Method for improving yield of bhets after purification by vacuum sublimation

Abstract

The present application belongs to the technical field of BHET purification, and relates to a method for improving the yield of BHET after vacuum sublimation purification, wherein an inhibitor is added in the process of vacuum sublimation of raw materials containing BHET to inhibit the polymerization reaction of BHET, so as to improve the yield of BHET after purification; the inhibitor is bis-(3-methyl-1-imidazole) ethylene bis(bistrifluoromethylsulfonyl) imide, and the chemical structural formula is: compared with the case where no inhibitor is added, the yield of BHET after purification is increased by more than 13%. The method of the present application can not only improve the purity and yield of BHET after vacuum sublimation purification, but also reduce the energy consumption in the purification process to a certain extent.

Description

Technical Field

[0001] This invention belongs to the field of BHET purification technology and relates to a method for improving the yield of BHET after vacuum sublimation purification. Background Technology

[0002] PET glycol alcoholysis is a common PET recycling method. The product, BHET, needs to be purified to obtain high-purity monomers for use in recycled PET or other applications. However, the vacuum sublimation purification process requires high vacuum (typically <1000 Pa) and high temperature (approximately 150~180℃), which can cause BHET molecules to polymerize, forming dimers or higher polymers, leading to a decrease in BHET purity and recovery rate. Existing technologies mainly include the following methods for preparing high-purity BHET:

[0003] Patent CN104710601B discloses a method for separating and purifying BHET, which mainly achieves this by controlling the solubility of BHET: first, the alcoholysis system is cooled to crystallize for coarse separation, and then water is introduced for recrystallization purification. Although this method can improve the purity of BHET to a certain extent, aqueous recrystallization requires low-temperature conditions far below room temperature, resulting in long processing time, high energy consumption, and difficulty in solvent recycling; especially for complex systems containing a large number of colorants, modifier fragments, and anionic and cationic impurities, it is difficult to obtain ideal high-purity BHET by aqueous recrystallization alone.

[0004] Patent CN119591497A discloses a low-temperature continuous production process for high-purity BHET. This process utilizes a highly active catalyst to lower the alcoholysis temperature of PET, achieving continuous production. It effectively removes metal residues through steps such as hydrolysis, filtration, purification, and cooling crystallization, ultimately yielding high-purity BHET. However, this method still relies on the traditional hydrolysis + filtration + cooling crystallization + drying process, making it difficult to completely remove trace organic impurities. Consequently, while the final BHET purity is high, it is typically difficult to consistently exceed 98%. Especially when processing complex waste PET (such as textiles / bottle flakes containing multiple additives), impurities are more noticeable, resulting in poor product color and transparency. Furthermore, the entire purification process introduces a large amount of water, requiring subsequent drying and dehydration, increasing energy consumption and wastewater treatment burden. Moreover, the entire process does not involve high-temperature vacuum refining steps such as vacuum sublimation, preventing further improvement in purity to ultra-high levels (>99%), thus limiting its application in high-end recycled PET.

[0005] Patent CN119677710A describes a method for purifying BHET from a crude BHET solution (containing dissolved barrier polymer impurities). This method uses water as the antisolvent for the barrier polymers, gradually adding water in at least two stages while simultaneously reducing the weight ratio of EG to water and lowering the temperature. This significantly reduces the solubility of the barrier polymers, effectively removing these impurities and improving the purity of BHET in one step. However, this method relies on multi-stage water addition and temperature control to remove impurities. During temperature control, BHET is prone to spontaneous polymerization to form dimers, especially in complex solutions containing barrier polymer impurities, where the polymerization risk is higher, resulting in limited purity improvement and an additional separation step.

[0006] Patent CN120289291A discloses a purification method for BHET monomer obtained from the depolymerization of waste polyester with ethylene glycol. This method employs a continuous process combining microwave-induced thin-film evaporation separation with molecular distillation purification, effectively solving problems such as long process flow, poor continuity, numerous side reactions, and low purity and yield inherent in traditional gas-liquid equilibrium separation methods. However, this method requires sophisticated equipment and is prone to increasing energy consumption.

