Liquid titanium-based polyester catalyst and method for preparing the same

Abstract

The application discloses a liquid titanium-based polyester catalyst and a preparation method thereof, and relates to the field of titanium-based catalysts. The catalyst takes titanate as a main raw material and is prepared through modification of an ionic liquid coordination modification; the modified ionic liquid is prepared through quaternary ammonium and ion exchange reactions and is composed of imidazole cations, phosphoric acid groups and bistrifluoromethylsulfonylimide anions. The preparation method comprises the following steps: first, an intermediate chloride is prepared through quaternary ammonium reaction of N-methyl imidazole and 3-chloropropyl phosphoric acid diethyl ester; then, the modified ionic liquid is obtained through ion exchange reaction to replace anions; finally, the modified ionic liquid is subjected to coordination reaction with the titanate to prepare the liquid titanium-based polyester catalyst. The liquid titanium-based polyester catalyst prepared by the application has excellent hydrolysis resistance, catalytic activity and thermal stability performance and is suitable for the industrial production requirements of the polyester.

Description

Technical Field

[0001] This application relates to the field of titanium-based catalyst technology, specifically to a liquid titanium-based polyester catalyst and its preparation method. Background Technology

[0002] Polyester (PET) is the world's largest-volume and most widely used synthetic polymer material. With its excellent mechanical properties, processing performance, chemical stability, and recyclability, it is widely used in textile fibers, food packaging, optical films, engineering plastics, and medical consumables. The core of polyester industrial production involves a three-step process: esterification of terephthalic acid (PTA) and ethylene glycol (EG), pre-condensation, and final condensation. The catalyst is the key material determining reaction efficiency, product quality, production costs, and environmental compliance. Currently, the mainstream catalyst systems in polyester industrial production are mainly divided into three categories: antimony-based catalysts, germanium-based catalysts, and titanium-based catalysts. Antimony-based catalysts offer moderate catalytic activity, few side reactions, strong process adaptability, and low cost. However, they contain the heavy metal antimony, which may result in high heavy metal content in the produced polyester, posing a threat to human health. Germanium-based catalysts boast high catalytic activity, good product color, and few side reactions, but their global reserves are extremely low and production costs are extremely high, hindering large-scale application. Titanium-based catalysts offer high catalytic activity, no heavy metal risk, and a wide range of raw material sources, along with lower costs. However, traditional titanium-based catalysts, such as titanates, are prone to hydrolytic deactivation, resulting in high grayness in polyester products. Furthermore, the excessively strong Lewis acid at the titanium center leads to increased yellowness in the polyester products, requiring stringent storage and usage conditions. To address the shortcomings of traditional titanium-based catalysts, current technologies utilize coordination modification of titanates and the addition of additives to solve these problems. Chinese invention patent application CN119552351A discloses a liquid titanium-based catalyst, its preparation method and application. The catalyst uses titanium as the main catalyst to react with hydroxycarboxylic acid in an alcohol solvent. At the same time, an organosilicon ester is added to introduce silicon. An antioxidant, a phosphate ester and another metal acetate are added to the obtained reaction intermediate for further reaction. The catalyst is prepared by heating to remove small molecules.

[0003] However, existing technologies still suffer from insufficient hydrophobicity of modified titanate esters. Modified titanate esters can still undergo hydrolysis and have poor thermal stability. Therefore, there is an urgent need to develop a titanium-based polyester catalyst with strong hydrolysis resistance, good thermal stability, and good catalytic activity. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the primary objective of this application is to provide a liquid titanium-based polyester catalyst and its preparation method. This application involves a quaternization reaction between N-methylimidazolium and diethyl 3-chloropropyl phosphate to introduce a diethyl phosphate propyl side chain onto the imidazolium ring, generating an intermediate chloride. Then, an ion exchange reaction is performed between the intermediate chloride and lithium bis(trifluoromethanesulfonyl)imide to prepare a modified ionic liquid. Finally, the phosphate group on the modified ionic liquid is used as a ligand to coordinate with the titanium atoms in the titanate ester, yielding the liquid titanium-based polyester catalyst.

[0005] To achieve the above objectives, this application provides the following technical solution: In a first aspect, this application provides a liquid titanium-based polyester catalyst, which is obtained by coordinating and modifying an ionic liquid on a titanate molecule; the modified ionic liquid is mainly composed of imidazole cations and phosphate groups, and bis(trifluoromethanesulfonyl)imide anions are introduced through ion exchange.

