An electrochemical method for hydrogenation of phthalate plasticizers

Abstract

This invention discloses an electrochemical method for the hydrogenation of phthalate plasticizers, belonging to the field of fine chemicals. Specifically, using an electrochemical method with platinum-containing metal as the cathode material, a liquid-liquid-solid three-phase system is constructed and rapidly stirred: organic phase-acidic aqueous electrolyte-catalyst. This successfully achieves the electrocatalytic hydrogenation of phthalate plasticizers to 1,2-cyclohexanedicarboxylate with a selectivity greater than 99%. The entire process requires no external hydrogen input, can proceed at room temperature and atmospheric pressure, and features mild conditions, simple process, and high catalyst stability, aligning with the development direction of green chemical synthesis. Simultaneously, the hydrogenation product can be directly separated from the electrolyte, achieving spontaneous purification.

Description

Technical Field

[0001] This invention belongs to the field of fine chemicals and relates to an electrochemical method for hydrogenating phthalate plasticizers. Background Technology

[0002] Phthalate esters are widely used as plasticizers, and their use is increasing year by year. They are used in hundreds of products, including toys, food packaging materials, detergents, nail polish, and hairspray. However, phthalates are fat-soluble compounds that can enter the human body through various pathways and have toxic effects on multiple organs, especially harmful to the health of pregnant women and infants. Therefore, the development of non-toxic, biodegradable plasticizer alternatives has attracted much attention. Compared with phthalate esters, 1,2-cyclohexanedicarboxylate, obtained by directly hydrogenating its aromatic ring, has higher safety and plasticizing properties and is expected to replace phthalates, becoming a current research hotspot.

[0003] Among existing preparation technologies, the thermocatalytic hydrogenation of phthalates to 1,2-cyclohexanedicarboxylate has been widely reported. For example, Chinese invention patent CN108435242A discloses a supported catalyst for the hydrogenation of phthalates, using molecular sieves as a support and ruthenium, platinum, and palladium as active components, with the addition of additives. Although the catalyst prepared by the above method has good stability and the precious metals are not easily lost, the catalyst preparation process is complex and the reaction conditions are relatively harsh, with a temperature of 200℃ and a hydrogen pressure of 5 MPa. Chinese invention patent CN113332977A discloses a ruthenium-based catalyst prepared by supercritical fluid deposition for the selective hydrogenation of phthalates in water. This method has a simple product-solvent separation process and avoids the use of organic solvents, but the reactor pressure reaches 2000-2500 psi during catalyst preparation, and the reaction conditions are even more harsh, with the hydrogen pressure reaching up to 10 MPa.

[0004] In contrast, electrocatalytic hydrogenation can be carried out at room temperature and atmospheric pressure, using water as the hydrogen source and powered by electricity generated from renewable energy sources such as solar and wind power. This represents a shift from harsh processes to milder conditions, meeting the requirements of sustainable energy production. Therefore, developing a greener and milder process for the electrohydrogenation of phthalates to 1,2-cyclohexanedicarboxylate is particularly necessary. However, phthalates are soluble in organic solvents but insoluble in water, making them unsoluble in conventional aqueous electrolytes and preventing the reaction from occurring. While organic solvents can dissolve phthalates, their lack of conductivity prevents electron transport at the electrode-organic phase interface, and the organic phase cannot drive hydrogen ion movement, thus limiting its practical application. Therefore, introducing an organic phase into an aqueous electrolyte to construct a new system is expected to simultaneously solve the solubility problem of phthalates and the conductivity problem of the electrolyte, thereby promoting the electrochemical reaction. Summary of the Invention

[0005] This invention provides an electrochemical method for the hydrogenation of phthalate plasticizers, avoiding the operational hazards and catalyst deactivation issues associated with traditional thermocatalytic hydrogenation of phthalate plasticizers, which rely on hydrogen as the hydrogen source and require high temperature and pressure. It also effectively reduces the use of organic solvents. By constructing a liquid-liquid-solid three-phase system, a small amount of water-insoluble organic solvent is added to dissolve the phthalate plasticizers. An acidic electrolyte aqueous solution serves as the medium for conduction and hydrogen ion transport, forming two phases as the electrolyte. Rapid stirring triggers the reaction through three-phase mass transfer at the catalyst electrode surface, successfully achieving highly selective electrocatalytic hydrogenation of phthalate plasticizers to 1,2-cyclohexanedicarboxylate. The entire process uses water as the hydrogen source, requiring no external hydrogen input, and can be carried out at room temperature and atmospheric pressure. The reaction conditions are mild, the process is simple, and it aligns with the basic principles and development direction of green chemistry. Furthermore, the hydrogenation product can be directly separated from the electrolyte, achieving spontaneous purification.

