Method for preparing PET (Polyethylene Terephthalate) through polycondensation of high-purity monohydroxyethyl terephthalate under mild condition

Abstract

The invention discloses a method for preparing PET (Polyethylene Terephthalate) by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions, which comprises the following steps of: stirring and reacting monohydroxyethyl terephthalate and ethylene glycol in inert atmosphere at the temperature of 180-230 DEG C for 3-12 hours; and adding a condensation polymerization catalyst into the reaction product, and stirring and reacting for 2-6 hours in an inert atmosphere under the conditions that the temperature is 260-290 DEG C and the vacuum is 40-150 Pa to prepare PET (Polyethylene Terephthalate). According to the preparation method, high-purity monohydroxyethyl terephthalate is used as a polymerization monomer raw material, and PET which meets the current commercially available product standard and is prepared under mild conditions is explored. In addition, based on the current situation that the preparation cost of the high-purity monohydroxyethyl terephthalate is too high, the invention provides a low-cost method for preparing the high-purity monohydroxyethyl terephthalate under a mild condition.

Description

Technical Field

[0001] This invention relates to the field of polyethylene terephthalate (PET) preparation technology, and to a method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions. Specifically, it relates to a method for preparing high-purity monohydroxyethyl terephthalate under mild conditions, and a method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions. Background Technology

[0002] PET is a widely used plastic in daily life. Due to its excellent mechanical properties and thermal stability, it is extensively used in bottles, films, and fibers, holding an important position in the plastics industry. This leads to a large amount of waste PET. Untreated waste PET can take 450 years to completely degrade in the natural environment, during which time microplastics (<5mm) are formed and dispersed in the environment, posing a significant threat to the ecological environment and human health. Recycling waste PET is an effective treatment method. Current recycling methods are divided into energy recovery and material recovery according to the purpose of recycling. Energy recovery mainly involves incineration to recover energy, but the recovery value is low, and the incineration process easily produces toxic and harmful gases, causing secondary pollution to the environment. Material recovery is divided into physical recycling, chemical recycling, and biological recycling. Among them, physical recycling is mostly used to recover uncontaminated single waste materials. Repeated processing can cause a decline in PET performance, which is considered downgrading recycling. Chemical recycling relies on heat or solvents to depolymerize PET into raw material monomers, or through the design and control of the degradation system, to obtain higher-value chemicals / new materials, showing higher economic, social, and environmental benefits, and is mostly considered upgrading recycling. There is already a lot of research on the chemical recycling of waste PET, but chemical recycling methods usually involve the use of large amounts of solvents or harsh reaction conditions such as high temperature and high pressure, and most of the recovered monomers need to be further separated and purified before they can be reused.

[0003] Bio-recycling utilizes naturally occurring enzymes to depolymerize and recycle PET. Due to its mild reaction conditions and low energy consumption, it has attracted increasing attention. It is generally believed that during the enzymatic hydrolysis of PET, PET hydrolases attack PET, releasing monoethyl terephthalate (MET). MET is further hydrolyzed into terephthalic acid and ethylene glycol by MET hydrolases. In their research project on PET recycling, the inventors' group, aiming for a closed-loop recycling of PET, explored the possibility of MET as an intermediate product in both depolymerization and polymerization processes. Based on previously reported literature and the bio-depolymerization and polymerization processes of PET, they discovered that MET can be used as a raw material for PET repolymerization. Using MET as a raw material for PET repolymerization would not only shorten the reaction pathways for both enzymatic hydrolysis and repolymerization but also improve atom utilization and reduce overall energy consumption. Therefore, if monohydroxyethyl terephthalate can be used directly as a raw material for PET polymerization, a closed-loop recycling of waste PET can be achieved after overcoming the technical difficulties of directly degrading waste PET to obtain monohydroxyethyl terephthalate.

[0004] However, the preparation of high-purity monohydroxyethyl terephthalate (MAT) currently faces challenges due to its complex process and high cost. Literature review reveals that only a few studies have developed enzymatic synthesis routes for MAT, but these are small-scale, costly, and the highest purity of the obtained MAT is only 86.7%, which is insufficient to meet the requirements for polymerization dosage and purity (Erika EQD, Pereira SIC, ...). AD, et al. Novel efficient enzymatic synthesis of the key-reaction intermediate of PET depolymerization, mono(2-hydroxyethyl terephthalate)-MHET. [J]. Journal of biotechnology, 2022, 358, 102-110. However, the existing chemical synthesis methods suffer from the significant drawback of requiring large amounts of catalyst and still have the problem of small feed amounts, making large-scale synthesis impossible.

[0005] Furthermore, current PET production mainly employs direct esterification or transesterification. Direct esterification involves the direct reaction of terephthalic acid and ethylene glycol under high temperature and pressure to produce ethylene terephthalate, followed by polycondensation. The ideal esterification temperature is 255-265℃, and the esterification time is typically over 10 hours. Transesterification involves the transesterification of methyl terephthalate and ethylene glycol between 150-160℃ to produce ethylene terephthalate, followed by polycondensation. Direct esterification is characterized by high temperature and long processing time, resulting in high energy consumption. While transesterification has a lower reaction temperature, it requires the addition of a transesterification catalyst and produces toxic methanol tail gas, necessitating additional tail gas treatment and generating additional energy consumption. Therefore, a method for producing PET under milder conditions would effectively reduce industrial energy consumption and align with current green and environmentally friendly sustainable development trends. Summary of the Invention

[0006] This invention addresses the problems identified in the prior art by providing a method for preparing PET via polycondensation of high-purity monohydroxyethyl terephthalate (MAT) under mild conditions. This method utilizes MAT as the monomer raw material and explores a method for preparing PET that meets current commercially available product standards under mild conditions. Furthermore, considering the currently high cost of preparing high-purity MAT, this invention provides a low-cost method for preparing high-purity MAT under mild conditions.