[0006] Preferably, the titanate is any one of tetrabutyl titanate, tetraisopropyl titanate, and tetran-n-propyl titanate.

[0007] Secondly, this application provides a method for preparing a liquid titanium-based polyester catalyst, comprising the following steps: S1. Under a nitrogen atmosphere, N-methylimidazole was added to the reactor, stirring was started, and under reflux conditions, diethyl 3-chloropropyl phosphate was added dropwise. After the addition was complete, the temperature was raised to react. After the reaction was completed, the intermediate chloride was obtained by vacuum distillation and washing to remove impurities. S2. Under a nitrogen atmosphere, anhydrous dichloromethane was added to the reactor, and stirring was started. The intermediate chloride was added and stirred until a homogeneous solution was formed. Then, a dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was added dropwise and the reaction was stirred. After the reaction was completed, the reaction solution was washed with deionized water to remove chloride ions. The organic phase was collected, dried, and then the dichloromethane was removed by rotary evaporation. Finally, it was dried under vacuum to obtain the modified ionic liquid. S3. Under a nitrogen atmosphere, anhydrous ethylene glycol was added to the reactor, stirring was started, and modified ionic liquid and triethylamine were added. The mixture was stirred until dissolved, and titanate was added dropwise. After the addition was complete, the temperature was raised to react. After the reaction was completed, the mixture was distilled under reduced pressure and cooled to room temperature to obtain a liquid titanium-based polyester catalyst.

[0008] It should be noted that before the reaction, the reaction vessel needs to be dried and purged with nitrogen to remove moisture and oxygen from the vessel and avoid affecting the reaction process.

[0009] Preferably, in S1 and S3, the pressure of the nitrogen atmosphere is 101.375~101.475 kPa.

[0010] It should be noted that titanates are prone to hydrolysis when exposed to water, thereby losing their catalytic activity. Therefore, it is necessary to isolate moisture during the preparation process. S1 and S3 use a slightly positive pressure nitrogen atmosphere to prevent the infiltration of external air and avoid affecting the reaction process.

[0011] Preferably, in step S1, the mass ratio of N-methylimidazole to diethyl 3-chloropropyl phosphate is (15~17):(40~52).

[0012] Preferably, in step S1, the stirring speed is 280~320 rpm, the rate of adding 3-chloropropyl phosphate diethyl ester is 0.01~0.05 mL / s, the temperature is controlled at 35~45℃ during the addition process, the heating rate of the reaction is 5℃ / 30 min, the final temperature is 80~87℃, and the reaction time is 20~26 h.

[0013] It should be noted that in step S1, after the final reaction, the vacuum distillation temperature was 80℃ and the pressure was -0.098MPa. The remaining liquid after distillation was washed three times with ethyl acetate to remove organic impurities, and finally dried under vacuum at 60℃ to obtain the intermediate chloride. Controlling the temperature at 35~45℃ during the dropwise addition process can avoid local overheating and reduce the formation of by-products. At the same time, a gradient temperature increase is used to avoid the formation of polyalkylation by-products during the reaction due to rapid temperature rise, thereby improving the purity of the intermediate chloride.

[0014] Preferably, in step S2, the mass ratio of anhydrous dichloromethane, intermediate chloride, and lithium bis(trifluoromethanesulfonyl)imide in dichloromethane is 100:(38~42):(138~142); and the concentration of the lithium bis(trifluoromethanesulfonyl)imide in dichloromethane is 1.8-1.9 mol / L.

[0015] It should be noted that in step S2, the hydrophilic chloride ions are replaced with strongly hydrophobic bis(trifluoromethanesulfonyl)imide ions through anion exchange reaction. These ions have extremely low surface energy and excellent hydrophobicity, and can form a dense hydrophobic layer around the active center of titanium.

[0016] Preferably, in step S2, the stirring speed is 240-260 rpm, the rate of adding the lithium bis(trifluoromethanesulfonyl)imide solution in dichloromethane is 0.1-0.2 mL / s, and the stirring reaction time is 10-14 h.

[0017] It should be noted that in step S2, the temperature for rotary evaporation to remove dichloromethane is 40℃ and the pressure is -0.095MPa, while the temperature for vacuum drying is 100℃ and the pressure is -0.098MPa.

[0018] Preferably, in step S3, the mass ratio of anhydrous ethylene glycol, modified ionic liquid, triethylamine and titanate is 100:(15~17):(0.025~0.032):(9~11).