[0006] The technical solution of the present invention:

[0007] An electrochemical method for hydrogenating phthalate plasticizers, comprising the following steps:

[0008] An H-type electrolytic cell with a diaphragm was used as the reaction apparatus; the organic phase consisted of phthalate dissolved in a completely water-insoluble organic solvent; the inorganic phase consisted of an aqueous solution of an inorganic acid as the medium for conducting electricity and transporting hydrogen; the two phases were rapidly stirred to form the cathode electrolyte for electrolysis, realizing the hydrogenation of phthalate to 1,2-cyclohexanedicarboxylate; the phthalate content in the cathode electrolyte was 0.001M to 0.01M, the concentration of the inorganic acid was 0.1M to 0.5M, the volume ratio of the inorganic phase to the organic phase was 4:1 to 2:1, and the stirring speed was 1000rpm to 1500rpm; constant current electrolysis was used, with an electrolysis current of -25mA to -100mA, an electrolysis time of 1.5 to 12h, and a reaction temperature of room temperature.

[0009] The phthalate is one of dimethyl phthalate, diethyl phthalate, and dipropyl phthalate.

[0010] The cathode material in the reaction device is one of platinum, platinum-ruthenium, or platinum-rhodium.

[0011] The anode material in the reaction apparatus is one of platinum, platinum-titanium, copper, carbon rod, or carbon fiber.

[0012] The organic solvent is one of n-heptane, cyclohexane, and decahydronaphthalene.

[0013] The inorganic acid mentioned is a common strong inorganic acid, such as sulfuric acid, hydrochloric acid, or perchloric acid aqueous solution, and there is no limitation here.

[0014] The beneficial effects of this invention are: the method requires no external hydrogen input, operates under mild conditions, is simple to operate, and has high catalyst stability. By constructing a liquid-liquid-solid three-phase system, the water-insoluble organic phase is used to dissolve phthalate plasticizers and stabilize them in the electrolyte; the inorganic acid aqueous solution acts as a medium for conduction and hydrogen ion transport, generating adsorbed hydrogen on the electrode surface as the hydrogen source for the reaction. Rapid stirring triggers the reaction through three-phase mass transfer at the catalyst electrode surface. Simultaneously, the product can be spontaneously purified to the organic phase, avoiding the cumbersome process of separating the product in the aqueous phase. This provides a valuable pathway for the conversion and utilization of phthalates at room temperature and the highly selective synthesis of 1,2-cyclohexanedicarboxylate. Attached Figure Description

[0015] Figure 1 The results represent the stability of the electrocatalytic hydrogenation of dimethyl phthalate to 1,2-cyclohexanedicarboxylate using platinum-ruthenium alloy as the cathode material, n-heptane as the organic phase, and an organic phase to water phase ratio of 1:2. Detailed Implementation

[0016] The present invention will be described in detail below through embodiments. The embodiments described below are exemplary and are only used to explain the present invention, but the present invention is not limited to these embodiments.

[0017] Example 1 (Influence of Organic Phase Type)

[0018] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm H-type electrolytic cell. The aqueous phase was a 0.2 M sulfuric acid aqueous solution, and the organic phases consisted of n-heptane, decahydronaphthalene, cyclohexane, ethyl acetate, and 1,4-dioxane. Dimethyl phthalate was added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. Since 1,4-dioxane is soluble in water, it did not separate into layers. The anolyte was a 0.2 M sulfuric acid aqueous solution. A platinum-ruthenium alloy was used as the cathode material, and platinum as the anode material. Electrolysis was performed at a constant current of -50 mA for 4 hours with a stirring speed of 1200 rpm. After electrolysis, the solution was allowed to stand to allow separation. The upper organic phase was then quantitatively analyzed by gas chromatography. The electrolyte solution using 1,4-dioxane as the organic phase was extracted with ethyl acetate, and the extracted solution was then quantitatively analyzed by gas chromatography. The effects of organic phase type on the conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faradaic efficiency were investigated, and the results are as follows:

[0019]

[0020] The results above show that the reaction conversion and Faraday efficiency significantly decrease when using ethyl acetate and 1,4-dioxane. Ethyl acetate is similar in properties to n-heptane, but compared to n-heptane, decahydronaphthalene, and cyclohexane, which are completely insoluble in water, ethyl acetate is slightly soluble in water. As the reaction time increases, the ethyl acetate content in the aqueous electrolyte increases, and the increased organic matter content in the aqueous electrolyte reduces the migration rate of hydrogen ions, leading to a decrease in the rate of adsorbed hydrogen generation at the cathode and a reduction in reaction performance.

[0021] 1,4-Dioxane, used as a co-solvent, can stabilize the substrate in the aqueous electrolyte. However, since 1,4-dioxane and sulfuric acid solution are in one phase, the hydrogen ion transport rate and the reaction rate for generating adsorbed hydrogen are significantly reduced compared to pure sulfuric acid solution. Compared to a liquid-liquid-solid three-phase reaction system, water-insoluble products generated in situ on the electrode surface are difficult to remove, whereas with a three-phase reaction system, the products can be directly separated from the electrolyte.

[0022] Example 2 (Influence of Electrolysis Current)

[0023] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm-equipped H-type electrolytic cell. The aqueous phase was a 0.2 M sulfuric acid solution, and the organic phase was n-heptane. Dimethyl phthalate was added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M sulfuric acid solution. Platinum-ruthenium alloy was used as the cathode material, and platinum-titanium alloy as the anode material. Constant current electrolysis was performed at 1200 rpm under conditions of -25 mA, -50 mA, -75 mA, -100 mA, and -300 mA. The reaction time was calculated based on the transfer of the same amount of charge (540 C). After electrolysis, the solution was allowed to stand for separation. The upper organic phase was then quantitatively analyzed by gas chromatography to investigate the effects of electrolytic current on the conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faraday efficiency. The results are as follows:

[0024]

[0025] The results above show that when the electrolysis current is large and the same amount of charge is transferred, the conversion rate and Faraday efficiency of the reaction decrease significantly. Although a large current will simultaneously drive the main reaction and the competing reaction (i.e., the hydrogen evolution reaction), the hydrogen evolution reaction, which requires only two electrons, is more likely to occur under these conditions than the hydrogen addition reaction, which requires six electrons. Therefore, the rate of the hydrogen evolution reaction is significantly accelerated.

[0026] Example 3 (Effect of stirring speed)

[0027] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm H-type electrolytic cell. The aqueous phase consisted of 0.2 M hydrochloric acid, and the organic phase was n-heptane. Dimethyl phthalate was added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M hydrochloric acid solution. A platinum-ruthenium alloy was used as the cathode material, and copper as the anode material. Electrolysis was performed at constant current for 3 hours at -50 mA with stirring speeds of 800 rpm, 1000 rpm, 1200 rpm, and 1500 rpm. After electrolysis, the solution was allowed to stand for phase separation. The upper organic phase was then quantitatively analyzed by gas chromatography to investigate the effects of stirring speed on the conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faraday efficiency. The results are as follows:

[0028]

[0029] The results above show that when the stirring speed is low, the conversion rate and Faraday efficiency of the reaction decrease significantly. Since n-heptane is insoluble in water, excessively low stirring speeds prevent sufficient contact between the aqueous and organic phases. This results in hydrogen ions generated by water electrolysis at the anode failing to diffuse rapidly to the cathode catalyst surface after passing through the diaphragm, reducing the number of hydrogen ions gaining electrons to form adsorbed hydrogen. Simultaneously, the exchange rate of the three phases on the catalyst surface decreases significantly. Excessively high local aqueous phase content near the electrodes hinders the approach of reactants and the desorption of products, while excessively high local organic phase content hinders the hydrogenation reaction.

