An apparatus and method for the co-conversion of carbon dioxide and lignin to prepare organic acids.

Abstract

This invention discloses an apparatus and method for the synergistic conversion of carbon dioxide and lignin to prepare organic acids. The apparatus includes: a reaction chamber for containing a lignin reaction solution; a gas inlet unit for introducing carbon dioxide-containing gas into the reaction chamber and forming microbubbles; and a plasma generation unit for generating non-thermal plasma at the interface between the microbubbles and the lignin reaction solution inside the reaction chamber, enabling simultaneous carbon dioxide reduction and lignin oxidation to generate organic acids. This apparatus uses carbon dioxide and lignin as the main green carbon sources, achieving efficient conversion and preparation of organic acids without the need for external photocatalysts, metal catalysts, or sacrificial agents. It produces few byproducts, has a simple system, low energy consumption, and simultaneously realizes the resource utilization of carbon dioxide and the high-value conversion of biomass, conforming to the concepts of green chemistry and carbon neutrality.

Description

Technical Field

[0001] This invention belongs to the field of nonthermal plasma chemistry and carbon resource utilization technology, specifically relating to an apparatus and method for the synergistic conversion of carbon dioxide and lignin to prepare organic acids. Background Technology

[0002] Organic acids, such as oxalic acid, formic acid, and acetic acid, are important platform chemicals, widely used in fine chemicals, pharmaceutical synthesis, food additives, and metal processing. Currently, industrial applications mainly rely on petrochemical routes or strong oxidation processes, which generally suffer from harsh reaction conditions, high energy consumption, numerous byproducts, and severe environmental pollution, making it difficult to meet the demands of green and low-carbon development.

[0003] In recent years, non-thermal plasma technology has been widely explored for the catalytic reduction of carbon dioxide (CO2) due to its ability to efficiently generate active electrons, free radicals, and excited-state species at ambient temperature and pressure. However, existing plasma CO2-H2O systems mainly produce C1 products (such as formic acid and carbon monoxide), exhibiting low carbon-carbon coupling efficiency and making it difficult to efficiently synthesize C2 and higher organic acids. Meanwhile, lignin, as the most abundant renewable aromatic polymer in nature, has a complex structure and diverse composition, and its direct utilization faces problems such as poor solubility and complex reaction pathways. Therefore, in existing research and process validation, representative lignin model compounds (such as guaiacol and vanillic acid) are typically used to characterize its reaction behavior. However, in advanced oxidation or plasma treatment, these model compounds are mostly mineralized into CO2 and water, or converted into low-value-added aromatic oxidation products, and no efficient conversion pathway for aliphatic organic acids has yet been established.

[0004] It is worth noting that CO2 reduction requires electron donors, while lignin oxidation provides both electrons and active intermediates. Theoretically, if these two processes could be coupled in a specific reaction environment, an internal cycle of electron transfer could be achieved, promoting C–C bond formation and increasing the yield of organic acids. However, current technologies lack reactor design and process methods capable of simultaneously controlling gas-liquid-solid multiphase interface reactions and achieving precise synergy between CO2 reduction and lignin oxidation.

[0005] Therefore, developing a method and dedicated apparatus for the efficient and selective preparation of organic acids such as oxalic acid, formic acid, and acetic acid through bubble plasma coupling of CO2 reduction and lignin oxidation at ambient temperature and pressure is of great significance for promoting the recycling of carbon resources and the high-value conversion of biomass. Summary of the Invention

[0006] To overcome the aforementioned problems, this invention proposes an apparatus and method for the synergistic conversion of carbon dioxide and lignin to prepare organic acids. The apparatus includes: a reaction chamber for containing a lignin reaction solution; a gas inlet unit for introducing carbon dioxide-containing gas into the reaction chamber and forming microbubbles; and a plasma generation unit for generating non-thermal plasma at the interface between the microbubbles and the lignin reaction solution inside the reaction chamber, enabling simultaneous carbon dioxide reduction and lignin oxidation to generate organic acids. This apparatus uses carbon dioxide and lignin as the main green carbon sources, achieving efficient conversion and preparation of organic acids without the need for external photocatalysts, metal catalysts, or sacrificial agents. It produces few byproducts, has a simple system, low energy consumption, and simultaneously realizes the resource utilization of carbon dioxide and the high-value conversion of biomass, conforming to the concepts of green chemistry and carbon neutrality.

[0007] Specifically, the object of the present invention is to provide the following aspects:

[0008] On one hand, an apparatus for preparing organic acids is provided, the apparatus comprising:

[0009] The reaction chamber is used to contain the lignin reaction solution;

[0010] The air intake unit is used to introduce carbon dioxide-containing gas into the reaction chamber and form microbubbles;

[0011] A plasma generating unit is used to generate non-thermal plasma in the interface region between the microbubbles and the lignin reaction solution inside the reaction chamber, so that carbon dioxide reduction and lignin oxidation occur simultaneously, generating organic acids. Optionally, the concentration of lignin in the lignin reaction solution is 1-50 mM.

[0012] Optionally, the intake unit is an insulated connecting fastener with a cubic three-way structure.

[0013] Optionally, the three ports of the insulating connector are used to connect to an external gas source, a quartz tube, and a high-voltage discharge electrode, respectively.

[0014] Optionally, the quartz tube is arranged axially along the reaction chamber.

[0015] Optionally, the quartz tube is a cylindrical tube with an open top and a hollow interior.

