A method for continuously electro-oxidizing cyclohexanone droplets in a microfluidic system to produce adipic acid

Abstract

The application belongs to the technical field of organic electrochemical synthesis, and discloses a method for continuously electro-oxidizing cyclohexanone droplet microfluidic to adipic acid, which comprises the following steps: introducing an aqueous solution containing electrolyte as a continuous phase and cyclohexanone as a dispersed phase into a droplet microfluidic electrochemical reactor; controlling the flow rates of the continuous phase and the dispersed phase, so that the cyclohexanone is wrapped by the continuous aqueous phase to form an oil-in-water droplet flow pattern in the cross-shaped microchannel; the formed aqueous phase liquid film enables the continuous aqueous phase to continuously contact the surface of the working electrode; during the flowing process, the cyclohexanone is dissolved into the continuous aqueous phase through the droplet interface and migrates to the vicinity of the working electrode; after the flow pattern is stabilized, a constant current or a constant potential is applied to the droplet microfluidic electrochemical reactor, and the cyclohexanone dissolved into the aqueous phase is oxidized on the surface of the electrode to generate adipic acid. The application can improve the supply capacity of the cyclohexanone on the reaction interface, strengthen the interface mass transfer, inhibit the competitive side reaction, and improve the faradic efficiency and the space-time yield of the adipic acid.

Description

Technical Field

[0001] This invention belongs to the field of organic electrochemical synthesis technology, specifically, it relates to a method for the continuous electro-oxidation of cyclohexanone to produce adipic acid. Background Technology

[0002] Adipic acid is an important raw material for the production of nylon 66, polyurethane, plasticizers, and various fine chemicals, and has broad industrial application value. In recent years, the electrochemical method for preparing adipic acid by using renewable electricity to drive the electro-oxidation of cyclohexanone has attracted attention due to its mild conditions, controllable process, and potential for green manufacturing.

[0003] However, cyclohexanone has low solubility in conventional aqueous electrolytes, making it difficult to meet the substrate supply rate requirements of efficient continuous electro-oxidation processes. Existing processes typically employ a single-phase premixed feed method, where cyclohexanone is first dissolved in an aqueous phase containing electrolyte, and then the resulting single-phase system is continuously introduced into the electrochemical reactor for reaction. This method is directly limited by the upper limit of the thermodynamic solubility of cyclohexanone in the aqueous phase, resulting in a limited initial substrate concentration. During continuous electro-oxidation, as cyclohexanone is continuously consumed at the anode interface, when the diffusion replenishment rate of the bulk liquid phase to the boundary layer near the anode is lower than its electrochemical consumption rate, the concentration of cyclohexanone near the electrode drops rapidly, and the system enters a state of significant mass transfer limitation. Under these conditions, the target oxidation reaction is inhibited, while competing side reactions are further enhanced, leading to a decrease in adipic acid Faraday efficiency, poor selectivity, and limited reactor capacity per unit volume.

[0004] To address these issues, existing methods typically attempt to improve the apparent dispersion or solubility of the substrate in the system by adding co-solvents, surfactants, or increasing mixing intensity. However, these methods often increase the difficulty of subsequent separation and recovery, raise process complexity and operating costs, and are not conducive to the engineering scale-up of continuous electrolysis processes.

[0005] Therefore, how to improve the reaction interface supply capacity of cyclohexanone in the continuous electro-oxidation process without significantly increasing the system complexity and post-processing burden, alleviate the problem of mass transfer limitation and enhanced competing side reactions caused by low solubility, and thus improve the Faraday efficiency and continuous production capacity of adipic acid, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the limitations of existing single-phase premixed feed continuous electro-oxidation methods, which are constrained by the upper limit of the static solubility of cyclohexanone in the aqueous phase, leading to insufficient substrate supply at the reaction interface, enhanced competing side reactions, and decreased Faradaic efficiency of adipic acid during continuous conversion, this invention provides a method for the continuous electro-oxidation of adipic acid using cyclohexanone droplets in a microfluidic system. This method directly and continuously introduces cyclohexanone as a dispersed phase in its original liquid form into the droplet microfluidic electrochemical system. The cyclohexanone in the dispersed phase continuously dissolves at the droplet interface and enters the continuous aqueous phase, further continuously feeding it to the boundary layer near the anode. This improves the cyclohexanone supply capacity at the reaction interface, enhances interfacial mass transfer, suppresses competing side reactions, and increases the Faradaic efficiency and space-time yield of adipic acid.