[0007] To achieve the above objectives, the present invention employs a technical solution consisting of the following technical measures.

[0008] In one aspect, the present invention provides a method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions, mainly comprising the following steps:

[0009] (1) Add monohydroxyethyl terephthalate and ethylene glycol to the reaction vessel in a molar ratio of 1:(1~2), mix and stir under an inert atmosphere and at a temperature of 180~230℃ until the solid is completely melted, and then continue to stir and react under the above conditions for 3~12h.

[0010] Wherein, the purity of the monohydroxyethyl terephthalate is at least 97%;

[0011] (2) The reaction product obtained in step (1) is added to the polycondensation catalyst and mixed and stirred under an inert atmosphere and at a temperature of 260-290°C until the solid is completely melted. Then, the above conditions are maintained and the reaction is stirred under a vacuum of 40-150 Pa for 2-6 hours to prepare PET.

[0012] The main inventive point of this invention lies in utilizing high-purity monohydroxyethyl terephthalate as a monomer raw material to explore the preparation of PET materials under mild conditions. Compared with the existing technology that mainly uses direct esterification or transesterification to prepare PET, the technical solution provided by this invention does not require an esterification reaction catalyst and can carry out the esterification reaction at a lower temperature (180-230℃) and normal pressure, avoiding the side reaction of ethylene glycol etherification. This facilitates the control of the content of diethylene glycol, a byproduct in PET preparation, resulting in high-quality PET.

[0013] The method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions provided by this invention has the following polymerization route:

[0014]

[0015] The content of terminal carboxyl groups is an important factor affecting the final polymerization effect. Therefore, the conversion rate of monohydroxyethyl terephthalate is defined as the esterification rate, which is used to represent the conversion rate of terminal carboxyl groups. The effect of different conditions on the esterification rate was evaluated through single-variable experiments.

[0016] Through the above single-variable experiment, it was found that the molar ratio of monohydroxyethyl terephthalate and ethylene glycol in step (1) directly affects the esterification rate. Therefore, in one preferred technical solution, the molar ratio of monohydroxyethyl terephthalate and ethylene glycol in step (1) is 1:(1.4~1.6).

[0017] Through the above single variable experiment, it was found that the temperature condition of 180-230℃ in step (1) directly affects the esterification rate. When the temperature exceeds 200℃, the esterification product is observed to turn yellow, which significantly affects the color of the PET finally prepared. Therefore, in one of the preferred technical solutions, the temperature condition in step (1) is 180-200℃.

[0018] Through the above single-variable experiments, it was found that the stirring reaction time in step (1) significantly affects the esterification rate. Furthermore, when the stirring reaction time exceeded 8 hours, the esterification product was observed to turn yellow, significantly affecting the hue of the final PET. Therefore, in one preferred technical solution, the stirring reaction time in step (1) is 5–8 hours. Similarly, the stirring reaction time in step (2) is 2–6 hours. When it exceeds 5 hours, the color of the resulting PET is observed to deepen, and it has no significant effect on increasing the molecular weight of PET.

[0019] Furthermore, existing technologies primarily employ direct esterification or transesterification methods to prepare PET, requiring the addition of an esterification catalyst as a variable. When tetrabutyl titanate is selected as the esterification catalyst, a comparison with the method without catalyst reveals that the esterification rate does not change significantly, but the esterification product with added tetrabutyl titanate exhibits a higher degree of yellowing, significantly affecting the color of the final PET. Therefore, the method for preparing PET through high-purity monohydroxyethyl terephthalate polycondensation under mild conditions provided by this invention, compared to existing technologies, eliminates the need for an esterification catalyst.

[0020] In this article, the polycondensation catalyst mentioned in step (2) is a conventional additive used in the preparation of PET. Those skilled in the art can directly refer to existing technologies (such as direct esterification or transesterification to prepare PET) to select a suitable polycondensation catalyst and its amount.

[0021] To better illustrate the present invention and provide a technical solution for reference, the polycondensation catalyst in step (2) is selected from any one of tetrabutyl titanate, anhydrous zinc acetate, antimony trioxide, antimony acetate, antimony glycolate, germanium oxide, aluminum glycolate, and tetraethyl germanate; the amount of the polycondensation catalyst added is 300 to 1000 ppm of the total mass (the total mass of the reaction product and the polycondensation catalyst in step (1)).

[0022] In this document, purity is a conventional chemical term referring to the content of the main component in a compound. In the field of PET preparation technology, those skilled in the art know that higher purity monomer raw materials are more conducive to obtaining higher quality PET.

[0023] However, as described in the background section, the existing high-purity monohydroxyethyl terephthalate has problems such as complicated preparation process and high cost. Currently, the price of monohydroxyethyl terephthalate reagent that meets the requirement of at least 97% purity in step (1) is about RMB 1900 / g.