[0016] Optionally, the plasma generating unit includes a high-voltage discharge electrode and a grounding electrode, which together form a spark discharge structure through a quartz tube and its micropores to generate microbubble plasma.

[0017] Optionally, the device further includes a power supply unit for maintaining plasma discharge.

[0018] In a second aspect, a method for preparing organic acids according to the apparatus described in the first aspect, the method comprising:

[0019] Step 1: Inject the lignin reaction solution into the reaction chamber;

[0020] Step 2: Through the air intake unit, carbon dioxide-containing gas is introduced into the reaction chamber, forming microbubbles;

[0021] Step 3: Non-thermal plasma is excited in the interface region between the microbubbles and the wood reaction liquid inside the reaction chamber, so that carbon dioxide reduction and lignin oxidation occur simultaneously and organic acids are generated.

[0022] In step 2, the gas flow rate is 5-100 mL / min.

[0023] The beneficial effects of this invention include:

[0024] (1) The apparatus for preparing organic acids provided by the present invention realizes the synergistic conversion reaction of carbon dioxide and lignin. The entire reaction is carried out at room temperature and pressure, requiring only 20-60 W low power drive. No external photocatalyst, metal catalyst or sacrificial agent is needed. It realizes the synergistic enhancement of greenhouse gas resource utilization and high-value conversion of biomass. It has few by-products, a simple system, and low energy consumption, which is in line with the concept of green chemistry and carbon neutrality.

[0025] (2) The apparatus for preparing organic acids provided by the present invention provides a through-hole micropore on the side wall of a quartz tube near the lower end, so that CO2 gas forms microbubbles with uniform size and stable trajectory in the form of a horizontal jet; at the same time, the discharge area is limited to the gas-liquid interface near the micropore outlet, ensuring that the newly formed bubbles are activated by non-thermal plasma the moment they leave the orifice, which greatly improves the energy utilization efficiency.

[0026] (3) In the method for preparing organic acids provided by the present invention, lignin is oxidized and ring-opened by ·OH free radicals in the liquid phase under the action of plasma, and releases electrons, while CO2 is reduced to ·CO2 by high-energy electrons in the bubbles. - These active intermediates significantly promote the C–C coupling pathway, generating C1–C2 organic acids such as oxalic acid, formic acid, and acetic acid with high selectivity. Attached Figure Description

[0027] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0028] In the attached diagram:

[0029] Figure 1 A schematic diagram of the apparatus for preparing organic acids according to the present invention is shown;

[0030] Figure 2 This diagram shows the structure of the intake unit of the present invention;

[0031] Figure 3 This diagram shows a comparison of organic acid yields at different guaiacol concentrations in Example 1.

[0032] Figure 4 This figure shows a comparison of organic acid yields under different CO2 gas flow rates and concentrations in Example 2.

[0033] Figure 5 The graph shows a comparison of organic acid yields under different discharge powers in Example 3.

[0034] Explanation of reference numerals in the attached figures

[0035] 1-Reaction chamber;

[0036] 2-Intake unit;

[0037] 21 - First Port;

[0038] 22 - Second port;

[0039] 23 - Third port;

[0040] 24 - First plug;

[0041] 25 - Second plug;

[0042] 26 - First nut;

[0043] 27 - Second nut;

[0044] 28 - Third nut;

[0045] 29 - Third plug;

[0046] 3-Quartz tube;

[0047] 4-High-voltage discharge electrode;

[0048] 5 - Grounding electrode;

[0049] 6- Circulating water inlet;

[0050] 7-Circulating water outlet;

[0051] 8-Micropores. Detailed Implementation

[0052] The following will refer to the appendix. Figures 1 to 5Specific embodiments of the invention will be described in more detail below. While specific embodiments of the invention are shown in the accompanying drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0053] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0054] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship in the working state of this invention, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0055] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0056] On one hand, according to the present invention, an apparatus for preparing organic acids is provided, wherein the apparatus is used to realize a synergistic reaction of carbon dioxide reduction and lignin oxidation, preferably using a bubble plasma method to realize the synergistic reaction of carbon dioxide reduction and lignin oxidation. The apparatus includes:

[0057] Reaction chamber 1, which is used to contain the lignin reaction solution;

[0058] The air intake unit 2 is used to introduce carbon dioxide-containing gas into the reaction chamber 1 and form microbubbles;

[0059] A plasma generating unit is used to generate non-thermal plasma in the interface region between the microbubbles and the wood reaction liquid inside the reaction chamber 1, thereby promoting the simultaneous reduction of carbon dioxide and oxidation of lignin, and generating organic acids.

[0060] A power supply unit, used to maintain plasma discharge;

[0061] Quartz tube 3 is used to restrict the direction of gas flow, so that it can only escape from the bottom micropore, forming a spark discharge and generating microbubble plasma;

[0062] The circulating cooling unit is used to introduce a cooling medium to control the temperature of the reaction system within the range of 10-60°C.

[0063] In this invention, the above-described reaction apparatus is used to perform a synergistic conversion reaction between carbon dioxide and lignin. First, a liquid-phase reaction system containing soluble lignin is added to the reaction chamber 1. Then, carbon dioxide-containing gas is introduced into the inner cavity of the quartz tube 3 through the gas inlet unit. The gas flows downward along the wall of the quartz tube 3 and is horizontally ejected from the micropore 8 near the lower end of its side wall, entering the liquid-phase reaction medium to form uniformly sized, continuously rising microbubbles.