[0030] Example 4 (Effect of the ratio of organic phase to aqueous phase)

[0031] Electrolytes with aqueous and organic phase volume ratios of 4:1, 3:1, 2:1, and 1:1 were prepared and added to the cathode chamber of a diaphragm-equipped H-type electrolytic cell. The aqueous phase consisted of 0.2 M perchloric acid, and the organic phase was n-heptane. Dimethyl phthalate was added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M perchloric acid solution. A platinum-ruthenium alloy was used as the cathode material, and a carbon rod as the anode material. Electrolysis was performed at a constant current of -50 mA for 3 hours with a stirring speed of 1200 rpm. After electrolysis, the solution was allowed to stand for phase separation. The upper organic phase was then quantitatively analyzed by gas chromatography to investigate the effects of the aqueous to organic phase ratio on the conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faraday efficiency. The results are as follows:

[0032]

[0033] The results above show that when the organic phase content is too high, the conversion rate and Faraday efficiency of the reaction decrease significantly. Excessive organic phase content reduces the probability of hydrogen ions generated by anodic electrolysis contacting the cathode after passing through the diaphragm, significantly decreasing the rate of adsorbed hydrogen formation and hindering the hydrogenation reaction.

[0034] Example 5 (Effect of Inorganic Acid Concentration)

[0035] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm-equipped H-type electrolytic cell. The aqueous phase consisted of 0.2 M potassium sulfate solution, 0.1 M, 0.2 M, and 0.5 M sulfuric acid aqueous solutions, respectively. The organic phase was n-heptane, with dimethyl phthalate added to the organic phase to a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M sulfuric acid aqueous solution. A platinum-ruthenium alloy was used as the cathode material, and carbon fiber as the anode material. Electrolysis was performed at a constant current of -50 mA for 3 hours with a stirring speed of 1200 rpm. After electrolysis, the solution was allowed to stand for separation. The upper organic phase was then quantitatively analyzed by gas chromatography to investigate the effects of inorganic acid concentration in the aqueous electrolyte on the conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faraday efficiency. The results are as follows:

[0036]

[0037] The results above show that when the concentration of inorganic acid in the electrolyte is low, the conversion rate and Faraday efficiency of the reaction decrease significantly. Since the potassium sulfate solution contains only potassium ions and sulfate ions, the kinetics of hydrogen ions moving to the cathode after passing through the membrane to generate adsorbed hydrogen are slow, and the reduced amount of adsorbed hydrogen as a hydrogen source significantly decreases the reaction rate.

[0038] Example 6 (Influence of Cathode Material)

[0039] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm H-type electrolytic cell. The aqueous phase was a 0.2 M sulfuric acid aqueous solution, and the organic phase was n-heptane. Dimethyl phthalate was added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M sulfuric acid aqueous solution. Platinum, platinum-ruthenium, platinum-rhodium alloy, and copper were used as cathode materials, respectively, and platinum was used as the anode material. Electrolysis was performed at a constant current for 3 hours at -50 mA with a stirring speed of 1200 rpm. After electrolysis, the solution was allowed to stand for separation. The upper organic phase was taken and quantitatively analyzed by gas chromatography to investigate the effects of the cathode material on the conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faraday efficiency. The results are as follows:

[0040]

[0041] The results above show that when copper is used as the cathode material, the conversion rate and Faraday efficiency of the reaction are zero. This is because copper lacks the ability to adsorb and activate benzene rings in electrocatalytic reactions, preventing the reaction from occurring.

[0042] Example 7 (Effect of substrate concentration)

[0043] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm H-type electrolytic cell. The aqueous phase was a 0.2 M sulfuric acid solution, and the organic phase was n-heptane. Dimethyl phthalate was added to the organic phase to achieve concentrations of 0.005 M, 0.01 M, and 0.02 M in the electrolyte solution. The anolyte was a 0.2 M sulfuric acid solution. A platinum-ruthenium alloy was used as the cathode material, and platinum as the anode material. Electrolysis was performed at a constant current of -50 mA for 3 hours with a stirring speed of 1200 rpm. After electrolysis, the solution was allowed to stand for phase separation. The upper organic phase was then quantitatively analyzed by gas chromatography to investigate the effects of substrate concentration on the conversion rate, selectivity of 1,2-cyclohexanedicarboxylate, and Faraday efficiency. The results are as follows:

[0044]

[0045] The results above show that when the substrate concentration is too high, the conversion rate and Faraday efficiency of the reaction decrease. When the dimethyl phthalate concentration is too high, the electrode surface cannot provide enough active sites, resulting in a large amount of residual substrate remaining in the organic phase and unable to participate in the reaction effectively.

