PEG-CQDs / ZnO composite catalyst and preparation method and application thereof

Abstract

The present application relates to the technical field of polyester synthesis catalyst, and is a PEG-CQDs / ZnO composite catalyst, a preparation method and application thereof, the preparation method of the PEG-CQDs / ZnO composite catalyst comprises the following steps: adding an activator and a stabilizer to the aqueous solution of carbon quantum dots, then adding polyethylene glycol to react, and obtaining PEG-CQDs powder after drying the reaction product;PEG-CQDs powder and soluble zinc salt are mixed in a reaction solvent, and silane coupling agent is added, and the PEG-CQDs / ZnO composite catalyst is obtained after reaction.The catalyst system can effectively promote the polyester polycondensation reaction under the condition of lower addition amount, has a significant inhibitory effect on the side reaction, and has good dispersity and stability, so as to realize the multiple balance between the polycondensation reaction efficiency, the side reaction control and the catalyst recyclability.

Description

Technical Field

[0001] This invention relates to the field of polyester synthesis catalyst technology, specifically a PEG-CQDs / ZnO composite catalyst, its preparation method, and its application. Background Technology

[0002] Since the industrialization of PET in the 1940s, this material has rapidly become a cornerstone material in packaging, textiles, electronics, and other fields due to its excellent mechanical properties, gas barrier properties, transparency, and processability. Global annual PET production capacity has exceeded 100 million tons, with China, as the world's largest polyester producer, accounting for over 50% of the total capacity. Low-melting-point copolyesters are a class of functional polyester materials that lower their melting point by introducing a third monomer to disrupt the regularity of the PET molecular chain (Sun Wenlu. Preparation and Performance Study of Low-Viscosity, Low-Melting-Point PET Copolyesters [D]. Shanghai: Donghua University, 2022.). Compared to conventional PET (melting point approximately 250-260℃), the melting point of low-melting-point copolyesters can be controlled to 110℃ to 210℃, giving the material excellent heat-sealing properties, flexibility, and processing fluidity. It is widely used in hot melt adhesives, heat-shrinkable films, composite fibers, medical dressings, and 3D printing (Xu Dongsheng, Li Ya, Zhang Long. Development process and performance study of ultra-low melting-point PET copolyesters [J]. Anhui Chemical Industry, 2025, 51(3):64-67+87.). Statistics show that the global annual demand for low-melting-point polyesters is growing at a rate exceeding 8%, indicating a broad market prospect. However, the synthesis of low-melting-point copolyesters faces unique technical challenges. The core issue lies in the mismatch between catalyst performance and the copolymerization activity of the third monomer. Therefore, the performance of the catalyst directly determines the reaction rate, degree of polymerization distribution, degree of side reactions, and the color, thermal stability, and heavy metal residue of the final product. Thus, catalyst selection is not only a technical issue but also a strategic choice concerning industrial sustainability.

[0003] Antimony-based catalysts (such as antimony trioxide and antimony glycolate) have long dominated the PET polycondensation catalysis field due to their high catalytic activity, fewer side reactions, and excellent product color. However, antimony is a heavy metal element with significant biotoxicity and environmental accumulation. During the dyeing and finishing processes and alkali reduction treatment of polyester fibers, antimony easily leaches from the fibers and is discharged with wastewater, causing continuous pollution to water bodies and soil, ultimately threatening human health through the food chain. With increasingly stringent global environmental regulations, especially China's export controls on antimony since 2024, the application of antimony-based catalysts is facing increasingly severe environmental pressure and supply chain risks. To replace antimony-based catalysts, researchers have developed environmentally friendly catalyst systems such as titanium-based and aluminum-based catalysts. Titanium-based catalysts are highly anticipated due to their extremely high catalytic activity and lack of heavy metal toxicity. However, existing titanium-based catalysts still have significant drawbacks in practical applications: First, titanium-based catalysts exhibit strong catalytic activity in both the main and side reactions of esterification and polycondensation, significantly increasing the degree of side reactions during the reaction process. This leads to a high b* value in the color of the synthesized PET product, resulting in obvious yellowing and severely affecting the product's optical quality and market competitiveness. Second, to solve the yellowing problem, phosphorus-containing stabilizers are usually added to suppress side reactions. However, phosphorus-based stabilizers are prone to precipitation and flocculation in the reaction system, which in turn reduces the product's whiteness (L* value). Furthermore, the introduction of stabilizers increases raw material costs and process control difficulty. Third, titanium-based catalysts lack stability in water and thermal environments, are prone to hydrolysis and deactivation, and require strict anhydrous conditions in the polymerization process. Although aluminum-based catalysts have lower toxicity, their catalytic activity is significantly lower than that of conventional antimony-based and titanium-based catalysts. To achieve comparable polymerization efficiency, the catalyst dosage needs to be greatly increased, but this increase in catalyst dosage leads to deterioration of the product's color.