[0064] After the aeration stabilizes, the power unit is activated. The micropores at the bottom of the quartz tube generate stable microbubble non-thermal plasma (i.e., plasma with an electron temperature much higher than the gas temperature, producing active species at room temperature) with the surrounding liquid. This plasma concentrates and generates high-energy electrons, hydroxyl radicals (·OH), atomic oxygen (O), and superoxide anions (·O2) on the microbubble surface and at the gas-liquid interface. - Active species such as CO2 are present. In this environment, carbon dioxide molecules within the bubbles are reduced by high-energy electrons to produce CO2. - The process involves reducing intermediates; simultaneously, lignin in the liquid phase undergoes ring-opening and demethoxylation reactions under the action of strong oxidizing species such as ·OH, releasing electrons and small organic fragments. Notably, the oxidation of lignin can provide an electron source for the reduction of CO2, forming an "oxidation-reduction coupling" cycle near the bubble interface, significantly promoting the occurrence of C-C coupling reactions, thereby efficiently generating C1-C2 organic acids. These C1-C2 organic acids refer to organic carboxylic acids containing 1 to 2 carbon atoms in their molecules, such as formic acid (HCOOH), acetic acid (CH3COOH), and oxalic acid (HOOC-COOH).

[0065] During the reaction, the circulating cooling unit continuously supplies cooling medium to control the temperature of the reaction system within the range of 10-60℃, thus preventing local overheating that could lead to side reactions or equipment damage.

[0066] Testing showed that the device of this invention achieved the simultaneous conversion of CO2 and lignin at room temperature and pressure, indicating that the bubble plasma environment effectively promoted the synergistic reaction of the two, realizing the joint high-value utilization of carbon resources and biomass.

[0067] In this invention, the volume of the reaction chamber 1 is not strictly required and varies depending on the required amounts of reactants and products. Typically, the volume of the reaction chamber 1 is 200-500 mL, and a liquid phase containment zone is formed inside to contain the lignin reaction solution.

[0068] In the lignin reaction solution, the concentration of lignin is 1-50 mM, preferably 10 mM. The constraints for selecting this parameter are: if the concentration is too low, the oxidation reaction will not provide enough electrons, making it difficult to effectively promote CO2 reduction; if the concentration is too high, it may lead to an increase in the viscosity of the reaction system, a decrease in mass transfer efficiency, and some model compounds may undergo over-mineralization under strong oxidizing conditions, reducing the yield of organic acids.

[0069] In this invention, the lignin includes natural lignin and industrial lignin. During experimental verification, lignin model compounds can be used as representative substrates to characterize the transformation behavior of typical lignin structural units in a plasma system. The lignin model compounds are selected from any one of guaiacol, vanillic acid, p-hydroxybenzoic acid, and syringaldehyde, with guaiacol being preferred. Guaiacin is one of the most typical structural units in lignin, containing a benzene ring, methoxy group, and hydroxyl group. Its oxidative degradation pathway is well-defined, readily generating carboxyl intermediates, which is beneficial for the formation of C1-C2 organic acids. Simultaneously, its good water solubility facilitates the preparation of homogeneous reaction systems, promoting experimental repeatability and industrialization.

[0070] In one embodiment, lignin is first dissolved in water to form a lignin reaction solution with a concentration of 1-50 mM (preferably 10-50 mM, most preferably 10 mM). Then, the reaction solution is injected into the reaction chamber 1, with the liquid level at a height of 4-8 cm. The liquid level must be sufficient to allow the bubbles to rise sufficiently and pass through the plasma discharge region (located near the lower end of the quartz tube 3, specifically the electric field region formed by the outer wall of the quartz tube 3 and the liquid phase), while preventing liquid splashing or short circuits caused by excessive liquid level contact with the high-voltage discharge electrode 4. If the liquid level is too low, the microbubbles cannot effectively contact the discharge region; if it is too high, it may affect the uniformity of gas distribution or cause inconsistent electrode immersion depth, affecting discharge stability.

[0071] Furthermore, the reaction chamber 1 is a quartz glass container, which has excellent resistance to plasma corrosion, high dielectric strength, and optical transparency. This ensures the safety of high-voltage discharge and facilitates real-time observation of bubble movement and discharge morphology. Simultaneously, the thermal expansion properties of the quartz material are matched with those of the quartz tube 3, which serves as the dielectric layer in the device, contributing to improved long-term system reliability.

[0072] In this invention, the air intake unit 2 is connected to the quartz tube 3, and is used to introduce carbon dioxide-containing gas into the inner cavity of the quartz tube 3, and discharge it into the reaction chamber 1 through the micropores 8 on the side wall of the quartz tube 3, forming microbubbles. The air intake unit 2 is preferably an insulated connecting fastener with a cubic three-way structure. The overall dimensions of the three-way body are 12-15 mm × 12-15 mm × 12-15 mm, for example, 15 mm × 15 mm × 15 mm. It is made of polytetrafluoroethylene (PTFE) or ceramic material, has excellent electrical insulation properties and corrosion resistance, and is suitable for long-term operation in high-pressure plasma environments.

[0073] Furthermore, the three ports of the insulating connecting fastener are respectively used to connect to an external gas source, quartz tube 3, and discharge electrode.

