Preparation method and application of waste polyester alcoholysis system

Abstract

The present invention discloses a waste polyester alcoholysis system and its application. The preparation method of the present invention comprises the following steps: (1) mixing commercial boron nitride and carbon material in a certain mass ratio, and then ball milling the mixture in a ball mill to obtain a uniformly mixed powder mixture of boron nitride and carbon material; and (2) sieving and separating the product in (1), and calcining the mixture in a tubular furnace under a nitrogen atmosphere for a certain period of time to obtain carbon-doped boron nitride. The advantages of the present invention are simple preparation method, low cost, stable catalyst and mild reaction conditions. The present invention overcomes the shortcomings of traditional PET alcoholysis catalysts such as metal ion overflow and secondary pollution, provides a new idea for the green and efficient treatment of PET waste, and has great practical application prospects.

Description

Technical Field

[0001] The invention relates to a method for preparing a catalyst, in particular to a waste polyester alcoholysis system, and belongs to the field of waste polyester degradation and regeneration. Background Art

[0002] Due to social development and increasing demands for human life, global polyester production continues to grow. Among the most common polyester materials, polyethylene terephthalate (PET) accounts for the largest share due to its desirable physical properties, such as durability, flexibility, water resistance, and lightness. According to statistics, nearly 73 million tons of PET were produced globally in 2020, but only 15% was recycled. The majority of the rest was dumped as waste in landfills or into the ocean, causing serious environmental pollution to water, air, and land. Therefore, recycling and reuse of PET is necessary, which not only avoids environmental pollution but also alleviates the shortage of petroleum resources.

[0003] PET recycling is generally categorized into four pathways: primary, physical, energy, and chemical. Primary recycling requires high raw material purity and relies solely on manual labor, which is both costly and inefficient. During physical recycling, mechanical stress and high temperatures can break PET molecular chains, leading to side reactions and a decrease in the viscosity and molecular weight of the recycled PET. Energy recovery generates dust and harmful gases, which can cause serious environmental pollution. Chemical recycling uses solvents to depolymerize PET molecular chains, converting them into monomers or oligomers, which are then purified and reused as chemical raw materials, achieving a true recycling cycle. Because the ester bonds in PET are easily depolymerized by many nucleophilic reagents, chemical recycling can be categorized into hydrolysis, alcoholysis, and aminolysis, depending on the solvent type. Currently, alcoholysis is the most widely studied and widely used method for PET chemical recycling. Glycolysis is the most widely used alcoholysis method for PET chemical recycling, as it is mild, requires minimal equipment, and is easily scalable. The final product, bis(hydroxyethyl) terephthalate (BHET), is produced. Patent application number CN202110360655.X discloses a method for the catalytic degradation of polyethylene terephthalate (PET): PET fragments and a catalyst (zinc compound) are uniformly mixed to obtain a mixture; the mixture is heated, and a degradation reaction occurs under the combined action of water vapor and the catalyst. After the reaction is complete, solid and liquid products are obtained. This invention uses zinc oxide, or a compound that decomposes to form zinc oxide during heating, as a catalyst. The zinc oxide then activates the C=O bonds in the PET polymer chain to produce terephthalic acid and ethylene glycol (EG), providing a new path for the high-value-added conversion and reuse of waste PET. Patent application number CN201410018035.8 discloses a method for the alcoholysis of polyethylene terephthalate (PET) catalyzed by a superbase ionic liquid. The method involves using a superbase ionic liquid as a catalyst and a mixture of one or more of ethylene glycol, diethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, and heptanediol as a solvent. The PET is alcoholyzed at a catalyst dosage of 0.05% to 30% of the solvent's mass, a reaction temperature of 120°C to 250°C, a pressure of 1 atm, and a reaction time of 0.5 to 10 hours. This method offers rapid reaction speed, high conversion rates, and mild conditions. The catalyst is highly active, recyclable, and the product is easily separable. However, the catalytic efficiency and environmental performance of this alcoholysis system remain to be improved. Therefore, to address the challenges associated with catalytic PET alcoholysis, it is necessary to develop a new, green and environmentally friendly alcoholysis catalyst that catalyzes PET alcoholysis without the use of metal or metal compound catalysts, does not cause secondary pollution to the reaction system, and exhibits excellent catalytic performance, thereby improving PET conversion and BHET yield.

