A method for preparing glycolide using industrial-grade glycolic acid aqueous solution and the obtained glycolide

Abstract

This invention discloses a method for preparing glycolide using industrial-grade glycolic acid aqueous solution and the obtained glycolide. The method includes: step 1, reacting purified glycolic acid aqueous solution to obtain glycolic acid oligomers; and step 2, depolymerizing the glycolic acid oligomers to obtain glycolide. In this method, industrial-grade glycolic acid aqueous solution obtained from the coal-to-ethylene glycol route is used as raw material. Glycolic acid oligomers are obtained by mixing the glycolic acid aqueous solution with a catalyst and then performing a prepolymerization reaction. The glycolic acid oligomers are then formed into a homogeneous melt, and vacuum distillation is performed to obtain a light-colored crude glycolide product. This method increases the glycolide content and reduces the acid value, dimer, and polymer content of the crude glycolide product, achieving better technical results.

Description

Technical Field

[0001] This invention belongs to the field of glycolide preparation, and particularly relates to a method for preparing glycolide using industrial-grade glycolic acid aqueous solution and the obtained glycolide. Background Technology

[0002] Polyglycolic acid (PEG), an aliphatic polyester, possesses high biodegradability and good biocompatibility. It can be hydrolyzed in organisms and metabolized by microorganisms in natural environments, ultimately decomposing into water and carbon dioxide. Furthermore, PEG exhibits good mechanical properties such as heat resistance and tensile strength, and demonstrates excellent gas barrier properties when used in films and sheets. Therefore, PEG is expected to replace commonly used biodegradable polymers in medical polymer materials, agricultural resource materials, and various packaging or container materials. PEG has already found applications in surgical sutures, artificial skin and blood vessels, bone fixation and repair, controlled drug release, and tissue engineering.

[0003] There are two methods for preparing polyglycolic acid (PEG). One method involves direct dehydration and polycondensation of glycolic acid. PEG obtained by this method has a low molecular weight, making it difficult to process into molded materials. The other method involves heating and decomposing the glycolic acid polycondensation polymer to obtain cyclic PEG. Ring-opening polymerization of this cyclic PEG yields PEG with molecular weights ranging from tens of thousands to hundreds of thousands, which can meet the requirements of subsequent processing. Summary of the Invention

[0004] The technical problem to be solved by this invention is a method for preparing glycolide using industrial-grade glycolic acid aqueous solution. In their research on the impact of glycolic acid aqueous solution on the quality of crude glycolide obtained from the depolymerization reaction, the inventors discovered that industrial-grade glycolic acid aqueous solution, especially that obtained via the coal-to-ethylene glycol route, contains not only impurities such as methoxyacetic acid, diethylene glycol, oxalic acid, and sodium chloride, but also aldehydes, ketones, and ethers such as glyoxylic acid, formaldehyde, acetaldehyde, acetone, butyraldehyde, butanone, glyoxal, hydroxyacetaldehyde, hydroxyacetaldehyde glycol ether (HOCCH2OCH2CH2OH), and hydroxyacetaldehyde diethylene glycol ether (HOCCH2OCH2CH2OCH2CH2OH). Consequently, the crude glycolide prepared from these raw materials via the depolymerization reaction has a darker color, lower glycolide content, higher acid value, and higher content of dimers and polymers, thus affecting the quality of the crude glycolide product. Through in-depth research, the inventors discovered that the presence of aldehydes, ketones, and ethers is the main cause. Specifically, after extensive experimental research, the inventors found that aldehydes, ketones, and ethers in the glycolic acid aqueous solution raw material are impurities that enter the crude glycolide during the prepolymerization and depolymerization reactions, resulting in a decrease in the yield of crude glycolide. Aldehydes oxidize to form acids in acidic environments, which increases the acid value of crude glycolide. An excessively high acid value is difficult to reduce in subsequent refining processes, ultimately leading to a high acid value in the refined glycolide product, which does not meet the requirements for glycolide polymerization. Furthermore, it also increases the synthesis of dimers and polymers. Ketones and ethers are not easily removed during the refining process, resulting in a decrease in the purity of the refined glycolide, which also fails to meet the requirements for glycolide polymerization.