[0007] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0008] This invention provides a method for the continuous electro-oxidation of cyclohexanone droplets to adipic acid using microfluidic technology, based on a droplet microfluidic electrochemical reactor. The droplet microfluidic electrochemical reactor includes a counter electrode chamber flow channel plate, a counter electrode, an ion exchange membrane, a working electrode, and a working electrode chamber flow channel plate. The surface of the working electrode chamber flow channel plate is provided with cross-shaped microchannels, the four ports of which are two continuous phase inlets, one dispersed phase inlet, and one outlet. The two continuous phase inlets are arranged opposite each other, as are the dispersed phase inlet and the outlet. The surface of the cross-shaped microchannels is covered with an inorganic hydrophilic thin film layer.

[0009] include:

[0010] Step S1: An aqueous solution containing electrolytes is introduced into the cross-shaped microchannel as a continuous phase through the continuous phase inlet, and cyclohexanone is introduced into the dispersed phase through the dispersed phase inlet.

[0011] Step S2: Due to the extremely hydrophilic surface of the cross-shaped microchannel and the symmetrical shearing of the dispersed phase by the continuous phase due to the cross-shaped structure of the cross-shaped microchannel, the flow rates of the continuous phase and the dispersed phase are controlled to form an oil-in-water droplet flow pattern in the cross-shaped microchannel, in which cyclohexanone is wrapped by the continuous aqueous phase. The aqueous phase film formed thereby allows the continuous aqueous phase to be in continuous contact with the surface of the working electrode. During the flow of the oil-in-water droplet flow pattern, cyclohexanone dissolves into the continuous aqueous phase through the droplet interface and migrates to the vicinity of the working electrode.

[0012] Step S3: After the water-in-oil droplet flow pattern stabilizes, a constant current or constant potential is applied to the droplet microfluidic electrochemical reactor, and the cyclohexanone dissolved in the aqueous phase undergoes an oxidation reaction on the working electrode surface to generate adipic acid.

[0013] Step S4: Collect the reaction liquid flowing out of the outlet and separate it to obtain adipic acid.

[0014] Preferably, in the cross-shaped microchannel: the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, are all of the same length and are shorter than the length of the branch between the liquid outlet and the cross intersection.

[0015] Preferably, in the cross-shaped microchannel: the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, have a width of 0.2-1.0 mm; the branch between the liquid outlet and the cross intersection has a width of 0.2-1.6 mm; and the depth of the cross-shaped microchannel is 0.2-1.6 mm.

[0016] Furthermore, both the counter electrode chamber flow channel plate and the working electrode chamber flow channel plate are high-transparency polymers, using at least one of polymethyl methacrylate or polycarbonate.

[0017] Furthermore, both the counter electrode and the working electrode are made of conductive material with flat surfaces, and are led outward through metal current collectors to achieve external power supply.

[0018] Furthermore, gaskets are respectively provided between the counter electrode chamber flow channel plate and the counter electrode, and between the working electrode and the working electrode chamber flow channel plate. The gaskets are made of PTFE material and are corrosion resistant.

[0019] Furthermore, the surface water contact angle of the cross-shaped microchannel after surface modification with an inorganic hydrophilic film layer is less than 50°.

[0020] Preferably, the inorganic hydrophilic film layer is a transparent amorphous silicon dioxide film.

[0021] More preferably, the inorganic hydrophilic thin film layer is prepared by physical vapor deposition using magnetron sputtering.

[0022] More preferably, the preparation method using magnetron sputtering physical vapor deposition includes: placing the working electrode chamber flow channel plate having the cross-shaped microchannels in the magnetron sputtering deposition chamber, using 99.999% pure silicon dioxide as the sputtering target, and evacuating the base vacuum of the magnetron sputtering deposition chamber to 6 × 10⁻⁶. -4 Under conditions of Pa below 30-50 sccm, high-purity argon gas flow rate stable at 30-50 sccm, and working pressure maintained at 1.0-4.0 Pa, sputter continuously for 5-30 min with 50-200 W RF power.

[0023] Preferably, in step S1, the continuous phase is an aqueous solution containing an alkaline electrolyte, wherein the alkaline electrolyte is selected from at least one of KOH, NaOH, K2CO3, and Na2CO3, and the concentration of the alkaline electrolyte is 0.1-2.0 M.