[0024] Given the current high price of the monomer raw material, this invention also provides a method for preparing high-purity monohydroxyethyl terephthalate under mild conditions, mainly comprising the following steps:

[0025] (I) Add potassium terephthalate monomethyl ester and ethylene glycol to the reaction vessel in a molar ratio of 1:(15-25), add esterification catalyst, and after the solid is completely dissolved under an inert atmosphere, continue to stir the reaction for 3-5 hours under an inert atmosphere and at a temperature of 100-120°C.

[0026] (II) Adjust the pH of the reaction product obtained in step (I) to 2-5, stir at room temperature until the precipitation reaction is complete, and then filter, wash, dry and purify in sequence to prepare monohydroxyethyl terephthalate.

[0027] The final prepared monohydroxyethyl terephthalate can achieve a purity of over 97%.

[0028] In this article, the esterification catalyst mentioned in step (I) is a conventional additive used in the preparation of PET. Those skilled in the art can directly refer to existing technologies (such as direct esterification or transesterification to prepare PET) to select a suitable esterification catalyst and its amount.

[0029] To better illustrate the present invention and provide a technical solution for reference, the esterification reaction catalyst in step (I) is selected from any one of tetrabutyl titanate, anhydrous zinc acetate, antimony trioxide, antimony acetate, antimony glycolate, germanium oxide, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene; the amount of the esterification reaction catalyst added is 300 to 600 ppm of ethylene glycol.

[0030] To prepare monohydroxyethyl terephthalate with higher purity and yield under mild conditions, the effects of different conditions on purity and yield were evaluated through single-variable experiments. It was found that the preparation of monohydroxyethyl terephthalate under the mild conditions provided by this invention is less prone to side reactions and impurities, thus better facilitating the avoidance of diethylene glycol byproduct formation during subsequent PET preparation.

[0031] Through the above single-variable experiment, it was found that the specific selection of the esterification reaction catalyst in step (I) affects the yield of monohydroxyethyl terephthalate. In one preferred technical solution, the esterification reaction catalyst in step (I) is preferably tetrabutyl titanate.

[0032] Through the above single-variable experiment, it was found that the temperature conditions in step (I) affect the purity of monohydroxyethyl terephthalate. In one preferred technical solution, the temperature conditions in step (I) are preferably 100-110°C.

[0033] Through the above single-variable experiment, it was found that the stirring reaction time in step (I) affects the purity of monohydroxyethyl terephthalate. In one preferred technical solution, the stirring reaction time in step (I) is preferably 3.5 to 5 hours.

[0034] Through the above single-variable experiments, it was found that the choice of acidic reagent used to adjust the pH in step (II) affects the yield of monohydroxyethyl terephthalate. When a weak acid solution is used to adjust the pH, the yield of monohydroxyethyl terephthalate decreases. Therefore, in one preferred embodiment, the pH adjustment to 2-5 in step (II) is performed using a strong acid solution, such as hydrochloric acid or sulfuric acid.

[0035] Typically, the steps (II) involving filtration, washing, drying, and purification follow conventional principles in chemical processes, and those skilled in the art can perform the specific operations based on common knowledge.

[0036] The present invention has the following beneficial effects:

[0037] 1. This invention provides a method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions. This method utilizes high-purity monohydroxyethyl terephthalate as a monomer raw material and explores a method for preparing PET that meets the standards of currently commercially available products under mild conditions. Furthermore, based on the current situation where the preparation cost of high-purity monohydroxyethyl terephthalate is too high, a low-cost method for preparing high-purity monohydroxyethyl terephthalate under mild conditions is provided.

[0038] 2. The preparation method of this invention has mild conditions and simple process. Compared with traditional polymerization methods, it reduces the esterification temperature and esterification pressure. Under these conditions, the side reaction of ethylene glycol etherification will not occur, which is conducive to controlling the production of diethylene glycol and does not produce the toxic byproduct methanol. The overall route is more environmentally friendly and the overall quality of the final product is improved.

[0039] 3. The PET ultimately prepared by this invention meets the current standards for commercially available products, and its thermal and tensile properties are comparable to those of commercially available PET. In one technical solution, the glass transition temperature of the prepared PET is 78.1℃, the 5% thermal weight loss temperature is 389.7℃, and the tensile strength reaches 70.94MPa. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the synthetic route for preparing high-purity monohydroxyethyl terephthalate under mild conditions in Example 3 of the present invention.

[0041] Figure 2 The images show the 1H NMR spectrum (a) and 1C NMR spectrum (b) of the high-purity monohydroxyethyl terephthalate prepared in Example 3 of this invention.

[0042] Figure 3The images show photographs of the reaction products obtained in step (I) of Examples 3 and 6 of this invention, as well as the crude product obtained in step (II) of Example 3 before purification and the final high-purity monohydroxyethyl terephthalate. From left to right, the images are: the reaction product obtained in step (I) of Example 3, the reaction product obtained in step (I) of Example 6, the crude product obtained in step (II) of Example 3 before purification, and the final high-purity monohydroxyethyl terephthalate obtained in Example 3.

[0043] Figure 4 These are photographs of the reaction products obtained in step (1) of Examples 9, 12, and 15 of this invention.