[0005] To address the problems of dark color, low glycolide content, high acid value, and high dimer and polymer content in crude glycolide prepared from industrial-grade glycolic acid aqueous solution obtained via the coal-to-ethylene glycol route in existing technologies, this invention provides a purification method for industrial-grade glycolic acid aqueous solution, a method for preparing glycolide using it, and the obtained glycolide. By controlling the amount of aldehydes, ketones, and ethers in the glycolic acid aqueous solution raw material, the prepared glycolide product has the advantages of light color, high glycolide content, low acid value, and low dimer and polymer content.

[0006] One objective of this invention is to provide a method for preparing glycolide using an industrial-grade aqueous glycolic acid solution, comprising: step 1 of reacting a purified aqueous glycolic acid solution to obtain glycolic acid oligomers, and step 2 of depolymerizing the glycolic acid oligomers to obtain glycolide.

[0007] In a preferred embodiment, in step (1), the industrial-grade glycolic acid aqueous solution is an industrial-grade glycolic acid aqueous solution obtained from the coal-to-ethylene glycol route.

[0008] In a further preferred embodiment, in step (1), the industrial-grade glycolic acid aqueous solution contains at least one impurity selected from aldehydes, ketones, and ethers; preferably, the aldehydes are selected from at least one of formaldehyde, acetaldehyde, butyraldehyde, glyoxal, and hydroxyacetaldehyde, the ketones are selected from at least one of acetone and butanone, and the ethers are selected from at least one of hydroxyacetaldehyde ethylene glycol ether (HOCCH2OCH2CH2OH) and hydroxyacetaldehyde diethylene glycol ether (HOCCH2OCH2CH2OCH2CH2OH).

[0009] In a further preferred embodiment, the content of aldehydes in the industrial-grade glycolic acid aqueous solution is greater than 2000 ppm, and / or the content of ketones is greater than 700 ppm, and the content of ethers is greater than 400 ppm.

[0010] If any of the three impurities exceeds the specified value, it will affect the yield of glycolide products, as well as the acid value, dimer content, and cyclic polymer content of glycolide products.

[0011] Specifically, after extensive experimental research, the inventors discovered that aldehydes, ketones, and ethers in the glycolic acid aqueous solution raw material are impurities that enter the crude glycolide during the prepolymerization and depolymerization reactions, resulting in a decrease in the yield of crude glycolide. Aldehydes oxidize to form acids in acidic environments, which increases the acid value of crude glycolide. An excessively high acid value is difficult to reduce in subsequent refining processes, ultimately leading to a high acid value in the refined glycolide product, which fails to meet the requirements for glycolide polymerization. Furthermore, it also increases the synthesis of dimers and polymers. Ketones and ethers are not easily removed during the refining process, resulting in a decrease in the purity of the refined glycolide, which also fails to meet the requirements for glycolide polymerization.

[0012] Preferably, in the industrial-grade glycolic acid aqueous solution, the content of aldehydes is 2000-3000 ppm, the content of ketones is 500-1000 ppm (e.g., 600-900 ppm), and the content of ethers is 300-800 ppm (e.g., 400-800 ppm).

[0013] In a preferred embodiment, in step 1, the purification process is carried out using at least one of organic nanofiltration membrane filtration, distillation, or extraction, preferably organic nanofiltration membrane filtration.

[0014] Any one or a combination of two or more of the following methods can be used: organic nanofiltration membrane filtration, distillation, or extraction, as long as the content of aldehydes, ketones, and ethers in the purified glycolic acid aqueous solution can be controlled within the appropriate range defined by this invention.

[0015] In a further preferred embodiment, in step 1, the purification process employs organic nanofiltration membrane filtration, wherein the organic nanofiltration membrane retains compounds with a molecular weight greater than 150, and preferably, the average pore size of the organic nanofiltration membrane is 100-200 nm.

[0016] The average pore size of the organic nanofiltration membrane can be 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, or 200 nm. Organic nanofiltration membranes with these pore sizes can retain compounds with a molecular weight greater than 150.

[0017] In a further preferred embodiment, in step (1), the filtration process is carried out in the range of 0.5 to 3.0 MPa, preferably 1.0 to 1.5 MPa, to reduce the content of aldehydes, ketones and ethers.

