Process for the preparation of adipic acid using a microchannel reactor

Abstract

The application discloses a method for preparing adipic acid by using a micro-channel reactor, which comprises the following steps: taking cyclohexane containing an initiator and oxygen-containing gas as raw materials, and adding the raw materials into the micro-channel reactor to generate adipic acid through an oxidation reaction; wherein the initiator comprises a cyclohexyl functional group-containing free radical and a peroxide. The application proposes a new method for preparing adipic acid by using the micro-channel reactor, which takes cyclohexane containing an initiator and oxygen-containing gas as raw materials, and generates adipic acid through preheating, mixing, reaction, post-treatment and circulation in a short time of ten seconds to several minutes. The process of the application is a continuous reaction from feeding, preheating, mixing and oxidation reaction, and compared with a batch reaction, the process can greatly improve the conversion rate of cyclohexane, the selectivity of adipic acid and the production efficiency, effectively improve the safety of the oxidation reaction process, and can be used for industrial production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of organic synthesis, and particularly relates to a method for preparing adipic acid by using a micro-channel reactor. BACKGROUND

[0002] Adipic acid is an important organic dibasic acid, and is widely used in chemical industry, food industry, perfume and dye industry, etc.

[0003] At present, the cyclohexane method and the cyclohexene method are the two main methods for producing adipic acid in industry. The cyclohexane is catalytically oxidized by using oxygen as an oxidant to obtain cyclohexanol and cyclohexanone intermediates, and then the adipic acid is prepared by oxidizing the cyclohexanol and cyclohexanone intermediates by using nitric acid under the catalysis of copper and vanadium. The single-pass conversion rate of the cyclohexane needs to be controlled to be lower than 5% to ensure that the total selectivity of the cyclohexanol and cyclohexanone is maintained at a high level (70-90%), which leads to low product yield and complex process. The cyclohexene process mainly includes the hydration of cyclohexene to obtain cyclohexanol, and then the oxidation of the cyclohexanol by using nitric acid to obtain adipic acid. The cyclohexene is prepared by selectively catalytically hydrogenating benzene, and the method has the disadvantages of low selectivity and high catalyst cost. It can be seen that the two methods for producing adipic acid both have the disadvantages of complex process, low product yield and the use of nitric acid and catalyst, and especially the use of nitric acid leads to the emission of a large amount of nitrogen oxides, which causes very serious environmental pollution problems.

[0004] The method for preparing adipic acid by one-step oxidation of cyclohexane overcomes the shortcomings of the traditional process, avoids the emission of nitrogen oxides and is more green and environmentally friendly, and is the development trend of the adipic acid production process.

[0005] Although relevant research work has been reported, due to the stable C-H bond in the cyclohexane molecule, the one-step oxidation of oxygen to adipic acid requires the use of catalysts to shorten the induction period and improve the reaction rate. Yin et al. developed a hollow structure of manganese-doped titanium-silicon catalyst for pure oxygen oxidation of cyclohexane, with a conversion rate of 13.4%, but the selectivity of adipic acid was only 57.5% (Catalysis Communications 2015, 58, 46); Bal et al. synthesized a Co3O4@ZrO2 nanostructured catalyst, with a pure oxygen oxidation conversion rate of cyclohexane as high as 40%, and an adipic acid selectivity of about 43% (Molecular Catalysis 2022, 528, 112473); Ghiaci et al. prepared amorphous carbon nitride supported Cu catalysts, which showed excellent activity for oxygen oxidation of cyclohexane, but the selectivity of adipic acid was less than 52% (Chemical Engineering Journal 2019, 370, 1310); CN102816054B discloses a kind of activated carbon supported gold catalyst, in the condition of butanone as solvent and cyclohexanone as initiator, the conversion rate of cyclohexane is up to 45%, the selectivity of adipic acid is 55%; CN113754531B discloses a kind of nitrogen-containing aromatic hydrocarbon organic catalyst for solvent-free oxidation of cyclohexane, with a conversion rate of about 10% and an adipic acid selectivity of 80-90%. In addition, in order to improve the efficiency of cyclohexane oxidation to adipic acid, polar molecules such as acetic acid and acetonitrile are generally used as solvents, and aldehydes or ketones are used as initiators (Journal of Catalysis 2019, 378, 256; Nano Research 2022, 15, 7662; CN112209816B; CN112521266B). As can be seen, in order to improve the efficiency of cyclohexane oxidation to adipic acid, catalysts, solvents or initiators are generally used, but this will also bring problems such as high production cost and difficult separation. At the same time, the current cyclohexane oxidation process uses a tank reactor, which has a large liquid holdup and intense local heat release, and improper operation can easily cause explosions and other major safety accidents.