[0019] It should be noted that in step S3, ethylene glycol, as the reaction solvent, exhibits good miscibility with the modified ionic liquid, forming a homogeneous reaction system. Simultaneously, it can form weak coordination with titanium atoms, stabilizing the titanium centers. The added trace amount of triethylamine neutralizes the trace organic acids generated in the system, inhibits the hydrolysis of titanate esters, and improves the color of the final polyester product.

[0020] Preferably, in step S3, the stirring speed is 180~220 rpm, the titanate is added at a rate of 0.02~0.03 mL / s, the temperature during the addition process is controlled at 25~35℃, the rate of heating the reaction is 10℃ / 30 min, the final temperature is 78~83℃, and the reaction time is 3.5~4.5 h.

[0021] It should be noted that the temperature is controlled between 25 and 35°C during the dropwise addition of titanate to avoid localized overheating, which could lead to titanate hydrolysis. A gradient temperature increase is used because the reaction rate is slow at low temperatures, but excessively rapid or high temperatures can cause the alkoxy groups on the titanate to be replaced, generating byproduct impurities. The optimal temperature range for the coordination reaction is 78–83°C, which helps improve coordination efficiency. In step S3, the vacuum distillation process is carried out at 80°C, with the vacuum level gradually increased to -0.095 MPa to remove the byproduct n-butanol.

[0022] The beneficial effects of this application are: This application utilizes the quaternization reaction between N-methylimidazolium and diethyl 3-chloropropyl phosphate to graft phosphate groups onto the N-methylimidazolium molecule to obtain an intermediate chloride. Then, an ion exchange reaction is used to replace the chloride ions on the intermediate chloride with bis(trifluoromethanesulfonyl)imide anions, yielding a modified ionic liquid. Finally, the modified ionic liquid is grafted onto the titanate ester via a coordination reaction between the phosphate groups and titanium atoms, resulting in a titanium-based polyester catalyst with good hydrolysis resistance and thermal stability. The bis(trifluoromethanesulfonyl)imide anion forms a hydrophobic environment around the active titanium center, improving the catalyst's hydrophobicity. Furthermore, the coordination bond formed between the phosphate groups and titanium atoms can block easily hydrolyzed alkoxy sites on the titanium atoms, synergistically enhancing the catalyst's performance. The catalyst exhibits excellent resistance to hydrolysis. Furthermore, the high bond energy between the phosphate group and titanium atom on the catalyst molecule, along with the diethyl phosphate group acting as a bidentate ligand for titanium atoms, enhances coordination stability and improves the catalyst's thermal stability. Additionally, the bis(trifluoromethanesulfonyl)imide anion and imidazole cation themselves possess high thermal stability. Therefore, the catalyst molecule as a whole exhibits excellent thermal stability. Finally, the imidazole cation grafted onto the catalyst molecule effectively enhances its catalytic activity. In the polyester polycondensation reaction, the imidazole ring can transfer protons from the alcohol hydroxyl group to the ester carbonyl oxygen atom, lowering the activation energy of the polycondensation reaction. This allows it to synergistically catalyze the polyester polycondensation reaction with the active titanium atom, resulting in a titanium-based catalyst prepared in this application exhibiting stronger catalytic activity. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 A flowchart illustrating the preparation method of a liquid titanium-based polyester catalyst provided in this application. Detailed Implementation

[0025] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of this application. However, this application may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0027] The following specific embodiments further illustrate this point: Example 1 like Figure 1 As shown, this embodiment provides a method for preparing a liquid titanium-based polyester catalyst: S1. Under a nitrogen atmosphere with a pressure of 101.425 kPa, N-methylimidazole was added to the reactor, and stirring was started at a rate of 300 rpm. Under reflux conditions, 3-chloropropyl phosphate diethyl ester was added dropwise at a rate of 0.03 mL / s. During the dropwise addition, the temperature was controlled at 40 °C. After the dropwise addition was completed, the temperature was increased to 85 °C at a rate of 5 °C / 30 min, and the reaction was carried out for 24 h. After the reaction was completed, the intermediate chloride was obtained by vacuum distillation and washing to remove impurities. The mass ratio of N-methylimidazole to 3-chloropropyl phosphate diethyl ester was 16:45.