[0018] The filtration is carried out at room temperature and at pressures of 0.5 MPa, 1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa or 3.0 MPa.

[0019] In a preferred embodiment, the organic nanofiltration membrane is selected from organic membranes capable of retaining compounds with a molecular weight greater than 150, preferably but not limited to a polychlorotrifluoroethylene homogeneous ion exchange membrane.

[0020] In a further preferred embodiment, the nanofiltration membrane has a selective permeability greater than 95%, preferably greater than 97%. In this invention, the selective permeability of the membrane refers to the ratio of the amount of glycolic acid in the filtrate to the amount of glycolic acid in the feed solution.

[0021] The inventors discovered through research that when the aforementioned organic nanofiltration membrane was used to treat an aqueous solution of glycolic acid, the contents of aldehydes, ketones, and ethers all decreased significantly. The reason for this is likely that the solution system contains acidic substances, which cause aldehydes, ketones, and ethers to condense. For example, small molecular weight substances may react to form larger molecules that are retained. Thus, after filtration, only a portion of aldehydes, ketones, and ethers are removed. Although aldehydes, ketones, and ethers still exist after filtration, as long as they are controlled within the aforementioned range, their impact on the glycolide preparation reaction is far less than before filtration.

[0022] In a preferred embodiment, in step 1, the purified glycolic acid aqueous solution contains less than 2000 ppm of aldehydes, less than 700 ppm of ketones, and less than 700 ppm of ethers.

[0023] Through in-depth research, the inventors discovered that as long as the aldehydes, ketones, and ethers in the industrial-grade glycolic acid aqueous solution are controlled within the above-mentioned range, they have virtually no impact on the glycolide preparation reaction, and can achieve technical effects equivalent to pure glycolic acid or glycolic acid crystals.

[0024] In a preferred embodiment, in step 1, the reaction is carried out in the presence of a catalyst; preferably, the catalyst is selected from at least one of stannous octoate, stannous chloride, antimony trioxide, zinc oxide, and zinc acetylacetonate.

[0025] In a further preferred embodiment, in step 1, the ratio of the catalyst to glycolic acid is 0.05–1 wt%, preferably 0.1–0.5 wt%.

[0026] In a preferred embodiment, the reaction in step 1 is carried out as follows: the temperature is raised to 160-260°C for a pre-reaction, and then (while maintaining the system temperature) the vacuum degree of the reaction system is controlled to 0.05-10 kPa to continue the reaction for 0.5-10 h.

[0027] In a further preferred embodiment, the reaction in step 1 is carried out as follows: the temperature is raised to 190-240°C for a pre-reaction, and then (while maintaining the system temperature) the vacuum degree of the reaction system is controlled to 0.1-5 kPa to continue the reaction for 2-5 hours.

[0028] For example, the reaction described in step 1 is carried out as follows: first, the temperature is raised from room temperature to 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃ or 260℃ for a pre-reaction, and then (while maintaining the system temperature) the vacuum degree of the reaction system is controlled to 0.05kPa, 0.1kPa, 0.5kPa, 1kPa, 2kPa, 3kPa, 4kPa, 5kPa, 6kPa, 7kPa, 8kPa, 9kPa or 10kPa to continue the reaction for 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h.

[0029] Preferably, after the pre-reaction, the vacuum degree of the reaction system is controlled to 1-5 kPa and the reaction is carried out for 2-5 hours, and then the vacuum degree is reduced to 0.1-1 kPa and the reaction is carried out for 2-5 hours.

[0030] In a preferred embodiment, in step 2, the temperature is raised to 180–260°C, preferably 200–250°C, for melting treatment to form a homogeneous melt of glycolic acid oligomer.

[0031] For example, in step 2, the temperature is raised to 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, or 260°C for melting treatment to form a homogeneous melt of glycolic acid oligomers.

[0032] In a preferred embodiment, in step 2, the vacuum distillation is carried out as follows: the vacuum level is increased to 0.05–10 kPa, and the reaction is carried out for 0.5–10 hours.

[0033] In a further preferred embodiment, in step 2, the vacuum distillation is carried out as follows: the vacuum level is increased to 0.1-5 kPa, and the reaction is carried out for 2-5 hours.

[0034] For example, in step 2, the vacuum distillation is carried out as follows: the vacuum level is increased to 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 kPa, and the reaction is carried out for 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 hours.