[0074] like Figure 2 As shown, the first port 21 of the insulating connector is a radial through hole with a diameter of 3-3.5 mm and external threads on its outer surface. A third plug 29 is located at the end of the first port 21 furthest from the tee body. This plug is a tapered insulating part with a straight cylindrical inner hole whose inner diameter matches that of the first port 21, used to connect an external air supply conduit. Its outer contour is tapered, with the outer diameter gradually increasing from the inlet end (closer to the tee body) to the outlet end (farthest from the tee body). The third plug 29 is used in conjunction with a third nut 28: the third nut 28 is a tapered compression nut with internal threads, its inner cavity shape matching the outer contour of the third plug 29, and it can be connected to the external threads of the first port 21. During installation, the external air supply conduit is inserted into the outlet end of the third plug 29, and then the third plug 29 and the third nut 28 are sequentially fitted onto the first port 21, and the third nut 28 is rotated to engage with the external threads of the tee. As the nut is tightened, the third plug 29 undergoes elastic deformation under axial pressure, and its inner wall tightly covers the outer surface of the external air source conduit, forming a highly airtight connection.

[0075] The second port 22 and the third port 23 of the insulating connecting fastener are both axial through holes and coaxially connected. The second port 22 is used to connect to the upper end of the quartz tube 3; the third port 23 is used to connect to the high-voltage discharge electrode 4.

[0076] The axial channel between the second port 22 and the third port 23 has a tapered structure and is externally threaded. Its diameter gradually increases from the third port 23 (the connection end of the high-voltage discharge electrode 4) to the second port 22 (the connection end of the quartz tube 3) to accommodate connecting components of different sizes. The inner diameter of the end of the second port 22 is the same as the outer diameter of the quartz tube 3; the inner diameter of the end of the third port 23 is the same as the diameter of the high-voltage discharge electrode 4.

[0077] A first plug 24 is provided at the end of the second port 22. This plug is a tapered insulating component with a straight-through cylindrical inner hole whose inner diameter matches the outer diameter of the quartz tube 3. Its outer contour is tapered, with the outer diameter gradually increasing from the inlet end (closer to the tee body) to the outlet end (away from the tee body). The first plug 24 is used in conjunction with a first nut 26: the first nut 26 is a tapered compression nut with internal threads, and its inner cavity shape matches the outer contour of the first plug 24. During installation, the upper end of the quartz tube 3 is inserted into the outlet end of the first plug 24. Then, the first plug 24 and the first nut 26 are sequentially fitted into the second port 22, and the first nut 26 is rotated to engage with the external threads of the tee. As the nut is tightened, the first plug 24 undergoes elastic deformation under axial pressure, and its inner wall tightly covers the outer surface of the quartz tube, forming a highly airtight connection.

[0078] A second plug 25 is provided at the end of the third port 23. Its structure is similar to that of the first plug 24, but smaller in size, to accommodate the smaller diameter high-voltage discharge electrode 4. Its inner hole is a straight cylindrical hole with an inner diameter consistent with the outer diameter of the high-voltage discharge electrode 4; its outer contour is also tapered, with the outer diameter gradually increasing from the inlet end (closer to the tee body) to the outlet end (away from the tee body). The second plug 25 is used in conjunction with the second nut 27, which is also a tapered clamping nut with an inner cavity matching the outer contour of the second plug 25. During installation, the high-voltage discharge electrode 4 is inserted into the outlet end of the third port 23, and the second plug 25 and the second nut 27 are sequentially inserted into the axial through hole of the tee. The second nut 27 is then tightened, causing the second plug 25 to deform under pressure and fit tightly against the electrode surface, achieving mechanical fixation, gas sealing, and electrical insulation of the electrode. The inner hole of the first port 21 is interconnected with the axial through holes of the second port 22 and the third port 23, forming an internal channel between the three ports. Although the three-port channels are interconnected, due to the structure's excellent airtightness, gas can only escape through the micropores 8 on the lower quartz tube 3.

[0079] The first plug 24, the second plug 25, the third plug 29, the first nut 26, the second nut 27, the third nut 28, and the tee body are all made of highly insulating and corrosion-resistant materials such as polytetrafluoroethylene (PTFE) or ceramics to ensure that no surface discharge or chemical corrosion occurs in the high-voltage plasma environment.

[0080] The integrated three-way structure of the aforementioned air intake unit 2 achieves integrated connection between the gas channel, electrode leads, and reaction chamber through threaded clamping and a conical elastic sealing structure. Compared with the traditional multi-component splicing method, it eliminates the need for additional sealing rings, simplifies assembly and disassembly, and maintains good airtightness and insulation under high-voltage discharge conditions, effectively preventing gas leakage or surface flashover, and significantly improving the stability and experimental repeatability of the device.

[0081] In this invention, the quartz tube 3 is arranged axially along the reaction chamber 1. It is a cylindrical tube with an open top and a hollow interior. Its upper end is sealed to the second port 22 of the air inlet unit 2 to receive carbon dioxide-containing gas introduced from the outside.

[0082] Near the lower end of the sidewall of the quartz tube 3 (preferably 1-3 cm from the bottom), there are 1-8 through-holes 8 with a diameter of 50-200 μm (ideally 200 μm). These through-holes 8 serve as the gas outlets in this device. When gas enters the inner cavity of the quartz tube 3 through the gas inlet unit 2, it flows downward along the tube wall and is ejected horizontally at the through-holes 8 due to the local pressure difference, entering the liquid reaction medium in the reaction chamber 1 to form uniformly sized and stably rising microbubbles. If the diameter of the through-holes 8 is less than 50 μm, they are prone to clogging, and the gas resistance is too high, affecting the continuous generation of bubbles; if the diameter is greater than 200 μm, the bubble size is too large, the rising speed is too fast, shortening its residence time in the discharge zone and reducing the activation efficiency.

