Method for preparing BHET by catalyzing ethylene glycol alcoholysis of PET

Abstract

The invention belongs to the technical field of polyester recovery, and relates to a method for preparing BHET by catalyzing ethylene glycol alcoholysis of PET. According to the method, gamma-Fe2O3 (at) meso-SiO2-DBU is adopted as a catalyst, the gamma-Fe2O3 (at) meso-SiO2-DBU is composed of gamma-Fe2O3 (at) meso-SiO2 and DBU, the gamma-Fe2O3 (at) meso-SiO2 is of a core-shell structure, the core is gamma-Fe2O3, the shell is composed of meso-SiO2, the DBU and the shell form ion pairs through electrostatic attraction, and the ion pairs are enriched in mesopores of the meso-SiO2. The problem that the yield of BHET is low in an existing magnetic separation heterogeneous catalysis system is solved, the conversion rate of PET and the yield of BHET are effectively increased, and the catalyst is stable in structure and lasting in catalytic activity, can be magnetically separated and recycled and adapts to the strict reaction conditions for catalyzing ethylene glycol to depolymerize PET.

Description

Technical Field

[0001] This invention belongs to the field of polyester recycling technology and relates to a method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol. Background Technology

[0002] Polyethylene terephthalate (PET) is a thermoplastic polyester material synthesized by direct esterification of purified terephthalic acid (PTA) and ethylene glycol (EG) or transesterification of dimethyl terephthalate (DMT) and EG. Due to its excellent mechanical strength, chemical stability, barrier properties and transparency, it is widely used in food packaging, textiles, construction and medical fields, and is one of the world's highest-volume polyester plastics.

[0003] PET products are mostly for single use, and their waste accounts for 8.0% of global solid waste by weight and 12.0% by volume. It is extremely difficult to degrade in the natural environment, placing a heavy burden on the ecosystem. Against this backdrop, the recycling and reuse of PET waste has become a key research focus in the global plastics circular economy field. Among these efforts, converting PET into high-purity bis(2-hydroxyethyl) terephthalate (BHET) monomer through chemical depolymerization, which can then be directly remelted and polycondensed to prepare recycled PET (rPET) with properties comparable to virgin PET, offers significant advantages in atom economy, environmental friendliness, and achieving closed-loop recycling of PET. This represents a core research direction in the field of PET chemical recycling.

[0004] Ethylene glycol alcoholysis of PET is the mainstream process for its depolymerization to prepare BHET. The choice of catalytic system plays a decisive role in the alcoholysis reaction efficiency, BHET yield, and purity. Currently, commonly used catalytic systems for PET alcoholysis are divided into homogeneous and heterogeneous types. Among them, heterogeneous catalysts exist in solid form and form a multiphase system with the liquid-phase reaction system. They have advantages such as easy separation and recovery, recyclability, environmental friendliness, reduction of metal ion residues to improve product purity, good stability, and suitability for continuous operation. Therefore, they are widely used in PET alcoholysis recovery processes.

[0005] Heterogeneous catalysts capable of magnetic separation possess the inherent advantages of heterogeneous catalysts and can achieve rapid and efficient solid-liquid separation through an external magnetic field. This eliminates the need for complex filtration and centrifugation operations, significantly simplifying the catalyst recovery process after the reaction, reducing separation energy consumption and operating costs, and minimizing catalyst loss during the separation process. This also improves the catalyst recycling efficiency, making it a preferred option for PET alcoholysis catalytic systems.

[0006] For example, the literature (Sequential extraction, depolymerization and quantification of polyethylene terephthalate nanoplastics using magnetic ZIF-8 nanocomposites[J]. Chemical Engineering Journal, 2024, 490: 151453.) uses Nano-Fe@ZIF-8 with nano-iron-based magnetic particles as the core to endow the material with magnetic response separation ability, and ZIF-8 zeolite imidazole ester framework material as the shell to provide catalytic active sites for alcoholysis reaction, but its catalytic activity is limited, and the highest yield of BHET is only 49%.

[0007] In summary, existing heterogeneous catalytic systems capable of magnetic separation generally suffer from low BHET yields. There is an urgent need to develop a novel heterogeneous catalytic system that combines efficient magnetic separation characteristics, structural stability, and excellent catalytic performance to promote the industrialization of PET alcoholysis for BHET recovery. Summary of the Invention

[0008] The purpose of this invention is to address the problems existing in the prior art and to provide a method for preparing BHET by catalytic alcoholysis of ethylene glycol on PET.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol uses γ-Fe2O3@meso-SiO2-DBU as the catalyst. γ-Fe2O3@meso-SiO2-DBU is composed of γ-Fe2O3@meso-SiO2 and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene). γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 and the shell being composed of meso-SiO2 (mesoporous silica). DBU and the shell form ion pairs through electrostatic attraction and are enriched in the mesopores of meso-SiO2.

[0011] This invention specifically addresses the problems of low PET conversion and low BHET yield in heterogeneous catalytic systems in existing technologies. The specific reasons are as follows:

[0012] (1) In this invention, DBU is enriched in the mesopores of meso-SiO2. During the process of catalytic alcoholysis of PET to prepare BHET, meso-SiO2 can form a nano-confined microenvironment, which increases the local concentration of PET chain segments and ethylene glycol in the mesopores enriched with DBU, increases the collision frequency of reactants, significantly enhances the catalytic effect, increases the reaction rate, facilitates the full breaking of ester bonds in the PET molecular chain, promotes the reaction towards degradation, and thus significantly improves the conversion rate of PET.

[0013] (2) Fe exposed on the surface of γ-Fe2O3 3+ Having empty d orbitals, such as Figure 11 As shown, it can act as a Lewis acid and weakly coordinate with the carbonyl oxygen (C=O) of the ester group in the PET molecule, polarizing the C=O bond and enhancing the positive charge of the carbon atom; at the same time, the DBU in the mesopores of meso-SiO2 activates ethylene glycol, generating a more nucleophilic ethoxy anion (HOCH2CH2O). - Polarized PET carbonyl carbon is more susceptible to nucleophilic attack, significantly reducing the depolymerization energy barrier, such as Figure 12 As shown, the sp2 hybridized nitrogen atom in the DBU molecule can assist in guiding the ethoxy anion to attack the carbonyl carbon due to the bicyclic alkyl power supply effect and conjugation effect, thereby reducing the generation of by-products and improving the BHET yield.

