Application of halide ion mediation in preparation of glycollic acid through alkali-free oxidation of ethylene glycol

Abstract

The invention discloses an application of halide ion mediation in preparation of glycollic acid by alkali-free oxidation of ethylene glycol, an electrolyte containing halide ions is used as an electrolyte for preparation of glycollic acid by alkali-free oxidation of ethylene glycol, halide ion mediation is utilized, halide ions are activated into halide free radicals at a photo-anode, and the halide free radicals react with ethylene glycol to generate a halide-containing intermediate; and carrying out continuous elimination or hydrolysis reaction on the halogen-containing intermediate to obtain glycollic acid. The reaction process for preparing glycollic acid through alkali-free oxidation of ethylene glycol under the mediation effect of halide ions is mild in condition, a noble metal catalyst is avoided, extra alkali is not needed, the selectivity of a glycollic acid product is high, the catalyst and energy consumption cost is greatly reduced, and the method is expected to become an alternative route for preparing glycollic acid through traditional ethylene glycol oxidation.

Description

Technical Field

[0001] This invention belongs to the field of photoelectrocatalytic organic synthesis technology, specifically involving the application of a halide ion-mediated process in the alkali-free oxidation of ethylene glycol to prepare glycolic acid. Background Technology

[0002] Ethylene glycol, an important organic chemical raw material, can be produced from fossil resources, biomass, and waste polyester plastics. It boasts high yield, relatively low price, and contains multiple highly reactive hydroxyl groups, enabling it to participate in various chemical reactions. Converting ethylene glycol into high-value-added aldehydes, ketones, and carboxylic acids through catalytic oxidation is one effective way to address the low economic efficiency of its industrial production. The selective oxidation of ethylene glycol by a single hydroxyl group can generate multi-carbon products such as glycolaldehyde and glycolic acid. Among these, glycolic acid (priced at approximately 50,000 RMB / ton), obtained from the selective oxidation of ethylene glycol by a single hydroxyl group, has significant applications in fine chemicals, pharmaceuticals, polymer materials, food, and cosmetics.

[0003] Methods for the selective oxidation of ethylene glycol to glycolic acid mainly include biological and chemical methods. Biological oxidation offers high specificity and mild reaction conditions, but the required enzyme catalysts have stringent environmental requirements, hindering large-scale production. Traditional chemical oxidation methods are typically thermally driven, requiring specific temperatures, pressures, and additional oxidants, resulting in high energy costs and significant carbon emissions. In recent years, green and efficient electrocatalytic technology has provided new ideas for the targeted conversion of ethylene glycol and the sustainable production of hydrogen. However, existing electrocatalytic systems largely rely on water-soluble alkalis (mainly KOH and NaOH) and noble metal catalysts (such as Au, Pt, and Pd). Among these, the free hydroxide ions (OH-) in the alkaline solution... - Alkali-based catalysts can catalyze the formation of alkoxides and geminal glycol intermediates, accelerating reaction kinetics. While noble metals can generate reactive oxygen species (M-*OH) at lower voltages for the selective dehydrogenation of alcohol hydroxyl groups, these methods not only increase the difficulty of product purification and separation and exacerbate wastewater discharge problems, but also raise catalyst costs. Therefore, there is an urgent need to develop environmentally friendly and economical alkali-free non-noble metal catalytic systems to achieve efficient and selective oxidation of ethylene glycol.

[0004] In recent years, the conversion of solar energy into chemical energy through photoelectrocatalysis has become a hot research topic. This technology has greatly improved the problem of photogenerated carrier recombination in photocatalysis. Photogenerated electrons are transported to the cathode through the external circuit under the drive of an applied bias voltage to participate in the reduction reaction. At the same time, photogenerated holes remain on the surface of the photoanode to participate in the oxidation reaction.

[0005] Researchers both domestically and internationally have successfully utilized active species in photoelectrocatalytic systems to mediate the alkali-free oxidation of ethylene glycol and glycerol, providing an important experimental and theoretical foundation for the directional conversion of polyols under mild conditions. However, in these reaction systems, highly oxidizing reactive oxygen species (mainly active hydroxyl radicals) easily cause side reactions such as excessive oxidation of polyols and C / C bond cleavage, ultimately generating C1 products with lower added value. Summary of the Invention

[0006] This invention is proposed to overcome the shortcomings of the prior art, and its purpose is to provide an application of halide ion-mediated oxidation of ethylene glycol to prepare glycolic acid without alkali.

[0007] This invention is achieved through the following technical solution: An application of halide ion-mediated oxidation of ethylene glycol to prepare glycolic acid without alkali, wherein the halide ion acts as the mediating ion in the preparation of glycolic acid without alkali. The method for preparing glycolic acid by alkali-free oxidation of ethylene glycol is as follows: S1. Assemble the photoanode, cathode, and halogen-containing electrolyte into a photoelectrolysis cell; S2. Add ethylene glycol to the electrolyte on the photoanode side; S3. Under light intensity and bias conditions, halide ions are activated into halogen free radicals at the photoanode and react with ethylene glycol to generate halogen-containing intermediates. The halogen-containing intermediates are converted into glycolic acid through continuous elimination or hydrolysis reactions, while water is reduced at the cathode to produce hydrogen gas.