[0006] Micro-channel reactor is a continuous flow pipe reactor, which includes mixing, heat exchange, reaction, control and other modules. Compared with traditional conventional reactors, micro-channel reactor has extremely small mass transfer and heat transfer distance, which can significantly improve heat and mass transfer efficiency and space utilization, realize accurate control of reaction temperature, reaction time and material ratio, and has intrinsic safety. CN102746111A and CN112209816A disclose the use of micro-channel reactor for preparing adipic acid from cyclohexane oxidation, but both need to use organic solvent and catalyst, which undoubtedly brings high production cost, separation difficulty and other problems. At the same time, the reaction liquid in the above cases also needs to be evaporated and concentrated to obtain adipic acid crystals, which cannot directly obtain adipic acid solid product, which undoubtedly greatly increases the production energy consumption. SUMMARY

[0007] This section is intended to summarize some aspects of the embodiments of the present application and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the specification to avoid obscuring the purpose of this section, the abstract and the title, and such simplifications or omissions cannot be used to limit the scope of the present application.

[0008] The present application solves the problems of low efficiency, great safety hazard, use of nitric acid and catalyst in the prior art of preparing adipic acid from cyclohexane oxidation, and provides a method for preparing adipic acid from cyclohexane by one step based on non-catalytic oxidation using a micro-channel reactor. The method can realize high conversion of cyclohexane and selectivity of adipic acid above 90% in a short reaction time, and the solid product of adipic acid is directly crystallized and separated from the reaction liquid.

[0009] To solve the above technical problems, the technical solutions of the present application are as follows:

[0010] Cyclohexane containing initiator and oxygen-containing gas are used as raw materials, the proportion is adjusted by metering pump, and then the oxidation reaction is carried out in the micro-channel reactor. The reaction liquid is cooled and crystallized, and then solid-liquid separation is carried out to obtain crude adipic acid solid. At the same time, the initiator generated in situ circulates with the reaction liquid, and the fresh cyclohexane is supplemented to carry out the next round of oxidation reaction. The initiator includes peroxide or free radical containing cyclohexyl functional group, and the peroxide is cyclohexyl functional group-containing peroxide.

[0011] The study found that the introduced peroxide containing cyclohexyl functional group can be decomposed into free radicals at the reaction temperature to greatly promote the oxidation of cyclohexane to adipic acid. Compared with other types of peroxides (such as hydrogen peroxide, tert-butyl hydroperoxide, etc.), the use of peroxide containing cyclohexyl functional group or free radical cyclohexane oxidation rate is faster, and is more conducive to the generation of adipic acid. By selecting "peroxide containing cyclohexyl functional group or free radical", the oxidation of cyclohexane to adipic acid can be directly initiated and promoted, avoiding the use of catalysts and solvents, and greatly reducing the production cost.

[0012] In addition, it is found that the peroxide containing cyclohexyl functional group and free radicals can be generated in situ during the cooling crystallization process of the reaction solution at a suitable temperature. By setting a reaction solution circulation pipeline connecting the cooling crystallization device and the microchannel reactor, the reaction solution containing peroxide containing cyclohexyl functional group and free radicals is circulated into the reactor. The consumption and generation of peroxide containing cyclohexyl functional group and free radicals gradually reach a balance during the circulation of the cyclohexane oxidation process to adipic acid, and no additional initiator needs to be added in the subsequent circulation, further optimizing the process and reducing the production cost.

[0013] In the above technical solution, the peroxide is a peroxide containing cyclohexyl functional group, preferably the peroxide is one or both of cyclohexyl hydroperoxide and cyclohexanone peroxide.

[0014] In the above technical solution, the initiator is added to the reaction system and / or generated in situ during the reaction process.

[0015] In the above technical solution, the molar ratio of initiator to cyclohexane is 0.0002-0.08, more preferably the molar ratio of initiator to cyclohexane is 0.005-0.04.