[0014] (3) On the one hand, DBU and the shell form ion pairs through electrostatic attraction. The ion pairs are reversible and flexible, which can significantly improve the spatial accessibility of active sites and the mass transfer efficiency of reactants while ensuring the stable anchoring of DBU. On the other hand, the Si–O–Fe interface bond formed between meso-SiO2 and γ-Fe2O3 can stabilize the surface structure of γ-Fe2O3, suppress the magnetic nucleus phase transition under high temperature and alkaline conditions, avoid the loss of DBU during the reaction process, and ensure the continuous and stable existence of catalytic active sites. Therefore, the catalyst maintains high activity throughout the reaction, promotes the full progress of PET depolymerization, and ensures the continuous generation of BHET, ultimately achieving a dual improvement in PET conversion rate and BHET yield.

[0015] (4) γ-Fe2O3, meso-SiO2 and DBU are used to construct an efficient reaction micro-region integrating adsorption-activation-reaction-desorption, forming a ternary synergistic catalytic system of Lewis acid activation substrate-mesoporous confined enrichment-Brønsted base nucleophile generation.

[0016] The literature (Preparation of a novel Fe3O4@SiO2@propyl@DBU magnetic core-shell nanocatalyst for Knoevenagel reaction in aqueous medium[J]. Journal of Experimental Nanoscience, 2020, 15(1): 85-98.) discloses a heterogeneous catalyst with a similar structure to that of the present invention, but it has never been applied to PET alcoholysis in the prior art because:

[0017] Firstly, in Fe3O4@SiO2@propyl@DBU, DBU is covalently linked to SiO2 through a propyl chain. The covalent bonds are rigid and have large steric hindrance, resulting in poor accessibility of DBU active sites and hindered mass transfer of reactants. Furthermore, the covalent bonds are easily broken under high temperature and alkaline conditions, causing DBU loss and catalyst deactivation.

[0018] Secondly, the Fe3O4 magnetic core itself has an unstable structure and cannot form a stable synergistic catalytic system with SiO2 and DBU. Fe3O4 has an inverse spinel structure and contains Fe on its surface. 2+ with Fe 3+ Fe 2+ d 6 The electronic configuration results in Lewis acidity being significantly weaker than Fe in γ-Fe₂O₃. 3+ It cannot effectively polarize the PET ester bonds; and under high temperature and alkaline conditions, Fe... 2+ Easily oxidized, triggering magnetic nucleus phase transition, SiO2 shell damage, and Fe... 2 + The leaching contamination reaction system cannot construct a stable synergistic catalytic mechanism.

[0019] In the catalyst of this invention, DBU and meso-SiO2 form ion pairs through electrostatic attraction. These ion pairs are reversibly flexible, ensuring stable anchoring of DBU while significantly improving the spatial accessibility of active sites and the mass transfer efficiency of reactants, thus avoiding steric hindrance and deactivation problems caused by covalent bond immobilization. Simultaneously, the stable γ-Fe2O3 magnetic core, together with the Si–O–Fe interfacial bonds, ensures the catalyst maintains structural integrity and sustained activity in the high-temperature alkaline alcoholysis system. Furthermore, the γ-Fe2O3 has a vacant spinel structure, with its surface containing only high charge density Fe. 3+ Lewis is highly acidic and structurally stable under high temperature and alkaline conditions. It can form a stable and efficient ternary synergistic system with meso-SiO2 and DBU, which is suitable for the harsh reaction conditions of PET glycol depolymerization.

[0020] As a preferred technical solution:

[0021] The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol as described above has an average core particle size of 218-220 nm, an average shell thickness of 12-16 nm, an average shell pore size of 2.3-3 nm, and a DBU content of 1.2 wt%-8.83 wt% in γ-Fe2O3@meso-SiO2-DBU.

[0022] The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol as described above, the preparation process of γ-Fe2O3@meso-SiO2-DBU is as follows: γ-Fe2O3@meso-SiO2 is dispersed in a mixture of ethanol and water, DBU is added and mechanically stirred to react, and then post-processed (magnetic separation and drying) to obtain γ-Fe2O3@meso-SiO2-DBU;

[0023] The volume ratio of ethanol to water is 0.1:1-0.5:1, the mass-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water is 1g:155mL-5g:155mL, the stirring temperature is 50-60℃, the stirring speed is 200-300r / min, and the stirring time is 12-36h.

[0024] During the preparation process, water and ethanol in the system act as proton sources, enabling the protonation of DBU to generate [DBUH]. + The silanol groups (–Si–OH) on the surface of meso-SiO2 partially dissociate into –Si–O. - The two form –Si–O through electrostatic attraction. - …[DBUH] + Dynamic ion pairs.

[0025] The preparation process of γ-Fe2O3@meso-SiO2 for BHET via catalytic alcoholysis of PET as described above is as follows:

[0026] (a) A CTAB (hexadecyltrimethylammonium bromide, as a template agent) solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to the solution, and the mixture was mechanically stirred (to ensure uniform dispersion of γ-Fe2O3), ultrasonically treated, and then an ammonia solution was added dropwise. Subsequently, a TEOS (tetraethyl orthosilicate, as a silicon source) solution (ethanol as solvent) was added dropwise using a peristaltic pump (BT100-2J from Baoding Lange Constant Flow Pump Co., Ltd.). After continuous mechanical stirring, the product was post-treated (the product was separated by magnetic adsorption using a neodymium magnet, and then washed and dried with alternating ethanol / water) to obtain γ-Fe2O3@SiO2. γ-Fe2O3 can spontaneously form high-density surface hydroxyl groups in the reaction system, exhibiting strong chemical stability. It can undergo dehydration condensation reaction with TEOS hydrolysis products to form dense and uniform Si–O–Fe interfacial bonds, thereby strengthening the core-shell interface bonding strength.

[0027] The concentration of CTAB solution is 0.01-0.05 g / mL, the mass ratio of γ-Fe2O3 to CTAB is 5:3-5:1, the mass fraction of ammonia solution is 25%-28%, the mass-volume ratio of γ-Fe2O3 to ammonia solution is 2 g:1 mL-3 g:1 mL, the volume fraction of TEOS solution is 10%-50%, and the volume ratio of TEOS to ammonia solution is 1.1:1-1.5:1.

[0028] The ultrasonic treatment frequency is 30-50kHz, the ultrasonic treatment time is 10-30min, the TEOS solution drop rate is 0.8-1.5mL / min, the stirring reaction temperature is 25-35℃, the stirring reaction speed is 220-260r / min, and the stirring reaction time is 24-36h.

[0029] (b) γ-Fe2O3@SiO2 was placed in a muffle furnace (Shanghai Guier Machinery Equipment Co., Ltd. GR.BF12 / 11) and calcined in air atmosphere (the purpose of which is to remove CTAB so that SiO2 forms a mesoporous structure). After that, it was naturally cooled to 25-30℃ to obtain γ-Fe2O3@meso-SiO2.