[0008] In the above technical solution, the electrolyte containing halide ions is an aqueous solution of at least one of sodium bromide, sodium iodide, potassium bromide, potassium iodide, or tetrabutylammonium bromide.

[0009] In the above technical solution, the concentration of the halogen-containing electrolyte is 10 g / L ~ 100 g / L.

[0010] In the above technical solution, the photoanode catalyst constituting the photoanode is any one of titanium dioxide, bismuth vanadate, tungsten trioxide, zinc oxide, or iron oxide.

[0011] In the above technical solution, the photoanode catalyst surface is modified with layered bimetallic hydroxides (LDHs) to improve photoelectrochemical activity, and the layered bimetallic hydroxides are any one of NiFe-LDH, NiCo-LDH or CoAl-LDH.

[0012] In the above technical solution, the cathode catalyst constituting the cathode is at least one of transition metal oxides, transition metal phosphides, transition metal sulfides, transition metal nitrides, transition metal borides, platinum-based catalysts, palladium-based catalysts, ruthenium-based catalysts, rhodium-based catalysts, nickel-based catalysts, or copper-based catalysts.

[0013] In the above technical solution, the concentration of ethylene glycol in the photoanode side electrolyte is 6.2 g / L ~ 31 g / L.

[0014] In the above technical solution, the light intensity is 100 mW / cm². -2 ~ 500 mW / cm -2 The bias condition is 0 V ~ 2 V.

[0015] The beneficial effects of this invention are: This invention provides an application of halide ion-mediated oxidation of ethylene glycol to prepare glycolic acid without alkali. The halide ion-mediated oxidation process involves the capture of photogenerated holes by halide ions to generate halogen free radicals, which then react with ethylene glycol to obtain a halogen-containing intermediate. This intermediate is then subjected to a series of elimination or hydrolysis reactions to yield glycolic acid. The halide ion-mediated oxidation results in mild reaction conditions, avoids the use of precious metal catalysts, eliminates the need for additional alkali, exhibits high selectivity for the glycolic acid product, and significantly reduces catalyst and energy costs. This method holds promise as an alternative to the traditional ethylene glycol oxidation process for preparing glycolic acid. Attached Figure Description

[0016] Figure 1 This is a scanning electron microscope image of the NiCo-LDH modified titanium dioxide photoanode in Example 1 of this invention; Figure 2 This is the X-ray diffraction pattern of the NiCo-LDH modified titanium dioxide photoanode in Example 1 of this invention; Figure 3 This is the liquid phase spectrum of the ethylene glycol catalytic oxidation product in Example 1 of the present invention; Figure 4 This is a reaction pathway diagram of the preparation of glycolic acid by bromide ion-mediated photoelectrocatalytic alkali-free oxidation of ethylene glycol in Example 1 of the present invention.

[0017] For those skilled in the art, other related figures can be obtained from the above figures without any creative effort. Detailed Implementation

[0018] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0019] Example 1 An application of bromide ion-mediated oxidation of ethylene glycol to prepare glycolic acid without alkali is as follows: S1. Add 0.18 g of ethylene glycol to 6 mL of KBr (70 g / L) solution; S2. Use the KBr solution and KOH solution obtained in step S1 as electrolytes, respectively, and also use TiO2 / NiCo-LDH ( Figure 1 An electrolytic cell was assembled using platinum sheets and a photoanode as the photoanode and cathode, respectively, and then subjected to AM 1.5G illumination (100 mW cm⁻¹). -2 The reaction was carried out under a bias voltage of 1.0 V for 4 h. S3. After the reaction is complete, take 1 mL of the reaction solution and detect the product by high performance liquid chromatography. like Figure 3 As shown in the comparison with Comparative Example 1, the main product of the photoelectrocatalytic oxidation of ethylene glycol in KOH electrolyte is formic acid, with a glycolic acid selectivity of only 33.4%, while in KBr electrolyte, it still maintains a high glycolic acid selectivity of 78.3%.

[0020] Example 2 An application of bromide ion-mediated oxidation of ethylene glycol to prepare glycolic acid without alkali is as follows: S1. Add 0.18 g of ethylene glycol to 6 mL of tetrabutylammonium bromide (50 g / L) solution; S2. Using the solution obtained in step S1 as the electrolyte, BiVO4 and nickel sheets are used as the photoanode and cathode, respectively, to assemble an electrolytic cell. Then, under AM 1.5G illumination (200 mW cm⁻¹), the cell is subjected to electrolysis. -2 The reaction was carried out under a bias voltage of 0.8 V for 6 h. S3. After the reaction is complete, take 1 mL of the reaction solution and detect the product by high performance liquid chromatography. The selectivity for glycolic acid is 80.0%.