[0028] S2. Under a nitrogen atmosphere, anhydrous dichloromethane was added to the reactor, and stirring was started at a rate of 250 rpm. The intermediate chloride was added, and stirring was continued until a homogeneous solution was formed. Then, a dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was added dropwise at a rate of 0.15 mL / s. The reaction was stirred for 12 h. After the reaction was completed, the reaction solution was washed with deionized water to remove chloride ions. The organic phase was collected, dried, and then the dichloromethane was removed by rotary evaporation. Finally, the solution was dried under vacuum to obtain the modified ionic liquid. The mass ratio of the anhydrous dichloromethane, the intermediate chloride, and the dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was 100:40:140. The concentration of the dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was 1.84 mol / L.

[0029] S3. Under a nitrogen atmosphere with a pressure of 101.425 kPa, anhydrous ethylene glycol was added to the reactor, and stirring was started at a rate of 200 rpm. Modified ionic liquid and triethylamine were added and stirred until dissolved. Tetrabutyl titanate was added dropwise at a rate of 0.025 mL / s, and the temperature was controlled at 30°C during the dropwise addition. After the dropwise addition was completed, the temperature was increased to 80°C at a rate of 10°C / 30 min, and the reaction was carried out for 4 h. After the reaction was completed, vacuum distillation was performed, and the temperature was lowered to room temperature to obtain the liquid titanium-based polyester catalyst described in Example 1. The mass ratio of anhydrous ethylene glycol, modified ionic liquid, triethylamine and titanate was 100:16:0.03:10.

[0030] Example 2 like Figure 1 As shown, this embodiment provides a method for preparing a liquid titanium-based polyester catalyst: S1. Under a nitrogen atmosphere with a pressure of 101.375 kPa, N-methylimidazole was added to the reactor, and stirring was started at a rate of 280 rpm. Under reflux conditions, 3-chloropropyl phosphate diethyl ester was added dropwise at a rate of 0.01 mL / s. During the dropwise addition, the temperature was controlled at 45 °C. After the dropwise addition was completed, the temperature was increased to 87 °C at a rate of 5 °C / 30 min. The reaction was carried out for 20 h. After the reaction was completed, the intermediate chloride was obtained by vacuum distillation and washing to remove impurities. The mass ratio of N-methylimidazole to 3-chloropropyl phosphate diethyl ester was 15:40.

[0031] S2. Under a nitrogen atmosphere, anhydrous dichloromethane was added to the reactor, and stirring was started at a rate of 240 rpm. The intermediate chloride was added, and the mixture was stirred until a homogeneous solution was formed. Then, a dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was added dropwise at a rate of 0.1 mL / s. The reaction was stirred for 10 h. After the reaction was completed, the reaction solution was washed with deionized water to remove chloride ions. The organic phase was collected, dried, and then the dichloromethane was removed by rotary evaporation. Finally, the mixture was dried under vacuum to obtain the modified ionic liquid. The mass ratio of the anhydrous dichloromethane, the intermediate chloride, and the dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was 100:38:138. The concentration of the dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was 1.8 mol / L.

[0032] S3. Under a nitrogen atmosphere with a pressure of 101.375 kPa, anhydrous ethylene glycol was added to the reactor, and stirring was started at a rate of 180 rpm. Modified ionic liquid and triethylamine were added and stirred until dissolved. Tetraisopropyl titanate was added dropwise at a rate of 0.02 mL / s, and the temperature was controlled at 35°C during the dropwise addition. After the dropwise addition was completed, the temperature was increased to 83°C at a rate of 10°C / 30 min, and the reaction was carried out for 3.5 h. After the reaction was completed, vacuum distillation was performed, and the temperature was lowered to room temperature to obtain the liquid titanium-based polyester catalyst described in Example 2. The mass ratio of anhydrous ethylene glycol, modified ionic liquid, triethylamine and titanate was 100:15:0.025:9.

[0033] Example 3 like Figure 1 As shown, this embodiment provides a method for preparing a liquid titanium-based polyester catalyst: S1. Under a nitrogen atmosphere with a pressure of 101.475 kPa, N-methylimidazole was added to the reactor, and stirring was started at a rate of 320 rpm. Under reflux conditions, 3-chloropropyl phosphate diethyl ester was added dropwise at a rate of 0.05 mL / s. During the dropwise addition, the temperature was controlled at 35 °C. After the dropwise addition was completed, the temperature was increased to 80 °C at a rate of 5 °C / 30 min. The reaction was carried out for 26 h. After the reaction was completed, the intermediate chloride was obtained by vacuum distillation and washing to remove impurities. The mass ratio of N-methylimidazole to 3-chloropropyl phosphate diethyl ester was 17:52.