[0016] In the above technical solution, the residence time of the reactants in the microchannel reactor during the reaction process is 0.15-13 min, preferably the residence time is 2-7 min.

[0017] In the above technical solution, the oxygen-containing gas is a gas with an oxygen content of 5-100%, preferably the oxygen-containing gas is one of air, oxygen-poor, oxygen-rich, and pure oxygen.

[0018] In the above technical solution, the molar ratio of oxygen to cyclohexane is 0.05-0.8, preferably 0.25-0.55.

[0019] In the above technical solution, the oxygen-containing gas can be introduced together with cyclohexane at the inlet of the microreaction channel, or can be introduced in segments at different parts of the microreaction channel.

[0020] In the technical solution, the oxygen partial pressure in the oxidation reaction is in the range of 0.1-1.5 MPa, and preferably the oxygen partial pressure is in the range of 0.5-0.9 MPa.

[0021] In the technical solution, the oxidation reaction temperature of cyclohexane is 105-150℃, and preferably the reaction temperature is 125-145℃.

[0022] In the technical solution, the reaction liquid after oxidation is cooled, and the solidified adipic acid is separated by solid-liquid separation. The cooling crystallization temperature is in the range of 0-42℃, and preferably the crystallization temperature is in the range of 8-30℃.

[0023] In the technical solution, the reaction liquid after solid-liquid separation is recycled in whole or in part, mixed with the supplemented cyclohexane, and subjected to the next round of oxidation reaction. The mass ratio of the recycled reaction liquid to the total reaction liquid is in the range of 70-100%.

[0024] The present application has the following beneficial effects:

[0025] (1) The present application uses a continuous flow microchannel reactor, and the reaction time is shortened from several hours in a traditional tank reactor to tens of seconds to several minutes, while the selectivity of adipic acid is improved, and the production efficiency is significantly improved.

[0026] (2) The present application uses a non-catalytic oxidation method for preparing adipic acid from cyclohexane, which saves the cost of solvent and catalyst, and the adipic acid is directly precipitated from the reaction liquid in solid form, and the initiator can be generated in situ and recycled with the reaction liquid, so that the initiator does not need to be added additionally during the reaction process.

[0027] (3) The process of the present application is a continuous process from feeding, preheating, gas-liquid mixing and reaction. Compared with the conventional batch oxidation process, the auxiliary device is reduced, the holdup of the reactor is greatly reduced during industrial implementation, and the process safety is improved. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor. Among them:

[0029] Figure 1 is a process flow diagram of the microchannel reactor of the present application for one-step oxidation of cyclohexane to adipic acid.

[0030] Figure 2 is an infrared spectrum of the adipic acid solid prepared in Example 1 of the present application and an adipic acid standard sample.

[0031] Figure 3 is the XRD spectrum of adipic acid solid prepared in Example 1 of the present application and adipic acid standard.

[0032] Figure 4 is the infrared spectrum of adipic acid solid prepared in Example 2 of the present application and adipic acid standard.

[0033] Figure 5 is the XRD spectrum of adipic acid solid prepared in Example 2 of the present application and adipic acid standard.

[0034] Figure 6 is the gas chromatogram of cyclohexane oxidation reaction liquid in Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0035] In order to make the above objectives, features and advantages of the present application more apparent, specific embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0036] In the following description, a lot of specific details are set forth in order to provide a thorough understanding of the present application, but the present application can be practiced in other different manners from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of the present application, therefore the present application is not limited to the specific embodiments disclosed below.

[0037] Secondly, "one embodiment" or "embodiment" referred to herein means that specific features, structures or characteristics can be included in at least one implementation of the present application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an embodiment that is independent of or selected from other embodiments.

[0038] Reference Signs List Figure 1 The micro-channel reactor of the present application is used in the process flow of one-step oxidation of cyclohexane to adipic acid, and the micro-channel reactor mainly consists of a preheater, a mixer and a micro-reaction channel, and the following steps are taken:

[0039] (1) first, cyclohexane containing an initiator and oxygen-containing gas are punched into the preheating module according to a certain proportion for preheating;

[0040] (2) the preheated reaction raw materials pass through the gas-liquid mixing module to achieve high dispersion of oxygen in cyclohexane;

[0041] (3) the mixed gas-liquid two-phase enters the micro-reaction channel module for oxidation reaction;

[0042] (4) the oxidation reaction liquid is finally cooled and crystallized to obtain adipic acid solid product, and the remaining reaction liquid is recycled and punched into the preheating module together with the fresh cyclohexane to be supplemented for the next round of reaction.