[0046] Example 8 (Different Substrate Types)

[0047] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm H-type electrolytic cell. The aqueous phase was a 0.2 M sulfuric acid aqueous solution, and the organic phase was n-heptane. Dimethyl phthalate, diethyl phthalate, and dipropyl phthalate were added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M sulfuric acid aqueous solution. A platinum-ruthenium alloy was used as the cathode material, and platinum-titanium was used as the anode material. Electrolysis was performed at a constant current of -50 mA with a stirring speed of 1200 rpm. After electrolysis, the solution was allowed to stand for separation. The upper organic phase was then quantitatively analyzed by gas chromatography to investigate the effects of the reaction system on the conversion rate of different substrates, the selectivity of 1,2-cyclohexanediol ester, and the Faraday efficiency. The results are as follows:

[0048]

[0049]

[0050] Example 9 (Stability Test)

[0051] 30 mL of an electrolyte solution with a 2:1 volume ratio of aqueous to organic phase was prepared and added to the cathode chamber of a diaphragm-type H-cell electrolytic cell. The aqueous phase consisted of 0.2 M sulfuric acid, and the organic phase was n-heptane. Dimethyl phthalate was added to the organic phase to achieve a concentration of 0.01 M in the electrolyte. The anolyte was a 0.2 M sulfuric acid solution. A platinum-ruthenium alloy was used as the cathode material, and platinum as the anode material. Electrolysis was performed at a constant current of 1200 rpm under -50 mA conditions for 3 hours, with multiple cycles. After electrolysis, the solution was allowed to stand for separation. The upper organic phase was quantitatively analyzed by gas chromatography to investigate the stability of the cathode material. The conversion rate of dimethyl phthalate, the selectivity of 1,2-cyclohexanedicarboxylate, and the Faraday efficiency were calculated. The results are shown in the attached figure. Figure 1 As shown.

Claims

1. An electrochemical method for hydrogenating phthalate plasticizers, characterized in that, The steps are as follows: An H-type electrolytic cell with a diaphragm was used as the reaction apparatus. The organic phase consisted of phthalate dissolved in a completely water-insoluble organic solvent. The inorganic phase consisted of an aqueous solution of an inorganic acid as the conductive medium and hydrogen source. The two phases were rapidly stirred to form the cathode electrolyte for electrolysis, achieving the hydrogenation of phthalate to 1,2-cyclohexanedicarboxylate. The phthalate content in the cathode electrolyte was 0.001M~0.01M, the inorganic acid concentration was 0.1M~0.5M, the volume ratio of the inorganic phase to the organic phase was 4:1~2:1, and the stirring speed was 1000rpm~1500rpm. A constant current electrolysis method was used, with an electrolysis current of -25mA~-100mA, an electrolysis time of 1.5~12h, and a reaction temperature of room temperature. The organic solvent is one of n-heptane, cyclohexane, and decahydronaphthalene.

2. The electrochemical method for hydrogenation of phthalate plasticizers according to claim 1, characterized in that, The inorganic acid is sulfuric acid, hydrochloric acid, or perchloric acid.

3. The electrochemical method for hydrogenating phthalate plasticizers according to claim 1, characterized in that, The phthalate is one of dimethyl phthalate, diethyl phthalate, and dipropyl phthalate.

4. The electrochemical method for hydrogenation of phthalate plasticizers according to claim 1, characterized in that, The cathode material in the aforementioned reaction device is one of platinum, platinum-ruthenium, or platinum-rhodium.

5. The electrochemical method for hydrogenating phthalate plasticizers according to claim 1, characterized in that, The anode material in the aforementioned reaction device is one of platinum, platinum-titanium, copper, carbon rod, or carbon fiber.

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