[0004] Currently, catalysts used for the synthesis of low-melting-point copolyesters mainly follow the conventional PET catalytic system, each with its own problems. Antimony-based catalysts—primarily antimony trioxide (Sb₂O₃) and antimony glycolide—have been the industrial standard for PET polycondensation since the 1960s. Their success stems from three technical advantages: first, moderate and easily controlled catalytic activity, enabling stable catalytic polycondensation at conventional reaction temperatures (270℃-290℃) without triggering violent side reactions; second, low corrosivity to equipment and good compatibility with the reaction system; and third, significantly lower cost than alternatives such as titanium-based catalysts. These characteristics have made antimony-based catalysts virtually unshakeable for over half a century. However, the toxicity of antimony is redefining the value of this "gold standard." Although it exhibits some activity in the copolymerization of the third monomer, the issue of heavy metal toxicity cannot be ignored. Low-melting-point copolyesters are commonly used in hot melt adhesives, medical dressings, and other applications involving direct contact with the human body, making the risk of antimony migration a safety concern. Although titanium-based catalysts are non-toxic and highly active (Sun Bin, Wang Mingyi. Progress and Trends in the Application of Titanium-based Catalysts in Polyester Synthesis (Part 1) [J]. Textile Guide, 2019, (9): 38-49.), they suffer from the contradiction of "excessive activity and poor selectivity". In systems containing a third monomer, titanium-based catalysts indiscriminately catalyze various side reactions such as ethylene glycol etherification and third monomer cyclization, resulting in a significant increase in the product b* value (yellow-blue phase) (usually 5-8), excessive diethylene glycol content, poor hydrolytic stability, and easy generation of insoluble titanium oxide particles. Although diol titanium alkali metal complexes have high activity for both depolymerization and cocondensation, which can simplify the process, they still face challenges in color control and side reaction suppression. (Feng Yanwei. Synthesis of High-Temperature Hydrolysis-Stable Liquid Titanium Catalysts and Their Application in PBST Copolyester Synthesis [D]. Zhejiang University, 2023.) The essence of the above problem lies in the fact that the design concept of existing catalysts is "single-component functionalization," which makes it difficult to simultaneously meet the multiple requirements of low-melting-point copolyester synthesis for "catalyzing the copolymerization of the third monomer, suppressing side reactions, and ensuring product color." This calls for a completely new catalytic mode—through the synergistic design of multiple components, to achieve high selectivity and excellent color control while maintaining high catalytic activity.

[0005] Besides the inherent issues of catalytic efficiency and toxicity, existing catalyst systems generally face the challenge of high dosage in industrial applications. Whether antimony-based, titanium-based, or aluminum-based catalysts, their effective addition amount is typically in the range of 100–300 ppm. Catalyst dosage not only directly affects production costs but is also closely related to product color—catalyst residue and its resulting byproducts are significant contributing factors to the deterioration of PET chip color. How to reduce catalyst dosage while maintaining polymerization efficiency and product color quality has become a core technological bottleneck that urgently needs to be overcome in the field of polyester catalysis.

[0006] In conclusion, developing a novel polyester polycondensation catalyst that combines high catalytic activity, low toxicity, low dosage, and significant improvement in the color of PET products is of great practical significance for promoting the green transformation and sustainable development of the polyester industry. Summary of the Invention

[0007] This invention provides a PEG-CQDs / ZnO composite catalyst and its preparation method, which overcomes the shortcomings of the prior art and can effectively solve the problems of easy yellowing of existing PET catalysts, large catalyst dosage, high toxicity, and poor product color.

[0008] One of the technical solutions of this invention is achieved through the following measures: a method for preparing a PEG-CQDs / ZnO composite catalyst, comprising the following steps: Step 1: Add activator and stabilizer to the aqueous solution of carbon quantum dots, then add polyethylene glycol to react, and dry the reaction product to obtain PEG-CQDs powder; Step 2: Mix PEG-CQDs powder and soluble zinc salt in a reaction solvent, add silane coupling agent, and after reaction, obtain PEG-CQDs / ZnO composite catalyst.

