A process for the preparation of dimethyl sebacate by electrolysis of monomethyl adipate

Abstract

This invention provides a method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis, comprising: flowing an electrolyte containing monomethyl adipic acid tangentially over an anode surface, the anode comprising a conductive substrate and a catalyst layer disposed on the substrate surface, the surface roughness Ra of the catalyst layer being less than 0.05 μm; applying a time-varying current to the anode and cathode, wherein a single cycle of the time-varying current comprises alternating periods of high current density and low current density; the anode current density during the high current density period being 100~250 mA / cm². 2 The anolyte current density during the low current density period is no greater than 5 mA / cm². 2 During the high current density period, monomethyl adipic acid undergoes an electrolytic decarboxylation coupling reaction on the anode surface to generate dimethyl sebacate and produce bubbles; during the low current density period, the bubbles desorb from the anode surface and are expelled. The method of this invention enables instantaneous bubble desorption and rapid replenishment of reactants, thereby significantly improving the selectivity and current efficiency of dimethyl sebacate.

Description

Technical Field

[0001] This invention relates to the field of organic electrochemical synthesis technology, specifically to a method for preparing dimethyl sebacate (DMS) from monomethyl adipic acid (MMA) via Kolbe electrolytic decarboxylation coupling. Background Technology

[0002] Dimethyl sebacate, an important fine chemical intermediate, is widely used in plastics, rubber, lubricants, and polymer synthesis, serving multiple functions as a plasticizer, solvent, and organic synthesis intermediate. Under the backdrop of green chemistry and sustainable development, electrochemical synthesis has become a popular research direction in the chemical industry. The electrosynthesis of dimethyl sebacate using monomethyl adipic acid as a raw material via the Kolbe reaction offers advantages such as being environmentally friendly, having mild conditions, and being highly controllable.

[0003] Existing technologies typically focus on constructing micro / nano hierarchical structures or porous coatings on the electrode surface to increase the catalytic specific surface area. For example, CN120575249A discloses an anode material for the electrolysis of adipic acid monoester and its preparation method. Through multi-scale interface strengthening and nanostructure regulation, a porous ZrO2-TiO2 transition layer is constructed on a titanium substrate, and platinum nanoparticles are generated in situ to anchor the pores. Then, combined with pulse-ultrasound synergistic electroplating, a platinum / carbon nanotube composite catalytic layer is deposited to form a functionalized anode material. Summary of the Invention

[0004] This invention proposes a method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis. This method can achieve instantaneous desorption of bubbles and rapid replenishment of reactants, thereby significantly improving the selectivity and current efficiency of dimethyl sebacate.

[0005] To achieve the above technical objectives, this invention proposes a method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis, the method comprising: An electrolyte containing monomethyl adipic acid flows tangentially over the surface of an anode, the anode comprising a conductive substrate and a catalyst layer disposed on the surface of the substrate, the surface roughness Ra of the catalyst layer being less than 0.05 μm; a time-varying current is applied to the anode and cathode, a single cycle of the time-varying current comprising alternating periods of high current density and low current density; the anode current density during the high current density period is 100–250 mA / cm². 2 The anode current density during the low current density period is no greater than 5 mA / cm². 2 (i.e., 0~5mA / cm) 2 During the high current density period, monomethyl adipic acid undergoes an electrolytic decarboxylation coupling reaction on the anode surface to generate dimethyl sebacate and produce bubbles; during the low current density period, the bubbles desorb from the anode surface and are discharged.

[0006] In the Kolbe reaction for the electrolytic preparation of dimethyl sebacate from monomethyl adipic acid, the free radicals formed by decarboxylation at the anode need to be rapidly coupled to generate dimethyl sebacate. However, if the anode is maintained at a high potential, the free radicals are continuously oxidized before leaving the electrode surface, leading to the Hofer-Moest side reaction, polymerization byproducts, and electrode carbon buildup. Our research team has discovered through practice that in the Kolbe electrolytic decarboxylation coupling process for dimethyl sebacate, the bubble retention caused by intense gas evolution at the anode is a significant factor affecting the reaction efficiency and long-term stable operation of the system. The retained bubbles form a localized gas film covering, hindering the diffusion and escape of free radicals from the electrode surface into the bulk solution, prolonging the residence time of free radicals on the electrode surface, and making them more susceptible to secondary oxidation. Simultaneously, the bubble covering reduces the effective active area, necessitating an increase in potential to maintain the target current density, which in turn exacerbates the unfavorable operating conditions.

[0007] However, there are currently no reports addressing the "gas evolution-free radical diffusion-secondary oxidation" coupling limitation in the Kolbe decarboxylation coupling process. To solve the problem of bubble retention caused by severe anode gas evolution, traditional approaches tend to increase the specific surface area by roughening and porousing the electrode to improve reaction efficiency. However, under relatively high current density conditions, bubbles can stably adhere to the micropores, cracks, and nanoprotrusions of such rough electrodes, causing pinning, coalescence, and prolonged retention, which severely obscures the effective active area, leading to local voltage spikes, exacerbating local overheating, and side reactions. In the aforementioned technical solution, a specific electrolytic reaction environment for adipic acid monomethyl ester is constructed through the synergistic effects of an ultra-smooth interface (Ra < 0.05 μm), eliminating the root causes of bubble pinning, providing a bubble desorption time window with a time-varying current at a specific current density period, and physical stripping by tangential flow. Specifically: The above technical solution employs an anode with an ultra-smooth surface Ra<0.05μm and extremely low bubble adhesion work to reduce the adhesion and pinning of carbon dioxide bubbles on the anode surface during electrolysis. This means that bubbles formed during electrolysis under intense gas evolution conditions at high current density spontaneously slide off at extremely small sizes, exposing more of the actual active area. This achieves a balance between higher efficiency and low bubble adsorption by accelerating desorption. Furthermore, the above technical solution sets a specific high current density period to induce decarboxylation, but this period cannot be sustained for too long to avoid secondary oxidation by free radicals. This is combined with a specific low current density period to create a time window for free radicals to escape and couple. Even further, the electrolyte forms a high-shear surface flow on the anode surface, subjecting bubbles to tangential shear force from the initial stage of formation. Especially during the relaxation phase with varying current (low current density period), bubble adhesion decreases, and instantaneous bubble detachment can be achieved under the action of fluid shear force.