[0024] Preferably, in step S2, the volumetric flow rate ratio of the continuous phase to the dispersed phase is 4:1 to 4:7, more preferably 2:1 to 2:3.

[0025] Preferably, in step S2, the total flow rate of the branch between the continuous phase inlet and the cross-shaped microchannel and the branch between the dispersed phase inlet and the cross-shaped microchannel is 0.1–1.00 mL / min, more preferably 0.2–0.8 mL / min.

[0026] Preferably, the unreacted cyclohexanone in the reaction solution obtained in step S4 is used as the raw material in step S1.

[0027] Compared with existing single-phase premixed feed continuous electro-oxidation processes, the present invention has at least the following beneficial effects:

[0028] (i) Based on the cross-shaped structure of the cross-shaped microchannel and the inorganic hydrophilic film layer on its surface, this invention can force the aqueous continuous phase to preferentially spread and wet the entire cross-shaped microchannel and the surface of the working electrode. The organic dispersed phase is subjected to highly symmetrical hydrodynamic shearing action of the aqueous continuous phase in the cross-shaped microchannel, stably generating uniformly sized oil-in-water (O / W) discrete droplets. Thus, the aqueous continuous phase spread on the surface of the working electrode constructs a physically isolated aqueous liquid film, blocking the direct contact and disordered adhesion between the insulating organic phase droplets and the electrochemical reaction interface, eliminating the resistance change and electrode passivation caused by the organic insulating layer, ensuring the long-term stability of the reactor tank voltage, and providing the basic conditions for the electrochemical reaction.

[0029] (ii) The present invention adopts a cyclohexanone dispersion feeding method, which avoids the problem of single-phase premixed feeding being limited by the upper limit of initial solubility, and is conducive to improving the cyclohexanone supply capacity of the reaction interface.

[0030] (iii) The present invention utilizes the high specific interface area and short mass transfer path formed by the droplet microfluidic system to enable cyclohexanone to continuously dissolve at the droplet interface and replenish the boundary layer near the anode, thereby enhancing the interface mass transfer.

[0031] (iv) This invention helps to alleviate the problems of anodic polarization and enhanced competing side reactions caused by insufficient cyclohexanone, thereby improving the Faraday efficiency and selectivity of adipic acid.

[0032] (v) This invention combines the enhanced features of droplet microfluidic interface with the cyclohexanone dispersion feeding method, which is beneficial to maintain good adipic acid generation performance at a high operating current density and is suitable for continuous operation. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 (a) is a schematic diagram of the droplet microfluidic electrochemical reactor of the present invention, and (b) is a schematic diagram of the cross-shaped microchannel structure used for droplet formation; wherein, the reference numerals in the figures are as follows:

[0035] 1: Counter electrode chamber flow channel plate; 2: Counter electrode; 3: Gasket; 4: Ion exchange membrane; 5: Working electrode; 6: Working electrode chamber flow channel plate; 7: Cross-shaped microchannel; 701: Dispersed phase inlet; 702: Continuous phase inlet; 703: Liquid outlet; 704: Cross-shaped intersection.

[0036] Figure 2 This is a comparison graph showing the change of adipic acid Faraday efficiency with current density under single-phase premixed feeding conditions with different initial cyclohexanone concentrations in Example 1.

[0037] Figure 3 This is an optical photograph of the droplet flow pattern formed under the cyclohexanone dispersion feeding conditions in Example 2.

[0038] Figure 4 This is a schematic diagram illustrating the mechanism by which cyclohexanone droplets continuously feed into the boundary layer near the anode through interfacial dissolution in Example 2.

[0039] Figure 5 This is a comparison graph of the adipic acid Faraday efficiency of the cyclohexanone dispersion feeding method and the single-phase premixed feeding method in Example 2 at different current densities.

[0040] Figure 6 This is a comparison graph of the space-time yield of adipic acid under different current densities between the cyclohexanone dispersion feeding method and the single-phase premixed feeding method in Example 2.

[0041] Figure 7 In Example 3, at 10 mA cm -2 A comparison of the effects of different continuous phase to dispersed phase volume flow ratios on the adipic acid Faraday efficiency under various conditions.

[0042] Figure 8 In Example 4, at 10 mA cm-2 A comparison of the effects of different total flow rates on the adipic acid Faraday efficiency under the given conditions.