[0009] The following are further optimizations and / or improvements to one of the above-mentioned technical solutions: The above-mentioned carbon quantum dots were prepared according to the following steps: A carbon source is mixed with hydrogen peroxide (H2O2) at a mass ratio of 1:1 to 1:5 and reacted. The reaction product is centrifuged, filtered, dialyzed, and dried to obtain carbon quantum dots. The carbon source is one or more of weathered coal, citric acid, glucose, fructose, sucrose, and cellulose. Preferably, the carbon source is one or two of citric acid and weathered coal.

[0010] The concentration of carbon quantum dots in the aqueous solution of the above-mentioned carbon quantum dots is 0.1 mg / mL to 1 mg / mL.

[0011] The activator mentioned above is 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), and the amount added is 20 to 30 times the mass of carbon quantum dots.

[0012] The stabilizer mentioned above is N-hydroxysulfosuccinimide (NHS), and the amount added is 15 to 20 times the mass of carbon quantum dots.

[0013] The molecular weight of the aforementioned polyethylene glycol is between 400 and 4000.

[0014] The mass ratio of the carbon quantum dots to polyethylene glycol is (1 to 5): (15 to 20).

[0015] The silane coupling agent mentioned above is KH550, and the amount of silane coupling agent added is 10 to 50 times the mass of PEG-CQDs.

[0016] The soluble zinc salt is one or more of zinc acetate, zinc nitrate, zinc chloride, and zinc sulfate, and the amount of soluble zinc salt added is 680 to 920 times the mass of PEG-CQDs. Preferably, the soluble zinc salt is zinc acetate.

[0017] In step one above, the reaction temperature is 80℃ to 120℃, and the reaction time is 4h to 24h.

[0018] In step two above, the reaction temperature is 100℃ to 150℃ for 8 to 24 hours. This reaction can be performed using a solvothermal method, a microwave-assisted heating method, or a hydrothermal method.

[0019] The reaction solvent is one or more selected from deionized water, ethylene glycol, and ethanol. Preferably, the reaction solvent is ethanol.

[0020] The second technical solution of the present invention is achieved through the following measures: the PEG-CQDs / ZnO composite catalyst prepared by the preparation method of one of the technical solutions.

[0021] The third technical solution of this invention is achieved through the following measures: the application of a PEG-CQDs / ZnO composite catalyst in the catalytic polycondensation reaction of PET. The PET polycondensation reaction is a reaction in which diesters and diols undergo melt polycondensation to obtain a low-melting-point copolyester; or a reaction in which diesters, diols, and other monomers undergo melt polycondensation to obtain a low-melting-point copolyester, wherein the other monomers are one or more of dimethyl isophthalate, 1,2-butanediol, diethylene glycol, 1,6-hexanediol, and pentaerythritol; the amount of the PEG-CQDs / ZnO composite catalyst used is 50 ppm to 500 ppm of the theoretical polyester mass.

[0022] This invention provides a PEG-CQDs / ZnO composite catalyst and its preparation method. This catalytic system can effectively promote polyester polycondensation reaction under low addition conditions, while significantly inhibiting side reactions and exhibiting good dispersibility and stability, thereby achieving multiple balances between polycondensation reaction efficiency, side reaction control and catalyst recyclability. Detailed Implementation

[0023] This invention is not limited to the following embodiments, and specific implementation methods can be determined according to the technical solutions and actual conditions of this invention. Unless otherwise specified, all chemical reagents and chemicals mentioned in this invention are well-known and commonly used chemical reagents and chemicals in the prior art; unless otherwise specified, all percentages in this invention are mass percentages; unless otherwise specified, all solutions in this invention are aqueous solutions with water as the solvent, for example, hydrochloric acid solution is an aqueous solution of hydrochloric acid; room temperature in this invention generally refers to a temperature between 15°C and 25°C, generally defined as 25°C.

[0024] The present invention will be further described below with reference to embodiments: Example 1: The preparation method of this PEG-CQDs / ZnO composite catalyst includes the following steps: Step 1: Add activator and stabilizer to the aqueous solution of carbon quantum dots, then add polyethylene glycol to react, and dry the reaction product to obtain PEG-CQDs powder; Step 2: Mix PEG-CQDs powder and soluble zinc salt in a reaction solvent, add silane coupling agent, and after reaction, obtain PEG-CQDs / ZnO composite catalyst.

[0025] Example 2: As an optimization of the above example, carbon quantum dots were prepared according to the following steps: A carbon source is mixed with hydrogen peroxide (H2O2) at a mass ratio of 1:1 to 1:5 and reacted. The reaction product is then centrifuged, filtered, dialyzed, and dried to obtain carbon quantum dots. The carbon source is one or more of weathered coal, citric acid, glucose, fructose, sucrose, and cellulose; preferably, the carbon source is one or two of citric acid and weathered coal. The carbon quantum dots prepared by the above steps have graphitized carbon cores and their surfaces contain at least one functional group selected from hydroxyl, carboxyl, or amino groups.

