Method for preparing adipic acid through one-step oxidation of cyclohexane under catalysis of metal-based ionic liquid

Abstract

The invention discloses a method for preparing adipic acid by catalyzing one-step oxidation of cyclohexane through metal-based ionic liquid, which comprises the following steps: mixing the metal-based ionic liquid, cyclohexane and an organic solvent, heating to 100-160 DEG C in an oxygen-containing atmosphere, and reacting for 2-12 hours to obtain adipic acid, the organic solvent is at least one of acetonitrile, acetone and dichloromethane. The volume ratio of the organic solvent to the cyclohexane is (5-20): 1, and the dosage of the metal-based ionic liquid is 0.1-20 mol% of that of the cyclohexane. The method is a green synthesis process, the catalyst is novel, the process is simple and environment-friendly, the production process is easy to control, and industrial production and application of adipic acid are facilitated. The method is high in catalytic oxidation activity, and the selectivity of adipic acid is remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of chemical catalysis, and in particular to a method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid. Background Art

[0002] Adipic acid, also known as fatty acid, is the most valuable aliphatic dicarboxylic acid in industrial production so far. Adipic acid has the common properties of aliphatic dicarboxylic acids, including salt formation reaction, esterification reaction, amidation reaction, etc. Its primary use is to synthesize nylon 66. About 3 million tons of adipic acid are used as monomers for synthesizing nylon-66 each year worldwide. In addition, adipic acid is also the basic raw material for synthesizing polyester polyols, polyurethanes, adipic acid diester plasticizers, adiponectin, and producing advanced lubricants and food additives.

[0003] In response to the above needs, researchers at home and abroad have been exploring efficient and low-pollution adipic acid production processes. At present, the main synthesis processes include phenol method, cyclohexane method, cyclohexene method and butadiene method. Among them, the cyclohexane method is mature in technology and mild in operating conditions. It is the most widely used preparation method in the world. The cyclohexane method requires cyclohexane to generate cyclohexanone and cyclohexanol (KA oil) under the action of copper salt / ammonium vanadate as a catalyst, and KA oil is then oxidized to adipic acid by nitric acid. This route accounts for about 90% of the global adipic acid production. However, there are still some problems with the cyclohexane oxidation method. For example, in the first step of the reaction, in order to avoid the deep oxidation of KA oil, the conversion rate of cyclohexane must be sacrificed (low to 5%), resulting in high separation energy consumption; and in the second step of the reaction, using an excess of 50-60% nitric acid as an oxidant will generate a large amount of nitrogen oxides, leading to environmental problems such as greenhouse effect, ozone layer depletion, acid rain and haze. Therefore, it is urgent to develop a low-carbon, green adipic acid production route to solve the problems existing in the cyclohexane oxidation method. Summary of the invention

[0004] The object of the present invention is to provide a method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid, so as to achieve efficient one-step oxidation of cyclohexane to prepare adipic acid.

[0005] The present invention provides a method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid, comprising the following steps:

[0006] The metal-based ionic liquid, cyclohexane and organic solvent are mixed, heated to 100-160° C. in an atmosphere containing oxygen, and reacted for 2-12 hours to obtain adipic acid.

[0007] The organic solvent is at least one of acetonitrile, acetone and dichloromethane, the volume ratio of the organic solvent to cyclohexane is (5-20):1, the amount of the metal-based ionic liquid is 0.1-20 mol% of the cyclohexane, and the structural formula of the metal-based ionic liquid is any one of (1)-(6):

[0008]

[0009] n is an integer from 0 to 8.

[0010] Compared with the prior art, the invention has the following beneficial effects: the invention adopts metal-based ionic liquid to catalyze the one-step oxidation of cyclohexane to prepare adipic acid, effectively solving the problems of nitric acid equipment corrosion and nitrogen oxide pollution in the traditional two-step oxidation method of cyclohexane, as well as the problems of acetic acid solution corrosion of equipment, the need to add free radical catalysts and co-catalysts, and the complicated subsequent treatment in the one-step oxidation method. Under the catalysis of metal-based ionic liquid, cyclohexane maintains a good conversion rate and adipic acid selectivity, the conversion rate of cyclohexane is 12.04-15.16%, and the selectivity of adipic acid reaches 54.68-74.69%. DETAILED DESCRIPTION

[0011] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0012] The embodiment of the present invention provides a method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid, comprising the following steps:

[0013] The metal-based ionic liquid, cyclohexane and organic solvent are mixed, heated to 100-160° C. in an atmosphere containing oxygen, and reacted for 2-12 hours to obtain adipic acid.