[0043] Figure 9 The graph shows the results of the continuous operation stability test of the cyclohexanone dispersion feeding method in Example 5. Detailed Implementation

[0044] To further understand the content, features, and effects of this invention, the following embodiments are provided, and detailed descriptions are given below in conjunction with the accompanying drawings:

[0045] like Figure 1 As shown in (a), the present invention uses a droplet microfluidic electrochemical reactor to prepare adipic acid by electrooxidation of cyclohexanone. The droplet microfluidic electrochemical reactor includes a counter electrode chamber flow channel plate 1, a counter electrode 2, a gasket 3, an ion exchange membrane 4, a working electrode 5, and a working electrode chamber flow channel plate 6.

[0046] High-transparency polymethyl methacrylate (PMMA) was selected as the substrate for the counter electrode chamber flow channel plate 1 and the working electrode chamber flow channel plate 6. A conventional straight-line microchannel was engraved on the counter electrode chamber flow channel plate 1 using precision CNC machining; a cross-shaped microchannel 7 was engraved at the corresponding position on the working electrode chamber flow channel plate 6.

[0047] like Figure 2 As shown in Figure (a), the four ports of the cross-shaped microchannel 7 are a dispersed phase inlet 701, two continuous phase inlets 702, and a liquid outlet 703. The two continuous phase inlets 702 are arranged opposite each other, and the dispersed phase inlet 701 and the liquid outlet 703 are arranged opposite each other.

[0048] In some preferred embodiments, the branch length between a dispersed phase inlet 701 and the cross intersection 704, and the branch length between two continuous phase inlets 702 and the cross intersection 704 are the same and less than the branch length between an outlet 703 and the cross intersection 704.

[0049] In some preferred embodiments, to ensure a more precise distribution of shear and flow focusing forces of the two-phase fluid in the cross-shaped microchannel 7, the branch width between the dispersed phase inlet 701 and the cross-shaped intersection 704 is set to 0.2-1.0 mm, the branch width between the two continuous phase inlets 702 and the cross-shaped intersection 704 is set to 0.2-1.0 mm, and the branch width between the outlet 703 and the cross-shaped intersection 704 is set to 0.2-1.6 mm; and the uniform depth of the four branches of the cross-shaped microchannel 7 is controlled within 0.2-1.6 mm; the dimensional tolerances of width and depth are strictly controlled within the range of ±10 to 20 μm.

[0050] After ultrasonic cleaning and drying, the PMMA substrate with the working electrode chamber flow channel plate 6 engraved with cross-shaped microchannels 7 was placed in an RF magnetron sputtering deposition chamber. Pure silica was used as the target material for magnetron sputtering physical vapor deposition. The specific process parameters were precisely controlled as follows: Before starting deposition, the base vacuum of the deposition chamber was evacuated to 6 × 10⁻⁶. -4 Below Pa; then high-purity argon (Ar) is introduced as the working discharge gas, with the flow rate stabilized at 30-50 sccm; the working gas pressure inside the deposition chamber is locked at 1.0-4.0 Pa; the output power of the RF DC power supply is set to 50-200 W; the continuous sputtering time is 5-30 min.

[0051] After the above magnetron sputtering treatment, a silica film is formed on the surface of the cross-shaped microchannel 7. The droplet contact angle precision tester shows that after this sputtering treatment, the water contact angle of the surface of the cross-shaped microchannel 7 is sharply reduced from 95.6° (hydrophobic state) of the intrinsic polymer of the substrate to 13.4° (extremely hydrophilic state), successfully reversing the hydrophilicity of the surface of the cross-shaped microchannel 7.

[0052] All components are assembled in the following order: counter electrode chamber flow channel plate 1, counter electrode 2, gasket 3, ion exchange membrane 4, gasket 3, working electrode, and hydrophilically treated working electrode chamber flow channel plate 6. During assembly, the electrode assembly sandwiched between the counter electrode chamber flow channel plate 1 and the working electrode chamber flow channel plate 6 uses flat-surfaced hydrophilic carbon paper as the counter electrode 2 and working electrode 5. This flatness ensures a tight fit between the working electrode 5 and the edge of the cross-shaped microchannel 7, avoiding microscopic leakage or localized flow field disturbances caused by surface undulations. To achieve low-impedance external potential connection, a connection scheme is adopted where copper tape is attached to the edges of the counter electrode 2 and working electrode 5.