[0004] The unique polarity of the boron-nitrogen bond in boron nitride (BN) and the high specific surface area of ​​hexagonal boron nitride (h-BN) contribute to its excellent catalytic activity. The coexistence of boron atoms, acting as Lewis acidic sites, and nitrogen atoms, acting as basic sites, enables various chemical interactions between BN and PET. The basic nitrogen atoms can enhance the nucleophilic attack of EG by abstracting hydrogen from the hydroxyl group, while the acidic boron atoms promote the electrophilicity of the carbonyl group of PET, making it more susceptible to attack by the nucleophile EG. The presence of boron and nitrogen atoms as acidic and basic sites makes BN a bifunctional catalyst for the alcoholysis of PET. Furthermore, the π-π bond interactions between the two atoms can milden the reaction conditions. Furthermore, theoretical calculations predict that carbon atom substitution can introduce higher charge and spin density into BN, leading to enhanced catalytic activity. Furthermore, the presence of NC=N and C≡N facilitates the dispersion of the catalyst in EG, increasing the contact area between the catalyst and the solvent and accelerating the reaction rate. However, there is currently no research on the use of carbon-doped BN materials to catalyze the glycolysis of polyethylene terephthalate. Summary of the Invention

[0005] To address the shortcomings of traditional PET alcoholysis catalysts, such as low catalytic efficiency, poor reproducibility, and metal ion overflow, the present invention provides a waste polyester alcoholysis system. Using BN as the matrix material, defects are generated by ball milling, and then a carbon material is loaded on the defects to obtain a carbon material-doped boron nitride-catalyzed polyethylene terephthalate alcoholysis catalyst. Studies have found that this catalyst exhibits acid-base synergy: nitrogen atoms, as basic sites, can enhance the nucleophilic attack of ethylene glycol by extracting hydrogen from hydroxyl groups, while boron atoms, as acidic sites, promote the electrophilic properties of PET carbonyl groups, making them more susceptible to attack by the nucleophilic reagent EG. Furthermore, this catalyst has the advantages of being metal-free, free of secondary pollution, having good recyclability, and low cost, providing a new approach for the efficient alcoholysis of PET and promising application prospects.

[0006] A method for preparing a waste polyester alcoholysis system, the specific steps of which are as follows:

[0007] (1) Commercial hexagonal boron nitride and carbon material are mixed in a certain mass ratio, and then placed in a ball mill for 18-25 hours to obtain a powdery mixture of uniformly mixed boron nitride and carbon material;

[0008] (2) The product of step (1) is separated by sieving, placed in a tubular furnace and calcined for a certain period of time under a nitrogen atmosphere to obtain carbon-doped boron nitride.

[0009] Preferably, the carbon material in step (1) is one or more of graphite flakes, carbon nanotubes, or activated carbon fibers.

[0010] Preferably, in step (1), the mass ratio of boron nitride to carbon material is 1 to 16:1.

[0011] Preferably, the calcination temperature in the tubular furnace in step (2) is 800-1000°C.

[0012] A waste polyester alcoholysis system is used in a method for preparing the waste polyester alcoholysis system. The method comprises placing a certain amount of PET, EG, and carbon-doped boron nitride in a three-necked flask connected to a thermometer, a condenser, a stirrer, and nitrogen. The temperature is slowly raised to an appropriate temperature. After a certain reaction time, the reacted solution is cooled to 80-160°C and filtered while hot to separate the undepolymerized PET from the solution. The solid is then rinsed with deionized water to remove the filtered solid, dried, and weighed to obtain the mass of the undepolymerized PET. A large amount of deionized water is added to the filtrate, stirred, heated to 40-110°C, and filtered while hot to obtain a mixed solution of BHET, EG, and water. The mixed solution is cooled to room temperature, then stored at a low temperature of 3-15°C for 5-24 hours. The BHET is then filtered and dried to obtain the BHET, which is then weighed to obtain the actual BHET yield.

[0013] Preferably, the mass ratio of boron nitride doped with carbon material, PET, and EG is 1-15:5-60:17-140, the alcoholysis temperature is 150-260° C., and the alcoholysis time is 3-15 h.