[0014] The organic solvent is at least one of acetonitrile, acetone and dichloromethane, the volume ratio of the organic solvent to cyclohexane is (5-20):1, the amount of the metal-based ionic liquid is 0.1-20 mol% of the cyclohexane, and the structural formula of the metal-based ionic liquid is any one of (1)-(6):

[0015]

[0016] n is an integer from 0 to 8.

[0017] Specifically, the reactor is a high-pressure reactor with a temperature sensor. The metal ions in the metal-based ionic liquid are divalent cobalt ions.

[0018] Preferably, the metal-based ionic liquid is used in an amount of 2-10 mol% of cyclohexane, the volume ratio of acetonitrile to cyclohexane is (7-14):1, and the atmosphere can be air or oxygen. Preferably, the atmosphere is oxygen at a pressure of 0.5-5 MPa, more preferably, oxygen at a pressure of 1-2 MPa.

[0019] More preferably, the temperature is heated to 130-150° C. and the reaction is carried out for 2-4 hours.

[0020] Specifically, the cobalt-based imidazolium ionic liquid containing carboxylic acid groups in ionic liquids (1) and (4) needs to be prepared in two steps: a. Place an equimolar mixture of n-methylimidazole or pyridine and chloroacetic acid in an oil bath at 70°C under nitrogen protection and stir and reflux for 8 hours to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place it in a vacuum drying oven and dry it at 80°C for 12 hours. b. Add the product obtained in a to a 20 mL glass vial, add an equimolar amount of anhydrous cobalt chloride, and stir at 100°C for 2 hours to obtain a carboxylic acid functionalized cobalt-based ionic liquid. The cobalt-based ionic liquid containing hydroxyl groups in ionic liquids (2) and (5) needs to be prepared in two steps: a. Place an equimolar mixture of n-methylimidazole or pyridine and 2-chloroethanol in an oil bath at 70°C under nitrogen protection and stir and reflux for 5 to 6 hours to obtain a white viscous liquid. After the reaction is completed, the product is washed with ethyl acetate for 3 to 5 times to remove the unreacted raw materials, and then placed in a vacuum drying oven and dried at 80°C for 12 hours. b. The product obtained in a is added to a 20 mL glass vial, and then an equimolar amount of anhydrous cobalt chloride is added, and stirred at 110°C for 4 hours to obtain a cobalt-based ionic liquid containing a hydroxyl group. The cobalt-based ionic liquid containing a benzyl group in ionic liquids (3) and (6) needs to be prepared in two steps: a. An equimolar mixture of n-methylimidazole or pyridine and benzyl chloride is placed in an oil bath at 65°C under nitrogen protection and stirred and refluxed for 10 hours to obtain an orange viscous liquid. After the reaction is completed, the product is washed with ethyl acetate for 3 to 5 times to remove the unreacted raw materials, and then placed in a vacuum drying oven and dried at 80°C for 12 hours. b. The product obtained in a is added to a 20 mL glass vial, and then an equimolar amount of anhydrous cobalt chloride is added, and stirred at 110°C for 4 hours to obtain a cobalt-based ionic liquid containing a benzyl group.

[0021] Preferably, the structural formula of the ionic liquid is (1) and (2),

[0022]

[0023] n is an integer from 0 to 8.

