh 1g of 11,11'-bis(dipyrido[3,2-a:2,3-c]phenazinyl (abbreviated as bidppz) (organic precursor), add 2g of NaOH and mix and grind. Add 6ml of water and stir at 80℃ for 6h. Raise the temperature to 110℃ to evaporate the water, obtaining a brownish-black solid. Heat the obtained solid powder to 800℃ at 5℃ / min under nitrogen and maintain for 5h. Allow it to cool naturally to room temperature, wash with water until neutral, and then dry at 80℃ to obtain catalyst m-NOC-NaOH-15%, with a total non-metallic element content of approximately 15wt% and a specific surface area of ​​approximately 3600m². 2 ·g -1 .

[0025] Example 2

[0026] The catalyst preparation conditions were the same as in Example 1, except that NaOH was replaced with an equal mass of LiOH, resulting in catalyst m-NOC-LiOH-15%, with a total non-metallic element content of approximately 15 wt% and a specific surface area of ​​approximately 3600 m². 2 ·g -1 .

[0027] Example 3

[0028] The catalyst preparation conditions were the same as in Example 1, except that the organic precursor was replaced with 1g of triphenylphosphine, resulting in catalyst m-POC-NaOH-15%, with a total non-metallic element content of approximately 15wt% and a specific surface area of ​​approximately 3600m². 2 ·g -1 .

[0029] Example 4

[0030] The catalyst preparation conditions were the same as in Example 1, except that the organic precursor was replaced with 1g of p-tolueneboric acid, resulting in catalyst m-BOC-NaOH-15%, with a total non-metallic element content of approximately 15wt% and a specific surface area of ​​approximately 3600m². 2 ·g -1 .

[0031] Example 5

[0032] The catalyst preparation conditions were the same as in Example 1, except that 0.5 g of activated carbon was added to obtain catalyst m-NOC-NaOH-10%, with a total non-metallic element content of approximately 10 wt% and a specific surface area of ​​approximately 3400 m². 2 ·g -1 .

[0033] Example 6

[0034] The catalyst preparation conditions were the same as in Example 1, except that 1g of activated carbon was added to obtain catalyst m-NOC-NaOH-7.5%, with a total non-metallic element content of approximately 7.5wt% and a specific surface area of ​​approximately 3200m². 2 ·g -1 .

[0035] Comparative Example 1

[0036] The catalyst preparation conditions were the same as in Example 1, except that the catalyst calcination temperature was replaced by increasing the temperature to 400°C at a rate of 5°C / min under nitrogen atmosphere, yielding catalyst m-NOC-NaOH-15%-400 with a total non-metallic element content of approximately 15wt% and a specific surface area of ​​approximately 600m². 2 ·g -1 .

[0037] Comparative Example 2

[0038] The catalyst preparation conditions are the same as in Example 1, except that the catalyst precursor is replaced with 1g of activated carbon, resulting in catalyst m-OC-NaOH-15%, with a total non-metallic element content of approximately 15wt% and a specific surface area of ​​approximately 1800m². 2 ·g -1 .

[0039] Comparative Example 3

[0040] The catalyst preparation conditions are the same as in Example 1, except that NaOH is replaced with an equal mass of Ca(OH)2, resulting in catalyst m-NOC-Ca(OH)2-15%, with a total non-metallic element content of approximately 15wt% and a specific surface area of ​​approximately 1200m². 2 ·g -1 .

[0041] Comparative Example 4

[0042] The catalyst preparation conditions were the same as in Example 1, except that NaOH was not added, resulting in catalyst NOC-800-15%, with a total non-metallic element content of approximately 15 wt% and a specific surface area of ​​approximately 400 m². 2 ·g -1 .

[0043] Comparative Example 5

[0044] Commercially available nitrogen-doped carbon material was used as the catalyst. This material contained 7.5% of doping elements other than carbon, denoted as NC-7.5%, and had a specific surface area of ​​approximately 1000 m². 2 ·g -1 .