[0043] The content of cyclohexane, cyclohexanol, cyclohexanone, cyclohexyl adipate in the reaction solution was determined by gas chromatography analysis, and the content of adipic acid was determined by acid-base titration, and the related parameters were determined as follows:

[0044]

[0045] Example 1

[0046] The device used was a high-throughput microchannel reactor (including preheating, gas-liquid mixing, reaction module), which was referred to Figure 1 The connection mode of the microchannel reactor was determined, and the number of reaction modules was determined according to the flow rate and reaction residence time, and the heat transfer medium was heat conducting oil.

[0047] Cyclohexane containing 1% mass content of cyclohexanone peroxide and oxygen were pumped into each preheating module by a metering pump, and the molar ratio of oxygen to cyclohexane was 0.35;

[0048] The gas-liquid two-phase passed through the gas-liquid mixing module into the micro-reaction channel module, the reaction temperature was 130°C, the residence time was 5 min, and the reaction system pressure was controlled at 1.2 MPa by adjusting the back pressure valve at the outlet;

[0049] The collected reaction solution was cooled to 15°C, and after solid-liquid separation, the crude adipic acid solid was obtained, and the remaining cyclohexane reaction solution was recycled with the supplemented fresh cyclohexane for the next round of oxidation reaction.

[0050] The cyclohexane single-pass conversion rate, adipic acid selectivity, and cyclohexyl adipate selectivity of the first reaction and the subsequent nine cycles are shown in Table 1.

[0051] Table 1

[0052]

[0053] With the increase of the number of cycles, the cyclohexane single-pass conversion rate was stabilized at 14%, the adipic acid selectivity was stabilized at 92%, the cyclohexyl adipate selectivity was stabilized at 4%, and the crude adipic acid solid yield was stabilized at 12.9%.

[0054] The infrared spectrum and XRD spectrum of the crude adipic acid solid obtained after the ninth cycle are shown in Figure 2 , 3 It can be found that the infrared spectrum and XRD spectrum of the crude adipic acid are consistent with the spectrum of the adipic acid standard, Figure 1 indicating that the obtained adipic acid solid has high purity.

[0055] Example 2

[0056] The microchannel reactor used was the same as in Example 1.

[0057] Cyclohexane containing 1% mass content of cyclohexanone peroxide and oxygen were pumped into each preheater by metering pump, the molar ratio of oxygen to cyclohexane was 0.45;

[0058] The gas-liquid two-phase entered the micro-reaction channel through the gas-liquid mixer, the reaction temperature was 130℃, the residence time was 5 min, and the reaction system pressure was controlled to be 1.2 MPa by adjusting the back pressure valve at the outlet;

[0059] The collected reaction liquid was cooled to 15℃, and after solid-liquid separation, the crude adipic acid solid was obtained, and the remaining cyclohexane was recycled with the supplemented fresh cyclohexane for the next round of oxidation reaction.

[0060] The cyclohexane single-pass conversion rate, adipic acid selectivity, and cyclohexyl adipate selectivity of the first reaction and the subsequent nine cycles are listed in Table 2.

[0061] Table 2

[0062]

[0063] With the increase of the number of cycles, the cyclohexane single-pass conversion rate stabilized at 17%, the adipic acid selectivity stabilized at 91%, the cyclohexyl adipate selectivity stabilized at 4%, and the crude adipic acid solid yield stabilized at 15.5%.

[0064] The infrared spectrum and XRD spectrum of the crude adipic acid solid obtained after the ninth cycle are shown in Figure 4 , 5 It can be found that the infrared spectrum and XRD spectrum of the crude adipic acid are consistent with the spectrum of the adipic acid standard, Figure 1 indicating that the obtained adipic acid solid has high purity.

[0065] Example 3

[0066] The micro-channel reactor used was the same as in Example 1.