[0021] Example 3 An application of bromide ion-mediated oxidation of ethylene glycol to prepare glycolic acid without alkali is as follows: S1. Add 0.18 g of ethylene glycol to 6 mL of KBr (80 g / L) solution.

[0022] S2. Using the solution obtained in step S1 as the electrolyte, Fe2O3 and copper sheet are used as the photoanode and cathode, respectively, to assemble an electrolytic cell. Then, under AM 1.5G illumination (500 mW cm⁻¹), the cell is tested. -2 The reaction was carried out under a bias voltage of 1.2 V for 4 h. S3. After the reaction is complete, take 1 mL of the reaction solution and detect the product by high performance liquid chromatography. The selectivity for glycolic acid is 72.5%.

[0023] Comparative Example 1 A basic photoelectrocatalytic oxidation of ethylene glycol, specifically: S1. Add 0.18 g of ethylene glycol to 6 mL of KOH (40 g / L) solution.

[0024] S2. Using the solution obtained in step 1 as the electrolyte, and TiO2 / NiCo-LDH and platinum sheets as the photoanode and cathode respectively, an electrolytic cell is assembled. Then, under AM 1.5G illumination (100 mW cm⁻¹), the cell is subjected to electrolysis. -2 The reaction was carried out under a bias voltage of 1.0 V for 4 h. S3. After the reaction is complete, take 1 mL of the reaction solution and analyze the product using high-performance liquid chromatography (HPLC). Figure 3 As shown, the main product of the photoelectrocatalytic oxidation of ethylene glycol in KOH electrolyte is formic acid, while the selectivity for glycolic acid is only 33.4%.

[0025] The principle of this invention: This invention fully utilizes the important application of active halogen species in the breaking of CH bonds in organic compounds. The halogen-containing intermediates generated can be further converted into aldehydes, ketones or carboxylic acids through continuous elimination or hydrolysis reactions.

[0026] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. Use of a halogen ion in the production of glycolic acid by the oxidation of ethylene glycol in the absence of a base, characterized in that: The halogen ion acts as a mediation ion in the preparation of glycolic acid by glycol base-free oxidation; The method for preparing glycolic acid by glycol base-free oxidation is: S1, assembling a photoanode, a cathode and an electrolyte containing halogen ions into a photoelectrochemical cell; S2, adding glycol to the electrolyte on the side of the photoanode; S3, under the conditions of light intensity and bias, the halogen ion is activated into a halogen radical on the photoanode, and reacts with glycol to form a halogen-containing intermediate, and the halogen-containing intermediate is obtained by continuous elimination or hydrolysis to obtain glycolic acid.

2. Use of halide ion mediation in the preparation of glycolic acid from ethylene glycol oxidation without alkali according to claim 1, characterized by: The electrolyte containing halogen ions is an aqueous solution of at least one of sodium bromide, sodium iodide, potassium bromide, potassium iodide or tetrabutylammonium bromide.

3. Use of halide ion mediation in the preparation of glycolic acid from ethylene glycol oxidation without alkali according to claim 1, characterized by: The concentration of the electrolyte containing halogen ions is 10 g / L to 100 g / L.

4. Use of halide ion mediation in the preparation of glycolic acid from ethylene glycol oxidation without alkali according to claim 1, characterized by: The photoanode catalyst constituting the photoanode is any one of titanium dioxide, bismuth vanadate, tungsten trioxide, zinc oxide or iron oxide.

5. Use of halide ion mediation in the preparation of glycolic acid from ethylene glycol oxidation without alkali according to claim 4, characterized by: The surface of the photoanode catalyst is modified with layered double hydroxides (LDHs) to improve photoelectrochemical activity, and the layered double hydroxides are any one of NiFe-LDH, NiCo-LDH or CoAl-LDH.

6. Use of halogen ions in the preparation of glycolic acid by the oxidation of ethylene glycol in the absence of alkali according to claim 1, characterized in that: The cathode catalyst constituting the cathode is at least one of a transition metal oxide, a transition metal phosphide, a transition metal sulfide, a transition metal nitride, a transition metal boride, a platinum-based catalyst, a palladium-based catalyst, a ruthenium-based catalyst, a rhodium-based catalyst, a nickel-based catalyst or a copper-based catalyst.

7. Use of halogen ions in the preparation of glycolic acid by the oxidation of ethylene glycol in the absence of alkali according to claim 1, characterized in that: The concentration of glycol in the electrolyte on the side of the photoanode is 6.2 g / L to 31 g / L.

8. Use of halogen ions in the preparation of glycolic acid by the oxidation of ethylene glycol in the absence of alkali according to claim 1, characterized in that: The light intensity is 100 mW / cm -2 ~ 500 mW / cm -2 The bias voltage is 0 V ~ 2 V.

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