[0030] The calcination process involves a heating rate of 2-5℃ / min, followed by holding at 350-550℃ for 2-3 hours.

[0031] The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, as described above, is as follows: PET, ethylene glycol, and γ-Fe2O3@meso-SiO2-DBU are mixed evenly and then transferred to a 100mL hydrothermal reactor for alcoholysis. After post-treatment (after the hydrothermal reactor cools to room temperature, it is opened, the reactor liquid is poured into a beaker, 7 times the volume of ethylene glycol deionized water is added, and the mixture is magnetically stirred and heated to 90-95℃; the oligomers and catalyst are insoluble in water, while ethylene glycol and BHET are soluble in water; a neodymium magnet is placed at the bottom of the beaker, and the magnetic catalyst powder is magnetically adsorbed at the bottom of the beaker for recycling; the solution is poured out while hot; the solution is filtered while hot through a Buchner funnel to separate undegraded PET residue, oligomers, and BHET solution; the BHET solution is slowly crystallized at about 10℃ for 24 hours, and the crystallized product is separated by solid-liquid separation and dried to constant weight), thus obtaining BHET.

[0032] The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, as described above, has a PET to ethylene glycol feed mass ratio of 1:3-1:8, and the feed mass of γ-Fe2O3@meso-SiO2-DBU is 1.6%-4.8% of the PET feed mass.

[0033] The alcoholysis reaction is carried out at a temperature of 210-240℃ for 3-5 hours and at a pressure of 0.25-0.9 MPa (autogenous pressure of the reaction system).

[0034] The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol as described above, wherein the PET is in one or more of the following forms: film, bottle, fiber, and powder.

[0035] As described in any of the preceding methods, the method for preparing BHET by catalytic alcoholysis of PET with PET has a PET conversion rate of 99.98%-100% and a BHET yield of 80.27%-84.85%, which significantly improves both the PET conversion rate and the BHET yield compared to existing technologies.

[0036] Beneficial effects:

[0037] (1) The present invention uses γ-Fe2O3@meso-SiO2-DBU as a magnetic separation heterogeneous catalyst for catalytic alcoholysis of PET by ethylene glycol, which effectively solves the problem of low BHET yield in the existing magnetically separable heterogeneous catalytic system, while improving PET conversion rate, and taking into account the high efficiency magnetic separation characteristics and excellent catalytic performance of the catalyst.

[0038] (2) The meso-SiO2 catalyst in this invention has a porous structure, which can enrich DBU molecules and form a nano-confined microenvironment, significantly enhance the catalytic effect, increase the reaction rate, and promote the full breaking of ester bonds in the PET molecular chain.

[0039] (3) Fe on the surface of the γ-Fe2O3 catalyst in this invention 3+ It works synergistically with DBU in the mesopores of meso-SiO2 to significantly reduce the depolymerization barrier of PET, reduce the generation of by-products, and further improve the yield of BHET.

[0040] (4) The structural design of the catalyst in this invention can avoid the loss of DBU during the reaction process, ensure the continuous and stable existence of catalytic active sites, and allow the catalyst to maintain high activity throughout the reaction process, so as to promote the full progress of PET depolymerization reaction.

[0041] (5) In this invention, γ-Fe2O3, meso-SiO2 and DBU construct an efficient reaction micro-region integrating adsorption-activation-reaction-desorption, forming a stable ternary synergistic catalytic system that is suitable for the harsh reaction conditions of PET ethylene glycol depolymerization.

[0042] (6) In this invention, the γ-Fe2O3 magnetic nucleus of the catalyst forms a Si-O-Fe interface bond with meso-SiO2, which can stabilize the surface structure of γ-Fe2O3, suppress the magnetic nucleus phase transition under high temperature and alkaline conditions, and ensure the structural integrity of the catalyst under harsh reaction conditions.

[0043] (7) In this invention, the DBU of the catalyst and meso-SiO2 form an ion pair through electrostatic attraction. The reversible flexibility of the ion pair can significantly improve the spatial accessibility of the active site and the mass transfer efficiency of the reactants while ensuring the stable anchoring of DBU.

[0044] (8) The catalyst of the present invention has magnetic separation characteristics, which can achieve rapid solid-liquid separation through an external magnetic field, and can be recycled, simplifying the catalyst recovery process after the reaction and reducing separation energy consumption and operating costs. Attached Figure Description

[0045] Figure 1 Comparison of the X-ray diffraction (XRD) pattern of BHET prepared in Example 3 with the BHET standard PDF card (PDF#53-1689);

[0046] Figure 2 The nuclear magnetic resonance hydrogen spectra of BHET prepared in Example 3 and standard BHET (commercially available, purchased from Beijing Huawirui Chemical Co., Ltd.) 1 H NMR comparison diagram;

[0047] Figure 3 A comparison of the Fourier transform infrared (FT-IR) spectra of the BHET prepared in Example 3 and the standard BHET;

[0048] Figure 4A comparison of thermogravimetric (TG) analysis curves of BHET prepared in Example 3 and standard BHET;

[0049] Figure 5 A comparison of differential scanning calorimetry (DSC) curves of BHET prepared in Example 3 and standard BHET;

[0050] Figure 6 Here is a photograph of the BHET prepared in Example 3;

[0051] Figure 7 The XRD patterns of γ-Fe2O3, γ-Fe2O3@meso-SiO2 and γ-Fe2O3@meso-SiO2-DBU are compared with the standard PDF card of γ-Fe2O3 (PDF#80-2186), corresponding to Example 3;

[0052] Figure 8 The FT-IR comparison images are of γ-Fe2O3, γ-Fe2O3@meso-SiO2 and γ-Fe2O3@meso-SiO2-DBU, corresponding to Example 3;

[0053] Figure 9 Transmission electron microscope (TEM) image of γ-Fe2O3@meso-SiO2-DBU prepared in Example 3;

[0054] Figure 10 This is a schematic diagram of the structure of γ-Fe2O3@meso-SiO2-DBU prepared in Example 1; where 1 is γ-Fe2O3, 2 is meso-SiO2, and 3 is DBU;

[0055] Figure 11 For Fe 3+ Schematic diagram of the reaction mechanism of Lewis acid-catalyzed alcoholysis of PET in ethylene glycol;

[0056] Figure 12 This is a diagram illustrating the reaction mechanism of DBU organic base-catalyzed ethylene glycol alcoholysis of PET.