[0008] Furthermore, in the above technical solution, the anode current density is 100~250 mA / cm².2 During the high current density period, the monomethyl adipic acid carboxylate ion in the electrolyte undergoes a single-electron oxidative decarboxylation reaction at the anode interface, forming an alkyl radical intermediate and releasing carbon dioxide gas; the anode current density is no greater than 5 mA / cm². 2 The low current density period reduces the continuous oxidation intensity of the anode, reduces the further oxidation of the generated alkyl free radicals, increases the probability of them diffusing out of the electrode and undergoing coupling reactions, provides a time window for the alkyl free radicals to diffuse into the bulk solution and undergo coupling reactions to generate dimethyl sebacate, and at the same time reduces the continuous generation rate of carbon dioxide bubbles; combined with the shearing effect generated by the tangential flow, the carbon dioxide bubbles attached to the anode surface are desorbed, thereby restoring the effective reaction interface of the anode.

[0009] The above technical solution achieves a comprehensive technical effect of high selectivity, high current efficiency, low energy consumption and long life by combining an ultra-smooth anode interface, time-varying current and tangential electrolyte flow.

[0010] Based on the above technical solution, the duty cycle of the high current density period is 20%~70%, and the pulse frequency is 10~500Hz. By adjusting the duration ratio of the high and low current density periods, the high current density period can provide sufficient decarboxylation driving force, while the low current density period accounts for 30%-80% of the entire pulse cycle, ensuring sufficient time windows for free radical diffusion coupling and bubble desorption. The pulse frequency of 10~500Hz matches the bubble growth cycle, avoiding excessive bubble aggregation, and achieving efficient recovery of periodic active area in conjunction with tangential flow. In the example of this invention, the duty cycle of the high current density period is 30%~60%. Based on the above technical solution, the duration of the high current density period is 0.5~5ms, preferably 3~5ms; the duration of the low current density period is 0.5~20ms, preferably 4~8ms; and the duration of the low current density period is not less than the duration of the high current density period. The relatively short high current density period avoids continuous excessive oxidation of free radicals, while the relatively long low current density period ensures bubble desorption and free radical diffusion coupling. In an optional embodiment of the present invention, a combination of a high current density period of 3 to 5 ms or a low current density period of 4 to 8 ms is beneficial for bubbles to obtain sufficient fluid shearing time in a low adhesion state, thereby achieving efficient dynamic recovery of the active area.

[0011] Based on the above technical solution, the adhesion work of the bubbles generated on the anode surface is ≤0.05J / m. 2The bubbles, with a diameter of 5-50 μm, detach from the surface early in the nucleation stage, preventing them from growing and agglomerating to form a gas film, thus continuously exposing ≥90% of the actual active area. Low adhesion work allows for smooth contraction of the three-phase contact line without pinning, and combined with tangential flow shearing, achieves instantaneous peeling, shortening the bubble residence time to 0.04-0.3 s, or preferably 0.04-0.12 s. This creates suitable interfacial conditions for rapid diffusion and escape of free radicals, suppressing secondary oxidation side reactions. It should be noted that the "gas film" mentioned in this invention refers to a continuous or localized gas-covered area formed on the electrode surface during electrolysis. This gas-covered area can hinder effective contact between the electrolyte and the electrode surface, shield the electrode active sites, and increase the ohmic impedance of the electrode interface.

[0012] Based on the above technical solution, the electrolyte forms a tangential flow with a linear velocity of 1.5~5.0 m / s on the anode surface, which can generate a transitional or turbulent flow state with a Reynolds number of 2100~8000 in the microchannel, applying a shear stress of more than 10 Pa to the anode surface, so that the bubbles are subjected to tangential shear force and are instantly peeled off in the early stage of formation; preferably, the linear velocity is 2~3.5 m / s, which is beneficial to fully shear and peel off the bubbles while avoiding the increase in energy consumption and mechanical damage to the electrodes caused by excessive flow velocity; in addition, the electrolyte temperature is controlled at 45~60℃, preferably 50~55℃, which can maintain the stability of the electrolyte viscosity, so that the flow state is within a controllable range, while suppressing the side reactions of ester hydrolysis and excessive oxidation of methanol, and improving product selectivity.

[0013] Based on the above technical solution, the electrolyte is input through a slit nozzle. The slit width of the nozzle is 0.05~1.5mm, and the distance between the nozzle outlet and the anode surface is 0.5~3mm. This allows the electrolyte to be uniformly distributed on the anode surface in the form of a slit jet, forming a stable tangential flow field. The slit width is preferably 0.5~1mm or the distance is preferably 1~2mm. This allows for the matching of jet velocity and shear stress under the constraint of the microchannel inter-electrode spacing, avoiding both velocity attenuation and uneven distribution caused by the slit, and preventing the risk of blockage caused by narrow slits or close proximity. This ensures that the bubbles are fully sheared and stripped in the early stages of formation.

[0014] Based on the above technical solution, the electrolyte includes monomethyl adipic acid and an organic solvent, wherein the organic solvent includes at least one of methanol and acetonitrile; the initial concentration of the electrolyte is 0.5~2 mol / L, preferably 1~1.5 mol / L, and the initial pH value is 8.0~9.5, which can effectively inhibit ester hydrolysis and improve decarboxylation efficiency.

[0015] Optionally, the electrolyte is prepared under an inert atmosphere. In this invention, the inert atmosphere refers to a gaseous environment composed of gases that do not chemically interact with the reactants, such as a nitrogen atmosphere, or an atmosphere formed by gases of Group 0 elements in the periodic table (such as argon).