[0007] Patent CN118812354A discloses a novel continuous process for preparing BHET from recycled polyester alcoholysis, belonging to the field of waste polyester recycling technology. The process involves alcoholystolysis of waste polyester with a catalyst and diol, followed by filtration to obtain an alcoholysis liquid. This liquid is then fed into an evaporation chamber for thin-film evaporation, allowing the generated diol vapor to directly participate in the reaction as a low-molecular-weight alcoholysis feedstock, while simultaneously sublimating and purifying BHET. This process avoids the high energy consumption problems of additional heat sources and circulating cooling water required for traditional hot water dissolution and crystallization purification, and achieves the recycling of diol vapor, effectively solving the difficulties in diol handling and poor recycling performance in conventional alcoholysis reactions. However, the sublimation process of this method requires high-temperature conditions, and BHET is prone to polymerization to form dimers, leading to reduced product yield and purity, equipment blockage, and further increased energy consumption.

[0008] Patent CN115894223B describes a method for chemically recycling waste PET products using the phase change properties of BHET crystals. The method involves mixing PET products with a catalyst in ethylene glycol for alcoholysis to obtain an alcoholysis solution, cooling and crystallizing to obtain BHET-containing crystals, and then sublimating the crystals under reduced pressure and collecting high-purity BHET by condensation. This method significantly simplifies the waste PET recycling process, reduces energy consumption and costs, and overcomes the technical bottleneck of difficult decolorization and purification of BHET. However, this method requires high vacuum and high temperature (150~200°C) in the reduced pressure sublimation step, which makes BHET prone to thermal polymerization, forming dimers. This leads to a decrease in purity from the potential 99% to 85~90%, a yield reduction of 15~25%, and although the decolorization effect is improved, the polymerization problem remains unresolved, resulting in unstable product quality and additional purification requirements.

[0009] Patent TW110141568 discloses a method for decolorizing and purifying BHET material, comprising the following steps: first, adding a first dose of activated carbon to the BHET material for preliminary adsorption treatment, followed by a first cooling crystallization and filtration to obtain BHET crystals; then, using an oxidant to chemically react with the crystals to destroy dyes or impurities, followed by adding a second dose of activated carbon to adsorb the oxidation products generated in the reaction; finally, performing a second cooling crystallization, filtration, and drying to obtain a high-purity decolorized BHET product. However, this method relies on multiple steps of activated carbon adsorption and oxidant treatment for decolorization and purification. Firstly, the numerous steps naturally increase costs, and new impurities may be introduced during this process; secondly, the introduction of oxidants may induce additional side reactions, resulting in limited purity improvement, a complex process, a high risk of residual oxidation products, and limited application scenarios.

[0010] Patent WO2021032826A1 discloses a method for preparing a high-porosity BHET solid composition, comprising the following steps: first, providing an ethylene glycol aqueous solution containing BHET and BHET dimer; selectively crystallizing and separating the dimer to purify the mother liquor; subsequently, crystallizing BHET from the mother liquor and washing to obtain a high-purity crystalline filter cake; then granulating the filter cake to obtain granular material; finally, fluidized bed drying to remove moisture and retain the high-porosity structure, resulting in a high-purity, easily soluble, and stably transportable BHET solid granule product. However, this method does not reduce the formation of dimers; it only separates the BHET monomer from the dimer through purification. While the purity of BHET is indeed improved, the yield is not.

[0011] Therefore, it is of great significance to study a method to improve the yield of BHET after vacuum sublimation purification in order to solve the problems existing in the current technology. Summary of the Invention

[0012] The purpose of this invention is to address the problems existing in the prior art and provide a method to improve the yield of BHET after vacuum sublimation purification. By synthesizing a novel inhibitor, the polymerization reaction during the vacuum sublimation process of BHET is inhibited, thereby improving the yield of BHET after vacuum sublimation purification.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0014] A method to improve the yield of BHET after vacuum sublimation purification involves adding an inhibitor during the vacuum sublimation process of the BHET-containing raw material to inhibit the polymerization reaction of BHET, thereby increasing the yield of purified BHET.

[0015] The inhibitor is bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide, with the following chemical structural formula:

[0016] ;

[0017] Compared with the absence of inhibitors, the yield of purified BHET increased by more than 13%.