[0026] Example 3: As an optimization of the above example, the concentration of carbon quantum dots in the aqueous solution of carbon quantum dots is 0.1 mg / mL to 1 mg / mL.

[0027] Example 4: As an optimization of the above example, the activator is 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), and the amount added is 20 to 30 times the mass of carbon quantum dots.

[0028] Example 5: As an optimization of the above examples, the stabilizer is N-hydroxysulfosuccinimide (NHS), and the amount added is 15 to 20 times the mass of carbon quantum dots.

[0029] Example 6: As an optimization of the above examples, the molecular weight of PEG is 400 to 4000.

[0030] Example 7: As an optimization of the above example, the mass ratio of carbon quantum dots to polyethylene glycol is (1 to 5): (15 to 20).

[0031] Example 8: As an optimization of the above example, the silane coupling agent is KH550, and the amount of silane coupling agent added is 10 to 50 times the mass of PEG-CQDs.

[0032] Example 9: As an optimization of the above examples, the soluble zinc salt is one or more of zinc acetate, zinc nitrate, zinc chloride, and zinc sulfate, and the amount of soluble zinc salt added is 680-920 times the mass of PEG-CQDs. Preferably, the soluble zinc salt is zinc acetate.

[0033] Example 10: As an optimization of the above example, in step one, the reaction temperature is 80°C to 120°C and the reaction time is 4h to 24h.

[0034] Example 11: As an optimization of the above example, in step two, the reaction temperature is 100°C to 150°C for 8 to 24 hours. This reaction can be carried out using a solvothermal method, a microwave-assisted heating method, or a hydrothermal method.

[0035] Example 12: As an optimization of the above examples, the reaction solvent is one or more of deionized water, ethylene glycol, and ethanol. Preferably, the reaction solvent is ethanol.

[0036] The catalyst composition of this invention includes carbon quantum dots (CQDs), zinc oxide (ZnO), and polyethylene glycol (PEG). Carbon quantum dots are a class of zero-dimensional carbon-based luminescent nanomaterials with a size of less than 10 nm, possessing abundant surface functional groups (including carboxyl, hydroxyl, and amino groups) and a unique electronic structure. The composite catalyst of ZnO and carbon quantum dots is achieved by loading CQDs onto the ZnO surface and then modifying the composite catalyst with PEG.

[0037] Example 13: Application of the PEG-CQDs / ZnO composite catalyst in the catalytic polycondensation reaction of PET. The PET polycondensation reaction is a reaction in which diesters and diols are melt-polymerized to obtain low-melting-point copolyesters; or, a reaction in which diesters, diols, and other monomers are melt-polymerized to obtain low-melting-point copolyesters, wherein the other monomers are one or more of dimethyl isophthalate, 1,2-butanediol, diethylene glycol, 1,6-hexanediol, and pentaerythritol; the amount of the PEG-CQDs / ZnO composite catalyst used is 50 ppm to 500 ppm of the theoretical polyester mass.

[0038] Example 14: The preparation process of this PEG-CQDs / ZnO composite catalyst is as follows: Step 1: Synthesis of CQDs Weathered coal was passed through a 100-500 mesh sieve and reacted with hydrogen peroxide (H2O2) in a round-bottom flask at a mass ratio of 1:5 at 60°C for 12 hours. After oxidation by H2O2 and cooling to room temperature, the supernatant was obtained by centrifugation after the reaction was completed. Then, the supernatant was obtained by dialysis and drying to obtain purified carbon quantum dot powder.

[0039] Step 2: Synthesis of PEG-CQDs The purified carbon quantum dot powder was prepared into a 50 mL aqueous solution with a concentration of 0.6 mg / mL. The solution was stirred at 45 °C for 60 min, and then 0.9 g of activator EDC and 0.6 g of stabilizer NHS were added. Next, 0.6 g of PEG was added, and the mixture was reacted at 120 °C for 10 h. After the reaction was complete, unreacted PEG was removed by dialysis with deionized water, and the resulting product was freeze-dried to obtain PEG-CQDs powder. The molecular weight of the PEG was between 400 and 4000.