[0083] Preferably, by optimizing the size and position of the micropores 8, the generated bubbles have a uniform diameter distribution, typically 100-300 μm, effectively avoiding bubble size fluctuations and bubble aggregation, thereby ensuring the stability and controllability of the gas-liquid interface reaction. When there are multiple micropores 8, they are distributed parallel to each other along the axial position of the quartz tube 3. Considering the stability of bubble generation and the focusing effect of the discharge field, it is preferable to set one micropore 8.

[0084] Specifically, when there is only one micropore 8, its position is precisely at the beginning of the spark discharge region formed by the outer wall of the quartz tube 3 and the grounding electrode 5. Under this design, the newly formed microbubbles enter the strong electric field region the instant they leave the micropore 8, and are efficiently activated by the excited non-thermal plasma. This enables the simultaneous reduction of CO2 molecules in the gas phase and the oxidation of lignin in the liquid phase, significantly enhancing the electron transfer and CC coupling pathway between the two, thereby effectively improving the synergistic efficiency of CO2 reduction and lignin oxidation.

[0085] In this invention, the inner diameter of the quartz tube 3 is 6-8 mm, for example, 6 mm; and the outer diameter is 10-12 mm, for example, 10 mm. The constraint for setting these parameters is that the inner diameter must match the diameter of the high-voltage discharge electrode 4 and the size of the first plug 24. If it is too small, it will be incompatible; if it is too large, it will affect the uniformity of the discharge gap. These parameters ensure smooth gas flow, stable electrode insertion, and guarantee that the quartz tube 3 has sufficient mechanical strength and dielectric properties when used as a dielectric layer.

[0086] Furthermore, the length of the quartz tube 3 is 120-180 mm, for example, 150 mm. The constraints for setting this parameter are: if the length is too short, the lower end of the quartz tube 3 will not be fully immersed in the liquid phase, and the micropore 8 may be exposed to air, affecting bubble generation and discharge stability; if the length is too long, it will increase the complexity of the system and cause the gas to stay in the tube for too long, reducing the reaction efficiency; the above-mentioned length is preferred and is also suitable for conventional scales.

[0087] In this invention, the plasma generating unit includes a high-voltage discharge electrode 4 and a grounding electrode 5, which together form a spark discharge structure through the micropores 8 of the quartz tube 3. In other words, when a high-frequency AC voltage is applied, gas discharge breakdown occurs at the interface between the micropores and the liquid phase of the quartz tube 3, exciting numerous tiny, instantaneous microbubble plasma discharge channels, generating high-energy electrons, hydroxyl radicals (·OH), and superoxide anions (·O2). - Active species such as CO2 reduction and lignin oxidation are used to achieve synergistic transformation.

[0088] Specifically, the high-voltage discharge electrode 4 is a metal needle-shaped electrode with a diameter of 2-4 mm, such as a tungsten electrode. One end of the electrode is inserted into the quartz tube 3 through the axial through hole of the air intake unit 2 (passing through the second plug 25) and extends to a position 1-3 cm from the bottom of the quartz tube 3 without contacting the inner wall of the quartz tube; the other end is connected to the high-voltage output terminal of the power supply unit.

[0089] The grounding electrode 5 is a metal needle-shaped electrode, such as a tungsten needle, which is disposed on the inner wall of the reaction chamber 1 or immersed in the liquid phase, located outside the quartz tube 3, arranged parallel to the high-voltage discharge electrode, and 3-5 cm away from the outer wall of the quartz tube. The grounding electrode 5 is connected to the grounding terminal of the power supply unit through a wire to form a closed loop.

[0090] During operation, the power supply unit applies high-frequency AC or pulsed high voltage, generating microbubble spark discharge in the gas-liquid interface region between the micropore 8 of the quartz tube 3 and the grounding electrode 5. Since the quartz tube 3 acts as a dielectric layer, preventing arc formation, the discharge is distributed in numerous micro-discharge channels on the surface of the microbubbles and the surrounding liquid film, generating high-energy electrons, hydroxyl radicals (·OH), atomic oxygen (O), and superoxide anions (·O2). - Active species such as )

[0091] Specifically, because the gas is horizontally ejected from the micropores 8 on the side wall of the quartz tube 3, the newly formed microbubbles are immediately placed in a strong electric field region upon leaving the pores, and CO2 molecules are reduced to CO2 by high-energy electrons within the bubbles. - Intermediates are produced; simultaneously, lignin is oxidized and ring-opened by ·OH in the liquid phase, releasing electrons. The two form an "oxidation-reduction coupling" cycle at the gas-liquid interface, which significantly promotes C-C coupling reactions and efficiently generates C1-C2 organic acids such as oxalic acid, formic acid, and acetic acid.

[0092] In this invention, the device is a high-frequency AC or pulsed high-voltage power supply, whose positive output terminal is connected to the high-voltage discharge electrode 4 via a high-voltage wire, and whose negative ground terminal is connected to the ground electrode 5, forming a closed discharge circuit.

[0093] The power supply unit has a discharge power of 20-60 W, for example, 40 W; an output frequency of 30-50 kHz, for example, 40 kHz; and a sine wave waveform. The constraints for setting these parameters are as follows: if the discharge power is too low, the plasma intensity is insufficient, the yield of active species (such as ·OH and high-energy electrons) is low, making it difficult to drive CO2 reduction and lignin oxidation reactions; if the power is too high, it leads to local overheating, causing severe solvent evaporation and excessive mineralization of organic matter (complete oxidation to CO2), which in turn reduces the selectivity of the target organic acid; at a power of 20-60 W, the yield of organic acid steadily increases with increasing power; below 30 kHz, the discharge channel distribution is sparse, and the interface coverage is uneven; above 50 kHz, although the density of active species can be increased, it will significantly increase dielectric loss and device heating, placing higher demands on cooling and potentially causing electrode corrosion.