[0035] In a further preferred embodiment, the temperature is gradually increased to 260–320°C, preferably 270–300°C, during the vacuum distillation process described in step 2.

[0036] For example, during the vacuum distillation process described in step 2, the temperature is gradually increased to 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, or 320°C.

[0037] After obtaining glycolide using the method described in this invention, it can optionally be further purified using any prior art method for glycolide purification. Preferably, the purification may include recrystallization and drying.

[0038] In the method of this invention, industrial-grade glycolic acid aqueous solution obtained from the coal-to-ethylene glycol route is used as raw material. Glycolic acid oligomers are obtained by mixing the glycolic acid aqueous solution with a catalyst and then performing a prepolymerization reaction. The glycolic acid oligomers are then formed into a homogeneous melt and distilled under reduced pressure to obtain a light-colored crude glycolide product. This method increases the glycolide content and reduces the acid value, dimer, and polymer content of the crude glycolide product, achieving better technical results.

[0039] In this invention, the linear dimer refers to HOCH2COOCH2COOH with a molecular weight of 134; the cyclic polymers include tetramers to octamers with molecular weights ranging from 232 to 464, and the content of cyclic polymers with higher degrees of polymerization is negligible.

[0040] A second objective of this invention is to provide glycolide obtained using the preparation method described in one objective of this invention.

[0041] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0042] Compared with the prior art, the present invention has the following beneficial effects: In the method of the present invention, the industrial-grade glycolic acid aqueous solution obtained by the coal-to-ethylene glycol route is used as raw material. After mixing the glycolic acid aqueous solution with a catalyst, a prepolymerization reaction is carried out to obtain glycolic acid oligomers. Then, the glycolic acid oligomers are formed into a homogeneous melt and distilled under reduced pressure to obtain a light-colored crude glycolide product. This increases the glycolide content and reduces the acid value, dimer and polymer content of the crude glycolide product, thus achieving better technical results. Detailed Implementation

[0043] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0044] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0045] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0046] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0047] The raw materials used in the specific embodiments of this invention are commercially available.

[0048] The analytical method for determining glycolide products in this invention:

[0049] (1) Determination of impurity content in aqueous glycolic acid solution

[0050] Prepare a 1000 mg / L acetonitrile solution (containing 1% phosphoric acid). Take 20 μL of the glycolic acid aqueous solution sample or mix it with 1 mL of the above acetonitrile solution, heat in a water bath for 30 min, and then inject for analysis. Analyze using ultra-high performance liquid chromatography (UHPLC) with a C18 column, an injection rate of 0.4 mL / min, and a mobile phase of 0 min (5% ACN) -> 6 min (95% ACN). Detect at 365 nm using UV.

[0051] (2) Determination of glycolide content

[0052] The analysis was performed using a gas chromatograph 7890B. 0.5 g of sample was placed in a centrifuge tube and dissolved thoroughly in 25 mL of dimethyl sulfoxide. The mixture was centrifuged at 10000 rpm for 5 min. The supernatant was then analyzed. A DB-FFAP 30 m × 0.25 mm × 0.25 μm column was used, and a standard curve of (250–5000) ppm was calculated.

[0053] (3) Determination of glycolide acid value

[0054] The acid value of crude glycolide was determined using acid-base titration. The sample was dissolved in 20 mL of dry dimethyl sulfoxide. After dissolution, a few drops of bromophenol blue indicator solution were added, resulting in a yellow solution. Titration was performed using a standard concentration of sodium hydroxide in benzyl alcohol. The endpoint was reached when the solution color changed from yellow to green. The acid value was calculated by determining the volume of sodium hydroxide solution used to reach the titration endpoint.

[0055] (4) Determination of glycolide dimer and polymer

[0056] Using a Bruker Advance II 600MHz nuclear magnetic resonance spectrometer, with d6-DMSO or deuterated trifluoroacetic acid as solvent and tetramethylsilane (TMS) as internal standard, the chemical shift values ​​of the proton peaks on the methylene groups of different oligomers were determined from the NMR spectra, and the content of H in the oligomers relative to H in glycolide was calculated based on their integrated areas.