[0016] Further, the initial conductivity of the electrolyte is 20-30 mS / cm, preferably 25-27 mS / cm. Optionally, an alkaline solution (such as KOH) is slowly added during electrolysis preparation to adjust the initial conductivity of the electrolyte, while simultaneously making the electrolyte weakly alkaline to a suitable pH range. Optionally, the prepared electrolyte is filtered through a 0.1-1.0 μm (e.g., 0.45 μm) polytetrafluoroethylene filter membrane and then transferred to a storage tank, which is then sealed and protected from light. Optionally, the water content of the electrolyte is not higher than 1 wt%, preferably not higher than 0.8 wt%, which can suppress the ester hydrolysis side reaction of monomethyl adipic acid and its product dimethyl sebacate, and avoid the decomposition of trace amounts of water in the methanol solvent at high anodic potential, which exacerbates bubble retention; at the same time, the low water content reduces the hydration competition of the KOH electrolyte, improving selectivity and current efficiency.

[0017] Based on the above technical solution, the duty cycle of the high current density period is 20%~70%, preferably 30%~60%; the electrolyte forms a tangential flow with a linear velocity of 1.5~5.0 m / s on the anode surface, which allows the pulse relaxation period and the fluid shear force to form a sequential synergy: during the low current density period, the bubble adhesion work decreases significantly, the three-phase contact line is easy to slide, and at this time the tangential flow provides a continuous and stable shear peeling force, and the two superimposed achieve instantaneous and complete desorption of bubbles; optionally, the duration of the low current density period is not less than During the duration of the high current density period, the duty cycle of 30% to 80% during the high current density period makes the relaxation period sufficiently longer than the discharge period, providing a sufficient time window for bubble stripping and free radical diffusion coupling, while avoiding excessive relaxation leading to energy loss. Matching the linear velocity of 1.5 to 5.0 m / s, a transitional flow state or turbulent transition state with a Reynolds number of 2100 to 8000 is formed, which can provide sufficient shear strength while avoiding excessive turbulent mixing that affects selectivity, thus achieving a dynamic balance between the periodic and efficient recovery of active area and reaction stability.

[0018] Based on the above technical solution, the time-varying current is an asymmetric pulse current, preferably an asymmetric square wave pulse current.

[0019] Based on the above technical solution, the conductive substrate is any one of titanium sheet, nickel sheet, and niobium sheet, and the catalyst layer is a platinum plating layer, ruthenium plating layer, or iridium plating layer disposed on the conductive substrate.

[0020] Based on the above technical solution, the surface roughness Ra of the catalyst layer is less than 0.03 μm.

[0021] Based on the above technical solution, the porosity of the catalyst layer is less than 0.5%, and the thickness of the catalyst layer is 0.5~3μm, preferably 0.5~2μm.

[0022] Based on the above technical solution, the cathode can be any one of stainless steel, nickel, or graphite. It should be noted that this invention does not limit the specific thickness of the cathode; its thickness can be adjusted according to the electrolytic cell structure, electrode mechanical strength, and current carrying capacity to provide a stable electron conduction and reduction environment. Based on the above technical solution, the smaller the electrode spacing between the anode and cathode, the lower the ohmic resistance of the solution, reducing energy consumption and simultaneously enhancing mass transfer and improving reaction efficiency. However, micro-spacing is prone to bubble blockage. Optionally, the electrode spacing between the anode and cathode is 0.5~1.5 mm, which can achieve a balance between minimizing ohmic resistance and ensuring uniform flow field at the microchannel scale. Preferably, it is 0.8~1 mm, balancing low resistance with sufficient tangential flow space, allowing the electrolyte to form a stable shear layer between the electrodes, and enabling efficient bubble removal and continuous exposure of the active area in conjunction with the pulse relaxation period.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention uses an ultra-smooth anode with Ra<0.05μm to achieve a bubble adhesion work ≤0.05J / m. 2 The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis achieves instantaneous bubble desorption and rapid reactant replenishment, significantly improving the selectivity and current efficiency of dimethyl sebacate. Bubbles spontaneously desorb at extremely small sizes, continuously exposing ≥90% of the true active area. Combined with asymmetric pulsed current, the high current density period induces decarboxylation, while the low current density period creates an escape coupling time window for free radicals. Simultaneously, the bubble adhesion decreases, achieving instantaneous peeling under tangential high-speed shearing. Attached Figure Description

[0024] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The gas chromatographic detection results of the product obtained in Example 1 are shown. Detailed Implementation

[0025] To facilitate understanding of the present invention, a more comprehensive description will be provided below, along with preferred embodiments. However, it should be understood that these embodiments are merely for more detailed explanation and should not be construed as limiting the invention in any way, i.e., not intended to limit the scope of protection of the invention.

[0026] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0027] Furthermore, it should be noted that although the various steps of the preparation method of the present invention are described in a specific order in the description of the present invention, these orders are not restrictive. Without departing from the basic principles of the present invention, those skilled in the art can perform the steps in different orders.

[0028] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "multiple" or "at least one" means two or more.

[0029] All numerical designations, such as pH, temperature, length, flow rate, and range, are approximate values. It should be understood that, while not always explicitly stated, all numerical designations are preceded by the term "approximately." It should also be understood that, while not always explicitly stated, the reagents described herein are merely examples, and their equivalents are known in the art.

[0030] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1~5” is disclosed, the described range should be interpreted as including ranges “1~4”, “1~3”, “1~2”, “1~2 and 4~5”, “1~3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0031] Example 1

[0032] (1) Preparation of Pt / Ti anodes An industrial pure titanium plate with dimensions of 100mm×50mm×2mm was selected as the substrate. Mechanical polishing was performed sequentially using alumina polishing slurries with particle sizes of 1.0μm, 0.3μm and 0.05μm until the surface exhibited a continuous mirror reflection.