[0094] In this invention, the circulating cooling unit includes a jacketed cooling channel surrounding the outer wall of the reaction chamber 1. This channel is made of stainless steel or polytetrafluoroethylene (PTFE) and is integrally formed with the reaction chamber 1 or tightly fitted with a sealing ring. The cooling channel has a circulating water inlet 6 and a circulating water outlet 7, which are symmetrically arranged circumferentially along the jacket or axially offset to promote uniform flow of the cooling medium within the channel. The circulating water inlet 6 is located at the bottom or lower side of the jacket, and the circulating water outlet 7 is located at the top or upper side, forming a counter-current cooling path from bottom to top. The circulating water inlet 6 and the circulating water outlet 7 are respectively connected to an external constant-temperature circulating water bath via silicone tubes. By adjusting the cooling water temperature and flow rate, the temperature of the reaction system is stably maintained within the range of 10–60℃, preferably 25±2℃.

[0095] During operation, the cooling medium (preferably deionized water) flows into the jacket from the inlet, forming a uniform flow field on the outer wall of the reaction chamber. After absorbing the Joule heat generated by the discharge and the heat of reaction, it flows out from the outlet, achieving continuous heat removal. The cooling medium flow rate is controlled at 0.5-2 L / min, and the temperature is set at 15-30℃ to stably maintain the reaction liquid temperature at 10-60℃, for example, 25±2℃. This design not only effectively suppresses excessive mineralization of organic matter (such as complete oxidation to CO2) caused by localized high temperatures but also ensures the structural stability of the quartz tube 3 and the tee connector during long-term operation.

[0096] In this invention, CO2-containing gas enters the inner cavity of the quartz tube 3 through the air intake unit 2 and is directionally ejected from the micropores 8 on the side wall to form a single row of microbubbles; at the same time, the power supply unit drives the high-voltage discharge electrode 4 to generate spark discharge between the ground electrode 5, and the plasma active species are concentrated in the gas-liquid interface region near the outlet of the micropore 8.

[0097] In this environment, CO2 is reduced to ·CO2 by high-energy electrons inside the bubbles. - The active intermediates, such as lignin, are oxidized and degraded by ·OH in the liquid phase and release electrons; the two form a closed loop of "oxidation electron donation - reduction electron acceptance" through interfacial electron transfer, which not only inhibits free radical quenching, but also promotes the C-C coupling pathway, thereby generating C1-C2 organic acids such as oxalic acid and formic acid with high selectivity.

[0098] The entire process is carried out at normal temperature and pressure, without the need for external catalysts or sacrificial agents, truly realizing the green and synergistic transformation of carbon resource fixation and high-value utilization of biomass.

[0099] On the other hand, according to the method for preparing organic acids using the apparatus described in the first aspect provided by the present invention, the method includes:

[0100] Step 1: Inject the lignin reaction solution into reaction chamber 1;

[0101] Step 2: Through the air intake unit 2, carbon dioxide-containing gas is introduced into the reaction chamber 1, forming microbubbles;

[0102] Step 3: Non-thermal plasma is excited in the interface region between the microbubbles and the lignin reaction liquid inside the reaction chamber 1, so that carbon dioxide reduction and lignin oxidation occur simultaneously and organic acids are generated.

[0103] In step 1, the liquid level of the lignin reaction solution is controlled to be 4-8 cm to ensure that the micropores 8 on the side wall of the quartz tube 3 are completely immersed in the liquid phase and located at the starting position of the discharge area formed by the high-voltage discharge electrode 4 and the grounding electrode 5.

[0104] In step 2, carbon dioxide-containing gas is introduced into the inner cavity of the quartz tube 3 through the gas inlet unit 2. The gas is horizontally ejected from the micropores 8 on the side wall, forming microbubbles. The carbon dioxide-containing gas is CO2 or a mixture of CO2 and an inert gas such as Ar, with a CO2 volume fraction of 10%-100%; preferably, the carbon dioxide-containing gas is pure CO2. The gas flow rate is 5-100 mL / min, preferably 10-50 mL / min, for example, 20 mL / min. Appropriately increasing the CO2 gas flow rate helps to improve the total yield of organic acids; however, excessively high flow rates lead to a shortened bubble residence time, affecting reaction efficiency. 20 mL / min is the optimal CO2 gas flow rate, at which the organic acid yield reaches its best.

[0105] In step 2, the diameter of the micropore 8 at the lower end of the quartz tube 3 is 50-200 μm, and the generated microbubbles have a diameter of 100-300 μm, rising steadily in a single row.

[0106] In step 3, the power supply unit is turned on, and the circulating cooling unit is started simultaneously. After the reaction is completed, the power supply unit and gas supply are turned off to obtain a product solution containing organic acids. The power supply unit applies an AC voltage of 20-60 W and 30-50 kHz between the high-voltage discharge electrode 4 and the grounding electrode 5 to excite non-thermal plasma in the gas-liquid interface region on the outer wall of the quartz tube 3. At the same time, the circulating cooling unit introduces a cooling medium at a temperature of 15-30℃ at a flow rate of 0.5-2 L / min to stably control the temperature of the reaction system within the range of 10-60℃, preferably 25±2℃.