[0057] (5) Determination of the color of glycolide

[0058] The yellowness index was measured using a Hunter Lab Scan XE yellowness index meter (USA). The instrument was calibrated using a black glass plate and then a white calibration plate, following the instrument's instructions until calibration was successful. A certain amount of glycolide sample was placed in the sample cup, the light cover was placed on top, and the "Test Sample" button was pressed to begin the measurement. After the test was completed, the sample was taken three times, and the average value was recorded.

[0059] In the embodiments, a polychlorotrifluoroethylene homogeneous exchange membrane was used as the organic nanofiltration membrane, which was purchased from Zhejiang Qianqiu Environmental Protection Water Treatment Co., Ltd., and can retain compounds with a molecular weight greater than 150.

[0060]

Example 1

[0061] Using an aqueous glycolic acid solution A as raw material, its impurity content was tested, revealing aldehyde impurities of 3200 ppm, ketone impurities of 896 ppm, and ether impurities of 503 ppm. The raw material was then purified by elution through an organic nanofiltration membrane at an operating pressure of 1.4 MPa. After purification, the impurity content was tested again, revealing aldehyde impurities of 1080 ppm, ketone impurities of 520 ppm, and ether impurities of 320 ppm.

[0062] 600 g of 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and the water generated in the reaction was removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 1 kPa, and the reaction was continued for 3 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C, while the system pressure was reduced to 5 kPa under vacuum. The reaction was carried out at this pressure for 5 hours, during which the temperature was gradually increased to 290 °C. The crude glycolide product was condensed and collected, yielding 287.1 g, with a yield of 95.7%. The crude glycolide product was found to have a color of 13.2, a purity of 95.3%, an acid value of 260 μmol / g, and NMR analysis showed a dimer content of 0.68 mol% and a polymer content of 0.84 mol%.

[0063]

Example 2

[0064] Using glycolic acid aqueous solution B as raw material, its impurity content was tested, revealing aldehyde impurities of 4600 ppm, ketone impurities of 780 ppm, and ether impurities of 460 ppm. The raw material was then purified by elution through an organic nanofiltration membrane at an operating pressure of 1.3 MPa. After purification, the impurity content was tested, revealing aldehyde impurities of 1340 ppm, ketone impurities of 630 ppm, and ether impurities of 108 ppm.

[0065] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and the water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 1.5 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C while the system pressure was reduced to 0.1 kPa under vacuum. The reaction was carried out for 2 hours under this pressure, and the temperature was gradually increased to 290 °C during the reaction. The crude glycolide product was condensed and collected, yielding 285.3 g, with a yield of 95.1%. The crude glycolide product was found to have a color of 14.9, a purity of 94.7%, an acid value of 284 μmol / g, and NMR analysis showed a dimer content of 0.72 mol% and a polymer content of 0.93 mol%.

[0066]

Example 3

[0067] Using an aqueous glycolic acid solution C as raw material, its impurity content was tested, revealing aldehyde impurities of 2080 ppm, ketone impurities of 880 ppm, and ether impurities of 570 ppm. The raw material was then purified by elution through an organic nanofiltration membrane at an operating pressure of 1.1 MPa. After purification, the impurity content was tested, revealing aldehyde impurities of 977 ppm, ketone impurities of 560 ppm, and ether impurities of 235 ppm.

[0068] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 1 kPa under vacuum, and the reaction was carried out for 2 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C, while the system pressure was reduced to 3 kPa under vacuum. The reaction was carried out at this pressure for 4.5 hours, during which the temperature was gradually increased to 290 °C. The crude glycolide product was collected after condensation, yielding 283.8 g, with a yield of 94.6%. The crude glycolide product was measured to have a color of 15.3, a purity of 94.1%, an acid value of 295 μmol / g, and NMR analysis showed a dimer content of 0.75 mol% and a polymer content of 0.88 mol%.

[0069]

Example 4

[0070] Using glycolic acid aqueous solution D as raw material, its impurity content was tested, revealing aldehyde impurities of 2710 ppm, ketone impurities of 690 ppm, and ether impurities of 613 ppm. The raw material was then purified by elution through an organic nanofiltration membrane at an operating pressure of 1.5 MPa. After purification, the impurity content was tested, revealing aldehyde impurities of 840 ppm, ketone impurities of 617 ppm, and ether impurities of 164 ppm.