[0033] After polishing, the substrate was sequentially ultrasonically cleaned with acetone, anhydrous ethanol, and deionized water for 15 min each, dried with nitrogen, and then placed in a magnetron sputtering system for Pt coating deposition. The process parameters are as follows: base vacuum ≤ 5 × 10⁻⁶. -4 Pa, Ar gas pressure 0.4 Pa, sputtering power 180 W, sputtering time 40 min.

[0034] Surface roughness was measured using atomic force microscopy (AFM, Bruker Dimension Icon). According to ISO 4287:1997, the sampling length λc was 0.8 mm and the evaluation length was 4.0 mm. Measurements were taken in five different regions, and the average surface roughness Ra of the catalyst layer was 0.021 μm.

[0035] Surface and cross-sectional morphology were observed using field emission scanning electron microscopy (FE-SEM, Zeiss GeminiSEM 500). At 50,000x magnification, no through-cracks or pores larger than 50 nm were observed in multiple 20 μm × 20 μm regions. Cross-sectional statistical results showed that the Pt coating thickness was 1.52 ± 0.03 μm, with a porosity of less than 0.3%.

[0036] The contact angle of CO2 bubbles was measured using a contact angle meter under electrolyte conditions at 60℃, with an average value of 154°±2°. The bubble adhesion work was calculated to be 28.4±1.8 mJ / m² based on the Young-Dupré equation. 2 .

[0037] (2) Electrolyte preparation Under nitrogen protection, monomethyl adipic acid (MMA) was dissolved in anhydrous methanol to prepare a 1.2 mol / L solution. KOH was then added for partial neutralization, resulting in an initial electrolyte pH of 8.8 and a conductivity of 27 ± 2 mS / cm. The water content of the system was measured using a Karl Fischer moisture analyzer and found to be less than 0.8 wt%, to reduce the risk of side reactions and ester hydrolysis.

[0038] (3) Assembly of electrolysis equipment A diaphragm-free microchannel flow electrolyzer, with the tank body made of polytetrafluoroethylene (PTFE), was employed. The Pt / Ti anode and 316L stainless steel cathode were installed parallel to each other, with an electrode spacing of 0.8 mm. A slit-type tangential nozzle with a width of 0.5 mm was used at the inlet, with the nozzle outlet 1.0 mm from the anode surface. The nozzle outlet direction was parallel to the anode working surface, allowing the electrolyte to flow tangentially at high speed along the anode surface. A corrosion-resistant magnetic circulation pump formed a closed-loop circulation. The power supply was a programmable asymmetric pulse power supply, capable of independently adjusting the current density, frequency, and duty cycle during the conduction and relaxation phases.

[0039] (4) Electrolysis reaction process Start the circulation pump to stabilize the linear velocity of the electrolyte on the anode surface at 3.5 m / s. Set the asymmetric pulse parameters as follows: High current density period: current density 180 mA / cm² 2 Lasts for 3ms; Low current density period: current density 2mA / cm2 Lasts 4ms; Pulse period: 7ms; Pulse frequency: approximately 142Hz; Duty cycle of the high current density period in a single cycle: 42.86%.

[0040] The reaction temperature was controlled at 55±2℃. A high-speed microscopic imaging system was used to observe the bubbles at a frame rate of 10000fps. The results showed that the average bubble diameter was 8~35μm; the average residence time was 0.04~0.12s; the phase contact line continuously contracted, with no obvious pinning phenomenon.

[0041] (5) Product analysis and performance evaluation Quantitative analysis was performed using an Agilent 7890B gas chromatograph (e.g. Figure 1 The results are as follows: Monomethyl adipic acid (MMA) conversion rate: 91.2%; Dimethyl sebacate (DMS) selectivity: 95.1%; Current efficiency: 82.4%; Average cell voltage: 4.8V; Main byproducts: dimethyl adipate 1.8%, methyl valerate 1.2%, methyl ester heavy components 1.5%.

[0042] In this embodiment, after 50 hours of continuous electrolysis, no significant carbon buildup or loss of luster was observed on the anode surface. Cyclic voltammetry was used to measure the double-layer capacitance and calculate the electrochemical active area (ECSA). The results showed that after 50 hours of operation, the anode maintained more than 92% of its initial active area.

[0043] The synergistic effect of high-speed fluid scouring and ultra-smooth surface effectively reduces bubble coverage, while maintaining stable mass transfer and low tank voltage under high current density conditions.

[0044] Comparative Example 1 In this comparative example, an industrial Pt / Ti anode without fine polishing was used, with a surface roughness Ra of 0.42 μm, and a constant current DC mode (180 mA / cm²) was employed. 2 Electrolysis was performed, and the remaining control conditions were the same as in Example 1.

[0045] Test results showed: average bubble residence time: 0.8~1.5s; obvious bubble aggregation on the electrode surface; average cell voltage: 6.7V; current efficiency: 61.3%; dimethyl sebacate (DMS) selectivity: 79.6%.

[0046] After 20 hours of continuous operation, obvious white deposits and localized loss of luster appeared on the anode surface, and the current efficiency decreased by about 18%.

[0047] Comparative Example 2 This comparative example uses the same ultra-smooth Pt / Ti anode as Example 1, but employs a conventional constant current DC electrolysis mode (180 mA / cm²). 2 Electrolysis was performed, and the remaining control conditions were the same as in Example 1.

[0048] The test results are as follows: average cell voltage: 5.9V; current efficiency: 70.5%; dimethyl sebacate (DMS) selectivity: 86.8%.

[0049] After 20 hours of continuous operation, the anode surface maintained a good morphology, but a small amount of grayish-white deposits and traces of tiny bubbles appeared in local areas. High-speed microscopic observation revealed that CO2 bubbles were continuously generated on the anode surface under constant current mode, with some bubbles remaining on the electrode surface for an extended period of 0.3–0.8 s, and coalescing to form localized bubble-covered areas.