[0107] In step 3, under the influence of plasma, carbon dioxide inside the microbubbles is reduced by high-energy electrons to generate CO2. - The reaction produces reducing intermediates, while lignin in the liquid phase is oxidized by hydroxyl radicals (·OH) to open the ring and remove methoxy groups, releasing electrons and small organic fragments. These two components form a redox coupling cycle near the gas-liquid interface, promoting C-C coupling reactions and efficiently generating C1-C2 organic acids such as oxalic acid, formic acid, and acetic acid. After the reaction continues for 0.1-4 hours, the power supply and gas supply are turned off, the reaction solution is collected, filtered through a 0.22 μm filter membrane, and the concentration of the organic acid products is analyzed by ion chromatography (IC). The organic acids are C1-C2 organic acids, including at least oxalic acid, formic acid, and acetic acid.

[0108] Example

[0109] In the following examples, guaiacol was used as a representative substrate for lignin to verify the feasibility and effectiveness of the method of the present invention. However, these examples are merely exemplary and do not constitute any limitation on the scope of protection of the present invention.

[0110] The following examples use, for example Figure 1The apparatus shown undergoes a reaction. Its reaction chamber 1 is a quartz glass container with a volume of 200 mL. The quartz tube 3 is a hollow cylindrical tube with an open top, an outer diameter of 10 mm, an inner diameter of 6 mm, and a length of 150 mm. A 200 μm diameter through-hole 8 is formed on its side wall 1 cm from the bottom. The air inlet unit 2 is a cubic tee insulated connector made of polytetrafluoroethylene (PTFE), with overall dimensions of 15 mm × 15 mm × 15 mm. Its second port 22 is sealed to the upper end of the quartz tube 3 via a first plug 24 and a first nut 26. Its third port 23 is fixed to the high-voltage discharge electrode 4 via a second plug 25 and a second nut 27. The high-voltage discharge electrode 4 is a 3 mm diameter stainless steel needle electrode inserted into the quartz tube 3 and extending to a distance of 2 cm from the bottom. The grounding electrode 5 is a tungsten needle electrode, located on the inner wall of the reaction chamber 1, parallel to the outer wall of the quartz tube 3, with a spacing of 3 cm; the circulating cooling unit is a jacketed channel surrounding the reaction chamber 1, through which 25°C deionized water is introduced at a flow rate of 1 L / min; the power supply unit is a sinusoidal AC power supply with a discharge time of 15 min.

[0111] Example 1

[0112] Effect of guaiacol concentration on organic acid yield

[0113] Guaiacin (GUA) was dissolved in deionized water to obtain a GUA reaction solution with a concentration varying from 1 to 50 mM (1, 5, 10, 20, and 50 mM, respectively). 100 mL of each concentration of GUA reaction solution was injected into reaction chamber 1, with the liquid level controlled at 5 cm. This ensured that the micropores 8 were completely submerged in the liquid phase and located at the beginning of the discharge region formed by the high-voltage discharge electrode 4 and the grounding electrode 5, allowing the reaction to proceed.

[0114] The reaction process is as follows: CO2 gas is introduced into the inner cavity of quartz tube 3 through inlet unit 2, flows downward along the tube wall, and is ejected horizontally from the micropores 8 on the side wall, forming a single-row rising microbubble with a diameter of about 200 μm; simultaneously, the power supply unit is turned on, and a sinusoidal AC voltage of 40 W and 40 kHz is applied to excite stable microbubble non-thermal plasma in the interface region between the micropores 8 and the liquid phase of quartz tube 3. Active species such as high-energy electrons and ·OH are concentrated at the gas-liquid interface near the outlet of micropores 8; at the same time, CO2 gas with a flow rate of 20 mL / min is introduced to form a single-row rising microbubble; in this microenvironment, C1-C2 organic acids such as oxalic acid, formic acid, and acetic acid are efficiently generated; the circulating cooling unit runs continuously to maintain the temperature of the reaction system stably at 25±2℃ and suppress the occurrence of side reactions.

[0115] After reacting for 15 min, the power and gas supply were turned off, the reaction solution was collected, filtered through a 0.22 μm filter membrane, and the yield of organic acids was analyzed by ion chromatography (IC).

[0116] Figure 3 The effect of different guaiacol concentrations on the yield of organic acids was shown. The results showed that when the guaiacol concentration increased from 1 mM to 10 mM, the total yield of organic acids increased significantly. When the concentration exceeded 10 mM, the yield growth tended to slow down or even decrease, indicating that 10 mM was the optimal concentration. Therefore, this condition was selected as the preferred reaction condition for subsequent experiments.

[0117] Example 2

[0118] Effect of CO2 gas flow rate on organic acid yield

[0119] Guaiacin (GUA) was dissolved in deionized water to obtain a 10 mM GUA reaction solution. 100 mL of this solution was injected into reaction chamber 1, with the liquid level controlled at 5 cm. This ensured that the micropores 8 were completely immersed in the liquid phase and located at the beginning of the discharge region formed by the high-voltage discharge electrode 4 and the ground electrode 5, and the reaction was carried out.