[0024] When the catalyst is an ionic liquid (1) and (2), the anions in the carboxylic acid methyl imidazolium cobalt trichloride ionic liquid in (1) and the hydroxymethyl imidazolium cobalt trichloride ionic liquid in (2) are both cobalt-trichloride coordinated, and the charge density of the cobalt coordinated by three chloride ions is moderate, so that the cobalt center has a higher oxidation activity. Although the activity is slightly weakened compared with the cobalt coordinated by two chloride ions, it is beneficial to avoid the problem of low selectivity of adipic acid caused by too fast reaction rate. In addition, the cations in the ionic liquid all contain hydrogen bond donors (carboxylic acid groups or hydroxyl groups), and the hydrogen donor groups can react with the reaction intermediate cyclohexanone through hydrogen bonds to promote the further reaction of cyclohexanone to convert into adipic acid; at the same time, the hydrogen donor groups can also form hydrogen bonds with the carbonyl oxygen in the precursor substance (carboxylic acid free radical) of adipic acid, stabilize the carboxyl free radical, reduce its decarbonylation to form carbon dioxide and lower carbon chain byproducts (such as glutaric acid, succinic acid, etc.), thereby further improving the selectivity of adipic acid. When taking the value of n, n is preferably 1 or 2, because short-chain raw materials such as chloroacetic acid and 2-chloroethanol have less steric hindrance with imidazole or pyridine during preparation and higher yield; in addition, during the catalytic reaction, a shorter chain length is beneficial for the hydrogen-donating group to transfer the charge to the charge center of the imidazolium cation, which in turn is beneficial for the hydrogen atoms in the hydrogen-donating group to form stronger hydrogen bonds with the oxygen atoms in the intermediate.

[0025] Although cobalt salts (such as cobalt chloride) that are not coordinated with ionic liquids have strong oxidizing ability, too strong oxidizing ability is not conducive to controlling the reaction rate, and it is often easy to cause intermediate products such as carboxyl radicals to decarboxylate quickly to form short carbon chain by-products. After being coordinated by the anions (i.e., chloride ions) of the ionic liquid, the charge density of divalent cobalt increases and the oxidizing activity decreases, which is beneficial to controlling the reaction process and providing favorable conditions for increasing the selectivity of adipic acid. However, continuing to increase the coordination number of cobalt will greatly weaken the oxidizing activity of cobalt and greatly weaken the cyclohexane oxidation performance. Therefore, cobalt-based ionic liquids coordinated with appropriate anions are one of the key factors for efficient one-step oxidation of cyclohexane to prepare adipic acid.

[0026] The side chain group connected to N at position 1 in the imidazolium cation can prevent the coordination of the N atom with cobalt, thereby weakening the oxidation activity of cobalt. For example, the ionic liquid (1-hexyl imidazolium cobalt trichloride ionic liquid) formed by the coordination of proton imidazole without side chain (1-ethyl imidazolium chloride [Eim]Cl) with cobalt chloride has almost no catalytic activity as a catalyst.

[0027] The type of group on N in the imidazolium cation also reduces or increases the positive charge of the cation by donating or withdrawing electrons, thereby affecting the electrostatic interaction between the cation and the coordinated cobalt trichloride anion, indirectly affecting the charge density of cobalt, and thus affecting the oxidation activity of cobalt. For example, when the side chain group is an electron-withdrawing phenyl group, the positrons of the imidazolium cation can be increased, thereby increasing the electrostatic interaction between the anion and the cation, appropriately reducing the charge density of cobalt, and increasing the oxidation ability of cobalt. Therefore, the ionic liquid (1-benzyl-3-methylimidazolium cobalt trichloride ionic liquid) formed by the coordination of the imidazolium ionic liquid (1-methyl-3-benzyl imidazolium chloride [BzMMim]Cl) with a phenyl side chain and cobalt chloride also has a high oxidation activity as a catalyst. However, its side chain does not have a hydrogen-donating group, so it cannot form a hydrogen bond with the intermediate product, and the selectivity advantage of adipic acid is not obvious.

[0028] Embodiment 1:

[0029] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid

[0030] Preparation of 1-carboxymethyl-3-methylimidazolium cobalt trichloride ionic liquid:

[0031] Add 4.106g (0.05mol) of methylimidazole and 4.725g (0.05mol) of chloroacetic acid into a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain 1-carboxymethyl-3-methylimidazole chloride.

[0032] 1.7615 g (0.01 mol) of 1-carboxymethyl-3-methylimidazolium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0033] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0383 g (0.125 mmol) of 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was introduced, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 12.68%, and the adipic acid selectivity reached 73.92%.