[0053] To address the extremely high internal pressure drop caused by continuous two-phase flow in the microfluidic system, corrosion-resistant PTFE gaskets 3 with a thickness of 0.2 mm were embedded between the counter electrode chamber flow channel plate 1 and the counter electrode 2, and between the working electrode chamber flow channel plate 6 and the working electrode 5. An assembly torque of 1.2 N·m was uniformly applied using a torque wrench through a network of M5 mechanical fastening bolts distributed around the droplet microfluidic electrochemical reactor. Extreme fluid dynamics tests showed that this encapsulation system could withstand pressure drops up to 0.2 mL / min. -1 At a total injection flow rate, it can smoothly withstand an internal pressure drop of 0.4 MPa without any leakage.

[0054] The present invention will be further described below with reference to specific embodiments and comparative experimental data.

[0055] To eliminate the influence of reactor structure, electrode material and operating conditions on the result judgment, Example 1 and Example 2 used the same reactor type, the same electrode system, the same electrolyte composition, the same temperature conditions and the same product analysis method, only changing the cyclohexanone feeding method.

[0056] Example 1 (Comparative Example): Continuous Electro-oxidation Test of Single-Phase Premixed Feed

[0057] This embodiment uses a droplet microfluidic electrochemical reactor and closes the continuous phase inlet 702, feeding only the dispersed phase inlet 701.

[0058] Cyclohexanone was selected as the substrate, nickel hydroxide / carbon paper was selected as the working electrode 5, and platinum / carbon paper was selected as the counter electrode 2. Single-phase premixed feed systems with initial cyclohexanone concentrations of 0.1 M, 0.2 M, and 0.3 M were prepared using an aqueous solution containing 1.0 M KOH as the electrolyte, and continuous constant current electrolysis was carried out at room temperature.

[0059] like Figure 2 As shown, the test results indicate that as the current density increases from 5 mA cm⁻¹ -2 Increased to 30 mA cm -2 The Faradaic efficiency of adipic acid shows a decreasing trend; among them, the initial cyclohexanone concentration is 0.3 M and the current density is 10 mA cm⁻¹. -2 At that time, the adipic acid Faraday efficiency was 52%. These results indicate that the single-phase premixed feed method is limited by the upper limit of the thermodynamic solubility of cyclohexanone in the aqueous phase. At higher reaction rates, insufficient substrate supply at the reaction interface easily occurs, leading to suppression of the target reaction and enhanced competing side reactions. However, since the upper limit of the thermodynamic solubility of cyclohexanone in the aqueous phase is 0.3 M, it is impossible to enhance mass transfer in the reaction process by further increasing the initial cyclohexanone concentration.

[0060] Example 2: Construction of a cyclohexanone dispersion feed droplet pattern, and performance comparison between cyclohexanone dispersion feed and single-phase premixed feed.

[0061] The same droplet microfluidic electrochemical reactor and electrode system as in Example 1 were used, except that the feeding method was changed from single-phase premixed feeding to cyclohexanone dispersion feeding. That is, cyclohexanone was continuously introduced into the droplet microfluidic electrochemical reactor through the dispersed phase inlet 701; and an aqueous solution containing 1.0 M KOH was used as the continuous phase and continuously introduced into the droplet microfluidic electrochemical reactor through the continuous phase inlet 702.

[0062] By adjusting the flow rates of the continuous aqueous phase and the cyclohexanone dispersed phase, a stable oil-in-water droplet flow pattern is formed in the continuous aqueous phase, such as... Figure 3As shown. In this embodiment, the volumetric flow rate ratio of the continuous phase to the dispersed phase is 1:1, and the total flow rate is 0.4 mL / min.

[0063] In this feeding mode, the local concentration of cyclohexanone in the droplets is significantly higher than its thermodynamic equilibrium concentration in the continuous aqueous phase, thus forming a persistent concentration gradient at the interface between the droplets and the continuous aqueous phase. This promotes the continuous dissolution of cyclohexanone into the continuous aqueous phase during the reaction, and further feeds it to the boundary layer near the anode. The reaction mechanism is as follows: Figure 4 As shown.

[0064] The performance of the cyclohexanone dispersion-feed continuous electro-oxidation method constructed in Example 2 was tested under the same reactor specifications, electrode materials, electrolyte composition, and product detection conditions as in Example 1.