[0034] S2. Under a nitrogen atmosphere, anhydrous dichloromethane was added to the reactor, and stirring was started at a rate of 260 rpm. The intermediate chloride was added, and stirring was continued until a homogeneous solution was formed. Then, a dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was added dropwise at a rate of 0.2 mL / s. The reaction was stirred for 14 h. After the reaction was completed, the reaction solution was washed with deionized water to remove chloride ions. The organic phase was collected, dried, and then the dichloromethane was removed by rotary evaporation. Finally, the solution was dried under vacuum to obtain the modified ionic liquid. The mass ratio of the anhydrous dichloromethane, the intermediate chloride, and the dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was 100:42:142. The concentration of the dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was 1.9 mol / L.

[0035] S3. Under a nitrogen atmosphere with a pressure of 101.475 kPa, anhydrous ethylene glycol was added to the reactor, and stirring was started at a rate of 220 rpm. Modified ionic liquid and triethylamine were added and stirred until dissolved. Tetra-n-propyl titanate was added dropwise at a rate of 0.03 mL / s, with the temperature controlled at 25°C during the dropwise addition. After the dropwise addition was completed, the temperature was increased to 78°C at a rate of 10°C / 30 min, and the reaction was carried out for 4.5 h. After the reaction was completed, vacuum distillation was performed, and the mixture was cooled to room temperature to obtain the liquid titanium-based polyester catalyst described in Example 3. The mass ratio of anhydrous ethylene glycol, modified ionic liquid, triethylamine, and titanate was 100:17:0.032:11.

[0036] Comparative Example 1 This comparative example provides a method for preparing a liquid titanium-based polyester catalyst. Compared with Example 1, the difference is that bis(trifluoromethanesulfonyl)imide anions are not grafted during the preparation process. The remaining steps are the same as in Example 1 and will not be repeated here.

[0037] Comparative Example 2 This comparative example provides a method for preparing a liquid titanium-based polyester catalyst. The difference from Example 1 is that imidazole cations are not grafted during the preparation process. The remaining steps are the same as in Example 1 and will not be repeated here.

[0038] To demonstrate the superior performance of the proposed solution, the liquid titanium-based polyester catalysts prepared in Examples 1-3 and Comparative Examples 1-2 were tested using the following methods: Hydrolysis resistance: Take 10 mL of catalyst sample and add it to 100 mL of deionized water. Let it stand for 10 min and observe whether the catalyst undergoes hydrolysis. The test results are shown in Table 1.

[0039] Evaluation of polyester synthesis applications: 1000g of purified terephthalic acid and 430g of ethylene glycol were added to a reactor, along with a catalyst sample. The catalyst sample was added at a rate of 3μg / g of purified terephthalic acid, calculated based on titanium. Esterification was carried out at 245℃ and 0.25MPa. Esterification ended when the water output reached 95% of the theoretical value. The reactor was gradually evacuated to below 50Pa, and the temperature was raised to 280℃. When the polycondensation product reached the predetermined stirring motor power equivalent to an intrinsic viscosity of 0.675dL / g, the polycondensation reaction ended. After pressurization and discharge, the product was water-cooled and pelletized to obtain polyester chips prepared from each catalyst sample. The performance of the polyester chips was tested and is shown in Table 2.

[0040] Table 1. Hydrolysis resistance of catalysts prepared in Examples 1-3 and Comparative Examples 1-2

[0041] Table 2 Performance Evaluation of Synthetic Polyesters

[0042] As shown in Table 1, the liquid titanium-based polyester catalysts prepared in Examples 1-3 of this application exhibit good hydrolysis resistance. However, Comparative Example 1, lacking the hydrophobic effect of the bis(trifluoromethanesulfonyl)imide anion, shows decreased hydrolysis resistance, with partial hydrolysis occurring. Table 2 shows that the liquid titanium-based polyester catalysts prepared in Examples 1-3 demonstrate superior performance in polyester synthesis, exhibiting shorter polycondensation times, higher whiteness, lower yellowness, and lower terminal carboxyl group content in the synthesized polyester. Comparative Examples 1 and 2 show poorer performance, primarily because the catalyst prepared in Comparative Example 1 lacks the bis(trifluoromethanesulfonyl)imide anion, making it prone to hydrolysis. During polyester polycondensation, the catalyst content decreases, reducing polyester synthesis efficiency and increasing side reactions, leading to a decline in the overall performance of the synthesized polyester. Similarly, the catalyst prepared in Comparative Example 2 lacks the imidazole cation, resulting in reduced catalyst activity, consequently lower polyester synthesis efficiency, and increased side reactions, leading to a decrease in polyester performance.