[0044] Figure 5 These are photographs of the reaction products obtained in step (1) of Examples 16, 20, 5, and 18 of this invention.

[0045] Figure 6 These are photographs of the reaction products obtained in step (1) of Examples 23 and 25 of the present invention.

[0046] Figure 7 These are PET photographs obtained in Examples 12 and 15 of this invention. Figure (a) is the PET photograph obtained in Example 15; Figure (b) is the PET photograph obtained in Example 12.

[0047] Figure 8 Figure 26 shows a comparison of the viscosity of the PET finally prepared in Example 26 and Comparative Example 8 of this invention. Figure 26 shows a schematic photograph of the viscosity of the PET prepared in Comparative Example 8. Figure 27 shows a schematic photograph of the viscosity of the PET prepared in Example 26. Detailed Implementation

[0048] To further understand the present invention, preferred embodiments are described below with reference to examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims. Those skilled in the art can refer to the content of this document to appropriately improve the process parameters. In particular, it should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included within the scope of the present invention. The methods and applications of the present invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate changes and combinations to the methods and applications described herein without departing from the content, spirit and scope of the present invention to realize and apply the technology of the present invention. Although it is believed that those skilled in the art will fully understand the following terms, the following definitions are set forth to help illustrate the subject matter disclosed in the present invention.

[0049] In one aspect, the present invention provides a method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions, mainly comprising the following steps:

[0050] (1) Add monohydroxyethyl terephthalate and ethylene glycol to the reaction vessel in a molar ratio of 1:(1~2), mix and stir under an inert atmosphere and at a temperature of 180~230℃ until the solid is completely melted, and then continue to stir and react under the above conditions for 3~12h.

[0051] Wherein, the purity of the monohydroxyethyl terephthalate is at least 97%;

[0052] (2) The reaction product obtained in step (1) is added to the polycondensation catalyst and mixed and stirred under an inert atmosphere and at a temperature of 260-290°C until the solid is completely melted. Then, the above conditions are maintained and the reaction is stirred under a vacuum of 40-150 Pa for 2-6 hours to prepare PET.

[0053] In one embodiment, the molar ratio of monohydroxyethyl terephthalate and ethylene glycol in step (1) is 1:(1-2), for example 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2 or any range or point value therebetween; the temperature condition of 180-230°C, for example 180°C, 190°C, 200°C, 210°C, 220°C, 230°C or any range or point value therebetween; the stirring reaction of 3-12h, for example 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h or any range or point value therebetween.

[0054] The main inventive point of this invention lies in utilizing high-purity monohydroxyethyl terephthalate as a monomer raw material to explore the preparation of PET materials under mild conditions. Compared with the existing technology that mainly uses direct esterification or transesterification to prepare PET, the technical solution provided by this invention does not require an esterification reaction catalyst and can carry out the esterification reaction at a lower temperature (180-230℃) and normal pressure, avoiding the side reaction of ethylene glycol etherification. This facilitates the control of the content of diethylene glycol, a byproduct in PET preparation, resulting in high-quality PET.

[0055] The method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions provided by this invention has the following polymerization route:

[0056]

[0057] The content of terminal carboxyl groups is an important factor affecting the final polymerization effect. Therefore, the conversion rate of monohydroxyethyl terephthalate is defined as the esterification rate, which is used to represent the conversion rate of terminal carboxyl groups. The effect of different conditions on the esterification rate was evaluated through single-variable experiments.

[0058] Through the above single-variable experiment, it was found that the molar ratio of monohydroxyethyl terephthalate and ethylene glycol in step (1) directly affects the esterification rate. Therefore, in one preferred embodiment, the molar ratio of monohydroxyethyl terephthalate and ethylene glycol in step (1) is 1:(1.4~1.6).

[0059] Through the above single-variable experiment, it was found that the temperature condition of 180-230℃ in step (1) directly affects the esterification rate. When the temperature exceeds 200℃, the esterification product is observed to turn yellow, which significantly affects the color of the PET obtained in the final preparation. Therefore, in one preferred embodiment, the temperature condition in step (1) is 180-200℃.

[0060] Through the above single-variable experiments, it was found that the stirring reaction time in step (1) significantly affects the esterification rate. Furthermore, when the stirring reaction time exceeded 8 hours, the esterification product was observed to turn yellow, significantly affecting the hue of the final PET. Therefore, in one preferred embodiment, the stirring reaction time in step (1) is 5–8 hours. Similarly, the stirring reaction time in step (2) is 2–6 hours. When it exceeds 5 hours, the color of the resulting PET is observed to deepen, and it has no significant effect on increasing the molecular weight of PET.

[0061] Furthermore, existing technologies primarily employ direct esterification or transesterification methods to prepare PET, requiring the addition of an esterification catalyst as a variable. When tetrabutyl titanate is selected as the esterification catalyst, a comparison with the method without catalyst reveals that the esterification rate does not change significantly, but the esterification product with added tetrabutyl titanate exhibits a higher degree of yellowing, significantly affecting the color of the final PET. Therefore, the method for preparing PET through high-purity monohydroxyethyl terephthalate polycondensation under mild conditions provided by this invention, compared to existing technologies, eliminates the need for an esterification catalyst.