[0014] In the present invention, the acidic sites and basic sites of boron nitride are simultaneously combined with transition metals. The doping of transition metals enables the 3d orbital of the metal to interact well with the 2p orbital of the corresponding B or N site in boron nitride, thereby enhancing the catalytic activity and synergizing with the acidic and basic sites that have catalytic effects themselves to enhance the catalytic activity.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) The catalyst is simple to prepare, low-cost, and does not contain metal ions. It is an efficient and purely green catalyst.

[0017] (2) The catalyst contains both acidic and basic sites. C atom substitution can introduce higher charge and spin density into BN, making it exhibit stronger catalytic activity and can be used as a multifunctional catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the XRD pattern of the graphite flake-doped boron nitride catalyst prepared in the present invention;

[0019] Figure 2 This is an SEM image of the graphite flake-doped boron nitride catalyst prepared in the present invention;

[0020] Figure 3The depolymerization rate and BHET yield of polyethylene terephthalate alcoholysis catalyzed by graphite flakes doped with boron nitride prepared by the present invention are shown. DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with specific examples. These examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, after reading the content taught by the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalents also fall within the scope defined by the appended claims of the application.

[0022] Example 1:

[0023] (1) Commercial hexagonal boron nitride and graphite flakes were mixed at a mass ratio of 16:1 and then ball milled for 20 h to obtain a powdery mixture of a uniformly mixed boron nitride and carbon material;

[0024] (2) The product in (1) was separated by sieving, placed in a tube furnace, and heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere and calcined for 2 h to obtain graphite flake-doped boron nitride ( Figure 1 and Figure 2 ).

[0025] An application of a waste polyester alcoholysis system is applicable to a preparation method of a waste polyester alcoholysis system: 1.0g PET, 15.0mL EG, and 0.15g BN-C are placed in a three-necked flask connected to a thermometer, a condenser, a stirrer, and nitrogen, and the temperature is slowly raised to 200°C. The reaction time is set to 5h. The conversion rate of PET is 100%, and the BHET yield is 85% ( Figure 3 ).

[0026] Example 2:

[0027] (1) Commercial hexagonal boron nitride and graphite flakes were mixed at a mass ratio of 8:1 and then ball milled for 25 h to obtain a powdery mixture of boron nitride and graphite flakes;

[0028] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 800°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0029] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 200°C, and the reaction time is set to 3 hours. The resulting PET conversion rate is 91%, and the BHET yield is 76%.

[0030] Example 3:

[0031] (1) Commercial hexagonal boron nitride and graphite flakes were mixed at a mass ratio of 4:1 and then ball milled for 20 h to obtain a powdery mixture of boron nitride and graphite flakes;

[0032] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 1000°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0033] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 200°C, and the reaction time is set to 3 hours. The resulting PET conversion rate is 82%, and the BHET yield is 61%.

[0034] Example 4:

[0035] (1) Commercial hexagonal boron nitride and carbon nanotubes were mixed at a mass ratio of 16:1 and then ball-milled for 20 h to obtain a powdery mixture of uniformly mixed boron nitride and carbon nanotubes;

[0036] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon nanotube-doped boron nitride.

[0037] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 160°C, and the reaction time is set to 5 hours. The resulting PET conversion rate is 82%, and the BHET yield is 70%.

[0038] Example 5:

[0039] (1) Commercial hexagonal boron nitride, graphite flakes, and carbon nanotubes were mixed in a mass ratio of 4:0.5:0.5 and then ball-milled in a ball mill for 20 h to obtain a powdery mixture of uniformly mixed boron nitride and carbon materials;

[0040] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0041] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 200°C, and the reaction time is set to 5 hours. The resulting PET conversion rate is 95%, and the BHET yield is 78%.

[0042] Example 6:

[0043] (1) Commercial hexagonal boron nitride and carbon nanotubes were mixed at a mass ratio of 4:1 and then ball-milled for 20 h to obtain a powdery mixture of a uniformly mixed boron nitride and carbon material;

[0044] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0045] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 200°C, and the reaction time is set to 3 hours. The resulting PET conversion rate is 87%, and the BHET yield is 73%.