[0067] Cyclohexane containing 1% mass content of cyclohexyl hydroperoxide and oxygen were pumped into each preheater by metering pump, the molar ratio of oxygen to cyclohexane was 0.35;

[0068] The gas-liquid two-phase entered the micro-reaction channel module through the gas-liquid mixer, the reaction temperature was 130℃, the residence time was 5 min, and the reaction system pressure was controlled to be 1.2 MPa by adjusting the back pressure valve at the outlet;

[0069] The collected reaction liquid was cooled to 15℃, and after solid-liquid separation, the crude adipic acid solid was obtained, and the remaining cyclohexane was recycled with the supplemented fresh cyclohexane for the next round of oxidation reaction.

[0070] The cyclohexane single-pass conversion rate, adipic acid selectivity, and cyclohexyl adipate selectivity of the first reaction and the subsequent nine cycles are listed in Table 3.

[0071] Table 3

[0072]

[0073] The conversion of cyclohexane per pass stabilized at 12% and the selectivity of adipic acid and cyclohexyl adipate stabilized at 93% and 3%, respectively, and the solid yield of crude adipic acid stabilized at 11.2% as the number of cycles increased.

[0074] Example 4

[0075] Example 2 was repeated except that the molar ratio of oxygen to cyclohexane was changed to 0.25;

[0076] The conversion of cyclohexane was reduced to 8.1% in the first reaction;

[0077] The conversion of cyclohexane per pass gradually stabilized at 10.1% and the selectivity of adipic acid and cyclohexyl adipate were about 93% and 4%, respectively, in the subsequent nine cycles.

[0078] Example 5

[0079] Example 2 was repeated except that the reaction temperature was changed to 120°C;

[0080] The conversion of cyclohexane was 6.0% in the first reaction;

[0081] The conversion of cyclohexane per pass gradually stabilized at 8.4% and the selectivity of adipic acid was 93% in the subsequent nine cycles.

[0082] Example 6

[0083] Example 2 was repeated except that the reaction temperature was changed to 140°C;

[0084] The conversion of cyclohexane was 15.1% in the first reaction;

[0085] The conversion of cyclohexane per pass gradually stabilized at 18.4% and the selectivity of adipic acid was 86% in the subsequent nine cycles.

[0086] Example 7

[0087] Example 2 was repeated except that the residence time was changed to 2 min;

[0088] The conversion of cyclohexane was 6.3% in the first reaction;

[0089] The conversion of cyclohexane per pass gradually stabilized at 8.1% and the selectivity of adipic acid was 94% in the subsequent nine cycles.

[0090] Example 8

[0091] Example 2 was repeated, except that the residence time was changed to 7 min;

[0092] The first reaction had a cyclohexane conversion of 14.3%, and

[0093] The cyclohexane conversion of the subsequent nine cycles gradually stabilized at 17.3%, and the selectivity of adipic acid was 91%.

[0094] Comparative Example 1

[0095] Example 2 was repeated, except that a batch reactor was used.

[0096] Cyclohexane containing 1% cyclohexanone peroxide by mass was added to a high-pressure reaction kettle, and heated to 130°C;

[0097] Pure oxygen was then charged to a pressure of 1.2 MPa, and after 1 h of reaction, the reaction liquid was cooled to 15°C, and the oxygen was evacuated.

[0098] The cyclohexane oxidation had a single-pass conversion of 3.2%, and the selectivity of adipic acid was 12%, and no adipic acid solid crystallized out after cooling of the reaction liquid.

[0099] Comparative Example 2

[0100] Example 2 was repeated, except that no initiator was added.

[0101] The cyclohexane oxidation had a single-pass conversion of 1.0%, and the selectivity of adipic acid was 6%, and no adipic acid solid crystallized out after cooling of the reaction liquid, and the gas chromatography analysis results are shown in Table 1. Figure 6

[0102] Comparative Example 3

[0103] Example 2 was repeated, except that the initiator was selected to be hydrogen peroxide, and the content was 1% of the mass of cyclohexane (strong oxidant, control the amount of addition, otherwise there is a risk of explosion).

[0104] The cyclohexane oxidation had a single-pass conversion of 2.1%, and the selectivity of adipic acid was 22%, and no adipic acid solid crystallized out after cooling of the reaction liquid.

[0105] Comparative Example 4

[0106] Example 2 was repeated, except that the initiator was selected to be peracetic acid, and the content was 1% of the mass of cyclohexane (strong oxidant, control the amount of addition, otherwise there is a risk of explosion).

[0107] The cyclohexane oxidation had a single-pass conversion of 3.2%, and the selectivity of adipic acid was 26%, and no adipic acid solid crystallized out after cooling of the reaction liquid.