[0071] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and the water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 3 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 0.5 kPa, and the reaction was continued for another 2 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C while the system pressure was reduced to 2 kPa under vacuum. The reaction was carried out at this pressure for 3.5 hours, during which the temperature was gradually increased to 290 °C. The crude glycolide product distilled off was condensed and collected, yielding 285.0 g, with a yield of 95.0%. The crude glycolide product was found to have a color of 13.2, a purity of 95.1%, an acid value of 268 μmol / g, and NMR analysis showed a dimer content of 0.88 mol% and a polymer content of 0.91 mol%.

[0072]

Example 5

[0073] Using glycolic acid aqueous solution E as raw material, its impurity content was tested, revealing aldehyde impurities of 2480 ppm, ketone impurities of 720 ppm, and ether impurities of 420 ppm. The raw material was then purified by elution through an organic nanofiltration membrane at an operating pressure of 1.0 MPa. After purification, the impurity content was tested, revealing aldehyde impurities of 1800 ppm, ketone impurities of 408 ppm, and ether impurities of 207 ppm.

[0074] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 3 kPa under vacuum, and the reaction was carried out for 1 h. The system pressure was then reduced to 1 kPa, and the reaction was continued for 2 h to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C while the system pressure was reduced to 3 kPa under vacuum. The reaction was carried out at this pressure for 4.5 h, during which the temperature was gradually increased to 290 °C. The crude glycolide product distilled off was condensed and collected, yielding 284.4 g, with a yield of 94.8%. The crude glycolide product was found to have a color of 16.8, a purity of 93.7%, an acid value of 304 μmol / g, and NMR analysis showed a dimer content of 1.07 mol% and a polymer content of 1.56 mol%.

[0075]

Example 6

[0076] Using an aqueous glycolic acid solution F as raw material, its impurity content was tested, revealing aldehyde impurities of 2900 ppm, ketone impurities of 630 ppm, and ether impurities of 710 ppm. The raw material was then purified by elution through an organic nanofiltration membrane at an operating pressure of 1.5 MPa. After purification, the impurity content was tested, revealing aldehyde impurities of 1617 ppm, ketone impurities of 306 ppm, and ether impurities of 84 ppm.

[0077] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and the water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 4 kPa under vacuum, and the reaction was carried out for 1 h. The system pressure was then reduced to 1 kPa, and the reaction was continued for 4 h to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C while the system pressure was reduced to 4.5 kPa under vacuum. The reaction was carried out at this pressure for 5 h, during which the temperature was gradually increased to 290 °C. The crude glycolide product was condensed and collected, yielding 282.0 g, with a yield of 94.0%. The crude glycolide product was found to have a color of 15.7, a purity of 93.5%, an acid value of 323 μmol / g, and NMR analysis showed a dimer content of 0.81 mol% and a polymer content of 1.13 mol%.

[0078]

Comparative Example 1

[0079] Using glycolic acid aqueous solution A as raw material, its impurity content was tested. The content of aldehyde impurities was 3200 ppm, the content of ketone impurities was 896 ppm, and the content of ether impurities was 503 ppm.

[0080] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and the water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 1 kPa, and the reaction was continued for 3 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C, while the system pressure was reduced to 5 kPa under vacuum. The reaction was carried out at this pressure for 5 hours, during which the temperature was gradually increased to 290 °C. The crude glycolide product distilled off was condensed and collected, yielding 255.3 g, with a yield of 85.1%. The crude glycolide product was found to have a color of 38.6, a purity of 89.7%, an acid value of 503 μmol / g, and NMR analysis showed a dimer content of 2.75 mol% and a polymer content of 3.39 mol%.

[0081] [Comparative Example 2]

[0082] Using glycolic acid aqueous solution B as raw material, its impurity content was tested. The content of aldehyde impurities was 4600 ppm, the content of ketone impurities was 780 ppm, and the content of ether impurities was 460 ppm.

[0083] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 1.5 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C while the system pressure was reduced to 0.1 kPa under vacuum. The reaction was carried out for 2 hours under this pressure, and the temperature was gradually increased to 290 °C during the reaction. The crude glycolide product was condensed and collected, yielding 252.9 g, with a yield of 84.3%. The crude glycolide product was found to have a color of 43.8, a purity of 88.4%, an acid value of 516 μmol / g, and NMR analysis showed a dimer content of 2.16 mol% and a polymer content of 2.40 mol%.