[0018] As a preferred technical solution:

[0019] As described above, in one method to improve the yield of BHET after vacuum sublimation purification, the amount of inhibitor added is 10-15 wt% of the raw material containing BHET.

[0020] The method described above for improving the yield of BHET after vacuum sublimation purification uses BHET-containing raw materials obtained by depolymerizing waste polyester with ethylene glycol.

[0021] The method described above for improving the yield of BHET after vacuum sublimation purification shows that the purity of BHET after purification is above 99.5% as determined by LC-MS.

[0022] LC-MS and GPC analysis showed that the content of dimers and polymers in the bottom residue after purification was 37-40%, while the content of dimers and polymers in the bottom residue after purification without the use of inhibitors was about 60%. After the vacuum sublimation, a small portion of BHET was converted into dimers and polymers due to self-polymerization, and its boiling point was greatly increased. Under the experimental conditions, it did not sublimate but remained at the bottom of the container, which contained both BHET and dimers and polymers.

[0023] The method described above for improving the yield of BHET after vacuum sublimation purification involves a vacuum degree of <1000 Pa and a temperature of 150~180℃.

[0024] The method described above for improving the yield of BHET after vacuum sublimation purification involves TGA testing of the inhibitor, and the residual mass at 300°C is 97.99%.

[0025] The method described above for improving the yield of BHET after vacuum sublimation purification involves using N-methylimidazolium (NMI) and sodium bis(trifluoromethanesulfonyl)imide (NaTf2N) as raw materials, and synthesizing them in two steps through intermediate synthesis and anion exchange.

[0026] The method described above for improving the yield of BHET after vacuum sublimation purification includes the following specific preparation steps for the inhibitor:

[0027] (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide;

[0028] N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and dried under vacuum to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide.

[0029] (2) Anion exchange yields bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide;

[0030] Dissolve the intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) in deionized water, add an aqueous solution of sodium bis(trifluoromethanesulfonyl)imide under ice bath conditions, and stir at room temperature for 12-24 hours. After the reaction, the system will separate into layers. Separate and collect the lower liquid, and wash it several times with ice-cold deionized water at 0-10°C until no bromide ions (Br) are detected in the washing liquid. - After drying with anhydrous magnesium sulfate, the water was removed by rotary evaporation, and then vacuum dried to obtain bis-(3-methyl-1-imidazolium)ethylene bis(bistrifluoromethylsulfonyl)imide.

[0031] As described above, in a method to improve the yield of BHET after vacuum sublimation purification, in step (1), the molar ratio of N-methylimidazole to 1,2-dibromoethane is 2.15~2.3:1, N-methylimidazole is slightly in excess, the vacuum drying temperature is 60~80℃, the vacuum drying time is 18~24h, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide is 90~95%.

[0032] In the method described above for improving the yield of BHET after vacuum sublimation purification, in step (2), the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide is 1:2.2~2.3, the vacuum drying temperature is 70~100℃, the vacuum drying time is 24~48h, and the yield of bis-(3-methyl-1-imidazolium)ethylene bis(bistrifluoromethanesulfonyl)imide is 85~95%.

[0033] Invention principle:

[0034] In the BHET purification process under reduced pressure, this invention involves adding a self-designed inhibitor to the BHET-containing raw material, mixing thoroughly, and then performing reduced pressure sublimation. This significantly improves the yield of BHET after purification. The specific inhibition mechanism is as follows:

[0035] The polymerization of BHET into dimers mainly occurs through condensation polymerization, requiring free radical or ionic catalysis. The inhibitor of this invention, as a dicationic ionic liquid, exhibits high thermal stability and low volatility, remaining stable under high temperature and vacuum. Its imidazole cation can form hydrogen bonds or electrostatic interactions with the hydroxyl or carboxyl groups of BHET, blocking the extension of the polymerization chain; the Tf₂N anion provides a hydrophobic environment, reducing water-induced polymerization and improving the thermal stability of the ionic liquid. Simultaneously, this compound contains two imidazole cation units connected by an ethane bridge (C₂), forming a double positive charge structure. Compared to monocationic ionic liquids, the number of positive charges carried by each molecule is doubled, resulting in a significantly enhanced electrostatic attraction between positive and negative ions. This enhanced Coulombic network makes the movement and rearrangement of ions in the liquid state more difficult, thereby increasing flow resistance and viscosity. The viscosity of the ionic liquid can then regulate the diffusion of BHET molecules; by increasing the system viscosity, it regulates the diffusion of BHET molecules, thereby inhibiting the self-polymerization reaction of BHET. Specifically, this ionic liquid has a high viscosity, much higher than that of pure BHET melt. When a certain amount of inhibitor is added, the viscosity of the entire reaction system increases significantly. According to the Stokes-Einstein relation, the diffusion coefficient D is inversely proportional to the medium viscosity η (D ∝ 1 / η). Therefore, the increase in viscosity leads to a significant decrease in the diffusion coefficient of BHET molecules. Consequently, under high viscosity conditions, the average diffusion rate of BHET molecules slows down, and the probability of effective intermolecular collisions per unit time decreases accordingly. Since the self-polymerization of BHET is mainly a condensation process, the high viscosity medium restricts the long-range diffusion of end groups, reducing the occurrence rate of effective end-group collisions, leading to a decrease in the overall polymerization rate. This favors the sublimation of BHET rather than polymerization.

[0036] Beneficial effects:

[0037] (1) A method for improving the yield of BHET after vacuum sublimation purification of the present invention is to add an inhibitor during the vacuum sublimation process of BHET to suppress the polymerization reaction that may occur in BHET under high temperature and high vacuum. This method can not only improve the purity and yield of BHET after vacuum sublimation purification, but also reduce the energy consumption in the purification process to a certain extent. It can achieve efficient decolorization and purification without multi-step oxidation and adsorption.

[0038] (2) The present invention provides a method for improving the yield of BHET after vacuum sublimation purification, which does not require the participation of any solvent, thus reducing the impact on the environment and the difficulty of post-processing.

[0039] (3) A method of the present invention for improving the yield of BHET after vacuum sublimation purification is to introduce the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide, which effectively inhibits the polymerization reaction of BHET during vacuum sublimation purification, thereby reducing the dimer content and achieving high purity without multi-stage temperature control. Attached Figure Description

[0040] Figure 1 This is a TGA test result of the inhibitor in Example 1;

[0041] Figure 2 The 1H NMR spectrum of the inhibitor in Example 1;

[0042] Figure 3 The infrared spectrum of the inhibitor in Example 1 is shown. Detailed Implementation

[0043] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0044] The test methods involved in the performance indicators in the embodiments and comparative examples of this invention are as follows:

[0045] Purity, dimer and polymer content: The purity of purified BHET and the content of dimers and polymers in the bottom residue were obtained based on different retention times in liquid chromatography combined with mass spectrometry analysis. The product was dissolved in acetonitrile:water at a volume ratio of 8:2 for 24 h, with a concentration <1 mg / mL. The mobile phase in the liquid chromatography section was acetonitrile and water; gradient elution was used, with the acetonitrile volume fraction increasing from 20% to 80% in the mobile phase from 0 to 10 min, from 80% to 90% in the mobile phase from 10 to 15 min, and decreasing from 90% to 20% in the mobile phase from 15 to 20 min; the injection volume was 2 μL; the flow rate was 0.2 mL / min; for mass spectrometry: positive ion bombardment mode was used, with a detection range of 200–1400 m / z. Among them, the experimental instruments were: liquid chromatography-mass spectrometry (LC-MS) system, model Agilent LC-MS6120, manufactured by Agilent Technologies Inc.; the experimental reagents were: acetonitrile, spectral grade >99.8%, manufactured by Shanghai Maclean Biochemical Technology Co., Ltd.

[0046] Yield: .