[0040] Step 3: Preparation of PEG-CQDs / ZnO composite catalyst Using a solvothermal method, 10 mL of 0.6 mg / mL PEG-CQDs aqueous solution was first prepared and reacted with 5.48 g of zinc acetate dihydrate in ethanol solution. 0.3 g of silane coupling agent (KH550) was added, and the reaction was carried out at 140 °C for 8 h. After the reaction was completed, the reaction product was obtained by centrifugation, washing, and drying to obtain the PEG-CQDs / ZnO composite catalyst.

[0041] Example 15: The preparation process of this PEG-CQDs / ZnO composite catalyst is as follows: Step 1: Synthesis of CQDs Weathered coal was passed through a 100-500 mesh sieve and reacted with hydrogen peroxide (H2O2) in a round-bottom flask at a mass ratio of 1:3 at 70°C for 8 hours. After oxidation by H2O2 and cooling to room temperature, the supernatant was obtained by centrifugation after the reaction was completed. Then, the supernatant was obtained by dialysis and drying to obtain purified carbon quantum dot powder.

[0042] Step 2: Synthesis of PEG-CQDs The purified carbon quantum dot powder was prepared into a 100 mL aqueous solution with a concentration of 0.5 mg / mL. The solution was stirred at 45 °C for 60 min, and then 1.5 g of activator (EDC) and 1 g of stabilizer (NHS) were added. Then, 1 g of PEG was added, and the mixture was reacted at 100 °C for 16 h. After the reaction was complete, unreacted PEG was removed by dialysis with deionized water, and the resulting product was freeze-dried to obtain PEG-CQDs powder. The molecular weight of the PEG was between 400 and 4000.

[0043] Step 3: Preparation of PEG-CQDs / ZnO composite catalyst Using a solvothermal method, 15 mL of 0.5 mg / mL PEG-CQDs aqueous solution was first prepared and reacted with 5.48 g of zinc acetate dihydrate in ethanol solution. 0.2 g of silane coupling agent (KH550) was added, and the reaction was carried out at 120 °C for 12 h. After the reaction was completed, the reaction product was obtained by centrifugation, washing, and drying to obtain the PEG-CQDs / ZnO composite catalyst.

[0044] Example 16: The preparation process of this PEG-CQDs / ZnO composite catalyst is as follows: Step 1: Synthesis of CQDs Weathered coal was passed through a 100-500 mesh sieve and reacted with hydrogen peroxide (H2O2) in a round-bottom flask at a mass ratio of 1:1 at 80°C for 4 hours. After oxidation by H2O2 and cooling to room temperature, the supernatant was obtained by centrifugation after the reaction was completed. Then, the supernatant was obtained by dialysis and drying to obtain purified carbon quantum dot powder.

[0045] Step 2: Synthesis of PEG-CQDs The purified carbon quantum dot powder was prepared into a 175 mL aqueous solution with a concentration of 0.4 mg / mL. The solution was stirred at 45 °C for 60 min, and then 1.8 g of activator (EDC) and 1.2 g of stabilizer (NHS) were added. Next, 1.2 g of PEG was added, and the mixture was reacted at 80 °C for 24 h. After the reaction was complete, unreacted PEG was removed by dialysis with deionized water, and the resulting product was freeze-dried to obtain PEG-CQDs powder. The molecular weight of the PEG was between 400 and 4000.

[0046] Step 3: Preparation of PEG-CQDs / ZnO composite catalyst Using a solvothermal method, 20 mL of 0.4 mg / mL PEG-CQDs aqueous solution was first prepared and reacted with 5.48 g of zinc acetate dihydrate in ethanol. 0.1 g of silane coupling agent (KH550) was added, and the reaction was carried out at 100 °C for 16 h. After the reaction was completed, the reaction product was obtained by centrifugation, washing, and drying to obtain the PEG-CQDs / ZnO composite catalyst.

[0047] Comparative Example 1: ZnO.

[0048] Comparative Example 2: Industrially produced antimony-based catalyst, antimony trioxide (Sb₂O₃).

[0049] Comparative Example 3: The difference from Example 14 is that the second PEG modification step is not performed. Instead, pure carbon quantum dot powder is directly reacted with zinc acetate dihydrate under the action of silane coupling agent (KH550) to obtain CQDs / ZnO composite catalyst.