[0084] [Comparative Example 3]

[0085] Using glycolic acid aqueous solution D as raw material, its impurity content was tested. The content of aldehyde impurities was 2710 ppm, the content of ketone impurities was 690 ppm, and the content of ether impurities was 613 ppm.

[0086] 600 g of a 70% aqueous glycolic acid solution and 2.1 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a four-necked flask. The temperature was gradually increased from room temperature to 220 °C, and the water generated during the reaction was removed by distillation. At this temperature, the system pressure was reduced to 3 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 0.5 kPa, and the reaction was continued for another 2 hours to obtain glycolic acid oligomers, which were then cooled and pulverized. 300 g of glycolic acid oligomers were added to a four-necked flask, and the temperature was gradually increased from room temperature to 230 °C while the system pressure was reduced to 2 kPa under vacuum. The reaction was carried out at this pressure for 3.5 hours, during which the temperature was gradually increased to 290 °C. The crude glycolide product distilled off was condensed and collected, yielding 259.8 g, with a yield of 86.6%. The crude glycolide product was found to have a color of 41.3, a purity of 86.0%, an acid value of 596 μmol / g, and NMR analysis showed a dimer content of 1.54 mol% and a polymer content of 3.97 mol%.

[0087] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing glycolide using an industrial-grade aqueous solution of glycolic acid, comprising: Step 1 involves reacting a purified aqueous glycolic acid solution to obtain glycolic acid oligomers, and step 2 involves depolymerizing the glycolic acid oligomers to obtain glycolide; the purified aqueous glycolic acid solution is obtained by purifying an industrial-grade aqueous glycolic acid solution; the industrial-grade aqueous glycolic acid solution is obtained from the coal-to-ethylene glycol route, and contains at least one impurity among aldehydes, ketones, and ethers; the purification process is performed using an organic nanofiltration membrane, which retains compounds with a molecular weight greater than 150; the content of aldehydes in the purified aqueous glycolic acid solution is below 2000 ppm, the content of ketones is below 700 ppm, and the content of ethers is below 400 ppm.

2. The method according to claim 1, characterized in that, In the industrial-grade glycolic acid aqueous solution, the content of aldehydes is greater than 2000 ppm, and / or the content of ketones is greater than 700 ppm, and / or the content of ethers is greater than 400 ppm.

3. The method according to claim 1, characterized in that, The purification process employs organic nanofiltration membrane filtration; wherein the filtration process is carried out at a pressure of 0.5~3.0 MPa.

4. The method according to claim 3, characterized in that, The filtration process is carried out within the range of 1.0 to 1.5 MPa.

5. The method according to claim 1, characterized in that, In step 1, the reaction is carried out in the presence of a catalyst.

6. The method according to claim 5, characterized in that, In step 1, the catalyst is selected from at least one of stannous octoate, stannous chloride, antimony trioxide, zinc oxide, and zinc acetylacetonate.

7. The method according to claim 5, characterized in that, The ratio of the catalyst to glycolic acid is 0.05~1wt%.

8. The method according to claim 7, characterized in that, The ratio of the catalyst to glycolic acid is 0.1-0.5 wt%.

9. The method according to claim 1, characterized in that, The reaction described in step 1 is carried out as follows: the temperature is raised to 160~260℃ for pre-reaction, and then the vacuum degree of the reaction system is controlled to 0.05~10kPa to continue the reaction for 0.5~10h.

10. The method according to any one of claims 1 to 9, characterized in that, In step 2, the depolymerization reaction includes a melting process and a vacuum distillation process.

11. The method according to claim 10, characterized in that, Heat to 180~260℃ for melting treatment.

12. The method according to claim 11, characterized in that, Heat to 200~250℃ for melting treatment.

13. The method according to claim 10, characterized in that, The vacuum distillation is carried out as follows: increase the vacuum to 0.05-10 kPa and react for 0.5-10 hours.

14. The method according to claim 13, characterized in that, The vacuum distillation is carried out as follows: increase the vacuum to 0.1-5 kPa and react for 2-5 hours.