[0034] Embodiment 2:

[0035] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-hydroxyethyl-3-methylimidazole cobalt trichloride ionic liquid

[0036] Preparation of 1-hydroxyethyl-3-methylimidazolium cobalt trichloride ionic liquid:

[0037] Add 4.106g (0.05mol) of methylimidazole and 4.0255g (0.05mol) of 2-chloroethanol to a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain 1-hydroxyethyl-3-methylimidazole chloride.

[0038] 1.7615 g (0.01 mol) of 1-hydroxyethyl-3-methylimidazolium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 110° C. for 4 h.

[0039] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0366 g (0.125 mmol) of 1-hydroxyethyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was introduced, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 15.16%, and the adipic acid selectivity reached 74.69%.

[0040] Embodiment 3:

[0041] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-benzyl-3-methylimidazole cobalt trichloride ionic liquid

[0042] Preparation of 1-benzyl-3-methylimidazolium cobalt trichloride ionic liquid:

[0043] Add 4.106g (0.05mol) of methylimidazole and 6.329g (0.05mol) of benzyl chloride to a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain 1-benzyl-3-methylimidazole chloride.

[0044] 2.087 g (0.01 mol) of 1-benzyl-3-methylimidazolium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 110° C. for 4 h.

[0045] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0423 g (0.125 mmol) of 1-benzyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 12.08%, and the adipic acid selectivity reached 55.70%.

[0046] Embodiment 4:

[0047] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using carboxymethylpyridine cobalt trichloride ionic liquid

[0048] Preparation of carboxymethylpyridine cobalt trichloride ionic liquid:

[0049] Add 4.558g (0.05mol) of pyridine and 4.725g (0.05mol) of chloroacetic acid into a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain carboxymethylpyridine chloride.

[0050] 1.8566 g (0.01 mol) of carboxymethylpyridinium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0051] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0316 g (0.125 mmol) of carboxymethylpyridine cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in. The reaction temperature was set to 140°C and the reaction was carried out for 3 hours. The product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 12.40%, and the adipic acid selectivity reached 66.87%.

[0052] Embodiment 5:

[0053] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using hydroxyethylpyridine cobalt trichloride ionic liquid

[0054] Preparation of hydroxyethylpyridine cobalt trichloride ionic liquid:

[0055] Add 4.558g (0.05mol) of pyridine and 4.0255g (0.05mol) of 2-chloroethanol to a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain hydroxyethyl pyridine chloride.

[0056] Add 1.71.67 g (0.01 mol) of hydroxyhexylpyridinium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride into a 20 mL glass vial and stir at 110° C. for 4 h.

[0057] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0302 g (0.125 mmol) of hydroxyhexylpyridine cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in. The reaction temperature was set to 140°C and the reaction was carried out for 3 hours. The product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 13.52%, and the adipic acid selectivity reached 71.23%.

[0058] Embodiment 6:

[0059] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using benzylpyridine cobalt trichloride ionic liquid

[0060] Preparation of benzylpyridinium cobalt trichloride ionic liquid:

[0061] Add 4.558g (0.05mol) of pyridine and 6.329g (0.05mol) of benzyl chloride to a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain benzyl pyridine chloride.

[0062] 2.0568 g (0.01 mol) of benzylpyridinium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 110° C. for 4 h.

[0063] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0419 g (0.125 mmol) of benzylpyridine cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in. The reaction temperature was set to 140°C and the reaction was carried out for 3 hours. The product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 12.04%, and the adipic acid selectivity reached 54.68%.

[0064] Example 7

[0065] In the presence of acetone as the reaction solvent, 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid catalyzes the one-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid The preparation of the 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid is the same as in Example 1

[0066] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetone, and 0.0383 g (0.125 mmol) of 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was introduced, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 15.71%, and the adipic acid selectivity reached 64.08%.

[0067] Example 8

[0068] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid as reaction solvent

[0069] The preparation of 1-carboxymethyl-3-methylimidazolium cobalt trichloride ionic liquid is the same as in Example 1

[0070] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of dichloromethane, and 0.0383 g (0.125 mmol) of 1-carboxymethyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 15.76%, and the adipic acid selectivity reached 62.71%.