[0057] Figure 13 The 2p orbital photoelectron spectrum (Fe2p) of iron in γ-Fe2O3@meso-SiO2-DBU prepared in Example 1.

[0058] Figure 14 The 1s orbital photoelectron spectrum (N1s) of nitrogen in γ-Fe2O3@meso-SiO2-DBU prepared in Example 1. Detailed Implementation

[0059] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0060] The following are the calculation formulas and test methods for the relevant performance indicators in each embodiment and comparative example:

[0061] Intrinsic viscosity: Dissolve 0.125g±0.0005g of the sample in a phenol-tetrachloroethane mixture (mass ratio of phenol to tetrachloroethane = 6:4), and measure its intrinsic viscosity using an Ubbelohde viscometer at 25±0.05℃.

[0062] PET conversion rate = ;

[0063] In the formula, The feed mass of PET is expressed in grams. The mass of undegraded PET residue and oligomers after drying to constant weight is expressed in grams.

[0064] ;

[0065] In the formula, The mass of the BHET produced is expressed in grams. The molar mass of BHET is 254 g / mol. The feed mass of PET is expressed in grams. The molar mass of the repeating PET structural unit is 192 g / mol.

[0066] Example 1

[0067] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0068] (1) Preparation of materials;

[0069] CTAB aqueous solution: concentration 0.01 g / mL;

[0070] γ-Fe2O3;

[0071] Ammonia solution: 25% by mass;

[0072] TEOS solution: 50% by volume, solvent is ethanol;

[0073] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0074] DBU;

[0075] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0076] Ethylene glycol;

[0077] (2) Preparation of γ-Fe2O3@meso-SiO2;

[0078] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to it. After stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment.

[0079] The mass ratio of γ-Fe₂O₃ to CTAB was 2:1, the mass-to-volume ratio of γ-Fe₂O₃ to ammonia solution was 3.0 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.5:1. The ultrasonic treatment frequency was 50 kHz, the ultrasonic treatment time was 20 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 °C, the stirring speed was 220 r / min, and the stirring time was 24 h.

[0080] (b) γ-Fe2O3@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain γ-Fe2O3@meso-SiO2; wherein the heating rate of calcination was 5°C / min, and the temperature was raised to 350°C and held for 3h.

[0081] (3) Preparation of γ-Fe2O3@meso-SiO2-DBU;

[0082] γ-Fe2O3@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, γ-Fe2O3@meso-SiO2-DBU was obtained. The mass-to-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water was 1 g:155 mL. The stirring temperature was 55 °C, the stirring speed was 200 r / min, and the reaction time was 24 h.

[0083] like Figure 10As shown, the prepared γ-Fe2O3@meso-SiO2-DBU consists of γ-Fe2O3@meso-SiO2 and DBU. γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 1 and the shell being meso-SiO2 2. The average particle size of the core is 220 nm, the average thickness of the shell is 12 nm, and the average pore size of the shell is 3 nm. DBU 3 forms ion pairs with the shell of γ-Fe2O3@meso-SiO2 through electrostatic attraction and is enriched in the mesopores of meso-SiO2. The content of DBU 3 in γ-Fe2O3@meso-SiO2-DBU is 3.52 wt%.

[0084] (4) Preparation of BHET;

[0085] PET, ethylene glycol, and γ-Fe2O3@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:6, and the mass of γ-Fe2O3@meso-SiO2-DBU was 1.6% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 240℃ for 3 hours and a pressure of 0.9 MPa.

[0086] Tests showed that the conversion rate of PET was 100% and the yield of BHET was 84.85%.

[0087] like Figure 13 and Figure 14 As shown, X-ray photoelectron spectroscopy (XPS) analysis was performed on the γ-Fe₂O₃@meso-SiO₂-DBU prepared in this embodiment. The characteristic peak of Fe 2p³ / ₂ was located at 710.5 eV, and the characteristic peak of Fe 2p¹ / ₂ was located at 724.0 eV. These peak positions match the characteristic peak positions of γ-Fe₂O₃, proving the stability of the material's core structure. The N 1s binding energy of this sample significantly increased to 400.0 eV and exhibited a single symmetrical characteristic peak, indicating that all detected nitrogen elements were in the same chemical environment; this result confirms that the DBU has been completely protonated to form [DBUH]. + Positive charges accumulate on nitrogen atoms, reducing their electron cloud density. If DBU is physically adsorbed onto the material surface as a free base, the N 1s spectrum will show double peaks or broad peaks, corresponding to two nitrogen chemical environments; however, the single symmetrical N 1s characteristic peak in this sample further proves that DBU has been completely protonated. When DBU is introduced as a basic reagent, the silanol groups (Si–OH) on the meso-SiO2 surface undergo partial deprotonation to generate siloxy groups (Si–O). - DBU accepts protons to form [DBUH]. + and Si–O -Ion pairs are formed through electrostatic interactions.

[0088] Example 2

[0089] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0090] (1) Preparation of materials;

[0091] CTAB aqueous solution: concentration 0.03 g / mL;

[0092] γ-Fe2O3;

[0093] Ammonia solution: 25% by mass;

[0094] TEOS solution: 30% by volume, solvent is ethanol;

[0095] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.1:1;

[0096] DBU;

[0097] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0098] Ethylene glycol;

[0099] (2) Preparation of γ-Fe2O3@meso-SiO2;

[0100] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to it. After stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment.

[0101] The mass ratio of γ-Fe2O3 to CTAB was 2:1, the mass-to-volume ratio of γ-Fe2O3 to ammonia solution was 2.0 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 40 kHz, the ultrasonic treatment time was 10 min, the dropping rate of TEOS solution was 1.5 mL / min, the stirring temperature was 35 ℃, the stirring speed was 240 r / min, and the stirring time was 36 h.

[0102] (b) γ-Fe2O3@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain γ-Fe2O3@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 500°C and held for 2 hours.

[0103] The prepared γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 and the shell being composed of meso-SiO2. The average particle size of the core is 220 nm, the average thickness of the shell is 15 nm, and the average pore size of the shell is 2 nm.

[0104] (3) Preparation of γ-Fe2O3@meso-SiO2-DBU;

[0105] γ-Fe2O3@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, γ-Fe2O3@meso-SiO2-DBU was obtained. The mass-to-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water was 3 g:155 mL. The stirring temperature was 50 °C, the stirring speed was 240 r / min, and the reaction time was 12 h.

[0106] The prepared γ-Fe2O3@meso-SiO2-DBU consists of γ-Fe2O3@meso-SiO2 and DBU. DBU forms ion pairs with the shell of γ-Fe2O3@meso-SiO2 through electrostatic attraction and is enriched within the mesopores of meso-SiO2. The DBU content in γ-Fe2O3@meso-SiO2-DBU is 3.92 wt%.