[0062] In this article, the polycondensation catalyst mentioned in step (2) is a conventional additive used in the preparation of PET. Those skilled in the art can directly refer to existing technologies (such as direct esterification or transesterification to prepare PET) to select a suitable polycondensation catalyst and its amount.

[0063] To better illustrate the present invention and provide a reference embodiment, the polycondensation catalyst in step (2) is selected from any one of tetrabutyl titanate, anhydrous zinc acetate, antimony trioxide, antimony acetate, antimony glycolate, germanium oxide, aluminum glycolate, and tetraethyl germanate; the amount of the polycondensation catalyst added is 300 to 1000 ppm of the total mass (the total mass of the reaction product and the polycondensation catalyst in step (1)).

[0064] In one embodiment, the temperature condition of 260-290°C in step (2) is, for example, 260°C, 270°C, 280°C, 290°C or any range or point value between them; the stirring reaction is 2-6h, for example, 2h, 3h, 4h, 5h, 6h or any range or point value between them.

[0065] In this document, purity is a conventional chemical term referring to the content of the main component in a compound. In the field of PET preparation technology, those skilled in the art know that higher purity monomer raw materials are more conducive to obtaining higher quality PET.

[0066] However, as described in the background section, the existing high-purity monohydroxyethyl terephthalate has problems such as complicated preparation process and high cost. Currently, the price of monohydroxyethyl terephthalate reagent that meets the requirement of at least 97% purity in step (1) is about RMB 1900 / g.

[0067] Given the current high price of the monomer raw material, this invention also provides a method for preparing high-purity monohydroxyethyl terephthalate under mild conditions, mainly comprising the following steps:

[0068] (I) Add potassium terephthalate monomethyl ester and ethylene glycol to the reaction vessel in a molar ratio of 1:(15-25), add esterification catalyst, and after the solid is completely dissolved under an inert atmosphere, continue to stir the reaction for 3-5 hours under an inert atmosphere and at a temperature of 100-120°C.

[0069] (II) Adjust the pH of the reaction product obtained in step (I) to 2-5, stir at room temperature until the precipitation reaction is complete, and then filter, wash, dry and purify in sequence to prepare monohydroxyethyl terephthalate.

[0070] The final prepared monohydroxyethyl terephthalate can achieve a purity of over 97%.

[0071] In this article, the esterification catalyst mentioned in step (I) is a conventional additive used in the preparation of PET. Those skilled in the art can directly refer to existing technologies (such as direct esterification or transesterification to prepare PET) to select a suitable esterification catalyst and its amount.

[0072] To better illustrate the present invention and provide an embodiment for reference, the esterification reaction catalyst in step (I) is selected from any one of tetrabutyl titanate, anhydrous zinc acetate, antimony trioxide, antimony acetate, antimony glycolate, germanium oxide, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene; the amount of the esterification reaction catalyst added is 300 to 600 ppm of ethylene glycol.

[0073] In one embodiment, the molar ratio of potassium monomethyl terephthalate and ethylene glycol in step (I) is 1:(15-25), for example 1:15, 1:16, 1:17, 1:18, 1:19, 1:20, 1:21, 1:22, 1:23, 1:24, 1:25 or any range or point value therebetween; the temperature condition of 100-120°C, for example 100°C, 110°C, 120°C or any range or point value therebetween; and the stirring reaction for 3-5 hours, for example 3 hours, 4 hours, 5 hours or any range or point value therebetween.

[0074] To prepare monohydroxyethyl terephthalate with higher purity and yield under mild conditions, the effects of different conditions on purity and yield were evaluated through single-variable experiments. It was found that the preparation of monohydroxyethyl terephthalate under the mild conditions provided by this invention is less prone to side reactions and impurities, thus better facilitating the avoidance of diethylene glycol byproduct formation during subsequent PET preparation.

[0075] Through the above single-variable experiment, it was found that the specific selection of the esterification reaction catalyst in step (I) affects the yield of monohydroxyethyl terephthalate. In one preferred embodiment, the esterification reaction catalyst in step (I) is preferably tetrabutyl titanate.

[0076] Through the above single-variable experiment, it was found that the temperature conditions in step (I) affect the purity of monohydroxyethyl terephthalate. In one preferred embodiment, the temperature conditions in step (I) are preferably 100-110°C.

[0077] Through the above single-variable experiment, it was found that the stirring reaction time in step (I) affects the purity of monohydroxyethyl terephthalate. In one preferred embodiment, the stirring reaction time in step (I) is preferably 3.5 to 5 hours.

[0078] Through the above single-variable experiments, it was found that the choice of acidic reagent used to adjust the pH in step (II) affects the yield of monohydroxyethyl terephthalate. When a weak acid solution is used to adjust the pH, the yield of monohydroxyethyl terephthalate decreases. Therefore, in one preferred embodiment, the pH adjustment to 2-5 in step (II) is performed using a strong acid solution, such as hydrochloric acid or sulfuric acid.

[0079] Typically, the steps (II) involving filtration, washing, drying, and purification follow conventional principles in chemical processes, and those skilled in the art can perform the specific operations based on common knowledge.

[0080] The present application will be further explained in detail below with reference to embodiments. However, those skilled in the art should understand that these embodiments are provided for illustrative purposes only and are not intended to limit the present application.