[0120] The reaction process is as follows: CO2 gas is introduced into the inner cavity of quartz tube 3 through inlet unit 2, flows downward along the tube wall, and is ejected horizontally from the micropores 8 on the side wall, forming a single-row rising microbubble with a diameter of about 200 μm; simultaneously, the power supply unit is turned on, and a sinusoidal AC voltage of 40 W and 40 kHz is applied to excite stable microbubble non-thermal plasma in the region between the outer wall of quartz tube 3 and the liquid phase interface. Active species such as high-energy electrons and ·OH are concentrated at the gas-liquid interface near the outlet of micropore 8; different CO2 gas flow rates (5 mL / min, 10 mL / min, 20 mL / min, 50 mL / min, 100 mL / min) are used in this microenvironment to efficiently generate C1–C2 organic acids such as oxalic acid, formic acid, and acetic acid; the circulating cooling unit runs continuously to maintain the temperature of the reaction system at 25±2℃ and suppress the occurrence of side reactions.

[0121] After reacting for 15 min, the power and gas supply were turned off, the reaction solution was collected, filtered through a 0.22 μm filter membrane, and the yield of organic acids was analyzed by ion chromatography (IC).

[0122] Figure 4 The effect of different CO2 gas flow rates on the yield of organic acids was shown. The results showed that appropriately increasing the CO2 gas flow rate helped to improve the total yield of organic acids. However, excessively high flow rates led to a shortened bubble residence time, which affected the reaction efficiency. 20 mL / min was the optimal CO2 gas flow rate, and therefore this condition was selected as the preferred reaction condition for subsequent experiments.

[0123] Example 3

[0124] Effect of discharge power on organic acid yield

[0125] Guaiacin (GUA) was dissolved in deionized water to obtain a 10 mM GUA reaction solution. 100 mL of this solution was injected into reaction chamber 1, with the liquid level controlled at 5 cm. This ensured that the micropores 8 were completely immersed in the liquid phase and located at the beginning of the discharge region formed by the high-voltage discharge electrode 4 and the ground electrode 5, and the reaction was carried out.

[0126] The reaction process is as follows: CO2 gas is introduced into the inner cavity of quartz tube 3 through inlet unit 2, flows downward along the tube wall, and is ejected horizontally from the micropores 8 on the side wall, forming a single-row rising microbubble with a diameter of about 200 μm; the power supply unit is turned on simultaneously, using different discharge powers (20 W, 30 W, 40 W, 50 W, 60 W) while keeping the output frequency constant at 40 kHz, to excite stable non-thermal plasma of microbubbles in the interface region between the outer wall of quartz tube 3 and the liquid phase. Active species such as high-energy electrons and ·OH are concentrated at the gas-liquid interface near the outlet of micropore 8; at the same time, CO2 gas is introduced at a flow rate of 20 mL / min. In this microenvironment, C1–C2 organic acids such as oxalic acid, formic acid, and acetic acid are efficiently generated; the circulating cooling unit runs continuously to maintain the temperature of the reaction system stably at 25±2℃ and suppress the occurrence of side reactions.

[0127] After reacting for 15 min, the power and gas supply were turned off, the reaction solution was collected, filtered through a 0.22 μm filter membrane, and the yield of organic acids was analyzed by ion chromatography (IC).

[0128] Figure 5 The effect of different discharge powers on the yield of organic acids is shown, and the results show that the total yield of organic acids continues to increase with the increase of discharge power.

[0129] The present invention has been described in detail above with reference to preferred embodiments and exemplary examples. However, it should be noted that these specific embodiments are merely illustrative explanations of the invention and do not constitute any limitation on the scope of protection of the invention. Various improvements, equivalent substitutions, or modifications can be made to the technical content and embodiments of the present invention without departing from the spirit and scope of protection of the invention, and all such modifications fall within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. An apparatus for preparing organic acids, characterized in that, The device includes: The reaction chamber (1) is used to contain the lignin reaction solution; The air intake unit (2) is used to introduce carbon dioxide-containing gas into the reaction chamber (1) and form microbubbles; The plasma generating unit is used to generate non-thermal plasma in the interface region between the microbubbles and the lignin reaction liquid inside the reaction chamber (1), so that carbon dioxide reduction and lignin oxidation can be carried out simultaneously and organic acids can be generated.

2. The apparatus according to claim 1, characterized in that, Preferably, the concentration of lignin in the lignin reaction solution is 1-50 mM.

3. The apparatus according to claim 1, characterized in that, The intake unit (2) is an insulated connecting fastener with a cubic three-way structure.

4. The apparatus according to claim 3, characterized in that, The three ports of the insulating connector are used to connect to an external gas source, a quartz tube (3), and a high-voltage discharge electrode (4), respectively.

5. The apparatus according to claim 3, characterized in that, The quartz tube (3) is arranged axially along the reaction chamber (1).

6. The apparatus according to claim 5, characterized in that, The quartz tube (3) is a cylindrical tube with an open top and a hollow interior.

7. The apparatus according to claim 1, characterized in that, The plasma generating unit includes a high-voltage discharge electrode (4) and a grounding electrode (5), which form a spark discharge through the micro-hole (8) of the quartz tube (3) and generate microbubble plasma.

8. The apparatus according to claim 1, characterized in that, The device also includes a power supply unit for maintaining plasma discharge.

9. A method for preparing organic acids using the apparatus according to any one of claims 1-8, characterized in that, The method includes: Step 1: Inject the lignin reaction solution into the reaction chamber (1); Step 2: Through the air intake unit (2), carbon dioxide-containing gas is introduced into the reaction chamber (1) and microbubbles are formed; Step 3: Non-thermal plasma is excited in the interface region between the microbubbles and the wood reaction liquid inside the reaction chamber (1) to allow carbon dioxide reduction and lignin oxidation to occur simultaneously and generate organic acids.

10. The method according to claim 9, characterized in that, In step 2, the gas flow rate is 5-100 mL / min.

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