[0107] (4) Preparation of BHET;

[0108] PET, ethylene glycol, and γ-Fe2O3@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:3, and the mass of γ-Fe2O3@meso-SiO2-DBU was 2.4% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.5 MPa.

[0109] Tests showed that the conversion rate of PET was 99.99% and the yield of BHET was 84.83%.

[0110] Example 3

[0111] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0112] (1) Preparation of materials;

[0113] CTAB aqueous solution: concentration 0.01 g / mL;

[0114] γ-Fe2O3;

[0115] Ammonia solution: 25% by mass;

[0116] TEOS solution: 50% by volume, solvent is ethanol;

[0117] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0118] DBU;

[0119] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0120] Ethylene glycol;

[0121] (2) Preparation of γ-Fe2O3@meso-SiO2;

[0122] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to it. After stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment.

[0123] The mass ratio of γ-Fe2O3 to CTAB was 5:3, the mass-to-volume ratio of γ-Fe2O3 to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 30 kHz, the ultrasonic treatment time was 30 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 °C, the stirring speed was 240 r / min, and the stirring time was 24 h.

[0124] (b) γ-Fe2O3@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain γ-Fe2O3@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 550°C and held for 3h.

[0125] The prepared γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 and the shell being composed of meso-SiO2. The average particle size of the core is 218 nm, the average thickness of the shell is 16 nm, and the average pore size of the shell is 2.5 nm.

[0126] (3) Preparation of γ-Fe2O3@meso-SiO2-DBU;

[0127] γ-Fe2O3@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, γ-Fe2O3@meso-SiO2-DBU was obtained. The mass-to-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water was 3 g:155 mL. The stirring temperature was 55 °C, the stirring speed was 300 r / min, and the reaction time was 36 h.

[0128] The prepared γ-Fe2O3@meso-SiO2-DBU consists of γ-Fe2O3@meso-SiO2 and DBU. DBU forms ion pairs with the shell of γ-Fe2O3@meso-SiO2 through electrostatic attraction and is enriched within the mesopores of meso-SiO2. The DBU content in γ-Fe2O3@meso-SiO2-DBU is 8.83 wt%.

[0129] (4) Preparation of BHET;

[0130] PET, ethylene glycol, and γ-Fe2O3@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:8, and the mass of γ-Fe2O3@meso-SiO2-DBU was 3.2% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.7 MPa.

[0131] Tests showed that the conversion rate of PET was 100% and the yield of BHET was 84.82%.

[0132] Depend on Figure 1 As can be seen, the characteristic diffraction peak positions and relative intensities of the BHET prepared in this embodiment highly match those of the BHET standard PDF card, and there are no impurity phase diffraction peaks; Figure 2 As can be seen, the BHET prepared in this embodiment has the same peak position as the standard BHET in the proton NMR spectrum, and no impurity peaks appear; Figure 3 As can be seen, the infrared characteristic absorption peak of the BHET prepared in this embodiment coincides with that of the standard BHET; Figure 4 It is evident that the BHET prepared in this embodiment exhibits highly consistent thermogravimetric behavior with the standard BHET; Figure 5 It can be seen that the melting point of the BHET prepared in this embodiment deviates only slightly from that of the standard BHET; Figure 6 As can be seen, the BHET prepared in this embodiment is a white needle-like crystal, exhibiting the typical morphological characteristics of BHET after recrystallization.

[0133] Depend on Figure 7It can be seen that after γ-Fe2O3 is coated with meso-SiO2 and loaded with DBU, the characteristic peak shape and position of γ-Fe2O3 do not change significantly. Figure 8 The FTIR spectra shown further confirm the successful loading of DBU on the γ-Fe2O3@meso-SiO2 surface and its chemical interaction with meso-SiO2: compared to γ-Fe2O3 and γ-Fe2O3@meso-SiO2, γ-Fe2O3@meso-SiO2-DBU exhibits better performance in the 2900-2800 cm⁻¹ range. -1 A distinct C–H stretching vibration peak appears, which is attributed to the alkyl skeleton in the DBU molecule; more importantly, the Si–O–Si asymmetric stretching vibration peak (~1080 cm⁻¹) originally belonging to the shell is also present. -1 The intensity decreased, while the characteristic peak (~950 cm⁻¹) of the surface silanol group (Si–OH) was observed. -1 The significant weakening or even disappearance of [DBUH] indicates that DBU underwent an acid-base neutralization reaction with Si–OH, producing [DBUH]. + and Si–O - .

[0134] Depend on Figure 9 As can be seen, the γ-Fe2O3@meso-SiO2-DBU prepared in this embodiment exhibits a clear core-shell structure.

[0135] Figures 1-9 This indicates that the present embodiment successfully prepared a γ-Fe2O3@meso-SiO2-DBU core-shell magnetic catalyst with a structure that perfectly matches the design, stable crystal form, and effective loading of active sites. At the same time, the target product BHET with accurate molecular structure, crystal form highly consistent with the standard, no impurity byproducts, and excellent purity was also prepared.

[0136] Comparative Example 1

[0137] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0138] (1) Preparation of materials;

[0139] CTAB aqueous solution: concentration 0.01 g / mL;

[0140] Fe3O4;

[0141] Ammonia solution: 25% by mass;

[0142] TEOS solution: 50% by volume, solvent is ethanol;

[0143] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0144] DBU;

[0145] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0146] Ethylene glycol;

[0147] (2) Preparation of Fe3O4@meso-SiO2;

[0148] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. Fe3O4 was added to it, and after stirring and ultrasonic treatment, ammonia aqueous solution was added dropwise. Then TEOS solution was added dropwise. After continuous stirring and reaction, Fe3O4@SiO2 was obtained after post-treatment.

[0149] The mass ratio of Fe3O4 to CTAB was 5:3, the mass-to-volume ratio of Fe3O4 to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 30 kHz, the ultrasonic treatment time was 30 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 ℃, the stirring speed was 240 r / min, and the stirring time was 24 h.

[0150] (b) Fe3O4@SiO2 was calcined in a muffle furnace under a nitrogen atmosphere and then naturally cooled to 30°C to obtain Fe3O4@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 550°C and held for 3 hours.

[0151] The prepared Fe3O4@meso-SiO2 has a core-shell structure, with Fe3O4 as the core and meso-SiO2 as the shell. The average particle size of the core is 220 nm, the average thickness of the shell is 12 nm, and the average pore size of the shell is 3 nm.