[0050] The electrochemical active area (ECSA) of the electrode was tested by cyclic voltammetry. After 20 hours of continuous operation, the active area of ​​the electrode decreased by about 12.5%. At the same time, the average cell voltage increased from the initial 5.9V to 6.4V, the current efficiency decreased to 62.7%, and the DMS selectivity decreased to 81.4%.

[0051] After extending the operating time to 50 hours, the anode surface showed obvious loss of luster, a small amount of organic deposits formed in local areas, the electrode active area retention rate decreased to about 82%, and the reaction stability decreased significantly.

[0052] Comparative Example 3 Compared with Example 1, this comparative example uses an industrial Pt / Ti anode that has not undergone fine polishing, the surface roughness Ra of the catalyst layer is 0.42 μm, and electrolysis is performed using the same asymmetric pulsed current mode as in Example 1. The remaining control conditions are the same as in Example 1.

[0053] Tests showed that the average bubble residence time was 0.65~1.20s, and obvious bubble aggregation was observed on the electrode surface; the average cell voltage was 6.1V; the current efficiency was 68.7%; and the selectivity of dimethyl sebacate (DMS) was 82.5%.

[0054] After 20 hours of continuous operation, localized black deposits appeared on the anode surface, the electrode gloss decreased significantly, and the current efficiency decreased by about 15%.

[0055] As can be seen from Example 1 and Comparative Examples 1 and 3, under asymmetric pulsed current conditions, if the roughness of the anode catalytic interface is high, the micropores and protrusions will cause bubble pinning, reducing the desorption efficiency during the pulse relaxation stage and resulting in a decrease in the effective active area. Therefore, the combination of an ultra-smooth catalytic interface (Ra<0.05μm) with time-varying current and tangential electrolyte flow is an important condition for achieving highly selective electrolysis reactions.

[0056] Comparative Example 4 Compared with Example 1, this comparative example uses the same ultra-smooth Pt / Ti anode and asymmetric pulsed current conditions as Example 1, but changes the electrolyte flow mode by adjusting the direction of the slit nozzle to enter the anode surface vertically, so that the electrolyte cannot form a tangential high-speed flow along the anode surface. The other conditions are the same as in Example 1.

[0057] Tests showed that the average residence time of bubbles on the anode surface increased to 0.35~0.75s, and bubble aggregation and coverage occurred in some areas; the average cell voltage was 5.8V; the current efficiency was 71.6%; and the selectivity of dimethyl sebacate (DMS) was 85.3%.

[0058] After 20 hours of continuous operation, a small amount of white deposits appeared on the anode surface, and there was a significant decrease in activity in some areas of the electrode.

[0059] As can be seen from Example 1 and Comparative Example 4, relying solely on ultra-smooth electrodes and pulsed current cannot completely solve the problem of bubble retention under high current density. In this invention, the ultra-smooth anode interface, time-varying current combined with the shearing effect generated by tangential flow can further promote bubble desorption and enhance the transfer of reactants to the anode surface, thereby improving the raw material conversion rate and product selectivity.

[0060] Comparative Example 5 Compared with Example 1, this comparative example uses the same ultra-smooth Pt / Ti anode, tangential flow conditions and pulse current parameters as Example 1, but adjusts the pulse duty cycle to reduce the proportion of the low current density period in a single cycle to 9.1%, that is, the high current density period lasts for 5ms and the low current density period lasts for 0.5ms. The other conditions are the same as in Example 1.

[0061] Tests showed that the average residence time of bubbles was 0.20~0.45s, with some bubbles failing to desorb in time during the low current stage; the average cell voltage was 5.6V; the current efficiency was 73.2%; and the selectivity of dimethyl sebacate (DMS) was 87.1%.

[0062] After 20 hours of continuous operation, a small amount of carbon deposits appeared on the anode surface, and the electrode activity decreased by about 12%.

[0063] As can be seen from Example 1 and Comparative Example 5, if the low current density period is insufficient, it cannot provide a sufficient time window for free radical diffusion coupling and bubble desorption, resulting in increased secondary oxidation of free radicals and side reactions. Therefore, a low duty cycle is not conducive to improving reaction selectivity.

[0064] Comparative Example 6 Compared with Example 1, this comparative example uses the same ultra-smooth Pt / Ti anode and tangential flow conditions as Example 1, but increases the proportion of the low current density period to 83.3%, that is, the high current density period lasts for 3ms and the low current density period lasts for 15ms, and the other conditions are the same as in Example 1.

[0065] Tests showed that the average bubble residence time decreased to 0.05~0.15s, but the effective discharge time per unit time decreased; the MMA conversion rate decreased to 76.8%; the average cell voltage was 4.5V; the current efficiency was 74.6%; and the DMS selectivity was 90.2%.

[0066] After 20 hours of continuous operation, no obvious carbon deposits were observed on the anode surface, but the product formation rate was significantly reduced.

[0067] As can be seen from Example 1 and Comparative Example 6, while an excessively high proportion of low current phase is beneficial for bubble desorption, it reduces the effective electrochemical reaction time and decreases the reaction rate. Therefore, as can be seen from Example 1, Comparative Example 5, and Comparative Example 6, a suitable duty cycle can promote a balance between free radical generation and bubble desorption.

[0068] Comparative Example 7 Compared with Example 1, this comparative example uses the same ultra-smooth Pt / Ti anode and asymmetric pulsed current conditions as Example 1, but reduces the tangential flow velocity of the electrolyte on the anode surface to 0.5 m / s, while the other conditions are the same as in Example 1.

[0069] Tests showed that the average residence time of bubbles increased to 0.45~0.90s, and localized bubble coverage appeared on the anode; the average cell voltage was 5.9V; the current efficiency was 69.8%; and the selectivity of dimethyl sebacate (DMS) was 83.6%.