[0045] Example 7

[0046] The catalysts described in Examples 1-6 and Comparative Examples 1-5 were applied to the synthesis reaction of benzylidene under the same conditions:

[0047] 0.25 mmol of benzyl alcohol, 0.3 mmol of aniline, 80 mg / mmol of m-NOC-NaOH-15%, 14 mg / mmol of potassium carbonate, and 2 mL / mmol of n-heptane were added to a 10 mL Schlenk tube. After the tube was sealed with a magnetic stir bar and an air bulb was attached, it was placed in an oil bath at 110 °C and stirred. After the reaction was completed, the tube was cooled to room temperature, and 25 mg of biphenyl was added as an internal standard and 2 mL of 1,4-dioxane was added to dissolve and dilute the mixture. The tube was stirred for 3 min to mix thoroughly and then centrifuged. An appropriate amount of the supernatant was taken and analyzed by gas chromatography to calculate the conversion rate of benzyl alcohol and the yield of benzyl aniline.

[0048] The results are shown in Table 1. As can be seen from the table, the catalyst of this invention exhibits good selectivity for benzyleneaniline. Increasing the calcination temperature helps to improve the catalyst activity and product selectivity. Pretreatment with alkali metal hydroxides is necessary; multi-element doping is beneficial for improving benzyl alcohol conversion and imine selectivity.

[0049] Table 1. Preparation of imines by cross-coupling of benzyl alcohol and aniline

[0050]

[0051] Examples 7-18

[0052] Except for the different alcohol substrate, reaction temperature, reaction time, and solvent, the catalyst activity evaluation was the same as in Example 1. Specific conditions were:

[0053] 0.25 mmol of alcohol derivative, 0.3 mmol of aniline, 80 mg / mmol of m-NOC-NaOH-800, 14 mg / mmol of potassium carbonate, and 2 mL / mmol of solvent were added to a 10 mL Schlenk tube. A magnetic stir bar was placed inside, the tube was sealed, an air bulb was attached, and the tube was placed in an oil bath at a controlled temperature with stirring. After the reaction was complete, the tube was cooled to room temperature, and a vacuum pump was used to evacuate the atmosphere for 1 minute at room temperature to remove the solvent. Benzyl benzoate was added as an internal standard, and 1 mL of CDCl3 was added as a solvent. After mixing thoroughly, the mixture was centrifuged, and a suitable amount of the supernatant was measured using a 400 M nuclear magnetic resonance spectrometer. 1 The yield of the imine product was calculated by 1H spectrum analysis, and the product structure was confirmed by gas chromatography-mass spectrometry.

[0054] The results are shown in Table 2:

[0055] Table 2. Cross-coupling of different alcohol substrates with aniline to produce imines

[0056]

[0057]

[0058] Examples 19-22

[0059] Except for the amine substrate, reaction temperature, and solvent, the catalyst activity evaluation was the same as in Example 1. Specific conditions were:

[0060] 0.25 mmol of benzyl alcohol, 0.3 mmol of an amine derivative, 80 mg / mmol of m-NOC-NaOH-800, 14 mg / mmol of potassium carbonate, and 2 mL / mmol of solvent were added to a 10 mL Schlenk tube. A magnetic stir bar was placed inside, the tube was sealed, an air bulb was attached, and the tube was placed in an oil bath at a controlled temperature with stirring. After the reaction was complete, the tube was cooled to room temperature, and a vacuum pump was used to evacuate the atmosphere for 1 minute at room temperature to remove the solvent. Benzyl benzoate was added as an internal standard, and 1 mL of CDCl3 was added as a solvent. After thorough mixing, the mixture was centrifuged, and a suitable amount of the supernatant was measured using a 400 MHz nuclear magnetic resonance spectrometer. 1 The yield of the imine product was calculated by 1H spectrum analysis, and the product structure was confirmed by gas chromatography-mass spectrometry.