[0050] Application Example 1: The PEG-CQDs / ZnO composite catalyst obtained in Example 14 was used to catalyze dimethyl terephthalate (DMT) and ethylene glycol (EG). The specific steps are as follows: Step 1: Dimethyl terephthalate (DMT) and ethylene glycol (EG) are subjected to transesterification under nitrogen protection in a molar ratio of 1:1.2 until the amount of alcohol produced reaches 80% to 100% of the theoretical value, to obtain the prepolymer dihydroxyethyl terephthalate (BHET). Step 2: Add 200 ppm of PEG-CQDs / ZnO composite catalyst, 0.02 g of heat stabilizer triphenyl phosphate, and 0.2 g of antioxidant pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid) at a theoretical monomer mass ratio to the prepolymer. Carry out the polycondensation reaction under vacuum conditions of less than 100 Pa to 10 Pa and temperature of 260℃ to 280℃. Stop the reaction when the stirring power of the reactor reaches the set value, discharge the material, record the polycondensation time, and test the polycondensation time and color data at different polycondensation temperatures. The results are shown in Table 1.

[0051] As shown in Table 1, the PEG-CQDs / ZnO composite catalyst of this invention exhibits a relatively slow reaction rate at a polycondensation temperature of 260℃ and a polycondensation time of 90 min. At a polycondensation temperature of 265℃, the polycondensation time is shortened to 75 min, achieving optimal reaction efficiency, indicating that the catalytic activity is fully released within this temperature range. The optimal polycondensation temperature window for the catalyst of this invention is 265℃, within which the polycondensation time can be controlled within 75 min, resulting in excellent reaction efficiency. Based on the brightness L* value, except for the sample at 270℃ (L*=70.23), which is slightly lower, the L* values ​​of all other samples are above 72. The sample at 280℃ achieves an L* of 85.47, the highest among all samples, indicating excellent brightness and a high whiteness at this temperature. Based on the red-green hue a* value, the absolute values ​​of a* for all samples are less than 0.3, ranging from -0.27 to 0.21, indicating highly neutral hue and no visually perceptible red / green deviation. The samples at 270℃ and 280℃ had negative a* values ​​(-0.19 and -0.27), slightly greenish, which often helps neutralize the yellow hue and improve visual whiteness in practical applications. According to the b* value of the yellow-blue hue, the lowest b* value was 3.65 (275℃) and the highest was 4.99 (280℃), with all samples having a b* value below 5.0.

[0052] Compared to traditional antimony-based catalysts, the PEG-CQDs / ZnO composite catalyst of this invention produces products with equally excellent color. Compared to titanium-based catalysts (b* is typically 5 to 8, prone to yellowing), the PEG-CQDs / ZnO composite catalyst of this invention exhibits significantly superior color, solving the long-standing yellowing problem of titanium-based catalysts. Of particular note is that the sample at 275℃ has the lowest b* value of 3.65 among all samples, while L*=77.45 and a*=0.06, achieving an optimal balance in colorimetric indicators. The sample at 280℃ has the highest b* value of 4.99, still below 5.0, while L*=85.47 is the highest in the field, exhibiting high whiteness and a slightly yellowish appearance, which can also meet the needs of most applications.

[0053] The PEG-CQDs / ZnO composite catalyst of this invention exhibits excellent catalytic activity in the PET polycondensation reaction. As shown in Table 1, at a polycondensation temperature of 265°C, the polycondensation reaction time can be shortened to 75 min, significantly better than traditional antimony-based catalysts (typically 90 to 120 min) and some titanium-based catalysts (80 to 100 min). This indicates that the PEG-CQDs / ZnO composite catalyst of this invention can effectively reduce the activation energy of the polycondensation reaction, accelerate the reaction process, and improve production efficiency. More importantly, at a polycondensation temperature of 275°C, the product has a b* value as low as 3.65, while the L* value is 77.45 and the a* value is 0.06, achieving an excellent color balance of high brightness, neutral hue, and low yellowing. This hue level is comparable to that of high-quality antimony-based catalysts and significantly superior to existing titanium-based catalysts.

[0054] Application Example 2: Catalytic tests were conducted on the catalysts of Examples 14 to 16 and Comparative Examples 1 to 3. The polymerization time and color of the PET copolyester polycondensation reaction were tested at dosages of 200 ppm and 100 ppm, respectively. The results are shown in Tables 2 and 3. The polymerization steps were the same as in Application Example 1. In this application example, the polymerization temperature was 265°C, the molar ratio of dimethyl terephthalate (DMT) to ethylene glycol (EG) was 1:1.2, and the transesterification reaction was carried out under nitrogen protection.