[0070] After 20 hours of continuous operation, a small amount of carbon deposits appeared on the anode surface, reducing long-term operational stability.

[0071] As can be seen from Example 1 and Comparative Example 7, an excessively low tangential flow rate cannot generate sufficient shear force to effectively strip CO2 bubbles formed on the anode surface, resulting in gas-liquid interface coverage and reduced electrode utilization.

[0072] Comparative Example 8 Compared with Example 1, this comparative example uses the same ultra-smooth Pt / Ti anode and asymmetric pulsed current conditions as Example 1, but increases the electrolyte tangential flow rate to 8.0 m / s, while the other conditions are the same as in Example 1.

[0073] Tests showed that the average residence time of bubbles decreased to 0.03~0.08s, but due to local pressure fluctuations caused by high-speed flow and a decrease in reactant residence time, the average cell voltage increased to 5.7V; the current efficiency was 72.5%; and the DMS selectivity was 86.9%.

[0074] After 20 hours of continuous operation, no obvious carbon deposits were observed on the anode surface, but the energy consumption of the circulating pump increased and the system's operational stability decreased.

[0075] As can be seen from Example 1 and Comparative Examples 7 and 8, there is an optimal range for tangential flow velocity. Too low a flow velocity cannot effectively desorb bubbles, while too high a flow velocity will increase flow energy consumption and reduce the effective residence time of reactants at the electrode interface. Therefore, the optimal balance between bubble desorption and mass transfer enhancement can be achieved in the range of 1.5~5.0 m / s.

[0076] Comparative Example 9 Compared with Example 1, this comparative example uses a common Pt / Ti anode (Ra=0.42μm), electrolysis is performed in constant current DC mode, and tangential flow circulation is eliminated. Only static electrolysis conditions are used, and the other conditions are the same as in Example 1.

[0077] Tests showed that the average cell voltage reached 7.3V; the current efficiency decreased to 52.4%; and the DMS selectivity was only 70.8%.

[0078] After 10 hours of continuous operation, obvious black carbon deposits appeared on the anode surface, and the electrode lost its mirror-like luster; the electrolysis was terminated prematurely.

[0079] As can be seen from Example 1 and the comparative examples, the present invention achieves high selectivity, high current efficiency and long-term stable operation through the synergistic effect of reducing bubble adhesion through an ultra-smooth catalytic interface, providing a relaxation and desorption window through asymmetric pulses, and enhancing shear stripping through tangential flow.

[0080] Example 2

[0081] A method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis includes the following steps: (1) Preparation of Pt / Ni anode A 100mm × 50mm × 2mm industrial pure nickel plate was used as the substrate. It was mechanically polished using alumina polishing slurries with particle sizes of 0.5μm and 0.05μm until a mirror-like finish was achieved. The polished nickel substrate was then ultrasonically cleaned sequentially in acetone, anhydrous ethanol, and deionized water for 15 minutes each, and dried with nitrogen gas. After drying, the substrate was observed at a 45° angle. If no interference rainbow appeared, the substrate was considered clean.

[0082] A magnetron sputtering deposition machine with a DC sputtering source and a 99.99% pure platinum target was used to deposit a platinum layer on the surface of the polished nickel substrate. The sputtering process parameters were: background vacuum ≤ 6 × 10⁻⁶. -4 Pa, argon gas pressure 0.5 Pa, sputtering power 150 W, sputtering time 30 min.

[0083] Using an atomic force microscope (AFM, such as the Bruker Dimension Icon), and in accordance with the ISO 4287:1997 standard, the sampling length (λc) was set to 0.8 mm, the evaluation length to 4.0 mm, and the average value of 5 lines was measured. The surface roughness Ra value of the catalyst layer was measured to be 0.028 μm.

[0084] Field emission scanning electron microscopy (FE-SEM, Zeiss GeminiSEM 500) was used with an accelerating voltage of 10 kV, a working distance of 8 mm, and a magnification of 50,000 to observe the cross-section and surface morphology of the coating. No pores or cracks with a diameter or width greater than 100 nm were observed in any of the five different regions within a 20 μm × 20 μm area. The thickness of the platinum coating was measured to be 1.91 ± 0.02 μm based on the cross-section.

[0085] The static contact angle between the electrode surface and CO2 bubbles was measured using a contact angle meter in an electrolyte solution at 60℃. The average value of three measurements was 150±3°. The bubble adhesion work was calculated to be 35.1±2.3 mJ / m using the Young-Dupré equation. 2 .

[0086] (2) Electrolyte preparation Under dry nitrogen protection, monomethyl adipic acid (MMA) (purity ≥99.5%) was dissolved in anhydrous methanol to prepare a solution with an initial concentration of 1.0 mol / L. KOH was slowly added to adjust the initial conductivity of the solution to 25 ± 2 mS / cm. The molar ratio of KOH to MMA was 0.1:1 (i.e., 10% partial neutralization), simultaneously making the solution weakly alkaline. The initial pH of the solution was adjusted to 8.0–9.5 (measured pH = 8.5), which effectively inhibited ester hydrolysis and improved decarboxylation efficiency. The prepared electrolyte was filtered through a 0.45 μm polytetrafluoroethylene (PTFE) membrane and transferred to a storage tank, which was then stored in a sealed, light-protected environment throughout the process.

[0087] (3) Assembly of electrolysis equipment A diaphragm-free microchannel electrolytic cell, with the cell body made of polytetrafluoroethylene (PTFE), was employed. The prepared Pt / Ni anode and a 316L stainless steel cathode of the same size were installed parallel to each other within the electrolytic cell, with the electrode spacing controlled to 1.0 mm using PTFE gaskets. An inlet and an outlet were respectively provided at both ends of the electrolytic cell. The inlet was connected to a slit-type fluid distributor with a slit width of 1.0 mm and a nozzle outlet distance of 1.0 mm from the electrode surface, allowing the electrolyte to flow into the microchannel in a tangential direction parallel to the electrode surface. A circulation pump was used to pump the electrolyte from a storage tank into the electrolytic cell, forming a closed-loop circulation. A programmable asymmetric pulsed DC power supply with a pulse control module was used, electrically connected to the anode and cathode.