[0152] (3) Preparation of Fe3O4@meso-SiO2-DBU;

[0153] Fe3O4@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, Fe3O4@meso-SiO2-DBU was obtained. The mass-to-volume ratio of Fe3O4@meso-SiO2 to the mixture of ethanol and water was 3 g:155 mL. The stirring temperature was 55 °C, the stirring speed was 300 r / min, and the reaction time was 36 h.

[0154] The prepared Fe3O4@meso-SiO2-DBU is composed of Fe3O4@meso-SiO2 and DBU, and the DBU content in Fe3O4@meso-SiO2-DBU is 8.83 wt%.

[0155] (4) Preparation of BHET;

[0156] PET, ethylene glycol, and Fe3O4@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:8, and the mass of Fe3O4@meso-SiO2-DBU was 3.2% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.7 MPa.

[0157] Tests showed that the conversion rate of PET was 90.78% and the yield of BHET was 66.86%.

[0158] Compared to Example 3, Comparative Example 1 showed a significant decrease in PET conversion and BHET yield. This is because Comparative Example 1 replaced the magnetic core with Fe3O4, which has an inverse spinel structure and contains Fe on its surface. 2+ with Fe 3+ Fe 2+ d 6 Its electronic configuration makes its Lewis acidity significantly weaker than that of Fe in γ-Fe₂O₃. 3+ It cannot effectively activate the ester bonds of PET; at the same time, in the high-temperature alkaline alcoholysis system, Fe 2+ It is easily oxidized, which can cause magnetic nucleus phase transition and damage to the meso-SiO2 shell. It cannot form stable interfacial bonds and synergistic catalytic system, ultimately leading to insufficient depolymerization of PET and a significant decrease in the conversion rate of PET and the yield of BHET.

[0159] Comparative Example 2

[0160] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0161] (1) Preparation of materials;

[0162] CTAB aqueous solution: concentration 0.01 g / mL;

[0163] NiO;

[0164] Ammonia solution: 25% by mass;

[0165] TEOS solution: 50% by volume, solvent is ethanol;

[0166] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0167] DBU;

[0168] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0169] Ethylene glycol;

[0170] (2) Preparation of NiO@meso-SiO2;

[0171] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. NiO was added to the solution, and after stirring and ultrasonic treatment, ammonia solution was added dropwise. Then TEOS solution was added dropwise. After continuous stirring and reaction, NiO@SiO2 was obtained after post-treatment.

[0172] The mass ratio of NiO to CTAB was 5:3, the mass-to-volume ratio of NiO to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 30 kHz, the ultrasonic treatment time was 30 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 °C, the stirring speed was 240 r / min, and the stirring time was 24 h.

[0173] (b) NiO@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain NiO@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 550°C and held for 3 hours.

[0174] The prepared NiO@meso-SiO2 has a core-shell structure, with the core being NiO and the shell being composed of meso-SiO2. The average particle size of the core is 230 nm, the average thickness of the shell is 15 nm, and the average pore size of the shell is 2.5 nm.

[0175] (3) Preparation of NiO@meso-SiO2-DBU;

[0176] NiO@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, NiO@meso-SiO2-DBU was obtained. The mass-to-volume ratio of NiO@meso-SiO2 to the mixture of ethanol and water was 3 g:155 mL. The stirring temperature was 55 °C, the stirring speed was 300 r / min, and the reaction time was 36 h.

[0177] The prepared NiO@meso-SiO2-DBU consists of NiO@meso-SiO2 and DBU, with the DBU content in NiO@meso-SiO2-DBU being 8.83 wt%.

[0178] (4) Preparation of BHET;

[0179] PET, ethylene glycol, and NiO@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:8, and the mass of NiO@meso-SiO2-DBU was 3.2% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.7 MPa.

[0180] Tests showed that the conversion rate of PET was 65.65% and the yield of BHET was 47.71%.

[0181] Compared to Example 3, Comparative Example 2 showed a significant decrease in PET conversion rate and BHET yield. This is because Comparative Example 2 replaced the magnetic core with NiO, and NiO contains Ni... 2+ The Lewis acidity is much weaker than that of Fe with high charge density on the γ-Fe₂O₃ surface. 3+ It is almost impossible for it to effectively coordinate with the carbonyl oxygen of the PET ester group, thus failing to achieve ester bond polarization and lower the reaction energy barrier. This makes it difficult to exert the substrate activation effect of the Lewis acid site, resulting in the failure of the Lewis acid-mesoporous confined enrichment-Brønsted base nucleophile generation ternary synergistic catalytic system. Ultimately, the degree of PET depolymerization is extremely low, and the conversion rate of PET and the yield of BHET decrease significantly.

[0182] Comparative Example 3

[0183] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0184] (1) Preparation of materials;

[0185] CTAB aqueous solution: concentration 0.01 g / mL;

[0186] Co3O4;

[0187] Ammonia solution: 25% by mass;

[0188] TEOS solution: 50% by volume, solvent is ethanol;

[0189] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0190] DBU;

[0191] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0192] Ethylene glycol;

[0193] (2) Preparation of Co3O4@meso-SiO2;

[0194] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. Co3O4 was added to it, and after stirring and ultrasonic treatment, ammonia aqueous solution was added dropwise. Then TEOS solution was added dropwise. After continuous stirring and reaction, Co3O4@SiO2 was obtained after post-treatment.

[0195] The mass ratio of Co3O4 to CTAB was 5:3, the mass-to-volume ratio of Co3O4 to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 30 kHz, the ultrasonic treatment time was 30 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 ℃, the stirring speed was 240 r / min, and the stirring time was 24 h.

[0196] (b) Co3O4@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain Co3O4@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 550°C and held for 3 hours.

[0197] The prepared Co3O4@meso-SiO2 has a core-shell structure, with the core being Co3O4 and the shell being composed of meso-SiO2. The average particle size of the core is 225 nm, the average thickness of the shell is 14 nm, and the average pore size of the shell is 2.9 nm.

[0198] (3) Preparation of Co3O4@meso-SiO2-DBU;

[0199] Co3O4@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, Co3O4@meso-SiO2-DBU was obtained. The mass-to-volume ratio of Co3O4@meso-SiO2 to the mixture of ethanol and water was 3 g:155 mL. The stirring temperature was 55 °C, the stirring speed was 300 r / min, and the reaction time was 36 h.

[0200] The prepared Co3O4@meso-SiO2-DBU consists of Co3O4@meso-SiO2 and DBU, with the DBU content in Co3O4@meso-SiO2-DBU being 8.83 wt%.