[0047] The sources of some of the substances used in the preparation of this invention are as follows:

[0048] name Reagent grade Manufacturers Anhydrous zinc acetate Analysis of pure AR Shanghai Titan Technology Co., Ltd. Ethylene glycol Analysis of pure AR Shanghai Lingfeng Chemical Reagent Co., Ltd. Acetonitrile Analysis of pure AR Aladdin Biochemical Technology Co., Ltd. N-Methylimidazole >99% Shanghai McLean Biochemical Technology Co., Ltd. 1,2-Dibromoethane >99% Shanghai McLean Biochemical Technology Co., Ltd. Ethyl acetate 99% Shanghai McLean Biochemical Technology Co., Ltd. Sodium bis(trifluoromethanesulfonyl)imide >98% Aladdin Biochemical Technology Co., Ltd. Anhydrous magnesium sulfate 99.99% Shanghai McLean Biochemical Technology Co., Ltd.

[0049] Example 1

[0050] A method for improving the yield of BHET after vacuum sublimation purification includes the following steps:

[0051] (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide;

[0052] N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and then dried under vacuum at 65°C for 24 h to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide.

[0053] The mass-to-volume ratio of N-methylimidazole to acetonitrile was 10.6 g:200 mL, the molar ratio of N-methylimidazole to 1,2-dibromoethane was 2.15:1, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide was 90%.

[0054] (2) Anion exchange yields the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide;

[0055] The intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) was dissolved in deionized water. An aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was added under ice bath conditions, and the mixture was stirred at room temperature for 12 hours. After the reaction, the lower layer was separated and collected. The mixture was washed five times with ice-cold deionized water at 8°C until no bromide ions were detected in the washings. After drying with anhydrous magnesium sulfate, water was removed by rotary evaporation. The mixture was then vacuum dried at 70°C for 48 hours to obtain the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bis(trifluoromethanesulfonyl)imide), with the following chemical structure:

[0056] ;

[0057] The mass-to-volume ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to deionized water is 1 g:9 mL, and the concentration of the aqueous solution of sodium bis(trifluoromethanesulfonyl)imide is 0.15 g / mL; the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide is 1:2.2.

[0058] The yield of the inhibitor was 85%; such as Figure 1 As shown, the inhibitor was tested using TGA, and the residual mass at 300°C was 97.99%.

[0059] like Figure 2 As shown in the spectrum, the singlet at 9.21 ppm corresponds to the characteristic peak of the methylene hydrogen in the N-CH=N group of the imidazole ring. The multiplets at 7.75 ppm and 7.71 ppm are the characteristic peaks of the two methylene hydrogens in the -CH=CH- group of the imidazole ring, respectively. The characteristic peak of the methylene hydrogen in the alkyl chain bridging the two imidazole rings is at 4.75 ppm, while the singlet at 3.87 ppm is the characteristic peak of the methyl hydrogen in the imidazole ring. The solvent used was dimethyl sulfoxide, whose characteristic peak is at 2.5 ppm, and the water peak is at 3.3 ppm.

[0060] like Figure 3 As shown, 3110cm -1 The peak at 1341 cm⁻¹ is attributed to the CH stretching vibration on the imidazole ring, while the CN stretching vibration absorption peak is at 1341 cm⁻¹. -1 At point C=N, the stretching vibration occurs at 1518 cm. -1 The tensile vibration peak of CNC is at 663 cm⁻¹. -1 The S=O stretching vibration in anions is located at 1078 cm⁻¹. -1 (Strong absorption peak), S–N–S stretching vibration located at 780 cm⁻¹ -1 At this point, the C–F stretching vibration should be located at 1230 cm. -1 Location; 2930cm -1 The stretching vibration peak of the methylene group is still present at that point.

[0061] In summary, the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide has been synthesized.

[0062] (3) During the decomposition of waste polyester (manufacturer: Hangzhou Deyang Chemical Fiber Co., Ltd., brand: molecular weight 25000) by ethylene glycol, an inhibitor was added to suppress the polymerization reaction of BHET, thereby increasing the yield of purified BHET.

[0063] The mass ratio of ethylene glycol to waste polyester is 4:1, the depolymerization temperature is 175℃, and the depolymerization time is 180 min. The amount of inhibitor added is 10 wt% of the product obtained by depolymerizing waste polyester with ethylene glycol. The vacuum degree of sublimation is 150 Pa, and the temperature is 150℃.