[0071] Example 9

[0072] When n=8, 1-carboxyheptyl-3-methylimidazole cobalt trichloride ionic liquid catalyzes the one-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid

[0073] Preparation of 1-carboxyheptyl-3-methylimidazolium cobalt trichloride ionic liquid:

[0074] Add 4.106g (0.05mol) of methylimidazole and 8.933g (0.05mol) of chlorooctanoic acid into a 150mL three-necked flask, and place it in an oil bath at 70°C under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place it in a vacuum drying oven and dry it at 80°C for 12h to obtain 1-carboxyheptyl-3-methylimidazole chloride.

[0075] 2.6076 g (0.01 mol) of 1-carboxyheptyl-3-methylimidazolium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0076] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0488 g (0.125 mmol) of 1-carboxyheptyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and placed in a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was introduced, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 10.73%, and the adipic acid selectivity reached 70.82%.

[0077] Comparative Example 1

[0078] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid catalyzed by cobalt chloride

[0079] Cyclohexane oxidation (when the reaction conditions are the same): 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0142 g (0.125 mmol) of cobalt chloride were weighed separately and placed in a 25 mL high-pressure reactor. The gas in the reactor was replaced with oxygen three times, and then 1 MPa of oxygen was introduced. The reaction temperature was set to 140°C and the reaction was carried out for 3 hours. The product was detected by gas chromatography, and the acidic product was quantified after esterification. The cyclohexane conversion rate reached 15.41%, and the adipic acid selectivity reached 52.0%.

[0080] Cyclohexane oxidation (when the cyclohexane conversion rate is the same): 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0142 g (0.125 mmol) of cobalt chloride were weighed separately and placed in a 25 mL high-pressure reactor. The gas in the reactor was replaced with oxygen three times, and then 1 MPa of oxygen was introduced. The reaction temperature was set to 140°C and the reaction was carried out for 1.125 hours. The product was detected by gas chromatography, and the acidic product was quantified after esterification. The cyclohexane conversion rate reached 12.4%, and the adipic acid selectivity reached 37.8%.

[0081] Comparative Example 2

[0082] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-carboxymethyl-3-methylimidazole zinc chloride ionic liquid

[0083] Preparation of 1-carboxymethyl-3-methylimidazolium cobalt trichloride ionic liquid:

[0084] 1.7615 g (0.01 mol) of 1-carboxymethyl-3-methylimidazolium chloride and 1.363 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0085] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0391 g (0.125 mmol) of 1-carboxymethyl-3-methylimidazole zinc trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in. The reaction temperature was set to 140°C and the reaction was carried out for 3 hours. The product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 1.30%, and the adipic acid selectivity reached 15.20%.

[0086] Comparative Example 3

[0087] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-carboxymethyl-3-methylimidazolium ferric chloride ionic liquid

[0088] Preparation of 1-carboxymethyl-3-methylimidazolium ferric chloride ionic liquid:

[0089] 1.7615 g (0.01 mol) of 1-carboxymethyl-3-methylimidazolium chloride and 1.622 g (0.01 mol) of anhydrous ferric chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0090] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0423 g (0.125 mmol) of 1-carboxymethyl-3-methylimidazole ferric chloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification, and cyclohexane basically did not react.

[0091] Comparative Example 4

[0092] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-carboxymethyl-3-methylimidazole copper trichloride ionic liquid

[0093] Preparation of 1-carboxymethyl-3-methylimidazole copper chloride ionic liquid:

[0094] 1.7615 g (0.01 mol) of 1-carboxymethyl-3-methylimidazolium chloride and 1.0 g (0.01 mol) of anhydrous copper chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0095] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0345 g (0.125 mmol) of 1-carboxymethyl-3-methylimidazole copper chloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification, and cyclohexane basically did not react.