[0201] (4) Preparation of BHET;

[0202] PET, ethylene glycol, and Co3O4@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:8, and the mass of Co3O4@meso-SiO2-DBU was 3.2% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.7 MPa.

[0203] Tests showed that the conversion rate of PET was 90.22% and the yield of BHET was 79.11%.

[0204] Compared to Example 3, Comparative Example 3 showed a significant decrease in PET conversion and BHET yield. This is because Comparative Example 3 replaced the magnetic core with Co3O4, where Co3O4 contains Co... 2+ / Co 3+ The Lewis acidity is much weaker than that of Fe with high charge density on the γ-Fe₂O₃ surface. 3 + The coordination activation ability of the carbonyl oxygen of PET ester group is insufficient, which cannot fully polarize the ester bond and reduce the energy barrier of alcoholysis reaction, resulting in a significant reduction in the effect of the ternary synergistic catalytic system. At the same time, the interfacial bond formed by Co3O4 and the shell is less stable than the Si-O-Fe bond, and the structural stability is insufficient under high temperature alkaline system, which cannot continuously ensure the efficient operation of the synergistic catalytic system. In the end, the degree of PET depolymerization is extremely low, and the conversion rate of PET and the yield of BHET decrease significantly.

[0205] Comparative Example 4

[0206] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0207] (1) Preparation of materials;

[0208] CTAB aqueous solution: concentration 0.01 g / mL;

[0209] γ-Fe2O3;

[0210] Ammonia solution: 25% by mass;

[0211] TEOS solution: 50% by volume, solvent is ethanol;

[0212] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0213] APTES (3-aminopropyltriethoxysilane);

[0214] DBU;

[0215] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0216] Ethylene glycol;

[0217] (2) Preparation of γ-Fe2O3@meso-SiO2;

[0218] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to it. After stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment.

[0219] The mass ratio of γ-Fe2O3 to CTAB was 5:3, the mass-to-volume ratio of γ-Fe2O3 to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 30 kHz, the ultrasonic treatment time was 30 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 °C, the stirring speed was 240 r / min, and the stirring time was 24 h.

[0220] (b) γ-Fe2O3@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain γ-Fe2O3@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 550°C and held for 3h.

[0221] The prepared γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 and the shell being composed of meso-SiO2. The average particle size of the core is 220 nm, the average thickness of the shell is 14 nm, and the average pore size of the shell is 3 nm.

[0222] (3) Preparation of γ-Fe2O3@meso-SiO2@APT-DBU;

[0223] γ-Fe₂O₃@meso-SiO₂ was dispersed in a mixture of ethanol and water. APTES was added for the first stirring reaction, followed by DBU for the second stirring reaction. After post-processing, γ-Fe₂O₃@meso-SiO₂@APT-DBU was obtained. The mass-to-volume ratio of γ-Fe₂O₃@meso-SiO₂ to the ethanol-water mixture was 3 g:155 mL, and the mass ratio of APTES to γ-Fe₂O₃@meso-SiO₂ was 1:2. The first stirring reaction was carried out at 55℃, 300 r / min, and for 3 h; the second stirring reaction was carried out at 55℃, 300 r / min, and for 36 h.

[0224] The prepared γ-Fe2O3@meso-SiO2@APT-DBU consists of γ-Fe2O3@meso-SiO2 and DBU. The DBU is covalently bonded to the shell of γ-Fe2O3@meso-SiO2 via a 3-aminopropyl group. The DBU content in γ-Fe2O3@meso-SiO2@APT-DBU is 8.83 wt%.

[0225] (4) Preparation of BHET;

[0226] PET, ethylene glycol, and γ-Fe2O3@meso-SiO2@APT-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:8, and the mass of γ-Fe2O3@meso-SiO2@APT-DBU was 3.2% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.7 MPa.

[0227] Tests showed that the conversion rate of PET was 84.37% and the yield of BHET was 68.93%.

[0228] Compared with Example 3, Comparative Example 4 showed a significant decrease in both PET conversion and BHET yield. This is because in Comparative Example 4, DBU was rigidly connected to SiO2 via a propyl chain through covalent bonds. On the one hand, the large steric hindrance of the covalent bonds resulted in poor accessibility of the DBU active sites and hindered mass transfer of reactants, preventing the full exertion of catalytic activity. On the other hand, in the high-temperature alkaline system of PET alcoholysis, covalent bonds are prone to breakage, causing DBU loss and catalyst deactivation, which prevents the catalyst from playing a continuous and stable catalytic role. Ultimately, this led to insufficient PET depolymerization, resulting in a significant decrease in both PET conversion and BHET yield.

[0229] Example 4

[0230] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0231] (1) Preparation of materials;

[0232] CTAB aqueous solution: concentration 0.05 g / mL;

[0233] γ-Fe2O3;

[0234] Ammonia solution: 25% by mass;

[0235] TEOS solution: 10% by volume, solvent is ethanol;

[0236] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.5:1;

[0237] DBU;

[0238] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0239] Ethylene glycol;

[0240] (2) Preparation of γ-Fe2O3@meso-SiO2;

[0241] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to it. After stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment.

[0242] The mass ratio of γ-Fe2O3 to CTAB was 5:1, the mass-to-volume ratio of γ-Fe2O3 to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.25:1. The ultrasonic treatment frequency was 40 kHz, the ultrasonic treatment time was 25 min, the dropping rate of TEOS solution was 0.8 mL / min, the stirring temperature was 25 °C, the stirring speed was 240 r / min, and the stirring time was 24 h.

[0243] (b) γ-Fe2O3@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 30°C to obtain γ-Fe2O3@meso-SiO2; wherein the heating rate of calcination was 5°C / min, and the temperature was raised to 500°C and held for 3h.

[0244] The prepared γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 and the shell being composed of meso-SiO2. The average particle size of the core is 220 nm, the average thickness of the shell is 13 nm, and the average pore size of the shell is 2.6 nm.

[0245] (3) Preparation of γ-Fe2O3@meso-SiO2-DBU;

[0246] γ-Fe2O3@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, γ-Fe2O3@meso-SiO2-DBU was obtained. The mass-to-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water was 5 g:155 mL. The stirring temperature was 60 °C, the stirring speed was 240 r / min, and the reaction time was 24 h.

[0247] The prepared γ-Fe2O3@meso-SiO2-DBU consists of γ-Fe2O3@meso-SiO2 and DBU. DBU forms ion pairs with the shell of γ-Fe2O3@meso-SiO2 through electrostatic attraction and is enriched within the mesopores of meso-SiO2. The DBU content in γ-Fe2O3@meso-SiO2-DBU is 1.2 wt%.