[0064] The final purity of BHET was 99.7%; the content of dimers and polymers in the bottom residue after purification was 37%; and the yield of BHET increased by 14% compared with that without the addition of inhibitors.

[0065] Example 2

[0066] A method for improving the yield of BHET after vacuum sublimation purification includes the following steps:

[0067] (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide;

[0068] N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and then dried under vacuum at 60°C for 24 h to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide.

[0069] The mass-to-volume ratio of N-methylimidazole to acetonitrile was 10.6 g:150 mL, the molar ratio of N-methylimidazole to 1,2-dibromoethane was 2.2:1, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide was 92%.

[0070] (2) Anion exchange yields the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide;

[0071] The intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) was dissolved in deionized water. An aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was added under ice bath conditions, and the mixture was stirred at room temperature for 16 hours. After the reaction, the lower layer was separated and collected. The mixture was washed six times with ice-cold deionized water at 8°C until no bromide ions were detected in the washings. After drying with anhydrous magnesium sulfate, water was removed by rotary evaporation. The mixture was then vacuum dried at 80°C for 42 hours to obtain the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bis(trifluoromethanesulfonyl)imide), with the following chemical structure:

[0072] ;

[0073] The mass-to-volume ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to deionized water was 1 g:9.5 mL, and the concentration of the aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was 0.16 g / mL; the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide was 1:2.22.

[0074] The yield of the inhibitor was 87%;

[0075] (3) During the decomposition of waste polyester (manufacturer: Hangzhou Deyang Chemical Fiber Co., Ltd., brand: molecular weight 25000) by ethylene glycol, an inhibitor was added to suppress the polymerization reaction of BHET, thereby increasing the yield of purified BHET.

[0076] The mass ratio of ethylene glycol to waste polyester was 5:1, the depolymerization temperature was 180℃, and the depolymerization time was 225 min. The amount of inhibitor added was 11 wt% of the product obtained by depolymerizing waste polyester with ethylene glycol. The vacuum degree of the sublimation was 350 Pa, and the temperature was 160℃.

[0077] The final purity of BHET was 99.8%; the content of dimers and polymers in the bottom residue after purification was 37%; and the yield of BHET increased by 15% compared with that without the addition of inhibitors.

[0078] Example 3

[0079] A method for improving the yield of BHET after vacuum sublimation purification includes the following steps:

[0080] (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide;

[0081] N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and then dried under vacuum at 70°C for 22 h to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide.

[0082] The mass-to-volume ratio of N-methylimidazole to acetonitrile was 10.6 g:175 mL, the molar ratio of N-methylimidazole to 1,2-dibromoethane was 2.23:1, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide was 92%.

[0083] (2) Anion exchange yields the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide;

[0084] The intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) was dissolved in deionized water. An aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was added under ice bath conditions, and the mixture was stirred at room temperature for 20 h. After the reaction, the lower layer was separated and collected. The mixture was washed seven times with ice-cold deionized water at 8 °C until no bromide ions were detected in the washings. After drying with anhydrous magnesium sulfate, water was removed by rotary evaporation. The mixture was then vacuum dried at 90 °C for 36 h to obtain the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bis(trifluoromethanesulfonyl)imide), with the following chemical structure:

[0085] ;

[0086] The mass-to-volume ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to deionized water is 1 g:10 mL, and the concentration of the aqueous solution of sodium bis(trifluoromethanesulfonyl)imide is 0.18 g / mL; the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide is 1:2.25.

[0087] The yield of the inhibitor was 90%.

[0088] (3) During the decomposition of waste polyester (manufacturer: Hangzhou Deyang Chemical Fiber Co., Ltd., brand: molecular weight 25000) by ethylene glycol, an inhibitor was added to suppress the polymerization reaction of BHET, thereby increasing the yield of purified BHET.

[0089] The mass ratio of ethylene glycol to waste polyester was 7:1, the depolymerization temperature was 185℃, and the depolymerization time was 270 min. The amount of inhibitor added was 12 wt% of the product obtained by depolymerizing waste polyester with ethylene glycol. The vacuum degree of the sublimation was 550 Pa, and the temperature was 165℃.