[0055] As shown in Tables 2 and 3, the PEG-CQDs / ZnO composite catalysts prepared in Examples 14 to 16 of this invention exhibit superior overall performance compared to the comparative examples in the polycondensation reaction. One key advantage is the significantly shorter polycondensation time, resulting in improved reaction efficiency. At the same or lower addition levels, the polycondensation time of the PEG-CQDs / ZnO composite catalysts is significantly shorter than that of the comparative examples. For example, at 200 ppm, the polycondensation time of the PEG-CQDs / ZnO composite catalyst in Example 15 is only 70 min, which is 30% shorter than that of pure ZnO (Comparative Example 1, polycondensation time 100 min) and 22% shorter than that of the antimony-based catalyst (Comparative Example 2, polycondensation time 90 min). Even at a low addition level of 100 ppm, the PEG-CQDs / ZnO composite catalyst in Example 15 completes polycondensation in 60 min, far superior to Comparative Example 1 (polycondensation time 110 min) and Comparative Example 2 (polycondensation time 100 min). This indicates that the catalyst of the present invention has higher catalytic activity and can reach the target degree of polymerization in a shorter time, which is beneficial for reducing production energy consumption and increasing production capacity. Secondly, it maintains a high product viscosity to ensure polymerization quality; the viscosity of the polymers obtained in Examples 14 to 16 is consistently 0.61 dL·g. -1 Up to 0.71 dL·g -1 Within the range, especially in Example 14, the concentration reached 0.69 dL·g at both addition levels. -1 and 0.71 dL·g -1 This is the highest among all samples. In contrast, the viscosity of Comparative Example 1 is only 0.59 to 0.60 dL·g. -1 The high viscosity implies a higher molecular weight and better mechanical properties, indicating that the catalyst of this invention does not sacrifice the polymerization quality of the product while accelerating the reaction. Thirdly, it delivers excellent product color, especially a significant improvement in brightness (L* value). The L* values ​​of Examples 14 to 16 are all above 72.5 at both addition levels, with Example 15 reaching 73.28 and 73.87 at 200 ppm and 100 ppm respectively, the highest among all samples, significantly better than Comparative Examples 1 and 2. Meanwhile, the a* and b* values ​​of Example 15 remain at low levels, especially at 100 ppm where the b* value is only 3.29, indicating a lighter and purer product color, better thermal stability, and effective suppression of side reactions (such as thermal degradation or yellowing).

[0056] Application Example 3: Following the steps of Application Example 1, with color, molecular weight, and viscosity as objective functions, the optimal experimental group was selected. Under optimal conditions, a third monomer, diol, was added to synthesize low-melting-point PET copolyester, in order to verify that the PEG-CQDs / ZnO composite catalyst of the present invention also exhibits excellent catalytic performance in the synthesis of PET copolyester containing a third monomer.

[0057] In this application example, the PEG-CQDs / ZnO composite catalyst prepared in Example 15 was used at an addition amount of 100 ppm, and the polycondensation temperature was 265°C. DMT, EG, and the third monomer were mixed according to the steps in Application Example 1, wherein the molar ratio of DMT to diol (the sum of EG and the third monomer diol) was 1:1.2, and the third monomer accounted for 20% to 60% of the total mass of the diol. The third monomer in the synthesized low-melting-point PET copolyester was 1,6-hexanediol. The transesterification reaction was carried out under nitrogen protection, and the polycondensation time, viscosity, molecular weight, and color data were recorded. The results are shown in Table 4.

[0058] As shown in Table 4, the copolyester exhibits the best overall performance when the third monomer accounts for 50% of the total mass of the diol. As shown in Application Example 3, under this composition, the intrinsic viscosity of the copolyester reaches 0.63 dL / g, the highest among all samples; the polycondensation time is 90 min, the product's L* brightness is 83.67, and its a* hue is -0.09 (closest to neutral), demonstrating excellent overall performance. Compared to conventional PET without the addition of the third monomer, this copolyester shows significantly improved flexibility and impact resistance, making it suitable for applications requiring high flexibility, such as heat-shrinkable films and engineering plastics. These results demonstrate that the PEG-CQDs / ZnO composite catalyst of this invention has good compositional adaptability, allowing for flexible adjustment of the third monomer addition ratio according to different application requirements, thus broadening the product portfolio of PET polyesters. The PEG-CQDs / ZnO composite catalyst of this invention exhibits excellent catalytic efficiency under high third monomer content conditions, and the catalyst can effectively reduce the activation energy of high-proportion long-chain diol systems, which is beneficial for improving production efficiency and reducing energy consumption.

[0059] Compared with the prior art, the PEG-CQDs / ZnO catalyst of the present invention has the following advantages: (1) The PEG-CQDs / ZnO composite catalyst of the present invention is green and non-toxic, which is in line with the concept of sustainable development. The catalyst is prepared by polyethylene glycol (PEG) modified carbon quantum dots (CQDs) composite zinc oxide (ZnO). The entire preparation and use process is free of toxic heavy metals such as antimony, avoiding the problem of harmful substance residues, greatly reducing the safety risks and environmental burden of product use, and conforming to the development direction of green chemistry and circular economy.