[0043] In summary, the liquid titanium-based polyester catalyst prepared in this application has excellent catalytic activity, hydrolysis resistance, and thermal stability, which can effectively improve the whiteness of polyester and increase the polyester synthesis efficiency. It is suitable for the production of fiber-grade and bottle-grade polyester.

[0044] It should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application, and all such modifications and substitutions should be covered within the scope of the claims of this application.

Claims

1. A liquid titanium-based polyester catalyst, characterized in that, The titanium-based polyester catalyst is obtained by coordinating and modifying an ionic liquid on a titanate molecule; the modified ionic liquid is mainly composed of imidazole cations and phosphate groups, and bis(trifluoromethanesulfonyl)imide anions are introduced through ion exchange.

2. The liquid titanium-based polyester catalyst according to claim 1, characterized in that, The titanate is any one of tetrabutyl titanate, tetraisopropyl titanate, and tetran-n-propyl titanate.

3. A method for preparing a liquid titanium-based polyester catalyst as described in any one of claims 1 to 2, characterized in that, Includes the following steps: S1. Under a nitrogen atmosphere, N-methylimidazole was added to the reactor, stirring was started, and under reflux conditions, diethyl 3-chloropropyl phosphate was added dropwise. After the addition was complete, the temperature was raised to react. After the reaction was completed, the intermediate chloride was obtained by vacuum distillation and washing to remove impurities. S2. Under a nitrogen atmosphere, anhydrous dichloromethane was added to the reactor, and stirring was started. The intermediate chloride was added and stirred until a homogeneous solution was formed. Then, a dichloromethane solution of lithium bis(trifluoromethanesulfonyl)imide was added dropwise and the reaction was stirred. After the reaction was completed, the reaction solution was washed with deionized water to remove chloride ions. The organic phase was collected, dried, and then the dichloromethane was removed by rotary evaporation. Finally, it was dried under vacuum to obtain the modified ionic liquid. S3. Under a nitrogen atmosphere, anhydrous ethylene glycol was added to the reactor, stirring was started, and modified ionic liquid and triethylamine were added. The mixture was stirred until dissolved, and titanate was added dropwise. After the addition was complete, the temperature was raised to react. After the reaction was completed, the mixture was distilled under reduced pressure and cooled to room temperature to obtain a liquid titanium-based polyester catalyst.

4. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In S1 and S3, the pressure of the nitrogen atmosphere is 101.375~101.475 kPa.

5. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In step S1, the mass ratio of N-methylimidazolium to diethyl 3-chloropropyl phosphate is (15~17):(40~52).

6. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In step S1, the stirring speed is 280~320 rpm, the rate of adding 3-chloropropyl phosphate diethyl ester is 0.01~0.05 mL / s, the temperature is controlled at 35~45℃ during the addition process, the heating rate is 5℃ / 30 min, the final temperature is 80~87℃, and the reaction time is 20~26 h.

7. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In step S2, the mass ratio of anhydrous dichloromethane, intermediate chloride, and lithium bis(trifluoromethanesulfonyl)imide in dichloromethane is 100:(38~42):(138~142); the concentration of the lithium bis(trifluoromethanesulfonyl)imide in dichloromethane is 1.8-1.9 mol / L.

8. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In step S2, the stirring speed is 240-260 rpm, the rate of adding the lithium bis(trifluoromethanesulfonyl)imide solution in dichloromethane is 0.1-0.2 mL / s, and the stirring reaction time is 10-14 h.

9. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In step S3, the mass ratio of anhydrous ethylene glycol, modified ionic liquid, triethylamine, and titanate is 100:(15~17):(0.025~0.032):(9~11).

10. The method for preparing a liquid titanium-based polyester catalyst according to claim 3, characterized in that, In step S3, the stirring speed is 180~220 rpm, the titanate is added at a rate of 0.02~0.03 mL / s, the temperature during the addition process is controlled at 25~35℃, the rate of heating the reaction is 10℃ / 30 min, the final temperature is 78~83℃, and the reaction time is 3.5~4.5 h.

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