[0046] Example 7:

[0047] (1) Commercial hexagonal boron nitride and carbon nanotubes were mixed at a mass ratio of 8:1 and then ball milled for 20 h to obtain a powdery mixture of a uniformly mixed boron nitride and carbon material;

[0048] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0049] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 160°C, and the reaction time is set to 5 hours. The resulting PET conversion rate is 74%, and the BHET yield is 59%.

[0050] Example 8:

[0051] (1) Commercial hexagonal boron nitride and activated carbon fiber were mixed at a mass ratio of 16:1 and then ball-milled for 20 h to obtain a powdery mixture of uniformly mixed boron nitride and carbon materials;

[0052] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0053] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 200°C, and the reaction time is set to 5 hours. The resulting PET conversion rate is 82%, and the BHET yield is 65%.

[0054] Example 9:

[0055] (1) Commercial boron nitride and activated carbon fiber were mixed at a mass ratio of 8:1 and then ball milled for 20 h to obtain a powdery mixture of a uniformly mixed boron nitride and carbon material;

[0056] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0057] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 160°C, and the reaction time is set to 5 hours. The resulting PET conversion rate is 72%, and the BHET yield is 66%.

[0058] Example 10:

[0059] (1) Commercial boron nitride and activated carbon fiber were mixed at a mass ratio of 4:1 and then ball milled for 20 h to obtain a powdery mixture of uniformly mixed boron nitride and carbon materials;

[0060] (2) The product in (1) was separated by sieving, placed in a tubular furnace, heated to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 h to obtain carbon-doped boron nitride.

[0061] A waste polyester alcoholysis system is used, and is suitable for a method for preparing the waste polyester alcoholysis system: 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN-C are placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature is slowly raised to 200°C, and the reaction time is set to 3 hours. The resulting PET conversion rate is 75%, and the BHET yield is 58%.

[0062] Comparative Example 1:

[0063] Commercial hexagonal boron nitride was ball-milled in a ball mill for 20 h. The ball-milled product was sieved and separated, and then placed in a tubular furnace and heated to 900 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 2 h to obtain boron nitride with vacancies.

[0064] 1.0 g of PET, 15.0 mL of EG, and 0.15 g of BN were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature was slowly raised to 200°C, and the reaction time was set to 5 hours. The resulting PET conversion was 48%, and the BHET yield was 21%.

[0065] Comparative Example 2:

[0066] The graphite flakes were ball-milled in a ball mill for 20 h, the ball-milled product was sieved and separated, and placed in a tubular furnace and heated to 900 °C at a heating rate of 5 °C / min under a nitrogen atmosphere and calcined for 2 h.

[0067] 1.0 g of PET, 15.0 mL of EG, and 0.15 g of graphite flake powder were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature was slowly raised to 200°C and the reaction time was set to 5 hours. The resulting PET conversion was 45% and the BHET yield was 18%.

Claims

1. An application of a waste polyester alcoholysis system, characterized in that: First, a waste polyester alcoholysis system is prepared, which includes the following steps: (1) Commercial hexagonal boron nitride and carbon material are mixed in a certain mass ratio, and then placed in a ball mill for 18-25 hours to obtain a powdery mixture of uniformly mixed boron nitride and carbon material; (2) sieving and separating the product in step (1), placing it in a tubular furnace and calcining it in a nitrogen atmosphere for a certain period of time to obtain carbon-doped boron nitride; Then, a certain amount of PET, EG and boron nitride doped with carbon material are placed in a three-necked flask connected to a thermometer, a condenser, a stirrer and nitrogen, and the temperature is slowly raised to 150-260 ° C for alcoholysis, and the alcoholysis time is 3-15 hours.

2. The use of a waste polyester alcoholysis system according to claim 1, characterized in that: The mass ratio of boron nitride doped with carbon material, PET and EG is 1-15:5-60:17-140.

3. The use of a waste polyester alcoholysis system according to claim 1, characterized in that: The carbon material is one or more of graphite flakes, carbon nanotubes, or activated carbon fibers.

4. The use of a waste polyester alcoholysis system according to claim 1, characterized in that: The mass ratio of boron nitride to carbon material is 1 to 16:

1.

5. The use of a waste polyester alcoholysis system according to claim 1, characterized in that: The calcination temperature in the tubular furnace is 800°C to 1000°C.

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