[0060] (2) The PEG-CQDs / ZnO catalyst of the present invention still has excellent catalytic activity under low addition conditions, the catalyst active site utilization rate is high, and a small amount of addition can achieve or even exceed the reaction efficiency of traditional catalysts, effectively reducing the cost of catalyst use.

[0061] (3) The PEG-CQDs / ZnO catalyst of the present invention can still ensure product quality under low addition conditions, achieving reduced dosage and increased efficiency. At an addition of only 100 ppm, the intrinsic viscosity of the polymer can reach 0.71 dL·g. -1 It is superior to all comparative examples; the system has fewer side reactions and the product has good thermal stability, which can yield high-molecular-weight, low-yellowing, high-quality polyester products.

[0062] (4) It has a wide range of applicable systems and can be adapted to copolymerization systems containing a third monomer. The PEG-CQDs / ZnO catalyst of the present invention has good compatibility with various polyester polymerization systems. It can maintain high catalytic activity when a third monomer is added for co-condensation. The condensation process is stable and controllable. The resulting product has excellent viscosity and color index, which expands the application scenarios of the catalyst and provides a reliable solution for the green production of various functional polyesters.

[0063] The above technical features constitute various embodiments of the present invention, which have strong adaptability and implementation effect. Unnecessary technical features can be added or removed according to actual needs to meet the needs of different situations.

Claims

1. A method for preparing a PEG-CQDs / ZnO composite catalyst, characterized in that... Includes the following steps: Step 1: Add activator and stabilizer to the aqueous solution of carbon quantum dots, then add polyethylene glycol to react, and dry the reaction product to obtain PEG-CQDs powder; Step 2: Mix PEG-CQDs powder and soluble zinc salt in a reaction solvent, add silane coupling agent, and after reaction, obtain PEG-CQDs / ZnO composite catalyst.

2. The preparation method of the PEG-CQDs / ZnO composite catalyst according to claim 1, characterized in that... Carbon quantum dots are prepared according to the following steps: Carbon source and hydrogen peroxide are mixed and reacted in a mass ratio of 1:1 to 1:

5. The reaction product is centrifuged, filtered, dialyzed and dried to obtain carbon quantum dots. The carbon source is one or more of weathered coal, citric acid, glucose, fructose, sucrose and cellulose.

3. The preparation method of the PEG-CQDs / ZnO composite catalyst according to claim 2, characterized in that... The concentration of carbon quantum dots in the aqueous solution ranges from 0.1 mg / mL to 1 mg / mL.

4. The method for preparing the PEG-CQDs / ZnO composite catalyst according to claim 2 or 3, characterized in that... The activator is 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride, and the amount added is 20 to 30 times the mass of carbon quantum dots.

5. The method for preparing the PEG-CQDs / ZnO composite catalyst according to any one of claims 1 to 4, characterized in that... The stabilizer is N-hydroxysulfosuccinimide, and the amount added is 15 to 20 times the mass of carbon quantum dots.

6. The method for preparing the PEG-CQDs / ZnO composite catalyst according to any one of claims 1 to 5, characterized in that... The molecular weight of polyethylene glycol is 400 to 4000; or / and the mass ratio of carbon quantum dots to polyethylene glycol is (1 to 5): (15 to 20).

7. The method for preparing the PEG-CQDs / ZnO composite catalyst according to any one of claims 1 to 6, characterized in that... The silane coupling agent is KH550, and the amount of silane coupling agent added is 10 to 50 times the mass of PEG-CQDs; or / and the soluble zinc salt is one or more of zinc acetate, zinc nitrate, zinc chloride, and zinc sulfate, and the amount of soluble zinc salt added is 680 to 920 times the mass of PEG-CQDs.

8. The method for preparing the PEG-CQDs / ZnO composite catalyst according to any one of claims 1 to 7, characterized in that... In step one, the reaction temperature is 80℃ to 120℃ and the reaction time is 4h to 24h; or / and in step two, the reaction temperature is 100℃ to 150℃ and the reaction time is 8h to 24h.

9. A PEG-CQDs / ZnO composite catalyst prepared by the method according to any one of claims 1 to 8.

10. The application of the PEG-CQDs / ZnO composite catalyst according to any one of claims 1 to 8 in the catalytic polycondensation reaction of PET.

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