[0061] The results are shown in Table 3:

[0062] Table 3. Cross-coupling of benzyl alcohol and different amine substrates to produce imines

[0063]

[0064]

[0065] The catalyst described in this invention contains no metal elements, causes no metal pollution, has high oxidation efficiency for alcohols, high product yield, and wide substrate applicability; it uses air as an oxygen source, making it economical and environmentally friendly, and has excellent application prospects.

Claims

1. A method for preparing a non-metallic element-doped carbon catalyst, characterized in that, The catalyst is prepared by pretreating an organic precursor with an alkali metal hydroxide and then calcining it at high temperature. The specific steps are as follows: the organic precursor and the alkali metal hydroxide are mixed and ground to obtain a uniform solid powder. Water is added and the mixture is stirred at 70-90℃ for 4-6 hours. Then the temperature is raised to 100-160℃ to evaporate the water, resulting in a dry brown-black powdery solid mixture. The mixture is calcined under N2 at a temperature of 600-800℃ for 4-6 hours to obtain a black solid. After pulverizing, the solid is filtered and washed with water until the filtrate is neutral. After drying, the catalyst is obtained. The organic precursor is one or more of the following: 。 2. The preparation method according to claim 1, characterized in that, In the process of preparing the black solid in the method, the mass ratio of organic precursor: alkali metal hydroxide: water is 0.2~1.8:0.5~8:2~20.

3. The preparation method according to claim 2, characterized in that, In the process of preparing the black solid in the method, the mass ratio of organic precursor: alkali metal hydroxide: water is 0.8~1.2:1~4:4~10.

4. The preparation method according to claim 1, characterized in that, The alkali metal hydroxide is one or more of LiOH, NaOH, KOH, and CsOH.

5. A non-metallic element-doped carbon catalyst produced by the preparation method according to any one of claims 1-4.

6. The non-metallic element-doped carbon catalyst according to claim 5, characterized in that, The catalyst comprises carbon and non-metallic elements doped onto the carbon; the non-metallic elements in the catalyst include one or more of N, P, B, and O, with a total content of 5-20 wt%; the specific surface area of ​​the catalyst is 1500-3700 m². 2 ·g -1 .

7. A method for synthesizing a corresponding imine by cross-coupling an α-H-containing monohydric alcohol with a primary amine, characterized in that, Add the primary amine, α-H monohydric alcohol, base, solvent, and catalyst as described in claim 5 or 6 to the reactor; seal the reactor, supply 1-10 bar of air into the reactor, start stirring, and raise the temperature to 80-130°C. ； In the reaction mixture, the molar ratio of α-H-containing monohydric alcohol to primary amine in the reaction system is 1:1~2; In the reaction mixture, the solvent volume in the reaction system is 0.5-2 mL / mmol, based on the molar amount of alcohol. In the reaction mixture, the molar ratio of the α-H-containing monohydric alcohol to the base in the reaction system is 1:0.05-0.5; In the reaction mixture, the catalyst has a mass of 5-30 mg / mmol, based on the amount of alcohol.

8. The synthesis method according to claim 7, characterized in that, Supply 4-6 bar of air into the reactor, turn on the agitator, and raise the temperature to 100-120℃; In the reaction mixture, the molar ratio of α-H-containing monohydric alcohol to primary amine in the reaction system is 1:1.1~1.4; In the reaction mixture, the solvent volume in the reaction system is 0.8-1.2 mL / mmol, based on the molar amount of alcohol. In the reaction mixture, the molar ratio of the α-H-containing monohydric alcohol to the base in the reaction system is 1:0.15-0.25; In the reaction mixture, the catalyst has a mass of 15-25 mg / mmol, based on the amount of alcohol.

9. The synthesis method according to claim 7, characterized in that, The base is one of LiOH, NaOH, KOH, Li2CO3, K2CO3, and Cs2CO3; the solvent is one of n-heptane, dioxane, and tert-amyl alcohol.

10. The synthesis method according to claim 7, characterized in that, The reaction time is 8-16 h.

11. The synthesis method according to claim 10, characterized in that, The reaction time is 10-12 h.

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