[0088] (4) Electrolysis reaction process Start the circulation pump and adjust the valves to stabilize the linear velocity of the electrolyte in the microchannel at 2.0 m / s. Turn on the pulse power supply and set the asymmetric pulse parameters as follows: High current density period (conduction period): Current density 150 mA / cm 2 Duration: 5ms; Low current density period (turn-off period): Current density 0 mA / cm 2 Duration: 5ms; Pulse period: 10ms, frequency approximately 142.9Hz, duty cycle of 50% during the high current density period.

[0089] The electrolysis reaction is carried out under isothermal conditions, with the temperature inside the electrolytic cell maintained at 50±2℃ by an external circulating cooling water system. Nitrogen gas is continuously introduced for protection during electrolysis, and the gaseous products are automatically discharged through a gas-liquid separator above the liquid outlet.

[0090] High-speed microscopic imaging at a frame rate of 10,000 fps, combined with a 20x microscope lens, showed that the diameter of the bubbles generated on the anode surface was 10-50 μm, the average residence time was 0.05-0.2 s, and the three-phase contact line showed a continuous and smooth contraction, which was determined to be spontaneous slippage desorption.

[0091] After the reaction ran continuously for 10 hours, the power was stopped and the circulation pump was turned off. The anode plate was removed and its surface condition was observed: no bubble adsorption or white deposits were observed under natural light, and the electrode surface maintained a mirror-like gloss.

[0092] (5) Product analysis and performance evaluation The reaction solution sample was quenched with deionized water, and the internal standard n-dodecane was added. Quantitative analysis was performed using an Agilent 7890B gas chromatograph with an HP-5 capillary column and an FID detector. Chromatographic conditions: injector temperature 280℃, detector temperature 300℃, column oven initial temperature 60℃ for 2 min, then increased to 280℃ at a rate of 10℃ / min and held for 5 min.

[0093] The analysis results are as follows: Monomethyl adipic acid (MMA) conversion rate: 88.0%; Dimethyl sebacate (DMS) selectivity: 93.5%; Current efficiency: 78.6%; Average cell voltage: 5.1V; Main byproducts: dimethyl adipate 2.1%, methyl valerate 1.3%, methyl ester heavy components 1.8%.

[0094] It should be noted that the above description is a further detailed explanation of the present invention in conjunction with specific embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple improvements can be made without departing from the concept of the present invention, and all such improvements should be considered to fall within the scope of protection of the present invention.

[0095] Example 3

[0096] A method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis includes the following steps: (1) Preparation of Ir / Ti anode An industrial pure titanium plate with dimensions of 50 mm × 50 mm × 2 mm was selected as the conductive substrate. It was mechanically polished sequentially using alumina polishing slurries with particle sizes of 1.0 μm, 0.3 μm, and 0.05 μm until the substrate surface exhibited a continuous mirror-like reflection. The polished titanium substrate was then ultrasonically cleaned for 15 min each with acetone, anhydrous ethanol, and deionized water, dried under nitrogen, and placed in a magnetron sputtering system for Ir catalyst layer deposition.

[0097] The magnetron sputtering coating machine was used, and the process parameters are as follows: base vacuum degree ≤5×10 -4 Pa, argon gas pressure 0.45 Pa, sputtering power 160 W, sputtering time 35 min.

[0098] The surface roughness of the catalyst layer was measured using atomic force microscopy (AFM, Bruker Dimension Icon). According to ISO 4287:1997, the sampling length λc was 0.8 mm, the evaluation length was 4.0 mm, and measurements were taken in five different regions. The average surface roughness Ra of the Ir catalyst layer was found to be 0.038 μm.

[0099] The surface and cross-sectional morphology of the catalyst layer were observed using field emission scanning electron microscopy (FE-SEM, Zeiss GeminiSEM 500). No obvious cracks or through-holes were observed at 50,000x magnification. Cross-sectional statistical results showed that the Ir catalyst layer thickness was 1.35 ± 0.05 μm and the porosity was less than 0.5%.

[0100] The contact angle between CO2 bubbles and the surface of the Ir catalyst layer was measured using a contact angle meter under electrolyte conditions at 60℃, with an average value of 148°±3°. The bubble adhesion work was calculated to be 39.6±2.5 mJ / m² based on the Young-Dupré equation.

[0101] (2) Electrolyte preparation Under nitrogen protection, monomethyl adipic acid (MMA) was dissolved in anhydrous methanol to prepare an electrolyte with an initial concentration of 1.5 mol / L. KOH was then added for partial neutralization, adjusting the initial pH of the electrolyte to 9.0 and the conductivity to 26 ± 2 mS / cm. The water content of the system was measured to be less than 0.8 wt% using a Karl Fischer moisture analyzer. The prepared electrolyte was filtered through a 0.45 μm polytetrafluoroethylene (PTFE) membrane and transferred to a storage tank for later use.

[0102] (3) Assembly of electrolysis equipment A diaphragm-free microchannel flow electrolyzer, with the tank body made of polytetrafluoroethylene (PTFE), was used. The prepared Ir / Ti anode and 316L stainless steel cathode were installed parallel to each other within the electrolyzer, with the electrode spacing controlled to 1.0 mm using PTFE gaskets. A slit-type tangential nozzle with a width of 1.0 mm was installed at the inlet end of the electrolyzer, and the nozzle outlet was 1.5 mm from the anode surface, allowing the electrolyte to flow at high speed tangentially along the anode surface. A corrosion-resistant circulating pump was used to circulate the electrolyte between the storage tank and the electrolyzer.