[0108] Comparative Example 5

[0109] ​Example 2 was repeated except that cyclohexanone was used as initiator, and the content was 1% of the mass of cyclohexane;

[0110] The single-pass conversion rate of cyclohexane was 2.4%, the selectivity of adipic acid was 17%, and no solid crystallization of adipic acid was precipitated after cooling the reaction solution.

[0111] Comparative Example 6

[0112] Example 2 was repeated except that the crystallization temperature of the reaction solution after cooling was changed to 50°C, and the single-pass conversion rate of cyclohexane, the selectivity of adipic acid, and the selectivity of cyclohexyl adipate in the first reaction and the subsequent five cycles were listed in Table 4.

[0113] Table 4

[0114]

[0115] It can be found that the single-pass conversion rate of cyclohexane was reduced to about 9%, the selectivity of adipic acid was about 93%, the selectivity of cyclohexyl adipate was about 4%, and the yield of crude adipic acid solid was about 8.4%.

[0116] Comparative Example 7

[0117] Example 2 was repeated except that acetic acid was introduced as a solvent, and the volume ratio of the solvent to cyclohexane was 5:1.

[0118] The reaction results showed that the conversion rate of cyclohexane was only 4.3%, the selectivity of adipic acid was 20%, and no solid crystallization of adipic acid was precipitated after cooling the reaction solution.

[0119] Comparative Example 8

[0120] Example 2 was repeated except that cyclohexyl acetate was introduced as a solvent, and the volume ratio of the solvent to cyclohexane was 5:1.

[0121] The reaction results showed that the conversion rate of cyclohexane was only 3.9%, the selectivity of adipic acid was 21%, and no solid crystallization of adipic acid was precipitated after cooling the reaction solution.

[0122] This may be because the introduction of the solvent causes the peroxide cyclohexanone to be unstable and decompose prematurely, thereby weakening its ability to initiate the oxidation of cyclohexane;

[0123] On the other hand, the conversion rate of cyclohexane in the subsequent cycles was only 4.1%, which may be because the solvent hinders the generation and stable existence of peroxide cyclohexanone and free radicals during the cooling process.

[0124] It should be noted that the above examples are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalently replaced, without departing from the spirit and scope of the present application, which should be covered in the scope of the present application.

Claims

1. A method for preparing adipic acid using a microchannel reactor, characterized in that: include, Cyclohexane containing an initiator and oxygen-containing gas are added to a microchannel reactor and undergo an oxidation reaction in the absence of solvent and catalyst to produce adipic acid. The initiator includes free radicals or peroxides containing cyclohexyl functional groups.

2. The method as described in claim 1, characterized in that: The peroxide is a peroxide containing a cyclohexyl functional group, including one or both of cyclohexanone peroxide and cyclohexyl hydrogen peroxide.

3. The method as described in claim 2, characterized in that: The peroxide comprises a mixture of cyclohexyl hydrogen peroxide and cyclohexanone peroxide, wherein the molar ratio of the two is 0.01 to 100:

1.

4. The method according to any one of claims 1 to 3, characterized in that: The molar ratio of the initiator to cyclohexane is 0.0002 to 0.08:

1.

5. The method as described in claim 4, characterized in that: The residence time of the reactants in the microchannel reactor during the reaction process is 0.15~13 min.

6. The method as described in claim 5, characterized in that: The oxygen-containing gas is a gas with an oxygen volume content of 5-100%, including air, oxygen-deficient, oxygen-rich, and pure oxygen.

7. The method as described in claim 1, 5, or 6, characterized in that: The molar ratio of oxygen to cyclohexane is 0.05~0.8:

1.

8. The method as described in claim 7, characterized in that: The partial pressure of oxygen in the oxidation reaction ranges from 0.1 to 1.5 MPa, and the oxidation temperature of cyclohexane is 105 to 150 °C.

9. The method as described in claim 1 or 8, characterized in that: It also includes cooling the oxidized reaction solution and performing solid-liquid separation on the crystallized adipic acid solid, wherein the cooling temperature of the oxidized reaction solution is 0~42℃.

10. The method as described in claim 9, characterized in that: The reaction liquid after solid-liquid separation is recycled in whole or in part, and mixed with replenished cyclohexane for the next round of oxidation reaction.

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