[0065] like Figure 5 As shown, at a current density of 10 mA cm⁻¹ -2 Under the specified conditions, after continuous operation for 1 hour using a cyclohexanone dispersion feeding method, the Faradaic efficiency of adipic acid was 76%; in contrast, the Faradaic efficiency of adipic acid in the single-phase premixed feeding system in Example 1 was 52% under the same conditions. The results indicate that the cyclohexanone dispersion feeding method can significantly improve the Faradaic efficiency of adipic acid.

[0066] Furthermore, such as Figure 6 As shown, at a current density of 5 mA cm⁻¹ -2 ~30 mA cm -2 Within a certain range, the space-time yield of adipic acid under two feeding methods was compared. The results showed that the space-time yield of adipic acid under the cyclohexanone dispersion feeding method increased steadily with increasing current density, which was significantly better than that of the single-phase premixed feeding system.

[0067] The above results demonstrate that the present invention, by employing a cyclohexanone dispersion feeding method in a droplet microfluidic electrochemical reactor, enables cyclohexanone to continuously dissolve at the droplet interface and enter the continuous aqueous phase, and further continuously feed to the boundary layer near the anode. This effectively alleviates the mass transfer bottleneck problem caused by the static solubility limitation of cyclohexanone in traditional single-phase premixed feeding processes, thereby improving the Faradaic efficiency and space-time yield of adipic acid. Furthermore, comparative results under different continuous phase to dispersed phase flow ratios show that the flow ratio affects the continuous feeding capacity of the system and the Faradaic efficiency of the target product to a certain extent.

[0068] Example 3: Effect of different continuous phase to dispersed phase volumetric flow rates on Faraday efficiency

[0069] Under the same reactor specifications, electrode materials, electrolyte composition, temperature conditions, current density conditions, and total flow rate as in Example 2, only the volumetric flow rate ratio (Q) of the continuous phase to the dispersed phase was changed.w :Q o The effect of this on the Faraday efficiency of adipic acid was investigated.

[0070] At 10 mA cm -2 Under these conditions, the adipic acid Faraday efficiency of the system was investigated when the volumetric flow rates of the continuous phase to the dispersed phase were 4:1, 2:1, 1:1, 2:3, and 4:7, respectively. Figure 7 As shown, within the investigated flow ratio range, the system can achieve a stable water-in-oil droplet flow pattern; among which, the Faraday efficiency is higher than 75% when the flow ratio is 2:1-2:3, which is the preferred flow ratio in this invention. This result indicates that the flow ratio of the continuous phase to the dispersed phase not only affects the stability of the droplet flow pattern but also the ability of cyclohexanone to continuously feed to the continuous aqueous phase and the boundary layer near the anode via the droplet interface, thus affecting the Faraday efficiency of adipic acid.

[0071] Example 4: The effect of different total flow velocities on Faraday efficiency

[0072] Under the same reactor specifications, electrode materials, electrolyte composition, temperature conditions, current density conditions, and flow rate ratio as in Example 2, only the total flow rate was changed to investigate its effect on the adipic acid Faraday efficiency.

[0073] At 10 mA cm -2 Under these conditions, the adipic acid faradaic efficiency of the system was investigated at total flow rates of 0.1, 0.2, 0.5, 0.8, and 1.0 mL / min. For example... Figure 8 As shown, within the investigated total flow rate range, the system can achieve a stable water-in-oil droplet flow pattern; among which, the Faraday efficiency is higher than 75% when the total flow rate is 0.2-0.8, which is the preferred total flow rate in this invention. This result indicates that the total flow rate not only affects the stability of the droplet flow pattern but also the ability of cyclohexanone to continuously feed to the continuous aqueous phase and the boundary layer near the anode via the droplet interface, thus affecting the Faraday efficiency of adipic acid.

[0074] Example 5: Continuous Operation Stability Verification

[0075] At 10 mA cm -2 The method of the present invention was subjected to long-term continuous operation testing. During the test, the changes in reactor operating voltage, current response, adipic acid Faraday efficiency, and space-time yield over operating time were recorded.

[0076] like Figure 9As shown, the results indicate that the operating voltage of the system remained stable at 1.6 V during continuous operation for 12 h. This demonstrates that in the O / W droplet microfluidic system, the continuous aqueous phase spread on the surface of the working electrode 5 constitutes a stable physically isolating liquid film, effectively preventing the disordered adhesion and coverage of the insulating cyclohexanone droplets to the electrochemically active interface. Simultaneously, in terms of activity, the adipic acid Faraday efficiency remained at 75%–80%. These results demonstrate that the method of this invention not only improves the reaction performance of continuous electrooxidation of cyclohexanone to adipic acid but is also suitable for continuous and stable operation.