[0103] (4) Electrolysis reaction process Start the circulation pump and adjust the flow rate to stabilize the linear velocity of the electrolyte on the anode surface at 5.0 m / s. Electrolysis is performed using an asymmetric pulsed power supply, with the pulse parameters set as follows: High current density period: Anode current density is 230 mA / cm², duration is 5 ms; Low current density period: Anode current density is 1 mA / cm², duration is 8 ms.

[0104] The pulse period is 13 ms, the pulse frequency is approximately 76.9 Hz, and the duty cycle during the high current density period is 38.5%.

[0105] The reaction temperature was controlled at 55±2℃.

[0106] The behavior of bubbles on the anode surface during electrolysis was observed using a high-speed microscopic imaging system at a frame rate of 10,000 fps.

[0107] The results show that: The diameter of the bubbles generated on the anode surface ranged from 8 to 45 μm, and the average residence time was 0.06 to 0.15 s. During the bubble desorption process, the three-phase contact line continuously contracted, and no obvious bubble aggregation was observed.

[0108] (5) Product analysis and performance evaluation The electrolysis products were quantitatively analyzed using gas chromatography. The analytical results are as follows: Monomethyl adipic acid (MMA) conversion rate: 89.6%; Dimethyl sebacate (DMS) selectivity: 94.0%; Current efficiency: 80.1%; Average cell voltage: 5.0 V. Major byproducts include: dimethyl adipate 1.9%, methyl valerate 1.1%, and methyl ester heavy components 1.6%.

[0109] After running continuously for 30 hours, electrolysis was stopped and the anode was removed for observation.

[0110] The results show: No obvious carbon deposition or catalyst layer shedding was observed on the anode surface. The electrochemical active area (ECSA) of the electrode was measured by cyclic voltammetry, and it still maintained more than 90% of the initial active area after 30 h of operation.

[0111] The results above show that using an Ir / Ti ultra-smooth anode, under the synergistic effect of asymmetric pulsed current and tangential flow field, can effectively reduce the retention of bubbles on the anode surface during electrolysis, improve the efficiency of the decarboxylation coupling reaction of adipic acid monomethyl ester, and achieve highly selective preparation of dimethyl sebacate.

[0112] It should be noted that the above description is a further detailed explanation of the present invention in conjunction with specific embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple improvements can be made without departing from the concept of the present invention, and all such improvements should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis, characterized in that, include: An electrolyte containing monomethyl adipic acid flows tangentially over the surface of an anode, the anode comprising a conductive substrate and a catalyst layer disposed on the surface of the conductive substrate, the surface roughness Ra of the catalyst layer being less than 0.05 μm; A time-varying current is applied to the anode and cathode, the time-varying current including alternating periods of high current density and low current density; the anode current density during the high current density period is 100~250 mA / cm². 2 The anode current density during the low current density period is no greater than 5 mA / cm². 2 ; During the high current density period, monomethyl adipic acid undergoes an electrolytic decarboxylation coupling reaction on the anode surface to generate dimethyl sebacate and produce bubbles; during the low current density period, the bubbles desorb from the anode surface and are discharged.

2. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The duty cycle of the high current density period is 20%~70%, preferably 30%~60%, and the pulse frequency is 10~500Hz.

3. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1 or 2, characterized in that, The duration of the high current density period is 0.5~5ms, preferably 3~5ms; the duration of the low current density period is 0.5~20ms, preferably 4~8ms; and the duration of the low current density period is not less than the duration of the high current density period.

4. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The adhesion work of the bubbles generated on the anode surface is ≤0.05 J / m. 2 The diameter of the bubbles is 5~50μm; Preferably, the average residence time of the bubbles is 0.04~0.3s.

5. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The electrolyte forms a tangential flow with a linear velocity of 1.5~5.0 m / s on the anode surface, preferably a tangential flow with a linear velocity of 2~3.5 m / s; The electrolyte temperature is controlled at 45~60℃, preferably 50~55℃; Preferably, the electrolyte is in a transitional or turbulent flow state within the electrolytic cell, and its Reynolds number is 2100~8000.

6. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The electrolyte is introduced through a slit nozzle, so that the electrolyte flows tangentially across the anode surface. The slit width of the slit nozzle is 0.05~1.5mm, preferably 0.5~1mm; the distance between the nozzle outlet and the anode surface is 0.5~3mm, preferably 1~2mm.

7. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The electrolyte comprises monomethyl adipic acid and an organic solvent, wherein the organic solvent comprises at least one of methanol and acetonitrile, and the initial concentration of the electrolyte is 0.5~2 mol / L, preferably 1~1.5 mol / L, and the initial pH value is 8.0~9.5; Preferably, the initial conductivity of the electrolyte is 20~30 mS / cm, more preferably 25~27 mS / cm; Preferably, the water content of the electrolyte is not higher than 1 wt%, and more preferably not higher than 0.8 wt%.

8. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The duty cycle of the high current density period is 20% to 70%, preferably 30% to 60%, and the electrolyte forms a tangential flow with a linear velocity of 1.5 to 5.0 m / s on the anode surface.

9. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The time-varying current is an asymmetric pulse current, preferably an asymmetric square wave pulse current.

10. The method for preparing dimethyl sebacate from monomethyl adipic acid by electrolysis according to claim 1, characterized in that, The conductive substrate is any one of titanium sheet, nickel sheet, and niobium sheet, and the catalyst layer is a platinum plating layer, ruthenium plating layer, or iridium plating layer disposed on the conductive substrate; And / or, the surface roughness Ra of the catalyst layer is <0.03 μm; And / or, the porosity of the catalyst layer is less than 0.5%, and the thickness of the catalyst layer is 0.5~3μm, preferably 0.5~2μm; And / or, the cathode is any one of stainless steel, nickel, and graphite; And / or, the distance between the anode and the cathode is 0.5~1.5mm, preferably 0.8~1mm.

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