[0096] Comparative Example 5

[0097] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-hexylimidazole cobalt trichloride ionic liquid

[0098] Preparation of 1-hexylimidazole cobalt trichloride ionic liquid:

[0099] Add 3.004g (0.05mol) of imidazole and 3.2255g (0.05mol) of ethyl chloride to a 150mL three-necked flask, and place in a 70°C oil bath under nitrogen protection, stir and reflux for 8h to obtain a white solid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place in a vacuum drying oven and dry at 80°C for 12h to obtain 1-carboxymethyl-3-methylimidazolium chloride.

[0100] 1.2459 g (0.01 mol) of 1-hexyl imidazole chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 100° C. for 2 h.

[0101] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0318 g (0.125 mmol) of 1-hexylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in. The reaction temperature was set to 140°C and the reaction was carried out for 3 hours. The product was detected by gas chromatography. The acidic product was quantified after esterification, and cyclohexane basically did not react.

[0102] Comparative Example 6:

[0103] One-step oxidation of cyclohexane with molecular oxygen to prepare adipic acid using 1-ethyl-3-methylimidazole cobalt trichloride ionic liquid

[0104] Preparation of 1-ethyl-3-methylimidazolium cobalt trichloride ionic liquid:

[0105] Add 4.106g (0.05mol) of methylimidazole and 3.2255g (0.05mol) of chloroacetic acid into a 150mL three-necked flask, and place it in an oil bath at 70°C under nitrogen protection, stir and reflux for 6h to obtain a white viscous liquid. After the reaction is completed, wash the product with ethyl acetate 3 to 5 times to remove the unreacted raw materials, and then place it in a vacuum drying oven and dry it at 80°C for 12h to obtain 1-ethyl-3-methylimidazole chloride.

[0106] 1.4661 g (0.01 mol) of 1-ethyl-3-methylimidazolium chloride and 1.2984 g (0.01 mol) of anhydrous cobalt chloride were added into a 20 mL glass vial and stirred at 110° C. for 4 h.

[0107] Cyclohexane oxidation: 0.2104 g (2.5 mmol) of cyclohexane, 2.5 mL of acetonitrile, and 0.0346 g (0.125 mmol) of 1-ethyl-3-methylimidazole cobalt trichloride ionic liquid were weighed separately and put into a 25 mL high-pressure reactor. After replacing the gas in the reactor with oxygen three times, 1 MPa of oxygen was filled in, the reaction temperature was set to 140°C, the reaction was carried out for 3 hours, and the product was detected by gas chromatography. The acidic product was quantified after esterification. The cyclohexane conversion rate reached 8.39%, and the adipic acid selectivity reached 48.21%.

[0108] The performance of the catalysts obtained in Examples 1 to 9 and Comparative Examples 1 to 5 in catalyzing cyclohexane oxidation is shown in Table 1.

[0109] Table 1 Performance of selective oxidation of cyclohexane to adipic acid over different catalysts a

[0110]

[0111]

[0112] The above embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention. The protection scope of the present invention is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present invention within the essence and protection scope of the present invention, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present invention.

Claims

1. A method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquids, characterized in that: The steps include: The metal-based ionic liquid, cyclohexane and organic solvent are mixed, heated to 100-160° C. in an atmosphere containing oxygen, and reacted for 2-12 hours to obtain adipic acid. The organic solvent is at least one of acetonitrile, acetone and dichloromethane, the volume ratio of the organic solvent to cyclohexane is (5-20):1, the amount of the metal-based ionic liquid is 0.1-20 mol% of the cyclohexane, and the structural formula of the metal-based ionic liquid is any one of (1)-(6): n is an integer from 0 to 8.

2. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: The metal-based ionic liquid has the structural formula (1) or (2), n is an integer from 0 to 8.

3. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1 or 2, characterized in that: The n is 1 or 2.

4. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: The amount of the metal-based ionic liquid used is 2-10 mol% of cyclohexane.

5. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: The volume ratio of the solvent to cyclohexane is (7-14):

1.

6. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: Heat to 130-150°C and react for 2-4 hours.

7. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: The atmosphere is oxygen at a pressure of 0.5 to 5 MPa.

8. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: The organic solvent is acetonitrile.

9. The method for preparing adipic acid by one-step oxidation of cyclohexane catalyzed by metal-based ionic liquid according to claim 1, characterized in that: The structural formula of the metal-based ionic liquid is (2)

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