[0081] Example

[0082] The embodiments of this application will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be construed as limiting the scope of this application. Where specific conditions are not specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Where the manufacturers of reagents or instruments are not specified, they are all commercially available conventional products. This application should not be construed as being limited to the specific embodiments described.

[0083] 1. Raw materials

[0084] Potassium monomethyl terephthalate was purchased from Shanghai Titan Technology Co., Ltd.

[0085] Ethylene glycol, tetrabutyl titanate, anhydrous ethanol, antimony glycol, fumaric acid, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, and anhydrous zinc acetate were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0086] Hydrochloric acid (30% aqueous solution) was purchased from Chengdu Kelong Chemical Reagent Factory.

[0087] All reagents used above are of analytical grade. Unless otherwise specified, all water used in the experiment is high-purity water.

[0088] 2. Preparation method

[0089] A method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions mainly includes the following steps:

[0090] (1) Add monohydroxyethyl terephthalate and ethylene glycol to the reaction vessel in a molar ratio of 1:(1~2), mix and stir under an inert atmosphere and at a temperature of 180~230℃ until the solid is completely melted, and then continue to stir and react under the above conditions for 3~12h.

[0091] Wherein, the purity of the monohydroxyethyl terephthalate is at least 97%;

[0092] (2) The reaction product obtained in step (1) is added to the polycondensation catalyst and mixed and stirred under an inert atmosphere and at a temperature of 260-290°C until the solid is completely melted. Then, the above conditions are maintained and the reaction is stirred under a vacuum of 40-150 Pa for 2-6 hours to prepare PET.

[0093] A method for preparing high-purity monohydroxyethyl terephthalate under mild conditions mainly includes the following steps:

[0094] (I) Add potassium terephthalate monomethyl ester and ethylene glycol to the reaction vessel at a molar ratio of 1:20, and add esterification reaction catalyst. After the solid is completely dissolved under an inert atmosphere, continue to stir the reaction for 3 to 5 hours under an inert atmosphere and at a temperature of 100 to 120°C.

[0095] (II) Adjust the pH of the reaction product obtained in step (I) to 2-5, stir at room temperature until the precipitation reaction is complete, and then filter, wash, dry and purify in sequence to prepare monohydroxyethyl terephthalate.

[0096] 3. Testing Methods

[0097] (1) Nuclear magnetic resonance (NMR) analysis

[0098] Monohydroxyethyl terephthalate and PET were characterized using a Bruker Advance AV-II 400MHz nuclear magnetic resonance spectrometer with deuterated dimethyl sulfoxide (DMSO-d6) and deuterated trifluoroacetic acid (TFAA-d) as solvents.

[0099] (2) Thermogravimetric analysis (TGA)

[0100] The sample was tested using a NETZSCH TG 209F1 thermogravimetric analyzer, with the heating program set to a heating rate of 10℃·min. -1 The test temperature range is 40-700℃.

[0101] (3) Differential scanning calorimetry (DSC)

[0102] The sample was tested using a differential scanning calorimeter (DSC2500), with the heating program set to a heating rate of 10 °C / min.-1 The test temperature range is 20-280℃.

[0103] (4) Intrinsic viscosity test ([η])

[0104] Using a phenol / 1,1,2,2-tetrachloroethane mixture (1:1 volume ratio) as a solvent, PET was prepared to a concentration of 5 g·L⁻¹. -1 The viscosity of the solution was measured using an Ubbelohde viscometer in a constant temperature water bath at 25°C.

[0105] (5) Gas chromatography (GC)

[0106] The samples were analyzed using a TSQ 9610GC-MS / MS triple quadrupole gas chromatography-mass spectrometry system, following method 5.2.2 B (ethanolamine degradation method) as specified in GB / T14190-2017.

[0107] Example 1

[0108] Example 1 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is tetrabutyl titanate, the amount added is 500 ppm of ethylene glycol, and the reaction is carried out at 100°C for 3 hours with stirring.

[0109] Example 2

[0110] Example 2 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is tetrabutyl titanate, the amount added is 400 ppm of ethylene glycol, and the reaction is carried out at 120°C for 3 hours with stirring.

[0111] Example 3

[0112] Example 3 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is tetrabutyl titanate, the amount added is 400 ppm of ethylene glycol, and the reaction is carried out at 100°C for 4 hours with stirring.

[0113] After testing, such as Figure 2As shown, in the 1H NMR spectrum, the peak at 13.35 ppm corresponds to the carboxyl group, and the peak at 8.09 ppm belongs to the benzene ring. The characteristic peaks of the methylene group appear at 3.72 and 4.31 ppm, while the hydroxyl group appears at 4.94 ppm. In the 1C NMR spectrum, the peaks at 167.05 ppm and 165.70 ppm correspond to the two carbonyl carbons, respectively; the peaks at 135.23 ppm, 133.89 ppm, and 129.91 ppm correspond to the carbons on the benzene ring; and the peaks at 67.42 ppm and 59.46 ppm correspond to the two methylene carbons.

[0114] Example 4

[0115] Example 4 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is tetrabutyl titanate, the amount added is 300 ppm of ethylene glycol, and the reaction is carried out at 120°C for 4 hours with stirring.

[0116] Example 5

[0117] Example 5 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is tetrabutyl titanate, the amount added is 500 ppm of ethylene glycol, and the reaction is carried out at 100°C for 4 hours with stirring.