[0090] The final purity of BHET was 99.7%; the content of dimers and polymers in the bottom residue after purification was 38%; and the yield of BHET increased by 14% compared with that without the addition of inhibitors.

[0091] Example 4

[0092] A method for improving the yield of BHET after vacuum sublimation purification includes the following steps:

[0093] (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide;

[0094] N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and then dried under vacuum at 75°C for 20 h to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide.

[0095] The mass-to-volume ratio of N-methylimidazole to acetonitrile was 10.6 g:225 mL, the molar ratio of N-methylimidazole to 1,2-dibromoethane was 2.26:1, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide was 93%.

[0096] (2) Anion exchange yields the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide;

[0097] The intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) was dissolved in deionized water. An aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was added under ice bath conditions, and the mixture was stirred at room temperature for 22 hours. After the reaction, the lower layer was separated and collected. The mixture was washed eight times with ice-cold deionized water at 8°C until no bromide ions were detected in the washings. After drying with anhydrous magnesium sulfate, water was removed by rotary evaporation. The mixture was then vacuum dried at 95°C for 30 hours to obtain the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bis(trifluoromethanesulfonyl)imide), with the following chemical structure:

[0098] ;

[0099] The mass-to-volume ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to deionized water was 1 g:10.5 mL, and the concentration of the aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was 0.19 g / mL; the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide was 1:2.27.

[0100] The yield of the inhibitor was 93%;

[0101] (3) During the decomposition of waste polyester (manufacturer: Hangzhou Deyang Chemical Fiber Co., Ltd., brand: molecular weight 25000) by ethylene glycol, an inhibitor was added to suppress the polymerization reaction of BHET, thereby increasing the yield of purified BHET.

[0102] The mass ratio of ethylene glycol to waste polyester was 8:1, the depolymerization temperature was 190℃, and the depolymerization time was 315 min. The amount of inhibitor added was 14 wt% of the product obtained by depolymerizing waste polyester with ethylene glycol. The vacuum degree of the sublimation was 750 Pa, and the temperature was 170℃.

[0103] The final purity of BHET was 99.5%; the content of dimers and polymers in the bottom residue after purification was 40%; and the yield of BHET increased by 15% compared with that without the addition of inhibitors.

[0104] Example 5

[0105] A method for improving the yield of BHET after vacuum sublimation purification includes the following steps:

[0106] (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide;

[0107] N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and then dried under vacuum at 80°C for 18 h to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide.

[0108] The mass-to-volume ratio of N-methylimidazole to acetonitrile was 10.6 g:250 mL, the molar ratio of N-methylimidazole to 1,2-dibromoethane was 2.3:1, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide was 95%.

[0109] (2) Anion exchange yields the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide;

[0110] The intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) was dissolved in deionized water. An aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was added under ice bath conditions, and the mixture was stirred at room temperature for 24 hours. After the reaction, the lower layer was separated and collected. The mixture was washed 10 times with ice-cold deionized water at 8°C until no bromide ions were detected in the washings. After drying with anhydrous magnesium sulfate, water was removed by rotary evaporation. The mixture was then vacuum dried at 100°C for 24 hours to obtain the inhibitor bis-(3-methyl-1-imidazolium)ethylenebis(bis(trifluoromethanesulfonyl)imide), with the following chemical structure:

[0111] ;

[0112] The mass-to-volume ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to deionized water is 1 g:11 mL, and the concentration of the aqueous solution of sodium bis(trifluoromethanesulfonyl)imide is 0.2 g / mL; the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide is 1:2.3.

[0113] The yield of the inhibitor was 95%;

[0114] (3) During the decomposition of waste polyester (manufacturer: Hangzhou Deyang Chemical Fiber Co., Ltd., brand: molecular weight 25000) by ethylene glycol, an inhibitor was added to suppress the polymerization reaction of BHET, thereby increasing the yield of purified BHET.

[0115] The mass ratio of ethylene glycol to waste polyester was 10:1, the depolymerization temperature was 196℃, and the depolymerization time was 360 min. The amount of inhibitor added was 15 wt% of the product obtained by depolymerizing waste polyester with ethylene glycol. The vacuum degree of the sublimation was 950 Pa, and the temperature was 180℃.