[0077] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many specific modifications under the guidance of the present invention without departing from the spirit of the invention and the scope of protection of the claims, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A method for the continuous electro-oxidation of cyclohexanone droplets to adipic acid using microfluidic technology, characterized in that, This is based on a droplet microfluidic electrochemical reactor, which includes a counter electrode chamber flow channel plate, a counter electrode, an ion exchange membrane, a working electrode, and a working electrode chamber flow channel plate. The surface of the working electrode chamber flow channel plate is provided with cross-shaped microchannels, the four ports of which are two continuous phase inlets, one dispersed phase inlet, and one outlet. The two continuous phase inlets are arranged opposite to each other, as are the dispersed phase inlet and the outlet. The surface of the cross-shaped microchannels is covered with an inorganic hydrophilic thin film layer. include: Step S1: An aqueous solution containing electrolytes is introduced into the cross-shaped microchannel as a continuous phase through the continuous phase inlet, and cyclohexanone is introduced into the dispersed phase through the dispersed phase inlet. Step S2: Due to the extremely hydrophilic surface of the cross-shaped microchannel and the symmetrical shearing of the dispersed phase by the continuous phase due to the cross-shaped structure of the cross-shaped microchannel, the flow rates of the continuous phase and the dispersed phase are controlled to form an oil-in-water droplet flow pattern in the cross-shaped microchannel, in which cyclohexanone is wrapped by the continuous aqueous phase. The aqueous phase film formed thereby allows the continuous aqueous phase to be in continuous contact with the surface of the working electrode. During the flow of the oil-in-water droplet flow pattern, cyclohexanone dissolves into the continuous aqueous phase through the droplet interface and migrates to the vicinity of the working electrode. Step S3: After the water-in-oil droplet flow pattern stabilizes, a constant current or constant potential is applied to the droplet microfluidic electrochemical reactor, and the cyclohexanone dissolved in the aqueous phase undergoes an oxidation reaction on the working electrode surface to generate adipic acid. Step S4: Collect the reaction liquid flowing out of the outlet and separate it to obtain adipic acid.

2. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, In the cross-shaped microchannel: the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, are all of the same length and are shorter than the length of the branch between the liquid outlet and the cross intersection.

3. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, In the cross-shaped microchannel: the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection, each have a width of 0.2-1.0 mm; the branch between the outlet and the cross intersection has a width of 0.2-1.6 mm; and the depth of the cross-shaped microchannel is 0.2-1.6 mm.

4. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, Both the counter electrode chamber flow channel plate and the working electrode chamber flow channel plate are made of highly transparent polymers, using at least one of polymethyl methacrylate or polycarbonate; both the counter electrode and the working electrode are made of conductive materials with flat surfaces, and are led outward through metal current collectors to achieve external power supply; gaskets are respectively provided between the counter electrode chamber flow channel plate and the counter electrode, and between the working electrode and the working electrode chamber flow channel plate, and the gaskets are made of PTFE anti-corrosion gaskets.

5. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, The surface water contact angle of the cross-shaped microchannel after surface modification with an inorganic hydrophilic thin film layer is less than 50°.

6. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, The inorganic hydrophilic film layer is a transparent amorphous silicon dioxide film.

7. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic control according to claim 1, characterized in that, The inorganic hydrophilic thin film layer was prepared by physical vapor deposition using magnetron sputtering.

8. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, In step S1, the continuous phase is an aqueous solution containing an alkaline electrolyte, wherein the alkaline electrolyte is selected from at least one of KOH, NaOH, K2CO3, and Na2CO3, and the concentration of the alkaline electrolyte is 0.1-2.0 M.

9. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, In step S2, the volumetric flow rate ratio of the continuous phase to the dispersed phase is 4:1 to 4:

7.

10. The method for continuous electro-oxidation of cyclohexanone droplets to adipic acid via microfluidic micro-oxidation according to claim 1, characterized in that, In step S2, the total flow rate of the branch between the continuous phase inlet and the cross intersection, and the branch between the dispersed phase inlet and the cross intersection in the cross-shaped microchannel is 0.1–1.00 mL / min.

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