[0118] Example 6

[0119] Example 6 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is tetrabutyl titanate, the amount added is 500 ppm of ethylene glycol, and the reaction is carried out at 120°C for 5 hours with stirring.

[0120] Example 7

[0121] Example 7 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is 1,5,7-triazabicyclo[4.4.0]dec-5-ene, which is added at 5 mol% of ethylene glycol and the reaction is carried out at 120°C for 5 h with stirring.

[0122] Example 8

[0123] Example 8 is a high-purity monohydroxyethyl terephthalate sample prepared according to the above "2. Preparation method". The esterification reaction catalyst used in step (I) is anhydrous zinc acetate, the amount added is 5 mol% of ethylene glycol, and the reaction is carried out at 120°C for 5 h with stirring.

[0124] The purity of the samples in Examples 1-8 above was ≥97% after purification (recrystallization).

[0125] The monohydroxyethyl terephthalate used in the following examples is the high-purity monohydroxyethyl terephthalate prepared in Example 3.

[0126] Examples 9-15, Comparative Examples 1-2

[0127] Examples 9-15 and Comparative Examples 1-2 were prepared by referring to the above "2. Preparation Method" to obtain PET, and the stirring reaction time in step (1) was used as a variable, namely 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, and 9h as Comparative Examples 1, Comparative Example 2, Example 9, Example 10, Example 11, Example 12, Example 13, Example 14, and Example 15.

[0128] In the remaining conditions, the molar ratio in step (1) is 1:1.5 and the temperature is 180℃; in step (2), tetrabutyl titanate (total mass 600ppm) is added and mixed and stirred under an inert atmosphere and at a temperature of 260℃ until the solid is completely melted. Then, the above conditions are maintained and the mixture is stirred and reacted under a vacuum of 40-150Pa for 4 hours to prepare PET as a sample.

[0129] like Figure 4 , Figure 7 As shown in Table 1, with the continuous extension of the stirring reaction time, the color of the reaction product obtained by esterification in step (1) gradually turns yellow, and the esterification rate gradually increases. The color of the product obtained after polycondensation in step (2) also gradually shows a significant difference. The reaction conditions can be optimized based on the esterification rate and sample color.

[0130] Table 1 Comparison of esterification rates of reaction products from step (1) of Examples 9-15 and Comparative Examples 1-2

[0131]

[0132] Examples 16-20, Comparative Examples 3-7

[0133] Examples 16-20 and Comparative Examples 3-7 were prepared using the method described in "2. Preparation Method" above. The temperature conditions described in step (1) were used as variables, namely 180℃, 190℃, 200℃, 210℃, and 220℃, respectively, for Examples 16, 17, 18, 19, and 20. In step (1), tetrabutyl titanate (total mass 600ppm) was added as an esterification catalyst, and the temperature conditions were used as variables, namely 180℃, 190℃, 200℃, 210℃, and 220℃, respectively, for Comparative Examples 3, 4, 5, 6, and 7.

[0134] In the remaining conditions, the molar ratio in step (1) is 1:1.5, and the reaction is stirred for 6 hours; in step (2), tetrabutyl titanate (total mass 600 ppm) is added and mixed and stirred under an inert atmosphere and at a temperature of 260°C until the solid is completely melted. Then, the above conditions are maintained and the reaction is stirred under a vacuum of 40-150 Pa for 4 hours to prepare PET as a sample.

[0135] like Figure 5 As shown in Table 2, compared with the reaction products of tetrabutyl titanate (TBT) without the addition of the esterification catalyst, the addition of the esterification catalyst did not significantly affect the esterification rate, but the degree of yellowing was higher. Moreover, not adding the esterification catalyst is beneficial to saving resources and protecting the environment. Therefore, it was chosen not to add the catalyst during the esterification process.

[0136] Table 2 Comparison of esterification rates of reaction products from step (1) of Examples 16-20 and Comparative Examples 3-7

[0137]

[0138] Examples 21-25

[0139] Examples 21-25 are PETs prepared according to the above "2. Preparation method". The molar ratio mentioned in step (1) is used as a variable and is 1:1.3, 1:1.4, 1:1.5, 1:1.6, and 1:1.7 respectively as Examples 21, 22, 23, 24 and 25.

[0140] In the remaining conditions, the temperature condition described in step (1) is 180℃, and the reaction is stirred for 6 hours; in step (2), tetrabutyl titanate (total mass 600ppm) is added and mixed and stirred under an inert atmosphere and a temperature of 260℃ until the solid is completely melted. Then, the above conditions are maintained and the reaction is stirred under a vacuum of 40-150Pa for 4 hours to prepare PET as a sample.

[0141] like Figure 6As shown in Table 3, the esterification rate gradually increases with the increase of ethylene glycol dosage, reaching its highest point when the molar ratio of monohydroxyethyl terephthalate to ethylene glycol is 1:1.5. With further increases in ethylene glycol dosage, the esterification rate decreases, accompanied by a yellowing of the color. Therefore, a molar ratio of 1:1.5 is selected as the optimal dosage.