[0116] The final purity of BHET was 99.7%; the content of dimers and polymers in the bottom residue after purification was 37%; and the yield of BHET increased by 17% compared with that without the addition of inhibitors.

Claims

1. A method for improving the yield of BHET after vacuum sublimation purification, characterized in that: Adding an inhibitor during the vacuum sublimation of BHET-containing raw materials inhibits the polymerization reaction of BHET, thereby increasing the yield of purified BHET. The inhibitor is bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide, with the following chemical structural formula: ； Compared with the absence of inhibitors, the yield of purified BHET increased by more than 13%.

2. The method for improving the yield of BHET after vacuum sublimation purification according to claim 1, characterized in that, The amount of inhibitor added is 10-15 wt% of the raw material containing BHET.

3. The method for improving the yield of BHET after vacuum sublimation purification according to claim 1, characterized in that, The raw material containing BHET is a product obtained by depolymerizing waste polyester with ethylene glycol.

4. The method for improving the yield of BHET after vacuum sublimation purification according to claim 1, characterized in that, After purification, BHET has a purity of over 99.5%. The content of dimers and polymers in the bottom residue after purification is 37-40%.

5. The method for improving the yield of BHET after vacuum sublimation purification according to claim 1, characterized in that, The vacuum degree of sublimation is <1000Pa, and the temperature is 150~180℃.

6. The method for improving the yield of BHET after vacuum sublimation purification according to claim 1, characterized in that, The inhibitor was tested by TGA, and the residual mass at 300°C was 97.99%.

7. The method for improving the yield of BHET after vacuum sublimation purification according to claim 1, characterized in that, The inhibitor is synthesized from N-methylimidazolium and sodium bis(trifluoromethanesulfonyl)imide through a two-step process of intermediate synthesis and anion exchange.

8. The method for improving the yield of BHET after vacuum sublimation purification according to claim 7, characterized in that, The specific preparation steps of the inhibitor are as follows: (1) Synthetic intermediate 1,2-bis(N-methylimidazolium)ethane bromide; N-methylimidazolium was dissolved in acetonitrile, and 1,2-dibromoethane was added. The mixture was stirred at 70°C for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, and the solid filter cake obtained by vacuum filtration was washed with ethyl acetate and dried under vacuum to obtain the intermediate 1,2-bis(N-methylimidazolium) ethane bromide. (2) Anion exchange yields bis-(3-methyl-1-imidazolium)ethylenebis(bistrifluoromethanesulfonyl)imide; The intermediate 1,2-bis(N-methylimidazolium)ethane bromide obtained in step (1) was dissolved in deionized water. An aqueous solution of sodium bis(trifluoromethanesulfonyl)imide was added under ice bath conditions and stirred at room temperature for 12-24 h. After the reaction, the lower liquid was separated and collected. It was washed multiple times with ice-cold deionized water at 0-10 °C until no bromide ions were detected in the washing liquid. After drying with anhydrous magnesium sulfate, the water was removed by rotary evaporation and vacuum drying to obtain bis-(3-methyl-1-imidazolium)ethylene bis(bistrifluoromethanesulfonyl)imide.

9. A method for improving the yield of BHET after vacuum sublimation purification according to claim 8, characterized in that, In step (1), the molar ratio of N-methylimidazole to 1,2-dibromoethane is 2.15~2.3:1, the vacuum drying temperature is 60~80℃, the vacuum drying time is 18~24h, and the yield of the intermediate 1,2-bis(N-methylimidazole)ethane bromide is 90~95%.

10. A method for improving the yield of BHET after vacuum sublimation purification according to claim 8, characterized in that, In step (2), the molar ratio of intermediate 1,2-bis(N-methylimidazolium)ethane bromide to sodium bis(trifluoromethanesulfonyl)imide is 1:2.2~2.3, the vacuum drying temperature is 70~100℃, the vacuum drying time is 24~48h, and the yield of bis-(3-methyl-1-imidazolium)ethylene bis(bistrifluoromethanesulfonyl)imide is 85~95%.

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