[0248] (4) Preparation of BHET;

[0249] PET, ethylene glycol, and γ-Fe2O3@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:6, and the mass of γ-Fe2O3@meso-SiO2-DBU was 4% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 230℃ for 4 hours and a pressure of 0.7 MPa.

[0250] Tests showed that the conversion rate of PET was 99.99% and the yield of BHET was 83.83%.

[0251] Example 5

[0252] A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, the specific steps of which are as follows:

[0253] (1) Preparation of materials;

[0254] CTAB aqueous solution: concentration 0.01 g / mL;

[0255] γ-Fe2O3;

[0256] Ammonia solution: mass fraction 28%;

[0257] TEOS solution: 50% by volume, solvent is ethanol;

[0258] A mixture of ethanol and water: the volume ratio of ethanol to water is 0.2:1;

[0259] DBU;

[0260] PET: In fibrous form, with an intrinsic viscosity of 0.65 dL / g;

[0261] Ethylene glycol;

[0262] (2) Preparation of γ-Fe2O3@meso-SiO2;

[0263] (a) CTAB aqueous solution was ultrasonically dispersed to form a uniform micelle solution. γ-Fe2O3 was added to it. After stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment.

[0264] The mass ratio of γ-Fe2O3 to CTAB was 2:1, the mass-to-volume ratio of γ-Fe2O3 to ammonia solution was 2.5 g:1 mL, and the volume ratio of TEOS to ammonia solution was 1.1:1. The ultrasonic treatment frequency was 50 kHz, the ultrasonic treatment time was 20 min, the dropping rate of TEOS solution was 1.2 mL / min, the stirring temperature was 25 °C, the stirring speed was 260 r / min, and the stirring time was 36 h.

[0265] (b) γ-Fe2O3@SiO2 was calcined in an air atmosphere in a muffle furnace and then naturally cooled to 25°C to obtain γ-Fe2O3@meso-SiO2; wherein the heating rate of calcination was 2°C / min, and the temperature was raised to 500°C and held for 2h.

[0266] The prepared γ-Fe2O3@meso-SiO2 has a core-shell structure, with the core being γ-Fe2O3 and the shell being composed of meso-SiO2. The average particle size of the core is 219 nm, the average thickness of the shell is 12 nm, and the average pore size of the shell is 2.3 nm.

[0267] (3) Preparation of γ-Fe2O3@meso-SiO2-DBU;

[0268] γ-Fe2O3@meso-SiO2 was dispersed in a mixture of ethanol and water, DBU was added and the mixture was stirred and reacted. After post-processing, γ-Fe2O3@meso-SiO2-DBU was obtained. The mass-to-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water was 3 g:155 mL. The stirring temperature was 55 °C, the stirring speed was 240 r / min, and the reaction time was 24 h.

[0269] The prepared γ-Fe2O3@meso-SiO2-DBU consists of γ-Fe2O3@meso-SiO2 and DBU. DBU forms ion pairs with the shell of γ-Fe2O3@meso-SiO2 through electrostatic attraction and is enriched within the mesopores of meso-SiO2. The DBU content in γ-Fe2O3@meso-SiO2-DBU is 3.72 wt%.

[0270] (4) Preparation of BHET;

[0271] PET, ethylene glycol, and γ-Fe2O3@meso-SiO2-DBU were mixed evenly and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET was obtained. The mass ratio of PET to ethylene glycol was 1:6, and the mass of γ-Fe2O3@meso-SiO2-DBU was 4.8% of the mass of PET. The alcoholysis reaction was carried out at a temperature of 210℃ for 5 hours and a pressure of 0.25 MPa.

[0272] Tests showed that the conversion rate of PET was 99.98% and the yield of BHET was 80.27%.

Claims

1. A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol, characterized in that, The catalyst used is γ-Fe2O3@meso-SiO2-DBU, which is composed of γ-Fe2O3@meso-SiO2 and DBU. γ-Fe2O3@meso-SiO2 has a core-shell structure, with γ-Fe2O3 as the core and meso-SiO2 as the shell. DBU and the shell form ion pairs through electrostatic attraction and are enriched in the mesopores of meso-SiO2.

2. The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to claim 1, characterized in that, The average core diameter is 218-220 nm, the average shell thickness is 12-16 nm, the average shell pore size is 2.3-3 nm, and the DBU content in γ-Fe2O3@meso-SiO2-DBU is 1.2 wt%-8.83 wt%.

3. The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to claim 1, characterized in that, The preparation process of γ-Fe2O3@meso-SiO2-DBU is as follows: γ-Fe2O3@meso-SiO2 is dispersed in a mixture of ethanol and water, DBU is added and stirred to react, and after post-processing, γ-Fe2O3@meso-SiO2-DBU is obtained. The volume ratio of ethanol to water is 0.1:1-0.5:1, the mass-volume ratio of γ-Fe2O3@meso-SiO2 to the mixture of ethanol and water is 1g:155mL-5g:155mL, the stirring temperature is 50-60℃, the stirring speed is 200-300r / min, and the stirring time is 12-36h.

4. The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to claim 1, characterized in that, The preparation process of γ-Fe2O3@meso-SiO2 is as follows: (a) CTAB solution was ultrasonically dispersed to form a micelle solution, γ-Fe2O3 was added to it, and after stirring and ultrasonic treatment, ammonia solution was added dropwise, followed by TEOS solution. After continuous stirring and reaction, γ-Fe2O3@SiO2 was obtained after post-treatment. (b) After calcining γ-Fe2O3@SiO2 in a muffle furnace, it is cooled to 25-30℃ to obtain γ-Fe2O3@meso-SiO2.

5. The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to claim 1, characterized in that, The specific process is as follows: PET, ethylene glycol and γ-Fe2O3@meso-SiO2-DBU are mixed and then transferred to a hydrothermal reactor for alcoholysis reaction. After post-processing, BHET is obtained.

6. The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to claim 5, characterized in that, The mass ratio of PET to ethylene glycol is 1:3-1:8, and the mass of γ-Fe2O3@meso-SiO2-DBU is 1.6%-4.8% of the mass of PET. The alcoholysis reaction is carried out at a temperature of 210-240℃ for 3-5 hours and at a pressure of 0.25-0.9 MPa.

7. The method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to claim 5, characterized in that, PET is available in one or more of the following forms: film, bottle, fiber, and powder.

8. A method for preparing BHET by catalytic alcoholysis of PET with ethylene glycol according to any one of claims 1 to 7, characterized in that, The conversion rate of PET is 99.98%-100%, and the yield of BHET is 80.27%-84.85%.

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