[0142] Table 3 Comparison of esterification rates of reaction products in steps (1) of Examples 21-25

[0143]

[0144] The content of diethylene glycol, a byproduct, is a significant factor affecting PET quality; therefore, its formation should be minimized. Analysis of the diethylene glycol content in the esterification products revealed that the esterification products obtained under the esterification conditions of this invention do not contain diethylene glycol, which is beneficial for controlling the diethylene glycol content in the final PET.

[0145] The PET prepared in Example 12 was named R-PET, and its thermal and mechanical properties were compared with those of commercial PET, as shown in Tables 1 and 2. The thermal properties of the PET obtained in this invention are comparable to those of commercial PET.

[0146] Table 4 Thermal stability of R-PET and commercial PET under nitrogen and phenol.

[0147]

[0148] Table 5. DSC test data for R-PET and commercial PET

[0149]

[0150] Example 26

[0151] Example 26 describes another method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions, mainly including the following steps:

[0152] (1) Add monohydroxyethyl terephthalate and ethylene glycol to the reaction vessel in a molar ratio of 1:1.5. Mix and stir under an inert atmosphere and at a temperature of 190°C until the solid is completely melted. Then continue to stir and react under the above conditions for 1 hour. Then raise the temperature to 210°C and continue to stir and react under an inert atmosphere for 1 hour. Finally, raise the temperature to 230°C and continue to stir and react under an inert atmosphere for 1 hour.

[0153] Wherein, the purity of the monohydroxyethyl terephthalate is at least 97%;

[0154] (2) The reaction product obtained in step (1) is added to the polycondensation catalyst and mixed and stirred under an inert atmosphere and at a temperature of 260°C until the solid is completely melted. Then, the above conditions are maintained and the mixture is stirred and reacted under a vacuum of 40-150 Pa for 1 hour to prepare PET.

[0155] Comparative Example 8

[0156] In Comparative Example 8, PET was prepared according to the above "2. Preparation Method", but the stirring reaction time in steps (1) and (2) was shortened. The main steps included were as follows:

[0157] (1) Add monohydroxyethyl terephthalate and ethylene glycol to the reaction vessel in a molar ratio of 1:1.5, mix and stir under an inert atmosphere and at a temperature of 200°C until the solid is completely melted, and then continue to stir and react under the above conditions for 2 hours.

[0158] Wherein, the purity of the monohydroxyethyl terephthalate is at least 97%;

[0159] (2) After cooling the reaction product obtained in step (1) to room temperature, add the polycondensation reaction catalyst and mix and stir under an inert atmosphere and at a temperature of 260°C until the solid is completely melted. Then continue to maintain the above conditions and stir the reaction under a vacuum of 40-150 Pa for 2 hours to prepare PET.

[0160] like Figure 8 As shown, Comparative Example 8, after shortening the reaction time, yielded PET with lower viscosity. However, when a gradient heating method was used during esterification, the resulting polyester exhibited toughness and good viscosity.

[0161] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing PET by polycondensation of high-purity monohydroxyethyl terephthalate under mild conditions, characterized in that... The main steps include: (1) Add monohydroxyethyl terephthalate and ethylene glycol to the reaction vessel in a molar ratio of 1:(1~2), mix and stir under an inert atmosphere and at a temperature of 180~230℃ until the solid is completely melted, and then continue to stir and react under the above conditions for 3~12h. Wherein, the purity of the monohydroxyethyl terephthalate is at least 97%; (2) The reaction product obtained in step (1) is added to the polycondensation catalyst and mixed and stirred under an inert atmosphere and at a temperature of 260-290°C until the solid is completely melted. Then, the above conditions are maintained and the reaction is stirred under a vacuum of 40-150 Pa for 2-6 hours to prepare PET.

2. The method according to claim 1, characterized in that: The molar ratio of monohydroxyethyl terephthalate and ethylene glycol in step (1) is 1:(1.4-1.6).

3. The method according to claim 1, characterized in that: The temperature conditions described in step (1) are 180 to 200°C.

4. The method according to claim 1, characterized in that: The stirring reaction time in step (1) is 5 to 8 hours.

5. The method according to claim 1, characterized in that: The catalyst for the polycondensation reaction in step (2) can be selected from any one of tetrabutyl titanate, anhydrous zinc acetate, antimony trioxide, antimony acetate, antimony glycolate, germanium oxide, aluminum glycolate, and tetraethyl germanate; the amount of the polycondensation reaction catalyst added is 300 to 1000 ppm of the total mass.

6. A method for preparing high-purity monohydroxyethyl terephthalate under mild conditions, characterized in that... The main steps include: (I) Add potassium terephthalate monomethyl ester and ethylene glycol to the reaction vessel in a molar ratio of 1:(15-25), add esterification catalyst, and after the solid is completely dissolved under an inert atmosphere, continue to stir the reaction for 3-5 hours under an inert atmosphere and at a temperature of 100-120°C. (II) Adjust the pH of the reaction product obtained in step (I) to 2-5, stir at room temperature until the precipitation reaction is complete, and then filter, wash, dry and purify in sequence to prepare monohydroxyethyl terephthalate.

7. The method according to claim 6, characterized in that: The esterification reaction catalyst in step (I) is selected from any one of tetrabutyl titanate, anhydrous zinc acetate, antimony trioxide, antimony acetate, antimony glycolate, germanium oxide, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene; the amount of the esterification reaction catalyst added is 300 to 600 ppm of ethylene glycol.

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